Manufacturing Processes [2 ed.] 9788120340824

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Manufacturing Processes SECOND EDITION

J.P. Kaushish Formerly, Deputy Director Central Building Research Institute, Roorkee, and Faculty, University of Roorkee (now IIT Roorkee)

New Delhi-110001 2010

MANUFACTURING PROCESSES, Second Edition J.P. Kaushish © 2010 by PHI Learning Private Limited, New Delhi. All rights reserved. No part of this book may be reproduced in any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-978-81-203-4082-4 The export rights of this book are vested solely with the publisher. Second Printing (Second Edition)

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August, 2010

Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Mohan Makhijani at Rekha Printers Private Limited, New Delhi-110020.

Contents

PREFACE ............................................................................................................................................ xxv PREFACE TO THE FIRST EDITION ................................................................................................... xxvii

1

MANUFACTURING: PRINCIPLES AND PROCESSES ................................................................................. 1–21 1.1 1.2 1.3 1.4 1.5

INTRODUCTION ................................................................................................................ 1 CONCEPT OF MANUFACTURING ...................................................................................... 2 MANUFACTURING PROCESS ........................................................................................... 3 TECHNOLOGY ................................................................................................................... 3 PRODUCTION METHOD .................................................................................................... 3 1.5.1 Types of Production Method .................................................................... 4 1.6 PRODUCTIVITY ................................................................................................................. 4 1.6.1 Important Aspects for Increasing Productivity ....................................... 4 1.7 MANUFACTURING ATTRIBUTES ...................................................................................... 6 1.8 TOTAL QUALITY MANAGEMENT (TQM) .......................................................................... 6 1.9 PRODUCT DESIGN AND CONCURRENT ENGINEERING ................................................. 7 1.10 SELECTING PRODUCT MATERIAL .................................................................................. 8 1.10.1 Types of Materials .................................................................................... 8 1.10.2 Properties of Materials ............................................................................ 9 1.11 MANUFACTURING PROCESSES ....................................................................................... 9 1.11.1 Classification of Manufacturing Processes .......................................... 10 1.11.2 Other Manufacturing Processes ............................................................ 11 1.12 INTRODUCTION TO BASIC MANUFACTURING PROCESSES ........................................ 12 1.13 PRODUCTION SHOPS ..................................................................................................... 18 1.14 MODERN CONCEPTS OF MANUFACTURING ................................................................. 18 1.15 HUMAN-FACTORS ENGINEERING AND INDUSTRIAL SAFETY ..................................... 19 1.16 MANUFACTURING COST ................................................................................................ 20 REVIEW QUESTIONS .................................................................................................................. 20

2

MATERIALS: STRUCTURES AND PROPERTIES .................................................................................... 22–72 2.1 2.2

INTRODUCTION .............................................................................................................. 22 SELECTING MATERIALS FOR MANUFACTURING ......................................................... 23 iii

iv

Contents

2.3 2.4 2.5 2.6 2.7 2.8 2.9

METALS AND NON-METALS .......................................................................................... 23 NATURE OF METALS ...................................................................................................... 24 STRUCTURE OF METALS ............................................................................................... 24 PHYSICAL STATE OF METALS ...................................................................................... 25 SPACE LATTICE (OR CRYSTAL LATTICE) ..................................................................... 26 DENDRITES .................................................................................................................... 27 GRAIN AND GRAIN BOUNDARY ..................................................................................... 28 2.9.1 Effect of Grain Structure and Grain Size on Properties of Metals ....... 28 2.10 TYPES OF SPACE LATTICE ............................................................................................ 29 2.11 GRAIN REFINING ............................................................................................................ 30 2.12 IMPERFECTIONS IN LATTICE STRUCTURE OF A CRYSTAL ........................................ 31 2.13 SOLID-STATE DIFFUSION .............................................................................................. 32 2.14 CRYSTAL LATTICE UNDER STRESS ............................................................................. 33 2.15 DEFORMATION BY SLIP ................................................................................................. 33 2.15.1 Slip by Dislocation Movement ............................................................... 34 2.16 DEFORMATION BY TWINNING ....................................................................................... 35 2.17 WORK-HARDENING (OR STRAIN-HARDENING) ............................................................ 36 2.18 COLD-WORKING ............................................................................................................. 37 2.18.1 Metals Most Adaptable to Cold-working ............................................... 38 2.18.2 Cold-working Processes ........................................................................ 38 2.19 HOT-WORKING ................................................................................................................ 38 2.20 ALLOY ............................................................................................................................. 39 2.20.1 Purpose of Alloying ............................................................................... 39 2.20.2 Phases of an Alloy and Phase Diagrams ............................................... 40 2.21 PHASES OF STEEL ......................................................................................................... 41 2.21.1 Few More Terms Defined ....................................................................... 42 2.22 IRON-CARBON EQUILIBRIUM DIAGRAM ....................................................................... 42 2.23 PROPERTIES OF METALS AND ALLOYS ....................................................................... 45 2.23.1 Physical Properties ................................................................................ 45 2.23.2 Chemical Properties .............................................................................. 47 2.23.3 Mechanical Properties ........................................................................... 48 2.23.4 Fabricating Properties ........................................................................... 53 2.24 STRESSES AND STRAINS .............................................................................................. 56 2.24.1 Stress–Strain Diagram (or s–e Diagram) ............................................... 56 2.24.2 Engineering Stress and True Stress ..................................................... 60 2.24.3 Computing Various Tensile Properties from Tensile Test Results ....... 61 2.24.4 Compressive Strength ........................................................................... 62 2.24.5 Bending Stress ....................................................................................... 63 2.24.6 Shear Strength ....................................................................................... 63 2.24.7 Working Stress (or Design Stress) ........................................................ 63 2.24.8 Factor of Safety ...................................................................................... 64 2.25 INSPECTION, TESTING AND QUALITY CONTROL OF METALS ................................... 64 2.25.1 Tensile, Compression and Bending Tests ............................................. 65 2.25.2 Hardness Testing ................................................................................... 65 2.25.3 Impact Test ............................................................................................. 67 2.25.4 Fatigue Test ............................................................................................ 68 2.25.5 Creep Test .............................................................................................. 68 2.25.6 Non-destructive Inspection and Testing ............................................... 68 REVIEW QUESTIONS .................................................................................................................. 69

Contents

3

v

FERROUS METALS: IRONS AND STEELS ............................................................................................ 73–131 3.1 3.2 3.3 3.4

3.5

3.6

3.7 3.8 3.9 3.10 3.11 3.12 3.13

3.14 3.15 3.16

3.17

3.18

INTRODUCTION .............................................................................................................. 73 FERROUS AND NON-FERROUS METALS ....................................................................... 73 GENERAL TERMS ABOUT IRON AND STEEL ............................................................... 74 PIG IRON ......................................................................................................................... 75 3.4.1 Impurities in Pig Iron ............................................................................ 75 3.4.2 Use of Pig Iron ........................................................................................ 75 CAST IRON ...................................................................................................................... 75 3.5.1 Production of Cast Iron ......................................................................... 76 3.5.2 Composition of Cast Iron ...................................................................... 76 3.5.3 Properties of Cast Iron .......................................................................... 76 3.5.4 Effect of Graphite Shape on the Properties of Cast Iron ...................... 77 3.5.5 Applications of Cast Iron ....................................................................... 77 TYPES OF CAST IRON .................................................................................................... 77 3.6.1 Grey Cast Iron ........................................................................................ 78 3.6.2 White Cast Iron (or Chilled Cast Iron) ................................................... 79 3.6.3 Mottled Cast Iron ................................................................................... 80 3.6.4 Malleable Cast Iron ................................................................................ 80 3.6.5 Ductile Cast Iron (or Nodular Cast Iron) ............................................... 83 3.6.6 High Duty Cast Iron (or Meehanite Cast Iron) ...................................... 84 3.6.7 Alloy Cast Iron ....................................................................................... 86 3.6.8 Wrought Iron .......................................................................................... 88 MECHANICAL AND FABRICATION PROPERTIES OF CAST IRONS .............................. 89 HEAT TREATMENT OF CAST IRONS ............................................................................. 90 INDIAN STANDARDS FOR CAST IRONS ........................................................................ 90 ABRASION RESISTANT IRON CASTINGS ...................................................................... 91 STEEL VS CAST IRON .................................................................................................... 92 3.11.1 Composition of Steel ............................................................................. 92 CLASSIFICATION OF STEELS ........................................................................................ 92 CARBON STEELS (OR PLAIN CARBON STEELS) .......................................................... 93 3.13.1 Composition of Carbon Steels ............................................................... 93 3.13.2 Types of Carbon Steel ............................................................................ 94 ROLE OF CARBON IN STEELS ....................................................................................... 94 COMPARISON OF CAST IRON, MILD STEEL AND HIGH CARBON STEEL .................. 98 ALLOY STEELS ............................................................................................................... 99 3.16.1 Purpose of Alloying Steels .................................................................... 99 3.16.2 Effects of Alloying Elements on the Properties of Steel .................... 100 CLASSIFICATION OF ALLOY STEEL ........................................................................... 101 3.17.1 Nickel Steels ........................................................................................ 102 3.17.2 Nickel–Chrome Steels .......................................................................... 102 3.17.3 Manganese Steels ................................................................................ 102 3.17.4 Tool and Die Steels .............................................................................. 103 3.17.5 Special Alloy Steels ............................................................................. 104 3.17.6 Stainless Steels ................................................................................... 104 3.17.7 Corrosion Resistant Alloy Steels ........................................................ 105 3.17.8 High Speed Steels (HSS) ...................................................................... 106 CUTTING TOOL MATERIALS ........................................................................................ 106

vi

Contents

3.19

CUTTING ALLOYS ......................................................................................................... 107 3.19.1 Cast Alloys ........................................................................................... 108 3.19.2 Cemented Carbides .............................................................................. 108 3.19.3 Coated Tools ........................................................................................ 109 3.19.4 Oxide Ceramics Tools Material ............................................................ 109 3.19.5 Nitride Tool Materials .......................................................................... 110 3.19.6 Diamond Tools ..................................................................................... 110 3.19.7 Abrasives .............................................................................................. 110 3.20 HARD FACING ALLOYS ................................................................................................ 111 3.21 LOW TEMPERATURE APPLICATION ALLOYS ............................................................. 111 3.22 MAGNETIC ALLOY STEELS ......................................................................................... 111 3.23 MARAGING STEELS ..................................................................................................... 112 3.24 AUSTENITIC MANGANESE STEEL .............................................................................. 112 3.25 CLASSIFICATION OF STEELS BASED ON COMMERCIAL NAMES ............................. 112 3.26 EFFECTS OF STEEL MAKING PROCESSES ON QUALITY OF STEEL ........................ 114 3.26.1 Type of Steel ......................................................................................... 114 3.26.2 Grain Size ............................................................................................ 115 3.26.3 Effect of Hot-working on Metal ............................................................ 115 3.26.4 Effect of Cold-working on Metal .......................................................... 115 3.26.5 Effect of Heat Treatment ...................................................................... 116 3.27 FABRICATION CHARACTERISTICS OF FERROUS METALS ....................................... 116 3.27.1 Machinability ....................................................................................... 116 3.27.2 Formability ........................................................................................... 117 3.27.3 Weldability ........................................................................................... 117 3.28 RECOMMENDING STEELS AND CAST IRONS FOR MACHINE PARTS ....................... 118 3.29 MARKET FORMS OF SUPPLY OF PIG IRONS AND STEELS ....................................... 121 3.30 CODING OF IRONS AND STEELS ................................................................................. 122 3.30.1 Coding of Plain and Alloy Castings ..................................................... 122 3.30.2 Coding of Steels ................................................................................... 124 3.31 CLASSIFICATION OF STEELS AS PER INTERNATIONAL STANDARDS ..................... 127 3.31.1 AISI Method of Classification .............................................................. 127 3.31.2 SAE Method of Classification .............................................................. 128 REVIEW QUESTIONS ................................................................................................................ 128

4

NON-FERROUS METALS AND OTHER MATERIALS .......................................................................... 132–165 4.1 4.2

4.3

4.4 4.5

INTRODUCTION ............................................................................................................ 132 ALUMINIUM ................................................................................................................... 132 4.2.1 Selection of Aluminium Alloys ........................................................... 133 4.2.2 Properties of Aluminium ..................................................................... 133 4.2.3 Uses of Aluminium .............................................................................. 134 ALUMINIUM ALLOYS .................................................................................................... 134 4.3.1 Duraluminium ...................................................................................... 135 4.3.2 Y-Alloy .................................................................................................. 135 4.3.3 Aluminium Casting Alloys .................................................................. 135 4.3.4 Porous Aluminium .............................................................................. 135 BIS SPECIFICATIONS FOR ALUMINIUM AND ITS ALLOYS ........................................ 136 COPPER ......................................................................................................................... 136 4.5.1 Properties of Copper ............................................................................ 136 4.5.2 Uses of Copper ..................................................................................... 137 4.5.3 Types of Copper ................................................................................... 137

Contents

4.6 4.7 4.8 4.9

4.10 4.11 4.12 4.13

4.14

4.15 4.16 4.17

4.18

4.19

4.20 4.21 4.22 4.23 4.24 4.25 4.26 4.27 4.28 4.29

vii

BIS SPECIFICATIONS FOR COPPER AND ITS ALLOYS .............................................. 137 ALLOYS OF COPPER .................................................................................................... 137 COPPER–ZINC ALLOYS (BRASSES) ............................................................................ 137 COPPER–TIN ALLOYS (BRONZES) .............................................................................. 139 4.9.1 Beryllium Bronze (or Beryllium Copper) ............................................. 139 4.9.2 Phosphor Bronze ................................................................................. 140 4.9.3 Manganese Bronze ............................................................................... 140 4.9.4 Gun Metal ............................................................................................. 140 4.9.5 Bell Metal ............................................................................................. 140 4.9.6 Silicon Bronze ..................................................................................... 140 4.9.7 Aluminium Bronze .............................................................................. 141 ZINC AND ITS ALLOYS ................................................................................................. 141 TIN AND ITS ALLOYS ................................................................................................... 141 4.11.1 Solders ................................................................................................. 142 NICKEL .......................................................................................................................... 142 NICKEL ALLOYS ........................................................................................................... 142 4.13.1 Grouping of Nickel Alloys .................................................................... 143 4.13.2 Monel ................................................................................................... 143 4.13.3 German Silver ...................................................................................... 144 4.13.4 Constantan ........................................................................................... 144 4.13.5 Inconel ................................................................................................. 144 LEAD AND ITS ALLOYS ............................................................................................... 144 4.14.1 Uses of Lead ......................................................................................... 145 4.14.2 BIS Specifications for Lead ................................................................. 145 LIGHT METALS ............................................................................................................. 145 MAGNESIUM AND ITS ALLOYS .................................................................................... 145 4.16.1 Dow Metal ............................................................................................ 146 LOW MELTING METALS AND ALLOYS ........................................................................ 146 4.17.1 Alloy Becoming Fluid between 150 and 540°C ................................... 146 4.17.2 Alloys Becoming Fluid between 94.5 and 150°C ................................ 147 4.17.3 Alloys Becoming Fluid at Less than 94.5°C ........................................ 147 BEARING METALS ........................................................................................................ 148 4.18.1 Main Requirements (or Characteristics) of an Antifriction Bearing Metal ................................................................... 148 CLASSIFICATION OF BEARING METALS .................................................................... 149 4.19.1 Soft Metal Bearings .............................................................................. 149 4.19.2 Hard Metal Bearing (or Bearing Bronzes) ........................................... 150 4.19.3 Other Bearing Metals ........................................................................... 150 4.19.4 Innovative Antifriction Bearing Materials and Techniques ................ 150 CHROMIUM ................................................................................................................... 151 COBALT ......................................................................................................................... 151 OTHER NON-FERROUS METALS ................................................................................. 151 SUPERALLOYS ............................................................................................................. 152 REFRACTORY METALS AND ALLOYS ......................................................................... 152 SHAPE-MEMORY ALLOYS ............................................................................................ 153 PRECIOUS METALS ...................................................................................................... 154 METALLIC GLASSES .................................................................................................... 154 NANOMATERIALS ......................................................................................................... 154 METALS FOR NUCLEAR ENGINEERING APPLICATIONS ........................................... 154

viii

Contents

4.30

OTHER IMPORTANT ENGINEERING MATERIALS ....................................................... 155 4.30.1 Timber .................................................................................................. 155 4.30.2 Abrasive Materials ............................................................................... 155 4.30.3 Ceramics .............................................................................................. 156 4.30.4 Silica .................................................................................................... 158 4.30.5 Glasses ................................................................................................. 158 4.30.6 Glass Ceramics .................................................................................... 159 4.30.7 Graphite ............................................................................................... 159 4.30.8 Diamond ............................................................................................... 159 4.30.9 Plastics and Polymers ......................................................................... 160 4.30.10 Composite Materials ............................................................................ 160 4.31 TYPES OF COMPOSITE MATERIALS ........................................................................... 161 4.31.1 Fibre Reinforced Plastics (or Reinforced Plastics) ............................. 161 4.31.2 Metal–Matrix Composites (or Fibre–Metal Composites) ..................... 163 4.31.3 Ceramic–Matrix Composites ................................................................ 163 4.32 SUPERCONDUCTORS ................................................................................................... 163 REVIEW QUESTIONS ................................................................................................................ 164

5

FOUNDRY PROCESSES: MOLDING AND CASTING ............................................................................. 166–277 5.1 5.2 5.3 5.4 5.5

5.6 5.7

5.8 5.9

5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17 5.18 5.19 5.20 5.21

INTRODUCTION ............................................................................................................ 166 A FOUNDRY SHOP ........................................................................................................ 167 TYPES OF FOUNDRIES ................................................................................................. 167 PREFERENCE FOR CASTING OVER OTHER PRODUCTION METHODS ..................... 167 FOUNDRY PROCESSES ................................................................................................ 168 5.5.1 Molding ................................................................................................ 168 5.5.2 Casting ................................................................................................. 170 CASTING IN GREEN SAND MOLD ................................................................................ 171 FOUNDRY TOOLS AND EQUIPMENT ........................................................................... 173 5.7.1 Hand Tools ........................................................................................... 174 5.7.2 Containers ........................................................................................... 176 MOLD MATERIALS AND THEIR SELECTION .............................................................. 177 MOLDING SANDS (OR FOUNDRY SANDS) ................................................................... 178 5.9.1 Natural Sands ...................................................................................... 178 5.9.2 Synthetic Sands ................................................................................... 178 5.9.3 Special Sands ....................................................................................... 179 CONSTITUENTS OF MOLDING SANDS ........................................................................ 179 SAND PREPARATION AND CONDITIONING ................................................................. 180 CHARACTERISTICS (OR PROPERTIES) OF MOLDING SANDS ................................... 181 SOME COMMONLY USED SAND MIXTURES (OR MOLDING SANDS) ......................... 182 5.13.1 Molding Sand for Casting Different Metals ......................................... 183 FOUNDRY BLACKINGS ................................................................................................. 184 PATTERNS ..................................................................................................................... 184 PATTERN COLOURS ..................................................................................................... 184 MATERIALS FOR PATTERNS ....................................................................................... 185 PATTERN ALLOWANCES .............................................................................................. 185 TYPES OF PATTERNS ................................................................................................... 187 CORES ........................................................................................................................... 191 TYPES OF CORE ........................................................................................................... 193 5.21.1 Types of Core according to the Core Material ..................................... 194 5.21.2 Types of Core according to the Position of Core Print ....................... 194

Contents

5.22 5.23 5.24 5.25 5.26

5.27

5.28 5.29 5.30 5.31

5.32 5.33

5.34

5.35

5.36

5.37

ix

CHARACTERISTICS OF A CORE .................................................................................. 195 CORE MATERIALS ........................................................................................................ 195 MAKING OF CORES ...................................................................................................... 196 TYPES OF MOLDS ........................................................................................................ 198 MOLDING METHODS .................................................................................................... 204 5.26.1 Molding Methods according to the Mold Materials ............................. 204 5.26.2 Molding Methods according to the Method of Making a Mold ........... 205 CASTING METHODS ..................................................................................................... 208 5.27.1 Sand Mold Casting ............................................................................... 208 5.27.2 Metallic Mold Casting .......................................................................... 209 5.27.3 Centrifugal Casting .............................................................................. 213 5.27.4 Precision Casting Processes ............................................................... 215 5.27.5 Full-mold Casting or Expendable-pattern Casting .............................. 217 5.27.6 Continuous Casting ............................................................................. 218 5.27.7 Chill Casting ........................................................................................ 218 CLEANING OR FETTLING OF CASTING ....................................................................... 220 REPAIR OF CASTINGS .................................................................................................. 221 METALS AND ALLOYS USED IN CASTING .................................................................. 221 METAL MELTING FURNACES ....................................................................................... 222 5.31.1 Types of Furnaces ................................................................................ 222 5.31.2 Protecting the Melt from Dissolved Gases .......................................... 228 INSPECTION AND TESTING OF CASTING ................................................................... 229 CASTING DEFECTS AND THEIR REMEDY .................................................................. 230 5.33.1 Metallic Projection ............................................................................... 231 5.33.2 Cavities ................................................................................................. 231 5.33.3 Discontinuities .................................................................................... 232 5.33.4 Defective Surface ................................................................................. 232 5.33.5 Incomplete Castings ............................................................................ 233 5.33.6 Incorrect Dimensions and Shapes ...................................................... 233 5.33.7 Inclusions ............................................................................................ 233 TESTING OF MOLDING SANDS .................................................................................... 234 5.34.1 Grain-fineness (or Sand Grain Size) Test ............................................ 235 5.34.2 Permeability Test ................................................................................. 236 5.34.3 Strength Test ....................................................................................... 238 5.34.4 Clay Content Test ................................................................................ 238 5.34.5 Moisture Content Test ......................................................................... 238 5.34.6 Mold Hardness Test ............................................................................. 238 POURING METAL IN SAND MOLDS ............................................................................. 238 5.35.1 Pouring Temperature ........................................................................... 239 5.35.2 Pouring Rate ........................................................................................ 240 DESIGNING GATING SYSTEM IN SAND MOLDS ......................................................... 240 5.36.1 Gating System ...................................................................................... 240 5.36.2 Vertical Element (Sprue) in a Gating System ...................................... 242 5.36.3 Gating Ratio (or Gate Ratio) ................................................................. 244 5.36.4 Designing of Risers .............................................................................. 248 SOLIDIFICATION OF CASTINGS ................................................................................... 258 5.37.1 Solidification of Pure Metals ............................................................... 259 5.37.2 Solidification of Alloys ........................................................................ 260

x

Contents

5.38

HEAT LOSS FROM CASTINGS DURING SOLIDIFICATION .......................................... 261 5.38.1 Effects of Cooling Rate ........................................................................ 261 5.38.2 Solidification Time ............................................................................... 262 5.38.3 Volume Change (or Shrinkage) during Solidification ......................... 262 5.39 SHRINKAGE DEFECTS ................................................................................................. 264 5.39.1 Shrinkage Defects Caused during Solidification of Molten Metal ....... 264 5.39.2 Shrinkage Defects Caused during Cooling of Solidified Casting to Room Temperature .......................................................................... 265 5.40 DIRECTIONAL SOLIDIFICATION .................................................................................. 267 5.41 COMPARATIVE STUDY OF VARIOUS CASTING METHODS ......................................... 269 5.42 CONSIDERATIONS FOR GOOD CASTING DESIGN ...................................................... 270 REVIEW QUESTIONS ................................................................................................................ 270 OBJECTIVE TYPE QUESTIONS ................................................................................................. 273

6

METAL MACHINING: PROCESSES AND MACHINE TOOLS ............................................................... 278–500 6.1 6.2

6.3

6.4

6.5

6.6 6.7

INTRODUCTION ............................................................................................................ 278 6.1.1 Classification of Machining Processes ............................................... 279 CUTTING TOOLS AND THEIR NOMENCLATURE ......................................................... 279 6.2.1 Classification of Cutting Tools ............................................................ 280 6.2.2 Angles of a Single-point Cutting Tool ................................................. 282 6.2.3 Nomenclature of a Lathe Tool ............................................................. 285 MECHANICS OF METAL CUTTING ............................................................................... 286 6.3.1 Formation of Chip ................................................................................ 286 6.3.2 Types of Chips ...................................................................................... 289 6.3.3 Chip Control and Chip Breakers ......................................................... 291 6.3.4 Orthogonal and Oblique Cutting ......................................................... 291 6.3.5 Chip Thickness Ratio (or Cutting Ratio) ............................................. 293 6.3.6 Velocity Relationship in Orthogonal Cutting ...................................... 295 6.3.7 Forces Acting on Chip in Orthogonal Cutting (Merchant’s Analysis) .......................................................................... 296 6.3.8 Stress and Strain on the Chip ............................................................. 300 6.3.9 Forces of a Single-point Tool ............................................................... 301 6.3.10 Popular Theories on Mechanics of Metal Cutting .............................. 303 HEAT IN METAL CUTTING ........................................................................................... 304 6.4.1 Heat Generated in Metal Cutting ......................................................... 305 6.4.2 Factors Affecting Temperature in Metal Cutting ................................ 306 6.4.3 Measurement of Chip-tool Interface Temperature .............................. 306 CUTTING FLUIDS .......................................................................................................... 306 6.5.1 Purpose of Using Cutting Fluids ......................................................... 306 6.5.2 Properties of Cutting Fluid .................................................................. 307 6.5.3 Types of Lubricants ............................................................................. 308 6.5.4 Types of Cutting Fluids ........................................................................ 309 6.5.5 Oils and Compounds Suggested for Use for Different Metals and Machining Operations .................................................................. 309 TOOL FAILURE ............................................................................................................. 310 6.6.1 Reasons of Tool Failure ....................................................................... 310 MECHANISM OF TOOL WEAR ...................................................................................... 312 6.7.1 Abrasion Wear ...................................................................................... 313 6.7.2 Adhesion Wear ..................................................................................... 313

Contents

6.8 6.9 6.10

6.11

6.12

6.13

6.14

6.15

xi

6.7.3 Diffusion Wear ..................................................................................... 313 6.7.4 Chemical Wear ..................................................................................... 314 TOOL LIFE .................................................................................................................... 314 6.8.1 Factors Affecting Tool Life .................................................................. 314 COST CONSIDERATION IN MANUFACTURING ............................................................ 316 6.9.1 Elements of Cost ................................................................................. 317 ECONOMICS OF METAL MACHINING .......................................................................... 320 6.10.1 Minimum Cost per Piece ..................................................................... 320 6.10.2 Maximum Production Rate .................................................................. 322 6.10.3 Optimum Cutting Speed and Optimum Tool Life for Minimum Cost of Production and Maximum Production Rate ........................... 323 MACHINABILITY ........................................................................................................... 324 6.11.1 Improving Machinability ...................................................................... 325 6.11.2 Machinability Index ............................................................................. 326 6.11.3 Measurement of Cutting Forces .......................................................... 326 6.11.4 Numericals on Mechanics of Metal Cutting ........................................ 329 MACHINE TOOLS .......................................................................................................... 340 6.12.1 Functions of a Machine Tool ............................................................... 341 6.12.2 Types of Machine Tools ....................................................................... 341 PRODUCTION OF GEOMETRICAL SHAPES ON MACHINE TOOLS ............................ 341 6.13.1 Production of Round or Tapered (Conical) Surfaces Using a Single-point Cutting Tool .................................................................... 342 6.13.2 Production of Flat or Plane Surfaces Using a Single-point Cutting Tool ......................................................................................... 342 6.13.3 Production of Round and Flat Surfaces (or Contours) Using Multi-edged Cutting Tools ................................................................... 343 LATHE ........................................................................................................................... 343 6.14.1 Components of a Lathe ....................................................................... 344 6.14.2 Defining the Lathe Size ....................................................................... 349 6.14.3 Types of Lathe ...................................................................................... 350 6.14.4 Operations Performed on Lathe .......................................................... 351 6.14.5 Taper and Taper Turning ...................................................................... 354 6.14.6 Numericals on Taper Turning .............................................................. 357 6.14.7 Thread Cutting on Lathe ..................................................................... 358 6.14.8 Cutting Speed, Feed and Depth of Cut in Turning .............................. 361 6.14.9 Lathe Accessories and Attachments .................................................. 366 TURRET AND CAPSTAN LATHES ................................................................................ 370 6.15.1 Difference between a Turret Lathe (or Capstan Lathe) and Centre Lathe ........................................................................................ 371 6.15.2 Principal Parts of Turret and Capstan Lathes ..................................... 371 6.15.3 Types of Turret Lathes ......................................................................... 375 6.15.4 Size and Specifications of Turret and Capstan Lathes ....................... 375 6.15.5 Difference between a Turret Lathe and a Capstan Lathes .................. 376 6.15.6 Machining Operations Performed on Turret and Capstan Lathes ...... 376 6.15.7 Work Holding Devices .......................................................................... 377 6.15.8 Methods of Holding Tools .................................................................... 378 6.15.9 Common Tools and Attachments ........................................................ 379 6.15.10 Bar Feeding Mechanism ...................................................................... 379 6.15.11 Tooling Layout ..................................................................................... 381

xii

Contents

6.16

6.17

6.18

6.19

6.20

AUTOMATIC LATHES ................................................................................................... 383 6.16.1 Automatic and Semi-automatic Lathes ............................................... 384 6.16.2 Classification of Automatic Lathes ..................................................... 385 6.16.3 Single-spindle Automatic Machines ................................................... 385 6.16.4 Multi-spindle Automatic Machines ..................................................... 387 6.16.5 Classification of Semi-automatic Lathes ............................................ 389 DRILLING MACHINE ..................................................................................................... 390 6.17.1 Drilling Machine Operations ............................................................... 391 6.17.2 Types of Drilling Machines .................................................................. 391 6.17.3 Size of a Drilling Machine ................................................................... 394 6.17.4 Main Parts of an Upright Drilling Machine ......................................... 395 6.17.5 Work Holding Devices .......................................................................... 395 6.17.6 Drill Holding Devices ........................................................................... 396 6.17.7 Force System in Drilling ...................................................................... 396 6.17.8 Cutting Speed, Feed and Depth of Cut in Drilling .............................. 398 6.17.9 Numericals on Drilling ........................................................................ 400 SHAPER (OR SHAPING MACHINE) .............................................................................. 401 6.18.1 Working Principle of Shaper ............................................................... 402 6.18.2 Shaper Size .......................................................................................... 402 6.18.3 Types of Shaper .................................................................................... 404 6.18.4 Principal Parts ..................................................................................... 405 6.18.5 Operations Performed on Shaper ........................................................ 407 6.18.6 Quick Return Mechanisms of Shaper ................................................. 407 6.18.7 Feed Mechanism .................................................................................. 412 6.18.8 Work Holding Devices .......................................................................... 413 6.18.9 Shaper Tools ........................................................................................ 414 6.18.10 Cutting Speed, Feed and Depth of Cut ................................................ 415 PLANER (OR PLANING MACHINE) ............................................................................... 417 6.19.1 Principle of Working ............................................................................ 418 6.19.2 Size of a Planer .................................................................................... 418 6.19.3 Principal Parts ..................................................................................... 418 6.19.4 Types of Planers ................................................................................... 419 6.19.5 Quick Return Table Drive of a Planer .................................................. 421 6.19.6 Feed Mechanism .................................................................................. 422 6.19.7 Work Holding Devices .......................................................................... 423 6.19.8 Planer Operations ................................................................................ 423 6.19.9 Planer Tools ......................................................................................... 423 6.19.10 Cutting Speed, Feed and Depth of Cut ................................................ 424 MILLING MACHINE ....................................................................................................... 425 6.20.1 Operations Performed on a Milling Machine ...................................... 426 6.20.2 Working Principle ................................................................................ 427 6.20.3 Main Features and Principal Parts ...................................................... 427 6.20.4 Types of Milling Machine ..................................................................... 429 6.20.5 Fundamentals of Milling Processes .................................................... 433 6.20.6 Milling Machine Operations ................................................................ 435 6.20.7 Milling Cutters ..................................................................................... 436 6.20.8 Elements of a Plain Milling Cutter ...................................................... 439 6.20.9 Size of Milling Machine ....................................................................... 442 6.20.10 Cutter Holding Devices ........................................................................ 442 6.20.11 Work Holding Devices .......................................................................... 443

Contents

xiii

6.20.12 Milling Machine Attachments ............................................................. 443 6.20.13 Cutting Speed, Feed and Depth of Cut ................................................ 445 6.20.14 Force System in Milling ....................................................................... 447 6.20.15 Dividing Head or Indexing Head .......................................................... 449 6.20.16 Methods of Indexing ............................................................................ 452 6.20.17 Cutting of Helix on Milling Machine ................................................... 457 6.21 GRINDING MACHINES .................................................................................................. 460 6.21.1 Methods of Grinding ............................................................................ 461 6.21.2 Types of Grinding Machines ................................................................ 462 6.21.3 Size of a Grinding Machine ................................................................. 468 6.21.4 Grinding Allowance and Tolerance ..................................................... 469 6.21.5 Wet and Dry Grinding .......................................................................... 469 6.21.6 Grinding Wheel .................................................................................... 469 6.21.7 Abrasives .............................................................................................. 470 6.21.8 Grit, Grade and Structure of Wheels ................................................... 470 6.21.9 Bonds and Bonding Processes ............................................................ 471 6.21.10 Method of Specifying a Grinding Wheel .............................................. 472 6.21.11 Common Wheel Shapes ....................................................................... 473 6.21.12 Mounting and Balancing of Grinding Wheel ....................................... 474 6.21.13 Glazing and Loading of Wheels ........................................................... 475 6.21.14 Truing and Dressing of Wheels ........................................................... 475 6.21.15 Selection of Grinding Wheels .............................................................. 476 6.21.16 Cutting Speed, Feed and Machining Time ........................................... 477 6.21.17 Maximum Chip Thickness in Grinding (Cylindrical) .......................... 478 6.22 FINISHING ..................................................................................................................... 480 6.22.1 Finishing Processes ............................................................................ 480 6.22.2 Surface Finish in Machining ............................................................... 484 6.23 METAL CUTTING SAWS ................................................................................................ 486 6.24 SLOTTER ....................................................................................................................... 487 6.24.1 Main Parts of a Slotter ......................................................................... 487 6.24.2 Quick Return Mechanism for Ram ...................................................... 489 6.24.3 Operations on a Slotter ....................................................................... 491 6.24.4 Feed Mechanism .................................................................................. 492 6.24.5 Types of Slotters .................................................................................. 493 6.24.6 Slotting Tools ....................................................................................... 493 6.24.7 Cutting Speed, Feed and Depth of Cut ................................................ 493 6.25 BORING MACHINE ........................................................................................................ 493 6.26 BROACHING MACHINE ................................................................................................. 495 6.26.1 A Broach .............................................................................................. 496 6.26.2 Broaching Operations .......................................................................... 497 6.26.3 Advantages of Broaching ..................................................................... 497 6.26.4 Broaching Methods .............................................................................. 497 6.26.5 Types of Broaching Machines .............................................................. 498 6.26.6 Limitations of Broaching ..................................................................... 499 REVIEW QUESTIONS ................................................................................................................ 499

7

ELECTRIC AND GAS WELDING PROCESSES ..................................................................................... 501–609 7.1 7.2

INTRODUCTION ............................................................................................................ 501 PREFERENCE FOR WELDING ...................................................................................... 501

xiv

Contents

7.3 7.4 7.5

7.6 7.7 7.8

7.9 7.10 7.11 7.12

7.13 7.14 7.15 7.16

7.17 7.18 7.19

7.20

APPLICATIONS OF WELDING ...................................................................................... 502 WELDING—A METAL WORKING PROCESS ................................................................ 503 BROAD CLASSIFICATION OF WELDING PROCESSES ............................................... 503 7.5.1 Fusion (or Non-pressure) Welding Processes ..................................... 504 7.5.2 Pressure Welding Processes ............................................................... 504 7.5.3 Thermochemical Welding Processes .................................................. 505 7.5.4 Radiant Energy Welding Processes ..................................................... 505 7.5.5 Underwater Welding Processes ........................................................... 505 ARC-WELDING PROCESSES ........................................................................................ 505 SHIELDED METAL-ARC WELDING (SMAW) ................................................................ 506 MAIN FEATURES OF SHIELDED METAL-ARC WELDING ........................................... 507 7.8.1 Electric Arc .......................................................................................... 508 7.8.2 Arc Initiation ........................................................................................ 508 7.8.3 Arc Stability ......................................................................................... 509 7.8.4 Polarity ................................................................................................. 510 A FILLET WELD ............................................................................................................ 513 BUILDING A WELD PAD (OR BUILDING UP) ............................................................... 513 EFFECT OF WELDING VARIABLES ON WELD QUALITY ............................................ 514 ARC WELDING MACHINES ........................................................................................... 514 7.12.1 Need for a Welding Machine ................................................................ 514 7.12.2 Welding Machines for Shielded Metal-arc Welding ............................. 515 7.12.3 Comparison between AC and DC Arc Welding Processes .................. 516 V-I CHARACTERISTICS OF ARC WELDING PLANTS ................................................... 518 SIZE OF A WELDING MACHINE ................................................................................... 519 ARC WELDING STATION .............................................................................................. 519 ELECTRODES USED IN SHIELDED METAL-ARC WELDING ...................................... 522 7.16.1 Current Carrying Capacity of Electrodes ............................................ 522 7.16.2 Types of Electrodes .............................................................................. 523 7.16.3 Electrode Core Wire Materials ............................................................. 523 7.16.4 Constituents and Functions of Electrode Flux Coatings ................... 524 7.16.5 Bare vs Coated Electrodes ................................................................... 524 7.16.6 Types of Flux Coating .......................................................................... 525 7.16.7 Selection of Electrodes ........................................................................ 526 7.16.8 Classification and Coding of Electrodes ............................................. 526 WELDING POSITIONS ................................................................................................... 528 WELDING JOINTS—TYPES AND SELECTION ............................................................. 530 OTHER ARC WELDING PROCESSES ........................................................................... 534 7.19.1 Carbon Arc Welding ............................................................................. 534 7.19.2 Submerged Arc Welding (SAW) ............................................................ 535 7.19.3 Gas Metal-Arc Welding (GMAW) ........................................................... 536 7.19.4 Electroslag Welding (ESW) and Electrogas Welding (EGW) ................ 537 7.19.5 Gas Tungsten Arc Welding (GTAW) or Tungsten Inert Gas (TIG) Welding 538 7.19.6 Atomic Hydrogen Welding (AHW) ........................................................ 539 7.19.7 Plasma Arc Welding (PAW) ................................................................... 540 RESISTANCE WELDING ............................................................................................... 541 7.20.1 Principle of Resistance Welding .......................................................... 541 7.20.2 Variables of Resistance Welding ......................................................... 542 7.20.3 Advantages of Resistance Welding ...................................................... 543 7.20.4 Application of Resistance Welding ...................................................... 543

Contents

7.21

7.22

7.23

7.24

7.25

7.26 7.27

7.28 7.29 7.30 7.31

7.32 7.33 7.34

7.35 7.36 7.37 7.38 7.39

xv

RESISTANCE WELDING PROCESSES ......................................................................... 544 7.21.1 Spot Welding ........................................................................................ 544 7.21.2 Seam Welding ....................................................................................... 545 7.21.3 Projection Welding ............................................................................... 546 7.21.4 High Frequency Resistance Welding (HFRW) ...................................... 547 7.21.5 Resistance Butt Welding ..................................................................... 547 7.21.6 Percussion Welding ............................................................................. 550 THERMOCHEMICAL WELDING PROCESSES .............................................................. 550 7.22.1 Thermit Welding .................................................................................. 550 7.22.2 Atomic-hydrogen Welding ................................................................... 551 RADIANT ENERGY WELDING PROCESSES ................................................................ 552 7.23.1 Laser Beam Welding ............................................................................ 552 7.23.2 Electron Beam Welding ....................................................................... 553 SOLID-STATE WELDING PROCESSES ........................................................................ 554 7.24.1 Cold Welding ........................................................................................ 554 7.24.2 Diffusion Welding ................................................................................ 555 7.24.3 Ultrasonic Welding ............................................................................... 556 7.24.4 Explosive Welding ................................................................................ 557 7.24.5 Friction Welding and Inertia Welding .................................................. 559 7.24.6 Forge Welding ...................................................................................... 560 UNDERWATER WELDING ............................................................................................. 560 7.25.1 Wet Welding ......................................................................................... 561 7.25.2 Dry Welding .......................................................................................... 561 GAS WELDING .............................................................................................................. 562 7.26.1 Types of Gas Welding Process ............................................................. 562 OXY-ACETYLENE WELDING ........................................................................................ 563 7.27.1 Oxygen ................................................................................................. 563 7.27.2 Acetylene ............................................................................................. 564 7.27.3 Special Precautions for Acetylene Cylinder ....................................... 565 OXY-ACETYLENE FLAMES .......................................................................................... 566 7.28.1 Types of Oxy-acetylene Flames ........................................................... 567 BACKFIRE OR POPPING ............................................................................................... 568 FLASHBACK .................................................................................................................. 569 OXY-ACETYLENE WELDING OUTFIT ........................................................................... 569 7.31.1 Welding Torch (or Blow Pipe) .............................................................. 570 7.31.2 Pressure Regulators ............................................................................ 572 GAS WELDING ROD (OR FILLER ROD) ....................................................................... 573 GAS WELDING FLUXES ............................................................................................... 574 TECHNIQUES OF GAS WELDING ................................................................................. 575 7.34.1 Leftward (or Forehand) Welding .......................................................... 575 7.34.2 Rightward (or Backhand) Welding ....................................................... 576 PUDDLING ..................................................................................................................... 576 JOINTS USED IN GAS WELDING ................................................................................. 577 METALS WELDED BY GAS WELDING ......................................................................... 578 GAS WELDING—A REVIEW .......................................................................................... 578 WELD DEFECTS ........................................................................................................... 579 7.39.1 External Weld Defects ......................................................................... 579 7.39.2 Internal Weld Defects .......................................................................... 581

xvi

Contents

7.40

INSPECTION AND TESTING OF WELDMENTS ............................................................ 582 7.40.1 Inspection before Welding ................................................................... 582 7.40.2 Inspection during Welding .................................................................. 582 7.40.3 Inspection after Welding ...................................................................... 583 7.41 NON-DESTRUCTIVE TESTING OF WELDMENTS ........................................................ 583 7.41.1 Visual Inspection ................................................................................. 583 7.41.2 Magnetic Particle Inspection ............................................................... 583 7.41.3 Liquid Penetrant Inspection ................................................................ 584 7.41.4 Stethoscopic (Sound) Test ................................................................... 584 7.41.5 Leakage Tests ...................................................................................... 584 7.42 DESTRUCTIVE TESTS .................................................................................................. 587 7.42.1 Testing of Butt-welded Joints .............................................................. 587 7.43 THERMAL CUTTING OF METALS ................................................................................ 587 7.43.1 Oxygen Cutting Processes ................................................................... 588 7.44 WELDING OF IMPORTANT METALS ............................................................................ 591 7.44.1 Welding of Wrought Iron ...................................................................... 591 7.44.2 Welding of Cast Irons ........................................................................... 592 7.44.3 Welding of Carbon Steels ..................................................................... 593 7.44.4 Welding of Tool Steels ......................................................................... 594 7.44.5 Welding of Cast Steel ........................................................................... 594 7.44.6 Welding of Alloy Steels ........................................................................ 594 7.44.7 Welding of Stainless Steel ................................................................... 595 7.44.8 Welding of Aluminium and Its Alloys .................................................. 596 7.44.9 Welding of Copper and Its Alloys ........................................................ 596 7.44.10 Welding of Dissimilar Metals ............................................................... 597 7.45 WELDABILITY ............................................................................................................... 598 7.45.1 Weldability of Some Important Metals ................................................ 598 7.45.2 Effect of Heat on Welding and Heat-affected Zone (HAZ) ................... 599 7.45.3 Weld Cracking ...................................................................................... 602 7.45.4 Weld Decay ........................................................................................... 603 7.45.5 Dilution ................................................................................................ 604 7.45.6 Modes of Metal Transfer in Welding .................................................... 604 7.45.7 Residual Stress and Distortion in Weldments .................................... 605 7.46 WELDING FIXTURES .................................................................................................... 607 REVIEW QUESTIONS ................................................................................................................ 607

8

SOLDERING AND BRAZING ................................................................................................................. 610–623 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9

INTRODUCTION ............................................................................................................ 610 SOLDERING .................................................................................................................. 610 METALS JOINED BY SOLDERING ............................................................................... 611 SOLDERING JOINTS ..................................................................................................... 611 TYPES OF SOLDERS .................................................................................................... 611 FUNCTIONS OF SOLDERING FLUXES ......................................................................... 613 TYPES OF SOLDERING FLUXES AND THEIR COMPOSITIONS .................................. 613 8.7.1 Surface Cleaning Agents Used in Soldering ....................................... 614 STEPS IN SOLDERING OPERATION ............................................................................ 614 METHODS OF SOLDERING .......................................................................................... 614 8.9.1 Soldering Iron Method ......................................................................... 614 8.9.2 Soldering Torch Method ...................................................................... 615

Contents

xvii

8.9.3 Dip Bath Method .................................................................................. 616 8.9.4 Wave Soldering ..................................................................................... 616 8.10 TINNING, SWEATING AND FLOATING ......................................................................... 616 8.11 BRAZING ....................................................................................................................... 616 8.12 BRAZING vs WELDING ................................................................................................. 617 8.13 BRAZING OPERATION .................................................................................................. 617 8.14 BRAZING JOINTS .......................................................................................................... 617 8.15 METALS JOINED BY BRAZING .................................................................................... 618 8.16 FILLER METALS (OR BRAZING ALLOYS) .................................................................... 618 8.17 BRAZING FLUXES ......................................................................................................... 619 8.18 APPLICATIONS AND ADVANTAGES OF BRAZING ...................................................... 620 8.18.1 Tipping of Tool Bits .............................................................................. 620 8.19 BRAZING METHODS ..................................................................................................... 620 8.19.1 Torch Brazing ....................................................................................... 621 8.19.2 Furnace Brazing ................................................................................... 621 8.19.3 Dip Brazing .......................................................................................... 622 8.19.4 Induction Brazing ................................................................................ 622 8.19.5 Silver Brazing ....................................................................................... 622 8.20 BRAZE WELDING (OR BRONZE WELDING) ................................................................ 622 REVIEW QUESTIONS ................................................................................................................ 623

9

METAL FORMING: HOT- AND COLD-WORKING AND PRESS-WORKING ......................................... 624–692 9.1 9.2 9.3 9.4

9.5 9.6

9.7 9.8

9.9 9.10 9.11

9.12

INTRODUCTION ............................................................................................................ 624 METAL FORMING .......................................................................................................... 626 9.2.1 Classification of Metal Forming Processes ......................................... 626 WROUGHT PRODUCTS ................................................................................................. 627 HOT FORMING (OR HOT-WORKING) OF METALS ....................................................... 627 9.4.1 Temperature Range for Hot-working ................................................... 628 9.4.2 Advantages and Disadvantages of Hot-working .................................. 629 MAJOR HOT-WORKING PROCESSES .......................................................................... 630 HOT ROLLING ............................................................................................................... 630 9.6.1 Rolling Parameters and Their Effects ................................................. 633 9.6.2 Types of Rolling Mills .......................................................................... 636 9.6.3 Roll Design and Forms of Grooves in Rolls ........................................ 640 9.6.4 Defects in Rolled Plates and Sheets ................................................... 641 HOT FORGING ............................................................................................................... 642 9.7.1 Forces in Hot Forging .......................................................................... 643 HOT EXTRUSION .......................................................................................................... 644 9.8.1 Types of Extrusion Processes ............................................................. 645 9.8.2 Characteristics of Hot Extrusion ........................................................ 646 9.8.3 Extrusion Defects ................................................................................ 646 9.8.4 Extrusion Force ................................................................................... 647 EXTRUSION OF TUBING ............................................................................................... 648 PRODUCTION OF SEAMLESS PIPE AND TUBING ....................................................... 648 MANUFACTURING OF WELDED PIPES AND TUBES ................................................... 649 9.11.1 Intermittent Method of Making Butt Welded Pipes ............................. 649 9.11.2 Continuous Method of Making Butt Welded Pipes ............................. 650 9.11.3 Lap Welded Pipes ................................................................................. 650 HOT DRAWING .............................................................................................................. 650

xviii

Contents

9.13 9.14 9.15 9.16 9.17

HOT SPINNING .............................................................................................................. 651 COLD FORMING OR COLD-WORKING OF METALS .................................................... 652 MAIN FEATURES OF COLD-WORKING ........................................................................ 653 CLASSIFICATION OF MAJOR COLD-WORKING PROCESSES .................................... 654 COLD FORMING PROCESSES: ALTERING THE CROSS-SECTIONAL SHAPE ........... 654 9.17.1 Cold Rolling ......................................................................................... 654 9.17.2 Thread Rolling ..................................................................................... 655 9.17.3 Cold Forging or Cold Heading ............................................................. 656 9.17.4 Swaging and Rotary Swaging ............................................................... 656 9.17.5 Coining ................................................................................................. 657 9.17.6 Cold Drawing of Rods, Wires and Tubes ............................................. 658 9.17.7 Cold Extrusion ..................................................................................... 663 9.17.8 Impact Extrusion ................................................................................. 663 9.17.9 Hydrostatic Extrusion ......................................................................... 665 9.17.10 Cold Bending and Roll Bending .......................................................... 665 9.18 SHEET METAL FORMING (PRESS-WORKING) ............................................................ 666 9.18.1 Metals Used for Sheet Metal Forming (Press-working) ....................... 667 9.18.2 Lubrication in Press-working .............................................................. 667 9.18.3 Press .................................................................................................... 667 9.18.4 Die-assembly ........................................................................................ 669 9.18.5 Classification of Dies ........................................................................... 670 9.18.6 Types of Presses .................................................................................. 673 9.18.7 Safety in Press-working ....................................................................... 676 9.18.8 Theory of Shearing Metal .................................................................... 677 9.19 SHEET METAL SHEARING PROCESSES ..................................................................... 680 9.19.1 Shearing Operations ............................................................................ 680 9.20 SHEET METAL FORMING PROCESSES ....................................................................... 681 9.20.1 Bending Processes .............................................................................. 682 9.20.2 Stretch Forming ................................................................................... 685 9.20.3 Drawing Operations ............................................................................. 686 9.20.4 Metal Spinning ..................................................................................... 689 9.20.5 Stamping .............................................................................................. 690 9.20.6 Bulging ................................................................................................. 690 9.20.7 Hydro Forming ..................................................................................... 690 REVIEW QUESTIONS ................................................................................................................ 691

10

METAL FORGING: SMITHYING AND POWER FORGING ................................................................... 693–732 10.1 10.2 10.3 10.4 10.5 10.6

INTRODUCTION ............................................................................................................ 693 ADVANTAGES AND DISADVANTAGES OF HOT FORGING .......................................... 695 APPLICATIONS OF FORGING ....................................................................................... 696 METALS AND THEIR FORGING TEMPERATURES ...................................................... 696 10.4.1 Measurement of Forging Temperatures .............................................. 697 SMITHYING (OR HAND FORGING) ............................................................................... 697 10.5.1 Blacksmith’s Hearth or Forge .............................................................. 697 BLACKSMITH’S TOOLS ................................................................................................ 698 10.6.1 Supporting Tools .................................................................................. 698 10.6.2 Striking Tools ...................................................................................... 699 10.6.3 Holding Tools ....................................................................................... 700

Contents

xix

10.6.4 Finishing and Shaping Tools ............................................................... 701 10.6.5 Cutting Tools ....................................................................................... 702 10.6.6 Measuring Tools ................................................................................... 703 10.7 HAND FORGING OPERATIONS ..................................................................................... 703 10.7.1 Drawing Out or Drawing Down ............................................................ 703 10.7.2 Fullering ............................................................................................... 704 10.7.3 Setting Down ........................................................................................ 704 10.7.4 Upsetting or Jumping .......................................................................... 704 10.7.5 Notching ............................................................................................... 705 10.7.6 Chiselling or Cutting ........................................................................... 705 10.7.7 Bending ................................................................................................ 705 10.7.8 Swaging ................................................................................................ 706 10.7.9 Punching .............................................................................................. 706 10.7.10 Drifting ................................................................................................. 707 10.7.11 Forge Welding ...................................................................................... 707 10.8 DIE FORGING ................................................................................................................ 708 10.8.1 Dies Used for Forging .......................................................................... 709 10.9 POWER FORGING METHODS ....................................................................................... 713 10.9.1 Drop Forging ........................................................................................ 714 10.9.2 Hammer and Press Forging ................................................................. 714 10.9.3 Flashless Forging ................................................................................. 715 10.9.4 Isothermal Die Forging ........................................................................ 715 10.9.5 Upset Forging ....................................................................................... 715 10.9.6 Roll Die Forging ................................................................................... 716 10.9.7 Swaging (Rotary Swaging) .................................................................... 717 10.9.8 Swing Forging ...................................................................................... 718 10.10 FORGING EQUIPMENT ................................................................................................. 718 10.10.1 Forging Hammers or Power Hammers ................................................ 719 10.10.2 Forging Presses ................................................................................... 724 10.11 FORGING MACHINES .................................................................................................... 725 10.12 DEFECTS IN FORGINGS ............................................................................................... 726 10.13 HEAT TREATMENT OF FORGINGS .............................................................................. 728 10.14 COMPARATIVE STUDY OF METAL FORMING PROCESSES ....................................... 728 REVIEW QUESTIONS ................................................................................................................ 730

11

POWDER METALLURGY PROCESSES ................................................................................................. 733–745 11.1 11.2 11.3

11.4 11.5 11.6 11.7 11.8

INTRODUCTION ............................................................................................................ 733 OPERATIONS IN POWDER METALLURGY PROCESS ................................................. 734 PRODUCTION OF METAL POWDERS ........................................................................... 735 11.3.1 Reduction of Metal Oxides ................................................................... 735 11.3.2 Electrolytic Deposition ........................................................................ 735 11.3.3 Atomization ......................................................................................... 735 11.3.4 Mechanical Comminution or Pulverization ........................................ 736 PROPERTIES OF METAL POWDER .............................................................................. 736 BLENDING OF METAL POWDERS ............................................................................... 737 COMPACTION OF METAL POWDERS ........................................................................... 737 11.6.1 Methods of Compaction of Metal Powders .......................................... 738 SINTERING OF GREEN COMPACT ............................................................................... 741 SECONDARY OPERATIONS .......................................................................................... 741

xx

Contents

11.9 APPLICATIONS OF POWDER METALLURGY ............................................................... 742 11.10 ADVANTAGES AND LIMITATIONS OF POWDER METALLURGY ................................. 743 REVIEW QUESTIONS ................................................................................................................ 744

12

PLASTICS: MANUFACTURING AND APPLICATIONS .......................................................................... 746–772 12.1 12.2 12.3 12.4 12.5 12.6 12.7

INTRODUCTION ............................................................................................................ 746 ADVANTAGES AND LIMITATIONS OF PLASTICS ........................................................ 747 PLASTICS AND POLYMERS .......................................................................................... 747 POLYMERIZATION ........................................................................................................ 748 STRUCTURE OF POLYMERS ........................................................................................ 748 ADDITIVES IN PLASTICS ............................................................................................. 750 CLASSIFICATION OF PLASTICS .................................................................................. 750 12.7.1 Thermoplastics .................................................................................... 750 12.7.2 Thermosetting Plastics ....................................................................... 751 12.8 SELECTION OF PLASTICS ........................................................................................... 751 12.9 THERMOPLASTICS: CHARACTERISTICS AND USES ................................................. 752 12.10 THERMOSETTING PLASTICS: CHARACTERISTICS AND USES ................................. 757 12.11 PLASTICS AS REPLACEMENT OF OTHER MATERIALS ............................................. 759 12.12 MANUFACTURING OF PLASTIC PRODUCTS ............................................................... 760 12.13 CASTING ....................................................................................................................... 760 12.14 COMPRESSION MOLDING ............................................................................................ 761 12.15 TRANSFER MOLDING ................................................................................................... 762 12.16 INJECTION MOLDING ................................................................................................... 763 12.17 EXTRUSION ................................................................................................................... 763 12.18 ROTO MOLDING OR SLUSH MOLDING ........................................................................ 764 12.19 CALENDERING ............................................................................................................. 764 12.20 BLOW MOLDING ........................................................................................................... 765 12.21 FORMING OR SHAPING METHODS .............................................................................. 765 12.22 LAMINATING METHODS ............................................................................................... 766 12.23 REINFORCED PLASTIC MOLDING ............................................................................... 766 12.24 MACHINING OF PLASTICS ........................................................................................... 767 12.25 JOINING PLASTICS ...................................................................................................... 767 12.25.1 Joining Thermoplastics ....................................................................... 767 12.25.2 Joining Thermosetting Plastics .......................................................... 768 12.26 ELASTOMERS (RUBBERS) ........................................................................................... 768 12.27 INDUSTRIAL APPLICATIONS OF PLASTICS ................................................................ 768 REVIEW QUESTIONS ................................................................................................................ 770

13

NON-CONVENTIONAL MACHINING METHODS ................................................................................... 773–797 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9 13.10

INTRODUCTION ............................................................................................................ 773 TYPES OF NON-CONVENTIONAL MACHINING METHODS ......................................... 773 PROCESS SELECTION CONSIDERATIONS .................................................................. 774 ELECTRIC DISCHARGE MACHINING (EDM) ............................................................... 775 ELECTRON BEAM MACHINING (EBM) ......................................................................... 777 PLASMA ARC MACHINING (PAM) ................................................................................. 779 LASER BEAM MACHINING (LBM) ................................................................................ 780 ION-BEAM MACHINING (IBM) ....................................................................................... 783 ULTRASONIC MACHINING (USM) ................................................................................. 784 ABRASIVE JET MACHINING (AJM) .............................................................................. 787

Contents

xxi

13.11 ELECTROCHEMICAL MACHINING (ECM) .................................................................... 788 13.12 ELECTROCHEMICAL GRINDING (ECG) ....................................................................... 792 13.13 CHEMICAL MACHINING (CHM) .................................................................................... 794 13.14 PROCESS CAPABILITIES OF NON-CONVENTIONAL MACHINING METHODS ........... 795 REVIEW QUESTIONS ................................................................................................................ 795 OBJECTIVE TYPE QUESTIONS ................................................................................................. 796

14

AUTOMATION: TRANSFER MACHINING, MACHINING CENTRES AND ROBOTICS ........................ 798–811 14.1

INTRODUCTION ............................................................................................................ 798 14.1.1 Mechanization and Automation .......................................................... 798 14.1.2 Automatic Machines ............................................................................ 799 14.1.3 Justification for Adopting Automation ............................................... 799 14.1.4 Advantages of Automation .................................................................. 799 14.2 TRANSFER MACHINING ............................................................................................... 800 14.2.1 General Aspects ................................................................................... 800 14.2.2 Main Parts ............................................................................................ 800 14.2.3 Types of Transfer Machines ................................................................. 801 14.2.4 Advantages and Disadvantages ........................................................... 803 14.3 MACHINING CENTRES ................................................................................................. 803 14.3.1 General Aspects ................................................................................... 803 14.3.2 Characteristics of Machining Centres ................................................ 804 14.3.3 Types of Machining Centres ................................................................ 804 14.4 ROBOTICS ..................................................................................................................... 804 14.4.1 Introduction ......................................................................................... 804 14.4.2 Main Parts of a Robot .......................................................................... 805 14.4.3 Configurations of Robots .................................................................... 806 14.4.4 Types of Robots .................................................................................... 806 14.4.5 Motion System of Robot ...................................................................... 807 14.4.6 Some Terms Related to Robot ............................................................. 808 14.4.7 Power Source ....................................................................................... 808 14.4.8 Artificial Intelligence ........................................................................... 808 14.4.9 Robot Programming and Languages ................................................... 808 14.4.10 Robot Sensing and Sensors ................................................................ 809 14.4.11 Robots vs NC Machines ....................................................................... 809 14.4.12 Applications of Robots ........................................................................ 810 REVIEW QUESTIONS ................................................................................................................ 810

15

MANUFACTURING GEARS AND THREADS ......................................................................................... 812–839 15.1

INTRODUCTION ............................................................................................................ 812

Gears ..................................................................................................... 813 15.2 15.3 15.4 15.5 15.6 15.7

TYPES OF GEARS ......................................................................................................... 813 MATERIALS FOR GEARS .............................................................................................. 815 FORMS OF GEAR TEETH ............................................................................................. 815 GEAR TOOTH TERMINOLOGY ..................................................................................... 815 METHODS OF MANUFACTURING GEARS .................................................................... 817 GEAR FORMING ............................................................................................................ 818 15.7.1 Selecting a Form Gear Cutter for Cutting Spur Gear .......................... 819 15.7.2 Selecting Gear Cutter for Cutting Helical or Spiral Gears .................. 820

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Contents

15.8 15.9

15.10 15.11 15.12

15.13

15.7.3 Broaching of Gears .............................................................................. 821 15.7.4 Parallel Shaping of Gears .................................................................... 821 TEMPLATE METHOD .................................................................................................... 822 GENERATING METHOD ................................................................................................ 822 15.9.1 Gear Shaper Process ........................................................................... 822 15.9.2 Rack Planing Process .......................................................................... 823 15.9.3 Gear Hobbing Process ......................................................................... 824 CUTTING BEVEL GEARS .............................................................................................. 825 CUTTING WORMS AND WORM WHEEL ....................................................................... 826 GEAR FINISHING OPERATIONS ................................................................................... 826 15.12.1 Gear Shaving ........................................................................................ 826 15.12.2 Gear Burnishing .................................................................................. 827 15.12.3 Gear Grinding ....................................................................................... 827 15.12.4 Gear Lapping ........................................................................................ 828 15.12.5 Gear Honing ......................................................................................... 828 INSPECTION OF GEARS ............................................................................................... 828

Threads .................................................................................................. 829 15.14 STANDARD FORMS OF SCREW THREADS .................................................................. 829 15.14.1 Selection of a Thread Form ................................................................. 830 15.14.2 Classification of Threads ..................................................................... 830 15.15 METHODS FOR MAKING THREADS ............................................................................ 831 15.15.1 Production of External Threads .......................................................... 831 15.15.2 Production of Internal Threads ........................................................... 832 15.16 THREAD CUTTING ON LATHE ..................................................................................... 832 15.16.1 Setting Tool for Threading ................................................................... 832 15.16.2 Feeding the Tool .................................................................................. 832 15.16.3 Undercut .............................................................................................. 833 15.16.4 Thread Catching .................................................................................. 833 15.16.5 Cutting Right or Left Hand Threads .................................................... 835 15.16.6 Cutting Square or Acme Threads ........................................................ 835 15.16.7 Cutting Multi-start Threads ................................................................. 835 15.17 THREAD CHASING ....................................................................................................... 835 15.18 THREAD CUTTING WITH DIE HEADS ......................................................................... 836 15.19 THREAD MILLING ......................................................................................................... 836 15.20 THREAD ROLLING ........................................................................................................ 837 15.21 THREAD GRINDING ...................................................................................................... 837 15.22 THREAD TAPPING ........................................................................................................ 838 15.23 AUTOMATIC SCREW CUTTING MACHINES ................................................................. 838 15.24 INSPECTION AND MEASUREMENT OF THREADS ...................................................... 838 REVIEW QUESTIONS ................................................................................................................ 838

16

JIGS AND FIXTURES ............................................................................................................................ 840–874 16.1 16.2 16.3 16.4 16.5 16.6 16.7

INTRODUCTION ............................................................................................................ 840 A JIG .............................................................................................................................. 840 A FIXTURE .................................................................................................................... 842 DIFFERENCE BETWEEN A JIG AND A FIXTURE ........................................................ 843 ADVANTAGES OF USING JIGS AND FIXTURES .......................................................... 844 ELEMENTS OF JIGS AND FIXTURES .......................................................................... 844 PRINCIPLES OF JIG AND FIXTURE DESIGN ............................................................... 845

Contents

xxiii

16.8 16.9 16.10 16.11 16.12

LOCATION AND DEGREES OF FREEDOM ................................................................... 849 PRINCIPLES OF LOCATION .......................................................................................... 851 LOCATING DEVICES ..................................................................................................... 854 CLAMPING DEVICES .................................................................................................... 858 DRILL BUSHES AND AUXILIARY DEVICES ................................................................. 861 16.12.1 Types of Drill Bushes ........................................................................... 862 16.13 TYPES OF JIGS ............................................................................................................. 864 16.14 TYPES OF FIXTURES .................................................................................................... 868 16.15 ERRORS IN THE DESIGN AND USE OF JIGS AND FIXTURES ................................... 870 16.16 ECONOMICS OF USING JIGS AND FIXTURES ............................................................. 871 REVIEW QUESTIONS ................................................................................................................ 872

17

METAL JOINING PROCESSES: ADHESIVE BONDING AND MECHANICAL FASTENING ................ 875–888 17.1 17.2 17.3

INTRODUCTION ............................................................................................................ 875 JOINING PROCESSES FOR MASS PRODUCTION ........................................................ 876 ADHESIVE BONDING .................................................................................................... 877 17.3.1 Properties of Adhesives ....................................................................... 878 17.3.2 Nomenclature of Adhesives ................................................................. 878 17.4 CLASSIFICATION OF ADHESIVES ............................................................................... 878 17.4.1 Synthetic Organic Adhesives .............................................................. 879 17.4.2 Some Important Adhesives Used by Industry ..................................... 879 17.5 ADHESIVES AND TEMPERATURE ............................................................................... 880 17.6 APPLYING ADHESIVES ................................................................................................. 880 17.7 PROCESS CAPABILITIES OF ADHESIVE BONDING .................................................... 881 17.7.1 Major Advantages of Adhesive Bonding .............................................. 882 17.7.2 Major Limitations of Adhesive Bonding .............................................. 882 17.8 MECHANICAL FASTENERS .......................................................................................... 883 17.9 TYPES OF MECHANICAL FASTENER .......................................................................... 883 17.9.1 Threaded Fasteners ............................................................................. 883 17.9.2 Rivets ................................................................................................... 884 17.9.3 Washers and Retaining Rings .............................................................. 885 17.9.4 Pin Fasteners ....................................................................................... 885 17.9.5 Quick Operating Fasteners .................................................................. 886 17.9.6 Other Fastening Methods .................................................................... 886 17.10 GUIDELINES FOR USING MECHANICAL FASTENERS ................................................ 887 REVIEW QUESTIONS ................................................................................................................ 887

18

PROTECTIVE SURFACE TREATMENTS: CLEANING AND COATINGS ............................................. 889–904 18.1 18.2

18.3 18.4

INTRODUCTION ............................................................................................................ 889 METAL CLEANING ........................................................................................................ 889 18.2.1 Chemical Cleaning Processes ............................................................. 890 18.2.2 Ultrasonic Cleaning ............................................................................. 892 18.2.3 Mechanical Cleaning ........................................................................... 893 18.2.4 Oxy-acetylene Flame Cleaning ............................................................ 894 COATINGS OR FINISHES .............................................................................................. 894 CONVERSION COATINGS ............................................................................................. 895 18.4.1 Phosphate Coatings ............................................................................. 895 18.4.2 Chromate Coatings .............................................................................. 895 18.4.3 Anodic Coatings ................................................................................... 895

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Contents

18.5

ORGANIC COATINGS .................................................................................................... 896 18.5.1 Types of Organic Coatings ................................................................... 896 18.5.2 Methods of Application of Organic Coatings ...................................... 897 18.6 INORGANIC COATINGS ................................................................................................. 900 18.7 METALLIC COATINGS .................................................................................................. 900 18.7.1 Hot Dipping .......................................................................................... 900 18.7.2 Electroplating ...................................................................................... 901 18.7.3 Galvanizing .......................................................................................... 901 18.7.4 Tin Plating ............................................................................................ 901 18.7.5 Metallizing ........................................................................................... 901 18.7.6 Sputtering ............................................................................................ 903 18.8 ANODIZING OF METALS .............................................................................................. 903 REVIEW QUESTIONS ................................................................................................................ 903

19

BENCH WORKING: FITTING AND TRADITIONAL SHEET METAL WORKING ................................ 905–977 19.1 19.2

INTRODUCTION ............................................................................................................ 905 BENCH WORKING PROCESSES ................................................................................... 905 19.2.1 Laying Out and Marking Processes .................................................... 906 19.2.2 Cutting and Chipping Processes ......................................................... 906 19.2.3 Filing and Finishing Processes ........................................................... 907 19.2.4 Drilling and Reaming Processes ......................................................... 908 19.2.5 Threading Processes ........................................................................... 909 19.2.6 Measuring Processes ........................................................................... 909 19.2.7 Fitting and Assembling Processes ...................................................... 910 19.3 BENCH WORKING TOOLS ............................................................................................ 910 19.3.1 Job Supporting and Holding Tools ...................................................... 910 19.3.2 Striking Tools—Hammers ................................................................... 913 19.3.3 Cutting and Finishing Tools ................................................................ 915 19.3.4 Drilling and Reaming Tools ................................................................. 926 19.3.5 Threading Tools—Taps and Dies ......................................................... 932 19.3.6 Laying Out and Marking Tools and Measuring Tools ......................... 935 19.3.7 Miscellaneous Tools ............................................................................ 958 19.4 TRADITIONAL SHEET METAL WORKING ................................................................... 964 19.4.1 Metals Used for Sheet Metal ................................................................ 965 19.4.2 Hand Tools ........................................................................................... 966 19.4.3 Operations Involved in Sheet Metal Working ...................................... 969 19.4.4 Types of Seams in Sheet Metal Products ............................................ 972 19.4.5 Notches ................................................................................................ 973 19.4.6 Sheet Metal Working Machines ........................................................... 973 19.4.7 Sheet Metal Joints ............................................................................... 973 19.4.8 Laying Out Some Typical Forms .......................................................... 975 REVIEW QUESTIONS ................................................................................................................ 976

APPENDIX ....................................................................................................................................................... 979–989 BIBLIOGRAPHY ............................................................................................................................................... 991–992 INDEX ............................................................................................................................................................ 993–1007

Preface It is a matter of great pleasure for me to present the second edition of this book to the readers. The present text is a revised, updated and enlarged version of earlier text and is designed to meet the current requirements of revised syllabi of various engineering and diploma courses and other professional and competitive examinations. With the inclusion of supplementary information on certain topics covered already in the previous edition and the four altogether new chapters, the present book effectively covers new topics on mechanics of metal cutting, machine tools with their features and working, design of sand molds and their gating and riser systems and the basics of cooling of castings, theory of hot metal forming by rolling and forging, non-conventional machining methods, automation and transfer machining, machining centres, robotics, manufacturing of gears and threads and jigs and fixtures. A large number of numericals are included to help students in understanding fundamentals of various manufacturing processes. The addition of new solid-state welding processes, weldability, heat in welding, stresses and distortion of weldments and more information on non-destructive testing of welds has greatly supplemented the old text on welding. A new feature of the present edition is the inclusion of objective-type questions drawn from various competitive examinations such as Indian Engineering Services, GATE, etc. It is hoped that the readers will find this new revised edition much more useful. In the end, I wish to express my sincere thanks and appreciation to Dr. D.I. Lalwani, S.V.  National Institute of Technology, Surat for his valuable suggestions on the text matter on mechanics of metal cutting and machine tools. I also wish to express my sincere appreciation for the untiring efforts put in by my publishers in bringing out this book so nicely. J.P. Kaushish

xxv

Preface to the First Edition Manufacturing of marketable commodity by converting raw materials into different forms of usable products is considered the backbone of any industrialized country; the greater the level of manufacturing activity, the higher the standard of living of the people. After World War II, manufacturing technology and engineering has made rapid strides, transcending diverse disciplines, such as product design, material selection, manufacturing processes, process planning and production control, machinery and tooling, quality control and organizational management including sales and economic growth. It is due to this reason that the subject of manufacturing technology and processes is taught as one of the compulsory subjects, in some form or the other, in all engineering colleges/institutes. This book has been written to place in the hands of undergraduate engineering students and practising engineers a comprehensive state-of-the-art text on basic principles and operational procedures of general manufacturing processes. The text provides a comprehensive introduction to the fundamentals of basic manufacturing processes and related field practices, to help the students in developing an understanding of the important and often complex interrelationship among many technical and economical factors involved in manufacturing. Since material properties play a key role in manufacturing processes, the book starts with a discussion on these properties while laying emphasis on the influence of materials and processing parameters in understanding manufacturing processes and operations. This is followed by a detailed description of various manufacturing processes commonly used in industry. General tools and equipment and machine tools and techniques used in manufacturing have also been discussed in the text, adequately supplemented with line diagrams and detailed explanatory captions. Throughout, the text uses SI units. To educate and inform the students about the quality standards available for testing of materials and products, references from the Bureau of Indian Standards (BIS) Codes have been provided wherever considered necessary. During the preparation of this book, I have taken help from many excellent texts and for this, I feel indebted to the authors and publishers of these books. My sincere thanks are due to Mr Sewa Ram for assisting me and in providing constructive suggestions from time to time during the preparation of the manuscript. xxvii

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PREFACE TO THE FIRST EDITION

Finally, I wish to express my heart-felt gratitude to my wife, Manju Kaushish, who has borne long hours of inconvenience during the preparation of this text. My affectionate blessings are due to my grandchildren, for demonstrating utmost patience and calm whenever I was on the work. I would appreciate and welcome comments and suggestions from the readers for further improvement of the book. J.P. Kaushish

1 1.1

Manufacturing Principles and Processes

INTRODUCTION

Manufacturing is considered the backbone of any industrialized country as a country’s level of manufacturing activity is directly related to its economy. Usually, the level of manufacturing activity of a nation is an indicator of the living standard of the people of that nation. In the present competitive age of industrialization, manufacturing of high-quality products at the lowest possible cost requires an understanding of the often complex relationship among many interrelated factors of manufacturing such as product design, selection of materials and manufacturing process, planning and utilization of manpower and equipment. For better performance, the designs of products being manufactured have to be constantly improved by taking advantage of the unique characteristics of new emerging materials and manufacturing processes, which enable making and assembly of the products easier while striving for zero-based rejection and waste. Selecting the best materials and the best manufacturing process out of a very wide variety of available materials and processes has now become a major challenge as well as an opportunity for the manufacturing engineers. Also, not all of the innovations that increase efficiency of manufacture are only in the areas of materials, processes or machine tools, increased emphasis is now being given to the management and training of manpower which is always an important asset of any manufacturing enterprise. Personnel on production lines are being trained to become teams to take collective responsibility for the end product. Under this concept, the team’s responsibility includes determining the distribution of work, the occupation of machines, and the speed and quality of work. Manufacturing (or production) engineers have an important part to play. They have to interact with design and material engineers to optimize productivity and minimize production costs. They also have to interact with the industrial engineers in planning for plant-floor activities and plant layout, selecting material handling systems, time and motion study, production planning and scheduling, and the maintenance of equipment. Production engineers in cooperation with industrial engineers help in evaluating new technologies and their 1

2

MANUFACTURING PROCESSES

implementation for profitable production. One most important task of a production engineer is to organize manpower. He should, therefore, be familiar with the principles and practice of the organization and functioning of the industrial concerns. It may be emphasized that high production rates are always achieved when manufacturing operations are carried out in safety and relative comfort by satisfying shop personnel. The cost of the product usually determines its marketability and acceptance by customers. In this is hidden a real challenge for the manufacturing engineers as this calls for an innovative and creative approach to product design and manufacturing technology with the concept that quality must be built into a product. Value engineering has, therefore, become a powerful tool to evaluate the cost of each manufacturing step relative to its contribution to the value of the product. Computer-aided design (CAD) and computer-aided manufacturing (CAM) are among the most outstanding developments to help in this regard. Industrial safety and accident prevention continues to be the important area of industrial management. The procedures laid down under relevant Factory Acts and rules for safeguarding men, materials, machines and environment are of direct concern to manufacturers. The predominant ecological problems related to manufacturers include noise pollution, and water and air pollution.

1.2

CONCEPT OF MANUFACTURING

Manufacturing can be defined as the process of converting raw materials (and information such as design specifications) into a usable form of products or goods for human needs. The process of manufacturing mainly encompasses (a) designing the product, (b) selecting raw materials for the product and (c) deciding sequence of processes through which the product will be manufactured. The word production is often used in place of the word manufacturing. Also, the term manufacturing engineering (or production engineering) is used synonymously to describe the manufacturing area of an industrial activity. Manufacturing entails a large number of interdependent activities, consisting of distinct entities such as materials, tools, machines, power and human being. It thus integrates many disciplines of engineering and management. Manufacturing is a system in which product design is the starting stage and the delivery of the finished product in the market is the final step. Manufacturing is a complex system because it is composed of many diverse physical and human elements, some of which are difficult to predict and control, such as supply and cost of raw materials, market changes and human behaviour and performance. Manufacturing activities have to be responsive to various demands and expectations of the market, for example, (a) a product needs to meet proper design requirements of specifications and standard, (b) its manufacturing methods and technologies should be eco-friendly and economical, (c) quality consciousness should be built or integrated into the product at each stage of manufacturing, and (d) a manufacturing organization should constantly strive for maintaining higher levels of quality and productivity, making optimum use of its resources such as materials, machines and labour. Technology for design and manufacturing should be evaluated from time to time in view of new engineering materials, production methods, etc. It needs to be done so that the output per employee per hour in all phases is maximized

MANUFACTURING—Principles and Processes

3

and zero-based part rejection (for reduction in waste) is made an integral aspect of productivity.

1.3

MANUFACTURING PROCESS

A manufacturing process is the activity (or a combination of activities) of transforming a given material into a product of different forms and sizes and with or without changing the physical and mechanical properties of the product material. Examples of a manufacturing process include: casting, welding, bending, forming, rolling and heat-treatment. A manufacturing process is always accomplished with the help of a variety of tools, equipment and other devices or mechanical aids and the human effort.

1.4

TECHNOLOGY

Technology comprises the art and technique involved in any manufacturing process or other technical services where use of knowledge and input of trained personnel is made. Technology comprises efficient goal-oriented activities with application of scientific and other knowledge to practical tasks, involving people and organization, living things and machines.

1.5

PRODUCTION METHOD

A production method comprises a set of various types of manufacturing processes carried in some sequence on a material to transform it into the final shape of a marketable commodity. Depending on the specifications of the product (material, size, etc.), as also other factors (such as service requirements), there can be more than one production method that can possibly be thought of for making the product. For example, take the case of making an extremely large gear. According to one production method, it can be made from cast iron, following a couple of manufacturing operations in some sequence, such as making of wooden pattern, preparing mold, casting by pouring molten metal in the mold, trimming and finishing the casting, machining and grinding it to the final form of the gear. In yet another production method, the same gear can be made as a steel weldment wherein manufacturing processes involved will be cutting gear blank from steel plate, welding hub to the blank and turning and gear-teeth cutting and heat-treatment of the finished gear. Besides these two methods, there could be even more methods of producing the same gear. The choice of a particular production method, therefore, depends on several factors such as specifications of the product (material, finish, hardness, strength as recommended by the product designer), number of pieces to be made, available facilities in terms of equipment and skilled manpower, time constraint, and overall cost of manufacturing. Other functions, which are also important components of the production method, include material procurement, planning and control of manufacturing, arranging facilities of machines, jigs and fixtures as also inspection and quality control.

4

MANUFACTURING PROCESSES

1.5.1 Types of Production Method As already mentioned, a production method comprises a set of processes and techniques that are used to manufacture a product, and the choice for a particular production method can vary greatly in terms of selecting a manufacturing process or a systematic combination thereof, mainly in view of the specifications of the product and its quality. Determining a particular production method is the part of process planning, which is related to converting the design (of a product) into a final marketable form with minimum investment of time and cost. Basic forms of a production method (or system) are: 1. Piece or unit production: It concentrates on producing one or several articles like prototypes, experimental products, etc. which are seldom or never to be produced again in that enterprise. It is usually a more expensive method of production. 2. Lot or batch production: In this system, articles of identical shape and size are produced in a lot or batch. Special jigs and fixtures are used as an aid for speeding up the production and for providing ease and accuracy. Articles produced in lot production are obviously cheaper than made in the unit production system. 3. Mass production: It relates to the manufacturing of a large quantity of standard parts by using specialized workforce and incorporating the principles of interchangeable production. Mass production methods are designed specially for applications involving high-volume runs since this form of production is most economical. Mass consumer goods available in the market are produced by mass production methods. Since the whole system and assets are geared for the mass production of a particular product or a family of products having similarity in design and specifications, this method, therefore, limits the operational flexibility of an enterprise in entertaining great variations in the product varieties and specifications.

1.6

PRODUCTIVITY

In business venture, productivity relates to output divided by input. In the traditional way of considering productivity, the labour is taken as the most important input. But productivity needs to be analyzed in terms of other factors too and these are capital, energy and materials. Productivity is defined as the optimum use of all the resources which include: manpower, materials, machines, energy, capital and technology. Output per employee per hour in all the phases of production must be maximized. Productivity basically measures operating efficiency. Zero-based part rejection is an important part of productivity. A manufacturing organization must constantly strive for higher levels of productivity. Modern manufacturing organizations have a Standard Measurement System for productivity, which involves control and screening measures, strategic planning and performance measurements, etc.

1.6.1 Important Aspects for Increasing Productivity Among a large number of considerations that could be thought of for boosting up productivity and efficiency of a production unit, the following ones are of prime importance.

MANUFACTURING—Principles and Processes

5

1. Simplification: Simplification aims at determining possible limited number of grades, types and sizes of a product to be manufactured. This is done to minimize waste and exercise better control by simplifying production processes, thus leading towards interchangeable manufacturing of products. 2. Standardization: Standardization is yet another aspect of interchangeable manufacturing, resulting into higher outputs with minimum inputs. Standardization is quite involving an exercise as it aims at determining materials with most appropriate specifications, and finalizing most efficient manufacturing processes and other associated functions and services to support manufacturing. The process of standardization is a highly dynamic process, involving continuous revision and bringing improvements in existing standard in all respects. 3. Inspection and quality control: Inspection is concerned with checking how well the physical and other specifications of raw materials and other material inputs in production are as per the specifications required, as also the final product for its right dimensions, surface specifications and for other requirements specified in the product design. Quality control is concerned with the prevention of bad products and is to be exercised at various stages of production. Statistical methods of quality control are often used in production. 4. Interchangeability: To manufacture a large number of components absolutely identical in shape and dimensions using economical methods of production is rather difficult due to several factors such as defects in machine tools, defective measuring instruments and carelessness of the operator. To take care of these factors, the size of the components being manufactured is permitted to vary between certain specified range or limits of dimensions of the component. When a system of this kind is used such that any one component will assemble correctly with the corresponding mating component (maintaining a particular ‘fit’), both chosen at random, the system is called interchangeable system or a limit system. The property of the mating component because of which they fit easily without affecting the ‘fit’ is called interchangeability. The principle of interchangeability is usually applied to mass production wherein identical products are manufactured in large volumes. To help easy fitting of randomly picked up mating parts of a product under manufacture, some allowance or limit on dimensions is given on the male and female parts to facilitate their easy fitting when picked up at random for making final assembly of the product. 5. Mechanization and automation: Mechanization aims at reducing, to the extent possible, the human inputs in accomplishing any operation. Besides that, there are many jobs performed in industry which may put human lives at risk. These jobs are better performed by machines. Mechanization is done to increase the productivity of labour and for cutting down the time and cost of any industrial operation. Automation is, in fact, an extension of mechanization in which operations and controls are exercised mechanically (or electronically) without much help of the human element. Automatic machines may be semi-automatic or fully automatic type in view of the extent of the involvement of human factor in operation and control of the machine. Automation plays an important role in mass production. Automatic machines are designed to perform specific jobs only and thus have minimum flexibility of their use in other works.

6

MANUFACTURING PROCESSES

1.7

MANUFACTURING ATTRIBUTES

Manufacturing attributes are those aspects which are considered in the decision-making exercise for a manufacturing system and these are (a) cost, (b) time, (c) quality and (d) flexibility. The costs, related to manufacturing, include material cost, labour cost, equipment and facilities cost, maintenance and overheads cost, besides the cost of capital investment. Time factor refers to how quickly a manufacturing system can respond to the changes in product design, demand variations, etc. Quality refers to customer satisfaction and is achieved by ensuring how well the production processes meet design specifications of a product under production. Flexibility indicates the ability of a system to cope with external changes in the job to be processed and the internal changes like organizational disturbances and is determined by the sensitivity of the system to changes. In any organization, manufacturing programmes should be aimed at achieving certain well-accepted goals which include: meeting fully the design specifications and service requirements of the product; always making efforts towards finding most economical methods of manufacturing which should be eco-friendly also; building quality into the product at each stage of design and manufacturing; flexibility in the process of manufacturing to respond to rapid changes in the market in respect of product variety and quantity demand, besides continually striving for achieving higher levels of quality and productivity while maximizing human values such as operator safety, comfort and satisfaction.

1.8

TOTAL QUALITY MANAGEMENT (TQM)

Product quality is an essential feature for the marketability of the product. Public perception is that a good quality product is one that performs its intended functions reliably over a long period of time without failure or needing repairs. Quality has been defined as a product’s fitness for use and is a broad-based characteristic, giving the totality of features and characteristics that bear on a product’s ability to satisfy a given need. Quality control is a set of operational techniques which are used to fulfil the requirements for quality. Quality assurance is defined as all actions required to ensure that quality requirements are satisfied and thus is the total effort made by the manufacturer to ensure that the manufactured products conform to a detailed set of specifications and standards (covering dimensions, finish, tolerances, composition, colour, etc.). This way, quality assurance is the responsibility of everyone involved with the design and manufacture of the product. One of the important aspects of quality assurance is the analysis of product defects and their prompt elimination or reduction to acceptable levels. In a broader sense, quality assurance involves evaluating the product and customer satisfaction. The sum total of all these activities is called total quality control (TQC), or in a greater sense total quality management (TQM). As explained above, quality, in a broader sense, is a characteristic or property consisting of several well-defined technical and aesthetic considerations, technical considerations being objective and aesthetic considerations being subjective. Generally, a high-quality product ensures that it will function reliably and as expected over a long period of time. In fact, quality cannot be inspected into any unit of the product after its fabrication as is traditionally

MANUFACTURING—Principles and Processes

7

considered, and hence the inspection of products after they are made is being replaced these days with the conception that quality must be built into a product, right from the design stage to all other following stages of manufacturing the product. It may be noted that various manufacturing processes (through which a product passes) have great influence on the product quality, and it is with this reason that in a quality control programme, the emphasis should be on controlling the manufacturing processes and not the product. TQM is therefore a management system that emphasizes the concept that quality must be designed and built into a product so that defect prevention rather than defect inspection is the main goal. TQM is a system approach in that both management and worker make a concerted effort to consistently manufacture high-quality products. Total quality management and quality assurance is therefore the responsibility of each and every person involved in designing and manufacturing the product. TQM aims at the importance of management’s commitment to product quality and of pride of workmanship at all levels of manufacturing, as also the use of various techniques of quality control, such as statistical process control and control charts for on-line monitoring of the production of product components, so that the sources of quality problem, if any, may be quickly identified. Quality management and quality assurance standards are now available. International Organization for Standardization (ISO) 9000 series on Quality Management and Quality Assurance Standards as well as QS 9000 came up as a need-based outcome of the trends towards global manufacturing and competitiveness with dynamic multinational markets.

1.9

PRODUCT DESIGN AND CONCURRENT ENGINEERING

The aspect of product design covers both the designing of an altogether new product and revising the design of an already existing product. In both cases, the designer should clearly ascertain beforehand the functions and the performance expected of the product during use. Product design is a critical activity since major part of the cost of product development and manufacture is determined by the decisions made in the initial design stages. So far, the trend had been that the design and manufacturing activities took place sequentially (i.e. design first and manufacture later) rather than concurrently or simultaneously. In the recent trend of concurrent engineering design, the design approach integrates the design and manufacturing of the product with a view towards optimizing, considering simultaneously all the aspects of life cycle of the product such as design, development, manufacturing, use, disposal, recycling, etc. The job has now been simplified through the use of computer-aided design (CAD) and computer-aided manufacturing (CAM). These two terms are discussed below. 1. Computer-aided design (CAD): It involves the use of computers for creating design drawings and product models. Thus it allows the designer to conceptualize objects more easily, avoiding use of costly models or prototypes. CAD systems are capable of analyzing quickly and completely the designs of simple objects to highly complex structures such as an aeroplane. CAD is usually associated with interactive computer graphics (CAD system) and is a useful tool for mechanical design and geometrical modelling of products. These days, products are made directly from the CAD software output without making a prototype or model. Using computer-aided engineering

8

MANUFACTURING PROCESSES

techniques, the performance of a structure subjected to steady or fluctuating loads, or varying temperatures can be easily stimulated, analyzed and tested more accurately and the information developed is easily stored, retrieved or reprinted. Design modifications are best made by CAD system as standard components are stored permanently in the database and called up and positioned (or modified) on the drawing, where it is also possible to produce 3-D models for carrying out finite element analysis of the product under design. 2. Computer-aided manufacturing (CAM): It involves the use of computers and computer technology for assisting in all the phases of manufacturing a product, including process and production planning, scheduling, machining, quality control, and management. This is achieved by utilizing and processing large amount of information on materials and processes collected and stored in the database (of the organization). Engineers are assisted by the computer in programming numerical control of machines, robots for material handling, welding and assembling or for designing tools, dies, fixtures and maintaining quality control.

1.10

SELECTING PRODUCT MATERIAL

The selection of a product material depends on the consideration of the following factors: ●

Service requirements such as strength, type of loading (static or cyclic, etc.), wear, corrosion resistance, electrical properties, etc.

●

Manufacturing requirements such as ease of fabrication, casting, welding, or machining, finish required, method of forming (hot or cold), method of jointing various parts into an assembly, need for heat-treatment, etc.

●

Cost of material should be cheapest possible.

●

Availability of material should be easy and preferably from local market so as to reduce procurement and transportation cost.

There might be more than one solution possible for selecting the product material. The final selection is often done by the designer based on his/her experience regarding the user’s preference, aesthetic of product, possible frequency of occurrence of faults in the product during its use and availability of repair facilities, useful life, and consideration of other life cycle elements.

1.10.1

Types of Materials

Materials are now available in an ever-increasing variety, each possessing its own characteristics, advantages and limitations. General type of materials used in manufacturing products are given below. These are used either individually or in a combination of several materials in a product. ●

Ferrous metals: Cast irons and carbon steels and alloy steels of various types such as tool steel, stainless steel.

MANUFACTURING—Principles and Processes

9

●

Nonferrous metals: Aluminium, copper, magnesium, nickel, titanium, refractory metals, beryllium, zirconium, low-melting point alloys.

●

Plastics:

●

Ceramics and others: Oxides, nitrides, carbides, glass ceramics, glasses, graphites, diamond and diamond-like materials.

●

Composite materials or engineered materials: and ceramic-matrix composites, laminates, etc.

●

New materials: Superalloys (high-temperature alloys), superconductors, shapememory alloys, nanomaterials, amorphous alloys (glasses), etc.

1.10.2

Thermoplastics, thermosetting plastics and elastomers (rubber).

Reinforced plastics, metal-matrix

Properties of Materials

Properties of a material considered important for making a product are as follows:

1.11

●

Mechanical properties include strength, ductility, toughness, hardness, elasticity, fatigue and creep. The strength to weight and stiffness to weight ratios of materials are important for aerospace and automobile industry.

●

Physical properties include density, specific heat, thermal expansion, melting point, thermal conductivity, electrical and magnetic properties.

●

Chemical properties and their consideration are important for both normal environment and hostile environment. Important factors to be considered are: oxidation, corrosion, toxicity, flammability and general degradation of material properties under the effect of environment.

●

Manufacturing properties or fabrication properties of materials determine the ease of their casting, forming, welding, etc. in making a product for market use. The manufacturing methods used in processing a material to the desired shape of the product can adversely affect the properties of the final product, service life and efficiency and cost.

MANUFACTURING PROCESSES

A large number of manufacturing processes are available to produce parts (or products) of different shapes. There may be more than one method of manufacturing a part from the given material. Various manufacturing processes used for transforming metals into some usable products are based on some basic properties of metals, for example, the process of casting is based on the property of ‘fusibility’ or melting, forging on the property of ‘malleability’ and rolling or forming based on the property of ‘ductility’. Likewise, the process of machining is based on the property of ‘divisibility’, which is the capability of metal to get divided into small bits (chips), separated from the workpiece blank with the help of a hard cutting tool.

10

MANUFACTURING PROCESSES

1.11.1

Classification of Manufacturing Processes

Manufacturing processes are broadly categorized as: (a) Primary shaping processes (b) Secondary shaping processes Primary shaping processes

Primary shaping processes (also known as basic manufacturing processes) are probably the oldest manufacturing processes, practised by the craftsmen of older times. These are, however, still very popular and most used processes. Examples are: metal casting processes and hot forming processes or forging. Primary shaping processes are those processes which are used to produce or manufacture a product directly to its usable form (without any subsequent finishing or machining), and the products are made usually from those materials which are available in raw form, such as pig iron ingots or steel ingots. Primary shaping processes are, therefore, considered cheaper processes. Examples of products made by using primary shaping processes include cast products like cast-iron articles which can be sold directly in the market as cast and without any further processing on them (like machining, shaping or grinding). Other examples are hot-rolled metal products (called wrought metals) such as angles, channels, rods, bar stocks, I-sections, etc., which are also used in the market as direct outcome of hot-rolling processes of steel ingots. Primary shaping processes are further divided as follows:

Secondary shaping processes

Secondary shaping processes (or secondary manufacturing processes) are those processes which are usually carried out on the outcome of primary shaping processes like castings or hot-rolled products, for example, cold working processes carried on the hot-rolled products for drawing them into finished bars for further machining (or wire drawing). In many cases, products from the primary forming process (casting and hot-rolled products) are required to undergo further refinement in shape and size through various processes of metal removal (or metal cutting or metal machining) such as turning, milling, planing. Some

MANUFACTURING—Principles and Processes

11

other times, surface finish on the product is an essential requirement for many reasons (fatigue, etc.). Finishing processes such as buffing, lapping, etc. are done for providing surface finish on the product. Secondary shaping processes are broadly categorized as follows:

1.11.2

Other Manufacturing Processes

Other manufacturing processes are those which may not fall directly under the category of primary shaping processes or secondary shaping processes. These include: Joining or jointing process: Joining or jointing processes are used to join various parts together to form into an assembly of a product or machine. Joining is an all-inclusive term, covering processes such as welding, brazing, soldering, mechanical fastening, adhesive bonding, riveting and metal stitching. Processes affecting changes in the property of metals: Manufacturing processes often used to affect changes in the property of metals include heat-treatment, hot working, cold working, shot peening, burnishing, etc. Various processes of heat-treatment such as hardening, tempering and annealing impart different properties to the metal. Hot-working refines grain structure of metal and improves its ductility and impact resistance. Cold-working improves tensile strength, yield strength and hardness. Both shot peening and burnishing add compressive stress at the outer surface of the part, thereby improving its fatigue strength. Decorative and protective coating processes: These are used to provide a protective coating on the surface of a part to protect it against oxidation or corrosion when subjected to aggressive atmosphere. Surface coatings are provided for decorative purposes also to improve aesthetic of the product. Various such processes are electroplating, galvanizing, metallizing (metallic coating), organic and inorganic coatings. Inspection processes: Inspection processes form part of the quality control programme. These comprise: ● ●

Examining in-process or finished product to determine conformance to specifications Checking/testing mechanical and physical properties

12 1.12

MANUFACTURING PROCESSES

INTRODUCTION TO BASIC MANUFACTURING PROCESSES

In spite of innumerable advances that have been made in the methods and equipment used in manufacturing, the basic categories of manufacturing processes have remained relatively unchanged and these are: (i) casting and molding, (ii) shearing and forming, (iii) machining (metal removal), (iv) heat treating, (v) finishing, (vi) assembling or jointing and (vii) inspection. However, none of these processes is exclusive when the intent of the operation as well as action itself is considered. For example, although some polishing operations remove material, the intent of polishing is finishing. Casting is the process that produces a product of desired shape by pouring molten metal into a mold cavity made to required shape in the sand (Fig. 1.1). The molten metal solidifies in the cavity in the form of a product or casting and is taken out by breaking the mold. Primary advantages of using casting process are that a completely formed product is made in a single operation. Castings are also made by pouring molten metal in metallic molds (called permanent molds). Molding box (upper)

Molten metal Mold cavity

Molding sand

Sprue

Molding box (lower)

Fig. 1.1

K

Finished casting

Essentials of a casting process. After the casting is taken out of the mold, the sprue (unwanted excess metal) is cut off at point (K) to get a clear casting.

Molding involves forcing granular or powdered material (plastic) into the heated mold cavity under great pressure which, together with the application of heat, causes the material to fill the mold cavity completely. After cooling, the mold is opened and the finished cast product is taken out of the mold (Fig. 1.2).

Fig. 1.2

A molding process. The piston compresses the powdered or granular plastic material and injects it into the heated mold cavity.

MANUFACTURING—Principles and Processes

13

In addition, the term ‘molding’ in reference to sand molds (used for casting) is used for the operation of making the mold cavity in the foundry sand. Shearing or cutting comprises operations used to cut the material to a desired shape and size. These are further divided into punching, piercing, shearing, blanking, cut off, parting and trimming (Fig. 1.3).

Fig. 1.3 Various shearing or cutting operations.

Punching is used to cut hole or any shape into a workpiece. Piercing produces a raised hole rather than a cut hole. Blanking, cut off, parting, shearing and trimming produce blanks (raw material for the workpiece) or parts. Blanking is a type of punching where the punched out piece is the workpiece (or blank) and the scrap is the piece of sheet from which the blank has been cut off. Cut off, parting and shearing are similar operations with the difference that shearing produces no scrap (only blank) and is used when the cut edge is straight. Cut off is a blanking operation along the zig-zag line with no scrap, but parting is that blanking operation which produces both workpieces and scrap for different uses. Trimming removes excess material from the edges of a workpiece. Forming operations (cold forming) are those that shape the part by forcing it through a die or set of rolls of specific configuration and are carried out at room temperature. These

14

MANUFACTURING PROCESSES

include forming, roll bending, extruding, roll forming, wire drawing, drawing, and spinning. These are shown in Fig. 1.4.

Fig. 1.4

Cold forming operations: (a) Forming is a type of bending operation used for producing twoand three-dimensional shapes, (b) Roll bending produces two-dimensional shapes, (c) Extruding is forming of specific shapes by forcing solid blank through a die to produce a workpiece with different cross-sections, (d) Roll forming involves passing metal strips (0.1 to 20 mm thick) between two rollers to form desired shape, (e) Cold drawing of metals is a process of finishing hot rolled bars of various cross-sections by forcing them under tension through a hardened die, (f) Wire drawing is extrusion of a rod through a die to cause reduction in its cross-sectional area. Wires are drawn after pulling the rod through a set of several dies (of gradually reducing size) which finally reduce the rod to a wire, (g) Drawing is forming shapes from thin sheets, e.g. deep drawing of steel utensils with die and press, and (h) Spinning is shaping of thin sheets by pressing them against a pattern or form when sheet blank (workpiece) is rotated fast on a lathe.

MANUFACTURING—Principles and Processes

15

Forming operations (hot forming) change the shape of hot metals by pressure. The metal is brought to the viscous or plastic state by subjecting it to elevated temperature, and by the use of pressure it is made to flow (with or without a die) but without rupture. Four major hot-forming operations are: rolling, forging, extruding and upsetting (Fig. 1.5).

Fig. 1.5

A schematic diagram of four major hot working processes: (a) Rolling is used for making bars, angle irons, channels, and I-sections, (b) Forging is used to form different shapes by passing the red hot blank of metal through a set of dies (or die stations) arranged in succession to help forging down the hot blank gradually from its original cross-section (here round) to the final desired cross-section (square) of the workpiece, (c) Extruding is done to reduce cross-sectional area of workpiece by passing hot blank under pressure through a die, and (d) Upsetting involves increasing the size of a portion of the workpiece.

Machining (metal removing) processes remove excess metal from a workpiece to bring the workpiece to the desired shape and size of a product. These are categorized as below: ●

Hole making operations are drilling, reaming, boring and tapping (Fig. 1.6). Drilling makes holes, reaming enlarges (very little) the drilled hole to a precise size, boring enlarges the already made hole considerably with a boring tool, and tapping is used for thread cutting in the drilled hole.

Fig. 1.6

Various hole making operations, namely drilling, reaming, boring and tapping.

16

MANUFACTURING PROCESSES ●

Shape changing processes are turning, facing, shaping, planing, milling, threading, parting, and broaching (Fig. 1.7).

●

Sawing processes are used for cutting pieces from raw stock (Fig. 1.8).

●

Grinding is a fine finishing operation usually done after turning, milling or shaping (Fig. 1.9). Other finishing operations are: lapping, honing, super finishing, and buffing.

●

Unconventional methods of machining include electric discharge machining, electrochemical machining, ultrasonic machining, laser machining, etc.

Fig. 1.7

Metal removal or metal machining or shape changing processes: (a) Straight turning, (b) Taper turning, (c) Facing, (d) Plane or slab milling, (e) Groove milling, (f) Shaping, (g) Contour forming on a planer, (h) Broaching, and (i) Threading and parting (cut off).

Fig. 1.8

Sawing or cutting process: (a) Cutting with bandsaw, (b) Cutting with hacksaw (manual or power), and (c) Cutting with circular saw.

MANUFACTURING—Principles and Processes

17

Grinding wheel

+

+

+

Workpiece (a) Surface grinding

Fig. 1.9

(b) Cylindrical grinding

Finishing by grinding: (a) Surface grinder and (b) Cylindrical grinder.

Finishing processes improve characteristics, appearance, or durability of a surface. Finishing operations include cleaning, deburring, polishing, painting, plating and coating. Assembly or jointing processes are used for attaching or connecting together individual elements/components to form a complete assembly of the product. These include mechanical fastening with bolts and nuts, screws, rivets and wire stitches as also permanent jointing by soldering, brazing, welding or adhesive bonding (Fig. 1.10). Bolt

Screw Rivets

Wire stitches

(a) Without adhesive

Welding / Brazing

With adhesive

Brazing / Soldering (b)

Fig. 1.10

(c)

Assembly or jointing processes. Jointing with bolt and nut, screw, rivet and wire stitching is shown at (a) whereas with welding/brazing/soldering at (b). The process of adhesive bonding is shown at (c).

Heat-treatment processes modify the mechanical properties of metals to prepare them for applications that require properties different from those inherent in the base metal. These processes include hardening to increase hardness of the workpiece, case-hardening to increase hardness of the surface, tempering to make steel both tougher and harder and annealing to remove hidden stresses and improve grains. Inspection involves examining in-process or finished parts to determine their conformance to the specifications and includes physically measuring dimensions of parts, and checking physical and mechanical properties.

18 1.13

MANUFACTURING PROCESSES

PRODUCTION SHOPS

Production or manufacturing of the components of a product is carried out in different shops, called production shops or production departments or blocks. All of them share production according to the specific work performed in those shops. There are main shops like foundry shop, fabrication or welding shop, machine shop, and heat-treatment shop, which are directly engaged in manufacturing. Main shops are fully equipped with necessary facilities of machines and equipment used for the jobs or functions carried out in that shop. These shops make good use of productivity aids (jigs, fixtures, templates, gages), and are geared to mass production with high volume output using modern tools and equipment. There are service or support shops also, which may not take direct part in manufacturing but provide services and support for effective running of the main workshops. Tool room, electric shop, and vehicle section are examples of service shops. A workshop (or workshops) is a small version of the aforesaid production shops. It may have a set-up of various facilities of casting shop, machine shop, welding shop, fitting shop, but all at a small level. In technical institutions, facility of a workshop provides opportunity to the students to get familiar with various plants and equipment and tools used in different shops and to have practical training on the use of these facilities.

1.14

MODERN CONCEPTS OF MANUFACTURING

Net-shape manufacturing

Not all primary shaping or manufacturing processes result into the production of accurately finished components. Some additional operations (such as machining, grinding, superfinishing) are therefore required to bring the components (made by casting, welding or forging) to desired finish and dimensions. All such finishing operations add a lot to the cost of manufactured components. In the present trend of net-shape or near-net-shape manufacturing, the component is made, in the first operation, as close to the final required dimensions, tolerances and finish as is possible. Examples of net-shape manufacturing include powder metallurgy techniques, injection moulding of plastics, stamped sheet metal parts, and some forged and cast products. Flexible manufacturing system (FMS)

Flexible manufacturing system involves integration of various manufacturing cells (machining centres) into a large unit, all interfaced with a central computer system. Further, the machining centres are interconnected with an in-line transfer system, automatic handling systems, conveyors and cars, all under computer control. The system has numerically controlled machine tools magazines, tool changer, fixtures, etc. FMSs are capable of producing a wide variety of items in small batches and in a random order. It has the capability to remain flexible for both small orders of varying parts and large volumes of production. It is, however, very costly. Group technology

According to group technology concept, various types of components are grouped and produced by classifying them into families, based on similarity in their design and manufacturing processes used to make them. By doing so, component designs and process plans can be easily standardized and families of like parts can be produced efficiently and economically.

MANUFACTURING—Principles and Processes

19

Computer numerical control (CNC) machines

Operations on conventional machine tools are controlled manually by the operator. Numerical Control (NC) is a method of controlling the movements of a machine tool component by directly inserting coded instructions, in the form of numbers and letters, into the control unit of the machine tool. The control unit automatically interprets the data and converts them to output electrical signals. These signals, in turn, control the movements of various components of the machine tool. Data concerning all aspects of machining operations (locations, speeds, feeds, etc.) can be stored on punched cards and magnetic media (tapes, hard disks). During the operation of the machine tool, the specific information can be relayed from the storage devices to the machine tools’ control panel. NC machine tools have registered many advantages over conventional machine tools, important among them are: reduction in production time, optimization of cutting tool life, better quality of machining, possibility of making components with extremely complicated shapes that are otherwise impossible to achieve in conventional machining and quick and more accurate inspection. Computer Numerical Control (CNC) is a system in which a control microcomputer is an integral part of the machine tools’ system. The computer is used to perform all the basic NC functions as per the control program stored in the memory of the computer. This way, the machine tool control data comes direct from the computer memory. The CNC provides greater flexibility of working as the part program can be prepared at some distant place by the programmer and it may also include information obtained from drafting software packages and from machining simulations for ensuring that the part program is bug free. Besides that, the machine tool operator can easily program onboard computers and can modify the programs directly, prepare programs for different parts and can store the programs as well. This flexibility is not available with the NC machines. Just-in-time production (JIT)

The concept of JIT is that the supply of raw material (to be processed further) is made just-in-time (i.e. no delay), components are manufactured just-in-time, and sub-assembling of components into the final product is also done just-in-time to be delivered to the customer. JIT ensures that high-quality products are made in time and at low cost.

1.15

HUMAN-FACTORS ENGINEERING AND INDUSTRIAL SAFETY

For the development of safe and efficient working conditions in a manufacturing plant, it is essential to understand the interaction of human beings with machines and workplace environment. Human-factors engineering (or human engineering) concerns all aspects of human-machine interactions. Ergonomics is a synonymous term often used for human engineering. The human-factors engineering aims at maximizing the quality and efficiency of work and the human values such as the operator’s safety, comfort and satisfaction. It also aims at reducing the operator’s fatigue and stress. Safety is defined as a judgement of the acceptability of a danger. Danger is a combination of hazard and risk. Hazard is an injury producer and risk indicates the probability that an injury may occur. Therefore, the safety of a machine or workplace depends on the associated hazard and risk with machine operation. For treating hazards in the workplace, danger should be reduced to a reasonable level by proper

20

MANUFACTURING PROCESSES

designing of the machine and the workplace. Also, use safeguarding technology. There should be warning signs and labels. Besides that, there should be proper training of the workers with adequate instructions for operating a machine. Workers should be encouraged to use personal protective equipment. Employers are responsible for providing a reasonably safe workplace. Various safety and health standards are available for this purpose.

1.16

MANUFACTURING COST

Manufacturing cost of a product plays an important role in marketing the product successfully. The cost of any new product must be competitive with that of the similar products already existing in the market. It may be noted that irrespective of how well the new product meets design specifications and quality standards, it has to meet the criteria of economy in its cost for being competitive in the market. Manufacturing cost of a product is a combination of several costs such as cost of materials, labour, tooling costs, and fixed and capital costs. Material cost: Efforts should always be made for minimizing the manufacturing cost by optimizing the product design and specifications, selecting most economical product material but with desired properties to suit the product and the corresponding appropriate and economical manufacturing process. Labour cost: Always trained operators or labour should be preferred to help reduce rejection and for ensuring better quality of the product. Increased automation in manufacturing tends to minimize labour costs. Tooling costs: These include cost of tools, dies, jigs and fixtures, patterns, mold, etc. These largely depend on the complexity of the shape and design of the product, material and manufacturing process selected for the product and the production quantity. Making of complex shapes and dealing with product materials that are hard and difficult to machine always add to the tooling costs. Fixed costs and capital costs: These depend on the facilities rigged up in a particular plant. Major capital investment is involved in installing computer-controlled specialized machines, but these machines try to prove economical in the long run as they give much higher production rates and improved quality product reducing rejection costs. Fixed cost includes cost of electric power, fuel, rent, insurance, depreciation and interest and taxes, etc. This cost is not affected much by the production volume. Capital cost relates to the investment made in land, buildings, equipment and other major manufacturing facilities.

REVIEW QUESTIONS 1. What is the role of a production engineer in any manufacturing enterprise? 2. Why is it important to study the subject of manufacturing processes in engineering courses? 3. Define technology and manufacturing.

MANUFACTURING—Principles and Processes

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

21

What do you understand by a manufacturing system? What is a manufacturing process? Name three such processes. Differentiate between the production method and the manufacturing process. Discuss the different types of production methods used in industry. Define productivity. How is productivity related to production? What is mass production and where is it used? What are the aspects considered important for increasing productivity? Explain the difference between inspection and quality control in manufacturing. What is interchangeability? Why is it important in mass production? How does standardization help mass production? Discuss. Differentiate between mechanization and automation. What do you understand by manufacturing attributes? What is their importance? What is a manufacturing process? Name the two broad categories of manufacturing processes. What are primary shaping processes? Discuss with example. How are the secondary shaping processes different from the primary shaping processes? Name the different types of secondary shaping processes in use by the industry. Discuss the various assembling or jointing processes. Which of these are used for temporary assembly and which for permanent assembly? Name the manufacturing processes used for affecting the change in the property of metals. Name some decorative and protective coating processes. What is casting? How is it different from molding? Name a few shearing or metal cutting processes. Differentiate between punching and piercing. What are cold-forming and hot-forming operations? Give some examples of each. Name a few hole making operations. Explain the operation of turning, facing, threading, shaping, planing and milling. Name the operations often used for surface finishing of metals. What do you understand by a production shop? Name the main production shops and service (or support) shops in a manufacturing organization. What is a workshop? How is it different from a mass production shop? Write a short note on: (i) Group technology (ii) Flexible manufacturing system (iii) Net-shape manufacturing (iv) Total quality management (v) Human-factors engineering

2 2.1

Materials Structures and Properties

INTRODUCTION

The art of melting metals for making various products was probably discovered towards the end of the Stone Age. Since then, energies amounting to billions of man-hours have been spent in the multi-phase development of materials and their processing to make varieties of products for the service of mankind. Research and investigations also continued to study the properties and behaviour of materials under varying situations of loading and other applications. Why are some metals brittle and hard while others ductile and soft? Why is it that some metals stand well against high temperatures whereas others not? And many such questions could be answered only by studying the structures of metals. Structure or arrangement of atoms within a metal greatly influences the behaviour and properties of metals. Properties are the characteristics of any material indicating its behaviour and performance under different conditions of working and use of the material. Hence, the knowledge of structures of metals enables us in controlling and predicting the behaviour and performance of those metals. It also allows predicting and evaluating their properties. Properties may be categorized as physical, thermal, electrical, magnetic, chemical or mechanical properties. Fabricating properties of the materials, particularly the metals, refer to the ability or ease with which metal can be formed, cast, machined or welded and these are called formability, castability, machinability and weldability, respectively. A large variety of materials are used for different industrial and domestic applications. These are broadly categorized into metals and non-metals. Examples of metal include: iron, copper, aluminium, tin, zinc, lead, nickel, etc. Metallic alloys, which are combinations of two or more metals, are also available, for example, different types of steels, cast irons, brasses, bronzes. Metals and their alloys find much wider applications in construction, manufacturing and chemical industries because of their very good physical, chemical, mechanical and fabricating properties. Non-metallic materials are weaker in strength, are less ductile and possess poor thermal and electrical conductivity in comparison to metals. Wood, plastics, glass and refractory materials 22

MATERIALS—Structures and Properties

23

such as sands and clays are some typical examples of non-metals. Recent developments in the properties of non-metals have, however, presented them as a tough competitor of metals.

2.2

SELECTING MATERIALS FOR MANUFACTURING

Materials with certain specific properties are picked for manufacturing a product for engineering applications. This is done to ensure that the product made from the right material will function satisfactorily during its service. For example, a soft metal (as copper) with high electrical conductivity is required for making electrical wire. The material for a lathe cutting tool should possess both high hardness and strength even after being red hot during service. The choice of a right material is, therefore, very essential for a trouble-free performance of the product with minimum repairs and maintenance cost. Important factors considered in selecting a suitable raw material include: (i) service requirements covering consideration of expected loads and loading patterns (steady, fluctuating, impact loading), likely temperature to be encountered, order of wear and corrosion resistance expected, need for better electrical or magnetic properties or a preference for certain thermal or chemical properties of the material, (ii) geometrical requirements draw attention to the complexity of shape and size of the product, number of pieces to be manufactured, precision in dimensions and aesthetic considerations of the product, (iii) manufacturing requirements include preference for a particular fabrication technique for economy or other reasons, order of finish required, method of forming (hot or cold), method of jointing sub-assemblies and need for heat-treatment, (iv) economic consideration includes costs involved in design, manufacturing and promotion of sale of the product, whether the product is a monopoly item or has to compete with other similar products already existing in the market and the demand prospects, and (v) other factors include ready availability of the material from the local market, ease of repairing the product on occurrence of a fault, and the expected repair facilities available with skill of personnel.

2.3

METALS AND NON-METALS

For the selection of raw materials for manufacturing a product, an engineer is required to be well conversant with important properties of popular raw materials available in the market. He should also have the understanding of why different materials behave differently in service, and the principles involved. To achieve optimum blend of some important properties of materials, he may have to make good use of a variety of metals and non-metals for manufacturing purposes. Metals are the elements that possess certain properties (called metallic properties) such as metals are opaque, possess characteristic luster, are normally solid at room temperature except a few (like mercury), are good conductor of heat and electricity, are denser, stronger, ductile, and are polycrystalline consisting of a large number of fine crystals (or grains). Examples of metals include iron, copper, aluminium, nickel, zinc, magnesium, lead, etc. Non-metals are devoid of metallic properties and are characterized as brittle substances. They are weaker, less dense, and possess poor thermal and electrical properties. Examples of non-metals include glass, rubber, bricks, concrete, wood, plastics, sand, clays, etc.

24

MANUFACTURING PROCESSES

Since general manufacturing activities in most industries involve greater use of metals and their alloys in comparison to non-metals, the discussion in this chapter will, therefore, be restricted only to the properties of metals and their alloys.

2.4

NATURE OF METALS

For understanding the behaviour of metals under different working conditions of loading and environment as also their adaptability to various fabrication processes, it is desirable to have basic knowledge of the structure of metals, alloys and the effect of alloying elements and modes of failure of metallic components. All materials are made up of atoms. An atom is the smallest part of an element (iron, lead, copper, etc.) that retains the physical characteristics of the element. Matter (anything that occupies space) is a collection or agglomeration of atoms whose positions and behaviour determine the properties of that matter. An atom consists of a nucleus (with positive electric charge) and a number of electrons (negatively charged) which keep moving around the nucleus in some orbital path, some close to the nucleus and others away from it. Thus an atom is not a solid sphere but is largely an empty space. Element is a pure substance containing only one kind of atoms. Element is a simple form of metal that cannot be divided further into simple substances. The composition of an element is indicated by its atomic number (number of electrons around the nucleus) and atomic weight (equals to roughly the weight of the nucleus as the weight of electrons is negligible). The nature of a metal depends upon several factors such as the type of bonding between its atoms, its crystal structure, the size and condition of grains or crystals and the presence of various imperfections in the lattice structure of its crystal.

2.5

STRUCTURE OF METALS

All solids are broadly divided into two types of structures: (a) amorphous structure and (b) crystalline structure. Amorphous structure is that in which basic structural solid is a molecule and whose atoms are not arranged in some systematic order (Fig. 2.1). Non-metallic materials such as rubber, plastics and glass have amorphous structure. Crystalline structure is found in all metals (and some non-metals also). A crystal is a solid in which the constituent atoms or molecules are arranged in three dimensions in an orderly pattern (Fig. 2.2). Crystals can be of similar size or of different size. They consist of a regular orderly arrays of atoms in space and each array consists of rows and columns of atom. The geometry of arrays is called crystal structure of metals. Different metals solidify in different atomic patterns. Crystals are sometimes referred as grains. The size, shape and arrangement of grains in a piece of metal

Fig. 2.1 Amorphous structure of solids with atoms in a disorderly manner.

Fig. 2.2 Crystalline structure of solids having atoms arranged in a perfectly orderly manner.

MATERIALS—Structures and Properties

25

constitute its grain structure. The size of a grain depends on the temperature of metal when poured in a cavity for solidification, cooling rate and the nature of metal. Large grains are formed when metal is cooled slowly and smaller grains produced when cooled fast. As the metals solidify from the molten state, their atoms arrange themselves into various orderly configurations (called crystals) and the arrangement of atoms within the crystal is called crystalline structure. A single crystal may have a number of unit cells which are the building blocks of a crystal and where a unit cell is the smallest group of atoms showing the characteristic lattice structure of a particular metal. Lattice in general means an interlaced (or intercrossed) work or structure made of metal bars (here atomic arrangements) just like a ‘Jali’ with openings in an expanded metal. There are three basic crystal structures in metals: bcc, fcc and hcp, which will be discussed later. Most metals have polycrystalline structure made by the accumulation of a large number of crystals, which are separated by distinct boundaries, called grain boundaries. Each crystal comprises a large number of unit cells in it which differ in their orientation with their neighbouring crystals (Fig. 2.4). Note that the grain boundaries are never straight since they get distorted due to the interference caused among the growing crystals (during solidification of metal from molten state). Metals generally have isotropic nature, i.e. possess same properties in all directions. However, due to cold working, the metals may sometimes get strained such that their grains are elongated in one direction and contracted in the other with the metal properties in vertical direction different from those in the horizontal direction, a phenomenon called anisotropy.

2.6

PHYSICAL STATE OF METALS

When a metal is melted, the metallic atoms detach themselves from other metallic atoms and vibrate rapidly in random directions in the molten mass of metal. What is required for solidification of metal from liquid stage is that a sufficient number of atoms may exist in proper arrangement so that a crystal may grow. When the molten metal begins to solidify, crystals start forming independently of each other at various locations within the liquid mass of metal, but they have random and unrelated orientations (Fig. 2.4). Various crystals grow and join together in a dendritic columnar sructure, growth being in three-dimensional pattern such that the atoms exist in straight lines and in planes. The process of solidification is termed crystallization. Crystallization takes place at numerous places within the molten metal. This pattern is a three-dimensional one and repetitive and controls the external shape of the crystal (or grain). Metals always contain some impurities, metallic inclusions or carbides, and presence of these results into non-uniform condition for a metal during solidification. In case of pure metal with no impurities, the growth of crystals continues in all directions until contact is established with the neighbouring crystal. But this does not happen in practice because of the impurities in the metals. Because of these impurities, even if the metal is cooled down uniformly, crystals of solidifying metals tend to advance in a branching or tree-like manner called dendritic growth (or pine tree column structure). The crystals formed by dendritic growth are called dendrites (Fig. 2.3). This dendritic freezing is the pattern of ferrous alloys (steel and cast iron). Nearly all metals and alloys solidify dendritically. A boundary known as grain boundary is formed

26

MANUFACTURING PROCESSES

Fig. 2.3

Nucleation and growth of a dendritic columnar structure and formation of grains. Arrangement of atoms in a solidifying metal is in three-dimensional pattern, each pattern known as ‘unit cell’. Unit cells initially arrange themselves in straight lines, which are later joined by a number of small branch lines called ‘dendrites’, which are also composed of unit cells. Outward dendritic growth in three dimensions in liquid metal continues until it is stopped by the growth of adjacent structure, thus resulting into the formation of ‘grains’ and ‘grain boundaries’.

when growth of crystals is stopped by interference with adjacent crystals or inclusion of impurities. A crystalline solid may consist of a single crystal or several crystals separated by distinct boundaries (called poly-crystalline solid) as is in most metals.

2.7

SPACE LATTICE (OR CRYSTAL LATTICE)

When metals solidify from a molten state, their atoms arrange themselves into various orderly configurations, called crystals. The arrangement of atoms in a crystal is called crystalline structure, which has a number of arrangements of atoms (or three-dimensional patterns) such as body-centred cubic (bcc), face-centred cubic (fcc) or hexagonal close-packed (hcp) pattern. Each such arrangement of atoms which is repeated many times in the growth or formation of a single crystal is called a unit cell, which is the smallest group of atoms showing the characteristic lattice structure of a particular metal and has the inherent basic mechanical properties of the metal (which are attributed to the special geometric pattern or arrangement of atoms). The unit cell is a building block of a crystal as each crystal consists of millions of tiny unit cells. The regular repeated arrangement of atoms in three dimensions made by repeating a large number of unit cells is called space lattice or crystal lattice of the metal. The three sides or edges (length, width and height) of the unit cell including the three interfacial angles are called geometrical constants or lattice parameters of a crystal system. The three most important geometric arrangements of atoms in crystalline solids are as follows: (i) Body-centred cubic (bcc) lattice (ii) Face-centred cubic (fcc) lattice (iii) Hexagonal close-packed (hcp) lattice Metallic structures can be visualized as comprising metallic ions occupying fixed positions in a cloud of electrons moving along a number of paths. The interaction between atoms leads to the formation of a three-dimensional lattice structure, characteristic of all metallic materials.

MATERIALS—Structures and Properties

27

Atoms in a lattice occupy the corners of either cubic space or hexagonal arrangement (whereas space is less between the atoms and hence is close-packed). The shape and dimensions of the lattice structure play an important role in controlling the mechanical properties of metals. We can modify the lattice structure by adding atoms of some other metals known as alloying to improve the properties of the metal. In some cases, the same metal may also form different structures; allotropic form of iron is an example.

2.8

DENDRITES

Unit cells initially arrange themselves in a straight line and many small straight line branches of unit cells are formed on the initial straight line which follows a geometrical pattern (Fig. 2.3). The smaller branches of unit cells are called dendrites, which are surrounded by liquid metal during solidification. A schematic illustration of various stages during solidification of molten metal is shown in Fig. 2.4, wherein each small square represents a unit cell. Nucleation of crystals [Fig. 2.4(a)] at random sites in molten metal is the start of solidification process, which finally ends with the formation of different crystals or grains. Note the different orientations of unit cells in the neighbouring grains [Fig. 2.4(c)].

Fig. 2.4

Showing different stages during the solidification of a metal. The smallest square in any of the above figures represents a unit cell. A crystal (or grain) may have a large number of unit cells which are the building blocks of a grain. A grain is confined by its grain boundary. (a) Nucleation of crystal at random sites in molten pool of metal, (b) Growth of crystals as solidification of metal continues and (c) Note that the crystalline solids are the aggregate of small grains (or crystals) surrounded by grain boundaries such that each grain has a proper geometrical form of its millions of unit cells (shown by small squares in the above figure) inside it, but which differ in orientation with their neighbouring grains.

28 2.9

MANUFACTURING PROCESSES

GRAIN AND GRAIN BOUNDARY

The entire aggregate of three-dimensional growth of unit cells is surrounded by a separating boundary, called grain boundary, which separates this three-dimensional growth from a similar adjacent formation. Such an aggregate of unit cells, separated by the grain boundary, is called grain or crystal. All metals are made up of many crystals or grains. The strength and hardness of the metal depend upon its grain size. Large grain size is associated with low strength, higher ductility and low hardness (soft). Fine-grained metals are tougher, stronger and harder. The grain size is measured by counting the number of grains in a given area or by counting the number of grains that intersect a length of a line randomly drawn on an enlarged photograph of the grains (taken under microscope on a polished and etched specimen). According to the American Society for Testing and Materials (ASTM), the grain size number, n, is related to the number of grains, N, per sq. inch at a magnification of 100 ¥ (equal to 0.0645 mm2 of actual area) by the formula, N = 2(n–1) Thus smaller is the value of ASTM grain size number (n), lesser will be the number of grains/mm2 (or grains/mm3). For example, a grain size number 3 gives a coarser (bigger) grain than the grain size number 8. A grain size 7 is usually acceptable for steel sheets for sheet metal work. Grain size between 6 and 8 is considered fine grain size and that equals to about 3, as large grain size. The appearance of the external surface of the metal is affected by the grain size. Large grains produce a rough surface appearance on sheet metal or when a metal piece is forged. It may be noted that the grain boundaries have important influence on strength and ductility of metals. Since they interfere with the movements of dislocations, they influence strain-hardening. Grain boundaries are more reactive (because they have higher energy than atoms in the orderly lattice within the grains) than the grains themselves because the atoms along the grain boundaries are packed less efficiently (loosely) and are more disordered. The atomic packing in grain boundaries is imperfect and there exists a transition zone between two adjacent grains, which is not in alignment with any of these grains. Imperfect and lower packing of atoms in the transition zone along grain boundaries helps in atomic diffusion. At elevated temperatures, plastic deformation also takes place by means of grain boundary sliding (as in creep).

2.9.1 Effect of Grain Structure and Grain Size on Properties of Metals The properties of a metal largely depend on the arrangement of grains, their shape and more importantly their size. Metals with fine grained structure are more stronger, tougher and have better mechanical properties, have higher hardness and are easy to machine in comparison to the coarse-grained metals. Larger is the grain boundary, higher is the pile-up of dislocations, creating more hindrance to their movement requiring higher applied stress, to cause plastic deformation. Coarse-grained steels, however, harden better. While strength decreases with increase in grain size, the ductility, however, increases (Fig. 2.5). Effective size of grain is decided by the volume to surface area ratio. Grains get elongated during rolling and forging and effective grain size is reduced because surface area of each grain increases but the volume remains same.

MATERIALS—Structures and Properties

Fig. 2.5

2.10

29

Showing that while the strength of a metal decreases with increase in grain size, the ductility, however, increases.

TYPES OF SPACE LATTICE

There are three important types of space lattices as discussed below: (a) Body-centred cubic (bcc) lattice: There are atoms in each corner of the cube and one atom in the centre of the cube body [Fig. 2.6(a)]. Metals with bcc lattice are alpha iron (a-iron), chromium, tungsten, sodium, potassium, molybdenum, vanadium, niobium and tantalum. (b) Face-centred cubic (fcc) lattice: There are atoms at each corner of the cube and an additional atom at the centre of each face of the cube [Fig. 2.6(b)]. Metals with fcc lattice are gamma iron (g-iron), silver, gold, aluminium, copper, platinum, nickel and lead. Ductile metals in general have fcc lattice.

(b) fcc

(a) bcc

(c) hcp

Fig. 2.6

The three important space lattices showing the arrangement of atoms in a unit cell of a crystalline structure of the metal. In each case, left-hand side figure shows only the positions of atoms in each lattice in a geometrical configuration, whereas in actuality the atoms touch each other as shown in the right-hand side figures. (a) Body-centred cubic (bcc) lattice has nine atoms, eight on corners and one in the centre of the cube, i.e. each atom in bcc structure has eight neighbouring atoms. (b) Face-centred cubic (fcc) lattice has fourteen atoms, eight on corners and one in the centre of each face. (c) Hexagonal close-packed (hcp) lattice has seventeen atoms, twelve on corners, one at the centre of top and bottom face, and three in the centre of alternate vertical faces.

30

MANUFACTURING PROCESSES

(c) Hexagonal close-packed (hcp) lattice: Least ductile materials, such as zinc, beryllium, cadmium, cobalt, magnesium and titanium, have this structure [Fig. 2.6(c)].

2.11

GRAIN REFINING

Grain refining is the process for improving the size and shape of grains of metal. This is achieved by introducing a ‘modifier’ into the molten metal. The modifier helps refining the grains (getting finer grains) through crystallization without affecting the chemical composition of the metal. Aluminium oxide powder is used for grain refinement in steel production. Cast iron grains are refined through the process of ‘innoculation’ wherein innoculants added are ferrosilicon, pure magnesium, iron chromium alloys, etc. Grain refinement is sometimes done through mechanical working of metals like smith forging, die forging, rolling, drawing and other cold working processes involving plastic deformation. The other method of grain refinement is ‘crystallization process’, wherein the metal is heated to the ‘recrystallization temperature’. This leads to the formation of new grains with undistorted space lattice (Fig. 2.7).

Fig. 2.7

Showing the process of recovery, recrystallization and grain growth with corresponding changes in the properties of a metal.

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31

Commonly used metals have polycrystalline structure, i.e. they are composed of many crystals or grains in random orientation. When a polycrystalline metal having uniform equiaxed grains (i.e. with equal dimensions in all directions) is subjected to plastic deformation at room temperature (i.e. subjected to cold working), the grains become deformed and elongated. The deformation may be caused by compressive or tensile loads. During plastic deformation, the grain boundaries remain intact and the mass continuity is maintained. The greater strength demonstrated by the deformed metal is due to entanglement of dislocations with grain boundaries. Also, the increase in strength is higher for metals with finer grains (which have a larger grain-boundary surface area per unit volume of metal). Plastic deformation (cold working) also causes anisotropic behaviour (having properties in vertical direction different from horizontal direction) in the deformed metal. The effects of cold working can be reversed, residual stresses relieved and properties of metal can be brought back to their original level by annealing, i.e. by heating the deformed metal piece to a specific temperature range for a period of time. The following three events happen during such a heating process as shown in Fig. 2.7. 1. Recovery: It occurs at a certain temperature range below the recrystallization temperature of the metal. Residual stress in highly deformed regions are relieved, followed by the beginning of the formation of subgrain boundaries. In this event, there is no appreciable change in hardness and strength of the metal. 2. Recrystallization: It is the process in which at a certain temperature range, new equiaxed and strain-free grains are formed, replacing the old grains. The temperature for recrystalization ranges between approximately 0.3Tm and 0.5Tm, where Tm is the absolute melting point of the metal. Recrystallization results in the decrease of the density of dislocations, lowering of strength and increasing of ductility. Since lead, tin and zinc recrystallize at about room temperature, they do not, therefore, work-harden in cold working operations. Since the recrystallization depends on the degree of prior work hardening or cold working, the more is the cold working, the lower is the temperature required for recrystallization to occur. Recrystallization is a function of time as it involves diffusion (i.e. the movement and exchange of atoms across the grain boundaries). While loss in brittleness and increase in toughness is contributed by the process of recovery, the yield strength and ductility are restored by recrystallization, which can be defined as the nucleation and growth of strain-free grains out of the matrix of cold worked metal. In general, the properties of the recrystallized metal are those of the metal before cold working. This is of commercial importance because a work-hardened alloy (during drawing) may be recrystallized and redrawn further. 3. Grain growth: It is the process in which, when temperature of metal is increased beyond recrystallization, the grains begin to grow and their size may even become larger than the original grains. During the process of grain growth, metal becomes soft, losses strength and hardness but gains ductility.

2.12

IMPERFECTIONS IN LATTICE STRUCTURE OF A CRYSTAL

So far we have discussed ideal metal crystals with no defects. The actual metal crystals contain various defects and imperfections in the crystal structure, presence of which lowers down the strength and other properties of the metal (such as yield strength and fracture strength, electrical

32

MANUFACTURING PROCESSES

conductivity, etc.). The term defect or imperfection is used to describe any deviation from the orderly array of lattice points. When the deviation from the ideal periodic arrangement of lattice is localized to the vicinity of a few atoms, it is called a point defect. But when the defect extends through microscopic reasons of the crystal, it is known as lattice imperfection, may be a line defect or surface defect. Line defects propagate as a two-dimensional net in the crystal. Surface defects (or plane defect) arise from the clustering of line defects into a plane. In general, following discrepancies are found in crystal structures: ● ●

● ●

●

Line defects, called dislocations. Point defects, such as vacancy (which denotes missing atom), interstitial atom (which denotes extra atom in the lattice) and impurity (which may be a foreign atom that has replaced the atom of pure metal) (Fig. 2.8). Voids (which denotes volume or bulk imperfections). Inclusions (which may be non-metallic elements comprising oxides, sulphides, silicates, etc.). Grain boundaries (which are planar imperfections). Vacancy

Interstitial impurity

Fig. 2.8

(a)

Interstitial atom

(b)

Substitutional impurity

llustrating two types of ‘point defects’ in a crystal lattice, (a) Vacancy and (b) Interstitial. Substitutional impurity in a lattice is also shown. Note that a ‘point defect’ in a crystal lattice is totally localized and occurs when an imperfection is restricted only to the close neighbourhood of a lattice point, for example, one lattice atom is missing creating a ‘vacancy’ as shown at (a). In another case, an atom is occupying an abnormal position as shown at (b) causing interstitial defect. Sometimes, impurities (slag inclusions) occupy a lattice site from where a regular atom is missing causing ‘substitutional impurity’. Smaller atoms of slag occupy spaces between relatively larger and hot atoms of the parent crystal causing ‘interstitial impurity’.

The presence of these defects disturbs the uniformity of the metal lattice. Their presence is sometimes blessing also, for example, with presence of these defects it is easier to deform metals into various shapes, machining becomes easier with dislocations present in the metal.

2.13

SOLID-STATE DIFFUSION

Solid-state diffusion comprises entry of some portion of solid metal into another solid metal when both are in contact, for example, copper will diffuse through nickel when both are placed

MATERIALS—Structures and Properties

33

into intimate contact and heated to a high temperature. Vacancies move through the lattice producing random shifts of atoms from one lattice to another. Introduction of carbon atoms into an atomic lattice of iron is another example of solid-state diffusion. Carbon atoms being much smaller than iron atoms, some of them squeeze into the structure of iron atom. a-iron has a bcc lattice at room temperature and can contain only a few carbon atoms in ‘solution’, but at elevated temperature, the structure of iron changes to fcc lattice (g-iron) where there is a considerable increase in the number of carbon atoms that form the solid solution.

2.14

CRYSTAL LATTICE UNDER STRESS

When external loads are applied on metals, they undergo some deformation (changes in shape and size). The deformation may be of temporary nature or of permanent nature: (a) Elastic deformation is temporary and is recoverable or disappears with removal of load. (b) Plastic deformation is permanent or non-recoverable even after load is removed. When the crystal lattice of a metal is subjected to a stress below its elastic limit, the structure of the crystal yields a small amount temporarily and recovers on the removal of load causing the stress. This is elastic deformation. But if sufficiently large loads are applied, plastic deformation takes place, wherein elastic limit is crossed or exceeded. Plastic deformation in crystalline structures can occur in the following two ways: (i) Deformation by slip (ii) Deformation by twinning Some metals deform by slip, some by twinning, and some by both.

2.15

DEFORMATION BY SLIP

Crystals may be considered to be composed of a number of slip blocks (layers) resting over each other. These blocks get displaced in relation to each other under the effect of external force. The blocks of crystals are considered separated by thin layers, which are the most dense atomic planes and are known as slip planes [Fig. 2.9(a)]. Slip takes place along these planes only. Stress

Slip plane (a)

Fig. 2.9(a)

(b)

Stress

(c)

(d)

Showing the process of slip in a single crystal. (a) Crystal is composed of a number of layers or slip blocks resting over each other but separated by thin layers called ‘slip planes’, (b) Under the effect of stress, the slip blocks try to get displaced in relation to each other, (c) Slip takes place along the slip planes. The most close-packed direction within a slip plane is the ‘direction of slip’, and (d) New position of slip blocks after the process of slip is over.

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MANUFACTURING PROCESSES

The direction (within the slip plane), which is most close-packed, is called the slip direction as the atomic layers (or blocks) are displaced along this direction only. Metals having few slip planes and few directions of slip (like hcp) are very difficult to deform through forming processes. When slip planes are readily available, the metal is considered very formable (like fcc lattice). Dislocations are defects in orderly arrangement of atomic structure. Dislocations play most important role in explaining the discrepancy between the actual and theoretical strength of metals, since a slip plane of the crystal structure containing a dislocation requires less shear stress to allow slip than does a plane in a perfect lattice. The movement of an edge dislocation across the crystal lattice under a shear stress is shown in Fig. 2.9(b). 1 2 3 4 5 6 7

Slip plane

1¢ 2¢

3¢ 4¢ 5¢

6¢ 7¢

(a)

Fig. 2.9(b)

2.15.1

(b)

Showing the mechanism of slip by the movement of dislocation: (a) Atom movements near dislocation in slip and (b) Movement of an edge dislocation.

Slip by Dislocation Movement

When a dislocation is present in a crystal lattice, motion of the dislocation through the crystal lattice requires a stress far smaller than the theoretical shear stress. In a perfect lattice, all atoms above and below the slip plane are in minimum energy positions, and when a shear force is applied to the crystals, the same force opposing the movement acts on all atoms. But when there is a dislocation present in the crystal, the atoms that are well away from the dislocation are still in the minimum energy positions, but at the dislocation only a small movement of atoms is required. This will be clear from Fig. 2.9(b), wherein the extra plane of atoms at the edge dislocation is initially at 4 as shown at (a). Under the action of shear stress, a very small movement of the atoms to the right will allow the extra half plane to line up with the half plane 5¢, while at the same time cutting the half plane 5 from its neighbours below the slip plane. In this way, the edge dislocation line moves from its initial position between planes 4¢ and 5¢ to a new position between the planes 5¢ and 6¢. As the atoms around the dislocations are symmetrically placed on opposite sides of the extra half plane, equal and opposite forces oppose and assist the motion. Hence, there is no net force on the dislocation and stress required to move

MATERIALS—Structures and Properties

35

the dislocation is virtually zero. Under the effect of applied stress, the dislocation moves to the right, and when the extra half plane of atoms reaches a free surface [as shown at (b)], it results in a slip step of one atomic distance for the simple cubic lattice. It has been mentioned above that during the deformation by slip, one plane of atoms slips over an adjacent plane (slip plane) under a shear stress (applied shearing force on a unit cross-sectional area being sheared), Fig. 2.9(c). A crystal requires a certain amount of shear stress (critical shear stress) to undergo permanent deformation and thus there must be a shear stress of sufficient value within the crystal for plastic deformation to occur. Presence of dislocations along the slip plane reduces the requirement or value of shear stress needed to cause slip. The ratio of dimensions ‘y’ and ‘x’ in Fig. 2.9(c) has an important role in deformation by slip as the shear stress required to cause slip in a single crystal is directly proportional to the ratio x/y. As x/y reduces, the shear stress required to cause slip decreases. It can, therefore, be said that slip in a crystal takes place along the planes of maximum density or in closely packed planes and also closely packed direction. Slip system is a combination of slip plane and its direction of slip. Ductile metals have slip systems more than 5. In bcc crystals, there are 48 possible slip systems whereas in fcc crystals, the number of slip systems is 12. Accordingly, the bcc crystals should be more ductile than the fcc crystals, but it is really not the case because of the effect of x/y ratio in these two types of crystals. Since the x/y ratio in bcc crystals is relatively higher, the metals with this structure are, therefore, stronger but are ductile only moderately. In fcc structure, the x/y ratio being lower, the metals with this structure have moderate strength but are more ductile as they need much lesser shear force to cause slip in them.

Fig. 2.9(c)

2.16

Showing the plastic deformation of a single crystal under the effect of shear stress. The crystal structure before deformation is shown at (i), whereas after deformation (permanent) is shown at (ii). Here ‘y’ is the spacing of the atomic planes and ‘x’ is inversely proportional to the atomic density in the atomic plane.

DEFORMATION BY TWINNING

Apart from the deformation of metals by the phenomenon of slip, the second important mechanism by which metals deform is the process of twinning. It results when a portion of the crystal takes up an orientation that is related to the orientation of the rest of the untwined lattice in a definite symmetrical way, such that the twinned portion of the crystal is the mirror image

36

MANUFACTURING PROCESSES

of the parent crystal. The plastic deformation leading to twinning action is shown in Fig. 2.10. The off-set produced in deformation by slip is a multiple of the inter-atomic spacing, but in twinning, the off-set produced by sliding of one plane against its neighbour is a fraction-of-unit slip and this causes difference in the orientation between the twinned and untwinned regions in the crystal. This deformation is most common in hcp metals.

Fig. 2.10

2.17

Plastic deformation leading to a twinning action wherein all the atoms in a twinned region move a given amount and change orientation. Twinning is a ‘surface defect’.

WORK-HARDENING (OR STRAIN-HARDENING)

It has been mentioned earlier that the presence of dislocations along the slip planes helps in reducing the shear stress required to cause slip. But this may not be always true as the dislocations may sometimes get entangled and interfere with each other. Also, their movement is impeded by barriers such as grain boundaries, inclusions and impurities present in the metal. Presence of all these factors calls for a shear stress of increased value to cause slip which, in turn, results in an increase in the overall tensile and yield strengths of metal (a phenomenon known as work-hardening or strain-hardening). And the greater is the deformation, the greater will be the number of entanglements of dislocations and hence greater will be the increase in the strength of the metal being strained. The process of deformation by slip is also known as shear deformation and begins at the points of imperfections or dislocations and twinning. As deformation progresses under load, the crystal deforms further and more dislocations are formed along the slip planes. Under these circumstances, strong interactions arise as more dislocations are forced to intersect on various slip planes. All this increases resistance to further deformation. As a result of this, if slip should continue (i.e. movement of dislocations should continue), the shear stress required to

MATERIALS—Structures and Properties

37

move dislocations also increases. Strain-hardening or work-hardening is the increase in the shear stress and metal is said to be work hardened. In this condition, metals are harder, stronger, less ductile and have reduced electrical resistance. When a metallic alloy is plastically deformed, its yield strength increases with an increase in strain. Thus, a controlled amount of cold-working (plastic deformation at room temperature) may be used to increase the strength of the metal.

2.18 COLD-WORKING Only a few metals are easily adaptable to cold-working. Cold-working involves deforming the metal to a plastic stage at room temperature or below recrystallization temperature. The extent to which a metal can be cold-worked depends upon its ductility. Slip occurs in the metal structure during cold-working. It has been discussed earlier that metals are crystalline in nature and are made up of irregularly shaped, different sized grains such that the orientation of atoms in a particular grain is uniform but differs from its adjacent grains. During the process of cold-working, grain structure changes and results in fragmentation of grains, movement of atoms and lattice distortion (Fig. 2.11). During the process, slip planes develop through lattice structure at points where atomic bonds are weakest, resulting into the distortion of the whole block of atoms. Greater loads are required to deform a metal in cold-working as it does not get permanently deformed until the stress exceeds the elastic limit. Since there is no recrystallization of grain during cold-working range, no recovery from grain distortion or fragmentation takes place in cold-working. Strain-hardening consequently results due to atomic dislocation, lattice distortion and fragmentation of grains. Residual stresses are then set up in a cold-worked metal piece.

Fig. 2.11

Cold-working and its effect on grain structure. Cold-working results in elongated, distorted and fragmented grains. Recovery (for releasing internal stresses) and recrystallization (for formation of new grains) are the post-treatments given to the metal for improving its grains after being coldworked.

Advantages

(i) Ultimate strength and yield strength increase. (ii) Hardness increases but ductility decreases.

38

MANUFACTURING PROCESSES

(iii) Surface finish is improved and close tolerances can be maintained. (iv) It is an ideal method for increasing hardness of those metals which cannot be hardened by heat treatment. Disadvantages

(i) Only ductile metals can be cold-worked. (ii) Residual stresses get developed. Over-working of metals results in brittleness and requires annealing. (iii) Grain structure is distorted and fragmented and needs heat treatment. (iv) Only small sized components are easy to cold-work as larger ones need more force.

2.18.1

Metals Most Adaptable to Cold-working

The following metals are easily cold-worked in the form of sheets and other forms: (i) (ii) (iii) (iv) (v) (vi) (vii) (viii)

2.18.2

Mild steels (with low carbon content) Copper Brass Bronze Aluminium bronze Stainless steel Monel metal Duralumin along with several other aluminium alloys

Cold-working Processes

(a) Shearing—blanking, punching, perforating, trimming, slitting. (b) Drawing—wire drawing, tube drawing, embossing, stretch forming. (c) Squeezing—cold rolling, coining, riveting, stamping, cold forging, thread rolling, knurling. (d) Bending—bending of bars, angle bending, roll forming, seaming.

2.19 HOT-WORKING Hot-working involves deforming a hot metal which is heated above crystallization temperature (800°C for steel). Hot-working is mostly carried out above this temperature. Less energy inputs are needed in hot-working than in cold-working. Hot metal easily deforms and also very large amount of deformation can be detained in the hot metal before cracking. Note the effect of hot-working on microstructure of metal (Fig. 2.12) and compare it with cold-working. Advantages

(i) Coarse grains are refined and fine grain structure is obtained. (ii) Porosity in metal is eliminated. (iii) Ductility and impact resistance is improved.

MATERIALS—Structures and Properties

Fig. 2.12

39

Hot-working results in the recrystallization or refinement of coarse grains into a fine grain structure.

(iv) Large deformations (change in shape and size) in the metal are easily and more rapidly obtained. (v) Process is rapid and requires less power. Disadvantages

(i) Tooling and handling of hot metal are difficult and costly. (ii) Close tolerance on dimensions is not possible. (iii) Resulting finish is poor due to oxidation of surface at high temperature.

2.20 ALLOY An alloy is a substance with metallic properties and is made up of two or more elements, with at least one element being a metal. The metal present in larger proportion in the alloy is called base metal, whereas other elements present are called alloying elements. Pure metals are rarely used because they do not possess good physical and mechanical properties for engineering purposes. Hence alloying elements are added to the pure metal to impart it certain desired properties, for example, steel is an alloy of iron and carbon wherein carbon is an alloying element added for giving greater strength, hardness and other properties to resulting steel. Other examples of an alloy are: various cast irons, brasses, bronzes, etc.

2.20.1

Purpose of Alloying

Alloys are formed to serve the following main purposes: (i) To increase strength, toughness and hardness, and also hardenability. (ii) To impart special properties like resistance to heat and high temperature, resistance to corrosion and oxidation.

40

MANUFACTURING PROCESSES

(iii) To lower down the melting point as in case of fusible alloys such as solders used for soldering purposes. Tin melts at 232°C and lead at 322°C, but the solder (alloy of tin and lead) melts at 205°C. (iv) To produce a metal having certain characteristics to help in making it adaptable to various manufacturing purposes. Low carbon steels have good forming ability to get easily formed in bending, wire drawing and spinning. Machinability is promoted by addition of lead and sulphur in low carbon steels.

2.20.2

Phases of an Alloy and Phase Diagrams

A phase is a part of an alloy and is a homogeneous, physically distinct and mechanically separable portion of an alloy and possesses its own distinct composition and physical and chemical properties. Hence if there is a change in the phase, there will be corresponding changes in the properties. Various forms in which a solid alloy can exist are termed different phases, for example, solid solution or intermetallic compound, or a mechanical mixture. Among several phases, one phase is solid solution. This phase is characterized in that one component of alloy retains its crystal lattice while the other component does not, but it gives up its atoms in the lattice of first component. So, a solid solution consisting of several components will have a single type of crystal lattice constituting a single phase. The component, whose crystal lattice is retained, is called solvent and the dissolved component is called solute. When alloying elements are added to pure metals, they alter the dimensions of the lattice structure of the pure metal, and may also even change the type of lattice. Sometimes, the solute atoms take the place of certain solvent atoms in the lattice structure producing substitutional solid solutions. The other times, the solute atoms fit themselves into the interstitial spaces of interstices between solvent atoms producing interstitial solid solutions as hydrogen gets dissolved into austenite forming interstitial solution with iron during welding. It is also possible for the solute atoms to join with the solvent atoms, forming an inter-metallic or mechanical mixture when elements of the alloy are insoluble in each other. The linear dimensions of interstitial space in fcc lattice is about 10% larger than bcc lattice. This is because temperature plays an important role in the formation of interstitial solid solutions. Carbon atoms being much smaller than iron atoms, at room temperature carbon atoms are partly squeezed into the lattice of iron atoms because iron has bcc structure at room temperature, but at higher temperature, iron changes to fcc structure thereby alloying more number of carbon atoms to form solid solution. Important phases present in carbon steels are: ferrite, cementite, austenite, and pearlite. Phases of an alloy are shown by a phase diagram (or constitutional diagram or equilibrium diagram), which is a graph and shows for a particular alloy system, the number of phases that are present at any temperature, with various compositions of alloying elements under equilibrium conditions. Equilibrium means that the state of a system remains constant over an indefinite period of time. The word ‘constitutional’ indicates the relationship among the structure, composition and the physical make up of the alloy. Temperature is plotted as ordinate and weight percentage of alloying element as abscissa.

MATERIALS—Structures and Properties

2.21

41

PHASES OF STEEL

Ferrite

Very limited interstitial solid solution of carbon in alpha iron (a-iron) is called ferrite (Fe). It is a solid solution of up to 0.022% carbon in solvent a-iron (shown by MKI, Fig. 2.13). As the solubility is negligible, the ferrite (designated as a-iron) is more close to pure iron. It is a bcc phase of iron and is very soft and highly ductile and highly magnetic. Even rapid cooling will not render this constituent very hard because carbon contents are very low. Cementite (Fe3C)

Iron and carbon combine together to form cementite which is iron carbide (Fe3C). In the structure of iron-carbon alloys, iron carbide is usually called cementite. Cementite consists of 6.67% carbon and is very hard (up to 1400 HB) and brittle, but has very high compressive strength and is magnetic below 200°C. It increases with increase in carbon percentage and is significantly present in steels and cast irons having carbon more than 0.8%. Austenite

Austenite (also called gamma iron, g-iron) is the interstitial solid solution of carbon in gamma iron (g-iron) with solid solubility of up to 2.11% carbon at 1148°C. Austenite is formed when steel is heated above upper critical temperature (Ac3, Fig. 2.13). Austenite is soft and non-magnetic and when cooled below 723oC, it changes into pearlite and ferrite. Ledeburite

Ledeburite is a eutectic mixture of austenite and cementite and is formed at about 1130oC. It contains 4.3% carbon. Pearlite

Pearlite is a mechanical mixture of ferrite (87%) and cementite (13%) by weight. Pearlite is composed of alternate white (ferrite) and dark (cementite) bands. It is obtained by transformation of austenite (having 0.8% carbon) during slow cooling at a constant temperature Ac1 (Fig. 2.13) and the size of pearlite depends upon the rate of cooling; faster rate of cooling will give fine pearlite. The strength of iron and steel is due to pearlite. Pearlite is easily machinable. Steels with 0.8% carbon carry a fully pearlite structure. Martensite

Martensite is the saturated solid solution of carbon in a-iron and is obtained by rapid quenching of steel from austenitic condition. The hardness of martensite and the rate of cooling required depend on carbon percentage in steels. Martensite is a desirable structure during hardening treatment of steels to make them hard and wear resistant. Troostite

The structure troostite is formed when martensite is reheated (tempered) in case of plain carbon steels between temperature 205 and 395°C. It is softer and less brittle than martensite and is less susceptible to distortion and cracking. Like pearlite, it is also a mixture of radial lamellas of ferrite and cementite.

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Sorbite

In sorbite, cementite is in a granular form and is softer and more ductile in comparison to troostite. This structure is obtained when martensite is tempered (heated) above 450oC. It is very close to pearlite. During tempering of martensite, if cooling rate is faster than that required for pearlite and slower than that required for troostite, sorbite structure will be obtained.

2.21.1

Few More Terms Defined

Eutectic mixture: When a liquid solution of an alloy containing such constituents, which are completely insoluble in each other, is cooled, the crystal of its constituents separates themselves from one another and forms a finely divided mechanical mixture (called eutectic mixture) while retaining the properties of the constituent elements, as they were in the mechanical mixture (alloy) formed by them prior to melting. Eutectoid: Eutectoid means most fusible. It is an alloy of two or more metals in which the proportion of the constituents is such that it has the lowest melting point in comparison to the constituent elements. At a point (carbon 0.8%) where all of the ferrite is in combination with carbon, the structure is pearlite and this combination of iron and carbon is known as eutectoid steel. Hypo-eutectoid steel: These are the steels that contain carbon less than 0.8%. The constituents of these steels are pearlite and ferrite when cooled slowly. Hyper-eutectoid steel: These steels have carbon more than 0.8%. The constituents of these steels on slow cooling are pearlite and cementite. Eutectoid steel:

2.22

They contain carbon exactly 0.8%.

IRON-CARBON EQUILIBRIUM DIAGRAM

The iron-carbon equilibrium diagram is known by two other names also, iron-carbon diagram and iron-carbide diagram. It is a phase diagram that shows a number of phases in iron-carbon alloys (plain carbon steels and cast irons) that are present at any temperature for varying percentage of carbon in steels and cast irons. The diagram is plotted with carbon percentage as abscissa and temperature as ordinate. The diagram is shown in Fig. 2.13. It is explained as follows: Know that: ● a-iron denotes ferrite. ● g-iron denotes austenite. ● Hypo-eutectoid steels contain carbon less than 0.8%. ● Hyper-eutectoid steels contain carbon more than 0.8%. ● Line OB represents pure iron. ● Point D (1536°C)—melting point of pure iron. ● Point P (1550°C)—melting point of cementite.

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43

Fig. 2.13 Iron-carbon equilibrium diagram. ●

●

●

Line DLG—With the fall of temperature of liquid metal along line DLG, austenite crystals separate from liquid melt. Line GP—With the fall of temperature along line GP, cementite separates from liquid. Alloy carrying carbon above 4.3%, solidify along the solidus line GP. Below this line, cementite precipitates from liquid alloy. Solidification of alloy completed along line GF, resulting into a phase, cementite and ledeburite. Line RJEGF is a solidus line, and represents the temperature line along which all iron-carbon alloys will solidify. Steels up to 2% carbon will solidify along RJE and cast iron having carbon more than 2% will solidify along EGF.

44

MANUFACTURING PROCESSES ●

●

●

●

●

● ● ●

Point G corresponds to 4.3% carbon. At this point, austenite and cementite precipitate from liquid alloy and form ledeburite (eutectic alloy). Point G is called eutectic point. Critical point (or line)—These are the temperatures at which structural changes occur in the alloy during heating and cooling. Ac—for heating, e.g. Ac3, Acm Ar—for cooling (not shown in diagram) Point I (912oC) represents a reversible transformation from a to g and g to a-iron. Line IH—or Ac3 along which austenite decomposes resulting into the separation of ferrite from austenite. Excess carbon that results from this decomposition forms cementite. Along the line (IH), critical points for heating are on line Ac3 and for cooling on Ar3 (not shown). Point H (723oC)—It corresponds to 0.83% carbon in solid phase and is called eutectoid point. At this point, ferrite and cementite separate simultaneously to form pearlite. The point H shows minimum temperature for the existence of austenite during cooling. Line HE—All temperatures along this line are designated by Acm where cementite separates from austenite. It is called upper critical temperature line for hypereutectoid steels and shows carbon solubility in austenite with temperature. Line KHN—All tempeatures along this line are designated by Ac1 for heating and by Ar1 for cooling (not shown). This line is designated as lower critical temperature line. This is the temperature line for phase equilibrium of austenite, cementite and ferrite and (pearlite). Point K—Indicates maximum solubility of carbon in a-iron (carbon—0.022%). Line KM—Shows variation in the solubility of carbon in a-iron. Region IKM—Region of formation of ferrite. Solubility of carbon in a-iron at point (K), temperature 723oC is 0.022% and it goes on reducing as temperature cools down, shown by line KM.

Note that: (a) If a steel having less than 0.8% carbon is cooled from austenite region (i.e. above line IH), the change in structure will occur along line IH, i.e. it will get transformed into a two-phase alloy comprising austenite (g-iron) and ferrite (a-iron). (b) If an alloy, having carbon exactly 0.8% is cooled from austenite region (i.e. above line IH), its complete transformation will occur at eutectoid point H, temperature 723oC and its structure will be changed to pearlite. (c) Hypo-eutectoid steels below the line IH (Ac3) will consist of a structure of ferrite plus pearlite. (d) Hyper-eutectoid steels below the line EH (Acm) will consist of a structure of pearlite plus cementite. (e) Cast irons having carbon less than 4.3% will have their structure consisting of pearlite and ledeburite.

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45

(f) Cast irons having carbon exactly 4.3% will have structure only ledeburite. (g) Cast irons having carbon more than 4.3% will have structure consisting of cementite and ledeburite below the line EF. (h) The line AC shows cast iron with carbon contents as 6.7% with purely cementite structure. This shows maximum capacity of carbon to combine with iron forming iron carbide (cementite). Any extra carbon (more than 6.7%) will float on the molten metal. Application of iron-carbon diagram

(i) Figure 2.13 indicates the presence of important phases of steels and cast irons in relation to the given temperatures and the percentage of carbon during heating and cooling of the alloy. (ii) It is of interest to metallurgists or others for studying the micro-structure of various carbon steels and cast irons. (iii) It also finds extensive application in understanding various processes of heat-treatment. (iv) With the help of Time Temperature Transformation (TTT) curves, the iron-carbon diagram can be helpful in controlling heat-treatment processes, aimed at receiving desired structures of steel with control of time and temperature. Limitation of iron-carbon diagram

Figure 2.13 cannot help in indicating the relative shape and size of two or more phases (such as pearlite, cementite) when they are simultaneously present in the structure in a particular temperature region.

2.23

PROPERTIES OF METALS AND ALLOYS

Properties are the characteristics of any material indicating its behaviour and performance under different conditions of working on it or during its use. Properties of metals can be broadly categorized as follows: 1. Physical properties 2. Chemical properties 3. Mechanical properties 4. Fabricating properties

2.23.1

Physical Properties

Physical properties are those properties which remain unchanged when metals are heated, electrical current is passed, the metal is melted or magnetized. Important physical properties include: density, porosity, thermal properties, electrical properties, magnetic properties, fusibility, refractoriness, etc. Some of these are elaborated in the following: Density of a material is its mass per unit volume. When density of a material is expressed in relation to that of water, this quantity is known as specific gravity, which has no units. A significant role played by density is the specific strength (i.e. strength-to-weight ratio) and

46

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specific stiffness (i.e. stiffness-to-weight ratio) of materials and structures. Weight saving is important for automobiles, aircraft, space vehicles and many other equipment where energy consumption and limitation of power and self-weight are important for cost saving and economy in use. Physical properties of important metals are given in Table 2.1. TABLE 2.1

Physical properties of metals

S. No.

Metal

Density (kg/m3)

Melting point (oC)

Specific heat (J/kg K)

Thermal conductivity (W/mK)

1. 2. 3. 4. 5. 6. 7.

Steels Aluminium Aluminium alloys Copper Copper alloys Nickel alloys Titanium alloys

6920–9130 2700 2630–2820 8970 7470–8940 7750–8850 4430–4700

1371–1532 660 476–654 1082 885–1260 1110–1454 1549–1649

448–502 900 880–920 385 377–435 381–544 502–544

15–52 222 121–239 393 29–324 12–63 8–12

Thermal properties include melting point, specific heat, thermal conductivity, heat radiation and coefficient of thermal expansion. These properties determine the behaviour of metals under varying temperature conditions. Thermal expansion of materials is an important consideration for designing a product. Generally, the coefficient of thermal expansion is inversely proportional to the melting point of the material. Thermal conductivity, in conjunction with thermal expansion, plays the significant role in causing thermal stresses, particularly in an assembly subjected to heat. A family of ironnickel alloys (Invars) has been developed with very low thermal expansion coefficients. These are called low-expansion alloys; low thermal characteristics of these alloys are often termed as Invar effect. Invar has typically iron 64% and nickel 36%. Super Invar has iron 64%, nickel 31% and cobalt 5%. Coefficients of thermal expansion of selected metals are given in Table 2.2. TABLE 2.2

Metal or alloy Invar Tungsten Grey cast iron Low carbon steel Medium carbon steel High carbon steel Copper Stainless steel Aluminium

Coefficient of thermal expansion of metals

Coefficient of thermal expansion (cm/°C × 10–6) 2 to 9 4.5 10.2 12.06 12.06 12.06 16.7 16.74 25

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Thermal conductivity is the ability of the metal to conduct heat from its one face to another as also to an adjacent body when in contact. Fusibility refers to the ease or difficulty with which a metal can be melt for shaping it into a product by the process of casting. Refractoriness is the property of a metal because of which it tries to resist deformation or change in shape when subjected to high temperature. Magnetic properties provide information regarding the response of the metal to an applied magnetic field so that its suitability as commercial, magnetic material for use in electrical equipment may be ascertained. Magnetic permeability, hysteresis, etc. are examples of magnetic property. Electrical properties comprise electrical conductivity, resistivity, dielectric strength, thermoelectric effect, etc. These properties determine the ability of a metal to easily allow or resist the flow of electricity through it. Electrical conductivity is the measure of the ease with which electrical current flows through a conductor. Electrical resistivity is the inverse of electrical conductivity. Larger is the resistivity of a metal, greater is the resistance offered by the metal to the flow of electricity. Conductors are materials with high electrical conductivity. Metals are good conductors of electricity. Insulators are materials with high resistivity. These are also called dielectrics. Superconductors are those metals which show the phenomenon of superconductivity. Superconductivity is the phenomenon of almost zero electrical resistivity. It occurs in some metals and alloys below a critical temperature, often near absolute zero (0 K, –273°C). The highest temperature at which superconductivity has been exhibited so far is 150 K (–123°C).

2.23.2

Chemical Properties

Chemical properties provide information regarding the reaction of metals when they come in contact with different environments like air, water, gases, fumes and various other chemical substances. Resistance to corrosion is an important chemical property. Corrosion refers to the deterioration of metals, ceramics and also plastics (where it is called degradation) when subjected to corrosive environment. Besides affecting adversely the surfaces of components and structures, corrosion also reduces their strength and structural integrity. Resistance to corrosion depends on the corrosive media and type of material. Corrosive media may be chemicals (acids, alkalis and salts), the environment (oxygen, polluted air and acid rain) and water (fresh or saltish). Usually the non-ferrous metals, stainless steels and non-metallic materials have higher corrosion resistance. General carbon steels and cast irons have poor corrosion resistance and hence should be protected by coatings (paints) and surface treatments (galvanizing, chrom-plating). Corrosion, when it appears in a localized area on the surface of the component, is called pitting. Corrosion may also be intergranular corrosion (along grain boundaries) and galvanic corrosion caused by the galvanic effect (two dissimilar metals form galvanic cell). Stress corrosion cracking happens in products having residual stresses and subjected to corrosive atmosphere.

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2.23.3

Mechanical Properties

Mechanical properties include: strength, elasticity, ductility, malleability, hardness, brittleness, abrasion resistance, toughness, impact strength, etc. Mechanical properties of a metal characterize its capacity and capability to withstand the action of variety of external loadings. These depend on its crystal structure, alloying elements, cold- and hot-working of the metal, heat-treatment, geometry and shape of the product and working conditions, like working at higher temperatures. Various factors affecting the mechanical properties of metals have been discussed in greater details elsewhere. Some mechanical properties of selected metals are given in Table 2.3. Strength: The resistance offered by the material against the deformation, caused in it by the external force, is known as the strength of material. It is the measure of the ability of a material to withstand external forces. Higher the strength, higher would be the resistance offered by the material to deformation and hence higher would be the amount of load the material can withstand without failure. Within a certain limit (elastic stage), the resistance offered by the material is proportional to the deformation caused by external forces and also this resistance is equal to the external force. Elasticity: It is the capacity of a material to resist deformation in its form, without any permanent change of form and to recover its original form after the removal of the external force responsible for creating the deformations. If after removal of the force the body comes back to its original shape and size (i.e. deformation disappears completely), the body is known as elastic body. Thus, the elasticity can also be said as the property of material by virtue of which it returns back to its original form after the removal of external force. Elasticity is sometimes referred as a type of tensile property of metal due to which it resists permanent deformation under applied force. Plasticity: It is that property of the metal because of which it can undergo permanent deformation without rupture or failure. This property of the material is necessary for forging, in stamping images on coins, and in ornamental works. Permanent deformation occurs only when the metal is stressed beyond the yield point, i.e. to the plastic stage. A material may be said to be perfectly plastic when no strain (deformation) disappears when it is relieved from load. In plastic state, the metal shows the phenomenon of ‘flow’, and this property of ‘flowing’ is utilized in forming various shapes or forms by the application of pressure or heat or both, as plasticity of metals generally increases by heating. The process of forming, shaping, extruding, rolling, etc. is based on the property of plasticity. Ductility: It may be defined as the ability of a material to withstand plastic deformation without rupture. It is the tensile property of metal that permits permanent deformation in it before fracture by tension and allows the metal to be drawn into smaller sections, like a wire drawn through a die. It is expressed in terms of percent elongation and percent reduction of area. Ductility is thought of in terms of bendability and drawability also. During ductile extension, a metal generally shows a certain degree of elasticity, together with a considerable amount of plasticity. Flowability, formability and workability are the terms sometimes used in place of ductility as they also indicate the ability of metal to undergo specific

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mechanical processes without rupture. Examples of common ductile metals include: copper, silver, steel, tin, etc. Malleability: It is the compressive characteristic of a metal that allows it to be easily rolled or hammered into plates or thin sheets without cracking through cold- or hot-working of the metal. Malleability is a property very similar to ductility but all malleable metals need not to be ductile also, for example, lead is malleable and can be rolled or hammered into sheets but cannot be drawn into wires. Metals in decreasing order of their malleability are: lead, soft steel, wrought iron, copper and aluminium. Brittleness: It is lack of ductility (rather opposite of ductility) and is the property of the metal to fracture without noticeable plastic deformation, i.e. with little or no ductility. Though brittle metals show lack of plasticity, they may be sufficiently strong, for example, cast iron which is brittle but fairly strong (i.e. has very good compressive strength). Glass and chinawares are other examples of a brittle material. Hardness: It is the property that is related to the wear resistance of a metal or its ability to abrade or indent another material. It also refers to the ability of cutting other softer material. The property of hardness is closely related to the metal strength (both tensile and compressive). Given the proper heat treatment, the hardness of steel will be determined largely by the carbon contents. However, the depth of hardness, known as hardenability, will be determined largely by the alloying elements. Abrasion resistance: It is the measure of the ability of a metal to withstand surface abrasions and wears. Generally, harder the metal, higher will be its resistance to abrasion and wear. But exceptions are there, for example, manganese steels may not be very hard but they have great resistance to abrasion and wear. Toughness: It is that property of a metal because of which a metal withstands bending or torsion and can absorb considerable amount of energy before fracture. Toughness is associated with impact strength of metal. Glass piece, which is brittle, will break immediately under a sudden impact load, whereas steel piece (tougher than glass) will absorb substantial amount of impact energy before failing. Toughness, thus, enables a metal to withstand both elastic and plastic deformation before failure. Impact strength: Impact strength or impact resistance of a metal is its capability to stand against shock or impact loading and vibrations. It involves consideration of both toughness and strength of a metal and is measured in terms of the resistance of the metal to fracture under impact loading. Stiffness: It is that property of the metal which makes it capable of resisting deflection or elastic deformation under the applied load. Therefore, the components which are required to deflect minimum during use should have higher stiffness. The measure of degree of stiffness of a metal is indicated by the value of its Young’s Modulus (or Modulus of Elasticity) for tensile and compressive loadings and by its Modulus of Rigidity for shear loadings. Rigidity: It is the term sometimes used in reference to the resistance offered by a metal or structure to vibrational deflections under dynamic loading.

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Strain energy: Whenever a body is strained, the energy is absorbed in the body. This stored energy, which is due to the straining effect, is known as strain energy and is equal to the work done by the applied load in stretching (or straining) the body. Resilience: It is defined as the total strain energy (or work done) stored in a strained body. Whenever the straining force is removed from the strained body, the body is capable of doing work. Proof resilience: It is the maximum amount of strain energy that can be stored in a body. The strain energy stored will be maximum when the body is stressed up to the elastic limit of its material. Fatigue strength: In practice, only a few machine components or structures may be subjected to static loading wherein load of a fixed amount acts continuously on the component, thus generating in it the stress of a fixed value. But when a machine component is subjected to variable or altering loads applied in a large number of repeated cycles, as a result of this, variable stresses are developed in the machine component. Under these circumstances, the component fails at a stress level much lower than the one attained for its failure under static constant loads. Failure of material under such type of circumstances is known as fatigue failure, and the stress at which material fails due to fatigue is known as its fatigue strength. Fatigue failure may occur even without any prior indication, but it is usually caused by means of progressive crack formation of very fine or microscopic size on the body of the component. The crack grows and propagates with every cycle of loading, which finally leads to a brittle fracture of the component. Fatigue is affected by chemical composition and microstructure of the metal, as also by the size, shape, surface finish, presence of stress raiser (notches, sudden change in size) in the body of the component. Fatigue properties of a component directly affecting its fatigue strength include endurance limit, nature of loading, surface finish, notch sensitivity, size factor and temperature. Surface treatment processes used to improve fatigue strength include: (a) (b) (c) (d) (e)

Cold-working Shot peening or hammering Surface hardening processes like carburizing and nitriding Surface plating with zinc, chromium and cadmium Adding residual compressive stresses to the metal surface (as by burnishing)

Fatigue limit or endurance limit: Fatigue limit or endurance limit is a well-defined value of stress in a metal and is the completely reversed bending stress below which the metal will not fail due to fatigue, even if application of variable or alternating loads is repeated on the metal for infinite number of times, usually up to 108 cycles. The endurance limit of a metal is always much lower than its normal yield stress. It depends on ultimate tensile strength and hardness of the metal. Creep: Creep is a plastic deformation, permanent in nature, and is that property of metal due to which it gets progressively deformed under a constant stress level and at a slow rate

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with the passage of time. Creep rate increases with the rise in temperature and with a higher stress level. Creep failure is like brittle failure without any necking. There is no correlation of creep property and mechanical properties of a metal at room temperature. If the creep deformation is allowed to continue, it may result in failure of the metal. Components of steam and gas turbines, power plants, furnaces, etc. often suffer from creep. At elevated temperatures, steels with coarse grain structures are more creep resistant than those with fine grains. Creep rates in steel are reduced (i.e. creep resistance improved) by adding alloying elements like nickel, cobalt, chromium, molybdenum, vanadium, manganese, tungsten and silicon. Residual stresses: Deformation in a component may be caused by subjecting the component to external loading (or cold-working). Some manufacturing processes such as welding and casting also cause deformation of the component during its cooling to room temperature. The deformation caused in a component may be uniform throughout the component or non-uniform also. Whenever a component is subjected to deformation that is not uniform throughout the component, residual stresses are developed. Residual stresses are the stresses that remain within a component after it has been deformed and all external deforming loads on the component removed. Because of non-uniform deformation during metal working processes (cold working, bending or drawing), most components develop residual stresses. When no external force is applied, the internal forces resulting from these residual stresses remain in the state of equilibrium. But whenever equilibrium is disturbed by machining or loading the outer layers of the component, the component will try to acquire a new shape (radius of curvature by warping) in order to balance the internal forces. Tensile residual stresses on the surface of the component are undesirable as they lower the fatigue life and fracture strength of the component. On the other hand, compressive residual stresses on the surface of the component are desirable to improve fatigue life, and these are introduced by shot peening or burnishing by rollers. Factors governing the mechanical properties of metals

A number of factors affecting the mechanical properties of metals are discussed here. (i) Crystal structure of metal: When loads are applied on a metal piece, deformation takes place due to slipping of atomic structure of the metal along its slip planes. The formability of the metal depends on the available number and directions of the slip. With few slip planes and direction of slips available, the metal is found difficult to form. And that is why metals with face-centred cubic (fcc) crystal lattice like copper, silver, etc. are easy to form. Further, metals with larger crystals (grain) have lower strength and are more deformable, whereas fine-grained steels have better strength. Imperfections and dislocations in crystal planes and inclusion of impurities (like slag) present in crystals cause compositional defects rendering the metal weak in strength. (ii) Alloying elements: Alloying elements in a metal have great effect on its mechanical properties. Common alloying elements used in steel include: carbon, nickel, chromium, manganese, tungsten, vanadium, etc. The inclusion of carbon helps in increasing tensile strength, hardness, impact strength; nickel increases toughness; chromium increases strength to suit high temperature applications.

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(iii) Working temperature: The working temperature affects the properties of metals as given below in case of wrought iron and steel. (a) The ultimate tensile strength in general (i) falls (slightly by 5%) with increase in temperature between the range of 90 and 130°C, but (ii) it rises to a value (10 to 15% higher than its value at 90°C) in a temperature range between 200 and 300°C, and (iii) it falls continuously with increase in temperature (see Fig. 4.1). (b) The elastic limit falls continuously with increase in temperature. (c) The elongation (i) falls with increase in temperature above normal to a value of about 130oC and then (ii) increases continuously with further rise in temperature. (d) The modulus of elasticity (E) decreases steadily with increase in temperature. (e) The toughness reduces and embrittlement (loss of toughness) increases beyond temperature of 450oC. (f) Creep increases with rise in temperature. (iv) Effect of heat treatment: Heat treatment involves heating and cooling of metals in specific ways to obtain certain desired properties. Heat treatment (a) relieves internal stresses in a metal that got developed in the course of passing through various manufacturing operations, (b) refines grains and their size ensuring improved mechanical properties, (c) helps altering the microstructure of metals, and (d) changes the surface chemistry of the product by adding or deleting elements such as carbon, thus increasing surface hardness of the product. (v) Cold- and hot-working: The process of cold-working is carried out at room temperature. Its example include: bending, drawing wires, extruding, punching, shearing (blanking), etc. Cold-working increases tensile strength, hardness and yield strength but decreases ductility, results in distortion and fragmentation of grain, arresting residual stresses which are detrimental to the strength of metals. Cold-working resulting into ‘strain hardening’ greatly modifies the yield point and ductility of the metal. This advantage is taken in cold-deformed mild steel bars used as reinforcement in concrete works. In a hot-working process, the heated metal undergoes plastic deformation while temperature is above the recrystallization temperature, say about 800oC in case of carbon steels. Examples of hot-working include: hammer forging, press forging, rolling and extruding. Many advantages claimed with hot-working include: coarse grains refined to fine grains with increase in strength, porosity eliminated, ductibility and resistance to impact improved. (vi) Geometry of product: If abrupt (sudden) changes of sections are brought in a product, the stress in the metal becomes unevenly distributed. That is why sharp corners and combination of sections of most uneven thickness, presence of holes, v-notch, and high roughness of the surface result in reducing the strength of the metal under load. An abrupt change of section thus lowers the value of the ultimate breaking load mostly in brittle or non-extendable metals such as cast iron or hard steel. (vii) Rate and type of loading: If the load on a metal piece is applied very slowly and not continuously but with pauses during which the metal has opportunity to strain-harden, smaller average strains (deformation) are observed on the metal piece

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than if load is applied more quickly but continuously. Similarly, the type of loading, for example, variable loading repeated number of times, greatly affects the fatigue strength of the metal.

2.23.4

Fabricating Properties

Fabricating properties of a metal refer to the ability or ease with which it can be formed, cast, machined or welded. These include: machinability, formability, weldability and castability. These provide information on the response or adaptability of metals to specific methods of fabrication. Various fabricating properties of a metal depend on different set of factors as discussed in the following. Machinability

Machinability refers to the ease with which the metal may be sheared in operations such as turning, drilling, reaming, sawing, threading, facing, etc. Ease of metal removal implies: (i) (ii) (iii) (iv)

that that that that

the forces acting against the cutting tool will be relatively low, the chips will be broken easily, a good finish will result, and the tool life will increase reducing its frequent resharpening or replacement.

Ease of machining is affected by properties of metal like hardness, tensile strength, chemical composition, microstructure, degree of cold work and strain hardening. Machine variables like cutting speed, feed, depth of cut, tool material and its form, cutting fluid, etc. also affect machinability. Formability

Formability refers to the ease with which a metal could be formed or wire drawn or bent. This property is indicative of the response and suitability of the metal for plastic deformation processes. The ability of metal to be forming is based on the ductility of the metal, which, in turn, is based on its crystal structure. Metals exhibit different responses when they are deformed under different conditions, for example, some metals show very good formability at high temperature, whereas when the same metals are deformed at room temperature, they exhibit poor response to the forming process. Some metals are deformed easily at slow speed but with higher speed of deformation, they break like brittle metals. When a metal is subjected to a stress below its elastic limit, the crystal structure of the metal will temporarily yield a small amount but will recover when load is released. But when sufficiently large load is applied, plastic deformation takes place in which the atomic structure of the metal has to slip. The slip takes place along certain crystal planes (called slip planes). It is with this reason that when slip planes are readily available in a crystal structure of a metal, the metal is considered very formable, but with few slip planes and directions of slip available, the metal is difficult to form. As already explained, metals with face-centred cubic (fcc) crystal lattice have greatest opportunity for slip and they are most ductile. Metals with fcc crystal lattice are: copper, silver, gold, aluminium and nickel. Other factors governing flowability (ductility or

54

MANUFACTURING PROCESSES

formability) of a metal are: grain size, alloying elements, hot- and cold-working, and softening heat treatment processes such as annealing and normalizing. The grain size or crystal size (in microstructure) in a metal has a great effect on formability. As stated above, deformation takes place along slip planes; the tendency to slip in any grain is obstructed by the resistance of opposing slip planes of adjacent grains. It is because of this that metals with large grains are better for heavy drawing, whereas those with small grains are recommended for shallow drawing. Hot- and cold-working tend to reduce the size of crystals. Grains are distorted in these processes and the amount of distortion becomes a determining factor in the ductility of metal. Generally, cold-worked crystals are more distorted than hot-worked and therefore cold-worked metals are usually less ductile than hot-worked. Alloying elements in pure metal reduce its ductility. Not only are the slip planes reduced with presence of alloys, but also in iron, carbide (formed due to alloying in steels) offers increased resistance to slip. Heat treatment processes may help in restoring most of the ductility when cold-worked metals are heated. Weldability

Although it may be possible to weld all metals by one welding process or the other, the real criteria for judging the weldability of metals are weld quality and ease with which the metal can be welded. The factors considered for deciding weldability are: (a) (b) (c) (d) (e)

Heat in welding Oxidation consideration Cooling rate of weldment (weld joint) Entrapment of gases Other considerations

(a) Heat in welding: Large heat and higher temperatures are involved in welding. The effect of heat in determining the weldability is related to the change in microstructure that results in a weldment (welded joint), for example, steels are sometimes considered weldable or non-weldable on the basis of the resulting hardness of weld. During welding, the metal deposited (by melting welding rod) may pick up carbon or other alloys besides impurities from the job metal or electrode, thus making the weld hard and brittle resulting into cracking upon cooling. On the other hand, the opposite may also occur, for example, tempered steels may lose their hardness after welding. (b) Oxidation consideration: Oxidation of the base metal (job metal) takes place because of very high temperature involved in welding. The metals that oxidize rapidly interfere with welding process, for example, aluminium melts at about 700oC but its oxide melts at 3000 to 4000°C; thus the oxide retards the flow of the molten metal in filling a weld joint. (c) Cooling rate of weldment: In metals with poor thermal conductivity, the rise of temperature in heat-affected zone (called HAZ, the area adjacent to weld) may cause development of entrapped residual stresses, which, on cooling, may result in poor

MATERIALS—Structures and Properties

55

strength and cause sudden unexpected failures under loading of the welded products. On the other hand, in metals with high thermal conductivity (as aluminium), heat loss is fast, giving reduced rate of welding. Hot shortness, a characteristic indicated by lack of strength of the job material at high temperature, may result in weld failure during cooling of certain metals. (d) Entrapment of gases: Gases may get entrapped in the weld deposits as these are produced due to burning of certain elements in the flux coatings of electrode. There may be already entrapped gases in the parent metal or these may be drawn from the atmosphere during welding. Entrapment of gases results in porosity, brittleness and reduced strength of the weld. In case of hydrogen embrittle phenomenon, hydrogen gets entrapped in the weld. This may sometimes result in catastrophic failure of weldment at much lower loads than those for which the weldment was designed to withstand. Alloy steels are best welded using low-hydrogen electrodes to reduce brittleness of the weld. (e) Other considerations: Other considerations regarding ascertaining the weldability include the need for pre-heating and post-heating the workpiece after welding as also the heat treatment of the welded piece. For example, low-alloy steels having carbon contents above 0.3% and total alloy contents above 3% are very difficult to weld, but these can be welded easily resorting to pre-heating and post-heating. Pre-heating and post-heating help reduce hidden stresses and development of cracks in the weld. Heat treatment of some metal pieces after they are welded ensures uniform structure both in the parent metal and in the weld. Castability

Casting of metals into different shapes and sizes by pouring molten metal into molds is a very common method of producing articles, particularly intricate shapes. Castability of a metal refers to the ease with which metal could be cast giving sound- and defect-free castings. Since the process of casting involves treatment of metals at molten state and their solidification in the molds, the castability of a metal is judged, to a large extent, based on the following factors: (i) (ii) (iii) (iv) (v)

Solidification rate Shrinkage Segregation Gas porosity Low hot strength

(i) Solidification rate: The ease at which a molten metal will continue to flow (to fill different hollow sections of the mold), after it has been poured in the mold, depends on its chemical composition and the pouring temperature, for example, grey cast iron, being very fluid, can be used for casting intricate shapes. The presence of phosphorus imparts greater fluidity to molten metal while too much sulphur reduces fluidity. The other important factor could be rate of solidification of the metal inside the mold, which helps in giving a particular crystal structure to the resulting casting, for example, grey cast iron is obtained by slow cooling of molten pig iron while the white cast iron is obtained by quick cooling of the same molten pig iron.

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MANUFACTURING PROCESSES

(ii) Shrinkage: Shrinkage is reduction in the volume of a metal when it passes from the molten to solid state, for example, steel shrinks during solidification where amount of contraction could be about 7% by volume, whereas grey iron contracts half as much. This fact has to be taken care of by the pattern maker to ensure castings of the right size. Alloying elements to be added to molten metal are available to effect control on shrinkage. (iii) Segregation: When metals start to solidify, tiny crystal structures, resembling pine trees, start to form at the mold edges and tend to exclude the alloying elements present in the molten metal. As a result of this, subsequent crystals that form are progressively richer in alloy elements as the metal solidifies. This makes the surface of the casting of different quality from the central position of the casting. This defect is overcome by very slow cooling of the casting or by subsequent heat treatment. (iv) Gas porosity: Some metals at molten state have affinity to combine with gases such as oxygen, nitrogen and others. This results in the formation of oxides and nitrides which present themselves as inclusions in the solidified castings. Some gases that get entrapped in the metal during solidification produce defects like void and honeycombing resulting into poor finish of castings and reduced strength. (v) Low hot strength: Consideration of hot strength of melts to be cast in a mold is important for designing the mold. Non-ferrous metals (aluminium, brass, etc.) have very low strength immediately after solidification (i.e. poor hot strength). In view of this, sharp corners (and bends) and abrupt changes in the thickness of the section to be cast should be avoided in the mold design to reduce possibility of development of hot tear and other flaws in the casting.

2.24 2.24.1

STRESSES AND STRAINS Stress–Strain Diagram (or s–e Diagram)

Prior to designing a component of a machine or structure, it is required to have thorough knowledge of the type and amount of loads and various working requirements the component has to meet during service. A material with proper mechanical and other properties matching to the expected service requirements is then selected to make a component or product out of it. This calls for having good knowledge of important properties of commonly available materials. One most popular method of knowing quite a large number of mechanical properties of a metal is the standard tensile test. It helps in determining different tensile properties, namely, ultimate tensile strength, yield strength, elastic limit, breaking strength, percentage elongation and modulus of elasticity and modulus of rigidity, etc. The tensile test consists of gradually loading a standard specimen (Fig. 2.14) of metal and noting down the values of loads applied and the corresponding strains produced in the metal until the specimen fractures. The test is usually carried out on a universal testing machine (Fig. 2.15). The distance between two reference points marked on the specimen is called gauge length and is taken as the original length for making various calculations of strain. Different values of stress are calculated by dividing the load applied by the

MATERIALS—Structures and Properties

Reduction of area

57

Elongation

Gauge length

(a)

(b)

Fig. 2.14 A standard tensile specimen showing ‘gauge length’ at (a) and elongation and reduction in crosssectional area of the specimen as a result of tensile loading at (b).

Fig. 2.15

Essentials of a universal testing machine (UTM) used for tensile, compressive and bending tests.

cross-sectional area of the specimen. The values of longitudinal strains are obtained by dividing the elongation produced by the gauge length. A graph (known as stress–strain diagram or curve) is later plotted between different values of stress (sometimes denoted by s) and the corresponding strain (denoted by e).

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MANUFACTURING PROCESSES

A stress–strain diagram for a mild steel test specimen under the tensile test is shown in Fig. 2.16. Various properties of the metal are described below with reference to this figure.

Fig. 2.16 Engineering stress–strain diagram for a ductile metal (mild steel). Note that the soft and ductile metals have a smaller angle f, i.e. larger strain for the same stress value.

Limit of proportionality (or proportional limit)

The point (O) on the stress–strain diagram shows the position when load (stress) is zero and so also the strain. From this onwards, the value of strain goes on increasing as stress is increased, until a point (A) is reached on the stress–strain curve. It will be seen that the line joining the point (O) and the point (A) is a straight line, which means that up to the point (A), strain produced in the metal is proportional to the stress and hence the metal obeys the Hooke’s law up to the point (A). The stress value at point (A) is known as the limit of proportionality or proportional limit. The limit of proportionality is defined as that limiting (maximum) value of stress up to which strain produced in a metal is proportional to stress, thus obeying Hooke’s law. It will be further seen that the stress–strain curve beyond point (A) is no longer a straight line. It is slightly curved. Therefore, the limit of proportionality is also defined as that stress beyond which the stress–strain curve begins to deviate from a straight line. Elastic limit

A material is perfectly elastic if deformation (strain) entirely disappears on removal of load. For every material, there is a limiting value of load (for a given resisting section or area) up to and within which the resulting strain entirely disappears when load is removed. The value of stress corresponding to the limiting load up to and within which metal shows elastic behaviour is known as the elastic limit of the metal. When the test piece is loaded beyond the limit of proportionality, i.e. point (A) (Fig. 2.16), you will meet on the stress–strain curve another point (B) which corresponds to the maximum value of stress up to which the metal shows its elastic behaviour. Therefore, the point (B) indicates the elastic limit of the metal under test. Sometimes, the elastic limit is defined as the

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59

maximum stress developed in the metal without any permanent deformation. It will be further noted that the line joining the point (A) and point (B) is not a straight line and is slightly curved, which shows that stress is not proportional to stress beyond point (A) although metal shows elastic behaviour up to the point (B), the elastic limit. Loading beyond the elastic limit damages the microstructure of the metal as there is a residual strain left in the metal on removal of load (or stress). Such residual strain is called permanent set and is a plastic deformation. Thus, loading beyond the elastic limit takes the metal into the plastic stage (permanent deformation). The elastic limit in ductile metals is little higher than the limit of proportionality. However, in some metals both elastic limit and limit of proportionality are just the same. The region of the stress–strain curve from zero stress value to the elastic limit is known as elastic region. Yield point

When the test piece is stressed beyond the elastic limit, i.e. point (B), the applied stresses cause plastic deformation, which is permanent in nature. Therefore, once the metal is stretched beyond the elastic limit, it will never come back to its original shape and size. It will be further noticed that beyond the elastic limit, the increase in strain is far more larger and rapid than the corresponding increase in stress. Also, the increase in strain does not bear the same direct proportionality with the corresponding stress. This continues till the yield point (C) is reached whereupon the strain increases even without any further increase in the stress. At the yield point (C), the metal is found to stretch suddenly. The stress at which this sudden stretch occurs is termed as yield point of the metal. In case of ductile metals, two distinct yield points are observed. There is ‘upper yield point’ (C) and ‘lower yield point’ (D). The upper yield point (C) corresponds to maximum stress preceding the extensive strain, and the lower yield point (D) refers to the stress following the sudden strain due to yielding. The stress corresponding to the lower yield point (D) is about 23% less than the stress at the upper yield point (C) in ductile metals. Ultimate stress

As mentioned above, the metal at the yield point (C) stretches suddenly. By this time, the metal has already suffered some permanent deformation. Further, from point (C) to point (D), it undergoes creep without any further increase in stress. The cross-sectional area of the specimen reduces and the length increases. Because of creep (or plastic deformation), the specimen undergoes strain or work-hardening due to slipping of the crystal planes of the metal (which means increase in load bearing capacity). This fact is clear from the stage between the point (D) and point (E) that the specimen becomes stronger to take large loads (stress) with, of course, corresponding increase in strain (elongation) and gradual reduction in the cross-section of the specimen. A stage (point (E)) is later arrived in tensile testing where the decrease in cross-section with increased strain attains a predominant position compared to the corresponding increase in load bearing capacity (i.e. stress). The work done during stretching the specimen is transformed largely into heat and the specimen becomes hot. At point (E), the stress, which attains its maximum value (i.e. largest load bearing capacity), is known as ultimate stress and is defined as the largest stress obtained by dividing the largest value of load reached in the test to the

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MANUFACTURING PROCESSES

original cross-sectional area of the test piece. It is also sometimes called ultimate tensile stress, which corresponds to the ultimate tensile strength of the metal. Breaking stress

At point (E), strain continues to increase slowly without any increase in the load (or stress), i.e. the phenomenon of creep takes place. The cross-sectional area of the specimen goes on reducing, resulting into the formation of a neck. Entire further deformation (extension in length of the specimen) takes place only in the neck area. Further extension (or deformation) of the metal is accompanied by reduction in stress, and the test specimen finally fractures at point (F), (in the neck) where the cross-sectional area becomes too small to sustain the stress. The stress that corresponds to point (F) or fracture point is known as breaking stress. The strength at this point is called fracture strength or breaking strength. It may be mentioned that necking before fracture takes place only in ductile metals. The stress–strain curve is terminated (i.e. specimen fails) before any necking develops in case of brittle materials.

2.24.2

Engineering Stress and True Stress

Engineering stress (or nominal stress) is calculated on the basis of the original area of the specimen. Engineering stress =

Load Original area

The maximum engineering stress in a tensile test is called tensile strength (or ultimate tensile strength, UTS) of the metal. Values of UTS, yield strength (Y) and modulus of elasticity (E) for some metals are given in Table 2.3. TABLE 2.3

Mechanical properties of metals

Metals

E (GPa)

Y (MPa)

UTS (MPa)

Steels Aluminium Copper and its alloys Molybdenum and its alloys Nickel and its alloys Tungsten and its alloys

190–200 69–79 105–150 330–360 180–214 350–400

205–1725 35–550 76–1100 80–2070 105–1200 550–690

415–1750 90–600 140–1310 90–2340 345–1450 620–760

Note:

The lower values of E, Y and UTS are for pure metals.

True stress is calculated on the basis of actual or instantaneous area of the specimen taking load. Instantaneous load True stress = Instantaneous area True stress, s =

L Ai

MATERIALS—Structures and Properties

True strain, e =

lf

Úl

o

61

lf dl = ln l lo

where s = True stress L = Load Ai = Instantaneous area e = True strain lf = Final length lo = Original length ln = Natural log

2.24.3

Computing Various Tensile Properties from Tensile Test Results

Based on information from the tensile test, various tensile properties of the metal that can be calculated are: elastic limit, yield strength, ultimate tensile strength, Young’s modulus of elasticity, percentage elongation, percentage reduction in area, breaking strength. These are calculated as follows with referene to Fig. 2.16. Maximum load at point (B) Original area of the specimen

Elastic limit =

Modulus (or Young’s modulus) of elasticity or coefficient of elasticity, Stress at a given point on line OA Strain at that point

E=

Load at yield point (C) Original area of the specimen

Yield strength =

Ultimate tensile strength (or tenacity) =

Ultimate load corresponding to point (E) Original area of the specimen Breaking load corresponding to point (F) Original area of the specimen

Breaking strength =

Final gauge length - Original gauge length × 100 Original gauge length

Percentage elongation = Percentage reduction in area =

Original area of the specimen - Final area at breaking point Original area of the specimen

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Stress–strain curves for various ferrous and non-ferrous metals are shown in Fig. 2.17.

Fig. 2.17

2.24.4

Stress–strain curves for tensile loading of (a) ferrous metals and (b) non-ferrous metals.

Compressive Strength

Cast

Intensity of compressive stress

M

iron

ild

ste

el

Compressive strength of a material is found by testing the specimen on a universal testing machine under compressive loading. The total strain (%) of a specimen just before rupture is found by direct measurement on the universal testing machine. A typical stress–strain diagram for compression is shown in Fig. 2.18 for several metals. Compressive strength is a measure

Wrought iron

Co

pp

Alu

er

min

ium

Compressive strain

Fig. 2.18

Stress–strain curves for compressive loading of a few common metals.

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63

of the extent a metal deforms under compression prior to rupture. Metals have generally the same elastic limit and modulus of elasticity (E) in direct compression as in tension, and the tension test being much easier to make than a satisfactory compression test, it is quite usual to rely on tension tests as an index of mechanical properties for nearly all metals.

2.24.5

Bending Stress

In bending, the outside fibres of the component (or beam) are placed in tension and the inner fibres in compression (Fig. 2.19), and stress goes to zero at neutral axis. Deflection of the beam depends on loading, geometry of the section, and modulus of elasticity of the metal. Bending stress is found as below: Bending stress =

M◊y I

where M—Bending moment y—Distance from neutural axis to the outer edge of the beam I—Area moment of inertia

Fig. 2.19 Transverse loading (bending) of a beam. Both tensile and compressive stresses have higher values at the outer fibres of the beam.

2.24.6

Shear Strength

The ultimate shear strength of a material is given by the maximum load it can withstand without rupture when subjected to shearing action. For determining this, a small test piece of sheet metal of known thickness is clamped over a die, and a punch is brought down on the test piece with a gradually increasing load until the material is completely punched through. Shear strength of mild steel varies from 60 to 80% of the tensile strength and is found as follows: Shear stress (or strength) =

Load

p ◊d ◊t

where d—Punch diameter t—Thickness of the test piece

2.24.7

Working Stress (or Design Stress)

While designing machine components (that are required to undertake various types of loading), it is desirable to keep the stress (allowed to be developed in the component) lower than the

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maximum or ultimate stress at which failure of the metal takes place. This stress is known as working stress or design stress.

2.24.8

Factor of Safety

Factor of safety is defined as the ratio of maximum stress to working stress. This provides safety margin (to avoid failure) by keeping working stress much lower than maximum or ultimate stress of the metal. In case of ductile metals like mild steel, where yield point is well defined, factor of safety is based on the yield point stress of the metal. Factor of safety =

Yield point stress Working or design stress

For brittle metals like cast iron, the yield point is not well defined and, therefore, factor of safety is based on the ultimate stress. Factor of safety =

Ultimate stress Working or design stress

The values of factor of safety largely depend on material and type of loading and may vary from 4 to 20.

2.25

INSPECTION, TESTING AND QUALITY CONTROL OF METALS

Inspection is concerned with how well the physical and other specifications of the metal (raw material) are being met. Inspection involves detection of defects in raw material so that the end product made out of it will function satisfactorily during use. Inspection also includes testing of materials for checking their physical, mechanical and fabricating properties. Mechanical testing includes: (a) Destructive tests like tensile and compressive test, hardness test, impact test, fatigue test and creep test wherein test piece is damaged in the test. (b) Non-destructive tests are limited to visual inspection and checking, radiographic test, ultrasonic test, magnetic particle test, etc. wherein test piece is not damaged during test. Test procedures are standardized and agencies involved in testing and certification are the Bureau of Indian Standards (BIS), the National Physical Laboratory (NPL) and the National Test House (NTH). Various tests carried out for inspection and ascertaining the qualities of the material are briefly described in the following. Quality control is concerned with the prevention of bad product. To achieve desired product standards of quality, the quality control plans and programmes are carried out right from the stage of production of raw materials like steel, brass, copper, etc. and later during their fabrication into some useful form or product. Statistical quality control methods are used for mass production.

MATERIALS—Structures and Properties

2.25.1

65

Tensile, Compression and Bending Tests

The tensile, compression and bending tests are carried out on a universal testing machine with a capacity of 200 tonnes or more. Tensile testing is more common than compression testing, which is done for a limited number of materials such as concrete, brick, ceramics, etc. A pulling load is applied for tensile specimen and a compressive load for compression test piece. Machines give continuous record of loads, deformation, etc. to help subsequent analysis and calculation of important quantities relating to tensile and compressive strength. Test procedures and specifications for test piece are kept as per relevant standards.

2.25.2

Hardness Testing

In selecting materials to withstand wear or erosion, properties often considered are hardness and toughness. Hardness enables the material to resist penetration and scratching. Hardness of a metal can be tested by several methods such as Brinell, Rockwell, Vickers and Shore scleroscope. Hardness gives a general indication of the strength of the material and its resistance to scratching and wear. A relationship has been established between ultimate tensile strength (UTS) and the Brinell Hardness (HB; formerly BHN) for steels as follows: UTS (MPa) = 3.5 (HB) where HB is in kg/mm2 as measured for a load of 3000 kg. Hot hardness is the hardness of materials at elevated temperatures and is an important factor considered for metal cutting tools, dies used for hot-working operation (forging, etc.), and casting operations. Hardenability is defined as the measure of the response of a metal to the process of hardening. It refers to the distance or depth within a specimen (normal to its surface) up to which appreciable hardness can be attained. Hardenability (i.e. depth of hardness) is determined largely by the presence of alloying elements (such as chromium, vanadium, molybdenum, etc.) in the hardened metal. It is tested by the Jominy end quench hardenability test. Brinell hardness test

Brinell hardness test is based on the area of indentation a steel or carbide ball (up to 10 mm diameter) makes in the surface of test specimen for a given load (varying up to 3000 kg) when loaded for 15 seconds on ferrous metals (steels and cast irons) and up to 30 seconds on nonferrous metals (brass, bronze, aluminium) specimen. After the load is released, the diameter of the spherical impression (Fig. 2.20) made in the surface of the test piece is found using Brinell microscope and the Brinell hardness number (HB; formerly BHN) is calculated as below: HB =

Load on ball (kg) Area of ball impression (mm)

where P—Applied load on ball in kg D—Diameter of ball in mm d—Diameter of impression in mm

2

=

2P È p D Í D - ( D 2 - d 2 ) ˘˙ Î ˚

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MANUFACTURING PROCESSES

Fig. 2.20 Brinell hardness test.

Brinell hardness number can be found directly from tables [such as made by the Society of Automotive Engineers (SAE) in relation to the diameter of the impression]. The magnitude of the HB is indicative of relative hardness of metals—higher the number, the harder the metal. This test is recommended for use on materials that are coarse-grained (or non-uniform) in structure since relatively large indenter ball gives better average reading over greater area. Rockwell hardness test

Rockwell hardness test is a much faster test and in this test also the Rockwell hardness number is determined through an indentation made under a load. The indenter is, however, much smaller in size, and it could be a steel ball used for soft metals or a diamond cone (called brale) used for hard metals. The principle of this tester is based on measuring the difference in penetration between a minor load (10 kg) and a major load (which is 60 kg for scale A, 100 kg for scale B, and 150 kg for scale C) and hence carried out in two stages. For softer materials like copper, brass, aluminium, grey cast iron and malleable cast iron, ball penetrator is used with 100 kg load on scale B to get Rockwell hardness number (HRB). For harder materials like white cast iron, hardened steel, etc., diamond brale penetrator is used under 150 kg load and the scale C is used to read Rockwell hardness number (HRC). Scale A, diamond brale and 60 kg load is used for testing extremely hard surfaces of carbides and case-hardened steel to get Rockwell hardness number (HRA). Microhardness tests

Knoop test: It is done when the requirement is to determine hardness over a very small area of the metal. Loads in this test vary from 25 gm to 5 kg and the indenter is a knoop diamond indenter, ground to a pyramidal form that makes an indentation having an approximate ratio between the long and short diagonals, 7:1. The knoop hardness number (HK; formerly KHN) is considered suitable for very thin and small specimen and for brittle materials such as carbides, ceramics and glasses. Vickers hardness test: In fact, this also comes under the category of microhardness test, with the difference that a square-based diamond pyramid indenter having 136° angle between

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67

opposite faces is used with loads varying from 5 kg to 120 kg. The Vickers hardness test gives Vickers hardness number (HV) and is a more accurate test. Shore scleroscope hardness tester

Shore scleroscope hardness test is also known as rebound test. Apparatus used is the shore-scleroscope. It is a fast and portable means of checking hardness. The hardness number is based on the height of rebound of a diamond tipped metallic hammer which falls freely from a given height. The amount of rebound is seen from the scale—the harder the material, the higher the rebound. Then the shore hardness number is obtained. Mohs hardness test

Mohs hardness test is a test that gives relative hardness of materials by comparing it with the metal of known hardness through scratching by file, etc. Mohs hardness values are used generally for designating hardness of minerals. The mohs scale is an arbitrary scale of hardness based on 10 selected minerals, for example, 1 is for talc, 7 for quartz, 9 for corundum and 10 for diamond. A material with higher mohs hardness number can always scratch on one with a lower number. Soft metals have mohs hardness number 2 to 3, hardnened steel about 6 and aluminium oxide (used as abrasive material for making the grinding wheels and cutting tools) as 9. The mohs scale is only a qualitative test. Durometer

Durometer is an instrument used for measuring the hardness of rubbers, plastics and other soft and elastic materials. In this test, an indenter is pressed against the surface and later a constant load (1 to 5 kg) is rapidly applied and the depth of penetration measured. The use of the above standard tests for hardness testing is summarized below: ● ● ● ● ● ● ● ●

Brinell for ferrous and non-ferrous metals, carbon and graphite Rockwell B for non-ferrous metals and thin sheet metals (minimum indentation) Rockwell C for ferrous metals Knoop for thin sections or small parts of hard metals such as carbides Vickers diamond pyramid for all metals Durometer for rubber and similar elastic materials Scleroscope for ferrous alloys Mohs for minerals

General characteristics of hardness testing methods, formulae for calculating hardness and chart for converting various hardness scales are given in Appendix.

2.25.3

Impact Test

Many parts of the machine need to be designed to stand impact loading and absorb the energy of impact within it through elastic action, thus providing damping effect. Impact test is done to determine the resistance to fracture (or rupture) against impact loads. Two tests developed to measure rupture strength (or toughness) are (a) Izod impact-toughness test and (b) Charpy impact-toughness test. Energy required to fracture a notched specimen is measured in these tests where measured energy is indicative of relative toughness of the material.

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2.25.4

Fatigue Test

Components subjected to fatigue loads are designed on the basis of fatigue strength (or endurance limit) of the metal, which is always lower than the yield strength of the metal. The most common fatigue test is the rotating beam test in which the test is performed on a rotating beam with a constant downward load, thereby subjecting the beam to bending moment. The rotation of specimen causes an alternate shift of the uniform bending stress from tension to compression in every 180o of rotation of the specimen.

2.25.5

Creep Test

In the creep test, the metal specimen is held under an applied load at a given temperature until it fails. Then a series of curves for different temperatures, stress levels and strains are plotted which can be helpful for the designers for selecting a metal for high temperature applications.

2.25.6

Non-destructive Inspection and Testing

In spite of taking all measures and precautions to maintain quality control of a product while passing through various manufacturing stages, the final product is still likely to have some invisible internal flaws or defects. Non-destructive testing methods are intended for detecting these internal flaws. In these methods, the test specimen is neither destroyed nor broken. Some common non-destructive methods of inspection and testing are discussed here. Visual inspection: As the name indicates, visual inspection with eyes is carried out on the products for checking their shape, size and surface condition (presence of crack and other surface defects, etc.) as per specifications required. The technique is fast, simple and cheap but the quality of results depends on the skill of the observer. Radiography: Radiographic techniques are employed for detection of internal flaws in the product by exposing it to X-rays, gamma rays or neutron beams. These radiations create shadow on a photographic film or fluorescent screen depending on the source of radiation. Absorption of the radiation and its extent depends on the microstructure of the metal or presence of flaws in the product. X-rays are short wavelength electromagnetic radiations capable of penetrating all metals. Gamma-rays are still shorter wavelength radiations with greater penetrating power than X-rays and hence used for flaw detection in thicker sections of metals and concrete components. The photographic film of the product test usually consists of darker and lighter areas wherein more darker portions of the film show the presence of voids and cracks. The principle involved is difference in density; the metal surrounding the defect (void etc.) is denser and hence shows up as lighter than the defect or flaws on an X-ray film. It is similar to the way bones and teeth show up lighter than the rest of the body in X-ray films. The neutron beam radiography is a more accurate and sensitive method for detection of flaws like inclusions, voids, etc. in aerospace equipment and other costly products. Ultrasonic testing: The ultrasonic detection technique is used for very small internal flaws in metallic components or other materials like concrete components, where it is used to detect flaws and variation in density of the product along different sections. A quartz crystal is used to produce ultrasonic vibrations. A transmitter probe is held on the smooth surface of the product under test, and a second receiver probe is placed little away from the first.

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For operation, ultrasonic wave is introduced into the metal and the time interval between transmission of the outgoing and reception of incoming signals is measured with a cathode ray oscilloscope. Liquid penetration test: It is used to detect the defects that extend up to the surface of the metal. The component to be tested is first cleaned and dried and a penetrant (liquid material) is applied to its surface. The penetrant gets entry into the component through surface discontinuities. In one method, the test piece is immersed in the solution of kerosene oil, transformer oil and terpentine oil. It is later washed in water and subsequently coated with the film of aqueous suspension of white clay and dried. The solution that might have gone into the cracks comes out of the cracks during drying. Sometimes, a cracked piece is just dipped in kerosene oil for some time. It is then wiped dry and rubbed with chalk. The oil that penetrates through the cracks appears as thin streaks along the cracks. Cracks in plastic components and other non-magnetic materials are detected by fluoroscopic crack detection. The test piece is immersed in a solution of kerosene oil and transformer oil. After that, it is wiped dry. Magnesium oxide is later sprinkled on the test-piece and it is then examined under ultraviolet ray. The liquid coming out of the cracks will wet the magnesium oxide. Magnetic particle crack detection: It is applicable only to ferrous metals, steels and cast iron. The article to be tested is first magnetized in the crack detector. It is dipped into a bath of magnetic particle powder (powder of iron). The magnetic particles will settle on the cracks if they are present.

REVIEW QUESTIONS 1. What are metals and non-metals? Give a few examples of each. 2. Why is it necessary for an engineer to have a background knowledge of important engineering materials? 3. Discuss the important factors that should be considered in selecting proper material for manufacturing a product. 4. Differentiate between an atom and an element. Name a few elements known to you. 5. What do you understand by the term bond in any element? What are the main types of bonds usually found in elements? 6. What is a metallic bond? Discuss its significance. 7. What are amorphous and crystalline structures in solids? Give examples of materials that have these structures. 8. Define crystal and crystal structure of metals. 9. Describe solidification of a ferrous metal with special reference to dendritic growth of crystals. 10. Define unit cell and space lattice. 11. Differentiate between grain and grain boundary. 12. What is a polycrystalline structure? Do the metals have this structure? 13. Discuss the different types of space lattice.

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14. 15. 16. 17.

What is bcc lattice? Name the metals that have this lattice structure. What is fcc lattice? Give examples of metals possessing this structure. What are important features of hcp lattice? Name the metals possessing this structure. Write the space lattice for the following metals: (i) gold (ii) lead (iii) iron (iv) sodium (v) copper (vi) magnesium (vii) vanadium (viii) aluminium

18. 19. 20. 21.

Discuss the effects of grain structure and grain size on the properties of metals. What is the effect of grain size on the strength and ductility of a metal? What do you understand by grain refining and recrystallization processes? What are imperfections in lattice structure? Name different types of imperfections in space lattice. Name the two important types of point defects in a crystal lattice. How does the presence of vacancies and interstitials affect the space lattice? Is their presence sometimes beneficial also? What are dislocations? Why do these occur? What are line-type defects in crystal lattice? Differentiate between edge dislocation and screw dislocation. Discuss with illustration the phenomenon of slip in a metal. What are slip planes and slip directions? How many types of deformations can a crystal lattice be subjected to? Differentiate between elastic deformation and plastic deformation. Discuss deformation by twinning. What is work-hardening or strain-hardening and how is it beneficially used in some engineering applications? What is cold-working? Discuss advantages and disadvantages of cold-working a metal. Name the metals that are most adaptable to cold-working. Name the important cold-working operations used in industry for various purposes. What is hot-working? Discuss advantages and disadvantages of this process. Name the treatments given to improve the quality of cold-worked metals. Write short notes on: (a) brittle fracture (b) ductile fracture (c) fatigue fracture (d) creep fracture

22.

23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.

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37. What is an alloy? Why alloying of metals is done? 38. What are different phases of an alloy? 39. What do you understand by the term ‘polymorphic forms of iron’? Explain with the help of a sketch. 40. What is the importance of curie point? 41. List down various phases present in carbon steels and cast irons. 42. Write short notes on: (i) cementite (ii) austenite (iii) pearlite (iv) martensite 43. Is it normally possible to have austenite structure at room temperature? 44. What do you understand by the term ‘Eutectoid’? 45. What are hypo-eutectoid steels? How are they different from hyper-eutectoid steels? 46. What is an iron-carbon equilibrium diagram? What is the use of this diagram? 47. What is the limitation of an iron-carbon equilibrium diagram? 48. What is the upper critical temperature line and what is its importance with reference to iron-carbon equilibrium diagram? 49. What is the lower critical temperature line and what does this signify? 50. What does the eutectic point signify in an iron-carbon equilibrium diagram? 51. What do you understand by the physical properties of metals? 52. What do the chemical properties of metals signify? 53. What do you understand by the mechanical properties of a metal? 54. Define the following: (a) Strength (b) Elasticity (c) Plasticity (d) Ductility (e) Abrasion resistance (f) Resilience 55. Differentiate between the following: (a) Ductility (b) Malleability (c) Brittleness 56. What are toughness and impact strength? Define them. 57. Differentiate between stiffness and rigidity. On what characteristic of the metal does the stiffness depend? 58. What is strain energy? Define proof resilience. 59. What is fatigue strength? What is its importance in designing machine component subjected to variable or reversible stresses? 60. How can the fatigue strength of a metal be improved? 61. Define the endurance limit of a metal. How is it determined? 62. Discuss the phenomenon of creep failure. 63. Discuss in detail the factors that affect the mechanical properties of metals. 64. What do you understand by fabricating properties of metals? 65. What is machinability? Discuss the factors that affect machinability of metals.

72 66. 67. 68. 69. 70. 71.

72. 73. 74. 75. 76. 77. 78. 79.

80.

81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91.

MANUFACTURING PROCESSES

What is machinability rating (or index)? Discuss the property of formability. What are the factors that affect formability of a metal? What do you understand by weldability? How weldability of a metal is decided? What are the factors that affect weldability of a metal? What is castability? What factors are considered in deciding castability of a metal? Define the following: (a) Load (b) Stress (c) Strain (d) Poisson’s ratio (e) Elastic limit (f) Hooke’s law What is stress? How many types of stresses are you familiar with? Discuss with the help of sketches. What is strain? How is the linear strain different from (a) lateral strain and (b) volumetric strain? Define elasticity. Which is more elastic—a rubber or a steel? Define modulus of elasticity. Is it possible to increase the modulus of elasticity of a metal? What is a stress–strain diagram? Define modulus of rigidity. Draw a stress–strain diagram for the following metals when subjected to a tensile load: (a) Mild steel (b) High carbon steel (c) Grey cast iron Draw a stress–strain curve for the following metals when tested under tension: (a) Aluminium (b) Copper (annealed and hard) (c) Brass (annealed and hard) Define the following terms in reference to the stress–strain diagram: (a) Limit of proportionality (b) Elastic limit (c) Yield point (d) Ultimate stress (e) Breaking stress Differentiate between engineering stress and true stress. What do you understand by percentage elongation and percentage reduction in area with reference to a tensile test specimen? What are bending stress and shear stress? Explain with the help of sketches. Name the general destructive tests and non-destructive tests carried out on a metal piece. What is their purpose? How are the tensile and compressive strength of a metal determined? How is the hardness of a metal judged? Describe common methods used for testing the hardness along with their suitability. What is an impact test? Why is it done? Name the two important impact tests used in practice. Describe, with the help of a sketch, the principle of (a) izod impact test and (b) charpy impact test. How is the fatigue test carried out? Discuss the salient features of a creep test. What is the importance of this test? Discuss the various methods of non-destructive testing.

3 3.1

Ferrous Metals Irons and Steels

INTRODUCTION

Among the materials used for common engineering applications, ferrous metals such as iron and steel have wide applications since they are abundant, economical and have unique magnetic properties. Ferrous alloys are easily formed to various shapes in the annealed state, and then can be heat-treated to be as hard as file, tough as the hook of a big crane or hardened and tempered to drill holes in other materials. Steel, the most important and popular ferrous metal, is unique in its ability to exist as a soft ductile material. It can be easily formed and machined and then, as a result of heat-treatment, can assume the role of a hard, tough material which resists change in shape. Ferrous metals are furnished to manufacturing industry in the shape of plates, bars, sheets, strips, castings, forgings, etc. The fact that iron-carbon alloys undergo various phase transformations largely accounts for the importance of steels and cast irons as engineering materials. The dominance of ferrous alloys in manufacturing stems from their wide range of properties with changes in carbon content. For example, as little as one part in one thousand (0.1%) changes pure iron to steel, which is a soft sheet steel used in drawing and forming processes. In mild steel, which is a general-purpose steel, carbon percentage may vary from 0.2 to 0.3%. The rail steel and other tool steels have carbon contents from 0.6 to 0.9%. As carbon percentage goes over 1%, the steels have the greatest ability to hold a sharp edge and hence such steels are used for making razors and wood chisels. The cast iron range extends from 1.5 up to 5% carbon. Although as much as 6% carbon can be dissolved in molten iron, only less than 2% can remain in solution in the solidified alloy.

3.2

FERROUS AND NON-FERROUS METALS

Metals can be broadly classified as (a) ferrous metals and (b) non-ferrous metals. Ferrous metals contain iron as the chief constituent. Other elements present in ferrous metals in varying proportions include carbon, silicon, manganese, sulphur, phosphorus, nickel, 73

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chromium, tungsten, cobalt, etc. Different types of cast irons, namely, grey cast iron, white cast iron, malleable cast iron, nodular cast iron and various steels like low, medium and high carbon steels, alloy steels, stainless steels and other special steels are the examples of ferrous metals. Non-ferrous metals do not contain iron. Examples of non-ferrous metals include aluminium, copper, zinc, lead, tin, silver, etc., as also a large number of their alloys like brass, bronze, gun metal. Non-ferrous metals are known for their special qualities such as light weight, good thermal and electrical conductivity, high resistance to corrosion, antimagnetic property and good machinability.

3.3

GENERAL TERMS ABOUT IRON AND STEEL

Pure iron is not available commercially. It contains other elements that affect its physical and mechanical properties. The amount and distribution of these alloying elements mainly depend on the method of manufacture. Commercial forms of iron and steel are given below in brief. Pig iron is the product of blast furnace where it is produced by the reduction of iron ores. Cast iron is produced by slow cooling of remelted pig iron with coke and limestone in a cupola or crucible furnace. One of the distinguishing features of all cast irons is that they have relatively high carbon contents. While steels range up to about 2% carbon, cast irons overlap with the steels and have carbon contents varying from 1.5 to 5% and silicon (up to about 3.5%). Cast iron is not appreciably malleable at any temperature as cast. The carbon in cast irons may be present in the form of iron carbide or free graphite. White cast iron contains cementite (iron carbide) in which carbon is present in the combined form with iron and is hard and brittle. It is obtained by quick cooling of pig iron. It gives a fracture of white colour as no graphite (free carbon) is present in it. Malleable cast iron is obtained by changing the iron carbide present in white cast iron to free or temper carbon through suitable heat treatment processes. In-got iron is the product of open-hearth process and contains very little carbon, manganese and other impurities. Wrought iron is the purest form of iron (up to 99.8%) and has very low carbon content (0.2% or less). It is made by melting white cast iron and passing an oxidizing flame over it. It has a fibrous structure with fine fibres of silicate slag distributed through the iron in the direction of rolling. Steel is a malleable alloy of iron and carbon having carbon contents up to 2%. The carbon present in steel is always in the combined form of iron carbide and never in the form of free graphite as it may be there in cast iron. Carbon steels have their properties primarily only due to the carbon contents. Alloy steels have alloying elements like nickel, chromium and tungsten which provide special properties to steels. Open-hearth steel, bessemer steel and electric furnace steel are the names of steel based on the process of its manufacturing irrespective of carbon contents.

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3.4

75

PIG IRON

Pig iron is the most basic form of iron used for making varieties of cast irons and steels. It is the crude form of iron extracted from iron ores by their smelting and reduction in a blast furnace, using raw materials such as iron ores, fuel (coke) and flux (limestone or dolomite).

3.4.1 Impurities in Pig Iron Pig iron can be regarded as an impure form of cast iron and is the raw material for making practically all iron and steel products. The following elements appear as impurity in pig iron. Carbon percentage varies from 4 to 4.5% in pig iron. The source of carbon in steels (and cast irons) is coal. Carbon present in pig iron is either in free graphite form or in the form of iron carbide. Carbon in pig iron increases hardness. Phosphorus present in pig iron may vary from 0.1 to 2%. It combines with iron forming Fe3P, which embrittles cast iron. Its amount should be kept as low as possible. The presence of phosphorus in small quantity in pig iron increases the fluidity of the molten iron. The presence of sulphur (0.4 to 1%) tends to make iron hard and produces unsound castings. The presence of manganese in iron, however, lowers the percentage of sulphur by forming manganese sulphide. The presence of silicon in pig iron varies from 1.0 to 4%. Silicon reduces both hardness and strength of pig iron. Silicon in steels promotes the decomposition of cementite to form graphite. It also increases electrical resistance in steels. Manganese quantity varies from 0.2 to 1.5% and its presence tries to reduce the quantity of sulphur present in pig iron by forming manganese sulphide, which is not as harmful as iron sulphide. Manganese increases tensile strength of iron and promotes combined carbon (carbide) resulting into hardness of cast iron.

3.4.2 Use of Pig Iron Due to the presence of impurities, pig iron is too brittle and has very low strength and poor ductility. It cannot be shaped into different forms by the processes of forging or hammering. It is mostly converted into various cast irons, wrought irons and steels for commercial use.

3.5

CAST IRON

Cast iron is widely used in the industry because of ease in casting and a wide range of properties. It may be mentioned here that the term ‘cast iron’ used in the following description is just a very general name (or main title name) of various cast irons which differ from each other primarily on account of the form of excess carbon (free carbon) present in them, which is governed by thermal conditions (e.g. rate of cooling and alloying elements). When a metal charge comprising pig iron, limestone (flux), coke, steel scrap and spoiled cast iron castings is melted in a cupola, poured into a mold and gives a casting on cooling and solidifying, it is known as cast iron (or cast iron casting). Cast iron is available in various forms such as grey cast iron, white cast iron, malleable cast iron, ductile cast iron and alloy cast iron. It is generally specified on the basis of its properties and

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not on chemical composition. The properties of cast iron are regulated by the amount, type and size and distribution of various carbon formations which are governed by casting design, chemical composition of metal, type and amount of steel and cast iron scrap added in a cupola, rate of cooling the metal in moulds and subsequent heat treatment of the castings.

3.5.1 Production of Cast Iron Cast iron is produced by melting pig iron billets in a crucible (for small quantities) or in a cupola (for large requirements of metal). The cupola is a vertical steel cylinder having a lining of refractory material inside. A cupola may have diameter up to 2 metres and height up to 13 metres. Cupola is a mini blast furnace, which helps in reducing and refining pig iron into cast iron.

3.5.2 Composition of Cast Iron The cast iron contains the following elements. Carbon in cast iron varies from 1.5 to 5%. It may be in the form of either free graphite or iron carbide. Carbon gives strength and hardness to cast iron. It is the basis for many good properties like high fluidity, high damping capacity, low notch sensitivity and good machinability. Silicon helps in keeping cast iron in graphitic condition (flakes) and thus makes it soft. Its content varies between 1.0 and 3.5%. Silicon acts as softener in cast iron. It reduces the ability of iron to retain carbon in iron carbide form and increases free carbon and decreases combined carbon. It increases fluidity of molten metal. More silicon is, therefore, needed for casting thin sections of cast iron to avoid formation of iron carbide (cementite) due to quick cooling of the thin sections. Increased amount of silicon helps avoid defects such as blow holes in big castings. Manganese in cast iron is found to vary between 0.5 and 1%. It enables more carbon to combine with iron and tries to retain maximum amount of carbon in iron carbide form and thus hardens the cast iron, increasing its wear resistance. Phosphorus imparts fluidity to molten cast iron and is present up to 1%. Increased amount of phosphorus is sometimes used for casting intricate shapes and sections. Sulphur is present in cast iron from 0.02 to 0.15%. Its presence results in unsound castings as it reduces the fluidity of the metal and makes it hard and brittle.

3.5.3 Properties of Cast Iron The character of cast iron is determined by the proportion of two forms of carbon present in it, the free graphite form and the chemically combined form (iron carbide or cementite). Cast iron, when completely fluid, contains carbon in combined state. Free carbon in graphite form is produced during cooling of the metal. With slow cooling, large graphite flakes are produced which render the cast iron soft and easy machinable structure, comprising ferrite, cementite, pearlite and graphite. The strength of cast iron is entirely due to its pearlitic structure. The cast iron has the following properties. ● ●

It is brittle and hard. Low resistance to tension.

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Very good in compression. Has no plasticity and hence cannot be forged or rolled. Very good vibrational damping characteristics. Has good machinability in general.

It will be seen that cast irons are available in large variety. The properties of various grades or types of cast iron will be discussed separately with each type.

3.5.4 Effect of Graphite Shape on the Properties of Cast Iron Graphite or free carbon in cast iron may be in the form of flakes or nodules (spheroids). Irrespective of these forms, graphite has very low cohesive strength and thus reduces tensile strength and ductility of cast iron. Cast irons are usually not considered for forming operations. Flakes present in cast iron are interconnected with their relatively sharp edges (stress raisers), which are potential areas of low ductility and weakness, and hence in the flake form, the tensile strength of cast iron is most adversely affected. However, the presence of graphite flakes gives cast iron capacity to dampen vibrations and ability to dissipate energy. Nodule form of graphite is more favourable as it imparts cast iron higher ductility and strength. Spheroidal form is ideal from the point of view of both ductility and strength.

3.5.5 Applications of Cast Iron Applications of different types of cast iron will be described later separately when a particular variety of cast iron is discussed. However, in general, cast iron finds application in the following: ● ● ● ● ● ● ●

3.6

Machine frames, columns, beds and bed plates Railings Pipes Bearing housings Cylinders of steam and automobile engines Flywheels Numerous other parts not subjected to tension and shocks

TYPES OF CAST IRON

Cast iron is produced in different grades or types with varying properties. Important types of cast iron are given below: (i) (ii) (iii) (iv)

Grey cast iron White cast iron (or chilled cast iron) Mottled cast iron Malleable cast iron

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(v) Ductile cast iron (or nodular cast iron or spheroidal graphite cast iron or high strength cast iron) (vi) High duty cast iron (or meehanite cast iron or inoculated cast iron) (vii) Alloy cast iron (viii) Wrought iron

3.6.1 Grey Cast Iron Grey cast iron is produced by melting together pig iron, steel scrap and coke in a cupola. It is then poured in the moulds. Grey cast iron is obtained by slow cooling of the molten metal during its solidification. Most of its carbon is in graphitic form, which is seen as small flakes in the fractured sections and is responsible for giving grey colour to iron. Generally, the grey cast iron castings have carbon 0.3 to 0.9% in the form of iron carbide (Fe3C), which gives strength and hardness to cast iron. The rest of carbon is in the form of graphite or free carbon. Since carbides give hardness and strength, a wide range of properties is possible. Elements present in grey cast iron include: carbon 3 to 3.5% (total), silicon 1.0 to 2.75%, phosphorus 0.15 to 1.0%, sulphur 0.02 to 0.15%, manganese 0.4 to 1.0%, and the remaining is iron. The role of silicon in grey cast iron is very important as it helps in keeping the cast iron in graphitic condition and makes it soft. Properties and uses of grey cast iron

As stated before, a series of cast irons is possible ranging from cast iron with all (or most of) the carbon in graphite form to cast iron with a good share of carbon in the combined form. Cast iron with all carbon in graphite form (ferritic structure) is soft, easily machinable and has high vibration damping capacity (i.e. loss of vibrational amplitude), and has high compressive strength and self-lubricating property that makes it highly suitable where sliding action is involved. However, such cast irons have poor tensile strength, ductility and impact resistance due to the weakening effect of the graphite flakes. These cast irons are used for beds of machine tools and other structural members loaded in compression. Cast irons having higher content of carbon in the carbide form are hard, brittle and have good wear resistance but are unmachinable. On the other hand, close-grained cast irons containing graphite and pearlite are the strongest, toughest and best finishing type. A high phosphorus grey cast iron is used for casting intricate shapes as it pours easily and is of cheaper and low quality. It is used for making covers of switch boxes, rain water goods and ornamental castings. The relative amount of free and combined carbon is determined by the variations in the composition of metal, melting practice, casting practice and the rate of cooling. Slow cooling of castings promotes formation of graphite while the fast cooling results in the formation of iron carbide (or cementite). ● ● ● ●

Melting point of grey cast iron varies between 1135°C and 1250°C. Tensile strength varies between 1500 and 4000 kg/cm2. Compressive strength is 3 to 4 times the tensile strength. Presence of free graphite provides natural lubricant and hence suitable for parts where sliding is involved.

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● ● ●

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Hardness varies between 180 and 300 HB. It is brittle and has no ductility and plasticity and cannot bear severe shocks. It cannot be forged. Little or almost negligible shrinkage. Very good machinability and fusibility. Good corrosion resistance and low coefficient of friction.

Grey cast irons are mostly used for structures (bed and columns) of machine tools and structural pacts under compression, brackets, water pipes and other general purpose castings like engine cylinder blocks, pipe fittings and agricultural implements. BIS specifications for grey cast iron

The Bureau of Indian Standards (BIS) has designated grey cast iron according to IS: 210–1970, wherein grey iron castings are designated by letters FG, followed by minimum ultimate tensile strength in kg/mm2, e.g. FG 15, FG 20, FG 25, FG 30, FG 40. Sometimes when the chemical composition is more important, then it may be specified as FG 20 Si 10 which means ultimate tensile strength of 20 kg/mm2 and silicon percentage as 10. The basic composition of grey cast iron is often described in terms of carbon equivalent (CE), which is a factor giving relationship of the percentage of carbon and silicon in iron to its capacity to produce graphite. CE = C1 + 1/3(Si% + P%) where C1 = total percentage of carbon. For example, if grey iron has carbon 3.3% and silicon 0.60% and phosphorus 0.1%, then CE would be: CE = 3.3 + 1/3(0.6 + 0.1) = 3.5

3.6.2 White Cast Iron (or Chilled Cast Iron) When the percentage of silicon in cast iron is very low, most of the carbon in the metal is in chemically combined form. The resulting cast iron is called white cast iron. Therefore, white cast iron (or chilled cast iron) has no graphite or free carbon and hence is white in colour. It is produced by (a) casting grey cast iron but cooling it very rapidly or (b) adjusting the composition of the metal such that the carbon and silicon contents are low. When white cast iron is produced by rapid cooling of grey castings against metal chills or in metal moulds (which loses heat fast), the product is called chilled grey cast iron. White cast iron can be produced by adjusting metal composition with the addition of elements like chromium, vanadium, or molybdenum which are carbide stabilizers and increase the formation of iron carbide in cast iron. Properties and uses of white cast iron

Various constituents of white cast iron castings include: carbon 1.7 to 2.4% (mostly in combined form), phosphorus up to 0.2%, silicon 0.85 to 1.2%, sulphur up to 0.12%, manganese up to 0.5%, and the remaining is iron. ●

White cast iron is very hard, brittle and wear resistant.

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●

●

●

●

●

Its fluidity is not up to the mark in molten state and hence not suitable for general foundry work. Hardness of 400 HB can be obtained by keeping silicon less than 1% and carbon about 2%. Addition of chromium (above 3%) prevents formation of graphite and imparts high temperature strength, better resistance to corrosion to white cast iron. If nickel (4.5%) and chromium (1.5%) are added to white cast iron, then its toughness and strength is virtually doubled with increase in hardness up to 700 HB. Being unmachinable, white cast iron is used in parts requiring high abrasion resistance like rim of a freight car wheel, railway wheel brake blocks, hammer mills, crushing rollers, balls and wear plates. Castings of white cast iron are used for subsequent conversion into malleable iron castings.

3.6.3 Mottled Cast Iron It is a mixture of grey cast iron and white cast iron and has the speciality that the carbon in it is present in both free and combined state, and is divided almost equally in these two forms.

3.6.4 Malleable Cast Iron Malleable cast iron is produced by giving long time heat treatment (annealing) to white cast iron castings. The annealing process consists of heating white cast iron castings slowly to about 870oC and then keeping them at this temperature for 25 to 60 hours, depending on the size of the casting and then cooling them slowly. The two common types of malleable cast iron are: (a) White-heart malleable cast iron and (b) Black-heart malleable cast iron. White-heart malleable cast iron is produced when white cast iron castings are packed in an oxidizing material to remove some carbon. Black-heart malleable cast iron is produced when white cast iron castings are packed in some inert material (such as ferrous silicate scale or slag) and annealed, the resulting structure of the castings consists almost entirely of graphite and ferrite. The idea behind annealing white cast iron is to precipitate the combined carbon gradually into free nodular graphite and then to drive it out, thus limiting the presence of total carbon contents to about 1.0%. White cast iron having all its carbon combined in the form of Fe3C (iron carbide or cementite) can only be produced at low silicon levels because silicon promotes tendency of graphitization. As white cast iron is annealed at temperature about 870oC or so, silicon causes iron carbide to break into iron and carbon in the form of irregular-temper carbon nodules. The resultant cast structure (malleable cast iron) is very ductile and has very good impact resistance, and can be easily machined and is more tougher with improved corrosion resistance in comparison to grey cast iron. Fe3C (Iron carbide)

Æ

3Fe (Iron)

+

C (Carbon graphite)

In the above, carbon is forming from the solid state and hence it forms up as fine nodules rather than flakes (which are present in grey cast iron and is the main cause of its weakness and

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brittleness). It may be noted that grey cast iron castings cannot be transformed into malleable cast iron castings through annealing, because graphite flakes of grey cast iron are very stable and these can go into solution only on melting (and not in solid state as is the case with white cast iron). Malleable cast iron casting not only possesses better characteristics than grey cast iron castings, it is superior to cast steel castings as well since all of the carbon present is in the free form which renders malleable cast iron better machinable than cast steel. Malleable cast iron is used in place of forged steel or wrought iron, where the intricacy of shape creates difficulty in forging. For structural parts, malleable cast iron is limited to relatively thin walls (less than 25 mm) because of large shrinkage and the need for rapid chilling to produce a white cast iron as cast structure. Since malleable cast iron is tougher and has more resistance to bending and twisting than grey cast iron, the former is, therefore, used for various automobile and agricultural components like tractor and plough components, gear housing, crank cases, levers, spanners, gear wheels, etc. Different types of malleable cast iron

Malleable cast irons as per BIS are classified as (a) White-heart malleable cast iron, (b) Blackheart malleable cast iron and (c) Pearlitic malleable cast iron. These are designated by letters WM (White-heart), BM (Black-heart) and PM (Pearlitic malleable) followed by minimum ultimate tensile strength in kg/mm2. These are covered under IS: 2108–1962, IS: 2640–1964 and IS: 2107–1962. (a) White-heart malleable cast iron: It is made from that white cast iron which has high carbon and sulphur contents, enabling the metal to be melted in cupola and later cast in sand moulds giving white cast iron castings without flake graphite. Silicon and manganese are kept low because silicon promotes graphitization of carbon, and manganese offsets the stabilizing effect of sulphur which helps in chilling the casting even in sand molds. A typical composition of furnace charge for white-heart malleable cast iron may be: C—3.2 to 3.6% Si—0.4 to 0.9% Mn—up to 0.4% S—0.1 to 0.3% P—0.1% (max.) During annealing, the castings are kept in an oxidizing and decarburing medium in boxes, usually kept in red haematite ore. The boxes are then annealed at temperature between 870°C and 1000°C for about 6 days and later slowly cooled in furnace. The carbon (in castings) gets diffused and oxidized. This way, the surface layer of casting is decarburized, but the central thick section of casting has a matrix of ferrite and pearlite in which very finely divided or nodular type graphite (called temper carbon) is found embedded. The amount of temper carbon in white-heart malleable cast iron is less than that present in black-heart malleable cast iron. White-heart malleable cast iron shows non-uniform mechanical properties, for example, thicker sections of castings are harder and stronger, less ductile and less

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impact resistant when compared to thin sections. The maximum thickness of casting is, therefore, limited to about 18 mm. White-heart malleable cast iron is used for making hardware items, pipe and pipe fittings, farm implements and automobile parts. (b) Black-heart malleable cast iron: White cast iron used for the black-heart malleable cast iron has lower contents of carbon and sulphur. A typical charge of metal for making black-heart malleable cast iron contains the following: C—2 to 2.6% Si—0.9 to 1.6% Mn—0.04 to 0.17% S—0.08 to 0.5% P—0.03 to 0.1% White cast iron to make black-heart malleable cast iron should have enough silicon to promote graphitization of iron carbide and sufficient manganese to offset the stabilizing effect of sulphur. The molten metal is very fluid and hence suitable for casting thin sections. The castings are packed in a neutral substance like sand and then kept in furnace for annealing at 870oC for 3 to 6 days. There is no decarburization at the outer surface of castings, and the lower carbon contents permit a higher silicon content without the risk of formation of graphite flakes in the centre of casting. In this case, carbon is not separated out and removed (as in case of white-heart malleable cast iron). During cooling of annealed castings, the combined carbon separates out but all of the carbon does not change into graphite, rather the microstructure of annealed casting comprises nodular graphite (temper carbon) embedded in a matrix of ferrite. The amount of temper carbon is more in this case in comparison to white-heart malleable cast iron. Black-heart malleable cast iron has ferrite matrix. The mechanical properties are uniform throughout the sections of the casting. Machinability is improved. It has higher impact resistance. The tensile strength may be about 3.6 tonnes/cm2 and elongation 15%. This metal is replacing steel in many applications for the reason of being cast into intricate shapes. Black-heart malleable cast iron is used in automobile and tractor parts for crank cases, rear axle housing, bearing housing, hubs, bush bearing, gear wheels, levers, spanners, agricultural implements, marine industry for ships components such as hinge, door keys, mountings, textile industry and machine component and joint fittings for gas and water pipe lines. (c) Pearlitic malleable cast iron: It is an improved version of black-heart malleable cast iron and is made by controlled heat treatment. Instead of having a ferrite matrix, it has a pearlitic structure, which is stronger and harder than ferrite. These irons are obtained by interrupting the second-stage annealing process, by introducing larger quantity of manganese which prevents graphitization of pearlite or by air cooling followed by tempering. The castings finally comprise a matrix of pearlite, martensite or sorbite instead of ferrite of black-heart malleable cast iron. Tensile strength is higher (about 5 tonnes/cm2 or more). Hardness is 200 to 260 HB. The ductility is, however, lower.

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It is machined better than steel, gives good finish, has good wear resistance and good fatigue strength, and better damping than steel. Pearlitic malleable cast iron replaces steel forgings for universal joint yokes, gears and small crank shafts.

3.6.5 Ductile Cast Iron (or Nodular Cast Iron) Other names of ductile cast iron are: spheroidal graphite cast iron or high strength cast iron. It is a relatively new and fastest growing ferrous alloy because of a wide range of properties; it can be stronger than mild steel, yet poured from a low cost melting furnace such as cupola. It is called nodular cast iron because the free graphite in this metal is in the form of tiny balls (spheroids or nodules) rather than the flakes as present in grey cast iron. Ductile cast iron is obtained by controlling composition of the metal and also by adding magnesium (in the form of magnesium-ferro-silicon) in the molten metal. The added element alters the surface tension of the graphite in the molten iron and causes it to condense into spheroids. In the form of tiny balls, graphite has no detrimental effect upon the mechanical properties of the matrix, so the strength of the ductile cast iron depends on the type of metallic matrix, for example, with a pearlitic matrix, it can have tensile strength up to 85 kg/mm2 which is equivalent to the strength of high carbon steel, but ductile cast iron is superior in castability and machinability. Having several times greater tensile strength than ordinary grey cast iron, as also highly increased ductility and shock resistance, ductile cast iron combines the advantages of grey cast iron, such as ready availability, ease of founding and better machinability, besides many of the product advantages of steel. The elimination of weakening effect of flake graphite gives magnesium-containing iron excellent engineering properties. It has particularly high tensile strength, elastic modulus, yield strength, toughness and ductility. Under stress, ductile cast iron behaves elastically like steel. Composition of ductile cast iron

Ductile cast iron is produced by adding magnesium (or cerium) in the ladle. A typical composition of ductile cast iron is as below: C—3.2 to 4.5% Si—1.0 to 4.0% Mn—0.1 to 0.8% P—0.1% Ni—up to 3.5% Mg—0.05 to 0.10% Nickel and manganese increase the strength of ductile cast iron but lower ductility. It has high fluidity, castability, strength, toughness, wear resistance, weldability, pressure tightness and machinability, besides high heat and corrosion resistance. According to BIS, spheroidal graphite cast iron (ductile cast iron) is designated by SG, followed by minimum ultimate tensile strength in kg/mm2 and percentage elongation, for example, SG 42/12 has ultimate tensile strength 42 kg/mm2 and percentage elongation 12%.

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Properties and applications of ductile cast iron

Properties and applications of ductile cast iron are given in Table 3.1. TABLE 3.1

Properties and applications of some typical ductile cast irons

Properties

Minimum tensile strength (kg/cm2)

Yield strength (kg/cm2)

% of elongation in 5 cm length

Hardness (HB)

Application

Pearlitic matrix, high strength as cast, can be easily hardened by flame or induction hardening

5815

4360

03

200–270

Gears, dies, rolls for wear resistance and strength

Ferritic matrix, excellent machinability and good ductility

4360

2910

10

140–200

Pressure castings, valve and pump bodies, shockresisting parts.

Fully ferritic matrix, maximum ductility

4360

2810

15

140–190

Navy shipboard and other shock resisting parts

Uniformly divided fine pearlitic matrix, excellent combination of strength, wear resistance and ductility, can be normalized or tempered

7270

5090

03

240–300

Gears, pinions, crank shafts, cams, guides, track rollers

Matrix of tempered martensite, maximum strength and wear resistance, may be alloyed for providing hardenability

8725

6545

02

270–359

Gears, pinions, crank shafts, cams, guides, track rollers

3.6.6 High Duty Cast Iron (or Meehanite Cast Iron) High duty cast irons are sometimes known by a trade name, Meehanite cast irons. In general, cast irons are known to have lower tensile strength, toughness and wear resistance, but these properties can be enhanced by improving matrix through controlling graphite distribution and its quantity and size. If the matrix is entirely of pearlite, then the metal will have good strength, machinability and hardness. The presence of lower cementite contents gives free ferrite with less hardness and strength. Similarly, the presence of cementite in excess of that required to produce

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pearlite increases brittleness and poor machinability. Different high duty cast irons are produced by (a) adding steel scrap (in the metal charge of cupola) to lower carbon contents, silicon and phosphorus, or (b) controlling graphitization through the rate of cooling, or (c) improving chemical composition. The two most common types of high duty cast irons are: (a) Semi-steel or steel-mix iron (b) Inoculated cast iron (or meehanite cast iron) Semi-steel or steel-mix iron

It may be noted that graphite (carbon) is a weakening element in cast iron. However, high graphite cast irons are needed for ease in casting and machining. Lower the carbon, stronger the cast iron. By adding steel scrap (10 to 20% of cupola charge), cast iron with low carbon contents with fine close grains can be produced. Such a product is called semi-steel. Inoculated cast iron (or meehanite cast iron)

Inoculated cast irons are produced by laddle addition of some graphitizing elements to white base iron, thereby getting finally the high strength grey cast iron castings. The method of using admixtures (or impurities or graphitizing agents) to get a fine-grained metal is called inoculation. Graphitizing agents are: ferrosilicon, silicon carbide or calcium silicide, etc. The trade name of inoculated iron is: meehanite cast iron or nickel tensile iron, covering a wide variety of ‘high duty cast irons’. These are, in fact, specially treated grey cast irons. Calcium silicide is used as a graphitizer. The grey iron selected for this should have low silicon and carbon (2.5–3.0%). The grey iron casting with this much carbon and low silicon content, when cast by usual method, will lead to produce white cast iron, but the use of calcium silicide enables the production of fine graphitic structure with excellent mechanical properties. Meehanite metal castings are available in a large variety, for example, (a) general engineering, (b) abrasion resisting, (c) heat resisting, (d) nodular type, (e) corrosion resisting type, etc. Properties and uses of meehanite cast irons are given below: ●

● ● ● ● ● ●

It has high strength, toughness, ductility, machinability and thus is different from normal grey cast iron. Tensile strength varies from 25 to 40 kg/mm2. Compressive strength is over 100 kg/mm2. Hardness varies between 193 and 223 HB. It can be welded by arc welding or gas welding. Better vibration damping property. Improved creep strength.

On account of the above special properties, meehanite cast iron is gradually replacing other cast irons, including malleable cast iron, steel castings and some non-ferrous alloys also. Castings of beds, pillars and columns of machine tools are made from meehanite cast iron. It is also used for hydraulic cylinders, brake drums, pump parts, gears and dies.

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3.6.7 Alloy Cast Iron The plain cast irons discussed so far contain a small percentage of silicon, manganese, sulphur and phosphorus. These cast irons are hard and brittle, have good compressive strength but lack tensile and transverse (bending) strength. These are also weak against shock loading. The properties of normal cast iron can be improved by adding some alloying elements to it, for example, addition of nickel imparts tendency to graphitization, thus avoiding hard or chilled spots in castings. Similarly, copper also promotes graphitization and it is added from 0.25 to 2.25%. Alloy cast irons are produced either in cupola by melting together alloying elements or by adding alloying metal to the pouring laddle after drawing the molten iron from the furnace. Air furnace or electric furnace is used as improved method of melting the alloyed metal. Alloying is done to bring improvement in hardness, strength, resistance to corrosion and high heat, besides improving the response of metal to heat treatment. The main alloying elements and their functions are given below. Nickel (Ni) improves machinability and can be added usually up to 5%. It increases both hardness and strength simultaneously and improves corrosion resistance. It promotes graphitization. Higher percentage of nickel (up to 18%) may be used for higher corrosion resistance. Chromium (Cr) is added up to 3%. It promotes formation of carbides and increases hardness. It increases corrosion resistance and heat resistance, as also wear resistance and tensile strength. When nickel and chromium are added in the ratio of 3:1 (total 4%), they neutralize the tendencies of formation of graphite and carbide, which result in a metal that has refined grains, increased hardness and strength without reduction in machinability. Molybdenum (Mo) is usually kept up to 1.5%. It increases strength and wear resistance but machinability is reduced as addition of molybdenum slows down the process of graphitization. It increases toughness. Vanadium (V) improves carbide formation and greatly increases strength and hardness. It is added up to 0.5%. Copper (Cu) promotes graphitization and is added up to 2.25% maximum. Types of alloy cast irons

Alloy cast irons have now become a general term often used for special purpose cast irons having special properties like wear resistance, high resistance to corrosion and heat. Sometimes, ductile cast irons are also considered alloy cast irons as they also contain nickel, chromium, copper, etc. as alloying elements. The alloy cast irons can be heat-treated also. There are several patented processes which give special purpose cast irons. These fall in four major categories: (a) general engineering, (b) heat resisting, (c) wear resisting and (d) corrosion resisting. Some special processed cast irons are specified by BIS as follows. AFG Ni 16 Cu 7 Cr 2

—

Austenitic flake graphite iron castings having Ni (16%), Cu (7%) and Cr (2%)

ASG Ni 20 Cr 2

—

Austenitic spheroidal graphite iron castings having Ni (20%) and Cr (2%)

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ABR 33 Ni 4 Cr 2

—

87

Abrasion resistance iron casting having minimum ultimate tensile strength 33 kg/mm2, and having Ni (4%) and Cr (2%)

Alloy cast irons are sometimes grouped as follows. Low nickel cast irons: In these cast irons, nickel is the dominating alloying element (used up to 2%). These are highly machinable cast irons as nickel is the promoter of graphitization and gives refinement of iron matrix. Little amount of chromium and molybdenum may also be added to these cast irons. A typical composition of low nickel cast iron contains the following alloying elements besides iron. C—3.2% Si—1.7% Ni—1.0% Cr—0.5% The low nickel cast irons have hardness 210 to 340 HB and are used for pressure vessel castings. Hard and heat-treating nickel cast irons: These contain higher amount of nickel (from 2.0 to 6.0%). These cast irons have high hardness (up to 400 HB) and hence these are high wear resistant and provide very good response to heat treatment. Two typical compositions are as given below: (a) Carbon—3.2%, Silicon—1.2% and Nickel—2.5%. It gives a hard grey cast iron. (b) Carbon—3.2%, Silicon—1.2% and Nickel—5%. It gives a martensitic grey cast iron which is difficult to machine. Ni-hard and Ni-white cast irons: These are very hard metals with high abrasion resistance; hardness may be 550 to 800 HB. The Ni-hard group contains nickel up to 5%, whereas Ni-white cast irons have lower percentage of nickel as shown below: Typical nickel-hard metal: C—3%, Si—0.5%, Ni—4.5%, Cr—1.5% Typical nickel-white metal:

C—3%, Si—0.5%, Ni—1.8%, Cr—0.8%

Alloy cast irons of the above groups are used in material handling equipment, sintering plant, glass and ceramic industry equipment. Austenitic cast irons: These are the cast irons that have very high contents of nickel (varying from 10 to 40%). Hardness may go up to 350 HB. A typical cast iron with high resistance to corrosion and heat (Ni-resist group) may have: C—2.8%, Si—1.5%, Ni—14%, Cr—2% and Cu—6%. A low-expansion alloy cast iron may have nickel up to 30%. Alloy cast irons have special properties like increased strength, high wear resistance, corrosion resistance and heat resistance. They have good response to heat treatment for purpose of hardening or stress relieving or annealing for improving the machinability. Alloy cast irons are extensively used for automobile components like cylinders, pistons, piston rings, brake drums, crankcases, components of air compressors, stone crushing and grinding machinery, pump casing (for higher corrosion) resistance, heavy machine tool beds and other parts, grate bars of boiler and furnaces.

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3.6.8 Wrought Iron Wrought iron is the purest form of iron containing iron up to 99.8%. It does not fuse like cast iron at elevated temperatures and hence is not suitable for casting. It is prepared from pig iron by burning out carbon, silicon, manganese, phosphorus and sulphur in a puddling furnace. These impurities are there in pig iron up to about 6% or more which, in the wrought iron, are brought down to about 1%. Carbon is reduced to about 0.2% or even less. Small amount of slag is incorporated while making wrought iron, which is uniformly distributed in wrought iron giving fibrous structure to it. Owing to the lack of carbon content, it cannot be hardened by heating and then quenching in water. It is very ductile and malleable and hence suitable for wire drawing and sheet making. The contents of phosphorus and sulphur in wrought iron need to be controlled to avoid defects like cold shortness (brittle, unsuitable for cold-working) and red shortness (brittle, unsuitable for hot-working). The maximum use of this metal is in forged articles. A typical composition of wrought iron may have: C—0.02% Si—0.12% S—0.018% P—0.02% Slag—0.07% The remaining is iron. The molten metal, free from impurities, is removed from puddling furnace as a pasty mass of iron and slag. Balls of this pasty mass, each 45 to 65 kg, are formed which are later mechanically worked to squeeze out the slag and to form it into commercial forms of plates, billets, bars, etc. However, with the increased use of mild steel, wrought iron is getting obsolete because its manufacturing is costly. Wrought iron is weak and soft but has very high toughness. The value of wrought iron is in its corrosion resistance and ductility. It can be rolled, drawn, forged and welded. When high toughness is needed or metal is to be bent cold or hot, wrought iron works well. Properties and uses of wrought iron ●

●

● ● ●

●

Wrought iron is very soft, has very good machinability, is highly ductile and has very high toughness. Since carbon contents are low, melting point of wrought iron is very high, which makes it unsuitable for casting. Ultimate tensile strength is 25–50 kg/mm2. It resists corrosion. It can be obtained in the form of plates, sheet, forged billets, bars and piping. It can be easily forged and welded. It is used for making pipes due to superior corrosion resistance and better welding and threading qualities.

It is used for making bars for stay bolts, engine bolts, railway couplings, rivets, water and steam pipes (being corrosion resistant). It is also used for making crane hooks, chains due to good weldability and high impact strength. It takes on and holds well protective metallic coatings and paints. Besides, it is used for general forging applications.

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3.7

89

MECHANICAL AND FABRICATION PROPERTIES OF CAST IRONS

Mechanical properties ● ● ●

● ● ●

Tensile strength varies between 13 and 53 kg/mm2 (or even more). Impact strength is low and hardness varies from 150 to 600 HB. Compressive strength of general cast irons is high, and particularly in case of grey cast iron, it is 3 to 5 times the tensile strength. Damping capacity (ability to absorb vibration) is high. Machinability of grey cast iron decreases as the tensile strength increases. High alloy cast irons of chromium, nickel and silicon type are especially resistant to acid corrosion.

Typical stress–strain relationship for principal cast irons is shown in Fig. 3.1. Since there is no definite yield point of ferritic cast irons, the value of yield strength is taken at a point of 0.2% elongation. Also, in certain cases, the tensile strength and hardness depend on the size of the section of the casing in view of cooling rate and its influence on the microstructure and consequently on strength and hardness of the cast iron.

Fig. 3.1 Stress–strain relationship of principal cast irons in tension.

Machinability ● ● ●

Ductile cast irons have very high machinability. White cast iron is very difficult to machine. Grey cast iron has good machinability.

Weldability

Most cast irons have low weldability. However, these can be generally welded by arc or gas welding. Pre-heating and post-heating may be sometimes needed.

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Corrosion resistance

Although cast irons are not resistant to rusting, the rust formation is, however, slower in comparison to steel. Cast irons with high silicon and high chromium are quite resistant to acids but have poor resistance to alkalies. High temperature application

General cast irons (particularly grey cast iron) can stand well up to 425oC. When heated above this temperature, grain growth, brittleness and distortion are caused. Oxidation and too much scaling take place if heated above 580oC. For higher temperature usage, cast iron should have lower carbon, lower silicon but more chromium. BIS Code IS: 3355–1974 covers grey cast iron castings for elevated temperatures. A typical cast iron can stand temperature up to 1000oC when its alloying elements are silicon, molybdenum, nickel, copper and vanadium.

3.8

HEAT TREATMENT OF CAST IRONS

Cast irons can be heat-treated to relieve stresses caused in them during solidification and cooling. It is done by heating the castings to about 450oC for couple of hours and then cooling slowly in furnace. The process is known as stress-relieving. Cast iron castings are sometimes annealed which softens the casting to facilitate machining. It involves heating the casting from 760°C to 825°C and maintaining this temperature for sometime and later cooling the casting slowly. Hardening of some special alloy cast iron castings is done to get high hardness at surface (through nitriding). Sometimes, full casting is hardened and later tempered.

3.9

INDIAN STANDARDS FOR CAST IRONS

Indian standards are available for various grades of cast irons, for example, IS: 210–1970 (Table 3.2) and IS: 3355–1974. The IS: 3355–1974 (first revision) covers grey cast iron castings for elevated temperatures, for example, the grey cast iron, grade-2, type E is recommended to stand temperature of up to 1000oC. It has carbon (3.5 to 4%), silicon (1.0 to 2.0%) along with other alloying elements, namely, molybdenum, nickel, copper and vanadium. TABLE 3.2

Classification or grade FG FG FG FG FG FG

15 20 25 30 35 40

General purpose grey cast iron castings (Based on IS: 210–1970)

Ultimate tensile minimum strength (kg/mm2)

Brinell hardness number (HB)

Application

15 20 25

149–197 179–223 197–241

Pulleys, sheaves, general duty Piston, cylinder, guide beds Piston rings (heat-treated), guide beds

30 to 40

207–241

Heavy duty guide beds (heat-treated)

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Good quality and strong components are made from cast irons grade FG 30 to FG 40. Ordinary cast iron can stand up to 400oC but when alloyed with chromium (up to 2.0%) and molybdenum (up to 0.5%), it can stand up to 700oC. For still higher temperature service, better grades of cast irons are available [IS: 3355–1974 (first revision)]. Various grades of malleable cast iron castings are given in Table 3.3. TABLE 3.3 Malleable cast iron castings (Based on IS Codes—IS: 2108–1962; IS: 2640–1964; IS: 2107–1962)

Code for designation

Grades

Minimum tensile strength (kg/mm2)

Brinell hardness (HB)

BM 35 BM 30 PM 70 PM 45 WM 42 WM 35

A C A E A B

35 30 70 45 42 35

149 163 241 to 285 149 to 201 217 217

BM—Black-heart malleable cast iron PM—Pearlitic malleable cast iron WM—White-heart malleable cast iron

3.10

ABRASION RESISTANT IRON CASTINGS

Abrasion resistant iron castings are basically white cast iron castings of different grades. Since these comprise alloying elements, these can be called ‘special service alloy cast irons’ also. Some typical types of abrasion resistant castings are given in Table 3.4. TABLE 3.4

Composition (%)

Total carbon Graphite carbon (max.) Silicon Manganese Nickel Chromium Molybdenum Sulphur Phosphorus

Abrasion resistant iron castings

Type-I Ni-Cr martensitic white iron

Type-II Cr-Mo white iron

Type-III high chromium white iron

3.0–3.6 0.10 0.3–0.8 0.3–0.8 3.3–5.0 1.4–2.5 0.75 (max.) 0.15 0.30

3.1–3.6 — 0.5–0.8 0.4–0.9 0.5 (max.) 14–18 2.5–3.5 0.06 0.10

2.3–1.0 — 0.2–1.5 1.5 (max.) 1.2 (max.) 24–28 0.6 (max.) 0.06 0.10

Abrasion resistant iron castings have hardness from 500 to 600 HB. These can be annealed to improve their machinability and are used for mining equipment, earthmoving machinery and other industrial applications where wear resistant castings are needed.

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3.11

STEEL vs CAST IRON

Steel is an alloy of iron and carbon with varying quantities of phosphorus and sulphur. Manganese is added to this alloy to work as a deoxidizer to minimize traces of oxygen. Silicon is also added to steel. Both manganese and silicon help in neutralizing sulphur, which is detrimental to steel. Carbon is present in steel in the chemically combined form (iron carbide or cementite). Steel differs from cast iron on two major accounts: (i) Amount of carbon present and (ii) Form in which carbon is present in the metal. In steel, the maximum amount of carbon that can be there is up to 2.0%, which is always present in the combined form, i.e. iron carbide (also known as cementite). Cast irons have relatively high carbon content. They overlap with steels somewhat because the carbon range in cast irons vary from 1.5 to 5%. Carbon in cast iron can be present in two forms: (i) Chemically combined form (i.e. iron carbide) or (ii) Free carbon or graphite form. Steel never contains graphite or free carbon. Carbon in steel exists in a very small quantity in the form of ferrite (solid solution of up to 0.025% carbon in solvent a-iron) and majority in the form of cementite or iron carbide. Higher the percentage of carbon, more would be cementite and hence harder and tougher will be the steel. Actually, as much as 6.0% carbon can be dissolved in molten iron, but only up to 2.0% carbon can remain in solution in the solidified alloy (as in case of steels) and the rest of carbon appears as free carbon or graphite (as in cast irons).

3.11.1

Composition of Steel

Steel has the following constituents. Carbon is the most important alloying element in steels and has great effect on its mechanical and physical properties. Steels normally contain 0.1 to 2.0% carbon. Steels with 0.1 to 0.8% carbon are used for general engineering purposes and with 0.9 to 1.6% carbon, used for higher strength components, tool and dies and wear resistance purposes. Iron content in most steels may be over 90%. Usually, alloys having 50% or more iron are categorized as steel and those below 50% are categorized as non-ferrous alloys. Manganese, phosphorus, silicon and sulphur are also present in varying quantities. Sulphur and phosphorus in steel come directly from pig iron. Manganese and silicon are added to steel during its production. Their proper adjustment along with varying carbon contents gives birth to a wide range of properties to steel. There are alloying elements also present in steel which are used to give steel some special properties. These include: nickel, chromium, molybdenum, vanadium, tungsten, copper, cobalt, etc.

3.12

CLASSIFICATION OF STEELS

Steels are classified in many ways based on different criteria as discussed in the following: (a) Carbon content: Based on the amount of carbon present, steels may be classified as dead mild steel, mild steel, medium carbon steel and high carbon steel. (b) Microstructure: Steel is sometimes categorized on the basis of its microstructure, for example, austenitic steel, pearlitic steel, martensitic steel, etc.

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(c) Alloying element: Based on the number and type of alloying elements, there are different categories of alloy steels such as low-alloy steels or high-alloy steels based on the total amount of alloying elements present. Steels are also categorized according to the principal alloying element, for example, nickel steel, chromium steel and manganese steel. According to the application, steel may be categorized as structural steel, tool steel, stainless steel or special purpose steel. (d) Method of steel production: Steels are named as electric furnace steel or open-hearth steel, based on the method of production. (e) Degree of deoxidation: Depending on the degree of deoxidation of steel during its production, steel may be categorized as killed steel, semi-killed steel, rimmed steel and capped steel. (f) Based on mechanical working of steel during production: Steel passes through various stages during its manufacture, from pig iron stage to the commercial form of steel. It passes through hot- and cold-working. Hence, steel is sometimes called wrought metal (hot-rolled) or cold-rolled. (g) Commercial forms of steel: There are numerous forms or names under which steel is available in the market, for example, mild steel, structural steel, machine or free cutting steel or cast steel, tool steel, forged steel, spring steel, stainless steel, heat-resisting steel, magnetic steel, high speed steel or cutting alloys. Details of different types of steels, their composition, properties and engineering applications have been discussed in the following.

3.13

CARBON STEELS (OR PLAIN CARBON STEELS)

Carbon steel is an alloy of iron (over 90%) and carbon, besides varying proportion of phosphorus and sulphur. Manganese, a deoxidizer, is added to this alloy to neutralize the sulphur which is detrimental to toughness and bending strength of steel. The limiting content of manganese in carbon steels is 1.65%. Both silicon and manganese help in deoxidizing sulphur. The content of silicon and copper in carbon steels is limited to 0.60% in each case.

3.13.1

Composition of Carbon Steels

Carbon steels comprise the following elements. Carbon may vary from 0.05 to 1.6%. Tensile strength of steel increases as the carbon content goes up to 0.83% and drops quickly beyond this. Among the properties most influenced by the carbon contents are: hardness, tensile strength, yield strength, impact strength, reduction of area and elongation. Tensile strength, yield strength and hardness increase with the carbon content, whereas impact strength, reduction in area and elongation are reduced (see Fig. 3.3). Ductility and weldabiity decrease with increase in the carbon content. Manganese contents in carbon steels vary from 0.2 to 0.8%. It acts as a deoxidizer for neutralizing the sulphur present in the metal. Both tensile strength and hardness increase with increase in manganese.

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Phosphorus contents vary from 0.005 to 0.12% (and maximum limited to 0.4%). It imparts tensile strength and hardness. Silicon varies from 0.1 to 0.35% and is the principal deoxidizer used in steels. Silicon helps in increasing grain size, thus giving deep hardening properties and also adaptability to case-carburizing. Sulphur is harmful in one sense as it lowers the toughness and bending strength of steel but, at the same time, improves machinability by imparting brittleness to chips during machining. Its contents are limited to 0.055%. Copper up to 0.25% is added in steels for increasing resistance to corrosion.

3.13.2

Types of Carbon Steel

Carbon steels are categorized based on the percentage of carbon present in any steel. Dead mild steels—Carbon less than 0.15% Mild steel—Carbon 0.15 to 0.3% Medium carbon steel—Carbon 0.3 to 0.8% High carbon steel—Carbon 0.8 to 1.6% High carbon tool steel: Plain carbon steels with carbon percentage 0.8 to 1.6 are used for making hand tools and general purpose machining tools for operating at low cutting speeds (about 12 m/min) because the tool looses its hardness at temperature above 200°C. It is with this reason that plain high carbon steel tools are not used for mass productions jobs. These tools are, however, cheaper, easy to forge and heat-treat. High carbon medium alloy steel: It is made by alloying plain high carbon steel with small amount of tungsten, vanadium, chromium, molybdenum, etc. Tools made of this steel show better hot hardness, wear resistance and impact strength. Tools can safely stand up to a temperature of 350°C using higher cutting speeds. Various plain carbon steels, their main characteristics and applications are discussed in Table 3.5.

3.14

ROLE OF CARBON IN STEELS

Carbon is the most important constituent of steel because its amount and the consequential microstructure decide many properties of steel like strength, hardness and the response to heat treatment. In steel, iron and carbon combine together to form iron carbide (Fe3C, known as cementite), and the amount of cementite present directly depends on the carbon contents in steel. Cementite consists of 6.67% carbon and is a hard and brittle structure. Steel, with very little carbon, has microstructure of uniform grains of ferrite, which is pure bcc iron with very small carbon in solution and is very soft and ductile. Effects of the carbon content on steel properties are described below in reference to Fig. 3.2 and Fig. 3.3. Ductility

As mentioned above, ductility of steel goes on reducing as the carbon contents are increased, for example, at 0.2% carbon contents, elongation in tensile test is about 32%, at 0.8% carbon,

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elongation is about 10%, at 1.2% carbon, elongation comes down to 2% only, and at 1.5% carbon, elongation is zero. The presence of manganese further reduces the percentage of elongation. Strength, hardness and machinability

Variation in the above properties with change in carbon contents in steel is demonstrated in Fig. 3.2 and Fig. 3.3.

Fig. 3.2

Illustrating the variations in average mechanical properties of rolled 25 mm dia bars of plain carbon steels, as a function of carbon content in steel.

Microstructure and related properties

Steel with zero (or very little) carbon has ferrite grain structure with low strength of about 25 kg/mm2 (Fig. 3.3). As the carbon contents increase, the microstructure has ferrite and pearlite

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Fig. 3.3

With increase in carbon contents in plain carbon steels, variation in tensile strength is shown at (a) and in microstructure, at (b). As carbon content increases from zero to higher values, steel structure changes from ferrite to pearlite and at carbon content 0.8%, entire structure of steel is pearlite only, as shown at (b). With further increase in carbon content beyond 0.8%, pearlite structure is gradually reduced to a combination of free cementite (iron carbide) and pearlite. The pearlite is a mechanical mixture of alternate layers of ferrite and cementite (iron carbide). The tensile strength in steel is due to only the pearlite structure. Above 0.8% carbon, higher the carbon contents, higher will be the proportion of free cementite with a simultaneous reduction in pearlite. Beyond carbon content 0.8%, tensile strength may reduce a little but the hardness goes on increasing as the proportion of cementite increases in the microstructure.

such that the pearlitic content goes on increasing as carbon percentage increases until at 0.8% carbon; the total structure of steel is pearlite only. The ultimate tensile strength is maximum (about 90 kg/mm2). However, beyond 0.8% to up to 1.2% carbon, steel has a pearlitic structure but free cementite also starts forming along grain boundaries. Cementite is hard and brittle and causes reduction in tensile strength due to localized brittleness. Hence, beyond 0.8 to 1.2% carbon, there is no increase in tensile strength (rather little reduction may be there), but there is increase in hardness. It may be emphasized that strength in steel is due to pearlite, which is a laminated structure or mechanical mixture consisting of alternate layers of ferrite and cementite. In the end, it may be repeated that both tensile strength and yield strength increase to maximum value only up to 0.8% carbon. There is no further increase in these two properties. However, hardness goes on increasing with increase in carbon. On the other hand, impact strength, elongation and reduction in area and hence the ductility as also weldability reduce with increase in carbon contents in general.

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Different types of carbon steels with their properties and applications are given in Table 3.5. TABLE 3.5

Different types of carbon steels with their properties and applications

Carbon (%)

Ultimate tensile strength (kg/mm2)

Hardness (HB)

Less than 0.15

—

—

Low carbon steels or mild steel

0.15 to 0.3

38 to 50

130 to 160

Good toughness, low strength and good weldability, good machinability, with 0.18% carbon, good impact resistance at subzero temperature

Lightly stressed machine parts, structural section (angles, channels, I-section), bridge components, small forgings, boiler plates, ship hub, rods, tubes, castings, stampings. These are casehardening steels

Medium carbon steel

0.3 to 0.8

40 to 80

160 to 250

Improved toughness and high tensile strength, higher hardness, good bending and torsion strength, good machinability and deep hardening property (up to C— 0.45%)

Heat-treated machine parts (gears, links, shafts etc.), loco tyres, torque tubes, drop forgings, agricultural implements, bright drawn bars, automobile parts, wire ropes, sprockets, clutch plate, large forging, dies, springs, strong steel castings

High carbon steel

0.8 to 1.0

90–100

250 to 280

More hardness and larger torque capacity, higher tensile and yield strength, reduced impact strength

Railway rails, drills, circular saws, punches, dies, chisels, lockpin, clutch disc, leaf spring, balls, keys, pins, harrows, coil spring, drills, taps, cold chisel

Name of steel

Dead steel

mild

Outstanding properties

Applications

Highly ductile, less Solid drawn tubes, strength welded tubes, thin sheets, wire rods with high surface finish, deep drawing objects, oil pans

(Contd.)

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TABLE 3.5

3.15

Different types of carbon steels with their properties and applications (Contd.)

1.0 to 1.4

About 100

280 to 330

—

Lathe tools, milling cutters, hot sets, dies, files, drills, razors, ball races and ball bearings, knives

1.5 to 1.6

About 100

350 and higher

—

Lathe and planner tools, scrapers, steel cutting saws, files, wire drawing dies

COMPARISON OF CAST IRON, MILD STEEL AND HIGH CARBON STEEL

The comparison of properties and uses of cast irons, mild steels and high carbon steels in general is given in Table 3.6. TABLE 3.6

S. No.

Properties and uses of cast iron, mild steel and high carbon steel

Cast iron

Mild steel Carbon 0.15 to 0.3%

High carbon steel (or up to 2%)

1.

Carbon 1.5 to 5%

Carbon 0.8 to 1.6%

2.

Crystalline granular structure Bright fibrous structure

Fine granular structure

3.

Hard and brittle and wear Tough and elastic resistance is outstanding in sliding friction

Tough and more elastic

4.

Can be hardened by sudden Can be easily case-hardened Can be hardened and tempered cooling during casting but cannot be tempered

5.

Cannot be forged

Can be easily forged and Can be easily forged but welded difficult to weld

6.

Cannot be magnetized

Can be magnetized

Can be magnetized

7.

Does not rust easily

Rusts quickly

Rusts quickly

8.

Neither malleable nor ductile

Malleable and ductile

Brittle and less ductile than mild steel

9.

Does not absorb shocks

Absorbs shocks easily

Absorbs shocks

10.

Melting point is 1135 to 1250oC

Melting point about 1500oC

Melting point 1400 to 1450oC

permanently

(Contd.)

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Properties and uses of cast iron, mild steel and high carbon steel (Contd.)

11.

Low tensile strength but Tensile strength better than Both tensile and compressive compressive strength is 3 to compressive strength strength better than cast iron 4 times of tensile strength and mild steel

12.

Used for making columns, bed plates, and brackets of machines not subjected to heavy shocks and tension

Used for high strength shafts, cutting tools, springs, forging dies, press dies, milling cutters, files, razors, drawing dies, lathe tools, taps and drills

It is not used for structural purposes in thin economical sections. It has better vibrational damping capacity (loss of vibrational amplitude) than steels

The vibrational damping The vibrational damping capacity is less than cast iron capacity is poorer than even mild steel

3.16

Used for all kinds of structural work in bridges, buildings, trusses, frames, girders, channels, angles, rounds, bolts, rivets, wire Also used for making water ropes and sheets pipe, sewers, lamp posts and carriage wheels for better corrosion resistance

ALLOY STEELS

In addition to iron and carbon, all plain carbon steels generally contain other alloying elements, namely, manganese, silicon and phosphorus in varying proportions. It was mentioned earlier that these three elements have limiting percentage in plain carbon steel. A steel is called alloy steel on two accounts: (i) When presence of one or more elements such as manganese, silicon and copper exceeds 1.65%, 0.6% and 0.6%, respectively, or (ii) When steel contains additional alloying elements such as nickel, chromium, molybdenum, tungsten, vanadium, etc. Alloy steels have properties different from plain carbon steels on account of these alloying elements.

3.16.1

Purpose of Alloying Steels

Alloying of steel is done for one or more of the following reasons: ● ● ● ● ●

●

To promote fine grain size, thus improving tensile strength and toughness. To improve machinability, workability and weldability. To increase hardenability and improve case-hardening properties. To increase corrosion resistance. To have resistance to scaling and oxidation at higher temperatures, i.e. to improve heat-resisting properties. To improve ability of steel to have resistance to softening, i.e. to maintain strength, shape and sharpness at elevated temperatures as required in case of cutting tools for lathe or other machine tools.

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Effects of Alloying Elements on the Properties of Steel

Table 3.7 explains the effects of alloying elements on the properties of steel. TABLE 3.7

Name of alloying element

Alloying elements and their effects on the properties of steel

Important qualities imparted

Comments

Nickel (Ni)

Provides toughness, impact strength, tensile strength, ductility, corrosion resistance and deep hardening. Also imparts hardness and fatigue resistance, good creep resistance at high temperature

Nickel is added to low and medium carbon steel as it is easily dissolved by ferrite than cementite. Nickel quantity is kept up to 3% for low carbon steels to make them tougher. Nickel contents more than 5% make steel brittle and hard

Chromium (Cr)

Improves corrosion resistance, toughness and hardenability due to formation of chromium carbides, improves resistance to abrasion, improves cutting ability and heat resistance

Increased hardness (to deeper depth) and toughness is the combined effect of chromium. Chromium up to 1.5% improves tensile strength, Cr 12% makes steel incorrodable, chromium can go up to 18% in steel

Manganese (Mn)

Deoxidizes, improves strength and hardness, and tougness, higher strength at elevated temperature, decreases weldability

Mn 1.0 to 1.5% increases strength and toughness; Mn (5% max.) gives hardness and wear resistance, but with carbon 0.8 to 1.5% and manganese (11 to 14%) steel is hard, tough and strong, after heat treatment

Silicon (Si)

Deoxidizes, gives resistance to high temperature oxidation, improves elastic limit, provides hardness and high magnetic permeability

Silicon varies usually up to 2%. Used for springs due to high elastic limit. Silicon up to 2.5% increases strength without decreasing ductility

Molybdenum (Mo)

Promotes hardenability, increases Molybdenum is important hardener tensile strength and creep strength of steel. Up to 0.2%, it improves at high temperature, increases wear fatigue properties resistance, heat and corrosion resistance and ability to deep harden

Vanadium (V)

Deoxidizes, promotes fine-grained structure, gives strength, toughness and improved hardening quality and hardness at elevated temperatures

Vanadium up to 0.2% with low and medium carbon steel increases tensile strength, elastic limit and fine-grained structure, increases endurance limit (Contd.)

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TABLE 3.7

101

Alloying elements and their effects on the properties of steel (Contd.)

Tungsten (W)

Increases strength, hardness, toughness, wear resistance, shock resistance to softening at higher temperature

Added to steel with chromium and manganese. Tungsten 3 to 10% makes steel hard, tough, wear and shock resistant. Higher contents of tungsten increases red hardness (or cutting hardness) at higher temperature

Cobalt (Co)

Improves hardness, toughness, Cobalt is a grain refiner. Cobalt may tensile strength, thermal resistance go up to 14%. With 35% cobalt, steel is highly magnetic. Steel with and magnetic properties 5% cobalt and properly hardened gives hard and tough cutting edge

Copper (Cu)

Improves strength and resistance to Its contents vary from 0.2 to 0.5% atmospheric corrosion

Important properties of steel and the alloying elements responsible for that are summarized in the following: ● ●

●

●

● ●

3.17

Higher tensile strength is due to nickel, molybdenum and cobalt. High hardness and abrasion resistance is due to chromium, manganese, molybdenum, tungsten, cobalt and vanadium. High hardness and heat resistance at elevated temperature is due to molybdenum, chromium, vanadium and tungsten (by forming carbides insoluble in ferrite). Higher strength at elevated temperature is due to manganese, nickel, silicon, chromium, molybdenum and tungsten. Corrosion resistance is due to nickel, chromium and copper. Improved machinability is due to sulphur, phosphorus and lead (as they form inclusions insoluble in ferrite).

CLASSIFICATION OF ALLOY STEEL

The classification of alloy steels into different categories is done on the basis of one or more than one consideration given below: ●

●

● ●

According to the application of steel, for example, tool steel, cutting tool steel, structural steel, stainless steel, heat-resisting steel and other special purpose steels. According to the principal alloying elements that are mainly responsible for imparting special properties to steel, such as nickel steel, chromium steel, manganese steel, etc. According to the internal structure like martensitic steels and pearlitic steels. Low, medium and high alloy steels are categorized based on total contents of the alloying elements. Low alloy steels contain total alloying elements up to 5%, medium alloy steels between 5 and 10% and high alloy steels more than 10%.

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High strength low-alloy steels (HSLA) have improved strength to weight ratio and are available mostly in sheet form. These are also available in plate, bar and structural shapes. These are characterized by low carbon contents (less than 0.3%) and a microstructure of fine-grained ferrite phase and a hard phase of martensite and austenite. These are used for light structure such as automobile body, transportation equipment as also mining and agriculture machines. HSLA plates and structural sections are used in ship, bridges and building construction. The main alloying element in these steels is niobium (also called columbium) which gives fine grains, high strength and impact toughness. Other alloying elements used include chromium, copper, molybdenum, nickel, vanadium, etc.

Other alloy steels are discussed in the following along with their uses.

3.17.1

Nickel Steels

Nickel steels have nickel up to 3.0% and carbon 0.2% to 0.35%. These have high tensile and fatigue strength and are used for ship parts and components subjected to shocks and fatigue, piston rods, locomotive forgings and axles. With nickel varying from 3.25 to 3.75% and carbon 0.50%, steel is used for making connecting rods, ball bearing races, valve seats and cylinder liners in automobile engines, drive shafts and heavy splined power transmission shafts. High nickel contents (24% or more) make steel highly non-corrodable and with least coefficient of expansion. Such steel is used for long span bridges. Invar (Ni 36% and iron 64%) and superinvar (Ni 31%, Co 5% and iron 64%) are used for making measuring instruments and surveyor taps and clock pendulum. All alloys having nickel 20.0 to 30.0% are non-magnetic and are used in electric machines.

3.17.2

Nickel–Chrome Steels

When chromium (up to 2.0%) is added to steel, it increases strength and hardness considerably but reduces ductility slightly. But with a combination of nickel–chrome, increase in strength and hardness can be obtained without losing ductility. One important example of nickel–chrome steel is austenite stainless steel having nickel (8.0%), chromium (up to 18.0%) and carbon (up to 0.3%). These are highly resistant to many acids and have high resistance to corrosion.

3.17.3

Manganese Steels

Manganese is one of the basic alloying elements of steel and is next only to carbon in frequency of use. Manganese steels are hard and have high abrasion resistance. These are used where high wear or abrasion is involved, besides strength and toughness requirement, as in case of cranks, connecting rods, agricultural machinery components, railway equipment, etc. One very common type of manganese steel contains carbon 0.15% and manganese less than 2%. This metal is very hard and less ductile. Manganese contributes largely to strength and hardness and little less to ductility. It also increases the depth of hardness. Effectiveness of manganese is dependent on carbon contents. For example, steels having carbon 1 to 1.4% and manganese 10 to 16% are very tough and surface-harden under repeated impacts and hence are used for railway-points,

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stone crushing rolls and dredger bucket. It is easily forged but cannot be machined. It is finished to shape by grinding.

3.17.4

Tool and Die Steels

Tool and die steels are special alloy steels designed for high strength, impact toughness and wear resistance at room and elevated temperature. They are used for (a) making cutting tools for machining metals (as tools for lathe, shaper, etc.) where plain high carbon steels do not work well, and (b) making hot and cold dies and punches and shears for various operations like forming, shearing, rolling, etc. They generally consist of high-speed steels (molybdenum and tungsten type), hot and cold work steels and shock-resisting steels. Alloy tool steels for making dies and punches

Alloy tool steels are of following general category. (i) Water-hardening tool steels are high carbon steels (C—0.70 to 1.30%) having vanadium (0.25%) also. These are tough, low cost and have good machinability. These are used for twist drills, files, shear knives, hammers, chisels and forging dies. (ii) Shock-resisting tool steels have combinations of silicon-molybdenum or siliconmanganese or chromium-tungsten. These have outstanding toughness and wear resistance and can be hardened and oil quenched. (iii) Cold-work tool steels are both oil and air hardening types; air hardening types are used for larger dies and tools. These can be easily machined. Hardness may be 55 to 65 HRC after heat treatment. (iv) Oil-hardening die steels are easy to machine and harden and are used for medium life tools and cold-working dies. (v) Hot-work tool steels are either tungsten based or chromium based and have fair machinability and wear resistance. These are used for blanking, forming, extrusion, hot-blanking dies, hot-punching dies and can stand working temperature up to 540°C. Hardness 30 to 60 HRC is obtained by heat treatment. Some typical tool and die steels for hot-work and cold-work are given here. Tool and die steels for hot-work: These steels contain chromium, molybdenum, vanadium and tungsten as main alloying elements. A typical composition is given below (element contents are in percentage): C—0.30 to 0.40 Si—0.80 to 1.20 Mn—0.25 to 0.50 Cr—4.75 to 5.25 Mo—1.20 to 1.60 V—0.20 to 0.40 W—1.20 to 1.6 These steels are used for hot extrusion dies, forging die inserts, hot shear blades, die-casting dies for copper and hot swaging die.

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Tool and die steels for cold-work: These steels include plain carbon and alloy steels capable of being hardened and tempered. These are used for making tools and dies for blanking, trimming, shearing and shaping. A typical composition (alloying elements in percentage) is given below: C—0.90 to 1.20 Si—0.10 to 0.35 Mn—0.25 to 0.50 Cr—0.40 to 0.80 Mo—0.25 max. V—0.20 to 0.30 W—1.25 to 1.75 Cutting tool materials

Cutting tool materials are used for making cutting tools for machining metals on lathe, shaper, milling machine, grinder, etc. Special characteristics and varieties of cutting tool materials have been discussed under Sections 3.17.8, 3.18 and 3.19.

3.17.5

Special Alloy Steels

Special alloy steels are the steels which have been developed with special qualities to serve specific purposes in their applications, for example, stainless steel, high speed steel, cutting alloys, heat-resisting steels, etc.

3.17.6

Stainless Steels

Stainless steels resist corrosion. A little amount of chromium (4.0 to 6.0%) in low carbon steels makes the steel resistant to general corrosion. If higher resistance to corrosion is needed, the chromium added is more than 12.0%. Hence, the steels containing chromium more than 12% are called stainless steels. These steels are slightly oxidizable as chromium forms on the steel surface a thin film of chromium oxide without affecting the colour and beauty of the basic material. This oxide film, in fact, protects the steel from further corrosion or oxidation. For temperature range from –234°C to above 980oC, this steel shows strength, toughness and corrosion resistance much superior to other metals. Stainless steels fall in the following categories. Ferritic stainless steels

Ferritic stainless steels are produced by modifying low carbon steels with addition of only the chromium, 18 to 30%. Sometimes, manganese (1.0 to 1.5%) and silicon (up to 1.0%) are also added. These have a stable ferritic structure as chromium is an effective ferrite stabilizer. This group can be hardened to some extent by cold-working but not by heat treatment. They possess bcc crystal structure and hence they are less ductile and less formable. They, however, weld well. Addition of silicon (up to 3%) makes them good heat resistant. These steels cannot be hot-worked (forged). Their use is restricted to a narrow range of corrosive conditions than the austenitic stainless steels, but they are good in strongly oxidizing conditions. These are used for

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105

non-structural applications such as dairy equipment, heat exchangers, chemical plants, food processing plants and cutlery. Martensitic stainless steels

Martensitic stainless steels have chromium between 12 and 17% and usually no nickel but have higher carbon content (up to 1.2%). The carbon dissolves in austenite and after quenching gives martensitic structure to steel. These can be hardened by heat treatment. These have high strength, hardness and fatigue resistance and good ductility and are used for valves, bolts, nuts and springs. It is magnetic. Martensitic stainless steels have moderate corrosive resistance and are used only under mild conditions such as weak acids, fresh water and the atmosphere. Austenitic stainless steels

The austenitic group is often referred as chromium-nickel alloy containing chromium 16 to 26% and nickel 6 to 22%. A composition having chromium 18% and nickel 8% is known as 18-8 stainless steel. The 8% nickel is sufficient to stabilize the high temperature, austenitic fcc structure such that upon cooling to room temperature, it remains same, i.e. austenitic. It is a high temperature strength alloy. Many grades of this steel are available. Generally, these steels are hardened by cold-working only and are non-magnetic, have excellent corrosion resistance but are susceptible to stress corrosion cracking. The austenitic stainless steels are more ductile than ferritic and martensitic stainless steels. They have better fabrication characteristics. It is the most popular group (and costliest too). Carbon contents in these steels may be 0.03 to 0.25%, nickel 3.5 to 22%, manganese 2% and silicon 1 to 2% besides chromium, 16 to 26%. Austenitic stainless steels are highly corrosion resistant and the best among the three types of stainless steels mentioned before. They have excellent formability, weldability and are used in utensils, chemical industry, aircraft industry, welded construction, furnace parts and many other usages, where deep drawn products or containers are needed. Stainless steel 18-8, type 304 (AISI Grade) is used for handling and storage of liquid helium, hydrogen, nitrogen and oxygen that exist at cryogenic temperatures. Precipitation–hardening (PH) stainless steels

Precipitation–hardening stainless steels contain chromium, nickel, copper, aluminium, titanium or molybdenum. They have good corrosion resistance and ductility along with high strength at elevated temperatures. These are used for structural components in aircraft and aerospace vehicles. Duplex structure stainless steels

Duplex structure stainless steels are mixtures of austenite and ferrite and have good strength and high resistance to corrosion in most environments, as also to stress-corrosion cracking. These are used mostly in water-treatment plants and heat-exchangers.

3.17.7

Corrosion Resistant Alloy Steels

The nickel alloy steels, nickel-chrome steels and stainless steels, already discussed, are all corrosion resistant alloy steels.

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3.17.8

High Speed Steels (HSS)

High speed steels are used for making cutters and tools for cutting and machining of metals. They retain their strength, form (shape and sharpness) and hardness (red hardness) at elevated temperatures (about 620°C) often encountered during cutting of metals. High carbon steel is used for making cutting tools for lathe and other machines but is suitable for lower cutting speeds only and thus metal removal rate is slow. Also, they are not able to cut harder and tougher materials. The high speed steel tools, on the other hand, can operate at cutting speeds two to three times higher than high carbon steel tools and retain their red hardness well up to 620oC. These are of different varieties. High speed steel (18-4-1 type) or tungsten high speed steel

The most popular high speed steel (HSS) is 18-4-1 high speed steel which has tungsten 18%, chromium 4% and vanadium 1%. It has carbon 0.7% and has a good combination of red hardness and shock and wear resistance. It is mostly used for cutting tools used on lathe, milling machine, shaper, slotter, broaches and drill bits. Cobalt high speed steel

Cobalt high speed steel is known as super high speed steel. It has improved red hardness and wear resistance. It contains cobalt 2–15% to increase hot hardness and wear resistance. A typical composition is: cobalt (12%), tungsten (20%), chromium (4%), vanadium (2%) and carbon (0.8%). Vanadium high speed steel

In abrasive resistance, vanadium high speed steel is better than 18-4-1 high speed steel and is used for tools for machining hard and difficult to machine metals. Molybdenum high speed steel

Molybdenum high speed steel has molybdenum (up to 6%), chromium (4%), tungsten (up to 6%), vanadium (2%) and higher carbon contents. It is very tough and has improved cutting properties, which has made it a more popular metal cutting material these days.

3.18

CUTTING TOOL MATERIALS

The cutting tool materials are those materials which are used for making tools for cutting metals and other materials. The characteristics of an ideal cutting tool material are as follows: ●

Hot hardness: The ability to retain its hardness at elevated temperatures is called hot hardness. The tool material must remain harder than workpiece material at elevated temperatures encountered during metal cutting.

●

Wear resistance: The tool material must withstand excessive wear (even though the relative hardness of tool-workpiece material changes during metal cutting).

●

Toughness: The term implies a combination of strength and ductility. The tool material should be capable of withstanding shocks and vibrations without breaking.

●

Coefficient of friction: The frictional coefficient at the chip-tool interface should be low for minimum tool wear and good surface finish on the job.

FERROUS METALS—Irons and Steels ●

●

●

107

Thermal conductivity and specific heat: Thermal conductivity of tool material determines its ability to conduct heat while the specific heat determines its ability to absorb and contain heat. Higher values of both of these are desirable so that heat generated at the cutting edge is absorbed and conducted away quickly without increasing the temperature of the tool. If the tool is used in the form of a bit or tip brazed on tool shank, then the tensile strength, coefficient of thermal expansion, etc. of the bit material should be very close to the shank material to avoid cracking of the bit during use. Cost and fabrication: The tool material should not be very costly. Its fabrication should be easy. The maintenance cost should also be low.

The choice of a proper tool material depends on the type of service to which the tool will be put. No tool material has all superior characteristics. Each one has its own characteristics which limit its field of application. There is a large family of cutting tool materials which comprise high-carbon steels, high speed steels, cast alloys and ceramic tool materials. Ceramics are compounds of metallic and non-metallic elements. A great variety of ceramic tool materials are now available, for example, oxide ceramics (alumina and zirconia), carbides (tungsten, titanium and silicon), nitrides (cubic boron, titanium, silicon and cermets), diamond, etc. Ceramic tools are used for machining high strength and harder materials often used for special applications. Ceramics have been discussed in detail in Chapter 4. Principal tool materials are discussed here under the following categories: 1. High carbon steels 2. High alloy tool steels (or high speed steels) (a) 18-4-1 high speed steel (b) Cobalt high speed steel (c) Vanadium high speed steel (d) Molybdenum high speed steel 3. Cutting alloys (a) Cast alloys (b) Cemented carbides (c) Coated tools (d) Ceramics or oxide tool materials (e) Diamond High carbon tool steels and high speed steels have already been described under Sections 3.13.2 and 3.17.8, respectively. Other cutting alloys (or tool materials) are discussed in the following.

3.19

CUTTING ALLOYS

Cutting alloys are, in fact, the cutting materials developed as superior to high speed steel. These are necessarily not ferrous type (such as high speed steels), can withstand much higher

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temperature (up to 1100oC), and thus are more suitable for cutting metals at much higher cutting speeds removing larger volume of metal at faster rate. These are usually made in small bits which are either brazed or clamped on a tool shank made of high carbon steel.

3.19.1

Cast Alloys

Cast alloys comprise cobalt, chromium, tungsten and carbon and are obtainable only in the cast condition (hence the name) because they contain no iron. Cast alloys are better known by their trade names of stellite, crobalt, black alloy, etc., stellite being more common. Stellites are superior to high speed steels because of higher red hardness (about 60 HRC up to 1000°C) and more toughness at higher temperature. It has cobalt (38 to 53%), chromium (30 to 35%), tungsten and/or tantalum (12 to 17%) and carbon (2.25 to 2.75%). Stellite tool cannot be machined or forged and hence they are only cast to the required shape by powder metallurgy or normal casting and later finished by grinding. These are preferred for machining cast irons, alloy steels, carbon steels and non-ferrous alloys. Stellite can also be deposited on steel tool shanks by oxyacetylene flame or small ‘tips’ (or bits) of stellite brazed on steel tool shank. It is a very brittle material.

3.19.2

Cemented Carbides

Cemented carbides are used in the form of bits only, which are available in different sizes and shapes like triangular, square, diamond, etc. and are brazed or clamped on steel shanks. These can stand much higher temperature (up to 1200oC) and are used with cutting speeds much higher than those used for high speed steel. Besides cutting tools, cemented carbides are also used for making dies for wire drawing and other operations. Cemented carbides are non-ferrous materials comprising mainly the carbides of tungsten, titanium and tantalum with cobalt as binder. These are powdered metallurgy products and hence sometimes called sintered carbides also. Cemented carbides are first cast to shape and later sintered. Tungsten is powdered and milled with carbon (gas coke) and later cobalt is mixed to work as a binder. The mixture is then pressed and sintered at 1490oC. Because of various alloying elements (other than tungsten), cemented carbides are available in different varieties. There are two most common types of cemented carbides: (a) Tungsten carbides (WC) and (b) Titanium carbide (TiC). The tungsten carbide (also called cemented carbide) is a composite material having tungsten carbide particles bonded together in a cobalt matrix. Tungsten carbide (hardness, 1800–2400 HK) tools are used for cutting steels, cast irons and abrasive non-ferrous materials. Tungsten carbides are usually compounded with carbides of titanium and niobium to improve the properties. Titanium carbide (TiC) has higher wear resistance (hardness, 1800–3200 HK) than the tungsten carbide, but is not as tough. In a matrix of nickel-molybdenum, TiC is capable of machining hard steels and cast iron at higher cutting speeds than that used with tungsten carbide tools. Inserts of carbides, when brazed or clamped on a steel shank, form a very good cutting tool. Silicon carbide (SiC) is also there having good resistance to wear (hardness, 2100–3000 HK), thermal shock and corrosion and retains strength at elevated temperature. It is used for high temperature components in heat engines and also as an abrasive for grinding wheels. Synthetic silicon carbide is made from silica sand, coke, saw dust, and sodium chloride.

FERROUS METALS—Irons and Steels

3.19.3

109

Coated Tools

The availability of high strength and tough workpiece materials (which were generally abrasive and chemically reactive with general tool materials and hence posed problems in their machining) has led to the development of coated tools. These tools are used with much higher cutting speeds, and have low friction and high resistance to cracks and wears. Machining time is thus reduced considerably and with a much longer tool life. The tools may be made of high speed steel or cemented carbides on which coating materials are applied or deposited. The coating materials include: titanium nitride (TiN), titanium carbide (TiC), titanium carbonitride (TiCN), aluminium oxide (Al 2O 3), etc. These coatings, generally in the thickness range of 2–15 mm, are applied on base tool materials (HSS or cemented carbides) by chemical-vapour deposition (CVD) method or physical vapour deposition (PVD) method. For example, titanium nitride coatings (golden colour) are given on high speed steel tools and carbide tools, drill bits and cutters. Coating of polycrystalline diamond on tungsten carbide and silicon-nitride inserts make effective tools for machining non-ferrous materials and abrasive materials such as aluminium alloys having silicon, fibrereinforced and metal-matrix composite materials and graphite with at least ten times more tool life over other coated tools.

3.19.4

Oxide Ceramics Tools Material

Among the oxide ceramics tools, aluminium oxide (alumina) tools are more common. These are very hard, and have high compressive strength. These are available in bits and can operate at cutting speeds faster than those used even for carbide tools. There are two main types of oxide ceramics: (a) Alumina (Al2O3), which is also called corundum or emery. (b) Zirconia (ZrO2) is zirconium oxide. (a) Alumina-based oxide ceramics: The alumina (aluminium oxide) is the most commonly used oxide ceramics either in pure form as raw material or mixed with other oxides such as zirconium oxide or titanium carbide to improve toughness and resistance to thermal shocks. The pure alumina has high hardness (2000–3000 HK), but has moderate strength and toughness. The raw materials are cold pressed into inserts which are later sintered. Pure alumina based ceramic tools have very high abrasion resistance and hot hardness, only next to diamond and cubic boron nitride tools. They are chemically more stable than high speed steel tools and carbide tools and have low tendency to form built-up edge on the cutting tool and hence give very good surface finish on steels and cast irons. These tools, however, lack toughness. Cutting with these tools should be done either dry or with sufficient amount of coolant to avoid thermal shocks and fracture of the tool. (b) Zirconia-based oxide ceramics have high strength and toughness, thermal expansion close to cast iron. These are used for heat engine components.

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MANUFACTURING PROCESSES

Nitride Tool Materials

Nitride tool materials are of the following main types: (i) Cermets (or black ceramics) (ii) Cubic boron nitrides (iii) Silicon-nitride based ceramics Cermets (or black ceramics) are combinations of ceramic phases bonded with metallic phases such as the hot pressed ceramics containing typically aluminium oxide (70%) and titanium carbide (30%). Other cermets may have molybdenum carbide, niobium carbide, tantalum carbide, etc. The cermets combine high temperature oxidation resistance of ceramics with toughness, thermal shock resistance and ductility of metals. Cermets are, in fact, the composite materials made with varying combinations of ceramics and metals. Besides being used for cutting tools, there are other variety of cermets made of various oxides, carbides and nitrides, which are used for high temperature applications such as nozzles for jet engines and brakes for aircraft. Cubic boron nitride (CBN) is next to diamond in hardness (4000–5000 HK) and is used as abrasive cutting material. It is made by bonding 0.5 to 1 mm layers of polycrystalline boron nitride to a carbide substrate by sintering under pressure, wherein CBN gives high wear resistance and cutting edge strength, and carbide provides shock resistance. CBN coated cutting tool bits are available that are brazed on a high carbon steel tool shank (or body). Silicon-nitride based ceramics (SiN) are composed of silicon nitride, aluminium oxide, titanium carbide, etc. Tools made of this material are tough, have high hot hardness and good thermal shock resistance. Sialon is SiN based material used for machining cast irons and nickel-based superalloys (but not steels because of chemical affinity to iron).

3.19.6

Diamond Tools

Diamond is the hardest known material. Polycrystalline diamond (PCD) tools or compacts, which contain synthetic crystals or industrial diamonds, are used for wire-drawing dies for fine wires, grinding wheels and abrasive coatings. Diamond tools can be run at cutting speeds about 50 times greater than that for high speed steel tools and at temperatures up to 1650°C. Besides being hard, diamond is incompressible and has very low coefficient of friction. Diamonds are used for producing high surface finish on soft materials that are normally difficult to machine. Single point diamond tools have been used in machining non-ferrous alloys, ceramics, precintered carbides, graphite, fibre glass and rubber.

3.19.7 Abrasives Abrasive grains in various forms—loose, bonded into wheels or stone, embedded in papers or cloths—find wide applications. These are used for grinding harder materials and where a superior finish is required on hardened or unhardened materials, metallic or others. Abrasive particles held together by a bonding material comprise the cutting edges in grinding wheels. Abrasive is a class of mineral: (a) natural abrasives such as emery, corundum and diamond

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111

dust, and (b) artificial abrasives such as carborundum which is silicon carbide. Emery is rough and durable and has about 70% aluminium oxide. Corundum contains about 90% aluminium oxide. Diamond dust mixed with oil or grease is used for polishing.

3.20

HARD FACING ALLOYS

In many applications, metal parts have to work against abrasive atmosphere, for example, blades of bulldozer, earth boring tools, clay grinding wheels, concrete mixers and many such applications where high wear resistance is required. Increasing hardness of a surface does not serve the purpose in many situations, particularly where wear is fast. Hence, the problem can be overcome by putting a layer (or facing) of harder material on the machine components subjected to wear. The process is called hard facing, and the material cladded on wearing surface is called hard facing material. Hard facing can be done either by electric arc welding (as stellite welding rods are available) or by gas welding (brazing) and is usually employed on medium-carbon steels, as they have good strength and are more easily hard faced. Otherwise, mild steel, grey cast iron or white fast iron can also be hard faced. Main hard facing alloys include: (a) Tungsten carbide composites in the form of inserts, (b) High chromium irons used for facing agricultural machinery, (c) Cobalt-base alloys as stellite, (d) Martensitic irons, (e) Nickle-base alloys, (f) Austenitic stainless steel and (g) Austenitic manganese steel, which is tough and used for hard facing power-shovel dipper teeth, jaw crushers, rail road frogs and crossings and provides both high wear and impact resistance simultaneously.

3.21

LOW TEMPERATURE APPLICATION ALLOYS

Sub-zero temperatures (cryogenic temperatures) have a marked effect on the properties of most metals. As the temperature reduces (below 0oC), the hardness, yield strength, ultimate tensile strength and modulus of elasticity of the metal increase but ductility decreases. Ordinary stainless steels have low impact strength at sub-zero temperatures. Austenitic stainless steel 18-8 type and some other chromium-nickel steels retain ductility even at extremely low temperatures (– 40°C). Chromium steels (Cr –12%) are often used up to –25°C. Aluminium and its alloys are also used for low temperature application as they retain strength and hardness unaffected at sub-zero temperatures. Berellium copper, phosphor bronze and tungsten and molybdenum alloys show good fatigue strength at low temperature.

3.22

MAGNETIC ALLOY STEELS

Magnetic alloy steels are rich in cobalt and tungsten and also have chromium and nickel. These are used for making permanent magnets for magnetos, loudspeakers and electrical measuring instruments. Cobalt may vary from 15 to 40%, tungsten up to 10%, chromium 1.5 to 9% and carbon up to 1.0%.

112 3.23

MANUFACTURING PROCESSES

MARAGING STEELS

Maraging steels are special type of steels because of their behaviour and response to heat treatment. These are basically dead mild steels (carbon less than 0.15%) to which are added higher amount of nickel (up to 25%) and cobalt and molybdenum such that when these steels are cooled from 815°C, these should have martensite structure when air cooled. Mechanical properties like yield strength and elongation are improved by age-hardening at 480°C. These steels find application when extremely high strength and good toughness are required. Besides that, they respond well to hot- and cold-working and hence ensure good mechinability and weldability.

3.24

AUSTENITIC MANGANESE STEEL

Austenitic manganese steel is also called Hadfield’s manganese steel. It is non-magnetic. It contains carbon 1.0% and manganese 12.0%. It is an austenitic steel because manganese is soluble in alpha and gamma irons. As cast, austenitic manganese steel is partly martensitic and hence is hard and brittle, but by quenching from a temperature of 1040°C, a homogenous austenite is retained with high toughness, strength and ductility. This steel is relatively soft but work-hardens easily on the surface, that is, it possesses the property of increasing hardness as metal is worked. It is because of this property that this steel finds application in marking power shovel teeth, rolls of stone crushers, rail road switch frogs and rail road crossing points.

3.25

CLASSIFICATION OF STEELS BASED ON COMMERCIAL NAMES

Low carbon steels

Low carbon steels are used when moderate strength is needed along with enough plasticity. Steels with carbon 0.05 to 0.10% are used for sheets, tubing and wire nails. Metal sheets are made either by cold-rolling or by hot-rolling; hot rolled sheets are thinner and have better mechanical properties. With addition of vanadium or columbium, high strength hot-rolled and cold-rolled sheets are made for marking tubes for components of automobiles, furniture and refrigerator by welding. Structural steels

Structural steels are intended for general engineering purposes for making structures, frames, bridges, industrial building trusses, transportation equipment and other components subjected to static and dynamic loadings. These can be divided as (i) Carbon steels and (ii) High strength low alloy steels. (i) Carbon steels: Most of the structural steels being used for general purpose are plain carbon steels. These are covered under IS: 1977–1969; IS: 2062–1969; IS: 226–1969 and IS: 961–1962. A typical St 42-S type structural steel has tensile strength between 42 and 54 kg/mm2 and carbon (0.25%), sulphur (0.055%) and phosphorus (0.055%). High tensile strength steels (St 58–HT), having minimum tensile strength of 58 kg/mm2, are also used, but these may not be welded. Carbon steels have in general excellent weldability and require neither pre-heat nor post-heat treatment.

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(ii) High strength low alloy steels (HSLA): These structural steels have alloying elements which enhance the hot-rolled strength. Low alloy structural steels (manganese and vanadium type) have tensile strength up to 50 kg/mm2. High strength low alloy columbium-vanadium structural steels (also called niobium-vanadium steels) have tensile strength up to 60 kg/mm2. These have high strength-to-weight ratio. Forging steels

Forging steels have carbon 0.30 to 0.40% and are readily forged. These are used for bolts, pins, crankshaft, connecting rods and are suitable for light loads. Bright steels

Bright steels are basically medium carbon steels (C—0.3 to 0.8%). Cold working of these steels gives them very clean and smooth surface with close dimensional accuracy. Bright steels are available in rounds, hexagonals, squares, bars, flats and other sections. These are used as available or can be machined. Free cutting steels (or machine steels)

Free cutting or free machining steels are used as raw material for mass production of bolts, nuts, screws and nails in automatic machines working at higher cutting speeds. These have exceedingly high machinability; some of them can have mechinability index (or rating) even more than 100%. Surface finish on these steels is very good. Two typical examples of these steels are given below, those having machinability index 150% and 100%. Machinability rating 150%

Carbon (0.10 to 0.16%), manganese (0.65 to 0.85%), sulphur (0.12 to 2.0%), phosphorus (0.08 to 0.12%) and lead (0.25%). Machinability rating 100% Carbon (0.10 to 0.16%), manganese (0.65 to 0.85%), sulphur (0.17 to 0.20%) and phosphorus (0.08 to 0.12%). Free cutting steels have higher sulphur contents in the form of manganese sulphide inclusions, causing chips to break short during machining. The presence of manganese and phosphorus hardens and embrittles the steel, which also contributes towards free machining and better finish. Spring steels

Spring steels are used for making helical and leaf springs. These are hardened and tempered after making the spring. For small springs, plain carbon steels are used. For large springs, alloy steel, chrome-vanadium type or silicon-manganese type is used. These steels have high elastic limit. Forged steel

Forged steel is that steel which has been hammered, drawn or pressed or hot rolled in the process of its manufacture. It is a general name of carbon steels. Hot rolled steel or products are called wrought metals also.

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Cast steel

Cast steel is that steel which has been cast into sand moulds to form finished or semi-finished products. Steel castings are used to replace forgings when only small quantities are required that do not justify the high cost of forging dies. Also, it is easier to cast large products which cannot be easily forged. If the configuration or shape of the product is intricate, in that case also it is easy to produce the product by casting rather than forging. Steel castings are categorized in the following types: (a) (b) (c) (d) (e)

Low carbon steel castings having carbon less than 0.2% Medium carbon steel castings having carbon 0.2 to 0.5% High carbon steel castings having carbon above 0.5% Low alloy steel castings having alloying elements totalling less than 8% High alloy steel castings having alloying elements totalling more than 8%

Most carbon steel castings are produced in low carbon steel or medium carbon steel. High carbon steel castings have higher tensile strength (up to 95 kg/mm2) and high hardness and are used for metal working dies and rolls. Such a typical casting may have carbon 1.7% (maximum), manganese 0.5 to 1%, silicon 0.2 to 0.8%, phosphorus 0.05% (maximum) and sulphur 0.05% (maximum). Boron cast steels have higher hardness and strength, without losing ductility and impact strength. Steel castings are covered under BIS Code (IS: 4843–1968).

3.26

EFFECTS OF STEEL MAKING PROCESSES ON QUALITY OF STEEL

While it is obvious that the composition of steel is of prime importance in determining engineering properties of steel, other considerations like type of steel based on the method of manufacture, grain size, hot- and cold-working, heat-treatment, etc. singly or in combination are usually employed to get desired properties in steel. This is explained in the following.

3.26.1

Type of Steel

Steel type is based on steel making process like open-hearth steel, bessemer steel, etc. In any steel making process, the main reaction is the combining of oxygen and carbon to form a gas. If the oxygen made available for this reaction is not removed by adding silicon or some other deoxidizers like aluminium prior to or during casting, the gaseous products continue to evolve from the casting during solidification, leading to the presence of blow holes in the cast steel ingots. Proper control of the amount of gas evolved during solidification determines the type of steel as (i) killed steel, (ii) semi-killed steel, (iii) rimmed steel and (iv) capped steel. (i) Killed steels: These are high quality steels deoxidized in the laddle with silicon and aluminium to such an extent that there is no gas evolution in the steel upon solidification, and they lie in the molds quietly as they cool. A killed steel is characterized by relatively uniform chemical composition and properties. It is used for forging, carburizing and other important purposes. Sheets and strips made of killed steel have excellent forming and drawing qualities.

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(ii) Semi-killed steel: In order to reduce cost, some steels are not fully deoxidized. This results in blow holes in steel on solidification which are taken care of during rolling. The steel deoxidized in this manner is called semi-killed steel. It is suitable for structural purposes as also for normal drawing operations (not deep drawing). (iii) Rimmed steel: In rimmed steel, the carbon contents are less than 0.1% and manganese contents less than 0.6%. It is the steel which is deoxidized still to lesser extent than semi-killed steel. During solidification of these steels, oxygen and carbon present in steel form carbon monoxide, which is freely evolved from the ingot at its outer rim. It is used for cold bending, cold forming and for making sheets and strips. (iv) Capped steel: If the above reaction (that of rimmed steel) is stopped after a short while by preventing mechanically further evolution of gas from the top of ingot, the steel is called capped steel. The gas evolution results in an outer skin on the ingot, which is clean and very low in carbon. The presence of this nearly pure iron skin enables the production of an excellent surface finish on rolled products. Sheets and strips are made from this steel.

3.26.2

Grain Size

The grain size of a steel refers to the austenitic grain size. When steel temperature is raised above critical range, different steels show wide variation in grain growth, and it depends upon the chemical composition and the deoxidation practice during steel making. For example, aluminium and ferrosilicon are the deoxidizers used for killed steels. Steels deoxidized by aluminium show slow rate of grain growth at over 900oC. While those deoxidized with ferrosilicon develop large austenitic grain size at this temperature. It is observed that finegrained steels do not harden as deeply and have less tendency to crack when quenched from high temperatures (above 900oC) as compared to the coarse-grained steels of identical composition. Fine-grained steels show better toughness. Generally, all alloy steels and high carbon steels are made having fine grains. However, the coarse-grained steels have better machinability (as free cutting steels).

3.26.3

Effect of Hot-working on Metal

During making of steel, it is cast into ingots by pouring and cooling the molten steel into steel or refractory molds, where the ingots cool slowly resulting into coarse-grained steel. Mechanical hot-working of ingots as a subsequent operation is very much needed to refine the structure. The hot-rolled steel becomes more ductile, stronger and tougher.

3.26.4

Effect of Cold-working on Metal

The properties of hot-rolled steel-sections are very much affected by subsequent cold-rolling. These improvements include: increase in tensile strength, yield strength, torsional strength, hardness and wear resistance. The effect of cold-working depends on the chemical composition, cross-section, method of steel making and type of hot-rolling treatment. In cold-working, increase in yield strength resulting from cold-working is proportionally greater than the increase in tensile strength. Cold-worked steel has improved machinability particularly in lower carbon

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grades. The ratio of yield strength to tensile strength is an influencing factor on machinability as high yield to strength ratio resulting in cold-worked steels minimizes the plastic flow of metals during machining, thus permitting much of the tool energy to be utilized in shearing the brittled chips.

3.26.5

Effect of Heat Treatment

The properties of hot-rolled and cold-rolled steel-sections are modified by heat treatment to suit various specific applications of steel. For example, a hot-rolled bar which gets hardened on its outer surface is ‘normalized’ to improve its machinability. Process-annealing and full-annealing are the heat treatment operations carried out during deep drawing of steel sheets for making utensils and other products. Stress-relieving is yet another heat treatment process carried out to get rid of hidden stresses in steel structures, particularly welded structures.

3.27

FABRICATION CHARACTERISTICS OF FERROUS METALS

As already explained earlier, the fabrication characteristic of a metal is its ability to be formed, cast, welded or machined. These are discussed in the following with respect to various ferrous metals (cast irons and steels).

3.27.1 Machinability Machinability is defined as the ease with which a metal can be machined and includes the factors like low cutting forces on tool, chips broken easily, longer tool life and good finish on the job. The following factors improve machinability. Factors that increase machinability: Uniform microstructure, small undistorted grains, spheroidal structure in high carbon steels, lamellar structure in low and medium carbon steel, hot-working of hard steel (such as medium and carbon steels), cold-working of low carbon steels, annealing and normalizing of steels, addition of lead, manganese, sulphur, phosphorus and absence of abrasive inclusions like aluminium oxide. Factors that decrease machinability: Non-uniformity of microstructure, large distorted grains, presence of abrasive inclusions, spheroidal structure in low and medium carbon steels, lamellar structure in high carbon steels, hot-working of low carbon steels, cold-working of higher carbon steels, quenching, carbon contents below 0.3% or above 0.6% and high alloy contents. Very low carbon steels do not machine well because of ferrite structure mostly, which is too soft to produce good shearing action. Alloying of lead or manganese that forms sulphides can break ferrite structure. Free-machining steels are lead-bearing, very low carbon steels. In case of alloy steels, those having low carbon contents have good machinability but machinability drops as the carbon content in alloy steels increases. As regards steel castings, these in general have the same machinability as comparable to wrought metals (hot-rolled). The skin of castings becomes hard and should be removed by pressure blasting prior to machining or the casting may be normalized. Grey cast iron has carbon in the range of 2.5 to 3.75% when graphite flakes cause discontinuity in ferrite structure, thus helping the chip to break easily improving machinability.

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117

Roughly, the normal hardness range of 130 to 230 HB will present no machining difficulty. Annealing of grey cast iron castings improves machinability. For harder cast irons, carbide tools should be used for machining. General grey cast irons have machinability rating about 110%. Malleable cast irons have machinability rating of about 120% and are the most readily machined metals in all the ferrous metals because of their uniform structure and the presence of nodular tempered carbon in their structure. Ductile cast irons are similar to grey cast iron in machinability as they form soft nonmetallic sulphides in steel. Appearing as inclusion in microstructure, they act to lubricate the tool tip ensuring production of non-clogging chips.

3.27.2 Formability The ability of steel to be formed is based on its ductility, which, in turn, depends on crystal structure (fcc crystal lattice structure provides greater formability). Other factors that govern flowability or ductility are grain size, hot- and cold-working, alloying elements and heat treatments like annealing and normalizing. As regards the grain size, the tendency to slip in any single grain is obstructed to a certain extent by the presence of oppositing slip planes in the adjacent grains. Smaller grain sizes are recommended for shallow drawing and large grains for heavy drawing. In hot-working, tremendous pressures are encountered which tend to reduce the size of crystal either by preventing the growth of crystals at elevated temperature or by breaking up existing crystals. Usually, the grains get distorted in the process and the amount of distortion of grains is the determining factor in the ductility of metal. Cold-working also results in distorted grains, rather more distorted than hot-worked steels. Therefore, the cold-worked metals are usually less ductile than hot-worked steels. Also, most alloying elements in a pure metal reduce its ductility, for example, steel, which is an alloy of carbon and iron, is less ductile than iron. Hence ductility of steel reduces as amount of carbon increases. Low carbon steels have good formability because of less carbon and alloys to interfere with slip planes. Medium carbon cold-rolled steels are too low in ductility for any practical degree of cold forming. Medium carbon hot-rolled steels are somewhat more ductile. Alloy steels are not suitable for forming operations. They can be forged. Some stainless steels are, however, quite formable even for deep drawing. There are cast irons, some of which are used for operations like coining and straightening of castings in a press.

3.27.3 Weldability Plain carbon steels (steel having carbon up to 0.15%) are most weldable. It is only as the carbon percentages increase that there is a tendency for the metal to harden and crack during welding. Low carbon steels (near 0.3% carbon) need to be so welded that extremely rapid cooling is avoided to check formation of martensite, which is a hard and brittle structure. Medium and high carbon steels will harden when welded if allowed to cool at speed in excess of the critical cooling rate. Pre-heating between 260 and 310oC and post-heating between 38 and 649oC will remove any of the brittle microstructures. Extra high carbon steels and tool steels (carbon 1.0 to 1.7%) are not welded by arc welding (high temperature welding). These are joined by brazing with a low-temperature silver alloy. Stainless steels are not particularly difficult to weld. They have low thermal conductivity

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and hence tend to localize the stress, and temperature build-up takes place at the weld joint. Arc weldings using inert gas protection for weld (as tungsten inert gas welding) are common methods of welding stainless steels. Brazing can also be done. Grey cast iron can be welded with electric arc or oxyacetylene torch; pre-heating may be needed in the latter case. Short welds in arc welding will keep heat at a low level. Slow cooling is essential to allow carbon to separate out in free graphite flakes and to avoid chilled weld area. Arc welding of grey cast iron is limited to repair works, but braze welding with bronze or nickel copper is frequently used in fabrication. Malleable cast iron castings are not considered weldable as grey cast iron castings since welding will destroy malleable properties. These can, however, be brazed. Ductile cast iron being a high carbon metal should be given special consideration as in case of grey cast iron castings. It can be welded by carbon-arc processes and by other fusion processes, either to itself or to other metals such as carbon steel, stainless steel, etc. This metal can be brazed also with silver or copper alloy.

3.28

RECOMMENDING STEELS AND CAST IRONS FOR MACHINE PARTS

To help selecting suitable material, steel or cast iron, for making a machine component, some recommendations have been compiled and given in Table 3.8 and Table 3.9. The selection is based on the duty or load to be encountered by the machine component during service. Material designation is given as per the Bureau of Indian Standards (BIS) recommendations, IS: 1570-1961. Equivalent British Standard (BS) specifications have also been given, designating steel as EN 8 or EN 207, etc. TABLE 3.8

S. No. 1.

Machine part GEAR

Recommending steels and cast irons for machine parts

Duty/Load

(a) High impact load, speed over 4 m/sec. (b) Medium load, speed up to 4 m/sec. (c) Medium load, speed up to 2 m/sec. (d) Medium load, speed up to 1 m/sec. (e) Low load, speed up to 0.3 m/sec.

Material

Heat treatment

Hardness

IS: 15 Mn Cr 65 (EN Carburizing 207) or IS: C 20 (EN 3)

HRC 56-62

IS: 40 Cr 1 (EN 18) Teeth hardening

HRC 45-50

IS: 40 Cr 1 (EN 18)

do

HRC 22-27

IS: C 40 (EN 8)

do

HRC 22-25

IS: C 40 (EN 8)

Normalizing

HB 179-207

(Contd.)

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FERROUS METALS—Irons and Steels

TABLE 3.8

2.

SPINDLE A—In ball or roller bearing ———

B-In bush bearings

3.

Recommending steels and cast irons for machine parts (Contd.)

IS: 15 MnCr 65 (EN Carburizing 207) or IS: C 20 (EN 3)

HRC 56-62

IS: 40 Cr 1 (EN 18) Hardening IS: C 40 (EN 8) do

HRC 35-42 HRC 22-25

(a) Medium load IS: 40 Cr 1 (EN 18) Hardening Drilling milling and lathe machines do (b) Light load, IS: C 40 (EN 8) Drilling and lathe machines

HRC 45-50

(a) Heavy duty as turret and automatic lathes (b) Medium load (c) Light load

SHAFT A—In bush Bending load or sliding bearing (a) Speed over IS: 15 Mn Cr 65 (EN Carburizing 207) 3 m/sec. (b) Speed up to IS: 40 Cr 1 (EN 18) Hardening

3 m/sec. (c) Speed up to 2 m/sec. (d) Heavy duty spline and plain, high strength and wear resistance

do

HRC 56-62 HRC 45-50

IS: C 40 (EN 8)

do

HRC 40-45

IS: 40 Cr 1 (EN 18)

do

HRC 35-42

B—In ball or roller (a) Medium load IS: 40 Cr 1 (EN 18) bearing spline shaft (b) Light load IS: C 40 (EN 8)

do

HRC 22-27

do

HRC 22-25

spline shaft 4.

5.

Feed shaft

HB 207-217

Crank, cam shaft

Lathe and vertical IS: 40 Cr 1 (EN 18) Hardening boring machine General duty IS: C 40 (EN 8) do

Worm shaft

(a) Heavy duty

IS: 15 Mn Cr 65 (EN 207) or IS: C 20 (EN 3)

HRC 56-62

(b) Light duty

IS: 40 Cr 1 (EN 18) Hardening

Carburizing

HRC 56

HRC 22-27 (Contd.)

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MANUFACTURING PROCESSES

TABLE 3.8

6.

Worm wheel

Recommending steels and cast irons for machine parts (Contd.)

(a) Heavy load (b) Medium load (c) Light load

7.

Claw coupling

When running

When stationary

Phosphor bronze General purpose bronze C.I. Grade 20

— —

— —

—

—

IS: 15 Mn Cr 65 Carburizing (EN 207) or IS: C 20 (EN 3) IS: C 40 (EN 8) Hardening

HRC 56-62

HRC 22-27

8.

Cam, roller, eccentric, pawl, stator of gear pump, etc.

Dimensional accuracy

IS: 105 Cr 1 (EN 31) Hardening

HRC 59-63

9.

Crane wheel

General duty

IS: C 55 Mn 75 Surface (EN 9) hardening

HRC 33-42

10.

Sprocket wheel

(a) Medium (impact load)

IS: 15 Mn Cr 65 (EN Carburizing 207) or IS: C 20 (EN 3) (b) Medium (non- IS: C 40 (EN 8) Surface impact load) hardening of teeth (c) Light load — C.I. Grade 20

HRC 56-62

HRC 45-50

—

11.

Pulley sheave

General duty

IS: C 20 (EN 3) or C.I. Grade 15

—

—

12.

Rack

Heavy load Medium load Light load

IS: C 40 (EN 8) C.I. Grade 25 General purpose bronze

— — —

— — —

13.

Spring

General duty

IS: C 70 (EN 42B)

14.

Friction disc

General duty

HRC 56-62 IS: 15 Mn Cr 65 (EN Carburizing Surface 207) hardening 0.6 or to 1 mm IS: C 20 (EN 3) depth

15.

Guide bed

Medium loading Heavy loading

C.I. Grade 15, 20 C.I. Grade 30

Hardening

Aging Aging

HRC 42-47

— HB 180-225 (Contd.)

121

FERROUS METALS—Irons and Steels

TABLE 3.8

16.

Piston ring

17.

Recommending steels and cast irons for machine parts (Contd.)

(a) For C.I. cylinder C.I. Grade 20 Heat or C.I. Grade 25 treatment Preferably addition of Ni—0.4%, P—0.6%

Piston

HRC 22-27

(b) For steel cylinder IS: C 40 (EN 8)

—

—

For steel cylinder

—

—

C.I. Grade 20

TABLE 3.9 Details of constituent elements (average %) present in metals recommended for machine parts

Material

C%

Mn%

Si%

C 20 C 40 15 Mn Cr 65 40 Cr 1 C 70

0.2 0.4 0.17 0.4 0.7

0.7 0.7 0.6 0.7 0.7

0.25 0.25 0.3 0.3 0.25

IS: 105 Cr 1 IS: C 55 Mn 75

1.0 0.55

0.4 0.7

0.25 0.25

IS: IS: IS: IS: IS:

C.I. C.I. C.I. C.I.

Grade Grade Grade Grade

15 20 25 30

Phosphorus Maximum % 0.5 0.4 0.3 0.3

Material Phosphor bronze General purpose bronze

3.29

Mo%

Tensile strength (kg/mm2)

— — 0.75 1.0 —

— — — — —

40 60 70 80 80

1.4 —

— —

— 70

Cr%

Sulphur Maximum % 0.17 0.12 0.12 0.12

15 20 25 30

Cu

Zn

Tin

Pb

P

Minimum strength (kg/mm2)

Remaining

—

—

4.0 6.0

0.8 1.2 —

24

do

9.0 11.0 4.0 6.0

4.0 6.0

15

MARKET FORMS OF SUPPLY OF PIG IRONS AND STEELS

Pig irons

Pig iron is available in several grades but the two more common grades are given below. ● Foundry quality (grade 1) ● Foundry quality and forge quality (grade 2) Foundry quality (grade 1): It has high percentage of carbon and silicon and is dark in colour. It is mechanically weak and is used for casting thin sections when strength is not the criterion. It is mixed with other grades of pig iron to improve its fluidity.

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Foundry quality and forge quality (grade 2): It is the most widely used grade for general purpose casting of wide range. Its utility is increased by adding steel scrap and other grades of pig iron to it. The forge quality of this pig iron is used for making wrought iron. The above pig irons are available in high manganese grades and low manganese grades. The pig iron of basic grade coke is used for steel making. Other grades of pig iron are standard basic and low silicon basic. Plain carbon steels

The market forms of plain carbon steel products consist of the following. Semis: These are available in ingots, slabs, blooms and billets. Sections: These comprise angles (equal and unequal type), channels, T-sections, telegraph channel and beams (I-sections). Rolled products: These include bars (dia 12 to 100 mm), flats of different thickness (45 × 6 mm to 250 × 25 mm) and squares (16–110 mm). Flat products: Wide and heavy plates, medium plates, hot-rolled sheets and coils. Track material: Fish plates, rails of various grades, bearing plate bars and crane rails. Special products: These include tin plates, pressed steel sleepers, wheels, axles, galvanized sheets, etc. Alloy steels and special alloys

These are available in the following three forms. (i) Forged products: Bars may be rounds (30 to 535 mm dia and 6 m long); square bars (30 to 420 mm and 6 m long) and flats (30 to 650 mm and 6 m long). Die blocks have length 1500 mm (max.), width (750 mm) and thickness (600 mm). Rings have outer dia (150 to 1250 mm) and inner dia (1200 mm max.) and thickness 13 to 440 mm. Discs in minimum 13 mm thickness and 150 mm dia and 110 mm thickness (max.) and 300 mm dia. (ii) Rolled products: These are hot-rolled round cornered square billets (dia 40 to 195 mm) and hot-rolled rounds (dia 22 to 125 mm). (iii) Sheet mill products (a) Hot-rolled sheets/plates 1.6 to 12 mm thick, width up to 1250 mm and length 2500 mm. (b) Cold-rolled sheets thickness 0.8 to 3.25 mm, width 1000 mm and length 2000 mm. These are properly heat-treated and pickled.

3.30 3.30.1

CODING OF IRONS AND STEELS Coding of Plain and Alloy Castings

The Bureau of Indian Standards (IS: 4843–1968) has given the following system of designating plain and alloy castings of irons and steels.

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123

Castings

System of designation of plain castings 1. Symbols indicating type of casting 2. Symbols indicating mechanical properties or Chemical composition

System of designation of alloy castings 1. Symbols indicating type of casting 2. Average carbon contents in hundredth of a percent 3. Symbols for significant chemical elements in descending order 4. Alloy index numbers for the average percentage of alloying elements

Symbols CS CSM FG AFG SG ASG BM PM WM ABR

: : : : : : : : : :

Steel casting Unalloyed special steel casting Grey iron casting Austenitic flake graphite iron casting Spheroidal or nodular graphite iron casting Austenitic spheroidal graphite iron casting Black heart malleable iron castings Pearlitic malleable iron castings White heart malleable iron castings Abrasion resistant iron castings

Examples Some designations of cast iron and steel castings and their explanations are given below: : Unalloyed steel casting, with minimum tensile strength 1250 N/mm2 : Unalloyed special steel casting with minimum tensile strength 350 N/mm2 FG 150 : Grey iron casting with minimum tensile strength 150 N/mm2 FG 35 Si 15 : Grey iron casting with minimum total carbon 3.5% and average silicon 1.5% SG 800/2 : Spheroidal graphite iron casting with minimum tensile strength 800 N/mm2 and minimum elongation 2% on gauge length equal to five times the diameter of bar BM 350 : Black heart malleable iron castings with minimum tensile strength 350 N/mm2 PM 700 : Pearlitic malleable iron casting with minimum tensile strength 700 N/mm2 CS 50 Cr 1 V 20 : Alloy steel castings with average percentage of carbon (0.50%), chromium (1%) and vanadium (2.0%)

CS 1250 CSM 350

The following IS Codes can be referred for more information. 1. Grey iron castings : IS: 210–1970

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2. Malleable iron castings

: IS: IS: IS: : IS:

2108–1962 2640–1964 2107–1962 1030–1962

3. Steel castings 4. Alloy steel castings for high temperature service : IS: 3038–1965 IS: 2856–1964

3.30.2

Coding of Steels

According to BIS Code (IS: 1962, part 1-1974), different steels can be designated as per either of the following bases: (a) On the basis of mechanical properties (b) On the basis of chemical composition Designating steels on the basis of mechanical properties

Both plain carbon steels and low alloy steels are designated on the basis of mechanical properties such as tensile strength and yield strength as per the following: (i) Letter Fe or FeE will be the first letter of code where Fe refers to classification based on minimum tensile strength and FeE for minimum yield strength. (ii) The above is followed by a figure giving minimum tensile or yield strength in N/mm2. (iii) It will then be followed by chemical symbols representing the elements signifying the steel. (iv) And in the end will be symbols indicating method of deoxidation, weldability, formability, surface finish, treatment, etc. ●

Treatment Method of deoxidation

●

Weldability

●

Surface condition (only selective)

●

Formability (sheets only)

●

Surface finish (sheet only) Elevated temperature properties Degree of purity

● ●

Symbol R—Semi-killed or rimmed steel K—Killed steel W—Fusion weldability W1—Weldable by resistance welding (but not fusion) Forged hot-rolled steel—No symbol S6—Bright drawn or cold rolled S7—Ground D1—Normal drawing quality D2—Deep drawing quality D3—Extra deep drawing quality 14 grades of finish are there (F1 to F14) H—Elevated temperature properties guaranteed Designated by P, it gives maximum contents of phosphorus and sulphur, for example, P25 means phosphorus and sulphur contents are 0.025% each. Word SP is used when they are not of same contents.

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125

Example (i) FeE 250 P 35

(ii) Fe 400 Cu K

: Steel with minimum yield strength 250 N/mm2, with sulphur and phosphorus both equal and individual up to a maximum of 0.035%, semi-killed type : Steel with minimum tensile strength of 400 N/mm2, sulphur and phosphorus both equal and each maximum up to 0.035%, killed type with copper as main alloying element

Designating steels on the basis of chemical composition

This system is more popular with engineers as they mostly specify steels on this basis in their recommendations for product design. (i) Designating carbon steels: According to BIS (IS: 1570–1961), the plain carbon steels are designated by the alphabet ‘C’, followed by numerals which indicate the average percentage of carbon in it. For example, C 40 refers to a plain carbon steel containing carbon 0.35 to 0.45%, i.e. carbon (0.40% on average), although other elements like manganese may be present. Sometimes, average percentage of some other alloying elements is also given in addition to carbon percentage, for example, C 55 Mn 75 means that the carbon content lies between 0.50 and 0.60% and the manganese content lies between 0.60 and 0.90%. (ii) For unalloyed steels: These are designated as per IS: 7598–1974 in the following order. (a) Figure giving 100 times the carbon percentage (b) C stands for carbon (c) Figure giving 10 times the average manganese percentage (d) Symbol indicating special characteristic (for example, G for guaranteed hardenability and W for fusion weldability) Example 40 C 5G: Carbon steel with carbon 0.4%, manganese 0.5% with guaranteed hardenability. 20 C 5W: Carbon steel with carbon 0.2%, manganese 0.5%, fusion weldable. Designating unalloyed tool steels

Unalloyed tool steels are designated in the following order: (a) Figure giving 100 times average carbon percentage (b) Alphabet T for tool steel (c) Figure giving 10 times average manganese Example 90 T 10: Tool steel with 0.90% carbon and manganese 1%. Designating unalloyed free cutting steels

The following example will clear how these steels are designated. Elements responsible for making steel free cutting include: lead (Pb), sulphur (S), tellurium (Te) and selenium (Se).

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The specification is: 30 C 12 Pb 15 K. It is explained as follows. 30 100 times average carbon %

C For unalloyed free cutting steels

Example 15 C 9 Pb 25 T 14:

12 10 times average Mn %

Pb Element making steel free cutting. It may be S, Te or Se

15 100 times percentage of Pb

K Special characteristic symbol

A free cutting steel with carbon 0.15%, manganese 0.9%, lead 0.25%, hardened and tempered (indicated by treatment grade 14).

Designating low and medium alloy steels (total alloying elements less than 10%)

Low and medium alloy steels are designated as per IS: 7598–1974 in the following order. (a) Figure giving 100 times average percentage of carbon (b) Symbol for alloying elements followed by a figure of its average percentage, which is multiplied by a standard multiplying factor as given below: Alloying element (i) Cr, Co, Ni, Mn, Si, W (ii) Al, V, Pb, Cu, Ti, Mo, etc. (iii) P, S, N

Multiplying factor 4 10 100

Mn symbol will be included only if it is greater than 1.0%. Example (i) 40 Ni 8 Cr 8 V 2 G: Alloy steel having carbon 0.40%, nickel 2%, chromium 2%, vanadium 0.2%, guaranteed hardenability. (ii) 20 Cr 4 Mo 2G: Alloy steel having carbon 0.20%, chromium 1%, molybdenum 0.2%, guaranteed hardenability. Designating high alloy steels (total alloying elements more than 10%)

These are designated in the following order. (a) Alphabet X designating high alloy steels (b) Figure giving 100 times average carbon percentage (c) Chemical symbol for alloying elements, each followed by a figure giving its average percentage (d) Symbol indicating surface condition Example (i) X 15 Cr 16 Ni 9 S6: Bright drawn high alloy steel having average carbon 0.15%, average chromium 16% and nickel 9%. (ii) X 12 Cr 18 Ni 12 H: High alloy steel having average carbon 0.12%, average chromium 18% and average nickel 12%, with special characteristics of elevated temperature properties.

FERROUS METALS—Irons and Steels

127

Designating alloyed tool steels

The designation of these alloys is done as described above for high alloy steel, except that the letter X will be substituted by T for low and medium alloy tool steels and XT for high alloy tool steels. Examples (i) T 30 Cr 5 Mo 1 V 3: Medium alloy tool steel having average carbon 0.30%, chromium 5%, molybdenum 1.0% and vanadium 3%. (ii) XT 70 W 18 Cr 4 V 2: High alloy tool steel having average carbon 0.70%, tungsten 18%, chromium 4% and vanadium 2%. Designating structural steels

Structural steels are designated by alphabets “St”, followed by a figure giving minimum ultimate tensile strength, which is followed by word like: O—for ordinary quality, S—for standard quality, W—for weldable, HT for high tensile steel. Example (i) St-32-0: Structural steel with minimum tensile strength (32–44 kg/mm2) and ordinary quality. (ii) St-42-S: Structural steel, min. tensile strength (42–54 kg/mm2), standard quality. (iii) St-58 HT: Structural steel, min. tensile strength (58 kg/mm2), high tensile steel (not welded).

3.31

CLASSIFICATION OF STEELS AS PER INTERNATIONAL STANDARDS

In every country there exists one or more organizations that work on testing and standardization of engineering materials and develop codes of practice for manufacturing and use of the materials. Given below are various such internationally known institutes or organizations. BIS ISO GOST AISI SAE ASTM ASTE DIN JIS

3.31.1

: : : : : : : : :

Bureau of Indian Standards International Standards Organization Russian Standards American Iron and Steel Institute Society of Automotive Engineers American Society of Testing Materials American Society of Tool Engineers German Standards Japanese Standards

AISI Method of Classification

(a) A capital letter prefix is used to indicate steel making process, for example, B C E

: Acid bessemer carbon steel : Basic open-hearth carbon steel : Electric furnace alloy steel

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(b) The above is followed by a series of four numericals out of which the first two indicate type of alloy and the last two approximate average carbon contents in points. The details of the first two numericals are as given below. 10 11 & 12 13 31 40 41 43 44 & 45 46 & 48

: : : : : : : : :

Carbon steels Free cutting steels Manganese steels Nickel chrome steels Molybdenum steels Chrome molybdenum steels Nickel chrome molybdenum steels Molybdenum steels Nickel molybdenum steels

Examples C-1040: Carbon steel, carbon (0.37 to 0.44) made in basic open-hearth

3.31.2

SAE Method of Classification

According to SAE method, steels are designated by a series of four digit numbers where the first digit gives type of steel, for example, 1. 2. 3. 4.

Carbon steels Nickel steels Nickel–chrome steels Molybdenum steels

5. 6. 7. 8.

Chromium steels Chromium–vanadium steels Tungsten steels Silicon–manganese steels

The second digit usually gives approximate percentage of predominating alloying element and the last two digits give average carbon contents in points. Example 2440: Nickel steel, average contents of nickel 4%, carbon 0.40%

REVIEW QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

How are metals classified? What is pig iron? How is it produced? What are the main impurities in pig iron? How are these removed? Why cannot pig iron be used as an engineering material like steel? What is cast iron? How is it produced? Discuss its properties and uses. What is the role of the shape of graphite in deciding the properties of cast iron? List down the various grades or types of cast iron in use in the market. What is grey cast iron? Discuss its composition, properties and uses. How is chilled cast iron (or white cast iron) different from grey cast iron? Discuss its uses. How is malleable cast iron produced? What is its use? How is it different from grey cast iron?

FERROUS METALS—Irons and Steels

129

11. Differentiate between white heart malleable cast iron and black heart malleable cast iron. Discuss their uses. 12. What is pearlitic malleable cast iron? Can it replace steel for some applications? 13. How does nodular cast iron differ from a malleable cast iron? 14. Why does ductile cast iron behave better than grey cast iron in terms of strength, toughness and ductility? 15. What is a high duty cast iron? How is it produced? 16. What are meehanite cast irons? How are they produced? Discuss their principal applications. 17. How is a meehanite cast iron better than grey cast iron? 18. What are alloy cast irons? Name the alloying elements and their role in improving the properties of cast iron. 19. What are low nickel cast irons? Are they machinable? 20. What are low-expansion alloy cast irons? 21. Discuss the uses of alloy cast irons. 22. Highlight the mechanical properties, machinability and weldability of the cast iron in general. 23. Name the elements that make cast iron suitable for high temperature services. Can you suggest a typical cast iron that can stand temperature up to 950oC? 24. Can the cast iron be heat-treated? If yes, name the heat treatment processes carried out on cast iron. 25. What is wrought iron? How is it made? Discuss important properties and uses of wrought iron. 26. How is the grey cast iron used for general casting purposes designated by the BIS? 27. What are abrasion resistant iron castings? How are they made? Where are they used? 28. Differentiate between steel and cast iron. 29. What are the main constituents of a steel? 30. How are steels classified? 31. What are carbon steels? Name the main constituents of the carbon steel. 32. Name the different types of carbon steel. Give their carbon percentages. 33. Give at least three applications of the following steels: (a) Dead mild steel (b) Mild steel (c) Medium carbon steel (d) High carbon steel 34. Discuss the importance of carbon in steel? How is its addition helpful in improving the properties of tensile strength, machinability and hardness? 35. What are alloy steels? Discuss the advantages of alloying the steel. 36. Name the main elements used for alloying steel. Discuss the role played by each element. 37. Name the elements used to improve the following characteristics of steel: (a) Higher tensile strength (b) Higher hardness and heat resistance (c) Corrosion resistance (d) Improved machinability

130 38. 39. 40. 41. 42.

43. 44. 45. 46.

47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63.

MANUFACTURING PROCESSES

How are the alloy steels classified? Write a short note on nickel alloy steels. What are invar and super-invar? How are the nickel–chrome steels different from nickel steels? Write a short note on manganese steel highlighting the role played by manganese in these steels. Suggest which steel should be used for the following applications. (a) Railway line points (b) Acid and corrosion resistance (c) Minimum coefficient of expansion (d) Dredger bucket (e) Low span bridges What are tool and die steels? Describe briefly. What are the main alloying elements in die steel for hot-work and cold-work? What is a stainless steel? Discuss its types and usages. Write a short note on the following. (a) Ferritic stainless steel (b) Martensitic stainless steel (c) Austenitic stainless steel (d) Maraging steel What are corrosion resistant steels? What are high speed steels? Briefly describe different types of high speed steel along with their alloying elements. What do you understand by cutting alloys? Differentiate between cast cutting alloys and cemented carbides. What are ceramic tool materials? What is the purpose of using hard facing alloys? What are heat resisting alloy steels? What are their main constituents? Name a few steels and other materials that can be used for sub-zero temperature applications. What are magnetic steels? What is an austenitic manganese steel? Discuss its characteristic features and uses. What are structural steels? Discuss their uses. What are free-cutting steels? Discuss. What is spring steel? How is it different from a bright steel? Differentiate between wrought iron and cast steel. Which different steels are used for making castings? Mention the main steps in which steel is made. What are different methods of making steel? Discuss the effect of the following on the properties of steel. (a) Type of steel based on degree of deoxidation (b) Hot-working (c) Cold-working (d) Heat treatment

FERROUS METALS—Irons and Steels

131

64. Write a brief note on the following fabricating characteristics of steels and cast irons. (a) Machinability (b) Formability (c) Weldability 65. What do you understand by machinability of steel? Discuss the factors that affect machinability. 66. Write a brief note on the formability of steels. 67. Which is more weldable and why? (a) Medium carbon steel (b) Carbon tool steel (c) Cast iron (d) Stainless steel (e) Structural steel

4 4.1

Non-ferrous Metals and Other Materials

INTRODUCTION

So far we have discussed ferrous metals and their alloys such as cast irons and steels. In all these metals, iron was present predominantly as the main base material. Non-ferrous metals (and alloys) do not contain iron, although in certain non-ferrous alloys very small amount of iron may be found. A combination of several properties which form sufficient basis for the preference for nonferrous alloys include: light weight, good corrosion resistance, good strength-weight ratio, higher electrical and thermal conductivity, ease of fabrication through casting, welding, cold forming and machining, and the choice of colours for good appearance. Examples of non-ferrous metals include aluminium, copper, zinc, lead, tin, magnesium, nickel, chromium, titanium and cobalt. In addition to this, there is a long list of non-ferrous alloys such as various brasses, bronzes, bearing metals, dow metal, duralumin, etc. Although non-ferrous metals comprise only about 20% by volume of all metals used in commercial production, some of them are of major importance, for example, aluminium used for aircraft bodies, cooking utensils and dairy industry; copper used for electricity and copper tubing for domestic water supply; zinc used for galvanized iron sheets and lead in storage batteries; chromium used for plating; titanium for jet engines and turbine blades and tantalum for rocket engines. The superalloys which are high temperature non-ferrous alloys find special use in rocket engines. Shape-memory alloys have unique properties of reversible behaviour, and they are used for space applications. Non-ferrous alloys in general are more expensive than steels and plastics. Most non-ferrous metals are rarely used as pure metal because of their poor engineering properties and high cost. However, their alloys are used quite extensively for varieties of reasons as discussed earlier.

4.2

ALUMINIUM

Aluminium and its alloys are most important non-ferrous metals. The most common aluminium ore is found in the form of hydrated aluminium oxide (called bauxite). Bauxite has impurities 132

NON-FERROUS METALS AND OTHER MATERIALS

133

like oxides of iron, silicon and titanium, and these are separated out by chemical refinement of bauxite through fusing it in electric furnace, where carbon is added to reduce the impurities. The pure aluminium oxide (free from impurities) is further processed through the electrolytic bath of molten cryolite, which reduces aluminium from its oxide. Aluminium is available in the market as wrought (hot rolled) or cast product in the form of ingots and notched bars for remelting. Aluminium, about 99.9% pure, is made by Hall-Heroult process. Aluminium melts at 660oC. Heat of combustion of aluminium in the oxygen is very high as finely powdered aluminium burns in air. Pure aluminium has low tensile strength (about 650 kg/cm2), which can be slightly improved by cold-working but the resulting strength is still very low for engineering applications. Pure aluminium finds little use in the production of castings, not only because of low strength but also because of its inferior casting qualities. Induction motor rotors and cable clamps are made from pure aluminium castings because of higher electrical conductivity of pure aluminium. Aluminium is mostly used in the form of its alloys.

4.2.1 Selection of Aluminium Alloys The selection of aluminium or its alloys is based on one or more of the following design considerations. (i) Weight-strength ratio, i.e. where lightness of the product is desirable. (ii) When ease of machining, fabricating (cutting and welding), or forming is of major importance. (iii) Where resistance to atmospheric corrosion or attack by certain chemicals is required. (iv) Where low electrical resistance is a requisite. (v) Where high heat and/or light reflectivity or low emissivity is needed. (vi) Where properties like acoustical deadness, non-toxicity to foods, non-sparking or nonmagnetic properties are desirable.

4.2.2 Properties of Aluminium Physical properties ● ● ● ● ● ● ● ● ●

Silvery white Lightness (density 2.7 gm/cm3 which is about one-third of iron) Good electrical and thermal conductivity (but next to copper) Good reflector of light Non-magnetic Resistant to atmospheric attack Highly ductile Melts at 660°C Tensile strength about 650 kg/cm2 which can be improved (up to double) by coldworking.

134

MANUFACTURING PROCESSES

Chemical properties ●

●

●

●

Resistant to atmospheric corrosion due to their oxide film (less than 0.02 micron) which thickens on heating and becomes impervious. Oxide coatings on aluminium through anodic coating (or anodizing) is a common process of providing a protective coating of oxide (up to 0.02 mm thickness) on the aluminium. Electro-polishing or brightening is done by anodic treatment in fluoboric acid as electrolyte, which gives very high reflectivity. Electroplating wherein aluminium can be electroplated with other metals by electrolysis.

4.2.3 Uses of Aluminium (i) Structural applications of aluminium are in aeroplanes, trains, ships, buses, roofing sheets for buildings, door and window frames, star rails, furniture, etc. These applications are due to the great property of aluminium of being resistant to atmospheric corrosion and weather, as also being highly adaptable to common manufacturing processes. (ii) Electrical industry employs aluminium for making cables, bus bars, induction motors using cast aluminium windings, rotors and conductors used for high speed turbine generators. (iii) Chemical and food processing industries use aluminium as it is resistant to many organic acids, minerals, salt solutions, sulphur and many other organic compounds. It is used for cooking utensils, milk and dairy industry, pressure cookers, etc. (iv) Brewery industry uses aluminium extensively for production and storage of breverages like beer. (v) Metallurgical industry wherein aluminium is used as deoxidizer for the production of irons and steels. It is also used for alloying steels for certain electrical, magnetic and oxidation resistant properties. (vi) Low-temperature applications (or cryogenic applications), such as storing of liquid oxygen or other liquid fuels for missiles or rockets, shelters and other equipment for sub-zero temperature (–30oC or so) encountered at Antarctica, are fulfilled satisfactorily by aluminium alloys because they are unusually ductile and resistant to shock loadings at sub-zero temperatures. With decrease in temperature, there is improvement in the tensile strength and yield strength of aluminium alloys. (vii) Process equipment used for rayons, plastic, petroleum and synthetic resin industry are generally made from aluminium.

4.3

ALUMINIUM ALLOYS

Aluminium is mostly used in the form of its alloys. Aluminium alloys are lightweight, resist corrosion, and have good electrical and thermal conductivity. These can be readily made by common production methods, i.e. casting, forging, machining, welding, etc. They lose their

NON-FERROUS METALS AND OTHER MATERIALS

135

strength at higher temperatures but are best suited for sub-zero temperature applications when their strength increases without loss of ductility. Aluminium alloys are produced in practically all of the forms such as plate, sheet, rod, wire, tube, forgings, castings, ingots, as well as rivets and screw-machine products. Rolled sections may be of regular standard structural shapes like channels, angles or may be of special design produced by extrusion. Forgings are made by pressing or drop-forging. Castings can be made into sand molds, cast iron molds, or die-casting. Various forms are fabricated into finished shapes and structured by drawing, stamping, spinning, hammering, machining, welding, brazing, riveting and soldering. Ease of fabrication is one of the reasons for the choice of aluminium alloys. Aluminium alloys are classified according to the method of shaping them, for example, wrought aluminium and cast aluminium and clad aluminium alloys. Few important aluminium alloys are discussed below.

4.3.1

Duraluminium

Duraluminium has aluminium 92% (minimum), copper 3.5 to 4.5%, magnesium 0.4 to 0.7%, manganese 0.4 to 7%, iron or silicon (not more than 0.7%) and the rest is aluminium. By age hardening process, it can attain tensile strength up to 43 kg/mm2. In the form of tube plate, sheet, rod or rivet, it is extensively used in the aircraft industry. The connecting rod for aero engine and automobiles is forged out of duraluminium. Duraluminium is as strong as steel but has only about one-third of its weight. Duraluminium has low resistance to corrosion and hence duraluminium sheets coated with pure aluminium are used for resisting corrosion. These sheets are named Alclad and are used in the aircraft industry.

4.3.2 Y-Alloy Y-alloy has aluminium 93%, copper 4%, nickel 2% and magnesium 1%. It is used as a casting alloy, although strips and sheets are also available. It is used for pistons of I.C. engines as the Y-alloy maintains its strength at elevated temperature also. By proper heat treatment, mechanical properties of the metal can be improved.

4.3.3 Aluminium Casting Alloys A general purpose aluminium casting alloy has aluminium 90%, copper 8%, iron 1% and silicon 1%. It may be sand cast, or pressure die cast and has good strength, hardness and machinability. In yet another type, zinc is 13.5%, copper 3% and the rest aluminium. This is also a general purpose type. There are aluminium–silicon alloys having silicon 5 to 15% and the rest aluminium. These have low shrinkage castings.

4.3.4 Porous Aluminium Blocks of aluminium are produced comprising aluminium powder 70–90% and the rest epoxy resin. These blocks are about 37% lighter than solid aluminium and have uniform permeability,

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which allows their use in applications where differential pressure has to be maintained, as also in vacuum holding of fixtures for assembly work.

4.4

BIS SPECIFICATIONS FOR ALUMINIUM AND ITS ALLOYS

The Bureau of Indian Standards (BIS) Codes for designating aluminium and its alloys are as follows. IS: 617–1959 gives specifications and uses of aluminium and its alloy ingots and castings for general engineering purposes. The aluminium (or alloy) ingot will be designated by letter ‘A’, followed by numbers, from 0 to 24 depending on the variety of ingot, for example, number 0 is for 99% pure aluminium for remelting, 1 for alloy ingots, 2 for pressure die-casting aluminium, etc. IS: 737–1965 covers wrought aluminium and its alloys.

4.5

COPPER

Copper is said to be the first metal used by the man and it is still the most useful metal. The major reasons of its importance are: (i) its very high electrical conductivity surpassed only by silver, (ii) very high heat conductivity, (iii) high resistance to corrosion, (iv) ease of working, i.e. forming, sheet-making, bending and fabricating properties, and (v) the alloys which it forms, the most important of which are brasses and bronzes, where brass is the second most commonly used non-ferrous alloy. Copper pyrites is the main ore for extracting copper. Ores are first roasted to remove water, carbon dioxide and sulphur, and later melted in reverberatory furnace where silica is added to help forming slag by interacting with impurities such as iron and alumina. The molten metal is further refined in a converter by blowing air which burns the impurities, and thus Blister copper with about 70% purity is produced. Electrolytic process is used to further refine the copper to a purity of about 99.9%. Copper is not used extensively in pure state because of it being very soft and weak. The majority of all pure copper is used mostly in the form of electrical conductors.

4.5.1 Properties of Copper ● ● ●

● ● ● ● ●

Tensile strength (cold-worked) ... about 35 kg/mm2 Hardness (cold-worked) ... about 90 HB Elongation percent on 50 mm length—50% for annealed copper and 5 to 20% for cold-worked copper High electrical conductivity (next to silver) High heat conductivity (next to those of gold and silver) Good corrosion resistance High ductility Light weight

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4.5.2 Uses of Copper (i) Copper bus bars are used for carrying heavy current over short distances, while bulk of copper is used for making wires for this purpose over longer distances. (ii) Used for making alloys such as brasses and bronzes. (iii) Used for bars of locomotive stay bolts, woven wire screen or clothe, and electrodes, parts of electrical switches. (iv) Copper sheets used for various cold operations such as stamping, spinnging, etc. (v) On account of high resistance to corrosion, copper sheets are used in chemical plants, food and brewing plants, also used for roofing, sheathing and flushing drains. (vi) Copper tubes used in radiators, refrigerators, air-conditioning equipment involving maximum heat transfer. (vii) Copper can be cold rolled for increasing its strength and hardness. Copper wire above 0.1 mm diameter is made by drawing from a hot-rolled rod without annealing.

4.5.3 Types of Copper Following are the forms in which copper is available in the market. (a) High conductivity (HC) copper is pure (purity 99.9%). (b) Best select (BS) copper is most widely used copper for industrial use. It is little lower in purity because of impurities present in it. (c) Arsenical copper has small amount of arsenic added to increase strength and resistance to oxidation. (d) Deoxidized copper is made by removing oxygen from copper by adding deoxidants (like phosphorus) to molten copper. Although presence of oxygen in small amount makes the copper slightly harder, oxygen is harmful if copper is welded as it embrittles the copper, rendering it useless.

4.6

BIS SPECIFICATIONS FOR COPPER AND ITS ALLOYS

IS: 2378–1974 covers characteristics of copper along with its alloys. Copper is designated by letters ‘Cu’, followed by letters like CATH, FRHC, etc., where CATH is for Cathode Copper and FRHC for Fine Refined High Conductivity Copper. Copper alloys are designated by symbol ‘Cu’, followed by next most significant elements and their percentage, etc.

4.7

ALLOYS OF COPPER

Copper has two main alloys: (a) Brasses and (b) Bronzes. Both these alloys are available in large varieties of compositions and properties.

4.8

COPPER–ZINC ALLOYS (BRASSES)

When copper is alloyed with zinc in varying proportions, the resulting material is known as brass, which is stronger than either of the material from which it is made. Brasses contain

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copper up to 70%, zinc 5 to 45%, besides small amount of aluminium, tin, manganese, lead, etc. to give special properties to the brass. Brasses have the following main characteristics: ● ● ● ●

Highly resistant to corrosion Good bearing metal Easy to cast and machine Can be cold-worked and hot-worked.

Main classification of brasses based on market names is given below: (a) Alpha brasses (b) Alpha-beta brasses (c) Leaded brasses (d) Tin brasses (e) Nickel–silver brasses Brasses with zinc up to 36% are very ductile and hence can be cold-worked. These are called alpha brasses. Those having zinc more than 36% are suitable for hot-working and are called alpha-beta brasses. Leaded brasses contain copper up to 88.5% and lead up to 3.25%. Tin brasses known as admiralty and naval brasses have tin 0.75 to 1.0%. Nickel–silver brasses have high percentage of nickel, added primarily for its influence on colour, besides improving mechanical and physical properties. Common brasses used in the industry are of the following types. (a) Muntz metal has copper 60% and zinc 40% and has tensile strength up to 35 kg/mm2. It is a general purpose brass which can be easily cast, rolled, extruded and stamped. It is used for marine fittings, pump parts, valves, taps, condenser tubes, fuses and other electrical machine parts and such items which require good resistance to corrosion. (b) Cartridge brass contains copper 70% and zinc 30%. Being strong and ductile, it is used for drawn products like cartridge cases, tubes, sheets, radiator shells and reflectors for head lamp. (c) Naval brass has copper 60%, zinc 39% and tin 1%. It is similar to cartridge brass but, due to tin, has greater strength and hardness but ductility reduces. With tensile strength 41 kg/mm2, it is used for fittings for ship components like window anchors. (d) Admiralty brass has copper 70%, zinc 29% and tin 1%, with tensile strength 68 kg/mm2. It can be cold-worked and drawn in tubes, sheets and bars and is widely used for bolts, nuts, ship fittings and other components exposed to sea water and also for steam condenser tubes. (e) Delta brass (or delta metal) has copper 55%, zinc 41%, lead 2% and iron 2%. It can be easily forged, rolled, extruded or cast. It is strong enough even to replace steel castings. It is used for parts of marine engines, chemical and hydraulic plants. (f) Beta brass has copper 50% and zinc 50%. It softens quickly on heating. It is used as a brazing solder. (g) Free cutting brass contains copper 60%, zinc 37% and lead 3% and is available as bar stock for free machining at higher speeds, on automatic lathes. (h) Red brass has zinc varying from 5 to 20% and the rest is copper. It can be easily brazed and is used for plumbing of pipe ends and hardwares, rivet, etc.

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(i) Precipitation hardening brass has copper 70%, zinc 30% (approx.) and small amount of nickel and aluminium and is used for gears, and formed parts and has the ability to harden after working. (j) Clock brass having copper 65%, zinc 34% and lead 1% is available in strips and has good bearing qualities for use in gears, pinions and other moving parts of clocks. (k) Silicon brass contains copper 80%, zinc 16% and silicon 4%. It has tensile strength of 6.8 tonnes/cm2. It is easily welded and die-cast or hot-worked (extruded, etc). It is used for parts of refrigerators and fire-extinguisher shells.

4.9

COPPER–TIN ALLOYS (BRONZES)

Alloys of copper with material other than zinc are known as bronzes. The main alloying element in bronzes is tin, although there are some bronzes that contain little or no tin. Other elements added in the bronze are: silicon, aluminium, manganese, phosphorus and nickel. Like brasses, bronzes are also hardened by cold-working. Bronzes are generally superior to brasses in terms of corrosion resistance and mechanical properties. Bronzes are the alloys of copper and tin, wherein addition of tin makes the alloy (bronze) harder and stronger than brass. Tin contents in bronze vary from 5 to 10% in general; bronzes with tin 8% are called cold working bronzes as these can be cold-worked, rolled or drawn, but those bronzes which have tin 10% are used in cast form only and are called hot-working bronzes. Tin has the tendency to oxidize when bronze is hot and this way the bronze becomes brittle. To avoid this phenomenon, some deoxidizers (like phosphorus and zinc) are added to bronzes to produce a metal with improved casting qualities and increased hardness and resilience. Phosphorus is added usually up to 0.5% but not more than 4%. Such bronzes are called phosphor bronze. The phosphor bronzes are suitable for making bearings, worm wheels and power-screws and welding rods. When zinc is used as deoxidizer, then gun metal is formed which is very useful for foundry work. In general, bronzes are hard and wear resistant and non-corrodable. These can be readily cast. Bronzes are used for hydraulic fittings, bearings, pump liners, valves, flanges, sheets, wires, and many drawn or stamped products. The most widely used bronzes are: beryllium bronze, phosphor bronze, manganese bronze, silicon bronze and aluminium bronze.

4.9.1 Beryllium Bronze (or Beryllium Copper) The most common alloy contains beryllium 2 to 2.75%. It is used when a non-ferrous, non-sparking or good electrical conductor with high strength (ultimate tensile strength on an average, 600 MPa) and high modulus of elasticity is needed. It is one of the best corrosionresistant spring materials when springs are subjected to vibrations and shokes and are required to be free from elastic drift. It may be cold-worked and heat-treated to make it hard. It can be sand cast also to give high strength castings. It is used for bellows, springs of all forms, X-ray windows, molds for plastics, bordon tubes, diaphragms and electrical contacts, as it has high electrical and thermal conductivity and high corrosion resistance. This metal does not produce

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spark when struck against stone and hence is used for making tools and equipment for mines where combustible gases are present.

4.9.2 Phosphor Bronze Phosphor bronze is an alloy of copper (95%) and tin (5%) deoxidized by phosphorus and is known for its strength (ultimate tensile strength, 325–960 MPa) and hardness which increases with increase in tin percentage (between 2 and 10%) but phosphorus may be up to 0.5% (but not more than 4%). It is used for different types of springs in electrical equipment. Drawn tubes are used in fuel system and instruments and welding rod. Cast phosphor bronzes are bearing bronzes having tin 10% and lead up to 1.25% and the rest copper. Phosphor bronze is tough and resistant to sea water and is used for bearings, pipe fittings, expansion joints, valves, pistons and bushings. There are gear bronzes having tin 13% for better strength. Phosphor bronzes can be sand cast. They have good wear resistance and load carrying capacity. Phosphor bronzes are used for heavy duty bearings, gears, worm-wheels, power-screws, springs and welding rods.

4.9.3 Manganese Bronze In comparison to phosphor bronze, manganese bronze is harder, stronger and has better anti-corrosion properties. It has poor response to cold-working. It contains copper 55 to 60%, zinc 38 to 42%, tin up to 1.5%, iron up to 2% and manganese up to 3.5%. It is used for propellers of ship, rudders, pump rods, worm-wheels and other high strength non-corrodable fittings.

4.9.4 Gun Metal When zinc is added to the bronze as deoxidizer, the gun metal is formed. One of the more common composition of the gun metal is: copper 88%, tin 10% and zinc 2% (which may go up to 5% in other varieties). Small amount of lead is also added to improve castability and machinability. Gun metals are very useful for foundry work. It is highly resistant to corrosion and is used for marine fittings, valves and pumps, general water service fittings, bushes, bearings, glands and steam pipe fittings.

4.9.5 Bell Metal Bell metal has copper 75 to 80% and the rest is tin. It has good ringing quality and is used for casting of bells and gongs.

4.9.6 Silicon Bronze Silicon bronze has silicon 2 to 4% and zinc or manganese up to 1% and the rest is copper. Sometimes lead (up to 0.5%) is added to improve machinability. Low silicon bronzes are easily cold-worked. Silicon bronzes can be cast, hot- or cold-worked, rolled and welded. It has high strength, toughness and corrosion resistance. It is used for tanks, boiler parts, marine hardware and used when high strength and resistance to corrosion are the main criteria.

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4.9.7 Aluminium Bronze Aluminium bronze has aluminium up to 14%, and the rest copper. Sometimes iron is also added, for example, a typical bronze having copper 89%, aluminium 9% and iron 2% has tensile strength of 65 kg/mm2, elongation 10 to 20% and hardness 160 HB. Aluminium up to 12% and iron (1.5 to 4%) increase the strength and refine the grains, and the bronze can be hardened by heat treatment to have good wear resistance as well as corrosion resistance and strength. Aluminium bronzes are used for parts of die-casting machine, pump rods, cams, rollers and sliders. In cold-working of aluminium bronzes, aluminium is kept up to 8% and the alloy is used in the form of tubes for condensers, heat exchangers, steam and chemical plants besides springs. In the hot-working aluminium bronze, aluminium is 8 to 14%. The metal can be forged, extruded, sand cast or die cast. It is used for gears, valve seats, cams, rollers, guides in I.C. engine.

4.10

ZINC AND ITS ALLOYS

Zinc-based alloys are widely used in the manufacture of carburetors, fuel pumps, door handles and household appliances. Zinc is a weak metal but has high corrosion resistance. It may be coated on steel by hot dipping (galvanizing) or electroplating (or electro-galvanizing). Zinc coating can be provided by hot spraying also. Rolled sheets of zinc are used for roofing purposes and for battery containers. Zinc has low melting point (419oC) and high fluidity. Zinc alloys are used for pressure die-casting. BIS has covered zinc and its products (sheets and strips) and zinc alloy ingots for die-casting under IS: 209–1966, IS: 713–1966, IS: 742–1966 and IS: 2258–1967. Major alloying elements in zinc-based alloys are: aluminium, copper and magnesium as they provide strength and dimensional control in castings. The main zinc alloys include cadmium zinc alloy, copper zinc alloy (brass), magnesium zinc alloy, lead zinc alloy and iron zinc alloy (for galvanizing and aluminium zinc alloy).

4.11

TIN AND ITS ALLOYS

Tin has a very low melting point (232oC). It is a soft metal but is brittle when cold. It is malleable at around 100oC when it can be rolled into sheets or drawn into pipes. Tin has good corrosion resistance against water and organic acids and hence is used as coating on steel containers for food and water like tanks, cooking utensils, etc. Tin finds application as an alloying element in soft solders, bell metal, bronzes and bearing metals. The most extensive use of tin, a silvery white lustrous metal, is as a protective coating on the steel sheets (called tin plates) used in making containers for food and other products. The low shearing strength of tin coating on steel sheets improves its performance in deep drawing and other general purpose press working. Tin-based alloys (called white metals) usually have copper, antimony and lead as alloying elements which provide hardness, strength and corrosion resistance. Another important alloy of tin is “babbitt” metal, which contains tin, copper and antimony and is used as journal-bearing material because of low adhesion and low friction

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coefficient. White metal and babbit metal are commonly used bearing material and these have been discussed in detail elsewhere in this chapter.

4.11.1 Solders Solder is the most important alloy of tin. It is used to join metal pieces together. Solder melts at a temperature which is lower than (a) the melting points of the alloying elements, and (b) the metal to be joined. For example, lead melts at 327oC and tin at 232oC but when alloyed in equal ratio, the solder produced melts at 205oC. Solders are categorized as (i) soft solder and (ii) hard solder. (i) Soft solders: These are tin-lead solders. Sometimes cadmium and bismuth are partly substituted for tin to make a solder for wetting copper and brass. The four typical compositions of soft solders in common use are: (a) (b) (c) (d)

Tin—50%, lead 50%, melting point 205oC Tin—40%, lead 60%, melting point 192oC Tin—66.6%, lead 33.4%, melting point 225oC Electrical solder—lead 5%, tin 95%, melting point 220oC

(ii) Hard solders: These contain copper and zinc with little tin and are used for joining copper and brass, etc. These are used for brazing purpose and called spelters. Generally, spelters are (i) alloys of copper and zinc (meting range 850–950°C) used for brazing cast irons and steel, (ii) alloys of silver and copper or silver-zinc for brazing any metal capable of being brazed (with melting range 600–850°C), and (iii) alloys of phosphorus–copper or phosphorus–silver and copper alloys for brazing copper and its alloys (with melting range 700–750°C).

4.12 NICKEL Nickel is a silver-white metal and is a major alloying element which imparts strength, toughness and corrosion resistance. Nickel can be easily cast, machined, drawn into wires, forged, welded and brazed. Nickel has high resistance to even highly corrosive solutions and may be acidic or alkaline. That is why it is used for manufacturing stainless steel (nickel–chrome) and for electroplating on base metal (steel) to provide corrosion-resistant surface. Nickel is a very important alloying element for steel where it increases its tensile strength, low-coefficient of expansion and anti-corrosion property.

4.13

NICKEL ALLOYS

Nickel is used extensively in stainless steel and in nickel-based superalloys. Nickel alloys are used in high temperature applications for parts of jet engines, rockets, etc., in coins, and in marine applications. Nickel being magnetic, nickel alloys are used in electromagnetic applications such as solenoids. The principal use of nickel is in electroplating of parts for better appearance and improved resistance for corrosion and wear. Nickel alloys have high strength and corrosion resistance at elevated temperature. Alloying elements in nickel are chromium, cobalt and molybdenum.

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Grouping of Nickel Alloys

The principal use of nickel is as an alloying element to ferrous and non-ferrous metals. Nickel alloys are stronger, tougher and harder than copper and aluminium alloys and are as strong as steel (but costlier than steel). Nickel alloys are highly resistant to corrosion and exhibit good heat resistance. These can be cold-worked, although frequent annealing may be required to remove work hardening effect. In view of their high heat resistance, ability to maintain strength at elevated temperatures and high corrosion resisting properties, these alloys are finding increasing use. Nickel alloys can be broadly grouped as below on the basis of their composition. (1) Nickel–molybdenum alloys are highly resistant to corrosion and are known by different trade names, e.g. Hastelloy A having nickel, molybdenum and iron. These alloys have high strength at elevated temperatures. Hastelloy C has nickel, chromium, molybdenum and iron. These alloys show good corrosion resistance and high strength at elevated temperatures. (2) Nickel–copper alloys are known with trade name monels, which contain nickel, copper and iron. These are easily cast, forged, machined or brazed. Monels are not affected by atmosphere and sea water and resist corroding effect of alkalies and acids. (3) Nickel–silver alloys, also called ‘German silver’, contain nickel, copper and zinc and are highly resistant to corrosion. (4) Nickel–iron alloys are obtained by adding varying amount of nickel into iron and are known for their magnetic and dilatation properties. Adding nickel to iron reduces magnetic properties of the resulting alloys. With nickel about 30%, the resulting alloy becomes practically non-magnetic. Such alloys are used for switchgear parts and transmission components. Adding nickel more than 30% gives an alloy with high magnetic permeability and low hysteresis losses. As regards dilatation properties, the coefficient of thermal expansion of nickel-iron alloy falls rapidly with increasing addition of nickel up to 25% and with nickel at about 35%, the coefficient of thermal expansion of the alloy becomes practically zero. Invar and superinvar are examples (Section 3.17.1). (5) Nickel–chromium alloys have higher resistance to oxidation which goes on increasing when chromium is added up to 20%, e.g. with chromium 20% and nickel 80%, the resulting alloys resist oxidation up to 1000oC. Syperalloys are the examples. Nickel– chromium alloys find use in heating coils for electric muffle furnaces, furnace bottoms, tubes for thermocouple sheaths, etc. Inconel with tensile strength up to 1400 MPa is an alloy used for high temperature applications where repeated heating and cooling is involved. Some important nickel alloys are described in the following.

4.13.2 Monel Monel is the most important alloy having average composition of nickel 67%, copper 28%, and iron, manganese and silicon combined (5%). It can be cold- and hot-worked, cast or welded. Monel resists corrosion of distilled water, salt water, foods and acids (except hydrochloric acid above 50°C). Its strength and thermal expansion are same as that of steel and is used for parts of machines that must resist corrosion. It is extensively used in chemical, marine, laundry,

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food-service steam valves and pipes, turbine blades and shafts. It resists oxidation up to 515oC, and at 285oC, it retains 75% of its strength at room temperature. Since the colour of monel is close to nickel, it is used for kitchen utensils and equipment and makes it competitive with stainless steels at several places. K-monel is a non-magnetic, age-hardening alloy containing aluminium about 3%. Its strength and hardness (in larger section) is comparable with heat-treated alloy steels. R-monel is a free-machining alloy adapted for use for automatic screws machines. It has sulphur (0.025 to 0.06%). H-monel has silicon (2.75 to 3.25%) with increased hardness but good ductility. S-monel has silicon (about 4%) and its hardness at as-cast is 320 HB, which can be increased to 350 HB by heat treatment.

4.13.3

German Silver

German silver is known as Ni-silver and has copper 60%, nickel 30% and zinc 10%. It is very soft, ductile and malleable with silvery appearance. It is used for jewellery, electrical contacts, resistance wires, etc.

4.13.4

Constantan

Constantan has high specific resistance and is unaffected by variation of temperature. It has nickel 45% and copper 55%. It finds application in thermocouples, wheatstone bridges, low temperature heaters and resistances.

4.13.5 Inconel Inconel contains nickel 80%, chromium 14% and iron 6%. It is used when high resistance to corrosion at elevated temperature is needed. This can stand repeated heating and cooling between 0 and 930°C. It is used for high temperature application and electrical appliances as resistance wire. Inconel can be made in sheet, strip, rod, wire or in cast form. It is highly resistant to corrosion up to 900°C. It is used for food processing industry, specially milk and its products. Nichrome and incoloy are yet other nickel-based alloys. Nichrome is used as resistance wire in electrical appliances, whereas incoloy is used as a high-temperature alloy. It has nickel 42%, chromium 13%, molybdenum 6%, titanium 2.4%, carbon 0.04%, and the rest is iron.

4.14

LEAD AND ITS ALLOYS

Lead is obtained from its ores which are found as oxides or sulphides. It is a soft and weak metal (tensile strength 150 kg/cm2). Lead is very malleable and ductile. It is very heavy (density 11.34 gm/cm3) and can be melted easily (melting point 327°C). Lead can be cast, rolled or extruded. It is less ductile because of poor tensile strength. Lead has high coefficient of thermal expansion and has very high anti-frictional properties because of which it is used in bearing metals. Environmental contamination or lead poisoning is a major concern. Lead is available commercially in the following forms: (a) Corroding lead which is pure, (b) Chemical lead having copper, silver and bismuth as impurities which, however, make lead

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resistant to corrosion (to sulphuric acid), (c) Tellurium lead has small amount of tellurium added to lead to make it with fine grain, higher tensile strength (about double), as also much greater resistance to corrosion, and (d) Antimony lead has better mechanical properties and is used for storage battery plates.

4.14.1 ●

● ● ● ● ● ● ● ●

●

●

●

4.14.2

Uses of Lead Lead is an alloying element in solders, steels and copper alloys and promotes corrosion resistance and machinability. Used for water pipes, roof sheets, paint industry and battery plates. It can be rolled into sheets, strips and made in pipes. Important element used for antifriction bearings. It is used with steel and other metals to produce free-machining characteristics. Lead sheets used for flooring in chemical plants and acid works. Because of its softness and ease in rolling, it is used as gasket material. It (with asbestos) is used as pads under machines and buildings for damping vibrations. It is a heavy weight metal and hence used as counter weights in various equipment like cranes. Used for lining purposes to resist sulphuric acid, sea coast atmosphere where lead is used for waste pipes for flow of sea water, for lining the refrigerator roof and aquarium. It is used in pulp and paper industry in the form of pipes for cooling sulphur-dioxide gas, for bleaching with hydrogen peroxide and several other applications. It is used as solid lubricant for hot metals forming operations.

BIS Specifications for Lead

Some of the BIS standards covering lead and its usages are as below. IS: 404–1977 lead pipes IS: 405–1977 lead sheets and strips IS: 25–1979 antifriction bearing alloys

4.15

LIGHT METALS

Aluminium and magnesium are called the light metals because aluminium has density 2.7 gm/cm3, which is about one-third that of iron. Similarly, magnesium has its density 1.74 gm/cm3, which is about two-thirds that of aluminium. Aluminium and its alloys have already been discussed. Magnesium and its alloys are discussed in the following.

4.16

MAGNESIUM AND ITS ALLOYS

Magnesium is obtained from several sources like sea water, natural salt brines and the ores (magnesite, dolomite) using electrolysis process. Magnesium is the lightest metal with density 1.74 gm/cm3 (about two-thirds weight of aluminium) and melts at 651oC. It can be cast in sand

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mold and pressure die-casting and can obtain good surface finish. It can be used for gear boxes and differential housings of the motor car. Magnesium castings are pressure tight. With addition of manganese (2%), magnesium can be formed into plates, sheets, drawn, spun and machined easily. Powdered magnesium burns easily in air. Magnesium with a specific gravity of only 1.74 is the lightest metal available for engineering applications. The chief source of magnesium is sea water, which contains about 0.3% magnesium. Magnesium is very ductile, easy to machine and adaptable to all usual methods of metal working. It can be sand cast, die-cast, painted, anodized and plated. It has resistance to alkalies and most oils. It is non-magnetic and has low electrical resistivity. Sand casting of magnesium finds extensive use in aircraft engines and aircraft landing wheels where light weight and high strength and shock resistance are needed. It is used for high speed rotating parts, gears housings, carburettor bodies, oil sumps and vacuum cleaner parts. Magnesium is a very ductile metal and is most readily machinable. Magnesium wire when heated in air burns with a dazzling white light. It is used for fire works, photography and signalling. The amount of cold work that can be done on magnesium alloys is limited but they can be hot-worked. Magnesium is alloyed readily with most elements except iron and chromium. Aluminium is the most commonly alloyed element, 3 to 10% for strength and hardness. More the aluminium, harder the alloy. Zinc (3%) and manganese (0.3%) are also added for greater resistance to corrosion. Commercially pure magnesium has so little tensile strength that it is of practically no value for use in construction, but some of its alloys have tensile strength in excess of 36 kg/mm2. Magnesium alloys should not be used over 200oC as its strength reduces considerably beyond that. The modulus of elasticity of magnesium is low (4680 kg/mm2) and hence it has good capacity for energy absorption. The most outstanding alloy of magnesium is dow metal.

4.16.1

Dow Metal

Dow metal has magnesium 85 to 95%, aluminium up to 12%, and little amount of manganese. Copper and cadmium are also added sometimes to impart high thermal conductivity. Dow metal can be cast, forged, rolled and drawn into wires, tubes, and extruded into sheets. It can be welded and machined. It is very light and is used for making crank cases, head and fuel tanks of aero engines and other parts of automobile engines and aircraft.

4.17

LOW MELTING METALS AND ALLOYS

Low melting metals or alloys are those which have relatively low melting point. Metals coming under this category are lead, zinc and tin and their alloys. Lead melts at 327°C, zinc at 419°C and tin at 232°C. The field of low melting point alloys includes all combinations of metals that have melting temperature below 540°C. This classification is further divided into the following categories.

4.17.1

Alloy Becoming Fluid between 150 and 540°C

Alloys becoming fluid between 150 and 540°C include zinc-based alloys, tin-based alloys and lead-based alloys.

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Zinc-base alloys are used in the manufacture of carburettor, fuel pump, door handles, household appliances, lathe gears and bower rotors. These alloys are easily cast; in fact more die-castings are made of zinc-base alloys than any other casting because of lower production cost and less maintenance of die (due to low temperatures involved relatively). A typical zincbase alloy has aluminium 4.1%, copper 2.7%, magnesium 0.03% and zinc 93.17%. It has melting point at about 390°C and tensile strength 1440 kg/cm2. Tin-base alloys are commonly called babbitt metals and are used for bearings. Main alloying elements are copper, antimony and lead. Antimony hardens these alloys and increases their antifriction properties. A typical alloy contains tin 89%, copper 3.5% and antimony 7.5%. Their melting point varies between 300 and 400°C. It is used extensively in automotive and aircraft bearings. Yet another alloy has tin 60%, copper 3.5%, antimony 10.5% and lead 26%. It is much cheaper than the former but is used for light duty bearings. All tin-base alloys are corrosion resistant and can be used in food handling equipment and for soda fountain hardware. Pure tin melts at 232°C. Lead-base alloys are composed principally of lead and antimony. Although lead melts at 327°C, the addition of antimony (17%) hardens the lead, reduces shrinkage but it also reduces the melting point of alloy to 294°C. Sometimes arsenic is also added to lead-base alloys. These alloys have low strength but are cheap and easy to cast. They are used for light duty bearings, battery parts, weights, X-ray shields and for non-corrosive atmosphere. Since shrinkage of leadbase alloys (and also tin-base alloys) is very low, this facilitates casting parts to accurate sizes and reduces danger of shrinkage cracks in castings. Solders are tin–lead alloys. Solders contain small amount of antimony and sometimes silver also. Though very wide range of solders are there, the most widely used solders are: (a) Tin solder having lead 38.1% and tin 61.9% with melting point 183°C, and (b) Plumber’s solder having lead 68% and tin 32% with melting point 253°C.

4.17.2

Alloys Becoming Fluid between 94.5 and 150°C

Bismuth is the common constituent of all the alloys of this category, as also the ultra-low-melting group (alloys becoming fluid at less than 94.5°C). The usual alloying elements are tin, lead and cadmium. Bismuth is one such element that does not shrink when it solidifies. Water and antimony are two other substances that expand on solidification but bismuth expands more than the former, i.e. 3.3% of its volume. This characteristic of bismuth alloys makes them excellent for casting as when they expand into mold, they pick up every detail. It is because of this property that bismuth alloys are used widely for anchors. The growth allows them to grip eyed, notched or bossed surfaces on which they are cast and to fill a number of spaces tightly, despite the absence of fusion or bond with the materials with which they come in contact. Anchoring of bearings, bushings and stationery parts in machinery in oversize holes or opening and pouring molten bismuth alloy to grip the component embedded. Dies and punches for blanking, shearing, trimming are secured in steel shell by use of these alloys.

4.17.3

Alloys Becoming Fluid at Less than 94.5°C

Alloys becoming fluid at less than 94.5°C have limited application so far, but the present trend shows their increasing use. Bismuth is the chief alloying element. These are used for

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fire-extinguishing systems, fusible plugs, production of molds and patterns for dentures, molds for plaster casting. The most common use is for bending thin walled tubing wherein this alloy is filled before bending, and after the bending operation is over, the alloy is melted out by immersing in boiling water.

4.18

BEARING METALS

Bearing is a component of machine supporting another moving (mostly revolving) component (known as journal or shaft). Although ball and roller bearings are there to support rotating shafts but the bearings made from antifriction metals are still extensively used for reasons of simplicity of design, low cost and large load carrying capacity. Since the antifriction materials used for bearings are mostly non-ferrous metals and their alloys, these only will, therefore, be discussed here (and not the materials for ball and roller bearing). The material selected for a bearing depends on factors such as loading, temperature, lubrication, maintenance and other service conditions.

4.18.1

Main Requirements (or Characteristics) of an Antifriction Bearing Metal

The following requirements should be fulfilled by a good bearing metal. (i) Loading a magnitude and the nature of load call for a particular material, for example, heavy loads need higher compressive strength of bearing metal to withstand the load without deformation or failure. The material needs to be ductile also to absorb pounding (or shocks) without cracking (due to being brittle). (ii) Fatigue strength is considered when load is of repetitive type or fluctuating type. The bearing material should not develop surface cracks under fluctuating or reversible load. This property is of major importance for bearings used in aircraft and automotive engines. (iii) Comformability is the ability of bearing metal to accommodate shaft deflections and inaccuracies of alignment of shafts and bearing. This is the usual problem encountered in the field. The bearing should take all this without excessive wear and heat. (iv) Embeddability is the ability of bearing material to accommodate (or embed) small particles of dust, etc. without scoring the journal or shaft moving in the bearing. For this, the bearing metal should have low modulus of elasticity and yield strength. (v) Bondability is an important consideration where many high capacity bearings are made of composite materials, i.e. where one or more layers of bearing metal are bonded with steel shell. Thus the strength of bond becomes important. (vi) Corrosion resistance is important because bearings are lubricated with oils or grease and the bearing metal should not corrode under the effect of these lubricants. Other factors for developing corrosion are: moisture, contaminated atmosphere and improper lubricant, i.e. that may result in acid formation in use. (vii) Temperature and thermal conductivity of the bearing metal should be high so as to permit rapid removal of heat generated on account of friction between the revolving

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loaded shaft and the bearing. The surface speed of shaft, amount and method of lubricant supply and conductivity of bearing metal give some idea of the temperature that will be developed inside the bearing. Depending on the size and type of bearing material, there is a limit on temperature development inside the bearing without losing its strength and developing excessive corrosion. (viii) Thermal expansion consideration needs the bearing material to have low coefficient of thermal expansion so that when bearing operates under higher temperatures, there is no undue change in the clearance between the shaft and the bearing hole. (ix) Metability or anti-seizure property: Metal to metal contact of shaft and bearing during operation is not desirable in a bearing system, and hence the bearing should be capable of forming an oil film in between the mating parts. The property of forming such an oil film is called metability, but in practice such a film does not always exist, particularly when a loaded shaft just starts moving. Hence bearing metal should be such that it allows metal to metal contact without welding or seizure of mating surfaces. This problem is aggravated further under higher temperatures inside the bearing. Anti-seizure property or resistance to galling or scoring is improved if the bearing metal has high thermal conductivity.

4.19

CLASSIFICATION OF BEARING METALS

The main component in all types of antifriction bearing alloys is tin. It reduces brittleness and increases compression strength. When tin is added in bronzes (which are also bearing metals), it makes them harder and stronger. General bearing metals can be divided into two broad categories. (1) Soft metal bearings (or white metal or babbitt metal bearings) (2) Hard metal bearings (or bronze bearings) IS: 25–1966 designates the antifriction bearing alloys according to the percentage of tin in them.

4.19.1

Soft Metal Bearings

Soft metal bearings are made from white metal which is an alloy of tin, lead and cadmium as main elements. These are further divided as below. (i) Tin-base white metal: The most common tin-base white metal is babbitt metal. In fact, babbitt metal is sometimes used in general term for soft lead and tin-base bearing metals also. The composition of babbitt metal is given below. Tin Copper Antimony

70 to 90% 2 to 24% 7 to 24%

In general, babbitt metals are prepared for higher speeds and fluctuating loads but of light type. These bearings have excellent anti-seizure properties. Babbitt metal cast liners (on rolled sheets) are used to form bearing halves. Babbitt may be bonded to a steel backing for forming a liner.

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(ii) Lead-base white metals: These have lead 87%, an average tin 4% and antimony 9%. Bearings for electrical machines, tramways and engines are the examples of lead-base bearings. The tin-base bearings are costlier than lead-base bearings but have better resistance to corrosion in acidic oils. Both tin-base and lead-base white metal bearings are widely used as bearing material. These are safe for general application for bearing pressure (on projected area) between 70 and 140 kg/cm2. In automobiles, thin liners of babbits (0.05 to 0.15 mm thick) are used bonded on steel shell. (iii) Cadmium-base white metal: These behave better at elevated temperatures in comparison to babbit metal bearings. The fatigue strength is high. A typical alloy of this type contains cadmium 95%, silver 5% and a trace of iridium. Copper up to 0.5% is added sometimes. These are used for car engines and other machines running at high speed.

4.19.2

Hard Metal Bearing (or Bearing Bronzes)

Hard metal bearings are the alloys of copper, tin and zinc and are used generally in the form of machined bushes pressed into the housing or shell. The bush may be one piece or in two halves which can be clamped in the housing (plummer block). The most common bronzes used are: (i) Gun metal and (ii) Phosphor bronze. Gun metal (copper 88%, tin 10%, zinc 2%) is a high grade bearing metal. Gun metal bearings can withstand high loads (100 kg/cm2) and high speed. Phosphor bronze (copper 70%, tin 16%, lead 14%) is used for heavy duty high loads pressure (140 kg/cm2).

4.19.3

Other Bearing Metals

Other bearing metals include cadmium–nickel bearings, silver bearings, copper-base heavy-duty bearings, cast iron bearings and non-metallic bearings. Cadmium–nickel bearings contain cadmium 1.25 to 3%, and the rest is nickel. These retain their bearing properties even at higher temperatures. Silver bearings contain lead 4% and the rest is silver. These show lowest coefficient of friction against iron (nickel alloy). Because of high fatigue strength, these are used in aircraft engines. Copper-base heavy duty bearings have copper 80%, silicon 10% and lead 10%. Cast iron bearings are used with steel journals and with lubricator; these work well up to a load of 35 kg/cm2 and speed 40 metres per minute. Non-metallic bearings are made of carbon-graphite, rubber, wood and plastic (nylon and teflon). The carbon-graphite bearings are self-lubricating, dimensionally stable over a wide range of operating conditions and chemically inert, and can operate at higher temperature than other bearings. These are used in food processing and other equipment where contamination by oil grease is prohibited.

4.19.4

Innovative Antifriction Bearing Materials and Techniques

Friction is the main cause of problems in bearings as it results in wastage of energy, wear and ultimately failures. Scientists have tried to solve friction problem in the following ways.

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(a) Instead of having an ideally smooth surface (of the bore in bearing), a preset relief with micro-depressions and gradients is created and thus oil pockets are formed to hold lubricants. (b) A unique antifrictional material (metal-fluoroplastic) has been developed consisting of a steel base, and a thin (0.3 mm) porous bronze layer, with its pores filled with a mixture of fluoroplastic and molybdenum disulphide. These bearings need no lubrication over a wide range of temperature. These are 10 to 15 times lighter and are half in outer diameter as compared to regular metal bearings. (c) Magneto-powders are developed to create very thin protective film from an elastic porous lubricant on the surface of part. (d) Using powerful laser radiations for a directional change in the properties of friction surfaces results in decrease in coefficient of friction and increase in micro-hardness of the friction surfaces. (e) Composites having a polymer or metal reinforced by carbon fibres, glass fibres, boron fibres or organic fibres help reducing size of bearings.

4.20 CHROMIUM By adding chromium to nickel, resistance to oxidation is increased. There is increase in electrical resistance also as (with 20% chromium) electrical resistance of nickel–chrome alloy is over ten times of the electrical resistance offered by nickel. Similarly, by adding chromium to nickel, tensile strength also increases and it remains high at elevated temperatures also. The alloys of chromium and nickel are used for electrically heated appliances working at temperatures above 850oC, for muffle furnaces in the form of wires for heating elements, as sheet or cast plate for furnace bottom, tubes for thermocouple sheaths, heating coils or elements (trade name Chromel or Nichrome) for furnaces in view of its high electrical resistance.

4.21 COBALT Cobalt was introduced as an alloy in tool steel as cobalt cutting tools were superior to high speed steel tools. Later, cobalt was used as a binder in cemented carbide tools to improve toughness. Cobalt is an important constituent of high temperature, high strength steels having damping properties and used for turbine blades subjected to vibrations. Cobalt and its alloys can be spot-welded, brazed or fusion welded. It is used in invar, a low expansion metal used for thermostat controls.

4.22

OTHER NON-FERROUS METALS

Other non-ferrous metals include metals such as titanium, zirconium, beryllium, niobium (columbium), tungsten, tantalum, etc. Some of these are discussed in the following, whereas niobium, tungsten and tantalum will be discussed later under the category of refractory metals and alloys.

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Titanium: Titanium is classed as light metal (60% heavier than aluminium), has high temperature properties, and is highly corrosion resistant. It can retain its strength up to 540oC. Titanium when alloyed with carbon (up to 0.4%), the tensile strength of titanium–carbon alloy reaches up to 75 kg/mm2. If tungsten is also added, strength may go up to 87 kg/mm2. Titanium–manganese alloys find use in the aircraft industry. Zirconium: Zirconium is found in nature as zirconium silicate. It oxidizes easily and is used for lining the high temperature furnaces as zirconium melts at 1852oC but its oxides melt at 2700oC. When alloyed with nickel, chromium, tin and iron, the corrosion resistance of zirconium is improved. Zirconium is a nuclear material. Beryllium: Beryllium is lighter than aluminium and is used as an alloying metal with copper and nickel to help increasing strength and modulus of elasticity. It has high thermal and electrical conductivity and good strength at high temperature. Beryllium copper alloys are strong and replace forged steel tools for use in explosive atmosphere as these alloys are non-sparking. Also, beryllium is used in aircraft and other space vehicles because it is light, strong, and has high modulus of elasticity.

4.23 SUPERALLOYS Superalloys are also called heat resistant or high temperature alloys. These are used in high temperature applications such as in jet engines, gas and steam turbines, rocket engines, tools and dies for hot-working of metals, nuclear and petrochemical industries, furnaces and missiles, etc. They show good resistance to corrosion, oxidation, erosion at high temperature, mechanical and thermal shocks and fatigue and creep. Most superalloys are generally used for service temperatures up to 1000°C or up to 1200°C for non-structural, non-load bearing components. Superalloys are categorized as follows. (a) Nickel-base superalloys: These are the most commonly used alloys. These are available in wide compositions with nickel 38 to 75%, chromium up to 27% and cobalt up to 20%. Common alloys in this category are available with trade names Hastelloy, Inconel, Astroloy, etc. (b) Cobalt-base superalloys: These contain cobalt 35 to 65%, chromium 19 to 30% and nickel 35%. These are not as strong as nickel-base superalloys but they retain their strength well at higher temperatures. (c) Iron-base superalloys: These have iron 32 to 67%, chromium 15 to 22% and nickel 9 to 38%. Incoloy series are common superalloys of this category.

4.24

REFRACTORY METALS AND ALLOYS

Refractory metals are those metals which have high melting point and maintain their strength at elevated temperatures. Because of these characteristics, these metals are used for rocket engines and gas turbines, as tool and die materials, and for nuclear power plants and chemical industry. The temperature range of their applications is 1100 to 2200°C.

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The four well-known refractory metals are: (a) (b) (c) (d)

Molybdenum Niobium (also called columbium) Tungsten Tantalum

Molybdenum: Molybdenum (Mo), a silvery-white metal, has high melting point (2620°C), a high modulus of elasticity, and good resistance to thermal shocks, besides good thermal and electrical conductivity. Most used refractory metal, molybdenum finds use in jet engines, heating elements, dies for die-casting and electronic components. Molybdenum is mostly alloyed with titanium and zirconium. It has unusual properties that make it useful in alloys of steel and cast iron and high temperature alloys. Its hardness varies from 150 to 250 HB at high temperatures. Molybdenum disulphide is used in greases and oils for lubrication. Niobium (Nb): Niobium, also called columbium, is soft and extremely ductile, has good formability and greater oxidation resistance than other refractory metals, with its melting point 2468°C. It is used for alloying materials for use in rockets and missiles, gas turbine blades, nuclear and chemical applications. Tungsten (W): Tungsten has the highest melting point of any metal (3410°C) because of which it is characterized by high strength at elevated temperatures. It has high density but is brittle at low temperatures. Because of high density, it is used as balancing weight material in mechanical system and watches. Tungsten and its alloys find use where temperatures are above 1650°C such as nozzle throat liners in missiles, hottest parts of jet and rocket engine, circuit breakers, electrodes for welding, filament wire in incandescent light bulbs, etc. Tantalum (Ta): It has high melting point (3000°C), good ductility and resistance to corrosion. Tantalum is used as an alloying element, with its main application being in electrolytic capacitors, electrical, electronic and chemical industries, thermal applications in furnaces and heat-exchangers.

4.25

SHAPE-MEMORY ALLOYS

Shape-memory alloys have unique property of reversible behaviour, that is, after being plastically deformed at room temperature into various shapes, they return to their original shapes upon heating. If a wire of this alloy is wound into the form of a helical spring, on heating it uncoils and returns to the original form of wire. These alloys are available in several compositions, for example, (i) nickel 55%, titanium 45%, (ii) copper–aluminium–nickel, (iii) iron–manganese–silicon, (iv) copper–zinc–aluminium, etc. These alloys are very ductile, have good corrosion resistance, and good electrical conductivity. These are used for space applications (fold them at room temperature for carrying and heat them to bring to original shape), for clamps, connectors, seals, eye glass frames, and antiscaled valves fitted in piping system, in sinks, tubs and showers to protect people from scalding (burning with hot water) by lowering the flow to a trickle within 3 seconds if water temperature reaches 47°C.

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PRECIOUS METALS

Precious metals (also called noble metals) are costly metals and include gold, silver and platinum. Gold (Au) is very soft and ductile with good anticorrosion property at any temperature and is used for reflectors, coinage, jewellery, dental work, electroplating and electrical contacts. Silver (Ag) is a ductile metal with highest electrical and thermal conductivity of any metal. It is used for electroplating, photographic film, electrical contacts, coinage and jewellery. Platinum (Pt) is a greyish white metal and is soft and ductile with good corrosion resistance at high temperatures. It is used for electrical contacts, filaments, nozzles, dies for extruding glass fibres, thermocouples, jewellery and electro-chemical industry.

4.27

METALLIC GLASSES

Metallic glasses are also called amorphous alloys, which do not have long range crystalline structure and have no grain boundaries, and their atoms are randomly and tightly packed. Because of resemblance of their structure with that of glasses, they are called metallic glasses. They contain iron, nickel, chromium, carbon, boron, phosphorus, aluminium and silicon. These alloys are available in the form of wire, ribbon, strip and powder. They have excellent corrosion resistance, good ductility and strength and very low loss from magnetic hysteresis, and hence are used for magnetic steel cores for transformers, generators, motors, magnetic amplifiers, etc.

4.28 NANOMATERIALS Nanomaterials have been developed to have their particle size in the order of 1–100 nm (where 1 nm = 10–9 metre) which make them superior to traditional materials in strength, hardness, ductility, corrosion resistance, besides making them more suitable for structural load bearing applications, and with unique electrical, magnetic and optical properties. Important compositions of these materials include carbides, oxides, nitrides, some metals and alloys, organic polymers and composite materials. These are available in various shapes such as nanopowders, nanowires, nanotubes, nanofilms, etc. Important applications of nanomaterials include: cutting tools and inserts of nanocrystalline carbides and ceramics; ductile and machinable ceramics; computer chips; ignitors for rockets; spark plug electrodes; high sensitivity sensors; high power magnets, etc.

4.29

METALS FOR NUCLEAR ENGINEERING APPLICATIONS

Uranium, thorium, plutonium, zirconium, titanium, beryllium and niobium are some of the important materials that find applications in the generation of nuclear energy in nuclear plants. Some of them are used as fuels, others as moderators, and some for structural purposes. Uranium is the most commonly used fuel. It is highly radioactive and easily oxidized. The natural uranium deposits carry two types of isotopes, uranium 238 and uranium 235. For fuel purpose, common elements alloyed with uranium are molybdenum, zirconium, chromium, etc.

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Plutonium is produced from uranium 238 and is easily oxidized. It is mainly used as fuel in fast breeding atomic reactors. Thorium is yet another material which is also radioactive and is used as fuel. Zirconium finds application in water cooled reactors. It has good weldability and fatigue resistance and hence is used for structural purposes. Titanium is also a strong material used for structural elements in reactors. Beryllium is used as a moderator, reflector and a source of neutron. It has good strength and high melting point (1283oC). Niobium has still higher melting point (2470oC) and has good compatibility with uranium. It is added to many base metals to make heat-resistant alloys as it offers very good corrosion resistance.

4.30

OTHER IMPORTANT ENGINEERING MATERIALS

Other important engineering materials (not falling in the categories of ferrous and non-ferrous metals and alloys) are discussed in the following. These include: (i) Timber (ii) Abrasive materials (iii) Ceramics (iv) Silica (v) Glasses (vi) Glass ceramics (vii) Graphite (viii) Diamond (ix) Plastics and polymers (x) Composite materials (reinforced plastics; metal-matrix and ceramic-matrix composites)

4.30.1 Timber Timber is the general name given to that wood which is good and suitable for engineering use. A good timber is strong, durable, dense, hard and free of defects. It takes up various woodworking operations (cutting, sawing, planing) easily. It also takes up polishing well. Wood as a forest product had been in use by mankind for building and household works right from the beginning of civilization. Even today, wood is an important and preferred material for many applications in buildings, furniture and other structures.

4.30.2

Abrasive Materials

The abrasives are small, non-metallic hard particles with sharp edges and irregular shapes. Abrasives when used in various forms (for example, as grinding wheel which is a bonded abrasive), are employed to remove or cut very small amount of material from the job surface as in grinding, polishing, honning, buffing and superfinishing operations on metals. Important abrasives used in manufacturing industry are as follows: (i) Aluminium oxide (Al2O3) (ii) Silicon carbide (SiC) (iii) Cubic boron nitride (CBN) (iv) Diamond

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The above first two are called conventional abrasives and the last two superabrasives. The natural abrasives are emery, corundum (alumina), quartz, diamond, etc. Since they contain unknown amount of impurities and give non-uniform properties, abrasives are, therefore, made artificially or synthetically for industrial use as described in the following. Aluminium oxide is made by fusing bauxite, iron filings and coke. It is available in several forms. Ceramic aluminium oxide (seeded gel), a new development, which is purest and harder than fused alumina, is used for making tools for machining difficult-to-grind materials. Silicon carbide is composed of silica sand, petroleum coke, and sodium chloride and made as black (less friable) and green (more friable). Particles of these carbides have higher friability (easily reduced to powder) than aluminium oxides and hence are more sharper, but with greater tendency to fracture. Cubic boron nitride (CBN), the second hardest known material (next to diamond), has special use in cutting tools and abrasives for grinding wheels. It is made only synthetically (like synthetic diamond) as it does not exist in nature. For cutting tools, CBN tool material is made by bonding 0.5–1 mm layer of polycrystalline cubic boron nitride to a carbide substrate (bulk or core material) by sintering under pressure. Diamond is the hardest known material. More details about the making of synthetic diamonds are given later.

4.30.3 Ceramics Sometimes we need materials with diversified properties. Metals usually fail to qualify but ceramics now have all those diversified properties such as high compressive strength, high elastic modulus, low thermal expansion, high temperature strength; hardness; wear resistance, inertness to chemicals; foods and environment and low electrical and thermal conductivity. Metallic and non-metallic materials discussed so far may not be suitable in view of these diversified properties. Ceramics which are the compounds of metallic and non-metallic elements are, however, considered the most suitable materials having the aforesaid diversified properties. The earliest use of ceramics was in pottery and bricks. These have now become important as materials for tool and dies, heat engines, exhaust port of engines, coated pistons and liners. Traditional ceramics are used for tiles, sewer pipe, pottery, etc. Porcelain, which is a white ceramics made of kaolin, quartz and feldspar, finds extensive use in appliances and sanitary wares. Fine ceramics or industrial ceramics are used for components of turbines, aerospace vehicle components, heat exchangers, semiconductors, seals and cutting tools. Ceramics have following characteristics compared to metals: high strength, brittleness, hardness at elevated temperatures, high elastic modulus, low toughness and density, low thermal expansion and low heat and thermal conductivity. Fused silica has high thermal shock resistance as it has virtually zero thermal expansion and thus avoids thermal cracking (or spalling). Ceramics find extensive use in electrical and electronics industry as they have high electrical resistance and dielectric resistance. Ceramics also find use in high temperature application because of their capability of maintaining strength and stiffness at elevated temperature. These are, therefore, used as cylinder liners, bushings, seals and bearings. Silicon nitride ceramic ball bearings are used in high performance spindle bearings of machine tools. Coating metals with ceramics is another use to reduce wear, control corrosion or to give thermal barrier as tiles on

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space shuttle (made of silica fibres with open cellular structure consisting of 5% silica and the rest air hence very light). Since ceramics have low density and high elastic modulus, they give lighter components with high strength and stiffness and reduced inertia forces (for engines, turbogeneraors, etc.). Bioceramics are now used for replacing joints in the human body as these are lighter, made porous, are inert to the human body and have high strength and stiffness. Oldest raw material for making ceramics is clay having fine sheet-like structure. Kaolin is the commonly used variety of clay, which consists of silicate of aluminium with alternating weakly bonded layers of silicon and aluminium ions. Flint (rock made of fine-grained silica, SiO2) and feldspar (crystalline mineral having aluminium silicate, potassium, calcium or sodium) are other raw materials used for making ceramics. Ceramics have a very large family. Different types of ceramics can be broadly categorized as below. (i) Oxide ceramics: (a) alumina (corundum or emery), (b) zirconia (ii) Carbides: (a) tungsten carbide, (b) titanium carbide, (c) silicon carbide (iii) Nitrides: (a) cubic boron nitride, (b) titanium nitride, (c) silicon nitride, (d) sialon, (e) cermets (iv) Silica (v) Glasses and glass ceramics (vi) Graphite (vii) Diamond The characteristics of various ceramics are discussed in brief in the following. Oxide ceramics: Alumina has high hardness, moderate strength and is most widely used ceramics for cutting tools, abrasives, electrical and thermal insulation. Zirconia has high strength and toughness with thermal expansion near to cast iron. It is suitable for heat engine components. Carbides: Tungsten carbide has high hardness, strength and wear resistance which mainly depends on the contents of cobalt used as binder. It is used for dies and cutting tools. Titanium carbide is not that tough as tungsten carbide. It has nickel and molybdenum as binder and is used for cutting tools. Silicon carbide has high-temperature strength and wear resistance and is, therefore, used for heat engine components. It is also used as abrasives. Nitrides: Cubic boron nitride is the second hardest material next to diamond and is used as abrasives and for cutting tools. Titanium nitride is a golden colour substance used as coating since it has low frictional characteristics. Silicon nitride has high resistance to thermal shocks and creep and is therefore used in heat engines. Sialon is made up of silicon nitride and other carbides and oxides. It is used as a cutting tool material. Cermets consist of oxides, carbides and nitrides. These are used in high-temperature applications. Silica: Silica imparts high resistance against high temperatures. The quartz exhibits piezoelectric effect. Silicates having various oxides are used in high-temperature non-structural applications.

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Glasses: They usually contain silica at least 50%. They have amorphous structure and are available with different mechanical and physical properties. Glass ceramics: These have highly crystalline structure with good resistance against thermal shocks. These are quite strong. Graphite: Graphite is the crystalline form of carbon with high electrical and thermal conductivity. It has good resistance against thermal shocks. Diamond: It is the hardest known substance which may be available as single crystal or polycrystalline form. It is used as cutting tool material and abrasives, as also for making dies for fine wire drawings. Some of the above ceramics have already been described as cutting tool materials (Chapter 3). Silica, glasses, graphite and diamond are discussed in the following.

4.30.4 Silica Silica is available in abundance in nature in the form of quartz, which is a hard and abrasive hexagonal crystal. Most glasses contain more than 50% silica. Piezoelectric effect is exhibited by silica (and some ceramics) in which there is a reversible interaction between an elastic strain and an electric field, and this property is used in making transducers (which convert strain from external load into electric energy). Quartz finds extensive use as an oscillating crystal of fixed frequency. Silica, when reacted with oxides of aluminium, magnesium, calcium, potassium and iron, gives silicates of different types such as clay, asbestos, mica, silicate glasses, etc. which have numerous industrial applications.

4.30.5 Glasses Glass, an amorphous solid, has structure of a liquid, i.e. it has been supercooled at a rate too fast to allow formation of crystals. All glasses contain at least 50% silica (known as glass former) with other additives (known as modifiers) as oxides of aluminium, sodium, calcium, magnesium, titanium, lithium, lead and potassium. Glasses are resistant to chemical attack and find application as window glass, containers, lighting and TV tubes, cookwares, fibre optics for communication by light (with little loss in signal power) employ special glasses. Glass fibres have high strength and are used in reinforced plastics. Glasses are available in various types, such as soda-lime glass (most common), lead-alkali glass, borosilicate glass, 96% silica glass, fused silica glass, etc. In terms of a thermal property (rather than mechanical), glasses may be hard or soft. Soda-lime and lead-alkali glasses are soft and the rest hard. Fused silica glass is costliest with the highest strength and resistance to thermal shocks and best chemical resistance and best impact-abrasion resistance but poorest hot workability. On the other hand, lead glasses have highest density, best electrical resistivity and hot workability, fair chemical resistance, low thermal resistance but low in cost. Soda-lime glasses are cheapest. Glass is brittle and is considered perfectly elastic with modulus of elasticity 55 to 90 GPa and hardness ranging from 350 to 500 HK (i.e. 5 to 7 on Mohs scale). Molten glass drawn into fibres (fibreglass) has tensile strength 0.2 to 7 GPa and is thus stronger than steel. Fibreglass is used for reinforcing plastics for making boats, automobile bodies, furniture, sports material,

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etc. Glasses have low thermal conductivity but high electrical resistivity and dielectric strength and very low thermal expansion. For example, both the titanium silicate glass (high silica type) and the fused silica glass have a clear construction and have nearly zero coefficient of expansion. Optical properties (such as reflection, absorption, refraction and transmission) are modified by varying composition and treatment of glasses.

4.30.6

Glass Ceramics

While glasses are amorphous (i.e. they do not have a long range crystalline structure and grain boundaries like metals), glass ceramics have high crystalline structure. Glass ceramics are made using high proportion of several oxides which give a structure combination of glass and ceramics. Most glass ceramics are stronger than glass and are not clear (like a glass) and are generally white or grey coloured. Since their thermal expansion coefficient is very low, they have good thermal shock resistance. Unlike conventional ceramics (with porous structure), glass ceramics have higher strength and are used for cookware, heat exchangers, housings for radar antennas and electrical and electronic appliances.

4.30.7

Graphite

Both graphite and diamond are forms of carbon and display unusual combinations of properties and hence have unique and emerging applications in industry. Graphite has high temperature and electrical applications. Graphite has a layered structure or sheet of close-packed carbon atoms and is a crystalline form of carbon. This structure gives graphite a low frictional property when used as a solid lubricant. Both strength and stiffness in graphite increase with temperature. Lamp black (black soot), used as a pigment, is an amorphous form of graphite. Graphite is brittle but has high electrical and thermal conductivity with good resistance to thermal shocks. It is used for applications such as electrodes, pencil lead (made of graphite and clay mixture), heating elements, motor brushes, high temperature furnace parts and fixtures, crucibles for melting and casting of metals and seals. Graphite has resistance to chemicals and hence is used in fillers for corrosive fluids. Graphite fibres are used for reinforcing plastics and other composite materials. Carbon foam having a cellular structure is used in aerospace structures.

4.30.8 Diamond Diamond is a covalently bonded form or structure of carbon and is the hardest known material (7000 to 8000 HK). Natural diamond is brittle and decomposes in air at about 700°C and contains unknown amounts of impurities and non-uniform properties. Synthetic or industrial diamond is produced by subjecting graphite to a hydrostatic pressure of 14 GPa at temperature 3000oC. Synthetic diamonds are identical to natural diamonds, have less impurities and hence are superior to natural diamond. Diamonds are used as cutting tool material, abrasive in grinding wheels for grinding hard materials, dressing of grinding wheels, wire drawing dies for drawing wires less than 0.06 mm size and for coatings for cutting tools and dies, metals, glasses, ceramics and plastics using various techniques such as chemical vapour deposition and others. One of the recent developments is diamond-like carbon (DLC). It is used as a diamond film coatings on tools and dies. Examples of diamond coated products include: wear faces of

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micrometers and calipers, scratch proof windows (as of aircraft), missile sensors for protection against sand storm, cutting tools such as inserts, drills, end mill cutters, surgical knives, razors, diamond coated speakers for stereo systems and fuel injection nozzles. Diamonds have unique properties such as very high hardness, wear resistance, high thermal conductivity, transparency to ultraviolet light and microwave frequencies which have enabled the use of diamond in the production of various aerospace and electronic components.

4.30.9

Plastics and Polymers

The word plastics is generally used as a synonym for polymers which are a major class of materials having very wide range of properties. When compared to metals, polymers are characterized by lower density, strength, elastic modulus, thermal and electrical conductivity but higher strength-to-weight ratio, higher resistance to corrosion, higher thermal expansion, noise reduction, and very wide choice of colours, transparencies and greater ease of manufacturing into complex shapes. Plastics are composed of polymer molecules and various additives and their properties depend on the structure, degree of polymerization and the additives (used to improve strength, flexibility, colour, flame retardation and various other properties). Polymers’ structure is modified to give a wide range of properties. Consumer and industrial products made of polymers (or plastics) include: packing, housewares, textiles, food and beverage containers, signs, medical devices, foams, paints, safety shields, toys, lenses, general appliances, electrical and electronics products (plastic coated electric wires, etc.), automobile bodies and components, aircraft components, sports goods, office equipment, etc. Plastics, being a very important modern engineering material with extensive use in manufacturing articles of domestic and industrial applications, has been dealt at length in a separate full chapter.

4.30.10 Composite Materials Composites are important engineering materials as they offer several outstanding properties as compared to conventional materials. A composite material is a combination of two or more chemically distinct and insoluble phases and the properties and structural performance of a composite are superior to those of its constituents when acting independently. For example, plastics are inferior to metals and alloys in strength, stiffness and creep resistance. When reinforcements of various types (glass fibres and graphite fibres) are embedded in plastics, a new material, reinforced plastic, is born, which has highly improved strength, stiffness, creep resistance, strength-to-weight ratio as well as stiffness-to-weight ratio. Composite materials with the potential of integrating various metals, ceramics, polymers, etc. appear to be well on the way towards eliminating some of the obstacles relating to strength and high-temperature performance. The word fibre is usually associated with thread-like materials as found in wool, cotton or hemp. The length of a fibre is at least 200 times greater than its effective diameter. The two well-known fibres used in composites are glass fibres and high strength wires of tungsten, molybdenum, steel and superalloys. Filaments are endless or continuous fibres. Glass filaments are formed by jets of air that pull glass strands from a spinneret onto a revolving drum.

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Whiskers are needle-like single crystals grown with almost no defect. These are short and discontinuous fibres of polygonal cross-sections made from copper, iron, graphite, silicon carbide, aluminium oxide, silicon nitride, boron carbide, beryllium oxide, etc. Whiskers grow from vapour or metallic depositions of gases or liquids on a surface. Oxide, carbide and nitride whiskers are of primary interest in composite-materials processing. Multiphase fibres consist of materials such as silicon carbide or boron carbide that are formed on the surface of a very fine wire substrate of tungsten. The maximum research in composites had been in the area of carbon and graphite fibres. Continuous graphite fibres (also called carbon fibres) are produced by oxidizing, carbonizing and graphitizing the polyacrylonitrile (rayon) materials at 2500°C. Density of graphite composite is half that of aluminium and one-fifth that of steel. The specific properties (strengthto-density and modulus-to-density ratios) of composites offer substantial performance advantages over those of commonly used metals. Graphite composites have low coefficient of thermal expansion, excellent wear resistance, long fatigue life, are electrically conductive, have excellent vibration damping characteristics with their flexural strength about 2.5 times that of steel. Besides the aerospace industry, graphite composites are used for tennis rackets, bicycle frames, automobile bumpers, guitars, etc. Metals lose much of their tensile strength with increasing temperature (Fig. 4.1). On the other hand, graphite offers tensile strength of 24,500 kg/cm2 to at least 2250°C; silicon carbide (or tungsten) has strength over 14,000 kg/cm2 at 1093°C; and alumina (Al2O3) has the highest strength above 1371°C (Fig. 4.2).

4.31

TYPES OF COMPOSITE MATERIALS

In the following are described the three main types of composite materials.

4.31.1

Fibre Reinforced Plastics (or Reinforced Plastics)

Fibre reinforced plastics are made using fibres of glass, graphite, armid or boron in a matrix of polyester or epoxy and have very high toughness and strength-to-weight ratio and stiffness-toweight ratio. The mechanical and physical properties of reinforced plastics depend on type, shape, length and orientation of fibres. Long fibres transmit loads more effectively through the matrix. Important properties of different types of fibres used in reinforced plastics are given in the following: (i) Glass fibres are most widely used, being least expensive. E-type fibres have tensile strength of about 3500 MPa and lowest cost. S-type fibres are costlier and have tensile strength of about 4600 MPa. E-CR fibres have higher resistance to elevated temperatures and acid corrosion. (ii) Graphite fibres and carbon fibres have low density, high strength and high stiffness but are costlier than glass fibres. Carbon fibres are usually 80 to 95% carbon whereas graphite fibres are more than 99% carbon. Conductive graphite fibres are available which give enhanced electrical and thermal conductivity to the reinforced plastic components.

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Fig. 4.1

Fig. 4.2

Effect of temperature on the ultimate strength of metals.

Showing that some materials behave much better than metals at elevated temperatures.

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163

(iii) Aramids are the toughest fibres with highest strength-to-weight ratio of all fibres. Absorption of moisture by these fibres degrades the properties of the composite. (iv) Boron fibres consist of boron deposited on tungsten fibres. They have high strength and toughness and high resistance to high temperature. Other fibres used are silicon carbide, silicon nitride, aluminium oxide, nylon, asbestos, etc. The matrix material used for reinforced plastics includes epoxy, polyester, phenolic, fluorocarbon, silicon, etc. Application of reinforced plastic includes: acid resistant tanks made of phenolic resin and asbestos fibres, boats made of epoxies with glass fibres, advanced composites made with glass or carbon fibre for high temperature (up to 300oC) application, components of aircraft and rockets, helicopter blades, automobile bodies and pressure vessels ladders, etc. Aluminium application in aircraft has been replaced by graphite-epoxy reinforced plastics with reduced weight and cost and with improved resistance to corrosion and fatigue.

4.31.2

Metal–Matrix Composites (or Fibre–Metal Composites)

Metal–matrix composites are made using fibres of graphite, boron, alumina, silicon carbide, tungsten, etc. in different matrix of aluminium, magnesium, titanium, superalloys, etc. They have higher resistance to high temperatures, higher elastic modulus, ductility and toughness. These composites find extensive use in the structures of helicopters, satellite, electrical contacts, jet engines, power reactors and high-temperature engine components.

4.31.3

Ceramic–Matrix Composites

Ceramic–matrix composites are the composites with a ceramic matrix (silicon carbide, silicon nitride, aluminium oxide, etc.) and have higher resistance to high temperatures (up to 1700oC) and corrosive environment. The carbon–carbon matrix composites are capable of retaining their strength up to 2500oC. Ceramic-matrix composites are used in jet engines, automobile engines, pressure vessels, cutting tools, dies for extrusion and wire-drawing, deep-sea mining equipment, etc.

4.32 SUPERCONDUCTORS The two main types of superconductors (materials exhibiting superconductivity) are as follows: (i) Low temperature superconductors (LTSC) are combinations of metals such as tin, titanium, niobium. (ii) High temperature superconductors (HTSC) are ceramics including copper oxides, etc. These are of greater practical application as they try to show superconductivity close to ambient temperature (referred above as high temperature). Manufacturing of superconductors poses great problems mainly because of inherent brittleness of both metals and ceramics. The ceramic superconductivity materials (available in powder form) are packed in silver tubes (silver having highest electrical conductivity) which are later mechanically worked and deformed to various shapes of wire, coil or tape, etc. Superconductors have major potential in energy saving in generation, storage and distribution of electrical power.

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REVIEW QUESTIONS 1. What are the unique properties of non-ferrous metals that make them so important for engineering applications? 2. What are the factors considered for selecting aluminium as an engineering material? 3. List down the important applications of aluminium. 4. How are aluminium and its alloys suitable for low temperature applications? 5. Discuss the main uses of copper. 6. Name the different forms in which copper is available in the market. 7. Write short notes on the applications of the following: (a) Pure aluminium (b) Zinc (c) Tin (d) Nickel 8. Why is lead called a heavy metal? Discuss its uses. 9. What are light metals? What is the use of magnesium and its alloys in industry? 10. What is the role of chromium when alloyed with nickel? 11. Write short notes on the following: (i) Chromium and non-ferrous nickel alloys (ii) Titanium (iii) Zirconium (iv) Molybdenum (v) Cobalt (vi) Beryllium 12. What are the two main alloys of copper? Name their constituents. 13. Write short notes on the following: (i) Naval brass (ii) Muntz metal (iii) Admiralty brass (iv) Silicon brass 14. What are bronzes? 15. How are the bronzes classified? Discuss their characteristics. 16. Write short notes on: (i) Phosphor bronze (ii) Silicon bronze (iii) Aluminium bronze (iv) Manganese bronze (v) Bell metal 17. What is gun metal? Give its composition and uses. 18. What is duraluminium? Why is it so important for aircraft industry? 19. What are Y-alloys? 20. Write a short note on non-ferrous nickel alloys and their grouping. 21. Write short notes on the following: (i) Monel metal (ii) German silver (iii) Constantant (iv) Inconel 22. What are low-melting point alloys? Describe briefly.

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165

23. What are solders? Discuss the types and uses of solders. 24. What are antifriction bearing metals? Discuss the main requirements of an antifriction bearing metal. 25. Write short notes on the following: (i) Babbitt metal (ii) Bearing bronzes (iii) Gun metal (iv) Non-metallic bearing metals 26. What is a self-lubricating bearing metal? 27. Name a few innovative antifriction bearing metals and techniques. 28. What are shape-memory alloys? 29. What are refractory metals? Where are they specially used? 30. What are different types of shape-memory alloys? 31. What are precious metals? Where are they used? 32. What are metallic glasses? 33. What are nanomaterials? Give their applications. 34. What are abrasive materials? Name important abrasive materials used in industry. 35. What are ceramics? Why have they become so important in industry? Discuss their general applications. 36. Name the various ceramics used for engineering applications. 37. Write short notes on the following giving their characteristics and uses: (a) Oxide ceramics (b) Carbides (c) Nitrides (d) Glasses (e) Glass ceramics (f) Graphites 38. What are plastics? Discuss their uses. 39. What are composite materials? Discuss their use in engineering applications. 40. What are fibres? Diferentiate between filament and whiskers. 41. What are graphite composites? 42. Name the major categories of composite materials. 43. What are fibre reinforced plastics? Write a short note on them. 44. What are metal–matrix composites? Discuss their important engineering applications. 45. What do you understand by superconductivity? What are superconductors? Discuss their role in industry.

5 5.1

Foundry Processes Molding and Casting

INTRODUCTION

The art of foundry (from Latin fundere means ‘melting and pouring’) is fundamental to civilization as it had been in use even during 4000–3000 BC, when bronze arrowheads were cast in open-faced clay molds. It is still the most popular method of manufacturing machine components and other products out of a variety of metals and alloys. The process of casting is known for its versatility of producing very heavy components (weighing in tonnes) on one hand, and highly complicated light components on the other. Metal casting is based on the property of liquid to take up the shape of the vessel which contains it. The process of metal casting (or simply casting) involves pouring of molten metal into a mold, which is a cavity formed in some molding material such as sand. The mold cavity exactly resembles in shape and size with the product to be made by casting. The molten metal, poured into the mold and allowed to freeze there, takes up the shape of the mold cavity and the product thus cast, is called a casting. The process of casting is based on the property of flowability of molten metal by virtue of which it flows into all parts and corners of the mold and on solidifying, takes the shape of the mold cavity. A mold cavity may be formed in some suitable molding material such as moist sand mixed with clay and other ingredients. It is then called a sand mold. A pattern with its shape and size similar to the desired casting is embedded in molding sand which is compacted around the pattern and thus the pattern is used to make a mold cavity (or mold) after being withdrawn from the sand. Besides the sand molds, metal molds (called permanent molds) are also used. Whereas a sand mold is used only once to produce a casting (since the casting is taken out of the sand mold only by breaking the mold), a metal mold is so designed that it is used repeatedly for producing a large number of castings (in thousands) without damaging the mold. Although most metals can be cast by one method or the other, the metals most adaptable to casting include: cast irons of all types, steels (carbon steels and alloy steels), stainless steel, non-ferrous metals (and their alloys) such as aluminium, copper, magnesium, zinc, brasses and bronzes and bearing metals. 166

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167

A large variety of products used in industry and domestic applications are made by casting; examples include: engine blocks; rail road equipment; machine tools bodies and other components; automotive components such as engine cylinders, pistons, crankshafts, flywheels, gears; household and agricultural equipment; pipes and plumbing fixtures, portable power tools and military equipment, etc.

5.2

A FOUNDRY SHOP

A foundry shop is that shop where molding (making of mold), metal melting and casting the molds and their related processes, such as fettling and cleaning of castings, are conducted. To function properly, the foundry shop is divided into the following departments. (i) Molding department, where operations related to making of molds and their baking are carried out. (ii) Core department, where operations related to making and baking of cores are carried out. (iii) Metal melting department, where melting of metal for casting purposes is carried out in a cupola or other furnaces. (iv) Casting treating or fettling department, where cleaning of castings and removing of gates, risers or fins from castings are done. (v) Quality control department is responsible for maintaining a particular quality standard of foundry products (casting) through proper inspection at various stages of production, right from material procurement to the inspection and testing of the castings.

5.3

TYPES OF FOUNDRIES

Foundries are usually categorized according to the type of metals or alloys cast in a particular foundry, for example, (a) (b) (c) (d)

5.4

Cast iron foundries involve casting of different types of cast irons. Malleable iron foundries are devoted to the casting of malleable cast iron products. Steel foundries involve casting of carbon steels and alloy steel products. Non-ferrous foundries involve casting of non-ferrous metals and alloys.

PREFERENCE FOR CASTING OVER OTHER PRODUCTION METHODS

A product can be made in several ways by following different methods of production such as casting, forging or machining. The choice of a particular production method depends on key factors such as mechanical and other properties of the final product, intricacy of its shape, size, dimensional tolerances, available manufacturing capability and market support, and the number of products to be made within a given time. In the following are given some distinct features of the casting process that make it a more favourable production method in many respects.

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(i) It is possible to cast the products with intricate external shapes and complex internal profiles making use of the ability of molten metal to fill completely the cavities in a mold. (ii) Casting may be advantageous when a large number of similar parts are produced, thus covering up the cost incurred on patterns. (iii) Casting permits the designer to place bulk of metal in a product where it is most needed and remove it from the place where it is in excess, for example, as in case of cast crankshafts and machine tool components. (iv) Casting is a suitable process for making very heavy components of structures or machine tool frames. Casting is easier and cheaper in such cases. (v) Parts requiring the property of damping of sound and mechanical vibrations are best made by casting, grey cast iron castings usually. (vi) Components made of refractory metals or highly creep resistant metals and alloys (for the parts of gas turbine) which are difficult to forge or machine are easily cast to close dimensional tolerances. (vii) Casting minimizes anisotropic qualities (or directional qualities found in most wrought or rolled and forged products) which render the rolled component poor in fatigue and impact in the transverse direction of rolling. (viii) Casting of precious metals such as gold and silver does not suffer from the problem of loss of material in the form of chips, flash or scrap. Castings usually suffer from problems such as internal porosity, dimensional variations due to metal shrinkage, solid or gas inclusions, etc. These can, however, be taken care of by proper designing of the cast product and using good foundry practice. Nevertheless, there may be times when it may not be advisable to go in for casting, such as in the following cases. (a) (b) (c) (d)

5.5

When When When When

a a a a

product product product product

can be easily stamped out on a punch press. can be deep drawn on a press. can be made by direct extrusion. is to be made from some highly reactive metals.

FOUNDRY PROCESSES

Various processes or operations carried out in a foundry shop for producing a metal casting can be broadly divided into two separate groups of activities, namely, (i) molding and (ii) casting. These are discussed in the following.

5.5.1 Molding A mold is a void or cavity created in a compact sand mass with the help of a pattern, which is the near replica of the casting to be made. This cavity is filled with molten metal, which, on solidification, results into a casting. The pattern nearly resembles with the shape and size of the casting to be made. The process of creating the cavity or making of mold (in the sand) is termed molding. It consists of compacting molding sand around a pattern (made of wood,

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169

plastic or metal) enclosed in a mold box or flask. Molding-related activities include: pattern making, preparing molding sand mixture, core making, baking of mold and core and preserving molds in case of dry sand molds and metallic molds. Essential features of a sand mold and few terms related to the mold are explained in the following in reference to Fig. 5.1.

Fig. 5.1

Illustrating salient features of a sand mold.

Pouring basin or pouring cup is an enlarged cup-shaped cavity made at the top end of a vertical sprue to help easy feeding of molten metal into the mold from the laddle during the process of casting. Sprue is a vertical entry for the molten metal to flow downward to runners and finally to the mold cavity. Runners are the channels to carry molten metal from the sprue to the mold cavity. Risers are specifically designed to store some additional metal during pouring so that when the casting shrinks during its solidification, the metal from the risers may flow to the casting to compensate for its shrinkage. Risers are designed to keep the metal in them in molten state until the casting is fully solidified. This way, the risers are the last to solidify. Risers also help in escaping out (from the mold) of the entrapped gases and slag which may be dissolved (or in suspension) with the molten metal. Risers may be open risers or blind risers. Open risers indicate the complete filling of the mold by overflowing the molten metal from the top of the risers during casting [Fig. 5.2(b)]. Core is the insert (made of sand) placed in the mold cavity to form hollow regions within the casting or to define the interior surfaces of casting. Cores are also used on the outside of casting to form features such as deep external pockets or contours. Cores are made of sand and some binding material and are usually baked for strength before use. Metal cores are used with permanent metallic molds. Vents are passages to carry off air and gases produced in the mold cavity when the molten metal comes in contact with the mold walls and the cores, since both are usually made from sands having additives which, on burning, generate lots of gases. Gates or ingates are the entry ends of the runners connecting the mold cavity or risers with the main runners.

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Various casting processes (discussed later) need different types of molds which may be broadly categorized as follows. (a) Expendable molds such as sand mold, plaster mold, shell mold, investment casting mold, etc., which are used only once (for casting) as these have to be broken up for removing the casting from them. (b) Permanent molds are made from steel or graphite and are so designed that they can be used repeatedly for a number of times to produce a large number of castings (as the casting can be removed from the mold without breaking it). Examples of such molds include: molds for die-casting, continuous casting and centrifugal casting. (c) Composite molds are made of two or more different materials, such as sand and graphite molds are used for casting aluminium alloy torque convertors. Salient features of a good sand mold A good sand mold should possess the following specific properties to ensure production of sound castings. (i) Properly designed gating system to provide adequate and turbulence free flow of metal in the mold. (ii) Generate minimum amount of gases on interaction of molten metal with mold. (iii) Good venting capability to vent out gases generated during casting. (iv) Good capability of resisting erosive action of hot metal flowing in the mold to ensure correct shape and size of casting. (v) Higher refractoriness to withstand high temperatures of molten metal without the fusion of sand. (vi) Good strength to resist weight of molten metal. (vii) Collapsibility for giving way to free contraction of solidifying casting for avoiding hot tears and cracks in casting. (viii) Capability to provide specific rate of cooling for the casting so that a casting of desired structure may be produced.

5.5.2 Casting The process of casting involves (a) pouring of molten metal into the mold cavity patterned after the product to be made, (b) allowing the metal to cool and solidify in the mold, and (c) removing the cast metal or casting from the mold. During casting, the molten metal may flow through a variety of passages (such as pouring basin, sprue, runners, risers and gating system) in reaching the actual mold cavity. Casting-related activities include: melting of metal and its handling during pouring into the molds, shake out (of the mold) for removing the casting from the mold, cleaning the casting surface by removing sand and cutting the gates and risers unwanted with the final casting, inspection and testing of castings and their heat-treatment in some cases. The process of casting is primarily a function of the type of mold used, not only in terms of the mechanics of making the mold and filling it, but also in terms of the metallurgical results brought about by the wide range of heat-extracting qualities of the mold materials. For example, metal molds have high rate of heat extraction which results in making tough fine-grained

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castings. On the other hand, plaster molds extract heat very slowly and hence are suitable for casting even thin sections (as they are easy to fill in plaster molds in comparison to metal molds wherein quick solidification of metal may choke the flow of metal). The resulting castings in plaster molds tend to be softer and coarse-grained. The normal sand molds lie in between these two types and hence are considered most versatile in performance.

5.6

CASTING IN GREEN SAND MOLD

The method of sand mold casting is an ancient process. It is still the most widely used process. Sand mold casting consists of (a) placing the pattern in the molding sand and making an imprint in the sand compacted around the pattern, (b) taking out the pattern from the sand leaving behind the mold cavity in the sand, (c) preparing the mold ready (for pouring metal) by cutting or making metal pouring basin, gating system and risers in the sand and connecting them to the mold cavity, (d) filling the resulting mold cavity in the sand with molten metal, (e) allowing the metal to cool in the mold until it solidifies, and (f) breaking away the sand mold and removing the casting. There are different types of sand molds, such as green-sand mold, skin-dry sand mold and dry-sand mold. The selection of a particular mold depends on the type and temperature of the metal to be cast, size of casting, surface finish and dimensional accuracy required on the casting. The outline of a green-sand mold casting process is given in the following. Steps involved in making a green-sand mold for casting a cylindrical block are described below with reference to Fig. 5.2(a).

Fig. 5.2(a)

Steps involved in molding a simple block.

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(i) Take the turnover board and sprinkle a little amount of parting sand on it. Place the mold box (drag) and pattern on the turnover board, pattern in the centre of mold box. Fill molding sand around the pattern to a height of about 13 cm using a hand shovel and press the sand evenly with rammer. Pour more sand till it covers the pattern completely and then ram the sand properly around the pattern and in the box. Then, fill and compact the sand in the molding box to its top. (ii) Add more sand so as to raise its level by about 4 cm over the edges of the mold box. Ramming of sand may be done using a flat rammer to make a flat surface of well-dressed sand extending about 6 mm above the box. (iii) Level up the top of the mold box using a strickle bar. Then sprinkle a little amount of parting sand on the top leveled face of the sand box. Use a venting rod to prick deep through the sand so as to provide a number of vent holes in the sand to allow easy escaping of the gases generated within the mold when the molten metal comes in contact with moist mold. (iv) Now lift up the sand filled mold box from the turnover board and turn it upside down and then place it again on the turnover board. Use a trowel to smoothen the sand surface and to make it in line with pattern face. Then sprinkle a little amount of parting sand on the face of the molding box (except the pattern). Place the cope on the drag and clamp both the cope and the drag together. Insert two tapered wooden rods, a gate rod (A) and a riser rod (B), in the sand of the drag. Then pour sand and fill the cope fully with sand. Ram the sand properly keeping both the rods (A) and (B) standing well vertical. Level the top surface with smoother. Vent the mold sand at number of places with the venting rod. Later, take out both the tapered rods (A) and (B) resulting in the formation of two holes, out of which one will work as ‘sprue’ for the entry of molten metal in the mold and another as the ‘riser hole’ for escaping the gases or slag in suspension with the molten metal. Later, using a gate knife, make the pouring cup at the top of sprue for receiving metal at the time of casting. (v) After removing the mold box clamps, pick up the cope box and turn it over and place it on the floor. Brush off all parting sand from the top face of drag and later dampen up the pattern edges (in contact with sand) with a water dipped swab. Gently drive a pattern lifter or spike in the pattern, loosen the pattern in the sand by tapping laterally all round and take it out of the sand. Make necessary connections or channels for the flow of molten metal from the bottom of pouring sprue to the mold cavity and also to the riser hole. Repair the mold with the help of trowel, cleaner or sleeker as per the conditions of breakages in the mold. Later, use a hand blower to clean the sand or dirt, if any, from the mold. Then carefully place the cope over the drag and clamp the two boxes together [Fig. 5.2(b)] and put some weight on the top of sand filled cope so that during the process of casting, due to the pressure of metal, the sand in the cope may not get disturbed. Weight should not cause hindrance to vents. This completes the operation of making the mold.

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173

Fig. 5.2(b) Sectional view showing (weights not shown). The state of mold just before pouring metal at (i) and metal pouring in the mold at (ii).

The next operation is casting, i.e. pouring molten metal in the mold through the pouring cup and seeing that it fills the mold completely and later comes to the top of riser hole. Allow the contents of the mold boxes to cool. After the solidification and cooling of the casting, take it out from the sand mold by breaking the mold. The cast product will have a shape as shown in Fig. 5.2(c). Remove the surplus metal of sprue, riser and runners (shown shaded) with the help of a chisel and hammer leaving the casting free. Grind the casting if so required. Pouring cup Riser Sprue

Runner

Casting

Fig. 5.2(c) Shape of casting as it leaves the mold box. The cast portion of riser, pouring cup, sprue and runner are removed to get the actual casting.

5.7

FOUNDRY TOOLS AND EQUIPMENT

A large variety of tools, gadgets and equipment are used for conducting various operations related to molding, casting and testing of mold materials and castings. These can be categorized as follows: (i) Hand tools (ii) Containers or flasks

174 (iii) (iv) (v) (vi) (vii)

MANUFACTURING PROCESSES

Equipment for machine molding Sand testing equipment Metal melting equipment Fettling and finishing equipment Inspection and testing equipment for castings

Only the hand tools and containers have been discussed in the following and all other equipment from the above list have been discussed elsewhere in this chapter.

5.7.1 Hand Tools Hand tools used by a molder are shown in Fig. 5.3. ● Shovel is used for mixing foundry sand and filling it in the mold boxes. ● Hand riddle is a sorting device used for riddling of sand to remove foreign matter (like nails and wires) when sand is sieved through it. ● Water sprinkler is a handy device for wetting and tempering the sand. ● Rammers are used for packing and compacting molding sand floor or molding sand in the molding box. Different types may include: floor rammer, hand rammer and peen rammer. ● Molding board or turnover board is used either for bench molding or for making a small mold on foundry floor. Wooden boards are quite common. ● Strickle bar or strikeoff bar is a flat bar of wood or metal and is used for striking off the excess sand from the mold box after ramming. It has one edge bevelled with the surface perfectly smooth. ● Vent wire is a thin steel rod with a pointed edge and is used to prick the rammed sand filled in the mold box for making small holes or vents for the escape of gases and steam during casting. ● Trowels of different shapes are used for the repair or mending the broken portion of the sand mold after the pattern is taken out from it. These are also used sometimes for filling sand in small mold boxes. ● Slicks are also used for repairing the mold, particularly the external or internal round and square corners of the mold. ● Brush usually made from cotton is used for clearing the parting sand from the surface of the pattern. ● Swab has small bulk of fibres and is used to dampen the outer edges of the pattern before it is taken out from the mold or compacted sand. ● Cleaners or lifters are finishing tools and used for repairing the mold in deep places after the withdrawal of pattern from the mold. They are also used to remove loose sand from the mold cavity. ● Beads are also used for repairing the mold. ● Draw spike or screw is a steel rod with a loop at one end and pointed or screwed portion on the other end. It is used to rap and lift or pull out the pattern from the mold.

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Fig. 5.3 Hand tools used by a molder. ●

●

●

●

●

●

●

●

Smoother is a rectangular wooden block used to smoothen the upper surface of the mold box filled with sand. Metallic smoothers are also used. Mallet is a wooden hammer used to drive draw spikes in the pattern for pulling it out of mold (or sand). Gate knife is used to make pouring basin or cup by enlarging and giving an appropriate shape to the metal receiving end of the pouring sprue in the mold. Sprue rod and riser rod are tapered cylindrical wooden rods used for making metal pouring sprue and riser hole in the cope (upper half of the mold). Bellows are used to blow out dust and sand particles from the mold cavity and thus clean it by air pressure. Shake bag is used to sprinkle parting compound on the mold and is made of cotton clothe. Gaggers are bent pieces of wires and rods used for reinforcing the downward projecting sand mass in a cope (or top portion of mold) in making large size molds. Nails and wire pieces are used to reinforce cores or thin projections of the sand in the mold.

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5.7.2 Containers Containers include (a) Molding boxes, (b) Ladles and (c) Crucibles. Molding boxes are used for making molds (or mold cavity) in the molding sand rammed in molding boxes. Both ladles and crucibles are used for melting and handling the molten metal during the process of casting. Molding boxes or flasks have box-like structure made of rectangular walls (sometimes circular also) and without any bottom or top cover. These are mostly made of cast iron although wood is also used sometimes. When a mold is made within two boxes, the upper box is called cope and the lower one drag [Fig. 5.4(a)]. When three molding boxes are used, the middle one is called cheek. Small molding boxes do not need strengthening cross bars for giving support to the molding sand in the cope part of the mold, but the bigger boxes used for large size molds (where the distance between mold walls is too large) essentially need the provision of cross bars [Fig. 5.4(b)]. A snap flask (Fig. 5.4(c)) is a wooden box hinged at one corner and having clamping arrangement at the opposite corner. It is used for bench molding of small jobs.

Fig. 5.4

Molding boxes (or flasks): (a) Rectangular type molding boxes, (b) A pair of molding boxes with strengthening cross bars and (c) A wooden snap flask.

The mold boxes carry projecting handles for holding of the mold as also the provision of clamping the cope and drag together (through the projecting portions or lugs on the mold boxes). Ladles are used to receive molten metal from the furnaces or cupola and to transport it to the casting place and also to pour the molten metal into the mold. These are made in different capacities, varying from 20 to 150 kg. A hand shank ladle handled by two persons is shown in Fig. 5.5(a). Very large size ladles, capacity up to 1000 kg, are handled by a crane. These have geared system to tilt the ladle for pouring molten metal into the mold [Fig. 5.5(b)], and these are used for very large castings where metal requirement is very high. Bottom pour ladles are also available. Crucibles are made of refractory materials (such as silicon carbide) and are used as metal melting pot [Fig. 5.5(c)]. The metal charge in broken pieces is placed in the crucible which is later placed inside a pit furnace or other furnace for melting the metal charge. Pouring of molten metal from the crucible may be done directly into the mold or by first receiving into a laddle and later poured in the mold.

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Fig. 5.5

5.8

177

(a) Hand shank laddle, (b) A tilting type geared crane ladle hanged to a trolley which can move to and fro on a monorail, (c) A crucible.

MOLD MATERIALS AND THEIR SELECTION

Molds are made from heat resisting materials such as sand, clay or graphite. Varieties of sands and clays mixed together with suitable binders provide a wide range of mold materials. Both green-sand molds (moist) and dry-sand molds (baked) are in fact the sand molds but the two types only differ in the state of the mold just before pouring metal. Permanent molds are metallic molds mostly used in die-casting and centrifugal casting processes. Plaster molds and ceramic molds are made from fast setting plasters, fibres and water mixture. Sometimes wax and mercury (Mercast process) are also used for making molds. The choice of a mold material depends mainly on the cost as mold should be made from cheaper and easily available materials. Molds are also selected on the basis of metal to be cast (high melting point or low melting point), for example, low melting point metals and alloys like aluminium, zinc, magnesium, lead, etc. are cast easily in metal molds by die-casting process. Size and shape of the casting is also important in selecting mold material with proper thermal conductivity. Metallic molds have higher thermal conductivity than sand molds and thus can produce castings at faster rate. But heavy and thick castings need extra time during solidification and cooling and hence these are best produced in sand molds. Among all the available mold materials, sand is the most versatile and commonly used mold material in all foundries since it fulfils the basic requirements of casting materials. It is also readily available. When properly mixed with suitable binders and additives, sand

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contributes to be one of the best and cheapest materials for making molds. All castable metals can be cast in sand molds irrespective of their size, shape and weight.

5.9

MOLDING SANDS (OR FOUNDRY SANDS)

Molding sands or foundry sands are the sand mixtures or molding mixtures used for making molds in a foundry shop. Different types of molds such as green-sand mold, skin-dry sand mold, dry-sand mold, CO2 hardened sand mold, etc. are all made from foundry sands of various types and grades. Molding sands are broadly classified in the following major groups, according to the nature of their origins.

5.9.1 Natural Sands Natural sands are collected from natural resources like rivers, lakes, seas or deserts. Natural sands constitute the major part of all foundry sands. They contain silica sand, clay substances and water. Silica sand has 80 to 90% silicon oxide (SiO2) obtained from quartz rocks or by decomposition of granite. The silicon oxide is characterized by having very high softening temperature and thus having high thermal stability, which imparts refractoriness, chemical resistivity and permeability to the molding sands to stand well when they come in contact with molten metal. Natural sands contain sufficient amount of binding clay (10 to 15% or more) which imparts bonding property, plasticity and cohesiveness to the silica particles in moist state. Water plays an important role in molding sands. When present up to 5 to 8%, water imparts green strength to the sand in the presence of clay, besides improving workability and permeability. Because of being moist, natural sand is also called green sand. The size and shape of silica sand grains are responsible for several important properties of the molding sand. The size of sand grains varies considerably over a wide range, from 50 microns to 3360 microns and accordingly the sands are classified as fine, medium or coarse grained. Fine sands are used for small and intricate castings but they have poor permeability and refractionness. These are, therefore, preferred for casting small size non-ferrous products which involve lower metal temperatures and production of lesser amount of gas in comparison to casting cast iron or steel. Fine sands with resonable amount of clay ensure good finish on non-ferrous castings. Medium size grains are preferred for light work. Large-grained sands are more permeable and hence used for large size castings. They have better refractoriness also. The shape of the sand grains may be angular, round, sub-angular or compounded. Round grains give poor strength to the mold but high permeability. Angular grains give better strength but reduced permeability of the molding sand. Sub-angular grains are less permeable than rounded grains. Compounded grains are in the form of hard lumps and their presence in molding sands is considered undesirable.

5.9.2 Synthetic Sands Synthetic sands are prepared from silica sands which are relatively clay free. They need some binder to make them suitable for foundry work. When mixed with suitable binders and additives, the silica sands are converted into synthetic sands. These are considered better

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sands (than natural sands) as their properties can be easily controlled by varying the contents of sand mixture. None of the natural sands possesses the required qualities to the extent necessary for being a good foundry sand, since these sands lack one or more importantly required properties which are always covered up by blending or mixing other sands or materials (like additives) with the natural sand, so that the resulting sand mixture turns out to be a good sand for foundry purposes. All foundry sands generally used for molding purposes are in a way blended sands. For example, a typically blended sand mixtures for making a green-sand mold for casting grey cast iron may contain: natural sand (river) 50 to 60%, clay 12 to 15%, bentonite 2 to 15%, coal dust 5 to 15% and water 4 to 6%.

5.9.3 Special Sands In addition to the natural sands and synthetic sands which are primarily based on silica sands, there are certain other varieties of special sands such as zirconite and olivine which are better in performance than silica sand but being costlier, these are used only for special applications.

5.10

CONSTITUENTS OF MOLDING SANDS

The principal constituents of a molding sand are: ● ● ● ●

Silica sand Binders Additives Water

1. Silica sand: It forms the bulk of the foundry sands and imparts refractoriness and other properties to the molding sand. 2. Binders: Binders give cohesiveness and strength to the sand to enable it to retain the shape of the mold cavity after the withdrawal of the pattern from the sand. Binders may be organic type (such as linseed oil, molasses, dextrin, pitch or resins) and inorganic type (such as clays or cement). Clay binders include: bentonite, limonite, fire clay, etc. Clay binders are used for both green- and dry-sand molding. Increased clay content adds to strength, hardness and toughness of the sand but reduces flowability and permeability of sand. 3. Additives: These are used to improve the properties of the sand, either by improving the existing properties of the sand or by imparting new properties to make the sand mixture more useful. Common additives are: coal dust, sea coal, cereals, silica flour, pitch, dextrin, molasses, wood flour, etc. Coal dust, when present (up to 10%) in the molding sand, reacts with oxygen present in sand pores at the time of casting and produces a reducing atmosphere of CO2 at the mold-metal interface, which gives smooth castings. Cereal flour improves strength and collapsibility of sand (to allow free contraction of casting). Silica flour increases hot strength of mold and decreases metal penetration into mold walls and thus gives smooth castings. Both dextrine and

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molasses improve dry strength (after baking the mold) and decrease metal penetration into mold walls. Additives such as wood flour, cereals and cellulose when added to molding sand work as cushion materials since they burn and form gases during casting, thereby giving rise to space to accommodate expansion of casting during its cooling. For example, wood flour and saw dust reduce expansion defects in grey cast iron castings by promoting mold wall movement and collapsibility. The grey cast iron expands (up to 2.5%) during solidification and cooling, because of the period of graphitization that occurs during the final stages of its solidification. 4. Water: Adding water to molding sand is called tempering. Water imparts workability to the sand for making the mold. Binders, such as clays, give green strength to the sand due to the presence of water. A green-sand mold may contain water around 5% whereas the sand for making a dry-sand mold (which is later baked) has higher water contents, up to 10%. Too high water causes blow holes in the castings. Low moisture sands are weak in green strength and produce defects such as scabs or roughness on the surface of castings. Properties of molding sand are affected by size, type and distribution of sand gains, amount of binding and additive materials and moisture contents. Amount of water added to molding sand is further governed by the type and amount of sand constituents and the condition in which the mold is to be used, for example, molding sand for a dry-sand mold has initially (before baking) more water contents than that of a green-sand mold. Normally water content varies from 3 to 10%. Effect of water on permeability and strength of molding sand is shown in Fig. 5.6(a).

Fig. 5.6(a)

5.11

Effect of moisture content on the permeability of sand having different types of grains shown at (i) and on the strength shown at (ii).

SAND PREPARATION AND CONDITIONING

As mentioned earlier, none of the naturally available sands possesses the desired qualities to the required degree for being considered a good molding sand. By the process of blending

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or mixing varieties of sands together and by adding binding materials or additives to this mixture, a new suitable sand mixture is produced for foundry purposes. New sands as well as used floor sand (which has been used several times in making molds) are properly prepared and mixed in a suitable ratio, for example, the ratio of used floor sand (old) to new sand may be more for light castings but it decreases for medium and heavy castings. Conditioning of the sand is essential. Proper conditioning means uniform distribution of clay bond and other additives over sand grains, even distribution of proper moisture and sorting out of foreign matter like nails, and other metal pieces (which might have been used for strengthening previous mold walls) by riddling and a thorough mixing of sand mass. The operation of conditioning is carried out manually by mixing the sand mixture and other additives with hand shovels. Modern foundries have appropriate equipment for conditioning of the sand such as circular pan sand mixer using rotating stone wheels and paddles. Testing of sands is also carried out for strength, permeability, moisture content, etc. to predict its performance during use.

5.12

CHARACTERISTICS (OR PROPERTIES) OF MOLDING SANDS

A good molding sand must possess the following characteristics, which are determined by the various constituents present in a particular molding sand mixture. 1. Refractoriness is the property which makes the molding sand capable of withstanding high temperatures of molten metal without the fusion of the sand. Silica sand present in the molding sand is primarily responsible for imparting this property. Also, the higher contents of impurities such as lime, magnesia, metallic oxide, etc. present in sands tend to lower the fusion point of silica sand. Larger grains of silica increase refractoriness. 2. Permeability or porosity is that property of sand which allows easy escape of gases and steam through the sand mold when molten metal comes in contact with moist sand having coal dust, oils, resins and other gas forming agents. Most molten metals also have dissolved gases in them which are evolved on solidification. Insufficient porosity of molding sand leads to several casting defects such as porosity, honeycombing, etc. Permeability depends on the shape and size of sand grains, the amount of clay, moisture contents and ramming of sand (around the pattern during molding) and venting of the mold. 3. Cohesiveness is the ability of sand particles to stick together and thus gives green strength to the moist molding sand in maintaining the shape of mold (or mold cavity) after the pattern has been withdrawn from the sand. Cohesiveness depends on the size, shape and distribution of sand grains, type and contents of clay and other bonding materials and the moisture contents. 4. Plasticity is that property of a molding sand by virtue of which it takes easily any desired shape (as per the pattern) under pressure and retains it after the pressure is removed. Fine-grained sands give good plasticity. Plasticity also depends on the clay contents, which absorb moisture and help improving the plasticity.

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5. Flowability is that property due to which the sand flows under the effect of ramming (during molding) to all portions or corners of the molding box and packs properly around the pattern while distributing the ramming pressure evenly on the sand in all parts of the molding box. 6. Adhesiveness provides the sand the capability of easily adhering to the surfaces of other materials such as the walls of molding boxes and thus helps in retaining the sand (filled in the molding box) in tact during molding (when mold boxes filled with sand may be lifted or overturned during the process). 7. Collapsibility is that property because of which a sand mold (or core) collapses automatically, giving way to free contraction of the solidifying casting and thus avoids hot tears and cracks in castings. Other properties may include: low coefficient of expansion; property of non-sticking with casting; non-reacting chemically with molten metal; low cost and easy availability.

5.13

SOME COMMONLY USED SAND MIXTURES (OR MOLDING SANDS)

Sand mixtures or molding sands of following types are commonly used for making molds and cores and to serve various other specific purposes. 1. Green sand, also known as tempered sand, refers to a moist molding sand usually available in foundry shops for general molding purposes. Molds made with this sand are called green-sand mold in which molten metal is poured during casting without any prior drying or baking of the mold. Green sand is a well-prepared foundry sand mixture made by mixing natural sand and other sands (including used floor sand) and additives and contains just enough moisture (up to 8% or so) to give sufficient bonding strength to the mold. Small and medium sized castings of ferrous and nonferrous metals are made in this sand as it is the least expensive foundry sand and takes less time in making the mold ready for casting (as no drying or baking of mold is involved). 2. Dry sand refers to that molding sand which at the time of making a mold has excess moisture but the same has been evaporated by drying the internal mold face or the full mold in an oven. The mold thus made is called dry-sand mold. The sands used for these molds are fine-grained and mixed with proper binders and additives that give strength to the mold on baking. Dry-sand molds are used for casting large size steel and cast iron components. 3. Floor sand or black sand or backing sand is that molding sand which is used over and over again for molding purposes, and is usually black in colour due to the addition of coal dust and also due to burning as a result of coming in contact with the molten metal repeatedly during casting. It usually forms the bulk of the molding sand as it supports the facing sand filled only to a certain depth around the pattern during molding. Floor sand is also mixed in certain proportion with the natural sand, clay or other binders and additives while preparing the new green-sand molding mixture.

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4. Facing sand is a fine textured sand used to form the face of the mold cavity when used around the pattern to a thickness of about 5 cm. A certain amount of floor sand and new molding sand properly tempered with moisture and suitable additives give a good facing sand. The facing sand has better strength and high refractoriness than the floor sand. 5. Parting sand is usually a natural dry silica sand sprinkled on to the pattern and also on the parting surfaces of the mold (when made in two or more boxes), so that the sand mass of the molding boxes may not stick to each other or to the pattern. A parting compound made from phosphate rock is also used sometimes as parting sand. 6. Core sand (or oil sand) is a large grain high silica sand bonded with organic binders such as linseed oil, light mineral oil, pitch or corn flour, etc. It finds use in making cores and molds for non-ferrous castings. 7. Loam sand comprises a mixture of ordinary clay (about 50%), fire clay and silica sand and water milled to a thin paste which, in the form of plaster, is applied on the face of a roughly carved mold made by using burnt clay bricks. The mold is brought to the final shape with a rotating sweeping pattern which molds the plastered face to the required shape and size of the mold. The plaster hardens on drying. Usually, symmetrical objects (cylinder, barrels, kettles) are made in such molds.

5.13.1

Molding Sand for Casting Different Metals

Molding sand for iron castings: For grey iron castings, mold material can be natural sand properly bonded but synthetic sand mixtures have proved better and are economical. Iron castings are made in green-sand molds, dry-sand molds and a variety of other sand molds discussed under Section 5.25. Molding sand for non-ferrous castings: Generally non-ferrous casting is done in green-sand molds. The composition of non-ferrous green sand is nearly the same as used in iron foundries except that a fine-grained sand with low clay contents is preferred. Any additive used in the sand should not be a clay material as non-ferrous alloys contract sufficiently during their transformation from liquid to solid state and hence mold material should be easily collapsible. The sand used for casting magnesium alloys should have low water contents and its grains should also be very fine. Special additives like borax, diethylene or glycol may be used with the sand. Plaster molds are used for precision castings of aluminium, zinc, copper and magnesium alloys. CO2 hardened sand molds are used for casting thin sections of non-ferrous metals. Shell molds are also used for non-ferrous casting. Molding sand for steel castings: Contraction of steel castings during freezing is fairly high, i.e. from 1.5 to 3% and hence shrinkage allowance on pattern should be more. Because of high temperature, coarser sand is preferred for steel casing. Steel casting is done in both green-sand molds and dry-sand molds. A typical composition of green-sand mixture for steel castings may have pure silica mixed with double quantity of reclaimed sand, bentonite up to 1.5% and dextrine up to 0.25% and cereals up to 0.2%. Moisture may be up to 3.5%. A typical dry-sand mixture for steel casting may have river sand 89%, bentonite 4.5%, dextrine 0.5% and water 6%. Oil sand (or pure silica sand) is used for casting large and complicated steel parts where good finish is required. Binding materials like gum, dextrine, linseed oil and molasses are

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added to pure silica sand. The molds and cores made from oil sand are baked before they are used for casting. At high temperatures, oil burns away as molten steel comes in contact with sand resulting in loss of bond of the molding sand. Consequently, the sand surrounding the metal collapses and allows for unrestricted contraction of the metal. Oil sand molds become more stronger when backed between 200 and 300°C. Compo sand molds are also used for producing large steel castings. Sand contains crushed materials like used crucible pots, old fire bricks, calcined clay, fire clay, etc. and is highly refractory having high permeability too. When heated to high temperatures, compo molds become extremely hard and strong. Silica flour mold facing is usually given before casting.

5.14

FOUNDRY BLACKINGS

Foundry blackings (or dressings or mold surface coatings and core facings) are applied to the mold face and core face to increase refractoriness and smoothness. Blackings are used dry (by dusting) or in the form of a paste (made using water and a binder) and applied with a brush or swab. Blackings are highly refractory materials such as plumbago (mineral graphite), coke powder, china clay, zircon flour or french chalk. Mold washes are slurries of fine ceramic grains applied over the mold faces to minimize fusing of the facing sand grains during casting. Talc powder with plumbago is the commonly used blacking in iron foundries. Blackings used in steel foundries comprise zircon flour, china clay, calcined magnesite with bentonite or gum as binder. For non-ferrous metal castings of small and medium size, no blacking is used on green-sand molds.

5.15 PATTERNS A pattern is used for molding a cavity (or mold) in the molding sand mixture such that the formed cavity is similar to the shape of the casting [Figs. 5.2(a) to 5.2(c)]. The pattern is a replica of the desired article to be cast but differs from the actual article in certain ways, for example, pattern carries (a) additional allowance in its dimensions to compensate for the metal shrinkage during casting; (b) allowances for machining or finishing the castings; (c) draft or taper on its exterior and interior surfaces for its easy removal from the molding sand; and also (d) additional projections (core prints) to produce seats in the mold for the setting of cores. Patterns are made from wood, metals, rubber, plaster, wax, plastics, etc. The selection of a particular material for making pattern depends on factors such as number of castings to be made, method of molding (hand or machine), quality of castings and degree of finish and dimensional accuracy desired on castings, design of the casting with possibilities of changes in the design of the pattern and expectations of repeat orders.

5.16

PATTERN COLOURS

The following information regarding the colour of the pattern is just a general guide only. Yellow—Core prints Red—Surfaces (of the resulting casting) to be machined

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Black—Surfaces (of the resulting casting) to be left unmachined Red strips on yellow base—Seat for loose pieces of the pattern Clear or no colour—Parting surface

5.17

MATERIALS FOR PATTERNS

Pattern material should be cheap and readily available. It should be hard, strong and light weight with resistance to corrosion. It should be capable of taking good surface finish. And it should be easy to work upon by normal manufacturing methods used in making the pattern out of it. The following materials are used for making patterns. 1. Wood is commonly used because it is cheap, easily available, can be shaped easily, light in weight and can have good surface finish. Wood, however, is affected by moisture and changes its shape and size on drying out. It also wears out quickly against abrasion by molding sands and hence has comparatively shorter life because of which wood patterns are not used for the production of large quantities of castings. The wood used for patterns include white pine, deodar, tun, walnut, teak, etc. 2. Metals used for making patterns include cast iron, brass, white metal, aluminium, etc. Metallic patterns are preferred when a very large number of castings are to be made. These patterns are costlier than wooden patterns but have much longer life. 3. Plaster of Paris and gypsum cement are quick setting compounds used for making patterns. They are easily cast into intricate shapes. Small patterns of complicated shapes are made of plaster of Paris by pouring and setting the plaster slurry into the pattern molds. 4. Plastics of different types are used for making patterns. Plastic patterns are lighter, stronger and dimensionally more stable than wooden patterns. Pattern surface being smooth, molding sand sticks less to the pattern. There is no moisture absorption by the pattern. Wooden patterns are, however, easy and quick to make and repair at cheaper cost. Among thermoplastics, polystyrene is commonly used for patterns (known as consumable pattern as heat of molten metal vaporizes the pattern which leaves behind the formed mold cavity). Thermosetting plastics such as phenolics and epoxies are also used for patterns. 5. Wax as a pattern material is used in investment casting process (or lost wax process).

5.18

PATTERN ALLOWANCES 1. Shrinkage allowance: Most metals, when they cool down from molten state to room temperature, shrink or contract in three different stages, namely, (i) Liquid contraction, (ii) Solidifying contraction and (iii) Solid contraction. The first two types of contractions (involving change from liquid to solid) are compensated by the feeding of metal from risers where the metal remains in molten state till the casting gets solidified. It is for the third type of contraction, the solid contraction (contraction of solidified casting to room temperature), for which allowance is provided on the patterns. Different metals have different contraction allowance. The solid contraction

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of the metal is influenced by the metal to be cast and the pouring temperature of the molten metal, design, geometry and size of casting, type of mold material and method of molding and resistance of mold to shrinkage. Although the contraction of the castings is volumetric contraction, the contraction allowance is, however, given in linear measures as shown in Table 5.1 which works as a general guide only. TABLE 5.1

Solid contraction allowance for different metals

Metal to be cast

Allowance (mm/metre)

Grey cast iron Malleable cast iron Steel Brass Aluminium Zinc Lead Copper, magnesium Silver White cast iron

10.5 10.5 21.0 16.0 16.0 24.0 24.0 16.0 10.0 21.5

2. Machining allowance: Machining allowance is given on a pattern only when the casting being made needs machining and finishing. This allowance is given in addition to the shrinkage allowance and varies from 2 to 5 mm according to the location and nature of casting, method of machining and the degree of finish required. In larger castings the machining allowance may be 12.5 mm or more. 3. Draft allowance: Patterns are given slight taper on their vertical surfaces (both external and internal) which are parallel to the direction of their withdrawal from the mold [Fig. 5.6(b)]. This taper or draft is expressed either in degrees or in terms of linear measurements, amount of draft being more on internal surfaces. The draft amount may vary from 10 to 25 mm per metre on external surfaces and from 40 to 70 mm per metre on internal surfaces.

Fig. 5.6(b) Tapering a pattern for its easy removal from the compacted sand mold without breaking the mold.

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4. Rapping or shake allowance: To take the pattern out of the compacted sand mold, the pattern is first rapped or shaken by striking over it side to side, so that the pattern surfaces may be made free of the adjoining sand walls of the mold. This operation obviously enlarges the size of the mold cavity and hence a negative rapping allowance is provided on patterns to compensate, at least in case of large castings.

5.19

TYPES OF PATTERNS

Patterns are of great variety because of their different designs, construction and methods of use. More commonly used types are discussed below. 1. Solid or single piece pattern: It is the simplest one piece pattern (Fig. 5.7) and can be molded in one box or two boxes depending on its shape.

Fig. 5.7

A solid pattern.

2. Two-piece or split pattern: The pattern is in two pieces which are connected with dowel pins (Fig. 5.8). The surface formed at the line of separation or splitting is called parting surface. The use of split pattern makes molding operation easy since many times casting design is such that the use of solid pattern offers difficulty in the making of mold and also in the withdrawal of the pattern from the sand.

Fig. 5.8

A split or two-piece pattern. The two halves of the pattern are shown at (a) whereas the casting made by this pattern is shown at (b).

3. Multipiece pattern: The multipiece pattern is made up of loose pieces (more than two) which may be necessary to be kept loose to facilitate the withdrawal of a typical shaped pattern from the mold. The loose pieces of the pattern connected together with dowels are removed separately from the mold cavity formed in separate mold boxes. A typical three-piece pattern and its molding in three boxes are shown in Fig. 5.9.

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Fig. 5.9

A multipiece (three-piece) pattern. The exploded view of the three-piece pattern is shown at (a) while the method of molding with the three-piece pattern is shown at (b) using three molding boxes.

4. Sweep pattern: A sweep is a template of wood (or other material) having its edge contour corresponding to the outer shape and size of the casting (Fig. 5.10). It is rotated around a central spindle as shown to sweep the desired shape in the sand. In most cases, it is used for casting symmetrical circular shapes of very large castings.

Fig. 5.10

A sweep pattern and its use in making a mold cavity in the sand.

5. Segmental pattern: It is used for molding parts that have circular ring-shaped large sections. The segmental patterns are the sections of a full pattern. Instead of using a full pattern (which in some cases may be too large), a part pattern is used. After molding at one place, the pattern is rotated to next adjacent location and molding is done. By repeating the same procedure, molding along the full circle is completed (Fig. 5.11).

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Fig. 5.11

189

A segmental pattern used for molding a ring-shaped casting. Bottom of the mold is first rammed and levelled and the spindle (or pivot) is erected vertical in the sand in the centre of mold. Segmental pattern is fastened to the spindle and sand is rammed underside the pattern. After levelling the sand, the first setting of pattern is thus completed. Later, the pattern is unfastened from the spindle and is drawn out of the sand. The next position of the pattern will be the one just adjoining its first setting and like that the process is continued till the complete periphery of the ring-shaped casting is molded.

6. Skeleton pattern: The skeleton pattern is used for casting very huge part. Instead of making completely solid form, the pattern is made of wooden frame and rib construction (skeleton) to form a partially exterior or interior outline of the casting so that a general contour and size of the desired casting may be produced (Fig. 5.12). The ribbed construction is filled with clay sand or loam and rammed. The proper contour (external) is made by using a template or strickle which removes excess sand out of the spaces around the ribs and also makes the proper contour. Skeleton pattern is built usually in two parts.

Fig. 5.12

A skeleton pattern for molding a large pipe with flanges at both ends shown at (a). The skeleton pattern (b) is set on the compacted and levelled sand floor with its flat face. Its hollow portions are filled with sand and rammed and the final cylindrical shape to the pattern is given by using the strickle (c). Molding flask is placed enclosing the pattern and the sand pattern dusted with parting sand. Later, molding sand is rammed in the flask. This way, half of the mold is prepared. Pattern may be drawn from the flask sand. Similarly, the other half of the mold may be prepared.

190

MANUFACTURING PROCESSES

7. Shell pattern: The shell pattern is mostly used for casting drainage fittings (straight or curved) for pipe works. The pattern is usually made of metal and mounted on a plate. It is parted along the centre line and both the pattern halves are doweled together accurately on the plate, one half on its top side and the other half at the bottom side of the plate. 8. Gated pattern: Gated patterns are used to form the mold cavity along with the cavity for gate, riser and runners because the patterns for gate and riser are also attached permanently with the regular pattern. These are used on molding machines. A typical multi cavity mold (four cavities) is shown in Fig. 5.13.

Fig. 5.13

Gated pattern composed of four different patterns to make a multicavity mold complete with gates, runners and risers.

9. Loose-piece pattern: A loose-piece pattern is used when a pattern has projecting parts which lie either above or below the main parting line in a mold. Such projections make lifting of the pattern from mold impossible. It is therefore necessary to make a pattern in such a way that the projecting portions should become loose-piece attached with a nail or dowel pin or taper fitting with the main pattern body and thus can be removed easily from the molding sand. The main part of the pattern and the loose pieces of pattern are removed from the sand separately (Fig. 5.14). 10. Match plate pattern: Match plate patterns are mounted on a metal plate (or wooden board) with gates and runners as an integral part of the pattern [Fig. 5.15(a)]. The pattern is split into two parts along the parting line formed by the metal plate. A match plate pattern usually comprises a group of patterns of the same or different types which may be joined together by runners, thus enabling the casting of several molds in one go. Shapes that necessitate molding in both the drag and cope are divided for parting and the portions (pattern halves) are placed on the opposite sides of the match plate. Two patterns, one for a cylindrical object and another for a rectangular block, are shown attached to one common match plate and their molding is also shown in Fig. 5.15(b). The match plate patterns find use on molding machines. They help in increasing the production rate of molding as several pieces are molded together in one go.

FOUNDRY PROCESSES—Molding and Casting

Fig. 5.14

Fig. 5.15

191

A loose-piece pattern for making a casting with a dovetail slot.

Match plate pattern showing: (a) Two views of a typical match plate pattern used for molding simultaneously the two objects of different shapes; patterns for a cylindrical casting and a block casting attached to the plate on its opposite sides, (b) A match plate pattern being used for making a mold using two mold boxes.

5.20 CORES A casting is sometimes required to be made hollow internally or having few holes, cavities, projections, undercuts or pockets on its external surface. This is achieved by using cores during casting or by subsequent machining of the casting. Usually, the large hollows in the castings are achieved by cores. Contours of the core are exactly similar to the cavities, pockets or holes it is supposed to create in the casting. The use of cores thus helps in eliminating much of machining (drilling, boring) to make a hole or cavity in the castings. Cores also give strength to the mold. Cores are made from special core sands. Figure 5.16 shows at (a) the casting (to be made) which is a pipe with flange on either end. Its pattern is shown at (b) with two additional end-projections, called core prints, which during molding help in making ‘impressions’ or cavities in the sand to support the core in the mold (with

192

MANUFACTURING PROCESSES

its extended length at its either end). The split-type core box used for making the core is shown at (c). The total length of the core is kept equal to the length of bore in the casting (pipe here) plus the lengths of two core prints. The core has to be properly seated in the mold in formed impressions in the sand and these impressions are formed by the core prints added on to the pattern at proper places (Fig. 5.17). Cores that consist of two or more parts made separately are pasted together after their baking to form the complete core.

Fig. 5.16

A split type core box for making a straight core for a two-flanged pipe shown at (a). The pattern for the pipe is shown at (b). The core box, in two halves, for making the core, is shown at (c). Note that the length of the core includes the length of the pipe and the length of both the core prints shown at the ends of the pipe pattern.

Fig. 5.17

Illustrating the use of a split type pattern for making a mold cavity in the sand. The split type (or two halves) pattern embedded and rammed in the sand in the cope and drag is shown at (i). Note that the core print forms the extended part of the pattern and is used to make a cavity or impression (extra to the mold) in the sand for supporting the core in the mold cavity. After the two pattern halves are taken out of sand leaving the mold cavity behind in the sand, the core is placed in position in the mold cavity as shown at (ii). Sprue hole, runner and gate are made in the sand for pouring molten metal. With a weight placed on the top of cope (to counteract the upthrust of molten metal during casting), the mold is ready for pouring metal.

A core is thus a body of sand prepared separately in a core box (Fig. 5.16). The core is placed in the mold cavity before the operation of casting (metal pouring) to form the interior surfaces of the casting [Fig. 5.17(ii)] and is removed from the casting (by breaking it) after it has solidified and cooled. Since the cores also have to face the molten metal during

FOUNDRY PROCESSES—Molding and Casting

193

casting, these should, therefore, possess good strength, permeability, capacity to withstand heat and also the collapsibility as is required in case of sand molds. Because of these factors, the cores are generally made of sand. As mentioned above, a core is set and anchored in the mold cavity with the help of core prints. However, the shifting of the core from its set position within the mold cavity is specially checked with the help of metal supports, called chaplets (Fig. 5.18). Different types of chaplets are shown in Fig. 5.19 which are metal pieces used as spacers and for anchoring and supporting the cores, sometimes for heavy cores, in addition to core prints.

Fig. 5.18

Setting and supporting a core inside a mold cavity. A core is supported in a mold either in the cavity formed in sand by the core prints as at (i) or with the help of chaplets or both as at (ii).

(a)

(b)

(d)

Fig. 5.19

5.21

(c)

(e)

Chaplets for supporting cores in the mold: (a) Stem chaplet, (b) Pipe chaplet, (c) Radiator chaplet, (d) Perforated chaplet and (e) Double headed or stud chaplet.

TYPES OF CORE

Cores are generally categorized according to (i) Core material and (ii) Position of core print (when core is placed in the mold).

194

MANUFACTURING PROCESSES

5.21.1

Types of Core according to the Core Material

1. Green sand cores are weak in strength and used only for light castings. A typical green sand mixture for the core may have: silica sand—75%, dung—10%, molasses— 15% and water. 2. Dry sand cores are the backed cores with refractory coating and hence are stronger. A typical dry sand core mixture may contain coarse grained silica sand, molding sand, and silica flour or wheat flour. The blacking used may be thin clay wash. 3. Oil sand cores are the most commonly used cores as they possess very good characteristics of collapsibility. These cores do not absorb water. Besides pure silica sands (also called sharp sands), they contain binders such as linseed oil, resins, molasses, cereal flour, etc. A typical composition of oil sand core may be: silica sand—95%, corn flour—1%, bentonite—1%, linseed oil—2% by volume of sand. Oil sand cores are baked to give higher strength. 4. Chill cores are the metallic cores often used on die casting machines. The chill core allows the cast metal to get solidified quickly. It is slightly tapered to help taking it out of the casting soon after the casting gets solidified. 5. Shell mold cores are made by shell mold process. A core box is heated and the sand consisting of 3 to 6% resin is blown into it. After sometime (which establishes the thickness of resin-sand shell), the unheated or unset interior sand–resin mixture is drained out. A shell wall (the core) 6 to 13 mm thick is stripped from the core box. The recovery of the core after casting consists of burning out resin at high temperature. 6. CO2 hardened cores are the sand cores hardened or cured by CO2. The carbon dioxide (CO2) reacts with sodium silicate constituent of the binder (used in core sand mixture) and produces silicon dioxide, which with water forms a cement like silica gel which binds together the sand particles. The core develops high strength during CO2 curing and hence can be used as soon as the core is removed from the core box after curing.

5.21.2

Types of Core according to the Position of Core Print

The position of the core print depends on the placement of the core in the mold. Some such typical placements are shown in Fig. 5.20 which gives the following types of cores. 1. Horizontal core is used in its horizontal position in the mold always at the parting line between two mold boxes. 2. Vertical core is set in the mold in its vertical position. Its top and bottom ends are supported in the seats or impressions formed by the core prints in the molding sand. 3. Balanced core has only one core print to maintain the alignment of the core and is often used to produce a blind cavity in the casting. The core print is large and heavy enough to balance the weight of the core in the mold. 4. Drop core or stop-off core is used when the cavity to be made in the casting is either above or below the parting line. 5. Hanging core or cover core hangs vertically in the mold with no support at its bottom end. The entire mold cavity has, however, to be contained in the drag only.

FOUNDRY PROCESSES—Molding and Casting

Core

195

Core

Mold Mold Horizontal core

Vertical core

Core

Core Mold

Core

Balanced core

Drop core

Fig. 5.20

5.22

Mold

Mold Hanging or cover core

Different types of cores.

CHARACTERISTICS OF A CORE

Since a core is also subjected to high temperatures of molten metal during casting, it should therefore have the following characteristics for effective performance. 1. Refractoriness or thermal stability: The core has to be highly refractory against burning or fusing of its constituents on coming in contact with molten metal to avoid casting defects such as rough surface and metal penetration (in the core). Blackings are applied on cores (to improve its refractoriness) before setting them into the molds. 2. Permeability: The constituents, present specially in core sands, are likely to generate large volume of gas on coming in contact with the molten metal during casting. It is for this reason that a core should be more permeable than mold. Core is, therefore, made with sands having larger grains. Vents are provided in the core for easy escape of gases. 3. Collapsibility: A core should be able to disintegrate (or collapse) shortly after the molten metal has solidified around it after casting. Collapsibility ensures freeness in the contraction of metal. This property is encouraged by using oil binders. 4. Strength: A core should have good strength (both green strength and dry strength) for its safe handling and placement in molds and later for standing well against the pressures generated by molten metal (and gases) during casting.

5.23

CORE MATERIALS

A number of sand mixtures or core materials as already discussed are used for making cores and the selection of a particular core mixture depends on the size of the core, degree of finish on the core, requirement of collapsibility, permeability and economics of making and baking of the core. A core mixture usually comprises some refractory core sand (or oil sand) with larger gains, binder to give strength to core after baking and some additives to give special

196

MANUFACTURING PROCESSES

properties to the core. A core sand comprises a variety of high refractory large grained pure silica sands (sharp sands) such as river sand, sea-shore sand and rock sand. These sands stand higher temperatures and resist well the penetrating action of molten metal during casting. These pure silica sands have high refractoriness but have no natural bonding material. A binder is, therefore, added to give green strength and hardness (after baking) to the core. Natural binders such as linseed oil and protein binders (such as gelatin and glue) are used. Synthetic binders used are thermosetting resins which give high strength, good collapsibility and resistance to moisture. In general, binders are categorized as follows: (a) Inorganic binders such as fire clay and silica flour. (b) Organic binders such as oils (linseed, soyabean, corn and cotton seed), cereals, pitch, dextrin, molasses and resins (in powdered or liquid forms). Molasses bond gives very hard surface to core on baking. Oils (both mineral and vegetable) are the commonly used binders.

5.24

MAKING OF CORES

Sand cores are molded in mold boxes, and later dried and backed. Core boxes, usually wooden, are made in two parts. A half core box (Fig. 5.21) is used to prepare a core in two halves which are later joined together (after baking) by cementing them to form a full core. A split core box (Fig. 5.22) is in two parts which are joined together with the help of dowel pins to make a complete cavity for making the core. Strickle-type core box is used for making cores which have unsymmetrical shapes (Fig. 5.23).

Half core Core box

Fig. 5.22

Half core box.

or e

bo x

Strickle

C

Fig. 5.21

Sand core

Fig. 5.23

Strickle-type core box.

Split core box.

FOUNDRY PROCESSES—Molding and Casting

197

Traditional cores include green sand cores, dry sand cores, loam sand cores and oil sand cores. Cores are also made from sand mixtures having sodium silicate solution. Such cores are hardened by exposing them to carbon dioxide. In addition to these, there are synthetic resin bonded cores, metallic cores (made of die steel or molybdenum alloy steel and used in die-casting), and the cores made by shell mold method. The general procedure for making sand cores is described in the following four steps. (a) Preparation of core sand: Core sand mixtures are prepared by thorough mixing of core sand ingredients and tempering with suitable amount of water during mixing. Thorough mixing of ingredients is done so that all the sand grains are covered by the binder. In small foundries, core sand mixtures are prepared by hand mixing but in mechanized foundries, roller mills (or mullers) and core sand mixers (vertical revolving arm type or horizontal paddle type) are used for achieving homogeneous and efficient mixing. (b) Molding of cores: Cores are made by either (a) hand molding or (b) machine molding. In hand process, the core sand mixture is filled and rammed in the core boxes manually. Steel rods or wires may be embeded in large size cores to give them strength and to prevent their deflection due to their own weight when placed in the mold. The cores thus prepared are removed from the core boxes and placed on metal plates for drying and later transferring to the oven for curing (or baking). The mechanized process of making cores involves the use of different types of equipment such as core blowing machines, core ramming machines and core drawing machines. The use of core blowing machines for blowing bonded core sand mixture in core molding boxes is by far the most prevalent core molding method. Small and medium sized cores are made on a mass scale using these machines. The core sand mixture is forced and compacted into the core boxes under the pressure of compressed air (about 7 kg/cm2). Air vents provided in core boxes allow the escape of compressed air after the compaction of the sand. The other type of core making machines are core ramming machines, which are automatic type production machines such as core jolting or core squeezing machines or sand slinging machines. These are used for filling core sand mixture in mold boxes and ramming it there, adopting several compaction methods such as jolting, squeezing or slinging. Core drawing machines are used when cores are made in the core boxes having very deep cavities. Sand is rammed in the core box which is later placed on a core plate mounted on the machine table, where rapping action on the core box is given by a vibrator, followed by the pulling up of the core box leaving the extruded core on the core plate. (c) Drying or curing of cores: The cores made by various methods are dumped on steel plates, and later sent for baking or curing to the core baking ovens to draw out the moisture from them and to make them hard and strong. The core baking ovens may be batch type or continuous type (for mass production). In batch type ovens, cores are placed on portable racks or drawers during their baking. The heating is done by gas, oil or coal. In continuous type ovens, cores are loaded on a conveyor or chain moving continuously and slowly through the oven. The dielectric (or radio frequency) heating ovens are also used.

198

MANUFACTURING PROCESSES

Baking of cores imparts them the hardness and strength to withstand high pressures and temperature of molten metal during casting. Resin bonded cores are baked at about 110oC and linseed oil cores at 200oC. Core baking results in the evaporation of moisture, and the oxidation of the film of organic substances (surrounding the grains of core sand), forming a plastic-like harder surface and a good bond among the sand particles thus giving strength to the core as a whole. Cores are also hardened (or cured) by exposing them to carbon dioxide gas when core sands are bonded with sodium silicate which on reaction with CO2 produces silica gel, a cement like material that provides rigid bonding to the sand particles and avoids the baking of cores. Cores are also made by shell mold method wherein sand mixture is bonded with resin. Components of larger and complicated cores made and baked separately are later joined by cementing to make the complete core. (d) Finishing of cores: The baked cores are finished to shape and size by removing casting fins and other extra material from the core body or surface. Finishing of cores is done either by rubbing them with some hard material or filling or by light chiseling if so required.

5.25

TYPES OF MOLDS

The molds are available in a large varieties. These may, however, be categorized as follows into three broad categories. (a) Expendable molds;

(b) Permanent molds;

(c) Composite molds

Expendable molds: These are made from refractory materials which are capable of withstanding high temperatures of molten metal, such as silica sand, gypsum plaster, ceramic or similar materials mixed with various types of binders. An expendable mold is used only once because after the casting has solidified, the mold has to be broken up to remove the casting. Examples of expendable mold include: various sand molds (green-sand mold, dry-sand mold, CO2 dried sand mold), core sand molds, shell molds, plaster molds, investment casting molds, etc. Permanent molds: Permanent molds are metallic molds which are capable of maintaining their strength and stability against high temperatures of molten metals. These are designed for easy removal of castings and without breaking the mold as is done in case of expendable molds. A permanent mold is thus easily used repeatedly for producing a large number of castings, for example, a permanent mold made of grey cast iron (in die-casting) may last over 5000 castings of aluminium. Examples of permanent molds include: permanent molds (used in chill casting of the components with hard and wear resistant surface), die-casting molds, pressure casting mold, centrifugal casting mold, continuous casting molds, etc. Composite molds: Composite molds are made of two or more different materials such as sand, graphite and metal. Such molds are made to take best advantage of the special properties of various constituents of materials of the composite mold. These molds have higher strength, better control on the rate of cooling of castings, and also optimize the overall cost of the process. Complex shaped turbine impellers are cast in composite shell molds. Graphite and sand composite molds are used for casting aluminium alloy torque converters.

FOUNDRY PROCESSES—Molding and Casting

199

There may be a very long list of different types of molds used in industry. Some of the more commonly used types are discussed in the following. 1. Green-sand mold: The term green-sand mold refers to the condition of a mold before pouring metal into it for casting and signifies that the mold (along with the bulk of sand mixture the mold is made of) is in the moist or damp condition (i.e. green condition) (2 to 8% water) when the metal is poured into the mold. The green-sand mold is made from a molding mixture of silica sand, clay, additives and water. It is easy and fast to make as no baking of the mold is required before pouring metal. It is, therefore, cheaper also. On account of being in green or moist condition, this mold is relatively weaker (than a dry-sand mold). All types of ferrous and non-ferrous metals are easily cast in green-sand molds for making small to medium size products. Because of poor strength and softness, these molds find favour or preference in casting intricate shapes of those metals which have high solid-shrinkage as the green-sand molds give way easily to the shrinking casting. A green-sand mold mixture for casting cast iron may consist of the following constituents. River sand Clay Bentonite Coal dust Water

50 to 60% 12 to 15% 2 to 15% 5 to 10% 4 to 8%

2. Dry-sand mold: The term dry-sand mold also refers to the condition of a mold before pouring metal and signifies that the mold (along with the bulk of sand mixture the mold is made of) is completely dried by baking in an oven or furnace before the molten metal is poured into it for casting. It may be noted that at the time of making the mold, the sand mixture (for making dry-sand mold) has enough moisture contents (rather more than that present in green-sand mold mixture) to provide ease in making and shaping the mold, but later the moisture of the completed mold is evaporated by baking it in the oven and thus converts it into a dry-sand mold. The baking of mold is done to have more strength and a hardened and non-erodable mold face before pouring the molten metal into it. Baked molds evolve less steam and gas during casting and therefore to get a good surface finish and dimensional accuracy on castings, dry-sand molds are made from fine-grained sands (which though give less permeable molds but because of backing of mold, no adverse effect is there on castings). Dry-sand molds are, however, costlier and hence used for large castings of steels and cast irons. A typical dry-sand mold mixture for casting cast iron may have the following composition. Floor sand River sand (fine) Clay Saw dust or dung Water

up to 50% 20 to 25% 5% 5 to 10% up to 10% or more

200

MANUFACTURING PROCESSES

The mold surface may be sprayed with molasses water before baking the mold between 150 and 300°C. Use of superior type of facing sand bonded with cereal floor or pitch gives good face strength to mold after baking. 3. Skin-dried mold: These are made from the green sand, and the mold surface or face is dried to a certain depth (12.5 to 25 mm) either by storing in hot air or by gas torches. The use of resin, linseed oil, molasses or corn flour as binder in the facing sand in these molds gives a very hard surface to the mold after drying. These molds are used for large castings of cast irons and steels. Air-dried mold is quite similar to the skin-dried mold but the mold faces of the mold are dried and hardened by exposing the green-sand molds to air, such that some moisture may get evaporated from the mold surface rendering it dried and hard. 4. Loam sand mold: These molds are used for extremely large castings which are symmetrical in shape, such as cylindrical or conical containers. A rough skeleton of the mold is first made using bricks reinforced with iron plates (Fig. 5.24). A loam mortar or loam sand is daubed over and plastered on the brick skeleton to make the mold face, which is later shaped to size and contour with a template or a sweep pattern. A refractory facing is later given to the mold face and the mold dried to get a strong mold. A loam sand mortar is prepared using clay, coarse silica sand, chopped straws, manure and fire clay milled with water. Large size cylinders, kettles, gear blanks, etc. are made by loam sand molding.

Fig. 5.24

Preparing a loam sand mold using a sweep pattern.

5. Plaster mold: A plaster mold is usually prepared for pouring metal by clamping together the two halves of the mold which are cast separately out of gypsum slurry, using a metallic (brass) match plate pattern (Fig. 5.25). The gypsum slurry comprises gypsum plaster, silica flour, silica sand and water. This slurry is poured on the pattern (one side of pattern, say top side) enclosed in a flask and is allowed to set on the pattern (in about 15 minutes). The unset slurry is later poured down from the flask by overturning the flask and the pattern removed away from the hardened slurry. The top half of the mold is thus prepared. Likewise, the bottom half of the mold is prepared by using the respective side of the pattern. Both the mold halves are later dried at 120 to 260°C in the oven and the complete mold is finally prepared by clamping the two mold halves. The metal is then poured. The casting is taken out by destroying the mold.

FOUNDRY PROCESSES—Molding and Casting

Fig. 5.25

201

Plaster molding. Pouring of gypsum plaster slurry over the match plate pattern enclosed in a flask is shown at (i). The dried mold halves (made of hardened plaster slurry) are being set (shown at (ii)) to form a mold cavity ready to receive the molten metal.

Plaster molds are used for precision castings of aluminium, zinc, copper and magnesium base alloys with finer surface details. These molds have low permeability and hence need special care for the escape of gases evolved during casting. The molten metal is therefore poured either in vacuum or under pressure. 6. Carbon dioxide hardened mold: The process of carbon dioxide molding is basically a hardening process for molds and cores. The molds are prepared from a clean and dry silica sand with 3 to 5% by weight of sodium silicate liquid base binder and moisture up to 3%. Wood flour, coal dust, pitch or graphite is added to mold sand mixture to increase collapsibility. The sand mixture is later packed around the pattern and gassing of carbon dioxide is done for 15 to 30 seconds before removing the pattern from the sand (Fig. 5.26). The CO2 reacts with sodium silicate, precipitating SiO2, which with water (in the mold) forms a silica jel, which is a cement-like material and binds the sand grains together giving strength and hardness to the mold. Na2SiO3 + CO2 + H2O Æ Na2CO3 + SiO 2 + H 2 O   

Silica jel

The CO2 hardened cores are also made in a similar way. The CO2 hardened molds are used for casting both ferrous and non-ferrous metals and preferred for casting thin sections such as sharp corners and cooling fins on a heat exchanger. CO2 hardened molds and cores can be stored for a longer period. 7. Core sand molds: Core sand is the name given to a sand mixture composed of fine sand mixed with either natural or synthetic resins and water. It is used for making cores as well as molds. After the mold has been prepared, it is baked at 177–204°C. Some core sands are mixed with furfuryl alcohol and alkyd resins. When this mixture is rammed into a heated cavity (mold), it sets there without baking. Core sand molds have more strength and can resist erosion of mold (against molten metal) better than green-sand molds. These are preferred for casting thin

202

MANUFACTURING PROCESSES

Fig. 5.26

Methods of gassing a mold with carbon dioxide: (a) Using a plywood cover and rubber strip, (b) Using a shower curtain, (c) Providing an entry to the gas so that it may pass through the mold sand to harden the mold face, (d) Using a hollow pattern and (e) Gassing through a sand core by evacuation of air from the pores of core, to replace this with carbon dioxide.

sections and intricate components. The best use is in cases where a large mold is made by joining identical sections together to form the complete mold. 8. Permanent mold or metallic mold: These molds are made of grey cast iron, steel, graphite and refractory metal alloys. Molds are usually made in two halves which are joined together to form a complete mold (Fig. 5.27). These molds give higher dimensional accuracy, better surface finish and high production rate of castings generally weighing less than 25 kg. They also promote finer grain structure in the castings because of their faster cooling of the cast metal. The metallic molds, however, provide a chilling effect on the casting surface rendering it hard. These molds are popularly known as dies (as used on die-casting machines) and these are used for casting both ferrous and nonferrous metals and alloys, but nonferrous alloys with lower melting points are more commonly cast in these molds (because of their lower refractoriness). Permanent molds in the form of dies, used on die-casting machines, will be discussed later. The white cast iron brake blocks for railway wagon wheels are cast in metallic molds mounted on a revolving circular table, where a metal pouring station is fixed at one location on the table, and the casting extraction station (by opening the mold) at another location. These molds are called permanent molds because of their very long working life and yield a large number of castings, for example, over 5,000 castings of aluminium by one mold before redressing the mold.

FOUNDRY PROCESSES—Molding and Casting Riser

203

Runner Hinge Core

Runner block Form block Casting Base block Hinge

Fig. 5.27

A permanent mold (shown one half and opened along a vertical dividing plane). The mold comprises several metallic blocks such as runner block (carrying runner and riser), form block (carrying mold cavity) and base block (forming base of the form block). For easy removal of casting, the runner and the riser are normally kept on the parting line (or plane) as shown.

9. Shell molds: These molds are prepared by pouring a mixture of sand and thermosetting resin over the heated surface of a metallic pattern, which results into the formation of a thin and rigid layer or shell of uniform thickness around the pattern, which, when separated from the pattern surface, forms one part of the shell mold and two such parts (or shells) are joined together to form the complete shell mold. The process of making a shell mold is shown in Fig. 5.28. A brass or aluminium pattern is clamped on a steel plate which also carries projections to produce runners, risers, etc. in the mold cast on this plate. The assembly of pattern is heated from 175 to 370°C and later sprayed with a silicon release agent to help easy removal of mold shell (to be made) from the pattern. A molding mixture comprising fine silica sand and 3 to 10% synthetic resin (thermosetting type such as phenol-formaldehyde) is prepared and dumped over the hot pattern and plate assembly held in a box and is allowed to remain in contact with hot pattern assembly for about 30 seconds. This results in the softening of the resin and formation of a shell or coating of uniform thickness around the pattern assembly. The remaining sand-resin mixture is dropped down by turning the box having the pattern assembly. Later, the assembly of pattern with the formed shell is cured by heating it to a temperature between 315 and 427°C for about three minutes. The shell thickness is kept usually between 5 and 10 mm and is controlled by the time the pattern is in contact with the molding mixture. The shell thus prepared forms only one half of the mold. Two such shells are produced and joined by clamping or adhesive to form a complete shell mold. High precision components with requirement of very good surface quality and close dimensional tolerances are cast in shell molds. Nearly all metals can be cast in these molds. Examples of shell mold cast products include: cylinder heads, connecting rods, gear housings and other mechanical parts.

204

MANUFACTURING PROCESSES

Pattern (hot)

Sand

Shells

Sand and resin Flask (i)

(ii) Mold Clamp

Shell

(iii)

Fig. 5.28

5.26

(iv)

(v)

Shell molding process: (i) Heated metal pattern clamped over the box having a mixture of sand and thermosetting resin. (ii) Box and pattern inverted and the heat in pattern gives an initial set to the resin next to pattern. (iii) After the box and pattern are righted, a shell of resin bonded sand is retained on pattern surface while unaffected sand falls down in the box. (iv) After the heating of shell along with the pattern is over in an oven, and the shell is finally set, it is ejected from the pattern usually by the ejector pins built into the pattern. (v) Two such shells are assembled with clamps to make a complete mold which is supported in a flask with metal shots or sand or other backing material. Shell mold is thus made ready to receive the metal.

MOLDING METHODS

A large number of prevalent molding methods can be grouped in the following two categories according to (a) materials used for mold and (b) methods used for making mold.

5.26.1

Molding Methods according to the Mold Materials

Following are the molding methods according to the mold materials: (i) Sand molding includes (a) green-sand molding, (b) dry-sand molding, (c) loam molding, (d) carbon dioxide molding, (e) core sand molding and (f) shell molding. (ii) Plaster molding (iii) Permanent or metallic mold Molding is essentially the process of making a mold. It consists of preparing the molding mixtures and then making the mold. The general procedure for preparing molding sand mixtures for sand molds has already been discussed. The process of making various sand molds (as mentioned above) with the details of mold materials have already been discussed under Section 5.25.

FOUNDRY PROCESSES—Molding and Casting

5.26.2

205

Molding Methods according to the Method of Making a Mold

Following are the molding methods according to the method of mold making: (i) (ii) (iii) (iv) (v) (vi) (vii)

Open mold method Floor molding or bedding-in method Bench molding Plate molding Pit molding Machine molding Flaskless molding

1. Open mold method: The method is suitable for making molds for flat products for which solid patterns are used, and the entire mold is made in the foundry floor sand bed without using any top or cope mold box. The upper surface of the mold is thus open to atmosphere. Products like floor plates, grills, railings, weights, large flywheels and other products with flat top are cast by this method. 2. Floor molding or bedding-in method: This method is used for making molds of products which are very large in size and for which the foundry floor sand bed is used as a drag. Large size green-sand molds, dry-sand molds and skin-dry sand molds are sometimes made by floor molding method. When a two-piece pattern is used, half of it is fully embedded in the floor sand and its top face is levelled with the compacted sand (around the pattern). Parting sand is later sprinkled on the pattern top and on the compacted floor sand around the pattern. The upper half of the pattern is then placed over the sand-embedded pattern half and the assembly enclosed with a cope flask. Sand is filled in the cope flask and compacted keeping provision for sprue and risers. Venting of the sand in the cope flask is done. Later, the cope flask is lifted off and the pattern halves are taken out from the floor sand bed and also from the cope sand. After putting the core in position, the cope flask is placed again over the half mold cavity in the floor sand bed and the mold is thus made ready for pouring the metal. Sometimes the floor sand bed is compacted and levelled to take up molding of component in which both drag and cope flasks are used and the floor sand bed just performs the function of a bed plate (or molding board) for molding. 3. Bench molding: Bench molding is carried out on a working bench using small size flasks and hence the method is suitable for making small molds. The molder works while standing. Green-sand molds, dry-sand molds and skin-dry sand molds can be made by this method. Bench molding may involve the use of two or more boxes. A wooden molding board is always used to support the molds filled with sand. A mold made by bench molding method, using two boxes, is shown in Fig. 5.29. When a number of molds, one above the other, are made using small boxes and having a common sprue to feed metal, the process is called stack molding [Fig. 5.29(A)].

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Fig. 5.29

Fig. 5.29(A)

Two box bench molding.

Stack molding.

4. Plate molding: It employs a metallic pattern consisting of a flat plate (match plate) with portions of a pattern permanently attached on both sides of the plate as shown in Fig. 5.30. The plate carries runners built in it and withdrawal of pattern is easy and quick as the plate (and pattern) overlaps the walls of the mold boxes. Molds for several identical parts of small size can be made using a number of patterns. The match plate with pattern is clamped with mold boxes for molding purpose. Pattern (half)

Pattern (half)

Runner Mold clamp

Pattern (half)

Runner

Plate Pouring cup

Fig. 5.30

Showing a typical match plate pattern used for making six castings in one go. Patterns are split along the parting line formed by the plate, cope patterns are on one side of the plate and drag patterns on the opposite side. The patterns carry a common runner and separate gates to feed metal. Molding is often done on molding machines. The pattern plate is coupled between the cope and drag with the help of two mold clamps.

5. Pit molding: Large castings that may not be accommodated in mold boxes are molded in pits made by digging the molding floor. The pit is lined with bricks or concrete and the desired shape of the mold is given by using a pattern. The bottom of the pit carries a well-rammed layer of cinder to allow escaping of gases during casting. The cope flask is placed over the pit containing the pattern. The cope is filled with sand and compacted and later provision of pouring sprue and riser, etc. is made in the cope sand. After lifting the cope, the pattern is removed from the sand

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bed (housed in the pit) and the cope placed back in position on the pit to make the mold ready for pouring. A large quantity of molten metal used in pit mold casting may generate large amounts of gases, giving a tendency of lifting the cope or bursting of mold. To take care of such a situation, a metallic bed plate may be provided at the pit bottom and is attached with steel rods with the cope flask, thus making the cope and the drag molds as one compact piece. 6. Machine molding: Hand molding is a slow and laborious process and gives variable hardness to the rammed mold. It is thus not very suitable for mass production. The use of molding machines, on the other hand, ensures higher production of better quality molds. Molding machines perform the following basic functions: (i) Ramming or compacting of sand in the mold boxes through the operations of jolting, squeezing or slinging the sand in the mold box. (ii) Rolling over or inverting the mold upside down. (iii) Rapping of pattern (embedded in compacted sand) by vibrating it. (iv) Withdrawing of pattern from the mold. Different types of molding machines are used in industry, for example, jolt machine, squeezer machine, jolt-squeezer machine, sand slingers, roll over machines, etc. A typical jolt roll-over pattern-draw molding machine used for making only the drag part of the mold is shown in Fig. 5.31. This machine performs several functions, for example, after ramming sand around the pattern kept in a drag [Fig. 5.31(a)], it rolls the mold over so that the bottom face of the mold is made available to remove the pattern by rapping it in the sand with air or electrically operated vibrator. Vibrator

Rocker arm

Clamp bar Jolt table

Drag

Draw table

Pattern plate Draw table Jolt table (a)

Fig. 5.31

(b)

A jolt roll-over pattern-draw molding machine used for making drag molds. (a) Drag mold box is placed on the pattern plate forming part of rocker arm. The mold box is sand filled and compacted by jolting. Top of the drag mold is covered with a plate which is clamped to the mold. (b) Mold is rolled over by 180o with the help of rocker arm. The draw table rises up and the mold is placed on the draw table. Mold is unclamped. Vibrator raps the pattern as the draw table comes down slowly, separating the mold from the pattern.

Sand slingers (Fig. 5.32) are used for filling and compacting large volumes of sand in the mold. In this machine, sand is thrown into the mold box with the help of a fast rotating impeller, which causes the packing of sand in the box through inertia. The slinger head can be easily moved forward and backward and turned around to

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Screen

Hopper Slinger head

Refused sand

Motor

Fig. 5.32

Molding boxes

Molding sand

A sand slinger.

fill sand in the entire area of the large mold. An average slinger can handle (and compact) up to 450 to 1800 kg of sand per minute. Slingers are used in filling pit molds. 7. Flaskless molding: When properly bonded and high strength molding sands (not less than 185 kPa in green state) are used and adequately rammed (ramming pressure up to 15 MPa) around the pattern, molded sand mass acquires adequate strength to maintain the integrity of mold. Molding machines are used for compacting the sand such that after the compacted sand mold is ready, the mechanized metallic molding flask is withdrawn with the help of a plunger leaving behind a mold with compacted sand body (and no molding flask). The process eliminates the need of molding flasks. It is used for making gas stove grills, brass valve bodies, malleable iron pipe fittings, etc. Castings are of high quality.

5.27

CASTING METHODS

Common methods of casting used in industry are given in the following.

5.27.1

Sand Mold Casting

Sand mold casting includes casting or pouring metal in sand molds such as green sand mold, dry sand mold, CO2 hardened sand molds, core sand molds, etc. Casting processes based on these molds have already been described while discussing these molds.

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Characteristics of sand mold casting ●

● ● ●

●

●

●

●

5.27.2

Almost any metal can be cast with no limit on size and shape. The molding method is, however, more suitable for moderate to large castings and unsuitable for thin sections. Low equipment and tooling cost. Economical for low-volume production. The casting has, however, rough surface, usually requiring machining for smoothening the surface. Surface finish of casting is affected by finish on pattern, sand structure, mold dressing, mold venting, etc. Since solidification of metal is under non-equilibrium conditions, the castings are susceptible to cooling cracks. By proper designing of the casting and using proper gates and risers and metal chills, shrinkage problems can be taken care of. Being porous to some extent, sand mold castings are not used for pressure tight vessels (generally used up to 10 kg/cm2 pressure). The structure of castings is loose and is less stronger than wrought products. Castings have good hardness. Internal stresses can be eliminated by avoiding sharp corners in castings and physical restraints (both in casting design and mold). Sand castings have poor ductility. As the grains in a sand mold casting are not close, the casting has lower density and poor strength.

Metallic Mold Casting

Metallic mold casting includes permanent mold casting (or gravity die-casting), slush casting, pressure casting (or low pressure permanent mold casting), die-casting (or pressure die-casting). 1. Permanent mold casting: Permanent mold casting is known by several names such as metallic mold casting, hard mold casting, or gravity die-casting. Unlike the sand mold which is destroyed every time when the casting is taken out of mold, the permanent mold is so designed that it lasts for several thousand castings. The process of casting by pouring metal in a permanent mold is called permanent mold casting. It is known as gravity die-casting because metal is poured in the mold (die) under hydrostatic pressure created by risers. The mold cavity and the gating system and risers, etc. for the flow of metal to mold are all made into the metal mold body and mold is usually made in two halves, hinged along one vertical edge. Mold may comprise several blocks or parts of mold, capable of being opened separately for easy removal of casting (along with risers, gates, etc.) (Fig. 5.27). Metal cores are used in permanent molds for creating holes or cavity. These have enough draft (taper) for easy removal from the mold soon after the metal just solidifies in the mold. Permanent molds are made from grey cast iron, steel, bronze, graphite, aluminium alloys and refractory metal alloys. Molds are sprayed or coated with some refractory slurry (graphite, lamp black or core oil). The permanent molds have high initial costs. Casting of low melting point alloys is preferred in these molds because of low refractoriness of the mold.

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Characteristics of permanent mold casting

(a) Castings have very close grain texture and decreased porosity (by half to that of sand castings). (b) Better surface appearance. (c) Castings superior in hardness and density and mechanical properties. (d) Higher dimensional accuracy. (e) Equipment costlier than sand molds but cheaper than die-casting. (f) With reduced porosity, castings used in pressure vessels (pressure up to 15 kg/cm2). (g) Castings are less susceptible to cooling cracks (and hence stronger) because of solidification of metal (in mold) under equilibrium conditions. (h) Less skilled labour required. (i) Production rate is higher and hence economical. (j) Require less space (than sand casting). (k) More suitable for small to medium size castings of non-ferrous metals. (l) Metal molds are unyielding against the contraction of molten metal during solidification and hence cause internal stresses in castings with the danger of shrinkage cracks. 2. Slush casting: During the solidification of molten metal poured in a mold, a thin solidified skin of metal begins to form at the surface of the cold mold walls and this skin gets thickened as the time passes. The slush casting is based on this principle. Hollow castings with thin walls are made in permanent molds by pouring metal in the mold and after the desired thickness of the casting (i.e. solidified skin of metal) is obtained, the mold is inverted and the remaining molten metal is poured out of the mold. The method is used for casting objects like lamp base, toys and other decorative or ornamental objects from low melting point metals such as zinc, tin or lead alloys. 3. Pressure casting: It is also known as low pressure permanent mold casting. In this method, the molten metal (heated in an electric kettle) is forced upward into a permanent mold or die (situated above the kettle) by maintaining a low air pressure on the molten metal in the kettle until the metal in the mold gets completely solidified. The method is used for high quality castings such as steel rail road or car wheels cast in graphite molds. Thin walled castings and castings with large projected areas are easy to be made by this method. 4. Die-casting or pressure die-casting: It is the method of rapidly producing metal components by forcing molten metal under pressure (0.7 MPa to 700 MPa) into a permanent mold called a die. Die-casting is a net-shape forming process wherein the casting requires no or little machining. Walls as thin as 0.38 mm can be die-cast. Die-casting process is usually adopted for casting zinc-, aluminium-, magnesiumand copper-base alloys. Dies used are made of hot-work die steels, chrome-vanadium or chrome-tungsten steel. A die is usually in two halves (or parts) with a vertical parting surface when closed. The part of die which remains always stationary is called cover die and which is movable is called ejector die. Metal cores are used for producing hole or cavity in the mold. Cores are placed in their set positions in

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the die cavity before pouring metal and are withdrawn soon after the cast metal has just solidified. Risers are not used in die-casting. Since molten metal is forced in the die under pressure and die being water cooled, the molten metal solidifies quickly avoiding the need of a riser. Some details of a typical die-casting machine are shown in Fig. 5.33.

Fig. 5.33

Details of die and other systems of a cold chamber die-casting machine. (a) Pouring metal into the chamber (or injection cylinder) when two halves of the die are set in closed-mold position. (b) Ejecting pins in operation to eject the casting along with sprue, etc.

Intricate castings are easily die cast. Since cooling rate of molten metal is high in die-casting (dies are water cooled), the process suits to mass production. Die cast products have excellent surface quality, close dimensional tolerances and features like screws, locating flanges or inserts of other metals are readily incorporated in the die cast products. A die-casting machine essentially consists of a die set having a fixed die half (cover die) and a movable die half (ejector die), mechanism for opening and closing the die, ejecting mechanism for castings, automatic system for insertion and removal of cores from the casting, means of forcing the molten metal in die as also the arrangement for the supply of molten metal for casting. The metal melting and supply system may, however, be either an integral part of the die casting machine or a separate system outside the main machine. Depending

212

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on the metal melting and its supply system, the die-casting machines are of following two main types. (a) Hot chamber die-casting machines: These have the metal melting chamber or system as the integral part of the main die-casting machine. A submerged plunger hot chamber die-casting is shown in Fig. 5.34. These machines are used for casting only the low melting point alloys such as zinc, tin, lead, etc. After the die halves are closed to make the mold cavity, the molten metal entered through the filling hole and collected into the goose neck metal container, is pumped into the die by the plunger operated by a hydraulic system and the pressure (up to 35 MPa) on molten metal (in the die) is maintained till the casting solidifies in the die.

Fig. 5.34

Schematic representation of a submerged plunger type hot chamber die-casting machine.

(b) Cold chamber die-casting machines: These have the metal melting chamber or system outside the main machine and the molten metal is brought from distance and poured into the injection cylinder by a hand ladle (Figs. 5.35 and 5.33). The plunger forces the molten metal into the die cavity at a pressure ranging from 20 to 70 MPa or more. Cold chamber die-casting machines are used for casting high melting point alloys such as aluminium, magnesium and copper. These metals are not cast in hot chamber die-casting machines since they tend to alloy with the steel plunger of the die-casting machine.

Fig. 5.35

Working of a cold chamber die-casting machine (horizontal chamber type).

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There are vacuum die-casting machines also which have come up as the modified version of the above mentioned two types of machines. With the incorporation of a vacuum system with the machines, the die cavity is kept completely free of air before metal is injected into it. Besides the total elimination of entrapped air from the die, the metal feeding pressures are also reduced considerably in these machines. Typical components made by die-casting include carburetors, valve bodies, transmission housings, hand tools, general purpose appliances, etc. Characteristics of die-casting process ●

●

● ●

● ● ● ●

●

● ● ●

5.27.3

Casting of thin sections with complex shape is possible, minimum thickness in zinc casting may be 0.5 mm and that for aluminium 0.9 mm. Process provides for precision manufacture of products with close dimensional control (tolerance of ± 0.07 mm for zinc up to 25 mm size). Very good surface finish. True shape as that of die cavity can be obtained and thus casting details are reproduced successfully with a high degree of precision. Sound castings and hence less defective castings and reduced machining cost. Less labour cost. Production rate very high (typically up to 800 castings per hour). True shape of die retained for longer period (for example, up to 100,000 castings of zinc and 75,000 castings of copper-base alloys). Cost of die and equipment is high. Die life reduces as metal temperature is high and hence limited range of non-ferrous alloys are die-cast properly. Size of casting is limited. Die-cast products may sometimes have porosity due to air entrapment. Maintenance of equipment is costly. Minimum economic quantity of production is to be decided judiciously as it is a mass production method.

Centrifugal Casting

The process of centrifugal casting (also called liquid forging) consists of rotating a mold (which may be of sand, metal or ceramic) at high speeds such that when molten metal is poured into the mold, it is thrown against the mold walls as it is directed outward from the centre under the action of centrifugal force. This results in the formation or deposition of a uniform thickness of metal all along the inside surface of the mold, where it remains until it cools and solidifies. The centrifugal casting is best suited for cylindrical parts such as cast iron pipes, cylinder liners, gun barrels, pressure vessels, gears, flywheels, etc. Besides round hollow section, a variety of other hollow symmetrical sections are also cast by this method. A typical centrifugal casting set-up is shown in Fig. 5.36(a). Metals cast with centrifugal casting system include: grey cast iron, cast steel and nonferrous alloys (specially copper-base alloys). Dual metal casting is also possible by this method. Very large castings (for example, length 10 m, diameter 1.8 m and weight 3.5 tonnes) are easily cast centrifugally. No core is used in this process.

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MANUFACTURING PROCESSES

Fig. 5.36(a)

Illustrating the principle of centrifugal casting to produce a high-grade casting by throwing heavier (colder) metal outward and forcing the less dense (hot) metal to remain near the axis of rotation, which further acts as feeder during solidification and thus promotes directional solidification. Impurities like oxides, non-metallic inclusions, slag, etc. being lighter try to congregate inward close to centre, as shown at (i) and are removed later by machining. The schematic representation of a horizontal centrifugal casting machine is shown at (ii).

Depending on the method of feeding metal in the molds and the location of molds with respect to the axis of mold rotation, centrifugal casting process is classified into the following three main types [Fig. 5.36(b)]. (a) True centrifugal casting (b) Semi-centrifugal casting (c) Centrifuging or pressure casting

Fig. 5.36(b)

Showing the basic difference between true centrifugal casting, semi-centrifugal casting and centrifuging.

In true centrifugal casting, the axis of rotation of the mold and that of the casting is same and horizontal. The casting produced is always centrally hollow and is made exclusively under the effect of centrifugal force. In semi-centrifugal casting, the component cast is usually large and has rotational symmetry as in case of pulleys, flywheels, discs, etc. Casting

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215

is done in a mold rotating about its vertical axis. The castings made by this method need not be centrally hollow. The process of centrifuging or pressure casting is applied to the casting of non-symmetrical objects such as brackets, bearing caps, etc. In this process, a number of molds are placed around a vertical central sprue for feeding metal and are all connected to this central sprue. The table carrying molds is rotated about the vertical axis of the sprue. In this case, the combined axis of rotation of the molds (or the table carrying molds) does not coincide with the axis of rotation of individual mold as the molds are situated at a distance from the central axis of the mold table. The molten metal in the central sprue is fed to all the molds under the effect of centrifugal force generated due to the rotation of the mold table.

5.27.4

Precision Casting Processes

Precision casting processes include investment casting (or lost wax process or precision casting), shell mold casting and ceramic mold casting. Precision casting processes are those which give castings high dimensional accuracy and excellent surface finish. The following processes are usually considered under this category. (a) (b) (c) (d)

Investment casting Shell mold casting Plaster mold casting Ceramic mold casting

(a) Investment casting: Investment casting, also known as lost wax process or precision casting, is a very old method of casting and dates back to even 4000 BC, but the process gained importance at industrial level only during World War-II for producing light (weighing from 1 g to 35 kg) and intricate components with close dimensional tolerances. Both ferrous and nonferrous alloys are cast by this process. High dimensional accuracy of the order of ±0.08 mm and smoothness of the order of 0.015 to 0.025 mm rms value can be obtained. Very thin sections of the order of 0.75 mm can be cast. The process is capable of reproducing surface details and dimensions with precision, especially for high melting point alloys and difficult to machine metals. Examples of investment cast products include cast superalloys components for gas turbine, gears, cams, ratchets, office equipment, jewellery, etc. The investment casting essentially consists of the following steps (Fig. 5.37). (i) Making of an expendable wax pattern by injecting wax (or thermoplastics, usually polystyrene) into a steel die or mold in two pieces. Cast solidified pattern is removed by opening the die. (ii) Connecting together a couple of cast patterns with previously cast wax gates (for metal flow in the molds) and thus preparing a tree with the aim of producing a stack of investment molds, followed by precoating of tree with fine refractory material. (iii) Preparing an investment mold (or molds) by investing (or dipping completely) the precoated wax pattern assembly into an investment material, which is a coarser and viscous refractory material slurry with some binder.

216

Fig. 5.37

MANUFACTURING PROCESSES

Steps involved in making a mold in investment or lost wax casting process. (a) Making a wax pattern by injecting molten wax into a metal die (made of two halves). (b) Forming a tree by welding (using hot spatula) several patterns together with the already prepared sprue, pouring cup and gates, all made of wax. (c) Precoating of wax tree by dipping it in an extremely fine refractory slurry either by spraying or dipping the pattern assembly in fine slurry and thus give an interface coating between pattern and coarse investment slurry. (d) A metal flask sealed to the pallet, houses the refractory coated (but dried) wax tree. The investment (which is a coarser and more viscous refractory slurry) is poured in the flask totally dipping the wax tree. The slurry filled flask is then vibrated for compacting the slurry properly around the wax tree. (e) After the investment has set around the wax tree, the investment mold thus formed is placed in inverted form in an oven maintained between 90 and 175oC to dry the investment and to melt out the wax pattern (or tree). Mold may be further heated between 650 and 1050oC to further dry the mold and to take out the wax completely from the investment mold and to bring the mold to the temperature at which it will receive the molten metal. (f) The hot mold is now ready to be poured immediately.

The investment slurry (or sometimes called coarser-investment slurry) is made of the following materials. (i) For non-ferrous castings, plaster of Paris and other gypsum products (ii) For steel castings, silica flour with binder such as ethyl silicate, sodium silicate and phosphoric acid (iii) Fine-grained silica sand with binder.

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(iv) The slurry sets around the pattern assembly. Later the pattern assembly is melted out of the set and hardened investment material, leaving behind an investment mold which is further heated in oven to clean out the traces of wax if any. (v) The molten metal is poured in heated molds to produce an investment casting. (b) Shell mold casting: Shell mold casting is also known as croning process after the name of its inventor J. Croning. It is famous for production of castings with very smooth surfaces and close tolerances at lower costs. Making of shell molds from sand and thermosetting resin has already been described (Fig. 5.28). It may be noted that the fine sand used in shell molds has much lower permeability than the sand used in green-sand molding. Also, the decomposition of shell-sand binder generates high volume of gas. Unless proper care is taken to vent out the entrapped gases and air, serious problems may arise in casting ferrous metals in shell molds. During casting, the mold may be supported or backed by sand filled in a flask. (c) Plaster mold casting: The plaster mold has already been discussed (Fig. 5.25). Because plaster molds have very low permeability, gases evolved during solidification of metal cannot escape easily. Therefore, the molten metal is poured either in vacuum or under pressure. Mold permeability is sometimes improved using foamed plaster containing trapped air bubbles. Castings made in plaster molds have good surface finish (63 to 125 RMS). Because these molds have low thermal conductivity than others, the castings cool slowly and a more uniform grains structure of castings is obtained with less warpage. Wall thickness of castings may be 1 mm to 2.5 mm. (d) Ceramic mold casting: The ceramic mold casting is a variation of plaster mold process, except that the mold material is a refractory slurry for high temperature applications. The slurry, comprising fine-grained zircon (Zr–SiO4), aluminium oxide, fused silica and some binding agent, is poured over the pattern (metallic or wooden) kept in a flask. After sometime, the set ceramic slurry facing is removed from the pattern and dried and baked. The mold (or ceramic facing) thus prepared is baked by fire clay to give strength to the mold (shaw process). Steel dies used for hot forging are made by shaw process. Ceramic molds are used for casting ferrous and nonferrous alloys, high temperature alloys, tool steels, etc. Castings have very good finish and dimensional accuracy. Examples of castings include dies for metal working, molds for plastic molding, cutters or impellers, etc.

5.27.5

Full-mold Casting or Expendable-pattern Casting

Full-mold casting is also known as lost foam process, evaporative-pattern or lost-pattern casting. The process uses a polystyrene pattern, which evaporates upon contact wth molten metal to form a cavity for the casting. The raw expendable polystyrene (EPS) beads are placed in a pre-heated aluminium die. The beads expand and take the shape of the die cavity. Beads are heated further to fuse and bond together to form the pattern. The pattern is later placed in a flask which is filled with sand and compacted around the pattern [Fig. 5.37(A)]. With pattern in position in sand, molten metal is poured onto the pattern which immediately vaporizes the pattern, resulting into a mold cavity which gets filled by metal. Fine castings

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MANUFACTURING PROCESSES

of ferrous and non-ferrous metals with complex shapes and good surface details are produced by this process. The process finds use in automotive industry for casting aluminium engine blocks, manifolds, cylinder heads, etc.

Fig. 5.37(A)

5.27.6

Full mold casting. Styrofoam pattern compacted in sand (a) and molten metal poured on the pattern (b).

Continuous Casting

The process of continuous casting consists of continuous pouring of molten metal into a horizontal or vertical mold or a die opened at both ends, cooling it there rapidly and removing the solidified product in a continuous form [Fig. 5.37(B)]. The process is used now-a-days to produce blooms, billets, slabs and tubings directly from molten metal in steel plants replacing the traditional casting of ingots in separate molds and later convert them into blooms or billets. The process of continuous casting is largely used for casting steel, brass, copper, bronze, aluminium, grey cast iron and alloy cast iron. The process provides continuous casting of very long lengths. Electromagnetic casting is a variant of continuous casting wherein molten metal is contained and solidified in an electromagnetic field (instead of a conventional mold) and the molten metal does not slide in contact with mold walls and the metal is solidified by direct water impingement. Casting has a very good surface finish.

5.27.7

Chill Casting

It is sometimes required that only the outer or inner surface of a cast iron casting should be hard enough to withstand surface wear while rest of the portion of the casting may remain soft. Examples include slide ways of machine tools, rolls, tramcar wheels and railway brake shoes. Required surface of the casting can be achieved hard by controlling the cooling rate of castings at that surface as the speed of cooling has a marked effect on the final hardness of cast iron. In normal cooling, graphite present in cast iron has enough time to separate itself from iron and thus the structure of casting will compose of free graphite flake which renders the casting soft and easy to machine. But when cooling rate is fast, graphite has no time to separate itself and it thus remains combined with iron in the form of iron carbide which is

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very hard and wear resistant. The more rapid the rate of cooling, the greater the depth of iron carbide formation in the casting. This is achieved by introducing a ‘chill’ (which is a metal piece with high thermal conductivity) in the mold. Chill forms that surface of the casting which is to be hardened and the result is known as ‘chilling’.

Fig. 5.37(B)

Showing the continuous casting of steel. The platform is kept about 20 metres above the ground level. The solidified metal descends at a speed of about 25 mm per second. The electric furnace ensures supply of clean molten metal to the tundish (which is a refractory lined pouring vessel of capacity about 5 tonnes). Impurities are skimmed off in tundish. The poured molten metal travels down through water cooled copper molds and begins to solidify into a path supported by pinch rolls. The metal pouring rate is controlled by X-ray system. Molds are coated with graphite or other solid lubricants. Molds are vibrated to reduce friction and sticking of metal with the mold. Solidified metal shell thickness at the exit of mold is about 18 mm or so, and therefore additional cooling is provided by water sprays along the path of travel of the solidifying metal.

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MANUFACTURING PROCESSES

For chill casting, molds are made in the usual way as done in sand molding but for the purpose of chilling, a chill of cast iron or steel is used as shown in Fig. 5.37(C). This is an example of composite mold of sand and metallic chills. For getting chilling effect, sometimes molds are made completely of metal as used for casting railway shoe brakes in which a number of steel molds are mounted on a circular table to facilitate pouring of metal in the molds; the table is rotated and molds are thus brought to the pouring position one after the other. Pre-heating of molds is sometimes done to reduce the chilling effect. Core chills of metals are used to make the bores of cast wheels and bushes hard and wear resistant.

Fig. 5.37(C)

5.28

Composite mold of sand and metallic chills for chill casting of a wheel.

CLEANING OR FETTLING OF CASTING

Cleaning and trimming of castings are required before they are subjected to other operations like machining or inspection. Cleaning or fettling is the operation of removing from casting the adhering sand, gates, risers, flash metal at joints, fins or cores and cleaning the casting surface. Gates and risers are removed by hammering, chipped off by chisel and hand or pneumatic hammer and castings cleaned by chipping, filing, shearing or sawing with abrasive wheel, flame cutting or grinding. Surface cleaning of castings involves removal of clinging and embedded sand particles and fins, etc. by impact or grinding. After fettling of gates and risers or fins, the adhering sand is removed by raping, wire brushing, tumbling, pickling and shot blasting. Raping involves hammering the casting to remove sand. In tumbling, castings are kept in a barrel with small sharp-cornered stars of white cast iron and the barrel rotated so that the impact of stars with casting may result in cleaning the castings. In the pickling process, castings are treated with acids such as hydrofluoric acid for cast iron castings, nitric acid for brass castings. Castings are later dipped in some alkaline solution and then in hot water. In shot blasting, large size sand particles or round or angular chilled iron shots are projected with high velocity against castings, the shots being carried by high velocity air-stream or mechanically hurled from a rotating impeller.

FOUNDRY PROCESSES—Molding and Casting

5.29

221

REPAIR OF CASTINGS

After the castings are fettled and cleaned, they are inspected for flaws. Visible surface defects in castings are repaired using some of the following methods and the casting is made serviceable after repairing. Cast iron castings are repaired by gas welding, electric welding, thermit welding or brazing. Porous spots are smoothened with air setting mixture of iron filings and air setting hardener. Steel castings are repaired by arc welding, thermit welding, inert metal arc welding or brazing. Porous spots are smoothened by impregnation with bakelite or resin. Non-ferrous castings are repaired by gas welding, arc welding (inert metal arc), soldering, brazing and impregnation with bakelite. Malleable iron castings are repaired by metal arc welding and gas welding.

5.30

METALS AND ALLOYS USED IN CASTING

The following metals and alloys are more commonly used for casting: 1. Grey cast iron represents the largest amount of all metals cast in foundries for engineering and other uses. It is used for engine blocks, machine tool beds, pipes, pulleys, gears and other machine parts. 2. Ductile or nodular cast iron is used for crank shaft, pipes, high strength and highly stressed parts of machines. 3. White cast iron is extremely hard and wear resistant and hence used for rolls in rolling mills, brake shoes for railway wagon wheels, rock crusher jaws, shot blasting balls, etc. 4. Malleable cast iron is used for rail road equipment, hardware, pipe fittings, gears, connecting rods, etc. 5. Cast steel is used for die blocks, heavy duty gear blanks, rail road wheel, gas turbine housings, pump, rock crusher jaws, etc. 6. Stainless steel castings are used for equipment and articles for high corrosive and high temperature applications. 7. Aluminium alloys are used for auto parts, engine blocks, decorative and architectural articles, kitchen and dairy industry utensils, equipment for aerospace industry and general portable power tools. 8. Magnesium alloys are lightest, have good corrosion resistance, and application is similar to aluminium alloys. 9. Copper alloys include bearing metals, brasses and bronzes and other articles for electrical industry. 10. High temperature alloys include titanium alloys, superalloys and other refractory metals.

222 5.31

MANUFACTURING PROCESSES

METAL MELTING FURNACES

The practice of metal melting is an important aspect in the casting process as it has a direct bearing on the quality of castings. A foundry man should be well versed with the operation and use of various metal melting furnaces. Besides that, he should also have a good knowledge of various alloying elements and other materials (such as flux and slag forming elements) added to the furnace charge. Different types of metal melting furnaces are available and the choice of a proper furnace depends on the following factors. (a) Furnace should be capable of controlling proper alloying of elements to control the properties of casting metal. (b) Operating temperature of the furnace should be higher than the metal to be melted. (c) Capacity of furnace to be adequate to meet the delivery requirement. (d) Furnace to have low initial investment and lower melting and maintenance cost.

5.31.1 Types of Furnaces Furnaces commonly used in foundries are of the following types. 1. Crucible furnaces: These are simplest and used for melting relatively small quantities of ferrous and non-ferrous metals. Crucibles vary in their sizes, up to 100 kg or even more. The crucible, made of silicon carbide, may be heated in natural draft coke fired pit furnace (Fig. 5.38) or oil or gas fired furnaces which may be stationary or tilting type. Induction type (electric) crucible furnaces are also used for heating metal in crucibles. The crucible filled with molten metal is taken out of furnace and is taken to casting site for pouring.

Fig. 5.38

A natural draft pit furnace.

2. Electric furnaces: These give high rate of production, much less pollution and the ability to hold molten metal at constant temperature for a long time for alloying purposes. These may be (a) direct arc type, (b) indirect arc type and (c) induction type. Essentials of these furnaces are shown in Fig. 5.39. Direct arc furnaces develop temperature up to 1925°C. Indirect arc furnaces, often used for melting copper and

FOUNDRY PROCESSES—Molding and Casting

Fig. 5.39

Three different types of electric furnaces:

223

(a) Direct arc, (b) Indirect arc and (c) Induction furnaces.

its alloys, develop lower temperatures. Induction type furnaces with still lower temperatures, are used for melting non-ferrous alloys. 3. Cupola and its operation: A cupola is a refractory lined vertical steel vessel [Fig. 5.40(a)] used for making cast iron by melting pig iron charge comprising pig iron, coke, limestone (flux) and steel scrap. Cupolas are capable of operating continuously, have high melting rate, are available in various sizes and capacities, and are easy and economical to operate. A properly designed and operated cupola is most economical to operate. Cupola is a mini blast furnace which helps in reducing (and refining) pig iron into cast iron. It is also used for the production of nodular cast iron and white cast iron for malleabilizing. Some copper-base alloys can also be melt in cupola. Refractory lining inside a cupola is done to safeguard against danger of melting down of cupola metallic shell. Cupolas are named ‘basic or acidic’ cupolas according to the type of refractory lining. Basic cupolas are lined with magnesia bricks (or dolomite plaster) and are used for making ductile cast iron. Sulphur contents in a melt in a basic cupola can be reduced to even 0.005%. Carbon pick up by melt is higher. Acid cupolas have fire clay brick lining and are cheaper and easy to operate. Normal cupolas used for general production of cast iron are all acid type which produce acidic slag involving the use of limestone as flux. The flux also helps in reducing sulphur contents in the cast iron melt as higher contents of sulphur reduces metal fluidity resulting in unsound, hard and brittle castings.

224

Fig. 5.40

MANUFACTURING PROCESSES

Outline of a cupola furnace (at (a)) and cross-section through a cupola showing its various zones (at (b)).

The sectional view of a cupola is shown in Fig. 5.40(b). It has a drop-bottom system to help discharging the debris from the cupola at the end of melt. The drop-bottom is made by joining two hinged semi-circular plates supported by a prop. In the beginning of cupola operation, a sand bottom is rammed properly upon the propped drop-bottom. Coke is placed on the sand bottom and fire is lit. Further charge of coke is added later if the first charge is found properly ignited. Ultimately coke charges are introduced in cupola till the coke bed level extends 0.75 metre or more above the tuyer level. This initial coke bed charge is used for supporting alternate charges of pig iron, flux and coke introduced later during operation of the cupola. The height of the initial coke bed charge is very important for the successful operation of the cupola. The coke bed is allowed to burn for about half-an-hour before charging the cupola with pig iron, coke and flux. Air for combustion of coke is supplied by air blower through the air blast pipe and enters the wind box surrounding the cupola. Air from the wind box passes through tuyeres (openings) that take air from the wind box to the combustion zone. Charges of pig iron and coke, etc. are fed into the cupola through the charging door in alternate layers of its constituents. The heat due to burning of coke melts pig iron. The flux (lime stone) takes out impurities from the metal in the form of slag which floats on molten metal and is taken out from the slag spout. The molten cast iron is periodically tapped into laddles for pouring purposes.

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225

Cupola charge: The material or charge fed to cupola comprises following: (a) Pig iron and scrap wherein the scrap consists of rejected cast iron castings and steel scrap. It is added to the charge to control the carbon contents of the final molten cast iron. (b) Fuel comprises a good quality coke (with low sulphur), anthracite coal or carbon briquettes. Coke is preferred to coal because coal is a highly complex organic material with undesirable elements, effects of which are difficult to control. The metal-fuel ratio (also called melting ratio) is usually kept 10 : 1, i.e. 10 parts of pig iron (including scrap) and 1 part of coke by weight. In other words, for melting one tonne of pig iron, 100 kg of coke will be usually required. The metalfuel ratio may vary from case to case. (c) Flux is used to refine the metal during the process of its conversion from pig iron to cast iron. The charge fed to cupola carries many impurities comprising mainly high melting point refractory materials such as oxides, rust, sand sticking to castings, coal ash, etc. By reducing sulphur contents of the charge, the flux helps in lowering down the melting point of these refractory materials. Reacting with the impurities, the flux forms a compound (called slag) which melts at lower temperatures and is lighter in weight such that in its molten state, the slag keeps floating on the liquid cast iron. From there, the slag is tapped out through the slag spout from time to time during the operation of the cupola. The most common flux used in cupolas is limestone (CaCO3). The quantity of flux used is 2 to 4% of the weight of iron charge. Other fluxes used include dolomite, sodium carbonate and calcium carbide. Flux in excess may damage cupola lining. Cupola zones are shown in Fig. 5.40(b). These are described as follows. 1. Well is the space between the bottom of tuyeres and sand bed at bottom and contains the molten cast iron. 2. Combustion or oxidation zone is above the top of tuyeres and up to a height of 15 to 30 cm. A large amount of heat is generated in this zone due to exothermic reaction. C + O2 Si + O2 2Mn + O2

Æ Æ Æ

CO2 + Heat SiO2 + Heat 2MnO + Heat

3. Reducing or protective zone extends from the top of combustion zone to the top level of coke bed. CO2 is reduced to CO in this zone. CO2 + C(Coke)

Æ

2CO – Heat

This zone provides a reducing atmosphere and protects oxidation of metal charge. 4. Melting zone is where solid iron is converted into molten state which trickles down through the coke bed to the well from where it is tapped out for pouring in the molds. The melting zone starts from the first layer of metal charge above the coke bed and stands upwards to a height of about 90 cm. In this zone, solid pig iron is

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MANUFACTURING PROCESSES

converted into molten cast iron. A significant part of carbon pickup by melt also occurs in this zone. 3Fe + 2CO Æ Fe3C + CO2 5. Pre-heating zone includes all the layers of cupola charges placed above the melting zone to the top of the last charge. The layers are heated by the upgoing gases. 6. Stack is the empty space above pre-heating zone and provides passage to the hot gases to escape into atmosphere. Jamming of cupola means the cut-off of air supply to the melting metal in cupola, and is an operational trouble. When temperature at tuyere level falls very low, the metal and slag solidify and stop the supply of air by chocking the tuyeres. This is known as temporary jamming. Sometimes when slag and metal are not tapped out regularly, their level may increase and they may overflow into the wind box and solidify due to contact of cold air. This is permanent jamming. It is a serious happening which may even render the cupola unserviceable. Importance of initial coke-bed height: The production of good quality and quantity of iron from a cupola very much depends on maintaining the correct initial coke-bed height throughout the entire heat as it affects both the temperature and oxidation of the metal. All melting of the metal should take place in that section of the cupola in which there is no free oxygen. If the coke-bed height is maintained too low, then oxygen of the draft will not be completely consumed by the coke and will thus pass up through the metal charges. This will result in the rapid oxidation of the metal. On the contrary, if the coke-bed height is too high, the initially formed carbon dioxide will combine with the carbon of the upper layers of the coke bed. This will produce carbon monoxide which will retard the development of maximum amount of heat in the top layers of the coke bed. Consequently the melting of metal will be retarded. It is, therefore, essential that a correct coke-bed height should be maintained to ensure favourable melting conditions and the coke in the bed that is consumed during melting should be replenished by the intermediate coke charges during the process of a heat. Hot blast cupola: In a hot blast cupola, hot blast of air (pre-heated from 200 to 400°C) is supplied to the cupola for combustion purpose which results in faster melting rates, saving in fuel consumption, attainment of higher temperatures with reduced oxidation losses of iron, silicon and manganese. Sulphur pickup is reduced. Hot gases coming out of the stack (from top of cupola) can be utilized for pre-heating the air blast. Advantages and limitations of cupola: With lower initial cost and on a small floor area, a cupola provides a continuous supply of molten metal with ease, efficacy and economy. Because of counter-flow pre-heating and close contact between metal and fuel, the process results into excellent heat-transfer. Cupola is the only furnace from which metal can be tapped continuously. Limitation in the use of cupola is that close control of temperature of molten metal is usually not possible. Also, there is some loss of elements such as silicon and manganese with pickup of sulphur which is undesirable.

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Loss and pickup of elements during cupola operation: During conversion of pig iron into cast iron through the cupola, some important following alloying elements from the melt are either lost or picked up. Carbon: As the molten cast iron trickles down through the coke bed, some carbon (usually 0.15%) is picked up by molten cast iron. The amount of carbon pickup depends on temperature and time of contact of molten iron with coke. Silicon: It gets oxidized in cupola with a loss of about 10% of total silicon of the charge. Inoculation of the metal in the ladle with ferrosilicon can compensate the loss of silicon. Silicon helps in keeping cast iron soft. Manganese: This is lost to an order of 15 to 20%. Loss can be made up by adding ferromanganese. Manganese increases wear resistance of cast iron. Sulphur: It is picked up by the metal from the coke in the range of 0.03 to 0.05%. Thermal efficiency of cupola =

Heat used in pre-heating, melting and superheating Heat input due to heat in coke + heat evolved in oxidation of C, Fe, Si, Mn + heat of air blast

(5.1)

The thermal efficiency of cupolas generally varies between 30 and 50%. Air for combustion of fuel in cupola: As already mentioned, coke is generally used as fuel in cupola. Coke is considered 88% pure carbon. For complete combustion of carbon, C + O2 Æ CO2 12 32 Æ 44 or 1 8/3 Æ 11/3 The above equation shows that for complete combustion of 1 kg of carbon, oxygen required is 8/3 or 2.67 kg. Since air contains about 23.2% of oxygen by weight, therefore the weight of air required to give 2.67 kg of oxygen will be 2.67/0.232, i.e. 11.5 kg of air. Thus for complete combustion of 1 kg of carbon, air required will be 11.5 kg. Taking weight of air as 1.28 kg/m3, volume of air required will be 11.5/1.28, i.e. 8.98 m3. Example 5.1: What will be the amount of air required for complete combustion in a cupola for a coke charge of 40 kg? 40 ¥ 88 Solution: 40 kg coke has , i.e. 35.20 kg of carbon. Here air required for complete 100 combustion of 35.20 kg of carbon will be: 35.20 ¥ 8.98 = 316.09 m3 (Ans.) Example 5.2: of 10 : 1. Solution:

Calculate air required to melt 500 kg of iron in a cupola with melting ratio

With a melting ratio of 10 : 1, 500 kg of iron will require 500/10, i.e. 50 kg of

50 ¥ 88 , i.e. 44 kg of carbon. Hence air required for combustion will be: 44 ¥ 8.98, 100 i.e. 395.12 m3 (Ans.)

coke or

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A casting is required to have the following composition: C—3.25% Mn—0.6% Si—1.8% S—0.1% P—0.5% Determine the weight of pig iron from pile A and pile B to be picked up in each metal charge if the charge (200 kg) is to contain pig iron —50%, foundry return —40% and purchased scrap —10%. Analysis of these metals is as follows: Example 5.3:

Metal Pig iron (pile A) Pig iron (pile B) Foundry returns Purchased scrap

Si% 2.40 1.40 1.70 2.20

Mn% 0.90 0.95 0.60 0.70

S% 0.05 0.05 0.06 0.07

P% 0.4 0.35 0.3 0.25

Solution: The calculations of the charge will be made on the basis of silicon contents only as various metals charged in a cupola are usually proportioned so as to contain a definite amount of silicon because this element has other important effects on properties of castings. Let X be the amount of pig iron selected from pile A. Weight of silicon in pig iron (pile A) = 2.4% of X = 0.024X Weight of silicon in pig iron (pile B) = 1.4% of (100 – X) = 0.014 (100 – X) Weight of silicon in foundry return = 0.017 ¥ 80 = 1.36 kg Weight of silicon in scrap = 0.02 ¥ 20 = 0.4 kg Loss of silicon during melting may be assumed to be 10%. The silicon contents required in the charge is 1.8% which is 90% of the silicon charged in the cupola. So the total amount of silicon in one metal charge will be: 100 ¥ 200 kg = 0.018 ¥ 90 Calculations are made as below: 100 (0.024X) + [0.014 ¥ (100 – X)] + 1.36 + 0.4 = 0.018 ¥ ¥ 200 90 or X = 84 kg. Hence pig iron from pile A will be 84 kg, from pile B will be 16 kg, foundry return —80 kg and scrap—20 kg. (Ans.)

5.31.2

Protecting the Melt from Dissolved Gases

A good amount of care against gases should be exercised during melting of metal to avoid production of defective castings. This is achieved by using a proper melting technique and a suitable furnace selected on the basis of chemistry of metal charge, delivery rate of molten metal, maximum temperature to be attained for melting the charge and type and size of metal charge (its ingredients) to be handled and overall economy of production of molten metal. The furnace design should ensure availability of proper atmosphere within the furnace to safeguard the melt against the attack of gases (oxygen, hydrogen, nitrogen, etc.) as they either

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229

get dissolved in the molten metal or form oxides and nitrides which are impurities. If these impurities are allowed to enter the mold cavity with molten metal, they generate serious defects (inclusions, surface defects, etc.) in castings. For example, oxygen and nitrogen produce oxides and nitrides by acting with molten metal in a cupola. Proper type of flux used with metal charge reacts with these impurities and produces lightweight slag, which is tapped out from time to time from the cupola and this way purity of melt is ensured. Hydrogen in a melt may be produced due to damp furnace, air, oil or grease. It has been observed that the amount of hydrogen dissolved in a melt is proportional to the square root of partial pressure of hydrogen in the atmosphere over the melt. The partial pressure of hydrogen is reduced by bubbling through it dry insoluble gases such as chlorine or helium in case of non-ferrous metal charge and carbon monoxide for ferrous metal charge. Nitrogen forms nitrides with molten ferrous and nickel base alloys and adversely affects the grain size. Use of vacuum melting technique is considered a more desirable choice in safeguarding against dissolving of gases in the melt. Gases absorbed in melt during casting lead to poor quality of casting generating defect such as blow holes, pin holes and porosity. Following are the possible ways or sources of gas generation and entrapment during casting. (i) Gases are mechanically trapped in a melt in the form of dissolved gases. They come from atmosphere within furnace, leakage of atmospheric air in furnace, raw materials of charge, furnace oil, etc. (ii) Gases may be developed in the melt due to variation in their solubility at different temperatures and phases. (iii) Gases are also produced (and entrapped in the melt) due to oxidation, burning or chemical reaction of molten metal with mold materials (binders and additives). Gases get dissolved in the melt at higher temperatures and are released (given off) on cooling. Gases normally exist in the molecular form but at higher temperature and in contact with metal, a good amount of gases gets dissolved in the melt in the atomic form and enters into the metal thus resulting into higher solubility of gases above the melting point of the metal. Solubility of gas falls steeply as the melt solidifies. Much of the gas bubbles out or is rejected at the solid–liquid interface to combine into molecules. But if this gas is somehow entrapped in the metal, it causes gas porosity, blow hole or pin holes in castings during solidification.

5.32

INSPECTION AND TESTING OF CASTING

The inspection and testing of castings are carried out to determine their quality and to detect the presence of defects in them, which may be external or internal type. Before transferring the castings from the foundry shop to the machine shop, it is essential to inspect them thoroughly. The stringency of the inspection depends on the cost and the anticipated service to which the casting will be subjected to, because the cost of inspection methods and service charges may be very high in some cases. Although thorough inspection of castings is done for external defects, many defects may not be visible from outside and casting may be defective from inside. In such cases, castings are subjected to internal inspection either by

230

MANUFACTURING PROCESSES

cutting through the sample castings or by resorting to other non-destructive methods of testing. The methods of inspection and testing can be grouped into two following categories: (i) Destructive methods of testing (ii) Non-destructive methods of testing Destructive methods of testing include those tests in which the casting that undergoes the test is rendered unserviceable after the test, i.e. it involves testing of castings to destruction. Such tests include: tensile test, compressive test, shear test, bend test, impact test, etc. One major drawback of destructive test is that locating the spot where cutting should be done for inspection is purely based on the personal judgement of the examiner. Also, the selected casting for such inspection may not be the true representative of the entire lot of defective castings. Non-destructive test methods include those tests which do not impair the usefulness of the casting after its testing. These tests include: 1. Visual inspection of casting with the naked eye or using magnifying glass. Surface defects such as cracks, swells, porosity, etc. are easily detected. 2. Inspection for dimensional accuracy is done using measuring instruments, particularly on those castings which are to be sent to machine shop. 3. Sound test or ring or vibration test is used to detect internal flaws. The ‘ring’ produced by casting by gentle hammering on it gives some idea of the soundness of casting. 4. Pressure or hydraulic testing is done on those castings which, during their service, will carry liquids or gases under pressure. Checking leakage of the liquid or gas filled in the casting under higher pressure is the aim of this test. 5. Magnetic particle test is done on those castings only which are made of ferromagnetic metals such as steel and cast iron. The test is done to detect discontinuities such as slag pockets, blow holes, cracks, etc. by making the casting a part of strong magnetic path and sprinkling magnetic powder (fine particles of steel or cast iron) on the casting. When the casting is magnetized, the defects distort the magnetic lime of flux slowing the presence and location of the defect. 6. Fluorescent penetration test is done to detect small surface cracks using a liquid dye penetrant on the casting surface. 7. Ultrasonic testing is capable of revealing the presence and location of internal defects like blow holes, voids, etc. 8. Radiography or X-ray tests are used for inspection of metals of all types and thickness ranging up to half a metre. Internal cracks, porosity, blow holes and slag inclusions are effectively detected. Gamma ray testing is used for thicker sections. For further details of the above tests, refer inspection and testing of weldments, Chapter 7.

5.33

CASTING DEFECTS AND THEIR REMEDY

Casting defects are those characteristics which generate imperfection in a casting exceeding the quality limits imposed by the design or service conditions of the casting. Defects are of

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231

three types: (i) most severe defects resulting in scraping or rejection of the casting, (ii) intermediate defects which can be repaired to make the casting functional but at the expense of some high cost, and (iii) minor defects which permit castings to be easily and economically repaired. Casting defects may be attributed to the following reasons: (a) Defects due to faulty mold: These include blow holes, scab, buckle, fusion, crusher, swells, shifts, shrinkage, hot tears, hard spot, etc. (b) Defects due to improper metal and mode of pouring: These include misruns, cold shut, shot metal, gas porosity, inclusion, etc. (c) Defects due to faulty design of pattern: These include warping, shrinkage, cracks, porosity, hot spots, hot tear, etc. The International Committee of Foundry Technical Association has developed a standard nomenclature of the following seven basic categories of casting defects. (i) (ii) (iii) (iv)

Metallic projections such as fins, flash and mismatch. Cavities including internal and exposed cavities, blow holes, pin holes, etc. Discontinuities such as cracks and hot tears. Defective surface such as fusion and penetration crushes, scabs, hard spots, surface folds, laps, scars, blister, etc. (v) Incomplete casting comprising misruns, cold shut and pour short. (vi) Incorrect dimensions and shape such as warping and shrinkage. (vii) Inclusions such as non-metallic particles and oxides in the metal matrix.

5.33.1

Metallic Projection

Metallic projections include small metallic projections (or extra metal) on the casting surface and are caused due to wrong molding practices or improper equipment. Mismatch is displacement of cope half of the casting with respect to drag half and is caused due to loose connecting pins of flasks or molding boxes. Lift and shifts are caused because of misplacemnt of cope or core. Fins and flash are thin projections of metal occurring at the parting line on casting surface. Poor kiss of the parting plane may cause partial runout of metal from the mold cavity. Swell is a localized enlargement of casting at its top side and is caused due to pressure of molten metal on soft rammed sand. Surface roughness is due to coarse molding sand. Proper flasks connecting gadgets and correct molding practice should be followed.

5.33.2 Cavities Cavities may be internal or external surface depressions or hollowness. Blow holes appear as internal voids having smooth round or oval holes as a result of entrapped gas, excessive moisture in sand, poorly baked cores and excessive use of organic binder giving high volume of gases during casting. Pin holes or gas porosity is sub-surface porosity caused by gases (hydrogen, oxygen, etc.) absorbed by molten metal, particularly hydrogen. The root causes for the formation of cavities in castings should be avoided.

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5.33.3 Discontinuities Discontinuities in the form of cracks (hot tears, also called hot cracks or hot pulls) are caused due to hindrance in the contraction of the casting. Hot tears [Fig. 5.41(a)] are caused because of low strength of metal after solidification, causing the metal to fail in coping up with the excessively high stress set up by solid shrinkage of the metal. The lack of collapsibility of mold or core is another cause. Hot spots are the portions of casting which solidify in the end and are often the cause of hot tear. The use of chills (metallic) helps in solidification of the hot spot area quicker and thus avoids cracking [Fig. 5.41(b)]. Cold cracks happen when casting has cooled below 260°C. Hot tear

Hot tear

Hot tear

Hot tear

Fig. 5.41(a)

Hot tears in castings of different shapes. Obstructions in the free shrinking of casting during its cooling is the main cause of hot tears.

Sand

Casting

Chill

Sand (i) Chill Chill

Porosity Porosity

Fig. 5.41(b)

5.33.4

Porosity

Chill

(ii)

Showing the use of chills, (i) internal chills and (ii) external chills, for eliminating porosity caused by shrinkage. Porosities are caused in sections where metal solidifies in the end. The use of chills at a particular location increases the rate of solidification in that critical section. Internal chills are preferably of same material as the casting and the external chills may also be of same material as casting or of iron, copper or graphite.

Defective Surface

Defects under this category are caused due to wrong molding, gate marking or core marking and metal pouring. Fusion and metal penetration are caused in the mold by very high

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233

temperature of the metal. Fusion appears as rough surface on casting. Penetration is the passage of molten metal through sand grains and appears as an intimate mixture of metal and sand. Crushers are due to mold surface being damaged during molding or placing the core. Hard spots are developed on the casting surface due to rapid cooling of that particular surface area (it may be frequent in the cast iron castings) and composition of metal may be responsible for it (manganese to be controlled as it is a carbide stabilizer generating hard spots). Cold shots are small spheres of metal appearing on casting surface almost distinct from casting. These are caused by streams of metal that are too cold to fuse properly. Hotter metal should be used. Scabs or washes are due to erosion or breaking down of some sand from mold or core surface and the recess thus made is filled by metal and the same appears on casting as extra metal. Scar is a shallow cavity on a flat casting surface caused by the liquid metal pressure, which succeeds in pushing back the mold sand at certain places. Blister is a shallow blow on a flat casting surface, covered by thin layer of metal. Correct molding procedures should be followed. Weights can be placed on the sand filled in cope to avoid swells. Buckles are found on high melting temperature metal castings. Due to heat of molten metal, surface of mold may expand but pieces of that do not fall (as in case of scab). The defect appears as an irregular long break of small width on casting surface. A rat tail is a minor buckle. Some additives should be used to reduce expansion characteristic of mold by improving its resistance against thermal shock.

5.33.5

Incomplete Castings

When the casting lacks metal supply during casting, it remains incomplete in its desired shape and size. Misruns and cold shut occur due to lack of fluidity of the molten metal and faulty design of castings, incorporating very thin sections where metal solidifies quickly thus blocking the metal flow. When metal fails to reach all sections of metal, the defect caused is misrun. Cold shuts occur when two streams of molten metal approach each other in the mold from opposite directions. Pour short is incomplete filling of mold due to less quantity of metal. Runout is drainage of molten metal from the mold cavity, thus leaving the mold unfilled.

5.33.6

Incorrect Dimensions and Shapes

Incorrect dimensions and shapes occur due to improper pattern with less shrinkage allowance and poor molding practice. Warping is the deformation of shape of casting. Metal shrinkage is volumetric shrinkage during solidification of metal. Drop appears as an irregular deformation on casting due to the falling of a portion of sand from the mold cavity and dropping into molten metal. Poor green strength of mold is the cause for this. Sand spots are the collected impurities at some spots and appear as irregularly shaped depressions. Some of the above defects are illustrated in Fig. 5.42 and Fig. 5.43.

5.33.7 Inclusions Inclusions are the impurities (oxides, sand, nitrides, etc.) in suspension in molten metal which, on solidification of casting, appear as inclusion defect that makes the casting poor in strength. Dross or sand inclusions are oxides held in suspension in the metal being cast.

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MANUFACTURING PROCESSES

Fig. 5.42

Illustrating some common defects in castings. The defects shown include scab, cold shut, wash, misrun, blister and scar.

These should be removed from the ladle before pouring. Sponginess and honeycombing (Fig. 5.43) appear as small voids in large numbers and in close proximity on the casting surface and are caused due to dirt or swarf dissolved with metal. Shot metal refers to the defect wherein balls of iron are found embedded in the metal in a fractured casting section. During pouring, if metal is cold (not having correct pouring temperature), it breaks into spaces or particles that freeze separately and get entrapped in the metal without fusing. Honeycombing

Blow holes Cavity (shrinkage defect)

Fig. 5.43

5.34

Some more casting defects, namely honeycombing, blow holes and cavity.

TESTING OF MOLDING SANDS

Essential characteristics of molding sands depend on the shape, size, composition and distribution of sand grains. Testing of molding sand is done either to forecast or to improve the performance of sand. The main tests conducted on a molding sand include the following: (i) Grain-fineness test (ii) Permeability test (iii) Strength test

(iv) Clay content test (v) Moisture content test (vi) Hardness test for mold and core

These are conducted on a sample of standard sand prepared exactly as per relevant standard of test.

FOUNDRY PROCESSES—Molding and Casting

5.34.1

235

Grain-fineness (or Sand Grain Size) Test

The test determines the size of sand grains and the distribution of grains of different sizes in a molding sand. Sands are first washed (to remove clay), dried and then tested for grain fineness either by mechanical sieving method or by any other standard method. In standard AFS (American Foundrymen’s Society) grain-fineness test, the mechanical sieving apparatus consists of eleven sieves each of 8 inch diameter and with their standard numbers and sizes of openings (Table 5.2). Foundry sand grading classifications according to Boswell are given in Table 5.3. In AFS grain-fineness test, dry-sand sample (50 gm) is placed in the upper most sieve and shaken for about 15 minutes. Due to the varying size of openings, certain amount of sand is retained by each sieve which when multiplied by 2 will express the weight of grain of various sizes as the percentage of original 50 gm sand sample. The observations of the test are tabulated as shown in Table 5.4. TABLE 5.2

AFS sieve details, multiplying factors and grain classification

Sl. No.

Sieve No.

Opening in inch

Multiplying factor

AFS grain fineness

Class of grain size

1 2 3 4 5 6 7 8 9 10 11 12

6 12 20 30 40 50 70 100 140 200 270 Pan

0.1320 0.0661 0.0331 0.0232 0.0165 0.0117 0.0083 0.0059 0.0041 0.0029 0.0021 Nil

3 5 10 20 30 40 50 70 100 140 200 300

10 to 15 15 to 20 20 to 30 30 to 40 40 to 50 50 to 70 70 to 100 100 to 140 140 to 200 200 to 300

10 9 8 7 6 5 4 3 2 1

TABLE 5.3

Grading of sand grains

Boswell’s classification Grade Gravel Very coarse sand Coarse sand Medium sand Fine sand Coarse silt Fine silt Clay grade

Particle size limits (mm) Greater than 2.0 Between 2.00 and Between 1.00 and Between 0.50 and Between 0.25 and Between 0.10 and Between 0.05 and Less than 0.01

1.00 0.50 0.25 0.10 0.05 0.01

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MANUFACTURING PROCESSES

To calculate the grain fineness, weight of sand, retained by each sieve and multiplied by 2, is further multiplied by a multiplier factor [Table 5.4]. The results thus obtained are added together to get the total products. Then, AFS fineness number will be =

Total product Total % wt. retained by different sieves

Observations of a grain-fineness test are recorded as in Table 5.4. A sample of 50 gm sand was washed, dried and then tested. TABLE 5.4

Typical calculations of AFS grain-fineness number

U.S. series equivalent No.

Amount of sand retained by each sieve (gm)

% wt. retained by each sieve (after multiplying by 2)

6 12 20 30 40 50 70 100 140 200 270 PAN

X X 0.05 0.1 0.25 14.0 23.1 8.9 0.8 0.0 0.02 0.02

X X 0.1 0.2 4.5 28.0 46.2 17.8 1.6 0.16 0.04 0.04

Total

98.64

Multiplier factor

Product

3 5 10 20 30 40 50 70 100 140 200 300

X X 1.0 1.0 135.0 1120.0 2310.0 1246.0 160.0 22.4 8.0 12.0 5015.4

AFS grain-fineness number =

Total product Total % wt. retained by different sieves

5015.4 = 51 98.64 So, the grain-fineness number of sand under test is 51. The grain-fineness number (51) obtained in the above test is the average grain size corresponding to a sieve number through which all the sand grains will pass through if they all were of same size. The grain size value falls generally between 40 and 220 for most foundry sands. =

5.34.2

Permeability Test

Permeability refers to the venting property of molding sands. Variation in permeability is caused by the shape and size of sand grains, type and amount of binder and moisture contents

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237

of sand. According to the condition of molding sand, there are three different types of permeability, for example, green, dry and baked permeability. Permeability is measured in terms of permeability number. Permeability is an absolute number defining the amount of air passed through a standard specimen of sand tested under standard conditions. The test specimen of molding sand (having diameter 50.8 mm and height 50.8 mm) is placed in a specimen tube. Air is passed through the specimen by keeping it in a permeability meter. Time taken for 2000 cm3 of air at a pressure of 10 gm/cm2 (980 Pa) to pass through the specimen is noted. Permeability number (P) is given as follows: P= where

V ◊H p◊ A◊t

(5.2)

V = Volume of air = 2000 cm3 H = Height of specimen = 50.8 mm = 5.08 cm p = Air pressure = 10 gm/cm2 A = Cross-sectional area of sand specimen =

p

4

¥ (5.08)2 =20.268 cm2

t = Time (minutes) for complete air to pass Permeability of sand specimen is tested by an apparatus called ‘permeability meter’. Effect of grain fineness on permeability of sands is shown in Fig. 5.44. AFS permeability numbers of sands used for casting different metals are given in Table 5.5.

Fig. 5.44 Table 5.5

Influence of grain fineness on permeability of sand.

AFS permeability numbers of sand used for different metals

Metal cast

AFS green permeability number of sand

Cast iron Steel (dry sand) Steel (green sand) Bronze Aluminium Magnesium

10 to 80 60 to 100 150 to 300 35 20 to 40 80 to 150

Note: The green permeability number of loam sands having moisture content up to 15% is usually less than 5 and of compo sand 40 and of oil sand (for core making) ranges from 50 to 200.

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MANUFACTURING PROCESSES

It is essential that a sand of proper permeability should be used to ensure sound castings. Low permeability is likely to cause the defects like blow holes and scabs and also difficulty in shaking out the casting from sand.

5.34.3

Strength Test

Standard specimen of sand is used for testing different strengths, namely, compression strength, shear strength and tensile strength. Universal sand strength testing machines are available. Both green compressive strengths and dry compressive strengths are tested. Green compressive strengths (load required to rupture sand specimen under compression) of general foundry sands lie between 30 and 160 kPa. Dry compression strengths of baked sand specimen lie between 140 and 1800 kPa.

5.34.4

Clay Content Test

Apparatus used for this test is called mud or clay content tester. Molding sand sample (50 gm) is taken and dried for an hour at a temperature 105 ± 5°C. The dried sample is placed in a glass beaker to which nearly 400 mL of water is added. After adding few drops of NaOH, the mixture is boiled for 3 to 4 minutes, followed by stirring of the mixture for 5 to 10 minutes. The mixture is then diluted by adding more water to it and the mixture left undisturbed for 10 minutes. Sand settles at the bottom and clay particles washed from sand float in the water which is siphoned off the beaker. This process is repeated till the water above the sand in the beaker becomes clear showing that all the clay of the molding sand has been removed. The sand is then taken out of the beaker and dried. The difference in weight of dried sand gives clay content of the molding sand.

5.34.5

Moisture Content Test

A sand sample weighing 50 gm is dried at a temperature of 105 to 110°C for 2 hours (expected time to dry the sample completely). The dried sample is weighed. Weight difference divided by weight of original sample (50 gm) when multiplied by 100 gives percentage of moisture contained in the molding sand. Moisture content test can be more speedily determined by an instrument called speedy moisture teller.

5.34.6

Mold Hardness Test

Handy hardness testers are available for testing the hardness of molds and cores. Hardness is measured in much the same way as followed in Brinell hardness test.

5.35

POURING METAL IN SAND MOLDS

The process of pouring molten metal into a mold cavity deserves serious considerations in regard to the temperature of molten metal, rate of filling mold, design and placement of gate and riser and use of chill blocks. Castings with non-uniform sections may create problems because of slow rate of heat dissipation from larger sections and thus posing problems in the

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239

directional solidification if risers of proper size and at proper locations are not placed to feed metal during solidification of metals, particularly of large freezing range. Chills also help in this regard. Pouring of molten metal into mold cavity calls for consideration of the following two main issues: (a) Pouring temperature of melt (b) Pouring rate of molten metal into mold

5.35.1

Pouring Temperature

Selection of correct pouring temperature is important in view of the following: (i) Ability of molten metal to flow in thin sections of casting. (ii) Use of higher pouring temperature leading to possibility of developing defects such as hot tearing in casting. (iii) Effect of temperature of molten metal on the metallographic structure of the cast product. (iv) Effect on fluidity of molten metal directly affecting the ability to flow and fill the molds of different configurations. If the pouring temperature is much higher (superheated) than the melting temperature of metal, fluidity of metal will increase. Besides filling the mold cavity, metal may enter between small voids in the particles of molding sand resulting into surface defects such as fusion and penetration. If the pouring temperature is lower, fluidity of molten metal decreases and metal may not properly fill the entire mold cavity and may result into casting defects such as misrun and cold shut. (v) Since solubility of gas in molten metal increases with increase in temperature of melt, the extent of super heats of metal should be kept low to minimize defects caused due to entrapped gases (blow holes, etc.). From the above it is obvious that the main considerations for pouring are proper fluidity of melt and minimization of gas solubility. Optimum pouring temperature is usually decided based mainly on the consideration of fluidity (or capability) of metal to fill the mold properly. Using proper gating system, supply and distribution of molten metal in the mold at proper rate and without much turbulence and temperature drop can be ensured. Entrapment of gases and slag can also be reduced. Pouring temperatures of common metals and alloys are given in Table 5.6. Pouring temperature should be close to melting point of the metal. TABLE 5.6

Pouring temperature of metals

Metal

Pouring temperature (°C)

Cast iron Steel Aluminium Brass (yellow) Copper Bronze Gun metal

1100–1300 1570–1650 700 760 900 1015 1010

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MANUFACTURING PROCESSES

5.35.2

Pouring Rate

Pouring rate or pouring time is the time consumed in pouring the molten metal into the mold cavity (including riser, etc.). Pouring rate is adjusted according to the metal and geometry (and size) of the casting. Low pouring rate results in longer filling time which may sometimes result in incomplete filling of mold due to solidification of metal at some places in the gating system. On the contrary, higher pouring rate may result into turbulence of molten metal and erosion of mold surface. Pouring rate is thus decided judiciously in view of the above facts.

5.36

DESIGNING GATING SYSTEM IN SAND MOLDS

5.36.1

Gating System

The gating system is the passage way that leads the molten metal into the mold cavity through a series of channels and comprises any or all of the following: pouring basin, sprue, runners, ingates, etc. A properly designed gating system provides the following: (i) It ensures smooth flow of metal into the mold by minimizing turbulence within the molten metal as it flows through the gating system. Turbulence causes aspiration of air and formation of dross (metal oxides). Sprue well helps a lot in this regard. Use of tapered sprue and proper streamlining of metal flow reduce erosion, premature cooling of metal and gas entrapment as the metal enters the mold. (ii) It traps contaminants [such as slag or dross (oxides) and other inclusions] in the molten metal by having contaminants adhering to the walls of gating system and thus preventing them from reaching the mold cavity. (iii) It delivers the molten metal at the best location to achieve proper directional solidification and optimum feeding of shrinkage cavity. (iv) It provides a built-in metering device to permit uniform standardized pouring time regardless of variations in pouring techniques. (v) It ensures filling of mold completely in minimum possible time. (vi) Gating system should be economical and easy to adopt. Various elements of gating system are described below in reference to Fig. 5.45.

Fig. 5.45

Elements of gating system of a sand mold.

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241

Pouring basin or cup is that in which molten metal from the ladle is poured for onward movement through sprue and runners to fill the mold cavity. Sprue is a vertical channel through which molten metal flows downwards from the pouring basin to runners and finally to mold cavity. A sprue well or bottom well or base is located at the bottom end of the sprue. It serves as a reservoir for molten metal and helps in dissipating the kinetic energy of falling stream of molten metal. Ceramic or wire mesh filters are sometimes placed at the sprue well to filter out inclusions and dross and eroded sand. Runners are generally horizontal channels that carry molten metal from sprue well to mold cavity or connect the sprue to the gates. Skim bob is an enlarged section at some place along the runner. This helps in trapping impurities such as dross or eroded sand and thus prevents them from going into mold cavity. Both light and heavy impurities are trapped by the skim bob. Choke is that part of the gating system which has the smallest cross-sectional area and helps in controlling the rate of metal flow, thus ensuring lower metal flow velocity in runners and minimizing sand erosion in runners. The velocity with which metal fills the mold is determined by the cross-sectional area of the gating system and the rate of metal pouring. Too high a pouring rate caused by too large a gating system results in sand inclusions by erosion and turbulence. Too slow pouring rate may result in solidification of metal before filling some sections of the mold. Choke is the minimum cross-section in the gating system. In other words, the choke is the section in the gating system where cross-sectional area times the potential linear velocity is at a minimum. When the gating system is choked at the bottom of the sprue, it is called non-pressurized system and is less reliable than a pressurized system in which the choke is at the gate. Gate (or ingate) is that portion of the runner through which metal enters the mold cavity. The gate acts to reduce the turbulence and erosion as metal enters the mold. Riser is that section of the gating system which serves as reservoir for the supply of molten metal to the solidifying casting. Riser is a hole molded or cut in the cope to allow molten metal to rise above the highest point in the casting. Riser also provides visual check to ensure filling up of mold cavity. A casting may be of very complex shape and size having combination of thin and thick sections. Such a casting is likely to have defects that get generated during solidification because the thin sections solidify first and the thick sections in the last. A hot spot is the point at which casting will freeze last. It then becomes essential that the hot spots should be fed up to the last moment till a casting is completely solidified. This is achieved by connecting these hot spots to a number of risers which act as storage of metal. Location of riser should be such that the riser ensures directional solidification. The heaviest section of casting solidifies last; the riser should be located to feed this section. More than one riser may be required for large castings as the number of risers and their location depend on the size and shape of casting. Risers may be of the following types in view of their location (Fig. 5.46). Top riser is located on the top of the casting. It has the smallest feeding distance and additional pressure head on molten metal. Side riser is filled last and hence contains the hottest metal. It is considered more effective than the top riser.

242

MANUFACTURING PROCESSES

Fig. 5.46

Different types of risers. Note a blind riser with sand core.

Open riser is any riser which is open to atmosphere at the top surface of mold. Blind riser is entirely surrounded by molding sand. Because of this, the blind riser looses heat slowly, thereby helping better directional solidification of casting. Since the metal skin may quickly form around the blind riser, a sand core may be used to admit atmospheric pressure to the interior of the blind riser.

5.36.2

Vertical Element (Sprue) in a Gating System

In the vertical part of the gating system (the sprue), the acceleration of gravity increases the flow velocity. In a free falling liquid, the cross-sectional area of the stream decreases as it gains velocity downwards. When we take a sprue of constant cross-sectional areas and pour metal through it, regions may develop where the liquid losses contact with sprue walls generating lower pressure regions. Aspiration, a process whereby air is sucked in or entrapped in the metal, may take place. In sand molds, it is required that pressure in the liquid stream does not fall below the atmospheric pressure to avoid entry of held-up gases in the mold into the stream of metal. But with a tapered sprue with its cross-sectional area smaller at the bottom, aspiration can be avoided. In fact, in ideal case, the sprue should have parabolic sides which, however, are not possible to make easily and hence tapered cylindrical sprue is used generally. Sprue design for negligible aspiration and turbulence should include provision of pouring basin of proper shape and size (Fig. 5.47). Molten metal if poured directly over sprue may erode the sprue wells. It is, therefore, poured in the pouring basin which, besides providing ease in pouring due to its larger area, also provides a constant pouring head. This makes the tapered sprue to run full of metal thus avoiding the possibility of aspiration, etc. A ceramic strainer is provided right below the exit of pouring basin to remove dross from the molten metal. To eliminate the aspiration effect, the shape of the sprue should ideally be made hyperbolic so that pressure within the sprue may not fall below the atmospheric pressure (thus create aspiration effect) which may suck gases (originating from the sand backings of organic compounds in the mold or core due to contact with molten metal) and allow them to mix with the molten metal stream, finally resulting into porous castings. Skim bob provided in the horizontal runner is a trap wherein heavy particles get trapped in the bottom of the skim bob and lighter impurities get settled in the upper section of the skim bob. To reduce turbulence in the gating, the provision of vena contracta is necessary where there is a sudden change

FOUNDRY PROCESSES—Molding and Casting

Fig. 5.47

243

Highlighting salient features of a top gate for feeding metal and the sprue design (tapered downward) for avoiding aspiration and turbulence.

in the flow direction of the metal. Similarly, the ceramic splash core provided at the bottom of the sprue helps in reducing the eroding force of molten metal stream from the sprue. It also checks carrying over of sand and other impurities to the mold cavity. According to the Bernoullis theorem, Vb =

2 ◊ g ◊ ht

(5.3)

where Vb = velocity of metal at sprue base or choke ht = height of sprue or pouring basin above the sprue base or head of metal at sprue base Based on the above expression, a sprue of suitable proportion can be designed. Let Ab = area of sprue at base At = area of sprue at top Vb = velocity of flow rate at base of sprue Vt = velocity of flow rate at top of sprue hb = head of metal at top of sprue (i.e. metal height in the pouring basin) Then, at the sprue top (or at entry to sprue), Flow rate = At ◊ Vt Similarly, at sprue base, Flow rate = Ab ◊ Vb But, Ab ◊ Vb = At ◊ Vt (being continuous flow) or

At = Ab

or

At = Ab

2 ghb Vb = Ab Vt 2 ght hb ht

(5.4)

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MANUFACTURING PROCESSES

The above expression gives relation between height and cross-sectional area at any point in the sprue. Area of sprue base can be calculated as follows: Volume of flow (Q) in a given time (t) = Ab ◊ Vb ◊ t = W/r or

Ab =

W c ◊ r ◊ t ◊ Vb

Since, Vb = 2 g ◊ ht , where ht = sprue height then, area of sprue base (or choke area) Ab will be: Ab =

W c ◊ r ◊ t ◊ 2 g ◊ ht

(5.5)

where W = weight of metal, kg c = flow efficiency factor r = density, kg/cm3 t = pouring time, sec g = 981 cm/sec2 ht = sprue height, cm To avoid the aspiration effect, a tapered sprue is used, and it is presumed that pouring basin at the top of the sprue is kept full of metal throughout the pour and in that case ht gives the head of metal at the sprue base up to the top level of metal in the pouring basin. But where only the sprue height is given, the ht = height of tapered sprue and the top area of the sprue at entry may be taken roughly equal to twice the sprue area at the bottom. If gating ratio given is 1 : 2 : 2, then in terms of choke area of sprue (Ab), total area of runner will be 2 ◊ Ab and total area of gates will also be 2 ◊ Ab.

5.36.3

Gating Ratio (or Gate Ratio)

Gating ratio refers to the ratio of area of sprue base (Ab), total areas of runner (At) and total area of gates (Ag). There are two types of gating systems, (a) pressurized and (b) non-pressurized or free flowing like the sewer. A pressurized system has a ratio of Ab : Ar : Ag as 4 : 3 : 2, i.e. areas Ar and Ag are smaller than Ab. In this system, the proportions of sprue, runner and gate cross-sectional area are so arranged that a back pressure is maintained on the gating system by a fluid film restriction at the gates as the system has less total cross-sectional area at the ingates to the mold cavity than at the sprue base. The pressurized system is kept full of metal and is usually smaller in volume for a given metal flow rate (than the non-pressurized system) and thus less metal is left in the gating system. The system should, however, be streamlined to avoid turbulence which is quite likely in this system. The pressurized system is used for molds for casting steel, iron, brass, etc. The system ensures rapid filling of the mold with sufficient pressure maintained on the sprue. In non-pressurized system or reverse choke system, the areas of runner and gates are enlarged after the sprue. This system reduces metal velocity, and thus the turbulence and

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245

aspiration are minimized but one must use tapered sprues, enlarged sprue base wells and pouring basins to achieve proper flow control. It should be ensured that the system runs full during pouring to eliminate separation effect and aspiration. The system is preferred for dross forming (or oxidizable) alloys like aluminium and magnesium alloys. However, in this system, the pouring is slow and it is difficult to keep the sprue filled with metal. Gating ratio like 1 : 2 : 2 or 1 : 3 : 3 or 1 : 4 : 4 will produce a non-pressurized system. A non-pressurized system with gating ratio 1 : 4 : 4 is shown in Fig. 5.48.

Fig. 5.48

A typical non-pressurized gating system using a gating ratio of 1 : 4 : 4.

Requirements of an ideal gating system

The gating system comprises all such passages through which molten metal enters the mold cavity, i.e. pouring basin, sprue, runner, risers and gates. An ideal gating system should fulfil the following requirements: (i) Entering velocity of molten metal (to the mold) should be least and free of turbulence to avoid erosion of mold. Metal should not absorb air or gases from the mold. (ii) The system should be streamlined by adopting gradual changes in the size of runner and gates, adopting generous radius at the bends to avoid turbulence. (iii) The portion of mold containing runner and gates should be rammed hard to avoid erosion of loose sand due to metal flow. (iv) A runner may be extended beyond the last gate (runner extension) to trap slag or dross from the first flow of metal (Fig. 5.48). (v) A tapered sprue minimizes aspiration effect. (vi) The gate should lead the metal to heavier sections of casting preferably below or through a riser. (vii) It should help developing such temperature gradients in the metal and the mold which will lead to directional solidification of the casting towards the risers.

246

MANUFACTURING PROCESSES

Types of gates

Depending on their positions with respect to mold cavity, gates in mold making are broadly classified into the following types: 1. Top gate 2. Parting line gate 3. Bottom gate 4. Side gate 1. Top gate (Fig. 5.47) provides entry of metal at the top of mold cavity which ensures availability of hottest metal at the top of casting for enabling proper temperature gradient for directional solidification from bottom towards the gate which works as a riser also. Drawbacks of this gate include erosion of mold due to dropping of metal, turbulence, excessive absorption of air and gases in the metal. Top gating is used for simple designs and large castings in grey cast iron made in erosion resistant molds but not for non-ferrous alloys since excessive dross will be formed by the agitation and turbulence in metal. Filling time for mold with top gate: Refer Fig. 5.49.

Fig. 5.49

Let

Am As ht hm g

= = = = =

Estimating mold filling time with a top gate.

cross-sectional area of casting, cm2 gate area, cm2 filling height, cm height of casting, cm 981 cm/sec2

Then, metal flow velocity at gate (Vg) = Filling time for the mold

2 ◊ g ◊ ht , cm/sec

=

Volume of mold Gate area ¥ velocity of metal at gate

=

Am ¥ hm , sec As ¥ Vg

(5.6)

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247

2. Parting line gate (Fig. 5.50) is the simplest to make and most practised by molders. The metal enters the mold cavity at the parting line. The sprue is connected to the mold through a runner and gate and hence it is easy to provide skim bob or skim gate to trap slag or sand in the metal. The choke helps in controlling the rate of flow of metal. This gate is not used for deep molds since in that case metal falling on the mold bottom from a height may cause erosion of mold.

Fig. 5.50

A parting line gate (skim gate).

3. Bottom gate [Fig. 5.51(a)] allows the molten metal to flow down to the bottom of the mold cavity in the drag. Metal enters at the bottom of the mold and later rises gently in the mold. Turbulence and erosion of mold are least. This gate is preferred for making molds for large steel castings. Mold filling time is little more. The biggest disadvantage with metal rising in the mold from its bottom to the top is the loss of its heat and when metal reaches at the top of casting, it becomes quite cold. Consequently, the metal at the bottom of mold (or casting) is much hotter than the metal in the riser (at the top of casting).

Fig. 5.51(a)

A bottom gate.

Fig. 5.51(b) Estimating mold filling time with a bottom gate

Filling time with bottom gate [Fig. 5.51(b)]: Filling time is longer with bottom gate because the metal is subjected to a decreasing head during filling. Hence in an increment of time dt, the metal height in mold will increase by dh and volume of metal will increase by Am ◊ dh when Am is mold cross-sectional area and the flow through the ingate in time dt will be Ag ◊ v ◊ dt (where Ag is ingate area and v is metal velocity at ingate)

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MANUFACTURING PROCESSES

and the velocity (v) of metal through the ingate will be 2g(ht - h), where ‘h’ is height of metal in the mold at any instant. Equating the increase in casting volume in time dt to the flow through the ingate in the same time interval,

2g(ht - h) ◊ dt

Am ◊ d h = Ag ◊

Ag

or

Am

dt =

dh 2 g ◊ (ht - h)

If tp is the time to fill the mold and hm be the height of mold cavity, then

1 2g Hence, filling time, tp =

hm

dh

Ú

ht - h

0

=

Ag Am

tp

Ú dt 0

2 Am ( ht - ht - hm ) Ag 2 g

For bottom gate, filling time È Area of mold ˘ 1 = Í ¥ ¥ 2( static head - static head - height of mold )˙ 2g ÎÍ Area of gate ˚˙

(5.7)

4. Side gate (Fig. 5.52) can solve some problems experienced with the bottom gate. In this gate system, the metal enters into the mold from the side and through a number of gates, starting first from the bottom most gate.

Fig. 5.52

5.36.4

Side gating system.

Designing of Risers

As already mentioned, a riser (also called feeder) is a hole cut or passage molded in the cope to permit the molten metal to rise above the highest point in the casting after the mold is full of metal. It allows a visual check for ensuring filling up of the mold cavity completely. In the initial stage of pouring, a riser permits air, steam or gases to escape out of mold. Later during the solidification of metal in the mold, the riser serves as a feeder of molten

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249

metal into the main casting to compensate for shrinkage, thus avoiding defects like cavity, porosity or hot tear in the casting. The design of the riser should ensure directional solidification of the casting maintaining proper temperature gradient within the solidifying casting. If no riser is provided, the solidification will start from the outer walls of the casting and the liquid metal thus left in the centre will be surrounded by a thick solidified metal shell. As the cooling of the casting will continue, the contracting liquid will result into voids in the centre of the casting. Risers thus provide thermal gradients from a remote chilled area to the riser. Size of riser is important and should be designed for minimum possible volume but should maintain a solidification time longer than that of casting. Since flow of metal from riser to solidifying casting is only during early part of solidification process, the volume of riser should, therefore, be much more than the shrinkage volume of casting and a value three times the shrinkage volume is considered adequate. Shape of riser: From functional considerations, the best shape for a riser is spherical (since it looses heat at the slowest rate) but due to the difficulties in the making of the shape, the next best shape is cylindrical. The riser should be large enough to solidify in the last and should be capable of holding liquid metal so long as the feeding to the casting is needed. It should be close to the heaviest section of the casting, preferably on top or at side and connected to the casting by a neck of metal called ‘gate’ for easy removal of riser from casting during fettling. A casting may have a shape which may be the combination of a couple of thick and thin sections. Each thick section should have its own riser. If the thinner section is not tapered to become larger towards heavier section, centre line shrinkage may occur. Chills at thinner sections may prevent such shrinkage promoting directional solidification from chill to the riser. Depending on the metal being cast, the volume of riser is kept 25 to 55% of the volume of casting. Several risers may be used with a casting having thin sections. Cylindrical-shaped risers are common as they loose low amount of heat, thus maintaining the riser hot and keeping the metals in molten state as long as possible. Following points may be considered in designing a riser. (i) A riser should be designed with minimum possible volume while maintaining the cooling rate slower than that of casting. (ii) Height of the riser should be tall enough so that a ‘pipe’ formed in it (because of feeding metal to other sections of the casting) may not penetrate the casting itself, i.e. height or depth of ‘pipe’ should be smaller than the height of riser [Fig. 5.99(a)]. Height of riser is usually kept either equal to or 1.5 times the diameter of the riser. Remember that shrinkage of casting is fed by the piping in the riser and the volume of ‘pipe’ is about 1/6th the volume of riser up to the depth of the pipe. (iii) The riser should solidify in the last after the casting has solidified. (iv) Volume of riser should be kept (a) at least three times the shrinkage volume of casting or (b) 25 to 55% of the volume of casting, as the case may be. Keeping much higher volume of riser than necessary is of no use.

250

MANUFACTURING PROCESSES

(v) Castings having high surface area to volume ratio require a riser larger than that arrived at by considering the cooling rate. (vi) Ratio of surface area to volume of riser should be much less than the ratio of surface area to volume of casting, or in other words, cooling time of riser should be more than cooling time of casting. Since cooling time is proportional to (V/A)2, the ratio 2 2 (V/A)riser > (V/A)casting or A/V of riser should be much smaller than A/V of casting, where A denotes surface area and V denotes volume. Size of riser: Riser size depends on the alloy poured and volume to surface area ratio of riser to that of the casting to be fed. Obviously to be effective, a riser must freeze more slowly than casting. Generally, Chvorinov’s rule is the basis for calculating the size of riser for short freezing range alloys such as steel or pure metals. There is no satisfactory method for calculating the size of riser for non-ferrous alloys. Chvorinov’s rule states that the solidification time (t) for an alloy is: ÊV ˆ t =CÁ ˜ Ë A¯

2

(5.8)

where t = solidification or freezing time V = volume of casting C = a constant (also called mold constant) reflecting mold material and metal properties A = cooling surface (or surface area) of casting The value of (V/A)2 and the freezing (solidification) time are available in handbooks. These are experimental values found for different alloys and different cylinder diameters. The value of (V/A)2 for a riser should be 10 to 15% larger than that of casting. Since V and A for casting are known, (V/A)riser can be found and by assuming a suitable value for the height to diameter ratio of cylindrical risers, the size of the riser can be found. The Chvourinov’s rule helps in arriving at the solidification time of castings. Positioning of risers: As regards the placement of risers, when A/V ratio of casting is low (as in case of cube and sphere), one central riser at the top of casting should be enough. But when A/V ratio is large (as in case of a bar and plate), more than one riser may be required. Past research has shown that for a steel plate of 100 mm thickness, one central riser of adequate size can ensure soundness for a distance of 4.5t for a plate casting wherein 2t is the riser contribution and 2.5t is from edge effect (Fig. 5.53). Maximum distance between two risers is kept 4t for plates but 1t to 4t for bars. Use of chill increases the feeding distance for plates to a total of 4.5t + 5 cm and for a bar to (6 t + t). Note that the distances are from the outside edge and not from the centre line of the riser. As a thumb rule, it can be said that a single riser is good enough if feeding length is less than 4.5 times the plate thickness (12–100 mm) for steel plates. Efficiency of risers can be increased (or in other words size of riser reduced) by (a) giving insulating cover (to the riser) made of plaster of Paris or rice hull which on burning gives ash which works as insulator or by (b) supplying additional heat to the riser using exothermic or heat liberating mixtures (i.e. mixture of metallic oxide and aluminium powder which on burning gives out heat).

FOUNDRY PROCESSES—Molding and Casting

Fig. 5.53

251

Feeding distance of risers for casting steel plates.

Example 5.4: A cylindrical riser is required for a steel plate casting of size 300 ¥ 180 ¥ 20 mm. Assume the ratio of the riser volume (Vr) to the casting volume (Vc) as 0.20 for the shape factor of the given casting. Design a proper riser taking solidification shrinkage of the casting as 8%. Solution: L + H 300 + 180 = = 24 t 20 Ratio of Vr/Vc = 0.20 for the above shape factor Here Vc = 300 ¥ 180 ¥ 20 = 108 ¥ 104 mm3 Hence, Vr = 0.20 Vc = 20/100 ¥ 108 ¥ 104 = 216 ¥ 103 mm Let us assume the diameter of the riser (dr) equal to its height (hr). Then,

Shape factor =

Vr = = or

p

p

4

p 4

dr2 ◊ hr dr2 ◊ dr

dr3 = 216 ¥ 103

4 or dr = 65 mm Hence, diameter of riser (dr) = 65 mm and height of riser (hr) = 65 mm.

(as hr = dr)

252

MANUFACTURING PROCESSES

Let us check for the height of riser according to the given shrinkage of casting, since the shrinkage in casting is fed by the piping formed in the riser. Taking the volume of pipe (Vp) formed in the riser (in sand molds) as 1/6th the volume of riser with its height equal to at least the depth of pipe (dp). Volume of pipe (Vp) = shrinkage volume of casting = 8% of Vc. Since volume of pipe (Vp) is 1/6th the volume of riser up to pipe depth (hp), 2

Volume of pipe (Vp) = or

hp =

1 Ê dr ˆ p ◊ hp 6 ÁË 2 ˜¯

8 300 ¥ 180 ¥ 20 ¥ 6 ¥ 4 ¥ = 156 mm 10 p ¥ 65 ¥ 65

Hence, height of pipe formed (hp) = 156 mm Since with this depth (or height) of pipe, the pipe will extend below the riser (assumed of height 65 mm in above) and into the casting. This is not acceptable. Hence, the height of riser should be little more than 156 mm in this case and diameter 65 mm. (Ans.) Example 5.5: Assuming a sprue height for 20 cm and a gating ratio of 1 : 2 : 2, design gating dimensions for an iron casting weighing 40 kg filled in 20 sec with a flow efficiency of 0.8. Take density of iron 8 ¥ 10–3 kg/cm3. Solution: Choke or bottom area of sprue (A). W 40 (A) = = = 1.5 cm3 -3 crt 2 ght 0.8 ¥ 8 ¥ 10 ¥ 20 ¥ 2 ◊ 981 ◊ 20 where W = 40 kg c = 0.8 r = 8 ¥ 10–3 kg/cm3 t = 20 sec ht = 20 cm g = 981 cm/sec2 Hence, choke area of sprue (A) = 1.5 cm2 For a gating ratio of 1 : 2 : 2, i.e. choke area (A): runner area (2): gate area (2A). Area of runner = 2A = 2 ¥ 1.5 = 3 cm2 Area of gates = 2A – 2 ¥ 1.5 = 3 cm2 4 Sprue choke dia at base = 1.5 ¥ = 1.9 = 1.4 cm (Ans.) p Take sprue well dia equal to twice the choke diameter, then well dia = 2 ¥ 1.4 = 2.8 cm. Taking sprue area at top twice that of choke area, Sprue area at top = 2A = 2 ¥ 1.5 = 3 cm2 Sprue diameter at top =

3¥

4

p

= 2 cm

(Ans.)

FOUNDRY PROCESSES—Molding and Casting

253

Example 5.6: Two castings, one sphere and the other cube, are of same metal and having same surface area. Find the ratio of solidification time for the sphere to that of the cube. Solution: Surface area (Asphere) = Surface area (Acube) According to Chvorinov’s rule, 2

ÊV ˆ Solidification time, t a Á ˜ Ë A¯ Let tsphere = solidification time for sphere, R = radius of sphere, tcube = solidification time for cube, a = side of cube. 2

tsphere tcube

È4 3˘ Í 3pR ˙ Ê Vsphere ˆ 16 p 2 R6 =Á = = Í ˙ Ë Vcube ˜¯ 9 a6 Î a3 ˚ 2

(Ans.)

Example 5.7: Considering uniform cooling in all directions, find the dimensions of a 90-mm cube after cooling down to room temperature. Take solidification shrinkage for the cast metal as 5% and solid contraction as 7%. Solution: Volume of casting = 903 = 729,000 mm3 Volume after solidification shrinkage 5 ˆ Ê = 729,000 Á 1 = 692,550 mm3 Ë 100 ˜¯

The metal further contracts by 7%; hence, 7 ˆ Ê Volume at room temperature = 692,550 Á 1 = 644,072 mm 3 Ë 100 ˜¯

Dimension of the cube side = (644,072)1/3 = 86.5 mm

(Ans.)

Example 5.8: Calculate solidification time for a cast metal piece having diameter 1000 mm and thickness 30 mm when mold constant is 2.1 sec/mm2. Solution: Volume of casting =

p 4

d2 ¥ h =

p 4

(1000)2 ¥ 30 = 23,571, 428 mm 3

p p È ˘ Surface area of casting = Í 2 ¥ d 2 + p dh ˙ 2 ¥ ¥ (1000)2 = 1,571, 428 mm 2 4 4 Î ˚ (neglecting h being much smaller than d) ÊV ˆ Solidification time (t) = C Á ˜ Ë A¯

2

2

È 23,571, 428 ˘ = 2.1 Í ˙ = 472.5 sec = 7.8 min (Ans.) Î 1,571, 428 ˚

254

MANUFACTURING PROCESSES

Example 5.9: Two solid castings, one sphere with radius R and another cylinder with diameter (d) equal to its height (h) and both having same volume, are to be sand cast. Find out which of the two will solidify faster. Solution: 4 p R3 3

Vsphere = Vcylinder = Since

Vsphere

p

d2 ◊ h =

4 = Vcylinder

p 4

d3

(as h = d )

4 p p R3 = d 3 3 4

Ê 16 ˆ d =Á ˜ Ë 3¯

or

1/ 3

◊R

Following Chvorinov’s rule, ÊV ˆ Solidification time, t a Á ˜ Ë A¯

2

4 / 3p R3 ÊV ˆ = = 0.33R ÁË A ˜¯ 4p R 2 sphere ÊV ˆ = ÁË ˜¯ A cylinder

and

p 4

d2 ◊ h

p p d ◊ h + 2 ◊ d2

=

(i)

d 6

4

Putting values of d in terms of R, d 1 Ê 16 ˆ ÊV ˆ = = Á ˜ ÁË ˜¯ A cylinder 6 6 Ë 3 ¯

1/ 3

R = 0.29R (ii) (a) From the above [(i) and (ii)], it is evident that cylindrical workpiece will solidify faster than the spherical workpiece. (b) Risers with a higher value of (V/A) loose heat at a slower rate. (Ans.) Example (a) (b) (c)

5.10: Determine which of the following castings should have least solidification time. A sphere with diameter (d) of 20 mm. A cylinder having diameter (d) and height (h) of 20 mm. A cube with its side (l) of 20 mm.

Solution: ÊV ˆ (a) Solidification time (t) = C Á ˜ Ë A¯

2

FOUNDRY PROCESSES—Molding and Casting

255

where V = Volume A = Surface area Èp 3 ˘ Í d ˙ Cd 2 C ◊ (20)2 = = 11.00 C = CÍ6 2 ˙ = 36 36 Î pd ˚ 2

tsphere

2

(b)

tcylinder

p 2 È ˘ 2 2 d ◊h Í ˙ Èd ˘ È 20 ˘ 4 = CÍ C C = = ˙ Í6˙ Í 6 ˙ = 11.00 C Î ˚ Î ˚ Í 2 ◊ p ◊ d 2 + p dh ˙ ÍÎ 4 ˙˚

2 2 È l3 ˘ Ê lˆ Ê 20 ˆ tcube = C Í 2 ˙ = C Á ˜ = C Á ˜ = 11.00C Ë 6¯ Ë 6¯ ÎÍ 6l ˚˙

(c)

It shows that all the three castings will have same solidification time. (Ans.) Example 5.11: What should be the area at the base of a down sprue to prevent aspiration of metal when height of sprue is 200 mm, top area of sprue 1100 mm2 and flow of liquid metal maintained at 1,700,000 mm3/s. Solution: Height of sprue (h) = 200 mm Area of sprue at top = 1100 mm2 Metal flow rate (Q) = 1,700,000 mm3/s g = 9815 mm/s2 Velocity of metal in down sprue (v) =

2g ◊ h

= 2 ¥ 9815 ¥ 200

= 1981.4 mm/s Area at the base of sprue = Q/v 1,700,000 = 857.9 mm 2 1981.4 = 857.9 mm2 (Ans.)

=

Example 5.12: Show that with a cylindrical side riser, longer solidification time will be registered when diameter and height of the riser are same. 2

ÊV ˆ A cylindrical side riser is shown in Fig. 5.46. Solidification time (t) μ Á ˜ , Ë A¯ where V = Volume of casting A = Surface area of casting Hence, for longer solidification time, V/A should be maximum, or in other words, A/V should be minimum.

Solution:

256

MANUFACTURING PROCESSES

Taking diameter and height of the riser as ‘d’ and ‘h’, respectively, p 2 p V = d ◊ h and A = pdh + 2 ◊ d 2 4 4 4V Putting the value of h = p d2 4V p A = pd 2 + d2 2 pd 4V p 2 + d 2 d For A to be minimum for a given V,

=

dA =0 dd

d È 4V p 2 ˘ + d ˙=0 dd ÍÎ d 2 ˚

or

4V

+ pd = 0 d2 –4V + pd3 = 0

or

–

or

d3 =

or

4V

p

Putting V in terms of d, d3 = hd2 d = h

or

Note: The above is true for cylindrical side riser only. When a top cylindrical riser is used, surface area will be: A = p dh +

p

d2 4 Proceeding as in above, it will be seen that diameter of riser (d) would be twice the height (h) of riser for longer solidification.

Example 5.13: Three metal pieces, one sphere, another cube and third cylinder (with its height equal to its diameter) have same volume. Which piece will solidify earlier and which at the end? Solution:

Taking volume as unity for all the three pieces, 2

ÊV ˆ Ê 1 ˆ Solidification time (t) = C Á ˜ = C Á 2 ˜ Ë A¯ ËA ¯

For sphere,

V=

4 3 Ê 3 ˆ p r or r = Á ˜ Ë 4p ¯ 3

1/ 3

FOUNDRY PROCESSES—Molding and Casting

Ê 3 ˆ A = 4p r 2 = 4p Á ˜ Ë 4p ¯

(i) For cube,

V = a3

or

257

2/3

= 4.84

a = 1

A = 6a2 = 6 (ii) For cylinder,

Ê 1 ˆ V = p r 2 ◊ h = 2p r 3 or r = Á ˜ Ë 2p ¯ Ê 1 ˆ A = 2p r 2 + 2p rh = 6p Á ˜ Ë 2p ¯

Since solidification time (t) a

1/ 2

2/3

= 5.54

1 ( A )2

tsphere = 0.0434C, tcube = 0.028C

and

tcylinder = 0.033C

Hence, the cube will solidify first and the sphere at the end. Accordingly, the cube-shaped castings will solidify at the fastest rate and the sphere-shaped castings will solidify at the slowest rate. (Ans.) Example 5.14: Calculate the time taken to fill up a cylindrical casting of 40 cm diameter and height 15 cm by top gating and bottom gating and using a sprue with gate diameter 2 cm. Head available for filling in both cases is 20 cm. Solution:

Fig. 5.54

(a) Top gating [Fig. 5.54(a)] Velocity at gate = Time taken to fill up mold =

p 4

p

4

◊ 402 ◊ 15 ◊ 22 ◊ 198

2 ◊ g ◊ 20 = 2 ◊ 981 ◊ 20 = 198 cm/sec Volume of mold Area of gate ¥ velocity of metal at gate

= 30.3 sec

(Ans.)

258

MANUFACTURING PROCESSES

(b) Bottom gating [Fig. 5.54(b)] Time to fill up mold

=

Area of mold 1 ¥ ¥ 2 ¥ ( Static head - Static head - Mold height ) Area of gate 2g

p = 4

p 4

=

◊ 402 ◊2

2

¥

1 2.981

¥ 2 ÈÎ 20 - 20 - 15 ˘˚

800 ¥ 2.27 = 41.0 sec 44.29

(Ans.)

Example 5.15: With a solidification factor of 0.97 ¥ 106 s/m2, find the solidification time for a spherical casting of 200 mm diameter. Solution: ÈV ˘ Solidification time (t) = C Í ˙ Î A˚

Given,

2

C = 0.97 ¥ 106 s/m2 Radius of spherical casting = 100 mm = 0.1 metre

Hence, 2

È4 ˘ 2 p ◊ (.1)3 ˙ Í 6È 1 ˘ = ¥ 0.97 10 t = 0.97 ¥ 106 Í 3 ˙ Í 30 ˙ ÍÎ 4p ◊ (.1)2 ˙˚ Î ˚ t = 1078 sec

5.37

(Ans.)

SOLIDIFICATION OF CASTINGS

After the molten metal is poured into a mold, a number of events take place during its solidification and cooling to the room temperature. Since solidification requires decrease in energy when liquid is replaced by solid, some super cooling or chilling below the freezing point of metal is, therefore, needed. It is provided by the mold walls. After the casting operation, when cooling starts, nucleation in the melt takes place first at the mold walls of the thinner sections. After nucleation, the structure grows until crystals or grains are formed. The crystals near the mold walls are smaller (due to fast cooling) and equiaxed (i.e. randomly oriented axes of crystals). As the metal solidifies further, crystals grow into the melt in a direction perpendicular to the mold walls. The grains join in a dendritic (or pine tree) long columnar structure which grows into the melt, with a mushy zone between the dendrites as they are thrust forwards towards the thermal centre of the casting, where in most cases,

FOUNDRY PROCESSES—Molding and Casting

259

equiaxed grains evolve [Fig. 5.57(a)]. The rapid cooling produces a solidified skin or shell or fine equiaxed grains along the mold walls. Those grains that have favourable orientation will grow preferentially in the form of columnar grains and those grains that have substantially different orientations are blocked from further growth. As time passes, the casting solidifies with rejection of the heat of fusion and with a volumetric shrinkage. Subsequent cooling to room temperature has further shrinkage in the casting.

5.37.1

Solidification of Pure Metals

A pure metal has a clearly defined single melting or freezing point (i.e. solidus temperature Ts) as shown in Fig. 5.55(a), for example, pure aluminium solidifies at 660°C. After the temperature of the molten metal drops to its freezing point, its temperature remains constant during solidification due to the release of latent heat and if super cooling of the metal has occurred, then temperature may also even increase slightly [Fig. 5.55(a)]. The temperature again starts falling steadily during cooling of the casting to room temperature. Freezing temperature is the temperature above which pure metals are completely liquid and below it completely solid. Resulting structure after solidification of pure aluminium is shown in Fig. 5.55(b).

Fig. 5.55

Solidification of a pure metal and the resulting structure. After pouring of molten metal in the mold, the temperature of metal falls steadily until freezing commences at a particular point (Ts). During solidification of pure metals, the temperature (Ts) remains more or less constant due to the release of latent heat. In fact, there may be even slight increase in temperature if super cooling has taken place. The temperature again starts falling steadily as the solidified metal cools down.

260 5.37.2

MANUFACTURING PROCESSES

Solidification of Alloys

Since the alloys do not have a sharply defined freezing temperature, their solidification takes place over a range of temperatures, i.e. solidification in alloys begins when temperature drops below the liquids Tl (Fig. 5.56), and is completed when it reaches the solidus, Ts and the metal remains in ‘mushy state’ between Tl and Ts. The two temperatures, Tl and Ts, vary with the composition of the alloy; the component of the alloy having higher freezing point starts solidifying first and thus the frozen metal along the mold walls has a different composition from that of the original alloy. Hence, solids separating out at different temperatures possess different compositions. A typical cast structure of an alloy is shown in Fig. 5.57(a) with an inner zone of equiaxed grains of a finer size which give a stronger structure than the dendritic one. This inner zone can be extended throughout the casting as shown in 5.57(b) by adding an inoculant or nucleating agent as done in case of meehanite cast iron for getting a fine-grained high strength high duty cast iron by adding calcium silicide (as graphitizer) to the white base cast iron.

Fig. 5.56

Ideal cooling curve of an alloy showing that the solidification of an alloy takes place over a range of temperature (i.e. between Tl and Ts, starts at Tl and finishes at Ts) as they do not have a sharply defined freezing temperature.

Mushy zone is that where both liquid and solid phase are present. It is described in terms of freezing range which equals (Tl – Ts), where Tl is liquidus temperature and Ts is solidus temperature. A short freezing range generally involves a temperature difference of less than 50°C and a long freezing range is greater than 110°C. Alloys have three types of solidification ranges (freezing ranges): (a) short, (b) medium and (c) long. Low carbon steels and pure metals are typical of the type (a); aluminium–silicon alloys are representatives of the type (b) and certain tin–bronzes are representatives of type (c). Since aluminium and magnesium alloys have wider mushy zone, they remain in a mushy state throughout most of the solidification process. In case of sand molds, heat extraction is considerably slower than the metal molds. It is because of this that in metal molds, a narrow mushy zone may quickly sweep through the cooling metal, whereas in sand molds, mushy zone may distinctly extend throughout the casting and for longer time.

FOUNDRY PROCESSES—Molding and Casting

Fig. 5.57

5.38

261

Showing cast structures of a solifidified metal alloy: (a) The three types of grain structures in cast metals are: chill, columnar dendrites and equiaxed. (b) Fully equiaxed structure of cast metal is stronger than the dendritic structure, and is obtained by laddle addition of some inoculant or nucleating agent as done in case of meehanite cast iron for getting a fine-grained high strength cast iron.

HEAT LOSS FROM CASTINGS DURING SOLIDIFICATION

A temperature distribution mechanism at the interface of mold wall and the liquid metal during solidification of metal castings is shown in Fig. 5.58. Heat is given off by the liquid metal through the mold walls to the surrounding air. The temperature drop at mold-metal interface and air-mold interface is due to the presence of boundary layers and imperfect contact at these faces. In actual practice, solidification process is very complicated because of complex mold geometry, composition of different alloys and their freezing phenomenon, etc. Risers provided with molds work as reservoir of liquid metal to supply the liquid metal to the cooling and contracting casting throughout the solidification period.

5.38.1

Effects of Cooling Rate

Cooling rates of a casting are directly related to its mass (in volume) and surface area and may be expressed as the ratio of surface area to volume or mass. The slow cooling of a large mass with a relatively small surface area will produce a considerably softer metal (of casting due to formation of large grains) than the fast cooling of a large thin section. Short local solidification times give finer structure having smaller dendrite arm spacing. As the grain size decreases (i.e. finer grains), the strength, impact and ductility of casting increase. With the

262

Fig. 5.58

MANUFACTURING PROCESSES

Showing the temperature distribution at the interface of the walls of the mold and liquid metal during the solidification of metals in a mold.

decrease of interdendritic shrinkage voids (microporosity) with finer grains, the tendency for the casting to crack (hot tear, etc.) during solidification also decreases. Normally, a fine grain size is desirable since higher ductility and impact strength values are obtained at a given tensile strength level. Large castings and ingots freeze with coarse grains and thin-sectional castings or castings made in metal molds develop fine grain due to rapid freezing.

5.38.2

Solidification Time

According to Chvorinov’s rule already discussed, the solidification time is a function of the volume of casting and its surface area, i.e. 2

È Volume (V ) ˘ Solidification time (t) = C Í ˙ Î Surface area (A) ˚ where C is a constant depending on mold material, material properties (latent heat, etc.) and temperature. As the solidification starts, a thin solidified skin begins to form at the mold walls. With lapse of time, the thickness of this film (tm) goes on increasing. With flat mold walls, this metal film thickness (tm) is proportional to the square root of time (t), i.e. tm a doubling the time will make the metal skin

5.38.3

t and hence

2 = 1.41 times (or 41%) thicker.

Volume Change (or Shrinkage) during Solidification

Melting of a metal from solid to liquid form results in the slight increase of volume on account of the structural breakdown of the crystal lattice. The reverse happens during solidification of a molten metal. Because of their thermal expansion characteristics, metals contract or shrink during solidification from molten state and later during cooling to ambient temperature. The shrinkage of metal brings dimensional changes which sometimes lead to the cracking of the casting.

FOUNDRY PROCESSES—Molding and Casting

263

Shrinkage is the result of the following three events: (a) Liquid contraction, i.e. contraction of molten metal as it cools prior to its solidification. (b) Solidifying contraction, i.e. contraction during the phase change from liquid to solid (release of latent heat). (c) Solid contraction, i.e. contraction of the solidified metal or shrinkage during the cooling of solidified casting to the ambient temperature. Shrinkage of metal during casting may cause serious defects, sometimes rendering the casting useless. To compensate for liquid–solid contraction (i.e. the contractions of types (a) and (b) above), a reservoir of liquid metal in the form of feeder or riser is provided which remains liquid until complete solidification. The third type of contraction, solid contraction [type (c) above], is taken care of by making an oversize pattern having proper contraction allowance or pattern maker’s shrinkage allowance added in the dimensions of the pattern (Table 5.1). Risers are designed and properly placed to ensure filling the mold cavity during solidification, and hence the volume of metal in the risers should be sufficient so that the metal in the riser solidifies last of all besides retaining heat long enough to feed the shrinkage cavity in the casting, equalizing the temperature in the mold to avoid casting strains. Riser requirements vary with the type of metal being cast; for example, grey cast iron needs less feeding than many other alloys because a period of graphitization occurs during the final stages of its solidification which causes an expansion that tends to counteract metal shrinkage. On the other hand, many non-ferrous alloys need elaborate feeding system to get a sound casting. It may be appreciated that if volume change on solidification for any metal is low, it is the easiest to cast since most problems in castings are experienced for this reason only. Because of this reason together with minimum requirement of feeder, the grey cast iron is the most preferred metal for casting purposes. Volumetric contraction during solidification (including both liquid contraction and solidifying contraction) of various cast metals is given in Table 5.7. The volumetric shrinkage in solidified castings in cooling to room temperature is given in Table 5.8. TABLE 5.7 Volumetric liquid contraction and solidifying contraction of various cast metals

Metal or alloy Aluminium Al–Cu Al–Si Carbon steels Copper Zinc Magnesium White cast iron Grey cast iron

Volumetric contraction (%) 6.6 6.3 3.8 2.5–3 4.9 6.5 4.2 4.0–5.5 Expansion up to 2.5

Note: Grey cast iron expands during solidification because of the relatively high specific volume of the graphite present in cast iron and when graphite precipitates as graphite flakes during solidification of cast iron, it causes a net expansion of the casting.

264

MANUFACTURING PROCESSES

TABLE 5.8 Volumetric solid contraction allowance for some metals cast in sand molds

Metal Grey cast iron White cast iron Aluminium alloys Magnesium alloys High manganese steel Phosphor bronze Aluminium bronze

5.39

Solid contraction (%) 0.83–1.3 2.1 1.3 1.3 2.6 1.0–1.6 2.1

SHRINKAGE DEFECTS

There are a number of problems or defects associated with the shrinkage of the castings; some are generated (a) when liquid metal freezes to solid state (for example, centre line shrinkage, dispersed microporosity and other problems) and (b) those associated with the cooling of a solidified casting to room temperature (for example, hot spots, hot tears and residual stresses).

5.39.1

Shrinkage Defects Caused during Solidification of Molten Metal

1. Centre line shrinkage occurs in short-freezing range alloys like steel. As solidification proceeds towards the central portion (hotter), the top molten metal feeds for the shrinkage. But if a casting of the simple shape as shown in Fig. 5.59(a) is cast without proper feeder, a central sinking surface is formed leading to a deep ‘pipe’ in the centre.

Fig. 5.59

Development of ‘pipe’ defect in the casting takes place as a result of shrinkage during solidification of metal with narrow freezing range and when the casting is without any riser (which is made to feed metal to the solidifying casting to avoid such defects as the ‘pipe’). Micro shrinkage cavities, as at (b), are caused in metals with wide freezing range and give porous structure.

2. Dispersed microporosity is associated with the metals having a wide freezing range such as high carbon steel, aluminium alloys and copper alloys. The defect occurs at the interstices between interlocking dendrite arms. The liquid metal solidifies and shrinks between dendrites and between dendrite branches. Porous regions or void may develop at the centre of thick section of casting because of the contraction as surfaces of the

FOUNDRY PROCESSES—Molding and Casting

265

thicker region begin to solidify [Fig. 5.59(b)]. Porosity caused by shrinkage can be considerably reduced by providing proper risers or feeders. Rate of solidification in critical regions can be increased by using internal or external chills or chill blocks [Fig. 5.41(b)] made of metal and used for heat dissipation, thus increasing rate of cooling. Chills also promote directional solidification.

5.39.2

Shrinkage Defects Caused during Cooling of Solidified Casting to Room Temperature

1. Hot spots are the last portions of the casting to solidify and they usually occur at points where one section joins another, or where a section is heavier than that adjoining it, for example, a corner of a right angle (Fig. 5.60). Hot spots are weak and metal may tear where they occur. Stress concentration around a hot spot is shown in Fig. 5.61 wherein for casting steel and other high-temperature metals, the hot spot shown encircled must be fed by molten metal (through proper feeder or risers) to avoid formation of cavities at the hot spot. Shape of the casting should avoid abrupt changes from heavy to thin section and should be such so as to make directional solidification possible. Besides improving the design of castings, occurrence of hot spots can be avoided by using chills.

Fig. 5.60

A right angle corner as shown at (i) is considered a bad design of casting because the hot spot at the joint may result in shrinkage cavity or possible cracking along section AA1. The design of the joint can be improved by giving generous curvature at the corner joint (as shown at (ii)).

2. Hot tears are cracks at various points in a casting brought about by internal stresses resulting from restricted contraction. The strength of metal near solidification or freezing point is very low. Stresses imposed at this temperature may lead to incipient flaws which later develop into well-defined cracks or hot tears upon cooling. Precautions must be taken to avoid stress concentrations caused by shrinkage stresses at weak points. Sharp angles and abrupt changes in cross-section contribute to large temperature differences within a casting, which may result in hot tear. Hot tears and excessive residual stresses in castings occur because sections of a casting are restrained from shrinking by the presence of either massive cores or hard rammed molds.

266

MANUFACTURING PROCESSES

Fig. 5.61

Stress conditions around a hot spot in a cross-joint. The area of the joint shown enclosed by a dotted circle should be fed through a riser during solidification. Sharp corners should be avoided (since they are stress risers) and improved by using fillet.

Care should be taken to avoid stress concentrations caused by shrinkage stresses at weak points. Take an example of a product, a high quality alloy steel casting, shown in Fig. 5.62. The design is one that is conductive to hot tearing at the natural hot spots in the inside corners. Cracks may occur in this radius area as this is hottest, weakest and subjected to highest stress concentration. Use of exothermic pads can solve the problem. The pads are made of exothermic mixture which comprises mill scales, aluminium powder, charcoal and refractory powder. When molten metal contacts the pads, they (pads) burn and the heat evolved due to exothermic reaction (during casting) keeps the metal in molten state long enough so that excessive strains are not built up during final stage of solidification and shrinkage, thus avoiding the possibility of hot tears.

Fig. 5.62

Showing the use of exothermic padding at inner corners A and B to prevent hot tears. Cracks may occur in these two regions (A and B) since these are hottest and weakest areas, and are exposed to high stress concentration.

Hot tearing is rupture of skin by thermal contraction and is observed where a thin section joins a thick section. The thin section solidifies first, whereas the thicker section still contains liquid metal. The thinner section which begins to solidify first contracts thermally pulling itself away from the large section which is still partly in molten state and the fracture results at the corner of the thicker section.

FOUNDRY PROCESSES—Molding and Casting

267

3. Residual stresses are developed when a casting is subjected to deformation that is not uniform throughout the part. These are the stresses that remain within a part after it has been cast and has no external forces acting on it. Residual stresses are also caused by temperature gradients within a body such as occurring during cooling of a casting. Different sections of a casting of non-uniform cross-section solidify at different rate resulting into non-uniform or varying amount of contraction in different sections of the casting. This produces high internal stresses and may cause cracking or tearing. Tensile residual stresses in the surface of a casting lower its fatigue life and fracture strength. Distortion of components or full casting may also be caused by residual stresses. Very complicated design of castings often lead to enclosed stress-active systems. The cast members that are parts of stress-active system should be designed slightly waved or curved (Fig. 5.63) as this reduces distortion resulting from contraction of casting during cooling. The wheel shown has rim, hub and spokes that may cool at different rates, thus subjecting the casting to considerable internal stresses. The spokes designed with a wave in them will, under stress, tend to flex, thus preventing tearing or distortion. Residual stresses can be relieved through heat-treatment also.

Fig. 5.63

5.40

Cast members (as at (i)) that are the parts of an enclosed stress-active system should be designed slightly waved (as at (ii)) to avoid hot tear.

DIRECTIONAL SOLIDIFICATION

Directional solidification of molten metal is that which starts from the thinnest section of the casting that solidifies first and continues progressively towards the risers which should be the last to solidify. Directional solidification is aimed at producing sound castings. As the molten metal solidifies in the mold, it shrinks in volume. Since all the parts of the casting do not cool at the same rate because of varying size of their sections, shrinkage cavities or voids are likely to be formed in certain regions (thermal centres) of the casting (Fig. 5.64). Proper risers should be located to promote directional solidification as the risers are the reservoirs of heat and feed metal and are used for sound castings, either to prevent centre line shrinkage or to compensate for shrinkage at the thermal centre. The formation of shrinkage cavity can, therefore, be avoided by using risers and chills.

268

MANUFACTURING PROCESSES

Fig. 5.64

Directional solidification and development of shrinkage cavity (as a result of hot spot) in an isolated heavy section shown at (i). The hot spot can be avoided by using a metal chill shown at (ii).

Besides the use of risers with proper design and their placement, another important aspect is the design of casting section which should be such that it allows the riser to fulfil the needs of supplying hot metal controlling the directional solidification of the casting. In a typical shaped casting (Fig. 5.65), directional solidification starts from the bottom end, whereas progressive solidification starts from all outer sides of the casting. When with properly maintained temperature differentials, the intersection of progressive solidification moves upwards into the hottest spot which should be within the riser, correct directional solidification will result. But suppose if the height of any section of the casting is too much in comparison to its cross-section, then progressive solidification rate may exceed the rate of directional solidification, resulting into a centreline porosity. Cross-section of the casting should, therefore, taper downwards (with smaller at the bottom end) to avoid such a situation.

Fig. 5.65

Showing the influence of the shape of casting on the directional solidification.

Chills [Fig. 5.41(b)] are metallic objects and have a higher heat absorbing capability than the sand molds. These are provided in the mold so as to increase the heat extraction capability of the sand mold and thus provide a steeper temperature gradient so that directional solidification as required in a casting may be obtained. Chills and risers placed on massive portions of the casting produce a joint effect which enables the production of a casting free of contraction voids. The directional solidification of a casting can be ensured by: (a) Proper designing and suitably positioning the gating system and risers. (b) Using chills (internal and external) in the molds.

FOUNDRY PROCESSES—Molding and Casting

269

(c) Using exothermic materials in the risers or in facing sand around certain portions of the casting. (d) Increasing thickness of certain sections of the casting by using exothermic padding.

5.41

COMPARATIVE STUDY OF VARIOUS CASTING METHODS

A comparative study of various casting methods is given in Table 5.9. TABLE 5.9 Process

Advantages

1

2

Comparative study of various casting methods Limitations

3

Material cast

Weight (kg) Min.

Max.

4

5

6

Surface Porofinish sity* (mm, Ra) 7

Section thickness (mm)

Shape complexity

Min.

Max.

8

9

10

11

Sand

Any metal cast, no limit to size, shape or weight, low tooling costs, easy operations

Finishing required, wide tolerances and coarse finish

All

0.05

No 5–25 limit

4

3

No limit

1–2

Shell mold

High production rate, good finish, close tolerances

Casting size limited, expensive patterns and equipment

All

0.05

100+

1–3

4

2

–

2–3

Expendable pattern

Complex shapes, cast in any metal and no limit of size

Patterns have low strength, costly for small quantities

All

0.05

No 5–20 limit

4

2

No limit

1

Permanent mold

Good finish and close tolerances, low porosity, high production rate

High mold cost, shapes Alu- 0.5 limited, not suitable for minhigh melting point alloys ium, brass, bronze, cast iron

300

2–3

2–3

2

50

3–4

Diecasting

Excellent surface finish and close dimensional tolerances, high production rate

Die cost high, casting size limited, good for non-ferrous metals

Al, (FV + 2FV1 + Ff1 + Ff 2)

(6.74)

FH is neutralized being equal and opposite. Frictional forces Ff1 and Ff 2 are too small in comparison to FV and FV1 and hence neglected. Thus for a balanced situation, it can be said that: P = FV + 2FV1

(6.75)

FZ is another force that acts in horizontal plane and normal to horizontal force FH and forms couple, then Turning moment = FZ ¥ S Torque (T) acting on drill is given as: T = C◊d◊f

9.7

where C = constant depending on cutting conditions d = drill dia, mm f = feed, mm/rev

kg

(6.76)

398

MANUFACTURING PROCESSES

Total power required for drilling = Power for rotating the drill + Power for feeding the drill =

T ◊N P◊ f ◊N + kW 975,000 612 ¥ 10 4

(6.77)

where N T P f

= = = =

drill speed, rpm torque on drill, kg thrust force, kg feed, mm/rev

6.17.8

Cutting Speed, Feed and Depth of Cut in Drilling

Cutting speed (V) is the peripheral speed of a point on the surface of the drill in contact with the workpiece. Choice of the cutting speed for a job depends on workpiece material, tool material, quality or finish of hole, availability of efficient cooling or cutting fluid, size and speed of drilling machine. V=

p dN

(6.78)

1000

where V = cutting speed (surface), m/min d = drill diameter, mm N = drill, rpm Average cutting speeds with HSS drill are given in Table 6.5. TABLE 6.5

Material 1. 2. 3. 4. 5. 6.

Cast iron Mild steel Medium carbon steel Brass and bronze Copper Aluminium

Average cutting speed with HSS drills

Cutting speed (rpm) 16–40 24–45 12–30 45–90 30–45 90 and up

Coolant Dry Soluble Soluble Soluble Soluble Soluble

oil oil oil or dry oil oil

Feed (f ) is the distance the drill moves into the workpiece in each revolution of the drill machine spindle and is expressed in mm/rev. If the total distance moved by drill into work, parallel to its axis, in one minute is considered, then feed can be expressed as feed in mm per minute. If N is rpm of drill, then, Feed in mm/min = Feed in mm/rev ¥ N

(6.79)

The factors that govern the feed rate are as follows: (i) Workpiece material, (ii) Depth of drilling, (iii) Available ranges of feed on the machine, (iv) Rigidity of drill machine, (v) Motor power, (vi) Drill size and (vii) Finish on the hole. Feed may vary from 0.01 to 0.3 mm/rev (or even more). Both cutting speed and rate of feed for twist drills are kept lower in comparison to other machining operations because the twist drill is relatively a weak tool. There are difficulties

METAL MACHINING—Processes and Machine Tools

399

in ejecting chips from the hole being drilled and inadequate cooling or lubrication of cutting edges of the drill. Depth of cut (d) in drilling is measured at right angles to the drill axis, i.e. the direction of feed and is numerically expressed as below: Depth of cut (d) =

Drill dia 2

(6.80)

Drilling time (see Fig. 6.92).

Fig. 6.92

Calculating drilling time.

Drilling time (T) or machining time = where T L f N

L , min N◊ f

(6.81)

= drilling time, min = length of axial travel of drill, mm = feed, mm/rev = rpm of drill

In above, L = l+a + b where l a d b

= = = =

thickness of workpiece approach of drill = 0.3d dia of drill tool over travel

Metal removal rate (MRR) in drilling Metal removal rate (MRR) is indicated by the volume of material in the hole. MRR is given by area of cross-section of hole times the tool travel rate through the metal. MRR = where d = drill dia, mm f = feed, mm/rev N = drill rpm

p ◊ d2 4

◊ f ◊ N , mm3/min

(6.82)

400 6.17.9

MANUFACTURING PROCESSES

Numericals on Drilling

Example 6.32: A twist drill of 30 mm dia is used in drilling a hole in a mild steel workpiece with following recorded data Vertical force (FV) = 50 kg Cutting force at lip (FV1) = 30 kg Feed rate = 0.6 mm/rev Drill speed = 500 rpm If value of constant (C) for mild steel is 0.36, then neglecting friction effect, find out: (a) Thrust force (b) Torque acting on drill (c) Power required for drilling Solution:

Thrust force (P) = = Torque (T) acting on drill = = = Total power for drilling = =

FV + 2FV1 = 50 + 2 ¥ 30 110 kg C ◊ d ◊ f 9.7 0.36 ¥ 30 ¥ 0.69.7 0.076 kg

T◊N P◊ f ◊N + 975,000 612 ◊ 10 4 0.076 ¥ 500 110 ¥ 0.6 ¥ 500 + 975,000 612 ¥ 10 4

= 5.3 ¥ 10–3 kW

(Ans.)

Example 6.33: Calculate drill rpm for drilling a hole of 25 mm diameter in a 15 mm thick plate with a cutting speed of 25 m/min. V=

Solution:

p dN 1000

V ¥ 1000 25 ¥ 1000 = 318.4 rpm (Ans.) = 3.14 ¥ 25 p ◊d Example 6.34: With a drill dia 20 mm, a hole is to be drilled in a plate 20 mm thick with a surface speed 60 m/min. Calculate drill speed (i.e. rpm) and feed per rev. or

N=

Solution: Feed =

N=

V ¥ 1000 60 ¥ 1000 = 955.4 rpm = 3.14 ¥ 20 p ◊d

total distance moved axially 20 + 0.3 ¥ 20 = 0.02 mm/rev = rpm 955.4

(Ans.)

Example 6.35: Four holes, each of 15 mm dia, are to be drilled along the periphery of a pipe, 30 mm thick. If cutting speed is 20 m/min and feed 0.2 mm/rev, calculate machining time for drilling.

METAL MACHINING—Processes and Machine Tools

Solution:

N=

(a) Drilling time (T) for one hole =

401

20 ¥ 1000 = 424.6 rpm 3.14 ¥ 15 L 30 + 0.3 ¥ 15 = 0.4 min = N◊ f 424.6 ¥ 0.2

For drilling 4 holes, machining time would be 4 ¥ 0.4 = 1.6 min

(Ans.)

Example 6.36: Given: hole diameter 20 mm; depth to be drilled 70 mm; feed 1.2 mm/rev; cutting speed 60 m/min, find out drill rpm, feed speed, cutting time and metal removal rate assuming tool approach and over travel as 5 mm. Solution:

Drill or spindle rpm (N) =

1000 V 1000 ¥ 60 = 955.4 rpm = 3.14 ¥ 20 p ◊d

Feed speed = f ◊ N = 1.2 ¥ 955.4 = 1146.4 mm/min Cutting or drilling time (T) = =

L and L = 70 + 5 = 75 mm N◊ f

75 1.2 ¥ 955.4

= 0.065 min

6.18

(Ans.)

SHAPER (OR SHAPING MACHINE)

Shaper is commonly used to produce flat surfaces (horizontal, vertical or inclined) by machining with the help of a reciprocating tool. But it can as well machine varieties of curved, odd and irregular surfaces composed of straight line elements. Shapers are capable of generating contour surfaces also. Due to limited length of its ram stroke, shaper is conveniently adapted to smaller jobs. Shaper is, however, a slow machine and considered suitable only for unit or batch production than for mass production. Shaper is many times preferred to other quick metal cutting machines. Although it may be slower in removing metal in comparison to other machines with multi-teeth cutters, the utility of installing it over other machines is more for a factory man because he can get nearly all kinds of surface-finishing done on this single machine along with ease of machining varieties of surfaces with minimum change over time since the set-up time for a shaper is much less for most of the jobs. Intricate fixtures and supporting devices are replaced by simple holding gadgets. Shaper tools are cheaper and simple. A shaper is particularly adapted to smaller jobs that may be held in its vice. Its tool head is so constructed that cuts in horizontal, vertical and angular directions are taken easily. Because of the ranges provided in a shaper for its stroke length and positions of stroke, adjustment of table in vertical direction, feeds in lateral, vertical, and angular directions, the shaper is sometimes more efficient for many jobs than a milling machine. In comparison to a planer, for shorter cuts, shaper costs less to buy, consumes less power, occupies less floor area and is easier to operate. Shaper is about one-third quicker in action than the planer.

402

MANUFACTURING PROCESSES

6.18.1

Working Principle of Shaper

On a shaper, job is fixed on the work table (i.e. job remains stationary) and the tool cuts while reciprocating over the job [Fig. 6.93(a)]. The tool is mounted on a reciprocating ram and the table which supports the job is fed normal to the tool motion at each stroke of the ram. The tool cuts in the forward stroke only except in case of a draw-cut shaper in which the tool cuts in backward stroke of the ram. The other stroke in both the cases remains idle as there is no cutting action in that stroke. Indexed feed to the job against the tool may be given in horizontal or vertical direction. Figure 6.93(b) shows main parts of a shaper.

Fig. 6.93 A shaper or shaping machine. The working principle of a shaper is shown at (a) and its various components are shown at (b) wherein 1. Base, 2. Bull gear, 3. Pinion, 4. Rocker arm, 5. Crank pin, 6. Job (or table) cross-feed arm, 7. Paul and ratchet arrangement of cross-feed of the table or job, 8. Table, 9. Ram, 10. Ram stroke position adjustment handle, 11. Table elevating screw, 12. Table support, 13. Ram locking handle, 14. Clapper box (swinging tool post), 15. Clapper box swiveling scale, 16. Tool feed handle, 17. Tool, 18. Column, 19. Cross-rail and 20. Saddle.

Some representative shaping operations are illustrated in Fig. 6.94(a) to Fig. 6.94(f).

6.18.2

Shaper Size

Shaper size is specified in terms of (a) maximum length of ram stroke, (b) table size, (c) table travel in horizontal and vertical direction, (d) number of ram strokes per minute, (e) type of quick return mechanism, (f) capacity of motor (HP), (g) weight, etc. Shapers are available with ram stroke varying from 200 (bench model) to 900 mm. The length of stroke also indicates the size of a cube that can be held and planed in the shaper. Further, in a 900 mm shaper, the length of stroke may be adjusted from 0 to 900 mm, the cross-feed adjustment of table will be 900 mm and the extreme bottom position of the cross-rail will permit the table to accommodate a workpiece of 900 mm height.

METAL MACHINING—Processes and Machine Tools

Fig. 6.94

Some typical examples of shaping operations.

403

404 6.18.3

MANUFACTURING PROCESSES

Types of Shaper

Shapers are categorized in a number of following ways: 1. According to the position of ram: (a) horizontal type (b) vertical type (c) travelling head type 2. According to the type of mechanism used for movement of ram: (a) crank type (b) hydraulic type (c) geared type 3. According to the table design: (a) standard type (b) universal type 4. According to the type of cutting stroke: (a) push type (b) draw type Horizontal shaper: In this shaper, the ram reciprocates in horizontal plane. The shaper is generally used to shape flat surfaces. Vertical shaper: A vertical shaper is sometimes called a slotter. It has its ram reciprocating in vertical plane. The work table is rotary type. The shaper may be driven by crank, hydraulic or screw. It is mainly used for cutting slots and keyways. Travelling head shaper: In this shaper, the ram carrying the tool while it reciprocates moves crosswise to give the required feed. The ram carries a tool slide. The ram itself is mounted on a saddle which also travels sideways. Heavy jobs are held static on the basement of the machine while the ram reciprocates and supplies the feeding movements (for both tool and job). Crank type shaper: This shaper is most commonly used. In this shaper, a single-point tool reciprocates equal to the length of stroke desired and the work is clamped on the table. A crank mechanism changes the circular motion of a ‘bull gear’ to reciprocating motion of ram. Hydraulic shaper: In this shaper, the reciprocating movement of ram is obtained by hydraulic power. Its working will be discussed later. The most important advantage of this shape is that the cutting speed and force of ram drive are constant from the very beginning to the end of cut. Besides, it offers great flexibility of speed and feed control. It eliminates shock and allows slip or slowing up of motion when the cutting tool is overloaded, protecting the parts or the tool from breakage. The shaper operates quietly without noise. Geared type shaper: A general shaper has a rack below its ram meshing with a pinion which is driven by a gear train and motor. It is not in common use. Standard or plain shaper: It is a light machine. The work table has only two movements, vertical and horizontal, to give the feed. Also, the table may or may not be supported at the outer end. Universal shaper: In this shaper, an extra support is provided at the front end of the table which can be readily adjusted for any height of the table and allows the table to be fed crosswise also. A universal table can be rotated a full circle on the saddle (Fig. 6.95).

METAL MACHINING—Processes and Machine Tools

Fig. 6.95

405

A universal shaper table.

Push type shaper: It cuts in forward stroke of ram and is most commonly used in the shops. Ram stroke may be up to 900 mm. Draw type shaper: It cuts in backward stroke of ram and is usually built very heavy. The tool is set in a reversed direction to that of a standard shaper. It allows heavier cuts with less vibration and strain on work table. Draw type shapers may have size up to 1800 mm.

6.18.4

Principal Parts

Principal parts of a commonly used crank driven standard shaper have been illustrated in Fig. 6.93(b). These are briefly described in the following: 1. Base: Shaper base is a heavy structure of cast iron. It supports all the other parts and assemblies (described in the following). It resists vibration and high compressive loads being of cast iron. Base is bolted down to the shop floor through foundation bolts. 2. Column (Fig. 6.96): It is a box structure of cast iron and houses the operating mechanisms of the machine. It also provides support for other parts of the machine, such as ram,

Fig. 6.96

Showing details of supporting structure (columns) and table feed mechanism. The table elevating screw (D) is operated with handle (M) through a set of two level gears (L).

406

MANUFACTURING PROCESSES

cross-rail, etc. Column consists of two vertical walls which are supported on the shaper base. Column has two horizontal machined guide ways (P) at its top face on which the ram reciprocates. Two more vertical guide ways (J) are machined on the front face of the column structure for supporting and guiding the movement of cross-rail block. The mechanism providing quick return motion to ram, speed reducing devices and ram stroke controlling system are also contained in the hollow of the column. 3. Cross-rail: The cross-rail block is mounted on the front two vertical guide ways (J) of the column (Fig. 6.96). The cross-rail block has two parallel horizontal guide ways (K) in the vertical plane and perpendicular to the axis of ram. The table may be raised or lowered to accommodate different sizes of jobs by rotating an elevating screw (D) which makes the cross-rail block to slide up and down on the vertical guide ways (J) of the column. A cross-feed screw (N) which is fitted within the cross-rail in horizontal position parallel to the top guide ways (K) of the cross-rail, actuates the table (saddle) to move horizontally in either direction on guide ways (K). 4. Saddle: The saddle (C) (Fig. 6.96) moves on the cross-rail block and carries the table (or work table) on it such that crosswise movement of the saddle by rotating the crossfeed screw (N) by hand or by power makes the table to move sideways. 5. Table: The table (Fig. 6.96) is firmly connected on to the saddle. It gets its crosswise and vertical movements from the saddle and cross-rail, respectively (as explained above in case of saddle). Table is a box structure casting having T-slots both on top and sides for clamping the job. 6. Ram: It is the reciprocating member with tool head mounted on its front face. Ram is semi-cylindrical in form with rigid structure heavily ribbed inside. It slides on two accurately machined guide ways (dovetail type) made on the top of column walls. The ram is connected to the reciprocating mechanism (quick return) housed inside the hollow of the column. Inside the ram is housed the mechanism for altering the ram positions with respect to the job. 7. Tool head or shaper head (Fig. 6.97) is mounted at the front end of ram and has the provision of being swivelled in any direction for shaping angular surfaces. The tool is held in tool post. The vertical feed of tool may be given with tool-feed handle (F) because its rotation makes the slide (E) to go up and down. Feed screw (I) has a graduated collar (J) for accurate adjustment of the depth of cut. The clapper block (D) carrying the tool post (G) is hinged with the clapper box through a pin (B). When tool cuts in forward stroke only, the clapper block gets a rigid support at its back because of the clapper wall, but in the return stroke the clapper block swings forward and moves the tool on the job surface without giving any cut or scratch. For swivelling the shaper head to any desired angle for machining taper surfaces like dovetail, etc., loosen the bolt (C) and tilt the shaper head to the required angle and later tighten the bolt (C). Tilting angle may be read from the graduations made on the shaper head. Apron may also be swivelled to different angles and clamped there with the help of the bolt (A).

METAL MACHINING—Processes and Machine Tools

Fig. 6.97

6.18.5

407

Shaper head (or tool head).

Operations Performed on Shaper

Shaper is used for machining of (i) horizontal flat surfaces, (ii) vertical flat surfaces, (iii) angular flat surfaces, (iv) irregular surface, (v) curved surfaces, (vi) slots, keyways, grooves, gear teeth, etc. (using indexing attachment). Some of the shaper operations have been illustrated in Fig. 6.94(a) to Fig. 6.94(h).

6.18.6

Quick Return Mechanisms of Shaper

Shaper is a reciprocating type machine tool wherein the rotary motion of motor is converted into reciprocating movement of ram by the mechanism housed within the column of the machine. In a standard shaper, tool cuts metal in its forward stroke only and the return stroke goes idle as no metal is cut during the return stroke. To reduce the total machining time, the return stroke should be performed by the machine quickly. Hence, shaper mechanism should be designed such that it may allow the ram (or tool) to move at a comparatively slower speed during the forward cutting stroke, whereas during the return or idle stroke, it may allow the ram to move at a faster rate. This feature is achieved in a shaper with the help of ‘quick return mechanism’ based on either of the following two methods: (a) Crank and slotted link mechanism (b) Hydraulic shaper mechanism Crank and slotted link mechanism

Main features of the crank and slotted link mechanism are shown in Fig. 6.98. Note the following main points: ∑ Bull gear (14) is a large gear mounted within the hollow of the column. It is driven by pinion (1) which itself gets power from motor through a set of gears which help in running of bull gear (14) at different speeds.

408

MANUFACTURING PROCESSES

Fig. 6.98

Crank and slotted link (rocker arm) mechanism.

∑ A radial or bull gear slide (16) is rigidly bolted to the centre of bull gear (14). The bull gear slide (16) carries a bull gear sliding block (10) which is capable of sliding inside the slot of bull gear slide (16) and wherein a crank pin (11) is rigidly fitted with the bull gear sliding block (10) such that the rotation of bull gear (14) causes the crank pin (11) to revolve at uniform speed, tracing the crank pin circle (19). ∑ A rocker arm sliding block (12) mounted upon the crank pin (11) is capable of sliding within the rocker arm or slotted link (9) which is pivoted at one end at rocker arm pivot (15) and the other end of rocker arm (9) is connected to ram block (8) through the floating pin and fork system. ∑ As the bull gear (14) rotates causing the crank pin (11) to rotate, the rocker arm sliding block (12) fastened to crank pin (11) will rotate tracing the crank pin circle (19) and at the same time moving up and down in the slot in the rocker arm (9) giving it a rocking movement which is communicated to the ram (2) and results into conversion of rotary motion of bull gear (14) into reciprocating motion of ram (2). Principle of quick return motion of the crank and slotted link mechanism is explained in Fig. 6.99 wherein: P = rocker arm pivot PM = position of rocker arm showing extreme backward position of ram stroke PN = position of rocker arm showing extreme forward position of ram stroke

METAL MACHINING—Processes and Machine Tools

409

C1 = crank pin position at the beginning of forward stroke of ram C2 = crank pin position at the end of forward stroke of ram R = crank radius

Fig. 6.99

Principle of quick return operation.

Note that: ∑ Forward cutting stroke takes place when crank pin rotates through the angle C1KC2. ∑ Return or idle stroke takes place when crank pin rotates through the angle C2LC1. Hence angle C1KC2 is larger than angle C2LC1. Because the angular velocity of the crank pin is constant, the return stroke is, therefore, completed within a shorter time (hence with a quick return motion). Time of cutting stroke Angle C1KC2 = Time of return stroke Angle C2 LC1

(6.83)

The ratio of cutting time to return time varies between 2 : 1, usually with practical limit of 3 : 2. Disadvantage of crank and slotted link mechanism is that in it the cutting speed and return speed are not constant throughout the stroke; they are minimum when rocker arm is at the two extremes and the speeds are maximum when rocker arm is vertical (Fig. 6.100). The massive bull gear works like a flywheel which helps in making the reciprocating movement of ram smooth and without jerks.

Fig. 6.100

Velocity diagram of crank and slotted link mechanism.

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MANUFACTURING PROCESSES

Adjusting length of stroke: Radius of crank (R) (Fig. 6.99), i.e. distance between crank pin (11) and centre of bull gear (14) (Fig. 6.98) can be varied by moving the bull gear sliding block (10) inside the slot of bull gear slide (16). This is done by rotating the bevel gear (18) placed at the centre of bull gear (14) with the help of a handle. The bevel gear (18) rotates bevel gear (17) which, in turn, rotates the lead screw (13) passing through the bull gear sliding block (10). Thus, rotation of lead screw (13) causes the bull gear sliding block (10) carrying crack pin (11) to be brought inwards or outwards with respect to centre of bull gear (14), i.e. results in varying the radius of crank (R). Closer the crank pin (11) is brought to centre of bull gear (14) (i.e. shorter the crank radius R), smaller will be the ram stroke. Maximum stroke length is obtained when crank pin (11) is shifted towards the farthest end of the bull gear slide (16). Adjusting the position of ram stroke or tool travel is illustrated in Fig. 6.101. Refer Fig. 6.98. Rotation of hand wheel (5) results in the movement of ram block (8) along the screwed shaft (3) as the ram block (8) acts like a nut. In order to set the position of ram stroke, the clamping lever (4) is loosened. By rotating the hand wheel (5), the screwed shaft (3) is rotated within the ram block (8). With the ram block (8) remaining fixed in position (due to rocker arm standing in a particular position), rotation of screwed shaft (3) will cause ram (2) to move forwards or backwards with respect to ram block (8) according to the direction of rotation of the hand wheel (5). In this way, the position of ram (2) may be adjusted with respect to the workpiece. The clamping lever (4) must be properly tightened after the ram stroke position adjustment has been done.

Fig. 6.101

Adjusting the tool travel.

Hydraulic shaper mechanism

The hydraulic shaper mechanism is illustrated in Fig. 6.102. Note that the ram is connected with the left hand end of the piston rod which moves in a hydraulic cylinder placed under the ram. A constant discharge oil pump is used to pump oil under high pressure which after passing through the valve chamber enters the hydraulic cylinder from right hand side and exerts pressure on the piston. This causes the ram to perform the forward stroke while any

METAL MACHINING—Processes and Machine Tools

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oil present on the left hand side of cylinder is discharged to the reservoir through the throttle valve. On the completion of forward stroke, shaper dog hits against the reversing lever. As a result of this, oil under high pressure is now pumped to the left side of the piston causing the ram to perform return stroke. The oil on the right side of piston is discharged to the reservoir. At the completion of return stroke, another shaper dog hits the reversing lever altering direction of stroke of piston and the cycle is repeated.

Fig. 6.102

Hydraulic shaper mechanism.

The quick return motion is obtained due to the difference in stroke volume of cylinder at both ends, the left hand end volume being smaller because of piston rod. Since the oil pump used is a constant discharge type, within a fixed period the same amount of oil will be pumped into the right or to the left hand side of cylinder. This means that same amount of oil will be packed within a smaller volume causing the oil pressure to rise automatically on the left hand side of cylinder which makes the piston rod (or ram) to move faster during return stroke. The stroke length and position are adjusted by shifting the position of reversing dogs. The throttle valve regulates the oil flow and is used to change cutting speed. Excess oil flows through the relief valve (when the throttle valve is partially closed) to the reservoir, thus maintaining uniform pressure during cutting stroke. Advantages of hydraulic shaper mechanism are as follows: ∑ Constant cutting and return speeds throughout the stroke, permitting cutting tool to work uniformly during cutting stroke. ∑ Length of stroke and its position relative to work may be changed quickly without stopping the machine. ∑ Ram movement can be reversed instantly anywhere in either direction. ∑ Infinite number of cutting speeds may be obtained.

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MANUFACTURING PROCESSES

∑ With a high rate of return of speed, greater number of cutting strokes are obtained within the range of cutting speed. ∑ Relief valve ensures safety to the tool and the shaper when the later is overloaded.

6.18.7

Feed Mechanism

Both down feed movement of tool and cross-feed movement of table (or job) to the tool are obtained in a shaper, but these feed movements are provided intermittently and during the end of return stroke only. For machining vertical or slant surfaces, tool feed is given by rotating the down feed screw of the tool head by hand. As regards the cross-feed of table, this is achieved by rotating table cross-feed screw either by hand or by power which causes the table to move sideways through a predetermined amount at the end of each return stroke, thereby bringing the uncut surface of the job in the direct path of the reciprocating tool. The principle of cross-feed (for table) mechanism is explained in Fig. 6.103. Note that ratchet wheel is keyed to the table cross-feed screw and the rocker arm (hinged at its bottom end) swings around freely on the cross-feed screw shaft. The rocker arm carries at its top end a spring loaded pawl which is straight and vertical on one side and slant on the other side. The driving disc is connected with bull gear of the shaper. As the bull gear rotates, the driving disc also rotates which, through the connecting rod, results in the rocking of the rocker arm on the fulcrum (or feed screw shaft). As the driving disc rotates through half of the revolution clockwise, the top end of the rocker arm also moves in clockwise direction and the spring loaded pawl (being slant) slips over the teeth of ratchet wheel but resulting in no movement or rotation of the ratchet wheel (or cross-feed screw), but when the driving disc rotates through the other half of the revolution clockwise, the top end of rocker arm moves in anticlockwise direction resulting into the engagement of the pawl with ratchet wheel teeth and causing the ratchet wheel to rotate in anticlockwise direction only. Thus, the table feed movement is effected only when the driving disc (or bull gear) rotates through half of the

Fig. 6.103

Automatic feed mechanism.

METAL MACHINING—Processes and Machine Tools

413

revolution, i.e. during the return stroke only. The table feed direction (or direction of rotation of ratchet wheel) can be obtained by rotating the pawl knob (after removing the locking pin) through 180° and thus reversing the pawl engagement position. As regards the amount of feed (coarse or fine), greater the value of R, the throw of crank pin, coarser will be the feed and vice versa. R is adjusted by shifting the crank pin from one position to another and locking it there.

6.18.8

Work Holding Devices

Various devices used to hold the workpiece on the shaper table during machining include the following. Note that the top and sides of the shaper table have a number of T-slots for clamping the work or other fixtures. ∑ Vice ∑ Directly clamped on table ∑ Angle plates ∑ V-block parallels, hold down devices, etc. ∑ Held between shaper index centre Figure 6.104(a) and Fig. 6.104(b) show the representative examples of the use of shaper vice and shaper index centre. Several other clamping devices used to support and clamp the workpiece directly on the shaper table are shown in Fig. 6.105.

Fig. 6.104(a)

Showing the use of shaper vice. Job is supported on the parallels and held in position by vice jaws (with the help of two hold downs). The vice lugs can be bolted on the shaper table.

Fig. 6.104(b)

Use of shaper centre for cutting splines or grooves.

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MANUFACTURING PROCESSES

Fig. 6.105

6.18.9

Devices to support and clamp the work during shaping.

Shaper Tools

Shaper tools are single-point cutting tools and are fairly similar to lathe tool except that shaper tools are more stronger and rigid to withstand shock experienced by the tool at the commencement of each cutting stroke. The effective rake and clearance angles in a lathe can be varied by raising or lowering the tool point in relation to work centre, but tool angles in a shaper cannot be changed by changing the tool position as in case of lathe because shaper tools are held rigidly in a position in relation to the job. Further, the shaper tools have less side and front clearance (usually 2 to 3° and 4°, respectively). A lathe tool needs sufficient side clearance angle as it is continually fed sideways but in a shaper tool as the feed is given at the end of cutting stroke, a very small clearance angle is required to give relief to the side cutting edge. A few common shaper tools are shown in Fig. 6.106.

Fig. 6.106

Shaper tools.

METAL MACHINING—Processes and Machine Tools

415

6.18.10 Cutting Speed, Feed and Depth of Cut Cutting speed in a shaper is the average linear speed of the tool (m/min) during the cutting stroke. It depends on the number of ram strokes per minute and the stroke length. The cutting speed of a shaper may be constant or variable depending on the design of quick return mechanism used. Cutting speed =

Length of cutting stroke Time taken by cutting stroke

(6.84)

Since it is difficult to measure exactly the time taken by cutting stroke, the ratio between the return time to cutting time and the number of double strokes per minute or rpm of bull gear are considered for calculating the cutting speed. Let L = length of cutting stroke, mm m = ratio between return time to cutting time. The value of m as 2 : 3 implies that tool is working 3/5th of time and idle stroke takes 2/5th of time. n = Number of double strokes of ram per minute (which equals to rpm of bull gear) v = cutting speed, m/min According to Eqn. (6.84), Length of cutting stroke (L ), mm Cutting speed (v), m/min L = 1000 ¥ v

Time taken by cutting stroke (min) =

and

m=

Time of return stroke Time of cutting stroke

Then return stroke time = m ¥ cutting stroke time m¥L = 1000 ¥ v L m¥L Time taken in completing one double stroke = + 1000 ¥ v 100 ¥ v =

L (1 + m) 1000 ¥ v

Number of double strokes per minute (n) or rpm of bull gear 1 = L (1 + m) 1000 ¥ v 1000 ¥ v or, n = L (1 + m) nL (1 + m) and cutting speed (v) = , m/min 1000

(6.85)

(6.86)

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MANUFACTURING PROCESSES

The cutting speed (v) obtained in above is the average cutting speed because of the assumption that the cutting stroke is completed at uniform speed. This may not be true for hydraulic shaper, but in a crank driven shaper, cutting speed and return speed are not uniform. Feed (f ) in shaper is the relative motion of workpiece in a direction perpendicular to the axis of ram reciprocation and is expressed in mm/double stroke on mm/stroke. Depth of cut (d) gives thickness of the material removed per cut and is given in mm. Machining time Let L = length of ram stroke, mm B = breadth of job, mm f = feed in mm/double stroke m = ratio of return time to cutting time v = cutting speed, m/min We know from Eqn. (6.85) that Time taken in completing one double stroke =

L (1 + m ) 1000 ◊ v

Total number of double strokes required to complete the work = Hence, time taken to complete machining =

B f

L ◊ B ◊ (1 + m ) 1000 ◊ f ◊ v

(6.87)

Metal removal rate (MRR) MRR = f ◊ d ◊ n ◊ L(1 + m), mm3/min (6.88) where f = feed/stroke, mm d = depth of cut, mm n = Number of double strokes/min L = length of stroke = Lj (job length) + 2 ¥ clear at each end m = return to cutting time ratio Average cutting time and feed for shaper work have been shown in Table 6.6. TABLE 6.6

Material Cast iron Mild steel Brass

Average cutting speed and feed for shaper work

High speed steel tool

Cemented carbide tool

v

f

v

f

18 24 48

2 1.5 1.25

30 45 60

0.125–0.5 0.125–0.5 0.25–0.35

where v = cutting speed, m/min f = feed, mm/double stroke

METAL MACHINING—Processes and Machine Tools

417

Example 6.37: Calculate the time to shape a plate 500 ¥ 900 mm size when the cutting speed is 10 m/min and return to cutting time ratio is 1 : 4. Take feed as 3 mm and clearance at each end 70 mm. Solution: The work is so arranged on the table that the tool takes cut over the 500 mm side as 900 mm is too big a size for normal shaper. Total length of stroke = 500 + 2 ¥ 70 = 640 mm Cutting time =

640 ¥ 60 = 3.84 sec 1000 ¥ 10

Return time 1 = Cutting time 4 3.84 = 0.96 sec 4 Total time for one complete double stroke = 3.84 + 0.96 = 4.8 sec 900 Number of double strokes required to complete the cut = = 300 3 4.8 Hence, time required to shape the work = ¥ 300 = 24 min (Ans.) 60

Hence, return time =

Example 6.38: Calculate the cutting speed if the shaper has stroke length 240 mm, number of double strokes per minute 40 and ratio of return to cutting time as 2 : 3.

Solution:

2ˆ Ê 40 ¥ 240 ¥ Á 1 + ˜ Ë n ◊ L ◊ (1 + m ) 3¯ Cutting speed (v) = = 1000 1000 = 15.93 m/min (Ans.)

Example 6.39: A workpiece 300 ¥ 200 mm size is to be machined on a shaper having cutting to return ratio of 3 : 2. If the cutting speed is 25 m/min, feed 2 mm/double stroke, clearance at ends as 30 mm and depth of cut 3 mm, find the material removal rate if available ram strokes are 40 per minute. Solution: MRR = f ◊ d ◊ n ◊ L(1 + m), mm3/min 2ˆ Ê = 2 ¥ 3 ¥ 40(300 + 30 + 30) ¥ Á 1 + ˜ Ë 3¯ = 1,43,424 mm3/min (Ans.)

6.19

PLANER (OR PLANING MACHINE)

Planer is a machine tool primarily used to produce plane and flat surfaces with a single-point cutting tool. The jobs normally handled on a planer are of larger size and heavier in weight because of which they are not capable of being handled and machined on a shaper, although the types of operations performed on a planer are similar to those performed on a shaper. The basic difference between a planer and a shaper is that in a planer the job, supported on the

418

MANUFACTURING PROCESSES

work table, reciprocates past the stationary cutting tool, with the feed supplied by the lateral movement of the tool, whereas in a shaper the tool mounted on a ram reciprocates over the job and the feed is given by the crosswise movement of the work table. Comparison between a planer and a shaper is shown in Table 6.7. TABLE 6.7

S.No. 1. 2. 3. 4. 5. 6. 7.

8.

6.19.1

Comparison between a planer and a shaper

Planer Work reciprocates horizontally Tool remains stationary during cutting More rigid, large and heavy machine and costlier also Occupies large floor area Capable of taking very heavy cuts and feeds Work setting on work table takes longer time and skill Capable of carrying several tools which can be employed simultaneously, thus increasing production Preferred for machining large size and heavy workpieces

Shaper Tool reciprocates horizontally Work remains stationary during cutting Comparatively lighter and cheaper machine Occupies less floor area Only light cuts and feeds are taken Work setting is easy and quick Usually only one tool is used

Preferred for machining small and lighter workpieces

Principle of Working

Working principle of a planer is shown in Fig. 6.107(a). In a planer, the tool is held stationary and the job, along with planer table, reciprocates past the tool during the cutting stroke. Cutting is done only in the forward stroke of the machine table and the return stroke of the table is idle and completed quickly (as in a shaper). Several tools can be made to work simultaneously on the job.

6.19.2

Size of a Planer

Length of planer table is almost equal to the table travel and other dimension given is the distance between two housings or columns, height from the top of table to the cross-rail in its uppermost position and maximum length of table travel. Double housing planer may vary from 750 mm ¥ 750 mm ¥ 2.5 m to 3000 mm ¥ 3000 mm ¥ 18.25 m. Other particulars such as number of speeds and feeds available, power, floor space, weight of machine, etc. are also given.

6.19.3

Principal Parts

Principal parts of a double housing (or column) standard planer are shown in a block diagram [Fig. 6.107(b)]. These are as follows: Bed: The bed is a boxlike casting, heavily ribbed crosswise. It carries two V-guide ways at its top on which the planer table reciprocates back and forth. Bed supports the housings and other structures. The hollow space within the box of the bed houses driving mechanism of the table. The bed is bolted down to floor.

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419

Fig. 6.107 A planer or planing machine. The working principle is shown at (a) and main parts are shown in block diagram at (b).

Table: The table supports the work and reciprocates along the V-guides made on the top face of the bed. The table is a heavy rectangular casting carrying slots for T-bolts and other clamping devices. Hydraulic bumpers are fitted sometimes at the end of the bed to stop the table from overrunning. Housings or columns or uprights: These are also rigid boxlike vertical structures placed on each side of the bed and fastened there. The front face of columns is accurately machined to provide precision ways on which the cross slide is made to slide up and down to accommodate jobs of different heights. The two side-tool heads or tool posts also slide upon the front face of the column. Cross-rail: It is also a rigid boxlike casting connecting the two columns or housings. The two elevating screws, in the two housings, are rotated by an equal amount to keep the crossrail horizontal in any position. The front face of the cross-rail carries tool head saddle and usually two tool heads or tool posts are mounted upon the cross-rail. Tool head: The tool head of a planer is similar to that of a shaper both in construction and in operation. Here the tool head refers to the two tool heads or tool posts which are mounted on the cross-rail.

6.19.4

Types of Planers

Most commonly used planers can be categorized as follows: (i) Standard or double housing planer (ii) Open side planer (iii) Pit planer (iv) Plate or edge planer (v) Divided table planer Standard or double housing planer is most widely used. Its length of bed is little over twice the length of the work table. It has two housings (or columns or uprights) and its main feature is shown in Fig. 6.108. The planer table is driven by mechanical or hydraulic devices.

Fig. 6.108

Standard double housing planer.

420 MANUFACTURING PROCESSES

METAL MACHINING—Processes and Machine Tools

421

Open side planer has the column or housing only on one side of the base and the cross-rail is suspended from the housing as a cantilever. This feature allows handling and machining of large and wide jobs. Pit planer is a massive unit and in this planer, the work table is stationary and the column carrying the cross-rail reciprocates on massive horizontal rails mounted on both sides of the table. This finds good use in machining very large work which cannot be accommodated on a standard planer and design saves much of floor area. Plate or edge planer is totally different in its design from that of an ordinary planer. The machine is specifically designed for use in squaring and bevelling the edge of steel plates used for ship building and pressure vessels. Divided table planer has two work tables on the planer bed which may be reciprocated separately or together. This design brings saving in setting time. To have a continuous production, one of the tables is used for setting up work, while the other table is used for machining work.

6.19.5

Quick Return Table Drive of a Planer

The return stroke of a planer is also faster than the cutting stroke and the following methods are commonly used to achieve this. (i) DC reversible motor (ii) Fast and loose pulley system (iii) Hydraulic drive system DC reversible motor

This method is commonly used on modern planers because it gives a wide range of table speeds and more responsive control. A special type of motor is used which can change its speed according to the field current applied. Motor speed can be changed very quickly from full speed forward to full speed reverse. Reversal is instantaneous. The main driving motor is fed with a variable voltage DC current of reversible polarity from a special motor generator powered from an AC supply. Electrical driving provides the facility of tripping and reversing. Trips mounted on one side of the planer table with a gap between them equal to the stroke length, actuate the switches to make the necessary change over and thus effect reversal of the table at the end of each stroke. The planer table has a rack fitted on its bottom and power from the driving motor to the table rack can be conveyed either with a train of reduction gears and the pinion engaging the rack or with a worm meshing with the rack (Fig. 6.109).

Fig. 6.109

Driving planer table with a DC reversible motor and worm and worm wheel speed reducer.

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MANUFACTURING PROCESSES

Fast and loose pulley system

This system is shown in Fig. 6.110. Ordinary planers still have this system of table drive. Note that a main driving shaft (7) with driving pinion (12) is placed below the table and a counter shaft (2) is placed at the top of the planer housing. The counter shaft (2) has two pulleys, smaller one (16) and bigger one (17). Similarly, on shaft (7), fast pulley [5(a)] and loose pulley [5(b)] are both bigger than pulley (16). On the other hand, fast pulley [15(a)] and loose pulley [15(b)] are both smaller than pulley (17). Power to the main driving shaft (7) for cutting stroke is obtained from pulley (16) through the fast pulley [5(a)] at lesser rpm than that of shaft (2). And power for return stroke is received by main driving shaft (7) through the fast pulley [15(a)] at rpm much higher than that of the shaft (2). It results in faster return stroke of the table. The shifting of belts from fast to loose pulleys and vice versa for driving table is achieved with the help of belt shifter (6) actuated by the lever (11) on which strikes the table reversing dogs at the end of each stroke. Note that the fast pulley [5(a)] is driven from pulley (16) with a cross-belt drive and hence the direction of rotation of fast pulley [5(a)] used for cutting stroke will be opposite to that of the fast pulley [5(b)] used for the return stroke.

Fig. 6.110 Schematic of planer table drive with fast and loose pulley. 1. Pulley to receive power from motor, 2. Counter shaft, 3. Cross-belt, 4. Open belt, 5(a). Fast pulley, 5(b). Loose pulley, 6. Belt shifter, 7. Main driving shaft, 8. Speed reducer, 9. Cam, 10. Lever, 11. Lever end where table type dogs strike, 12. Pinion, 13. Rack, 14. Planer table, 15(a). Fast pulley, 15(b). Loose pulley, 16. and 17. Pulleys.

Hydraulic drive system

Hydraulic drive mechanism of a planer is similar to that of a shaper already described.

6.19.6

Feed Mechanism

In a planer, feed is given intermittently and at the end of the return stroke. Both down feed and cross-feed are given by the tool. The cross-feed is given while machining horizontal surfaces on a job mounted on the table. The tool clamped on the tool head slides on the

METAL MACHINING—Processes and Machine Tools

423

cross-rail by a predetermined amount at the end of each return stroke, thus giving the required cross-feed. The down feed is used while machining vertical or angular surfaces by rotating the down feed screw of the tool head. Feed may be given by hand or power using the following methods: (a) By friction disc (b) By electrical device In a planer as the length of the table stroke is very long, the bull gear makes a larger number of revolutions in the forward cutting stroke and the same number of revolutions in the return stroke. It is different from that in a shaper. By friction disc, only a part of the revolutions of the bull gear is used to feed gearing at the end of the return stroke while in the rest of the period, the feed mechanism remains inoperative. In electrical feed drive, a separate motor is used to operate the feed mechanism. The motor is energized simultaneously with the table reversing system and rotates through a definite part of revolution, which may be half or one revolution only. At the set time, the electrical control trips off the supply of current and the motor is stopped.

6.19.7

Work Holding Devices

Since the planer is primarily intended to handle large and heavy jobs, work setting on the table is done very carefully and takes a lot of time. Work needs to be rigidly connected with the table using proper clamping devices but avoiding undue clamping pressure which may cause distortion of the work. Work holding devices used on planer are: (a) Standard clamping devices are devices such as vices, step blocks, T-bolts and clamps, jacks, stops, V-blocks, planer centres, stop pins and toe dogs. These are quite similar to those used on shaper but planer gadgets need to be stronger and bigger. (b) Special fixtures are designed and used as per the requirement of shaping a particular job.

6.19.8

Planer Operations

Operations performed on a planer are similar to that of a shaper with the only difference that planer is used for machining large and heavy jobs such as bases and tables of all kinds of machine tools, frames of engines, and other large structures and weldments. The common operations performed on a planer include planing of (a) flat horizontal surfaces, (b) flat vertical surfaces, (c) slant surfaces and dovetails, (d) curved surfaces and (e) slots, grooves, keyways and splines.

6.19.9

Planer Tools

Planer tools are all single-point cutting tools, similar in shape and tool angles to those used on a shaper except that planer tools are strong as they have to take up heavy cuts, coarser feeds during a long cutting stroke. Planer tools may be solid forged type or bit type employing bits of high speed steel, stellite or cemented carbide. Some of the planer tools are shown in Fig. 6.111.

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MANUFACTURING PROCESSES

Fig. 6.111

Planer tools.

6.19.10 Cutting Speed, Feed and Depth of Cut Cutting speed is the rate at which the metal is removed during forward cutting stroke and is expressed in m/min. Equation (6.86) holds good for calculating cutting speed of the planer. Feed is the distance the tool head travels at the beginning of each cutting stroke and is expressed in mm per double stroke. Depth of cut is the thickness of metal removed in one cut. It is measured by the perpendicular distance between machined and unmachined surfaces of the work. It is given in mm. Machining time for planing

In commonly available planers, the ratio of cutting time to return time generally varies from 2:1 to 4:1. If the cutting speed, feed, length of cutting stroke, width of job and number of double strokes per minute for a planing operation are known, then Machining time for one complete cut = where L B f m v

= = = = =

L ◊ B(1 + m ) 1000 ◊ v ◊ f

(6.89)

total length of stroke, mm width of job, mm feed, mm/double stroke ratio of return time to cutting time cutting sped, m/min

Cutting speed, feed and depth of cut in a planer are shown in Table 6.8. TABLE 6.8

Cutting speed, feed and depth of cut in a planer

Work material

v

High speed steel tool f

t

V

Cemented carbide tool f

t

Cast iron Steel Bronze Aluminium

15–28 9–23 38–46 46–61

0.8–3.1 0.8–3.1 0.8–2.4 0.8–2.4

3–25 3–25 3–12 3–12

42–77 36–82 Max. table speed

0.8–1.6 0.8–1.6 0.8–1.6 0.8–1.6

1.5–19 1.5–19 1.5–19 1.5–19

METAL MACHINING—Processes and Machine Tools

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Here, v = cutting speed, m/min f = feed, mm/double stroke t = depth of cut, mm Example 6.40: Find the time required in machining a slab of width 400 mm and length 3000 mm on a planer having cutting speed 20 m/min, return speed 75 m/min, machinery allowance 10 mm, tool approach angle 45°, cross-feed of tool 3 mm/full stroke, side over travel of tool 3 mm and total table over travel on both sides 275 mm. Solution: Given: B = 400 mm; Lj = 3000 mm; v = 20 m/min; vr = 75 m/min; machining allowance = 10 mm; tool approach angle (l) = 45°; f = 3 mm/full stroke length of table stroke (L) = 3000 + 275 = 3275 mm n ¥ L (1 + m ) then, cutting speed (v) = 1000 and

m=

cutting speed 20 = = 0.26 return speed 75

1000 ¥ v 1000 ¥ 20 = = 4.879 ª 5 (say) L (1 + m) 3275(1 + 0.26) where n = Number of full strokes/double strokes/min m = return stroke time/cutting stroke time or

n=

Cutting time (tm) =

job width (B) + side approach of tool + side over travel f ¥n

Here, side approach of tool = 10 cot 45° = 10 mm side over travel of tool = 3 mm then,

Machining time, tm =

6.20

MILLING MACHINE

400 + 10 + 3 = 27.5 min 3¥5

(Ans.)

A milling machine removes metal with a fast rotating multi-tooth cutter. As this machine yields high production of different varieties of jobs, in choice for production machines, it comes after the lathe. The normal production accuracy of a milling machine may be within 0.08 mm. A tool room milling machine may give still higher accuracy. It is preferable to machine smaller or moderate jobs on a milling machine because for machining larger jobs, it may perhaps be slower. Because of using multi-tooth cutters and various form cutters, a milling machine can be economically employed for generating varieties of surfaces quite speedily. Jobs needing turning operations should preferably be entertained on a lathe but those needing the machining operations involving indexing of their periphery are better done on a milling machine.

426

MANUFACTURING PROCESSES

6.20.1

Operations Performed on a Milling Machine

The following operations are normally performed on a milling machine: ∑ Machining of contours of infinite variety with straight or spiral elements ∑ Facing operations of all kinds ∑ Squaring shaft ends ∑ Making of hexagonal and other heads of bolts ∑ Forming of profiles of cams, dies and templates ∑ Slotting, slitting and all types of cutting ∑ Making keyways, grooves of variety, convex and concave surfaces ∑ Indexing operations of all kinds, e.g. cutting of flutes, splines and gears (spur, bevel, helical and worm wheels) ∑ Boring, reaming and threading Some of the above operations are illustrated in Fig. 6.112.

Fig. 6.112

Illustrating few examples of milling operations.

METAL MACHINING—Processes and Machine Tools

6.20.2

427

Working Principle

Working principle of a milling machine is shown in Fig. 6.113. The milling machine removes metal with a fast rotating multi-tooth cutter, each tooth having a cutting edge which removes metal from the workpiece when the workpiece fixed on the machine table is fed to the cutter longitudinally, transversely or vertically by operating the table feed accordingly. The multi-tooth cutter is known as milling cutter which has equally spaced peripheral teeth on it. The cutter teeth come in contact with the workpiece intermittently and machine it. The machine can hold more than one cutter at a time. The machine gives better surface finish and dimensional accuracy.

Fig. 6.113

6.20.3

Working principle of a milling machine.

Main Features and Principal Parts

Most common types of milling machines are generally either horizontal spindle type or vertical spindle type. Block diagram of a horizontal spindle column and knee type milling machine is given in Fig. 6.113(a) and that of a vertical spindle milling machine in Fig. 6.114. Brief description of the principal parts of a column and knee type milling machine is given in the following in reference to Fig. 6.113(a).

Fig. 6.113(a)

Horizontal spindle column and knee type milling machine.

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MANUFACTURING PROCESSES

Fig. 6.114 A vertical milling machine with a swivelling head which can be revolved (along with spindle head) in the vertical plane.

1. Base: It provides support to column and other parts which rest on it. Base is a grey iron casting and rigid in construction. It is connected to the shop floor through the foundation bolts. 2. Column: It is a box-shaped strong structure mounted vertically on the base. Column is the main supporting frame giving support to knee, work table, over arm, etc. and houses all the driving mechanisms for spindle and table feed. Column has accurately front vertical face with guide ways on which the knee slides up and down. 3. Knee: The knee provides support to saddle and table and can be slided up and down along the vertical front face of column with the help of knee elevating screw. The knee houses feed mechanisms of table and controls. The top face of the knee forms a slide way for the saddle to provide cross-travel of the table. 4. Saddle: It slides on guide ways made on the top face of the knee. The guide ways are for the movement of the table. Cross-feed to the saddle to move horizontally towards or away from the column face can be provided by hand or power. 5. Table: It rests on the guide ways on the saddle and travels longitudinally. T-slots for clamping jobs or fixtures are provided in the table. Table feed to move it horizontally over the saddle is achieved by hand or power. In universal machines, table may also be swivelled horizontally where the table is mounted on graduated circular base. 6. Over arm: It is mounted on the top of the column. It extends beyond the column face and serves as a bearing support to the arbor through the yoke. The length of the over arm is adjustable so that it may be set nearest to the cutter fitted on the arbor. 7. Spindle: Spindle is located in the upper part of the column and gets driving power from motor to transmit it further to the arbor (see Fig. 6.115). The front end of the spindle just projects from the column face and is provided with a tapered hole to accommodate various cutting tools and arbors.

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Fig. 6.115 Arbor assembly: 1. Draw bolt, 2. Lock nut, 3. Spindle, 4. Key block, 5. Arbor, 6. Setscrew, 7. Spacing collars, 8. Cutter, 9. Bearing bush.

8. Arbor: The arbor connected with spindle through draw bolt serves as an extension of the machine spindle on which milling cutters are securely mounted and rotated. It has taper shank for proper alignment with the spindle having tapered hole at its nose. Details of connecting the arbor with the spindle and mounting of a milling cutter on the arbor are shown in Fig. 6.115.

6.20.4

Types of Milling Machine

Milling machines are available in various designs covering a wide range of work and capacities. The choice for a particular machine depends on the nature and size of the work to be undertaken. Broad classification of milling machines according to their design features is as follows: (a) Column and knee type (b) Manufacturing or fixed bed type (c) Planer type (d) Special type (a) Column and knee type: It is the most commonly used machine for shop work. Main features of this type of machine have been shown in Fig. 6.113(a), whereas special features are shown in Fig. 6.116. The column and knee type machines may be further categorized as: 1. Hand milling machine (or hand miller) is the simplest and a small machine having only hand feed for the table and power rotation of the spindle (or cutter) which is horizontal. It is used for light work such as making slot, keyways and grooves. 2. Plain milling machine is relatively more sturdy and powerful than a hand miller. Table has both hand and power feed in longitudinal (i.e. right angle to spindle), cross (i.e. parallel to spindle) and vertical direction. This machine is intended for heavier milling operations. 3. Universal milling machine is called a universal machine because it can be adapted to a very wide range of milling operations. Besides the three table monuments as

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MANUFACTURING PROCESSES

that in a plain milling machine, the universal milling machine has an additional feature of work table which can be swivelled horizontally and fed at an angle to the spindle (or cutter). This machine may not be as strong and rigid as the plain milling machine and hence it finds use generally in tool rooms and special milling work since the universal milling machine is equipped with auxiliaries such as dividing head, vertical milling attachment, etc. that make it capable of milling gears, twist drills, milling centres, and drilling and shaping tools.

Fig. 6.116 Special features of a column and knee type milling machine: 1. Starting lever, 2. Over arm, 3. Over arm position control, 4. Arbor, 5. Table, 6. Speed change dial and crank, 7. Table hand feed (longitudinal), 8. Lever for rear table feed (power), 9. Knee clamp, 10. Vertical feed lever (power), 11. Vertical hand feed crank, 12. Knee elevating screw and telescope coolant return, 13. Feed change dial and crank, 14. Cross feed hand wheel, 15. Cross feed lever (power), 16. Front table feed engaging lever (power), 17. Handle, 18. Column.

4. Omniversal milling machine is special in the sense that the table, besides having all the movements of a universal milling machine, is capable of being tilted in vertical plane also. This feature makes the machine suitable for machining taper spiral grooves in reamers, bevel gears, etc. 5. Vertical milling machine has its spindle in vertical direction (Fig. 6.114). The machine may be plain or universal type. This machine is usually adapted for operations performed by using end mill cutters and face milling cutters.

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(b) Manufacturing or fixed bed type: These machines are relatively large, heavy and rigid. They differ from the column and knee type machines by construction of their table mounting. The table is mounted on the guide ways machined at the top of fixed bed such that the table is restricted to move at right angles to the horizontal spindles with no provision of cross or vertical adjustment. The cutter is mounted on the spindle and hence may be adjusted horizontally and its vertical feed given by moving the spindle head on two columns placed opposite to each other on the bed side (Fig. 6.117). Simplex, duplex and triplex machines have single, double and triple spindle heads, respectively. It is a production machine suitable for repetitive works.

Fig. 6.117

Duplex fixed bed miller.

(c) Planer type or plano-miller: It is used for heavy-duty works. It resembles a planer as it has cross-rail, cutter heads and columns or uprights (Fig. 6.118). There may be a number of independent spindles carrying cutters on the cross-rail along with two tool heads on the uprights. This is the most powerful milling machine and the modern plano-millers have high power driven spindles powered up to 100 HP ensuring tremendous metal removal capacity. (d) Special type milling machines: Special type milling machines have been developed for specific works. Some of these machines are described in the following: 1. Rotary table machine is a modified version of a vertical milling machine adapted for higher production as in this machine face milling cutters are mounted on two or more vertical spindle while a number of workpieces are clamped on the horizontal circular table capable of rotating around a vertical axis. Cutters may be set at different heights to suit different working operations to be done on the job simultaneously. 2. Drum milling machine is similar in principle to the rotary table machine except that its work supporting table is a drum rotating on horizontal axis and further that the workpieces are supported on both the faces of the drum such that face milling cutters, carried by three or more spindle heads, rotate in horizontal axis for removing metal from the workpieces. Finished workpieces are removed after one complete revolution of the drum.

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Fig. 6.118

Planer type milling machine.

3. Planetary milling machine has the workpiece held stationary and the revolving cutters, moving in a planetary fashion, finish the outer or inner cylindrical surfaces of the cylindrical workpiece. It is used for milling internal or external threads (Fig. 6.119).

Fig. 6.119 Illustrating working principle of a planetary miller. View at (a) is for external milling and that at (b) for internal milling.

4. Pantograph milling machine is capable of duplicating a job by using a pantograph system that permits the size of the workpiece reproduced to be smaller than, equal to or greater than the size of a template used for the purpose. 5. Profiling machine is virtually a vertical milling machine of bed type wherein spindle can be adjusted vertically and cutter head horizontally across the table. The cutter movement is guided by a hardened pin that moves following the outline or profile of the template mounted on the table. A profiling machine duplicates the full size of the template.

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6. Tracer-controlled milling machine reproduces irregular or complex shapes of dies or molds by synchronized movements of the cutter and tracing element.

6.20.5

Fundamentals of Milling Processes

Milling processes performed by different milling cutters can be grouped into the following broad categories: (a) Peripheral milling; (b) Face milling; (c) End milling. Fundamentals of cutting action of milling cutters used to perform the above processes are discussed in the following: (a) Peripheral milling or plain milling or slab milling results in the production of a machined surface parallel to the axis of rotation of the cutter [Fig. 6.121(a)]. In this process, cutting force is not uniform throughout the length of the cut by each tooth. Also, the quality of the surface generated and the shape of the chip formed depend on the rotation of cutter relative to the direction of feed movement of the work. Due to these factors, peripheral milling results into the development of vibrations during cutting. In view of the relative movement between the cutter and the workpiece, peripheral milling is of the following two types: (i) Up milling or conventional milling (ii) Down milling or climb milling Up milling or conventional milling is shown in Fig. 6.120(a). This is the process of removing metal by a milling cutter which is rotated against the direction of feeding of work to the cutter. Note the chip thickness which is minimum (zero) at the beginning of the cut and reaches to the maximum when the cut terminates. Because of this, the cutting force in up milling increases from zero to the maximum per tooth movement of the cutter. Since the cutting force is directed upwards, it tends to lift the workpiece from the table (or the work holding fixture). Further, as the cutting progresses and there is difficulty in pouring coolant on the cutting edge of cutter to flush out chips, there is accumulation of chips at the cutting zone and when chips are carried over with cutter, they spoil the work surface. Since the cutter teeth do not begin cutting as soon as they touch the work surface, in the first instance, sliding of cutter teeth takes place for a small distance on the work surface which results in waviness of the resulting machined surface of the work.

Fig. 6.120(a)

Up milling or conventional milling.

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Down milling or climb milling is shown in Fig. 6.120(b)). It is the process of removing metal by a milling cutter which is rotated in the same direction as the travel of the workpiece. Note the thickness of the chip which is maximum when the cutter tooth begins its cut and it reduces to the minimum (zero) when the cut terminates. In down milling, the cutter tooth starts removing metal as soon as it touches the work surface and without sliding (as in up milling). It will be observed that the cutting force in down milling is also variable throughout the cut, maximum when tooth begins to cut and minimum when tooth leaves the work. Since the cutting force is directed downwards, it tends to seat the work firmly in the fixture which suites more for machining thinner jobs being easily and firmly clamped in fixtures. Coolant is more effectively provided in the process, thereby avoiding overheating and accumulation of chips in cutting area. Better surface finish on the work is attained in climb milling as the cutter takes a chip of zero thickness at the end of the cut. There is, however, a tendency of the cutter to pull the work forwards.

Fig. 6.120(b)

Down milling or climb milling.

Although the down milling process seems to have several advantages, it cannot, however, be conducted on old machines having backlash error between the table feed screw and the nut. The backlash causes the work to be pulled below the cutter when the cut begins and leaves the work free when the cut is terminated. As this action is repeated during machining, vibrations in the machine are set up damaging the work surface considerably. It is because of this that down milling is performed only on those machines which are rigid and have backlash elimination arrangement. (b) Face milling is the operation performed by a face milling cutter to produce a flat machined surface perpendicular to the axis of rotation of the cutter [Fig. 6.121(b)]. In the process, the peripheral cutting edges of the cutter perform the actual cutting (or facing) and the face cutting edges only finish up the work. (c) End milling [Fig. 6.121(e)] is the combination of peripheral and face milling operations. The cutter has teeth both on the end face and on the periphery. When peripheral cutting edges are used for cutting, the direction of rotation and direction of helix of the cutter should be opposite to each other and when only the end cutting edges are used for cutting, the direction of rotation and the direction of helix of the cutter should be the same.

METAL MACHINING—Processes and Machine Tools

Fig. 6.121

6.20.6

435

Different milling operations.

Milling Machine Operations

Although various jobs are performed on a milling machine, the basic milling operations done on them fall under the following categories: 1. Plain milling or slab milling or peripheral milling in which a flat surface is produced by a rotating cutter with its axis parallel to the surface being machined [Fig. 6.121(a)]. Up milling and down milling are two types of plain milling.

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MANUFACTURING PROCESSES

2. Face milling in which a flat surface is produced at right angle to the axis of the cutter [Fig. 6.121(b)]. 3. Side milling in which a flat vertical surface is produced on the side of a workpiece using a side milling cutter. 4. Straddle milling in which flat vertical surfaces on both sides of a workpiece are produced using two side milling cutters mounted on the same arbor [Fig. 6.121(c)]. Distance between the two cutters is adjusted by using suitable spacing collars. Straddle milling is commonly used for making square or hexagonal surfaces. 5. Gang milling in which several plain milling cutters of same or different diameters and width may be used at the same time for production of several different parallel horizontal surfaces on the workpieces [Fig. 6.121(d)]. 6. End milling in which narrow slots, grooves or keyways are produced using an end mill cutter on a vertical milling machine [Fig. 6.121(e)]. 7. Angular milling in which angular surfaces such as angular grooves on V-blocks are produced using an angle milling cutter [Fig. 6.121(f)]. 8. Form milling in which irregular contours are made using form cutters [Fig. 6.121(g)]. 9. T-slot milling in which first a plain slot is cut on the work surface using a side and face milling cutter. Then the T-slot is cut feeding the T-slot cutter from one end of the plain slot [Fig. 6.121(h)]. 10. Dovetail milling in which dovetail surfaces are made using a dovetail cutter [Fig. 6.121(i)]. 11. Saw milling is the operation of production of narrow slot and grooves using a saw milling cutter. The process can be used for complete parting off also [Fig. 6.121(j)]. Besides the above, milling keyways, slots and grooves, cutting of all types of gears and worm wheels and milling of cam profiles, thread milling, helical milling of flutes in drills, etc. are also performed on milling machines.

6.20.7

Milling Cutters

A milling cutter is a revolving tool having one or several cutting edges or teeth of identical form equally spaced on the periphery of the cutter. The cutting teeth intermittently engage the workpiece and remove metal by relative movement of the cutter and workpiece. High speed steel or carbide milling cutters are quite common. These cutters may be solid (teeth integral with cutter body), inserted teeth or tipped solid type. Arbor type cutters have a central hole and keyway for mounting them on the arbor during use. Shank type cutters have straight or taper shanks directly inserted in the spindle nose for use. Facing type cutters are either bolted or attached directly to the spindle nose. Cutters may be right hand or left hand type. There are standard milling cutters which are conventional type and their dimension and details are standardized. Only these cutters and their types have been discussed in the following. Types of standard milling cutters

∑ Plain milling cutter ∑ Side milling cutter

METAL MACHINING—Processes and Machine Tools

∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑

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Face milling cutter Metal slitting saws Angle milling cutter End mills Shell end mills T-slot milling cutter Woodruff key slot milling cutter Formed cutter Involute gear cutter Fly cutter Thread milling cutter Tap and reamer cutter

1. Plain milling cutters have, on their periphery, straight teeth for light-duty operations and helical teeth for heavy-duty operations and wider cutters are used to mill flat surfaces parallel to the cutter axis [Fig. 6.122(a)]. 2. Side milling cutter [Fig. 6.122(b)] has teeth on the periphery and on one or both sides of the tool. Plain side milling cutters have teeth on periphery and both sides of the cutter. Staggered tooth side milling cutters are used for milling deep slots as they have teeth with alternating helix. Interlocking side milling cutters are made from two halves (i.e. two side milling cutters having teeth on one side). These cutters are used for milling wider slots of accurate width. 3. Face milling cutter of shell-end-mill type is shown in Fig. 6.122(c). It has teeth on both face and periphery. It is a general purpose facing tool. For facing bigger surfaces, inserted tooth facing cutter is employed which has cutting edge made of superior cutting tool material and inserted in the steel shank. 4. Metal slitting saws used for cutting off and slotting operations are plain metal slitting type [Fig. 6.122(d)] having teeth saw-tooth form on the periphery and used for fine slitting (up to 4.75 mm). Staggered teeth metal slitting cutters have their teeth staggered at periphery with alternating helix for slitting width up to 6.35 mm usually. The side teeth slitting cutters [Fig. 6.122(e)] are used for cutting deep and wider slots. They have teeth on both periphery and sides. 5. Angle milling cutters may be single or double angle cutters and are used to machine angles other than 90°. The cutting edges are formed at the conical surface around the periphery of the cutter [Fig. 6.122(f)]. 6. End mills have cutting teeth on the end as well as on the periphery. The teeth may be straight or helical (right handed or left handed). These are used for cutting slots, holes and narrow flat surfaces. End mill cutter may be taper shank or straight shank type. Taper shank end mill cutters may be double or multiple fluted having diameter from 10 to 63 mm. A right handed straight tooth end mill cutter is shown in Fig. 6.122(g) and a straight shank end mill cutter with helical teeth is shown in Fig. 6.122(h).

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7. Shell end mills have large diameters 40 to 160 mm and width 32 to 63 mm with bore 16 to 50 mm for mounting on a short arbor. They have teeth on periphery and at the end, teeth may be straight or helical, right or left handed. Shell end mills [Fig. 6.122(c)] are used for facing operations. 8. T-slot milling cutters are special forms of end mills for making T-slots [Fig. 6.122(i)]. 9. Woodruff key slot milling cutter is used for cutting woodruff slots and is similar to a thin small diameter plain milling cutter [Fig. 6.122(j)]. 10. Formed cutters are used to mill irregular surfaces similar to that formed on the periphery of various cutters, e.g. concave, convex or corner rounding shapes [Fig. 6.122(k)]. 11. Involute gear cutter, used for cutting gears, has formed edges which reproduce the shape of the cutter teeth on the gear blank. Cutter teeth may be of involute or cycloidal profile. An involute gear cutter is shown in Fig. 6.122(l). 12. Fly cutter is the simplest cutter as it may be just a cutting steel strip formed to a shape and hardened for light cutting. It is often used for experimental purpose or emergency when regular tools are not available. It is basically a formed cutter [Fig. 6.122(m)]. 13. Thread milling cutters mill threads of specific forms and worms. 14. Tap and reamer cutters are double angle cutters and used for making flutes and grooves in taps and reamers.

Fig. 6.122

(Contd.)

METAL MACHINING—Processes and Machine Tools

Fig. 6.122

6.20.8

439

Types of standard milling cutters.

Elements of a Plain Milling Cutter

Principal parts of a plain milling cutter are shown in Fig. 6.123. Land is the part of the back of tooth adjacent to the cutting edge which is relieved to avoid interference between the surface being machined and the cutter. A milling cutter may have tooth of three forms: (a) saw tooth, (b) form tooth and (c) inserted tooth. Saw tooth form is mostly used in plain cutters used for metal sitting and end mills. Form tooth cutters have different irregular shapes formed to their cutting edges. In case of inserted tooth cutters, cutting blades may be of high speed steel or carbide and are inserted and held rigidly on steel shank (Fig. 6.124).

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MANUFACTURING PROCESSES

Fig. 6.123

Fig. 6.124

Elements of a plain milling cutter.

Slabbing cutter (inserted tool bit type).

Cutter angles have great influence on the performance of a cutter. Various cutter angles of a plain milling cutter are described in the following: Helix angle is the cutting edge angle which a helical cutting edge makes with a plane containing the axis of a cylindrical cutter (Fig. 6.125). A cutter with helical teeth is preferred to plain saw-tooth type cutter having its teeth straight and parallel to its axis because the teeth of plain cutter strike the work simultaneously across the entire width of the cutter resulting in a hammering action. The milling operation is not smooth. In case of a helical cutter, the tooth engages with the work progressively, thereby giving a smooth and continuous action. It results in reduced power consumption and silent and smooth machining.

METAL MACHINING—Processes and Machine Tools

Fig. 6.125

441

Helix angle (a).

Relief angle helps in avoiding interference between the land of tooth and the work surface. This angle should be large enough to give less friction and wear on the tool with increased tool life and surface finish on work. The relief angle is kept about 7° for machining cast iron, 5° for steel and 15° for brass. Clearance angles are of two types: (a) primary clearance angle and (b) secondary clearance angle. The tooth land width may increase after repeated sharpening of the cutter and so the clearance angles are provided for maintaining the land width. Lip angle should be large enough because the larger the lip angle, the stronger will be the tooth. Its value depends on the rake and relief angles of the cutter. Rake angle (radial) serves the same purpose as served in case of a lathe tool. Rake angle on a milling cutter may be positive or negative (Fig. 6.126). High speed steel cutters are given positive rake generally, whereas the milling cutters with carbide tips have negative rake, the usual value of which is 10°.

Fig. 6.126

Milling cutter with rake angles positive, zero and negative.

Negative rake milling has the following advantages: ∑ Since at higher cutting speeds rake angle has little influence on the cutting pressure, hence higher cutting speeds are used with negative rake yielding higher production. ∑ Cutting forces in negative rake decrease as cutting speed increases. ∑ Tool tip becomes stronger to withstand higher forces effectively. ∑ No coolant is needed. Heat generated in cutting helps decreasing the cutting forces. ∑ Formation of built-up edge is avoided due to higher cutting speed. ∑ Chances of crater failure of tool are reduced.

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∑ Excellent finish is obtained. ∑ Disadvantages of machine speeds should be avoided as smooth machining is ensured by uniform cutting speed only.

6.20.9

Size of Milling Machine

Let us take the example of a column and knee type machine. Its size is given by the size of the table (length and width) and its maximum length of longitudinal, cross and vertical feed of the table. In addition to this, available numbers of spindle speeds, feeds, spindle nose taper, motor power, weight of machine, etc. are also considered.

6.20.10 Cutter Holding Devices The following devices are commonly used for mounting milling cutters on the spindle. 1. Arbor is a short shaft connected with the spindle by a draw bolt (Fig. 6.115) and cutters are mounted and keyed on the arbor. 2. Collet [Fig. 6.127(a)] is just a type of sleeve used for reducing the size of the taper hole at the nose of the spindle to help accommodating mounting of smaller size cutters with tapered shank. Spring collets are used to hold straight shank cutters [Fig. 6.127(b)]. 3. Adapter is also a form of the collet used on milling machines having standard spindle end. Cutters with shanks are mounted in the adapter (Fig. 6.128).

Fig. 6.127(a)

Fig. 6.127(b)

Milling machine collet.

Spring collet. The nose end of adapter is split by three slots to give spring action.

Fig. 6.128

Milling machine adapter.

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4. Bolting system is shown in Fig. 6.129 wherein a larger diameter face milling cutter is bolted with the spindle nose.

Fig. 6.129

Milling cutter bolted with spindle nose.

5. Screwed on cutters include small size cutters having threaded bore for being easily screwed on to the threaded nose of the arbor fitted with the spindle.

6.20.11 Work Holding Devices Work holding devices used on a milling machine include: ∑ Vices which may be a plain vice, swivel vice and tool maker’s universal vice. Plain vice is similar to parallel jaw vice. Swivel vice has a graduated circular base for rotating and adjusting the vice in suitable positions in a horizontal plane. A tool maker’s universal vice helps in adjusting the vice at different positions in a vertical plane (Fig. 6.130). ∑ Angle plates ∑ V-blocks ∑ Special fixtures

Fig. 6.130

Universal vice.

6.20.12 Milling Machine Attachments Special attachments are used on milling machine for performing different typical operations. Common attachments include: (a) Vertical milling attachment is used to facilitate the horizontal spindle milling machine to do facing on horizontal surfaces or for making grooves (Fig. 6.131). (b) Universal milling attachment is similar to vertical attachment with added features for swivelling the spindle about two mutually perpendicular axes.

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(c) Slotting attachment (Fig. 6.132) is used for converting rotational motion of the horizontal spindle machine into up and down vertical reciprocating motions. The slotting head is fitted to the spindle and is used for cutting slots. The attachment can be swivelled to any angular position.

Fig. 6.131

Vertical milling attachment.

Fig. 6.132

Slotting attachment.

(d) High speed milling attachment is a gearing system to help increasing the regular spindle speed by four to six times. The attachment is bolted to the face of the column. (e) Rack milling attachment is bolted to the face of the column when the job is fitted on the table. It is used for cutting rake teeth. The attachment has a gear train which enables the spindle axis to be set at right angles to the machine spindle in the horizontal plane to help cutting the teeth. (f) Circular milling attachment (Fig. 6.133) is a rotary table type work holding device bolted on the table. It provides a rotary motion to the workpiece in addition to longitudinal, cross and vertical motions.

Fig. 6.133

Hand-operated circular milling attachment.

(g) Dividing head attachment is used to change the angular position of the work in relation to the cutter. Operation like cutting a gear requires that the blank should be rotated through certain degrees after cutting one tooth so that gear blank may be brought in position for cutting the next subsequent tooth. The work is mounted on the dividing head which is bolted with the table. The work can be held in a chuck (with dividing head) or between centres of dividing head and foot stock (similar to tail stock). The attachment is used for cutting gears, flutes on twist drills, etc. The detailed description of the dividing head will be given later.

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6.20.13 Cutting Speed, Feed and Depth of Cut Cutting speed (V) is the peripheral speed of the cutter in m/min. p DN Cutting speed (V) = , m/min (6.90) 1000 where D = cutter diameter, mm N = cutter speed, rpm The cutting speed in a milling machine depends on work material, cutter diameter and number of cutter teeth, feed, depth of cut, width of cutter and use of coolant. Feed (f ) is defined as the movement of work relative to the cutter axis and is the rate at which the work is being fed to the cutter. Feed in milling operation is expressed in the following three ways: (i) Feed per tooth (fz), mm per tooth of cutter (ii) Feed per revolution (frev), mm per revolution of cutter (iii) Feed per minute (fm), mm, per minute The above three feeds are related as follows: (6.91) fm = N ¥ frev = fz ¥ Z ¥ N where Z = number of teeth in cutter N = cutter speed, rpm For milling mild steel, feed may vary from 0.03 mm/tooth to 0.25 mm/tooth. Highest possible feed should be employed for rough cutting. Depth of cut is defined as the thickness of the layer of the material removed in one pass of the work under the cutter. For common roughing, the depth of the cut may be up to 8 mm and that for finishing less than 1.5 mm. Material removal rate (MRR) is the volume of metal removed in unit time. MRR (mm3/min) = B ¥ d ¥ f (6.92) where B = width of cut, mm d = depth of cut, mm f = feed rate, mm/min Machining time (tm) is the time required for one pass of the width of the cut (B) for milling a surface and is given as: length of cut Machining time tm = feed rate =

L L = f fz ¥ Z ¥ N

where tm = time required to complete the cut in one pass, min L = length of the table travel to complete the cut, mm fz = feed per tooth, mm Z = number of teeth in a cutter N = rpm of cutter

(6.93)

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MANUFACTURING PROCESSES

Plain milling (Fig. 6.134) Let Lj = job length, mm la = approach length, mm lo = overrun of cutter, mm D = cutter diameter, mm d = depth of cut, mm B = width of work, mm

Fig. 6.134

Then,

Plain milling.

L = Lj + la + lo Approach length (la) =

d ( D - d ) , mm

(6.94)

Face milling (Fig. 6.135)

Approach length (la) = 0.5 ( D - D 2 - B2 ) , mm

Fig. 6.135

(6.95)

Face milling.

The overrun length of cutter (lo) can be taken from 1 to 6 mm. Total machining time (tm) =

L¥n , min f

where n = number of cuts required f = feed, mm/min L = length of table travel to complete the cut, mm

(6.95)

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6.20.14 Force System in Milling A milling machine removes metal with a multi-teeth cutter but note that all the teeth of the cutter do not perform cutting simultaneously; at a particular time, some teeth may be fully engaged in cutting, some engaged partially while others may not be engaged at all. Because of this, a continuous chip of uniform thickness is not formed and also the tool geometry keeps changing during cutting. This results in the generation of vibrations in machine tool, variation in cutting speed and poor finish on work surface. Tool life is also reduced as cutting force keeps varying. Consider the force acting on the cutting edge of a tooth of a slab milling cutter (Fig. 6.136). These include cutting force to shear metal, force required to overcome friction between margin of the cutter and the workpiece as also between chip and face of milling cutter. If the cutter is having helical teeth, then an axial force will also be there. In Fig. 6.136, P = peripheral or tangential force Pr = radial force R = resultant force

Fig. 6.136

Force system in milling operation.

The resultant force R is further resolved as: Ph = horizontal component Pv = vertical component Pa = axial thrust Then,

R=

Ph2 + Pv2 + Pa2

(6.97)

Power for milling

Chip thickness in milling varies from 0 to maximum or maximum to zero, depending on the method of milling. As a result of this, the cutting resistance of metal also varies. For finding

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the power required for milling, determination of mean cutting torque is necessary which is based on the mean or average cutting pressure or specific cutting pressure (i.e. cutting force per unit area of the chip cross-section). Mean or average cutting pressure (Pm) =

P Am

where P = tangential component Am = average area of chip cross-section P = P m ¥ Am

or

(6.98)

In the above equation, radial component Pr is not considered as it does not effect the torque. Further, Am =

b¥d¥ f 1000 ¥ V

(6.99)

P ¥V 75 ¥ 60

(6.100)

where b = width of cut, mm d = depth of cut, mm f = feed rate, mm/min V = cutter speed, m/min Power required for cutting is given by: Power (HP) =

The values of b, d, f and V may be known to find the value of Am from Eqn. (6.99). Pm can be found from standard tables corresponding to the value of maximum chip thickness which is given by: 2¥ f ¥ d ( D - d ), mm (6.101) Maximum chip thickness (tmax) = N¥Z¥D where D = cutter dia, mm d = depth of cut, mm Z = number of teeth in cutter N = rpm of cutter f = feed rate, mm/min Example 6.41: With a metal removal rate of 20 cm3/min, depth of cut 5 mm and width of cut 90 mm in milling operation, find out the table feed. Solution:

Metal removal rate (MRR) = depth of cut (d) ¥ width of cut (b) ¥ feed rate (f) 20 ¥ 103 = 5 ¥ 90 ¥ f

or feed (f ) =

20 ¥ 103 = 44.44 mm/min 5 ¥ 90

(Ans.)

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449

Example 6.42: A slot of 30 mm ¥ 30 mm is to be milled in a workpiece of 300 mm length using a side and face milling cutter of diameter 100 mm, width 30 mm and having teeth 20. Taking depth of cut 5 mm, feed per tooth 0.1 mm, cutting speed 35 m/min and over travel distance of 5 mm, calculate the time required for milling the slot. Solution: Given: Lj = 300 mm; D = 100 mm, Z = 20 teeth; d = 5 mm; feed per teeth (fz) = 0.1 mm; V = 35 m/min; over travel (lo) = 5 mm Machining time (tm) = where L = la + Lj + over travel and la =

L f

d ( D - d ) = 5(100 - 5)

= 21.79 mm Then L = 21.79 + 300 + 5 = 326.79 mm Feed rate (f ) = fz ¥ Z ¥ N = 0.1 ¥ 20 ¥ N = 0.1 ¥ 20 ¥ 92.28 = 184.56 mm/min where Then

N= machining time (Tm) =

100 ◊ V 1000 ¥ 35 = = 92.28 rpm pD p ¥ 120 L 326.79 = 1.77 min = f 184.56

(Ans.)

Example 6.43: A slot of 20 mm width and length 100 mm is being milled in a plate 100 ¥ 200 mm size with cutting speed 55 m/min, end mill cutter diameter 20 mm, number of flutes 8, feed per flute 0.01 mm, depth of cut 3 mm, find out the machining time per pass. Solution:

Given: Lj = 100 mm, V = 55 mm/min, D = 20 mm number of flutes = 8, feed per flute = 0.01 mm, d = 3 mm

D 20 = = 10 mm 2 2 Cutter over travel (lo) = 5 mm (say)

Here, cutter approach (la) =

Spindle rpm (N) =

100 ◊ V 1000 ¥ 55 = = 875.79 = 876 p ◊D p ¥ 20

Table feed (f), mm/min = fz ¥ No. of flutes ¥ N = 0.01 ¥ 8 ¥ 876 = 70 Machining time (tm) =

L la + lo + L j 10 + 5 + 100 = = 70 f f

= 1.64 min

(Ans.)

6.20.15 Dividing Head or Indexing Head The most familiar operations performed on a milling machine include forming of regular shapes such as squares, hexagons or octagons, cutting of flutes on drills and reamers or

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MANUFACTURING PROCESSES

cutting teeth on a gear blank. All these need rotation or indexing of workpiece through a certain angle between successive cuts. Indexing is, therefore, the operation of dividing the periphery of a work into any number of equal parts. Indexing is accomplished by using a special attachment known as dividing head or indexing head. Dividing heads are of three types: (i) plain or simple dividing head, (ii) universal dividing head and (iii) optical dividing head. Use of dividing head for cutting a spur gear is shown in Fig. 15.11. Plain or simple dividing head: Elements of a plain dividing head are shown in Fig. 6.137. It comprises a spindle which at its nose carries job holding devices such as three jaw chuck, face plate with centre and dog carrier. A worm wheel is rigidly fixed on spindle while an index crank is rigidly mounted on the worm shaft such that the rotation of index crank finally results in the rotation of the spindle (and thus the job mounted on the spindle). In a plain dividing head, its spindle rotates only around a horizontal axis.

Fig. 6.137

Elements of plain dividing head.

To rotate a job through a required angle, one needs (i) a device to rotate the job (here an index crank) and (ii) a source which can ensure that the job has been rotated through the desired angle (here an index plate). The index plate remains fixed and does not rotate while performing simple indexing operation. The amount of rotation of the spindle relative to the worm depends on the ratio between the revolutions of worm and the worm wheel. The most common ratio is 40 to 1 which means that 40 revolutions of index crank or worm will move the worm wheel or spindle through one complete revolution. Ratios other than this are also available with dividing heads. The index plate has several circles with different number of holes in each circle. Details of a typical index plate are given below: Index plate No. 1:

On one side, 49, 39, 29, 19 and 16 hole circles On the other side, 47, 37, 27, 17 and 15 hole circles

Index plate No. 2:

On one side, 43, 33, 23 and 20 hole circles On the other side, 41, 31, 21 and 18 hole circles

Index plate with several other combinations of hole circles are also available. Index pin is spring loaded and after every indexing it is set in the required hole of the appropriate circle. The index pin may move in the slot of the index crank to fit over a required hole circle. Two arms of the adjustable sector may be opened and locked to accommodate a required number of holes between them. Worm and worm wheel assembly are properly

METAL MACHINING—Processes and Machine Tools

451

accommodated inside the housing. The dividing head is mounted on the milling machine table such that the job to be indexed comes directly under the cutter (Fig. 15.11). Universal dividing head is the most commonly used type of attachment on a milling machine. It is used for (i) setting the work in horizontal, vertical or inclined positions relative to the milling machine table, (ii) turning the work periodically through a given angle for performing indexing of the work and (iii) imparting a continuous rotary motion to the workpiece for milling helical grooves. Dividing head spindle can be connected with the table feed screw through a gear train [Fig. 6.139(c)] to impart a continuous rotary motion to the workpiece for helical milling. Main elements of a universal dividing head are shown in Fig. 6.138(a) and setting the dividing head spindle at an angle is shown in Fig. 6.138(b).

Fig. 6.138(a)

Working mechanism of a universal dividing head. Crank is rigidly fixed at one end of worm shaft while the bevel gear (B) runs free on worm shaft. Index plate is bolted with gear (B) and can be locked against rotation with lock pin.

Fig. 6.138(b)

Universal dividing head showing spindle set at an angle.

Optical dividing head is used for high precision angular indexing of the job with respect to the cutter. For reading the angle, an optical system is built into the dividing head.

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MANUFACTURING PROCESSES

6.20.16 Methods of Indexing Following are the common methods of indexing: (i) Direct indexing (ii) Plain or simple indexing (iii) Differential indexing (iv) Angular indexing A dividing head with 40 to 1 ratio will be referred in all the discussions and calculations for different methods of indexing discussed in the following: (i) Direct indexing: For direct indexing, a dividing head must have an index plate directly fitted on the spindle (avoiding worm and worm wheel). Index plate for such a purpose has 24 holes. Crank may be rotated to divide the periphery of the job into the divisions 2, 3, 4, 6, 8, 12 and 24 directly. Such indexing is often used on indexing fixtures. (ii) Plain or simple indexing: General set-up of a dividing head is shown in Fig. 6.137. Different index plates with varying number of holes may be used to increase the range of indexing. Use the following relation for finding the angular movement of the crank. 40 (6.102) N where A gives the number of turns or parts of a turn through which the index crank must be rotated to obtain the required number of divisions (N) of the job periphery. The following rules will guide further. (a) If A is a whole number, then it denotes the number of full turns the crank is to be rotated for indexing one division. Start from any hole on the index plate and come back to the same hole for each full turn of the crank. (b) If A is only a fraction, use an index plate having a number of holes corresponding to the denominator of the fraction while the numerator will show the number of holes over which the crank is to be moved to index one division of the job. (c) If A is a whole number along with a fraction, then the whole number will denote the full turns through which the index crank should be rotated first and then for further rotation of the crank corresponding to the remaining fractional part of A, use rule (b) above. A=

These three cases will be clear from the following numerical examples: Example 6.44:

Indexing for 5 divisions.

40 40 = =8 N 5 So, rotate the crank 8 complete turns to index the job through one division.

Solution:

Here,

Example 6.45:

A=

Indexing for 120 divisions.

40 1 = 120 3 Multiply 1/3 by appropriate numbers so that the denominator should correspond to the available hole circles.

Solution:

Here,

A=

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453

1 10 11 14 and so on. = or or 3 30 33 42 Rotate the crank over 10, 11 or 14 holes in the hole circles of 30 or 33 or 42 holes, respectively. Any of these solutions may be used to index the job by one division.

So,

Example 6.46: Solution:

Indexing for six divisions.

Here, A =

40 4 = 6 in which 6 is a whole number and 4/6 is a fraction. 6 6

4 2 20 22 26 = = = = and so on. 6 3 30 33 39 One solution out of the several above is in which the crank will be rotated through 6 complete turns plus 20 holes on a 30 hole circle of the index plate.

(iii) Differential indexing: Available number of index plates with different hole circles sometimes limit the range of plain indexing. In such cases differential indexing is found useful. Between the indexing plate and the spindle of the dividing head, a certain set of change gears is incorporated. Dividing heads are provided with such standard set of gears. A standard dividing head was found to have the change gears with the following number of teeth. 14 (2 nos.), 28, 32, 40, 44, 48, 56, 64, 72, 86 and 100 Sometimes gears of 46, 47, 52, 58, 68, 70, 76 and 84 teeth are also provided. In the process of differential indexing, the index plate rotates itself in relation to the crank during the process of indexing. Therefore, the index plate locking pin, which was kept locked with the index plate while performing plain indexing, should be disengaged lest it might not be sheared off during the process of differential indexing. For making necessary calculations to find the change gears to be placed between the spindle and the worm shaft, use the following relation: Driver 40 = (n - N ) ¥ (6.103) Driven n where N is the number of divisions to be indexed and n is a number slightly greater or less than N. After simplification, the above relation will give the gear ratio between the gears to be placed on the spindle (Driver) and the worm shaft (Driven). Gears may be arranged in a simple train or in a compound train as the case may be. Because of the difference between N and n, the index plate will rotate itself while indexing, in a proper direction relative to the crank. Remember: When (n – N) is positive, the index plate must rotate in the direction in which the crank is rotated. If (n – N) is negative, the index plate must rotate in the opposite direction to that of the crank. Few rules are given below for finding the number of idler gears to be used with the change gear set. (a) With simple wheel train and (n – N) positive, use one idler gear. (b) With simple wheel train and (n – N) negative, use two idler gears.

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MANUFACTURING PROCESSES

(c) With compound wheel train and (n – N) positive, use no idler gear. (d) With compound wheel train and (n – N) negative, use one idler gear. For finding the movement of the index crank, simplify 40/n and multiply the numerator and denominator by such a number that the denominator gives a suitable hole circle and the numerator gives the number of holes to be passed over by the crank out of that circle. To make sure that the calculations made for the change gears and the crank movements are correct, apply the following rules: (i) Using a simple wheel train with one idler or a compound wheel train with no idler, the solution will be correct if: N = 40M – RM (ii) Using a simple wheel train with two idlers or a compound wheel train with one idler, the solution will be correct if: N = 40M + RM where N = Number of divisions to be indexed on the job M =

Hole circle chosen No. of holes in the circle

R=

Driver Driven

Example 6.47: Solution:

Indexing for 73 divisions.

Let us take n = 75 The crank movement =

40 40 8 8 ¥ 2 16 = = = = n 75 15 15 ¥ 2 30

or 16 holes from a 30-hole circle Gear ratio =

Driver 40 = (n - N ) ¥ Driven n

40 16 = 75 15 Using the standard gears out of the set mentioned above, we will have to try a compound

= (75 – 73) ¥

train. Driver 16 4 ¥ 4 4 8 4 14 = = = ¥ ¥ ¥ Driven 15 3 ¥ 5 3 8 5 14 32 56 ¥ 24 70 Thus, the gears of 32, 24, 56 and 70 teeth will be required. Spindle gear A(32T), second gear on stud C(24T), first gear on stud B(56T) and gear on worm D(70T) have been shown arranged in Fig. 6.139. =

METAL MACHINING—Processes and Machine Tools

Fig. 6.139

455

Compound gear train for differential indexing.

Here, (n – N) is positive and compound wheel train is used, so no idler will be used in the set of change gears. The solution will be written as: 16 holes Crank movement = 30-hole circles Driver 32 56 ; use no idler = ¥ Driven 24 70 Check: Here,

M =

16 30 and R = 15 16

30 16 30 ¥ = 73 which is equal to N 16 5 16 So, the solution is correct.

Then, 40 ¥

Example 6.48: Solution:

Indexing for 55 divisions.

Here, let n = 54 Then, crank movement =

40 40 20 = = n 54 27

i.e. 20 holes on 27-hole circles Driver 40 40 = (n - N ) ¥ (54 - 55) ¥ Driven n 54 =-

20 4 ¥ 5 4 ¥ 8 5 8 32 40 = = ¥ ¥ = ¥ 27 0 ¥ 3 9 ¥ 8 3 8 72 24

So, the change gears 32, 72, 40 and 24 teeth will be needed. As (n – N) is negative and compound wheel train is arranged, so one idler will be used. The solution will be written as: Crank movement =

20 holes 27-hole circles

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MANUFACTURING PROCESSES

Driver 32 40 ; use one idler = ¥ Driven 72 24

Check:

Here,

M=

27 20 and R = 20 27

27 20 27 + ¥ = 55 which is equal to N 20 27 20 So, the solution is correct.

Then, 40 ¥

(iv) Angular indexing: Instead of rotating the job through certain divisions of its periphery, it may sometimes be required to rotate it through certain angle. Angular indexing is used for that. Since 40 turns of index crank turn the work one complete revolution or 360°, then one turn of crank will rotate the job 360/40 = 9°. So rotation of the job is through 9° or 540 minutes or 32,400 seconds. To determine crank movement for angular indexing of the job, divide the angle (through which the job is to be rotated) by 9. In case the angle is in minutes or seconds, then divide the angle by 540 or 32,400, respectively. Example 6.49:

Indexing through 27°. 27 = 3 full turns of crank. 9 Indexing through 42° – 40¢.

Crank movement =

Solution:

Example 6.50:

Here, 42° – 40¢ = 42 ¥ 60 + 40 = 2560¢

Solution:

2560 20 =4 540 27 i.e. 4 full turns plus 20 holes on a 27-hole circle.

Crank movement =

1 Indexing through 8 ∞ . 4

Example 6.51(a): Solution:

8

1 4 1440 7 = 360 ¥ = = 43 divisions 4 33 33 11

Take 45 divisions. Spindle indexing for 45 divisions = Movement for 43 Now 40 – 38 Hence, 1

40 8 = 16 holes on a 18-hole circle = 45 9

7 Ê 480 ˆ 480 8 1280 26 ¥ = = 38 ÁË ˜¯ divisions = 11 11 11 9 33 33

26 7 =1 33 33

7 turns to be gained. 33

METAL MACHINING—Processes and Machine Tools

Gear ratio = ( NI - N ) ¥ =

457

40 Ê 7 ˆ 40 = Á 45 - 43 ˜ ¥ NI Ë 11¯ 45

15 40 40 8 ¥ 5 32 ¥ 40 ¥ = = = 11 45 33 11 ¥ 3 44 ¥ 24

No idler is required as the train is compound and turns are to be gained. Proof:

9 ˆ Ê 40 9 ˆ Ê N = 40 M – RM = Á 40 ¥ ˜ - Á ¥ ˜ Ë 8 ¯ Ë 33 8 ¯

= 45 –

15 7 = 43 11 11

6.20.17 Cutting of Helix on Milling Machine Helix is a curve generated by a line drawn or wrapped around a cylinder advancing uniformly along its axis for each revolution. Examples of helix are thread on a bolt or flute on a twist drill. Spiral is similar to helix but instead of being formed on a cylindrical surface, it is formed on a conical surface, for example, a thread on the wood screw. A helix, its lead and helix angle are shown in Fig. 6.139(a).

Fig. 6.139(a)

Tangent of helix angle (a) =

Showing helix angle and lead of a helix.

pD

L where D = out diameter of cylinder = pitch circle diameter in gear L = lead

The two primary conditions in milling helix are that (a) the work should be rotated and (b) fed simultaneously forwards under the rotating cutter fitted on the arbor. These conditions

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MANUFACTURING PROCESSES

are accomplished by connecting the worm shaft of the dividing head to the feed screw of the table through a set of gears. Lead of the milling machine is the distance travelled by the machine table corresponding to one revolution of the dividing head spindle (or work) with no speed change gears fitted between the worm shaft and the table feed screw. In other words, the lead of machine is the same as the lead of the helix it would cut if the worm shaft of the dividing head and the table feed screw were connected to each other by a 1:1 gear ratio. We know that 40 turns of worm are needed to rotate the dividing head spindle through 1 turn. With a 1:1 gear ratio as mentioned above between the work and the spindle, the feed screw will also rotate through 40 turns during the above period. Let ‘p’ mm be the pitch of feed screw, the table during the above period would have advanced through 40 pitch distance of feed screw. Thus, lead of machine = 40 ¥ p, mm Since feed screw pitch is a constant quantity, lead of machine will also be a constant. Milling helix

The set-up for a milling helix is shown in Fig. 6.139(b) wherein the table has been swung to a position which is at an inclination equal to the helix angle (a). Thus, the work or table will be fed to the cutter at an inclination of a° to their normal position.

Fig. 6.139(b)

Set-up for milling a helix.

Gearing ratio for cutting helix can be found as follows: Gear ratio = =

Driver Lead of the machine table in mm = Driven Lead of the helix to be cut in mm 40 ¥ lead of the feed screw in mm Lead of the helix to be fut in mm

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459

The above ratio will give either a simple gear train or a compound gear train. The use of idlers can be made. Note that, for cutting the right hand helix, the feed screw and worm shaft should rotate in the same direction and for the left hand helix, they should rotate in the opposite direction. The following should be considered while setting up work for the milling helix. 1. Table has been swung to the required helix angle. 2. Table has been swung in correct direction for the generation of right or left hand helix. 3. Dividing head is placed directly over the table feed screw. 4. Change gears are properly set. 5. Index plate lock pin has been withdrawn before starting the operation so that the index plate is free to revolve as the table moves. Example 6.51(b): It is required to mill 5 helical flutes with a lead of 762 mm. Job diameter is 76.2 mm. Pitch of table feed screw is 6.35 mm. Solution: For indexing 5 divisions on the job periphery, the crank movement will be 40/5 = 8 complete turns of the crank. Tangent of helix (a) =

pD

=

3.14 ¥ 76.2 = 0.314 762

L Then, a = tan–1 0.314 = 17½° So, the milling table will be set at 17½° to the normal position of the table.

Gear ratio =

Then,

Driver Lead of the machine 40 ¥ 6.35 1 = = = Driven Lead of the helix on the job 762 3

1 1 ¥ 24 24 = = 3 3 ¥ 24 72

Put a gear of 24 teeth on the table feed screw (driver) and the gear of 72 teeth on the worm shaft (driven). An idler gear can be used if needed. Gearing arrangement (compound gear train) for helical milling is shown in Fig. 6.139(c).

Fig. 6.139(c)

Gearing arrangement for helical milling.

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MANUFACTURING PROCESSES

A compound gear train is shown connected between the lead screw (2) and gear (10) where the table lead screw (2) is the driver and gear (10) is the driven. Motion of the table lead screw (2) is, thus, transmitted to the mitre gear (10) mounted on the sleeve (12). The index plate (9) is screwed on the other end of the sleeve (12) and the crank pin (13) is kept engaged into any one of the holes on the index plate. During helical milling, the lock pin at the back of the index plate is removed so that motion is carried from the mitre gear (10) to the worm shaft through the index plate (9). This way, the worm gear (8) is made to rotate. Since 40 turns of worm (7) are required to turn the worm gear (8) or the workpiece through one complete revolution, the change gear (1) between the table lead screw (2) and the shaft (11) is so arranged that when shaft (11) rotates through 40 revolutions, the table lead screw (2) will rotate by that number of revolutions which will cause lead screw (2) to move axially equal to the lead of helix to be cut. Thus, when the table is fed equal to the lead of helix being cut, the workpiece is rotated by one complete revolution. Change gear train (1) is arrived at as follows: Driver Lead of the machine = Driven Lead of the work

6.21

GRINDING MACHINES

Grinding is done for removing a very small amount of metal (nearly less than 1 mm) from the workpiece either to bring its dimensions within very close tolerances (± 0.02 mm) or to give a fine finish (0.1 µm Ra) on the work surface. The cutting tool used is an abrasive wheel which is rotated at a very high speed (typically 30 m/sec) and the work is fed to the grinding wheel or the grinding wheel is fed to the work to remove excess stock of metal from the work surface. The grinding wheel is made of fine grains of abrasive materials which project above the periphery of the wheel and are held together by a bonding material such that each individual and irregularly shaped projecting grain acts as a cutting element or a single-point cutting tool [Fig. 6.150)]. The grains during the rotation of the wheel remove very thin chips. As the section of chip removed is small and high cutting speeds are involved, the grinding operation results into a very good finish on the work surface and high accuracy in work dimensions. In most cases of metal cutting, grinding is a finishing operation only. The grinding can be divided into two types: (a) rough grinding often used to grind castings and weldments using portable grinders or pedestal grinder and (b) fine grinding or precision grinding applied to finishing of those materials which are too hard to be machined by other methods of metal cutting. Precision grinding is also used for producing surfaces on the job to attain higher dimensional accuracy and finish. Important features of grinding process include: grinding is an intermittent operation wherein discontinuous chips are produced; wheel has self-sharpening character due to automatic removal of worn-out abrasive grains and exposing of new grains during working; load on individual grain during cutting is non-uniform; effective rake angle of grains is highly negative and operation is associated with high specific cutting energy; high temperatures (above 2000°C) may be involved in grinding.

METAL MACHINING—Processes and Machine Tools

6.21.1

461

Methods of Grinding

In accordance with the type of surface to be ground, main kinds of grinding methods are as follows (Fig. 6.140): (i) External cylindrical grinding produces a straight or tapered surface on a workpiece when it is rotated about its own axis between centres as it passes lengthwise across the face of a revolving grinding wheel. (ii) Internal cylindrical grinding produces internal cylindrical holes and tapers. The work is chucked and rotated on its axis while the grinding wheel rotates against the work. (iii) Surface grinding produces flat surfaces and the work may be ground either by periphery or by end face of the grinding wheel. (iv) Face grinding is a method of grinding vertical flat surfaces and the wheel spindle may be vertical or horizontal. (v) Form grinding is done with specially shaped grinding wheels to grind formed surfaces as gear teeth, threads, splined shafts, dovetails, etc. (vi) Set wheel grinding is a method of grinding short workpieces without changing the cross-setting of the grinding wheel once set. (vii) In-feed or plunge grinding is also a method of grinding very short workpieces and involves the use of a grinding wheel having its face equal to or wider than the length of the surface to be ground and feeding the same into the work with no traversing motion of it. (viii) Centreless grinding is a method of grinding external and internal cylindrical surfaces in which the work is supported among a regulating wheel, a grinding wheel and a work rest blade. (ix) Snagging is an operation of grinding gates, sprues and fins on castings, forgings, removing scale, excess metal from steel billets and weldments. (x) Off-hand grinding is a rough grinding method in which work is held in hand and pressed against the rotating grinding wheel, for example, grinding a chisel on pedestal grinder. (xi) Creep feed grinding is another method of grinding in which a soft grinding wheel is used. The wheel revolves in position while the work is fed past the wheel at a very slow speed such that the material is removed only in a single pass. Ample amount of coolant is, however, used in this method. Some of the more commonly used grinding methods have been illustrated in Fig. 6.140.

Fig. 6.140 Showing different methods of grinding. A, B and C represent surface grinding, D represents face grinding, E, F and G show cylindrical grinding, and H and I show internal grinding.

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MANUFACTURING PROCESSES

6.21.2

Types of Grinding Machines

Grinding (a) (c) (e) (g)

machines may be broadly classified as follows: Portable grinder (b) Flexible shaft grinder Pedestal grinder (d) Surface grinder Cutter and tool grinder (f) Cylindrical grinder Internal grinder (h) Centreless grinder

(a) Portable grinder comprises a single grinding wheel (usually not more than 125 mm dia) mounted on the shaft of an electric motor (Fig. 6.141). It finds use in foundry shop and fabrication shop for general purpose fettling or cleaning of castings or welded structure. Portability of the grinder to the place of grinding is the main feature.

Fig. 6.141

A portable grinder.

(b) Flexible grinder (Fig. 6.142) is mounted on a trolley, and a flexible power shaft, extending from the motor shaft, carries the grinding wheel on its other end. It works the same way as the portable grinder.

Fig. 6.142

A trolley mounted flexible grinder.

(c) Pedestal or bench grinder (Fig. 6.143) is a double ended wheel grinder often used in fitting and machine shops for general purpose grinding, for example, grinding a chisel, sizing a piece of metal, etc. It carries two grinding wheels, one on each end of motor shaft; one wheel is for rough grinding and the other is for fine grinding. A pedestal grinder is just a bench grinder mounted on a pedestal stand made of brick work and concrete usually and of suitable height.

METAL MACHINING—Processes and Machine Tools

Fig. 6.143

463

A pedestal grinder or double-ended bench grinder.

(d) Surface grinder is used for grinding flat surfaces (Fig. 6.144). Surface grinding is effective for removing hard spots and seats from the work surface. Hard spots tend to blunt or impede the cutting tools in other machining methods. Surface grinding machine differs according to the shape of grinding wheel and motion given to the work table during working.

Fig. 6.144

A surface grinder.

Some common types of surface grinding machines are described in the following: 1. Horizontal spindle surface grinding machines [Fig. 6.145(a)] make use of the circumference of a straight grinding wheel and are able to handle a wide range of work needing super finish and extremely fine limits of accuracy.

Fig. 6.145(a)

Horizontal spindle surface grinding machines.

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MANUFACTURING PROCESSES

2. Vertical spindle flat grinding machines [Fig. 6.145(b)] are strongly built machines. They yield more output with cup type wheels than when using a straight wheel.

Fig. 6.145(b)

Vertical spindle flat grinding machines.

3. Disc grinding machines [Fig. 6.145(c)] are used for rough and semi-precision grinding. Rapid removal of metal is the main criterion in using these machines.

Fig. 6.145(c)

Disc type surface grinders.

(e) Cutter and tool grinder (Fig. 6.146) is used for grinding milling cutters, reamers, drills, etc. and is a precision machine often installed in tool rooms.

Fig. 6.146

A universal tool and cutter grinder.

(f) Cylindrical grinder is used primarily for grinding plain cylindrical parts, although it can be used for grinding contoured cylinder tapers, shoulders, fillets, cams and crank shafts. The work is held between centres. Working principle of a plain cylindrical centre type grinder is shown in Fig. 6.147(a) and its special features include (a) work revolves (ii) wheel revolves

METAL MACHINING—Processes and Machine Tools

465

(iii) work passes the wheel and (iv) wheel passes the work. The traverse of the work past the wheel and vice versa is controlled by dogs which cause the table or wheel to reverse at the end of each stroke. Two distinct types of grinding operation done on this type of grinder are (a) traverse grinding and (b) plunge grinding. In traverse grinding, work reciprocates as wheel feeds to produce cylinders longer than the width of the wheel [Fig. 6.147(a)]. In plunge type grinding, the work rotates in a fixed position as the wheel feeds to produce cylinder of a length equal to or shorter than the width of the wheel, as grinding a crank shaft [Fig. 1.47(b)]. Universal type cylindrical grinder is mostly used in tool rooms for grinding tools. The head stock supports the work by means of a dead centre and drives it by means of a dog or chuck. The other end of the job is supported by the tail stock. The head stock can be shrivelled at an angle in a horizontal plane [Fig. 6.147(c)].

Fig. 6.147(a)

Principle of cylindrical grinding (traverse grinding) shown at (i) and block diagram of a plain centre type cylindrical grinder shown at (ii).

Fig. 6.147(b)

Plunge grinding.

Fig. 6.147(c) Showing the movements of different components of a cylindrical grinder (universal type).

(g) Internal grinders are used to finish internal bores which may be straight or tapered, to the correct size, shape and finish. There are three main types of internal grinders: (i) chucking type (ii) planetary type and (iii) centreless internal type. In chucking type grinders, the work is chucked and rotated against the sense of rotation of the work [Fig. 6.148(a)]. In a planetary grinder, the work is mounted on the reciprocating table and is not revolved. Instead, the grinding wheel is given rotary and planetary motions to grind cylindrical holes

466

MANUFACTURING PROCESSES

[Fig. 6.148(b)]. Planetary internal grinders are used to grind holes in large, irregular shape and heavy work. Small wheels are used and hence these grinders run at very high speeds. In centreless internal grinding, work is supported by three rolls, pressure roll, supporting roll and regulating wheel, all the three moving in same direction, as shown in Fig. 6.148(c). The grinding wheel contacts the inside diameter of the work. The pressure roll can be swung aside to permit loading and unloading of work.

Fig. 6.148(a)

Working principle of a chucking type internal grinder.

Fig. 6.148(b)

Fig. 6.148(c)

Planetary internal grinder.

Centreless internal grinding set-up.

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(h) Centreless grinders are basically cylindrical grinders only but they differ from centre-type cylindrical grinders in that the work, instead of being held between centres, is supported by a combination of a grinding wheel, a regulating wheel and a work rest blade [Fig. 6.149(a)]. Work is revolved and traversed across the face to grinding wheel while being supported on the work rest blade. The cutting pressure of the grinding wheel and regulating wheel keeps the workpiece in contact with the work rest blade. As mentioned, the workpiece is not only revolved but also simultaneously given an axial movement by the regulating wheel and guides so as to pass between the wheels. For this, the axis of regulating wheel is inclined at an angle a (2 to 10°) vertically. Amount of metal to be removed determines as to how many times a workpiece has to pass between the wheels. Block diagram of a centreless grinder is given in Fig. 6.149(b).

Fig. 6.149(a)

Principle of through-feed centreless grinding.

Fig. 6.149(b)

Block diagram of a centreless grinder.

Depending on the method of feeding the work to the grinder, centreless grinding is of three types: (i) through-feed grinding, (ii) in-feed grinding and (iii) end-feed grinding. In through-feed grinding, the work has to be completely straight and cylindrical with no collar

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MANUFACTURING PROCESSES

or step. The work is passed completely through the space between the grinding wheel and the regulating wheel [Fig. 6.149(a)]. The process is used for grinding long slender shafts or bars. In in-feed grinding which is similar to form grinding or plunge grinding, the regulating wheel is drawn back so that work may be placed on the work rest blade. Then it is moved in to feed the work against the grinding wheel and the method is used for grinding shoulders, and formed surfaces [Fig. 6.149(c)]. The end-feed grinding is used to produce taper. Either the grinding wheel or the regulating wheel or both are formed to produce a taper [Fig. 6.149(d)]. Work is fed from one end longitudinally and as it advances, its surface is ground till it touches the end stop at farther end.

Fig. 6.149(c)

In-feed centreless grinding.

Fig. 6.149(d)

End-feed centreless grinding.

Centreless grinding has the following advantages: (i) Size of work is easily controlled. (ii) There is no chucking or mounting of work on mandrels or centres. (iii) There is no chattering of work as it is supported throughout its entire length during grinding. (iv) Process is continuous. Large grinding wheels are used to reduce error due to wheel wear. (v) A low order of skill is required to operate the machine. Besides the above, there are many other special grinding machines such as crank shaft grinder, piston grinder, roll grinder, thread grinder, etc.

6.21.3

Size of a Grinding Machine

Grinder size is specified according to: (i) Largest workpiece that can be handled (ii) Width and maximum travel of the table (iii) Power capacity (iv) Wheel diameter (v) Height of grinding head

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6.21.4

469

Grinding Allowance and Tolerance

Metal volume to be removed by grinding depends on the character of the work and the grinding machine used. ∑ On cylindrical grinders, grinding allowance may be 0.15 to 0.8 mm. ∑ On internal grinders, grinding allowance may be 0.1 mm for holes of 3 mm diameter and 0.8 mm for holes of 200 mm diameter. ∑ In centreless grinders, grinding allowance may be 0.25 mm for rough grinding and 0.02 to 0.05 mm for finish grinding. Tolerance as small as 0.02 mm is easily achieved by commercial grinding.

6.21.5

Wet and Dry Grinding

The temperature involved in grinding may be around 2000°C. A wet grinding has a coolant box which spreads a large amount of coolant (usually soda water coolant) over the work and wheel face and sides. This dissipates heat, and wheel life is increased and a good finish obtained on the work surface. On the other hand, dry grinding produces discolouration and burrs. Discolouration of work surface is due to heating. Burrs can be reduced by using wheel of finer grit which produces a lighter burr but the rate of wheel wear is more than wheels having coarser grit.

6.21.6

Grinding Wheel

A grinding wheel is a multi-teeth cutter made up of many hard particles known as ‘abrasives’ which have been crushed to leave sharp edges which do cutting. Abrasive particles or grains are mixed with a suitable binding material or bond which acts as a matrix or holder of these abrasives during the use of grinding wheel. Each individual and irregularly shaped abrasive grain acts as a cutting element (a single-point cutting tool). Figure 6.150 shows the projecting grains of abrasive material held firmly with the grinding wheel by the bond material. These grains, during high speed rotation of the grinding wheel, remove very thin and small chip of workpiece material and this results in a very good finish and high accuracy in dimension of the workpiece.

Fig. 6.150

Close-up of cutting action by abrasive grits during grinding.

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MANUFACTURING PROCESSES

Grinding ratio (G) is indicative of the performance of a grinding wheel and it is given as: Grinding ratio (G) =

Volume of metal removed Volume of wheel wear

For rough grinding, G is less than 10, for fine grinding, it varies between 10 to 60. The greater the grinding ratio, the better is the wheel. Characteristics of grinding wheel include those parameters which influence the performance of a grinding wheel and these are type and size of abrasive material, bond or binding material for abrasives, grade or strength with which bond holds the abrasive grains together and structure (dense or light) of the wheel.

6.21.7 Abrasives An abrasive is a substance that is used for grinding and polishing operations. Abrasives can be grouped as follows: (a) Natural abrasives include sandstone, emery, corundum and diamonds. (b) Artificial abrasives include silicon carbide and aluminium oxide. Natural abrasives: Sandstone or solid quartz is one of the natural abrasive stones. It is now used only for sharpening wood-working tools. Emery is a natural aluminium oxide having alumina 55 to 65%. Corundum is also a natural aluminium oxide (75 to 95%). Both emery and corundum have better hardness and abrasion action than quartz. Diamond is crushed to make abrasive grains for making grinding wheels to grind cemented carbide tools. On account of the lack of uniformity and impurities of these natural abrasives, only a very small percentage of grinding wheels are produced from natural abrasives. These have been successfully replaced by artificial abrasives. Artificial abrasives: Silicon carbide abrasive is made from silicon sand, powdered coke, salt and saw dust. The ‘green grit’ silicon carbide contains at least 97% silicon carbide and ‘black grit silicon carbide’ contains silicon carbide at least 95%. In hardness, it is next to diamond but is not as tough as aluminium oxide. The silicon carbide abrasives are used for grinding materials of low tensile strength such as cemented carbide, stone, ceramic materials, copper, aluminium, grey cast iron, etc. Aluminium oxide (Al2O3) abrasive is made by heating in an electric arc furnace mineral bauxite, hydrated aluminium oxide clay containing silica, iron oxide, titanium oxide, etc. mixed with ground coke and iron filings. It is tough and is not easily fractured. It is used for grinding materials with high tensile strength such as carbon steels, high speed steels, tough bronze, etc.

6.21.8

Grit, Grade and Structure of Wheels

Grain or grit indicates the size of the abrasive grain used in making grinding wheels. Grain size is denoted by a number indicating the number of meshes per linear inch (25.4 mm) of the screen through which the grains pass when they are graded after crushing. Coarser grain size varies from grain size number 10 to 24, medium grain size from 30 to 60, fine grain size from 80 to 180 and very fine grain size from 220 to 600. The grain size required depends on

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the amount of material to be removed, finish required and hardness of material to be ground. Wheels with coarse grains are used for faster removal of metal as also for grinding soft and ductile metals, whereas fine-grained wheels are used where good finish is required or material to be round is hard and brittle. Grade refers to the hardness (not the hardness of grain) or tenacity with which the binding material or bond holds the abrasive grains in place. Grade is denoted by letters A (softest grade) to Z (hardest grade) where terms ‘soft’ and ‘hard’ denote the resistance a bond offers to disruption of abrasives. Soft grade goes from A to H, medium grade from I to P and hard grade from Q to Z. The hard grade wheels are recommended for grinding soft materials and soft grade wheels for grinding hard materials. Structure refers to the relative spacing between the abrasive grains as they are not packed tightly in the wheel but are distributed through the bond. Structure is denoted by the number of cutting edges per unit area of wheel face as well as by the number and size of void spaces between grains. Structure provides chip clearance. Structure may be open or dense type and it is denoted by numbers, for example, dense structure varies from 1 to 8 and open structure from 9 to 15 or still higher. Open structure is suitable for grinding soft, tough and ductile materials and for heavy cuts, whereas dense structure is suitable for grinding brittle materials and for finishing cuts.

6.21.9

Bonds and Bonding Processes

Bond is an adhesive material used to hold abrasive grains together in the form of grinding wheels. The following bonds are commonly used in making grinding wheels of both silicon carbide and aluminium oxide. Vitrified bonding process: For making vitrified wheels, clay (felspar which is a fusible clay) and abrasive grains are thoroughly mixed together with water and the uniform mixture, thus produced, is poured into molds for drying. When the stuff is semi-dried capable of being handled, it is trimmed to correct shape and size. It is later heated or burnt in a kiln like a tile. During burning, the clay vitrifies, i.e. it fuses and forms a porcelain that surrounds and connects the abrasive grains. Over 70% of the grinding wheels are vitrified bonded because this bond imparts uniform structure to the wheel with good strength, high hardness even to the bond and good porosity, and allows high stock removal while maintaining cool cutting. The vitrified bonded wheels are, however, sensitive to impact and are poor in bending. These wheels are raddish brown in colour. They are affected by water, oil, acids or temperature. Vitrified bonded wheels are denoted by the letter ‘V’. Silicate bonded process: In this process, wheels are made by mixing abrasive grains with silicate or soda or water glass. The mixture is packed into molds and dried. The molded wheel blank is baked in a furnace at 260°C for several days. Silicate bonded wheels are light grey in colour. This bond releases the abrasive grains more readily in comparison to vitrified bond. Silicate wheels are waterproof and they give smooth cutting action. These are specially used for grinding edged tools or other operations where heat of grinding is controlled by proper cooling. This bond does not provide a bond as hard as that of a vitrified wheel. Silicate bonded wheels are designated by the letter ‘S’.

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Oxychloride bonding process: In this process, abrasive grains are mixed with oxide and chloride of magnesium and are made in the same way as the vitrified bonded wheels. Both wheels and wheel systems are made by this bond, the latter being used in disc grinding. Since this bond ensures cool cutting, grinding is done dry. These wheels are designated by the letter ‘O’. Resinoid bonding process: In this process, abrasive grains are mixed with synthetic resins and other compounds by molding and heating at 200°C. Wheels bonded with synthetic resins (bakelite and redmanol) are strong and run at very high speeds and are capable of removing metal at faster speed. These are used for precision grinding of cams, rolls, etc. Resinoid wheels are denoted by the letter ‘B’. Shellac bonding process: Wheels made by this process are often called elastic bonded wheels because the elasticity of this bond is greater than that of other types. Abrasives and shellac are mixed in heated containers and later pressed and baked in heated molds at 150°C. Shellac bonded wheels are not recommended for heavy cuts. They are used for grinding hardened steels and cams, thin sections, aluminium pistons and thin wheels for abrasive cutting off machines. A shellac bonded wheel is designated by the letter ‘E’. Rubber bonding process: In this process, abrasive grains are mixed with pure rubber and sulphur, and the mixture is rolled into sheets from which wheels are punched out and later vulcanized. Rubber bonded wheels are more resilient, poor in heat resistance and denser than resinoid bonded wheels. These wheels are used primarily for getting good finish. Extremely thin wheels can be made with this bond. Rubber bonded wheels are designated by the letter ‘R’.

6.21.10 Method of Specifying a Grinding Wheel Marking system for grinding wheels recommended by Bureau of Indian Standards (IS: 551–1989) is as follows: ∑ Prefix—manufacturer’s symbol for its own trade brand for abrasive used (use of prefix is optional) ∑ Abrasive type ∑ Grain or grit size (mesh number) ∑ Grade ∑ Structure (use is optional) ∑ Bond ∑ Suffix—manufacturer’s private marking for the type of wheel or bond (its use is optional) In addition to the above, the wheel size (diameter, thickness or width, bore size) is also given, for example, a wheel with marking as 220 ¥ 25 ¥ 30 W A40 L4 V18 indicates as follows: Wheel dia = 220 mm Wheel thickness = 25 mm Bore = 30 mm W = manufacturer’s profile to abrasive (it is optional)

METAL MACHINING—Processes and Machine Tools

A 40 L 4 V 18

= = = = = =

473

abrasive (Al2O3) grain size medium grade medium structure dense vitrified bond suffix denoting ‘bond type’ of the manufacturer

6.21.11 Common Wheel Shapes The most common shapes of grinding wheels are shown in Fig. 6.151(a). Grinding wheels are made in many shapes and sizes to adapt them for use in different types of grinding machines for performing different classes of work. Shapes of the wheels have been standardized; the more common are straight-side wheels, cylinder wheels, cup wheels and dish wheels. Size of wheels can also be referred to system of key letters so that their dimensional specifications may be written. Among the grinding wheels, straight wheels (type Nos. 1, 5 and 7) are generally used for cylindrical, internal, centreless, and surface grinding. Wheel diameter and width vary greatly depending on the class of work and machine power. Tapered wheel (type No. 4) is used for grinding flat surfaces and grinding is done by the end face of the wheel. Cup wheel (type No. 6) is used for grinding flat surfaces using its face or end, whereas the flaring cup wheel (type No. 11) is used in tool room. Similarly, dish wheel (type No. 12) is also used for precision grinding in tool room. Saucer wheel (type No. 13) is used for sharpening circular or band saws. Segmented wheels are used on vertical spindle, rotary and reciprocating table surface grinders and way grinders (used for machine tool bed way, etc.).

Fig. 6.151(a)

Standard grinding wheel shapes.

For grinding special contours, grinding wheels of straight wheel type are available with large variety of faces such as flat, pointed, convex, concave, etc. Mounted wheels and points [Fig. 6.151(b)] are small grinding wheels (diameter less than 50 mm) mounted firmly on to a steel spindle, mandrel or shank. They are made in various different shapes and sizes so as to enable grinding even in those places which are not easily accessible otherwise. They are used mostly on portable grinders.

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Fig. 6.151(b)

Mounted wheels and points.

6.21.12 Mounting and Balancing of Grinding Wheel Grinding wheels operate at very high cutting speeds and hence sufficient care should be taken while mounting a wheel on machine spindle. A typical method of mounting of grinding wheel is shown in Fig. 6.152. Grinding wheels are supplied with a central hole fitted with lead bush. Each wheel should be inspected for any crack and other defects. The lead bush should be neither loose nor tight on the spindle and also it should not extend beyond the sides of grinding wheel. Flanges should be used on both sides of the wheel and these should be of equal size. Central part of flanges should be relieved for proper holding of the wheel. A blotter (or washer) is used between abutting surfaces of wheel and inner face of flanges. Bolt and nut should be properly tightened.

Fig. 6.152

Mounting of a grinding wheel.

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For avoiding vibrations and chatter and undue wear of machine and to get good finish on the jobs, the grinding wheels should be properly balanced, because they run at very high revolutions. Wheels may be out of balance due to wear in wheel or part breakage of wheel. Large wheels need particular care for balancing. Balancing is usually done in the static position of the wheel by shifting the position of weights on one of the mounting flanges of the wheel and for this, wheel is mounted on mandrel placed on a balancing fixture for finding out the direction in which the weights are to be shifted.

6.21.13 Glazing and Loading of Wheels The sharp cutting points of abrasive grains in a wheel become dull after some use which results in the wheel face becoming smooth. Such a wheel, instead of biting into the work material, gives a rubbing action only. The phenomenon is known as glazing and the affected wheel is called a glazed wheel. This phenomenon happens more with hard wheels operating at higher speeds. The face or cutting edge of the wheel takes a glass-like appearance. Another problem may be that cut particles of the work material adhere with the wheel face. This loses sharpness of cutting edges and the wheel face becomes smooth. This phenomenon is known as loading and the affected wheel is called loaded wheel. A hard bond wheel working at a slower speed for grinding softer materials may suffer from this problem.

6.21.14 Truing and Dressing of Wheels Truing is the operation of changing the shape of the grinding wheel as it becomes worn from an original shape, owing to the breaking away of the abrasive and bond. Truing is done to make the periphery of the wheel concentric with its axis and to make its sides true and this way to recover the lost shape of its face. Truing is in fact done on glazed wheels. Dressing of wheel is done to recover proper cutting action of wheel face by removing the layer of dulled grains or grains clogged with foreign material. Dressing removes the loading of wheel. Although truing and dressing are done with same tools, these are done not for the same purpose. One method of truing a grinding wheel is by the use of a diamond tool, as used in the way as for dressing (discussed in the following). Most popular method is from truing with a crushing roll shaped to the desired profile. The crushing roll is forced against the revolving grinding wheel. Wheels trued by crushing cut faster and run cooler than those trued with a diamond tool as crushing produces a wheel with many sharp grains. Dressing removes the loading by breaking away the glazed surface so that sharp abrasive particles are again presented to work. Various types of dressing methods are used. Star dresser is a common type of wheel dresser (Fig. 6.153). It consists of a number of hardened steel wheels with points on their periphery. During use, the dresser is held against the face of the revolving grinding wheel and moved across the face to dress the whole surface. It is mostly used for dressing coarse-grain wheels. Cylinder dresser consists of solid steel cylinders with helical grooves and are used for dressing wheels used for cast iron or cylindrical grinding. Abrasive sticks are used for shaping wheel faces and dressing their wheels or rough dressing before applying a diamond dresser. Abrasive wheel dresser is silicon carbide grain wheel with vitrified bond. It is used for dressing wheels used for cylindrical and centreless grinding. For

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MANUFACTURING PROCESSES

precision and high finish grinding wheels, small industrial diamonds, known as bort, are used. The diamond or group of diamonds is mounted in a holder and used as shown in Fig. 6.154. To keep the diamond pointed and safe, the holder is held slant at 10° to 15°.

Fig. 6.153

Dressing a grinding wheel.

Fig. 6.154

Wheel dressing with a diamond dresser.

6.21.15 Selection of Grinding Wheels In selecting a grinding wheel for a particular use, the following factors are generally considered. (i) Material to be ground: Work material influences selection of abrasive, grain size, grade, structure and bond. ∑ Aluminium oxide abrasive is recommended for materials of high tensile strength and silicon carbide for low tensile strength materials. ∑ Fine grain is used for hard and brittle work materials and coarse grain for soft ductile materials. ∑ Hard wheel is used for soft material and vice versa. ∑ Dense structure is required for hard and brittle materials and open structure for soft and ductile. ∑ Majority of wheels are made with vitrified bond. Table 6.9 gives a general guide for grit and grade ranges for various classes of work. (ii) Amount of stock to be removed: Coarse-grained wheel is used for fast cutting and fine-grained wheel for fine finish. Also wide spacing is used for fast cutting and close for fine finish. (iii) Area of contact: It influences selection of grit size, grade and structure. Fine grain and close grain spacing are used where area of contact is small and coarse grain and wide spacing where area of contact is large. (iv) Type of grinding machine: It determines the grade of wheel. Heavy rigid machines take softer wheels than the lighter and more flexible types. (v) Wheel speed: It influences the selection of grade and bond. The higher the wheel speed in relation to work speed, the softer the wheel should be. Vitrified bond is usually specified for wheel speeds up to 2000 m/min and shellac or resinoid bonds

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for wheel speeds over 2000 m/min. For different types of grinding, wheel speeds are given in Table 6.10. (vi) Work speed: The work speed in relation to the wheel speed determines hardness of the wheel; the higher the work speed with respect to the wheel speed, the harder the wheel should be. (vii) Machine condition: Problems like wheel spindle loose in its bearings or shaky foundation would necessitate the use of harder wheels. TABLE 6.9

Grit and grade range for various classes of work

Class of work

Grit size

Grade

Fettling Tool grinding General rough work (off hand) Cylindrical Centreless Internal Tool and cutter Surface grinding (segments) Surface grinding (cup/cylinders) Surface grinding (straight wheel)

12–30 30–80 14–30 36–120 46–80 46–60 46–60 20–36 20–36 46–60

Q–T M–Q Q–S J–N J–N H–N I–M G–M G–K H–K

TABLE 6.10

Wheel speed for different types of grinding

(a) Vitrified bonded wheel

Surface speed (m/min)

Cylindrical Surface Internal Tool and cutter Centreless snagging (b) Resinoid bonded wheel Snagging

1500–2000 1200–1500 600–1800 1500–2000 1500–1800 2000–3000

6.21.16 Cutting Speed, Feed and Machining Time Cutting speed (v) is the relative speed of grinding wheel (vw) (peripheral speed) and workpiece. With sufficient approximation, p Dw ◊ nw , m/sec (6.104) v = vw = 1000 where Dw = diameter of wheel, mm nw = speed of wheel, rps Work speed (vp) is expressed as:

vp =

C ◊ d pz Tm ◊ t x ◊ S y

, m/min

(6.105)

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where

C T dp t S x, y, z and m

= coefficient depending on the type of grinding and work material = wheel life, min = work diameter, mm = depth of cut = feed, mm/revolution of work = exponents to be found together with C from handwork data. Speed of work (np) =

1000 ◊v p

pdp

, rpm

(6.106)

Feed in cylindrical grinding is longitudinal distance of work per revolution of work. It varies from 0.6 to 0.9 of the face width of wheel for rough grinding and from 0.4 to 0.6 of face width for finish grinding. Depth of cut is the thickness of layer of metal removed in one pass. It varies from 0.005 to 0.04 mm. Machining time (Tm) for cylindrical grinding:

Tm = where L i sl np K

L ◊i ◊ K , min sl ◊ n p

(6.107)

= length of longitudinal travel, mm = number of passes = longitudinal feed, mm/revolution = speed of work, rpm = coefficient, for rough grinding, K is equal to 1 to 2 and for finish grinding 1.3 to 1.7

Machining time (Tm) for plunge cut cylindrical grinding

Tm =

a ◊ K , min sc ◊ n p

(6.108)

where a = grinding allowance in each side, mm sc = cross-feed, mm/revolution

6.21.17 Maximum Chip Thickness in Grinding (Cylindrical) Figure 6.155 shows a magnified portion of the grinding wheel and work in contact with each other. Note that when an abrasive grain starts to penetrate the work, as at A, depth of cut is zero and it increases gradually as the wheel and the work revolve. Since the wheel revolves much faster than the work usually, the point of maximum cut depth is almost at the point where the wheel leaves the work. The maximum depth is known as ‘grain depth of cut (t)’. Let, d = diameter of work D = diameter of wheel v = surface velocity of work V = surface velocity of wheel T = time taken by a grain (on wheel) to move from A to B

METAL MACHINING—Processes and Machine Tools

Fig. 6.155

479

Determining maximum chip thickness.

Hence, arc AB = V ◊ T During the time T, a point on workpiece at A will move only up to C. Then, arc AC = v ◊ T Hence, ACB (shaded area) becomes the chip with its maximum thickness of CD. Since AC is a very small arc, it could be treated as straight line, then CD = AC sin(a + b) = v ◊ T sin(a + b) Here, a and b are the angles subtended by the arc of the contact at the centre of wheel and work. Let N be the number of grains per unit length of the wheel circumference, then Maximum chip thickness per grit or grain depth of cut (t) =

CD v ◊ T ◊ sin (a + b ) = N ¥ arc AB N ◊V ◊T

1 v (6.109) ¥ ¥ sin(a + b ) N V The above equation shows that grain depth of cut varies directly as work speed, inversely as wheel speed and directly as sin(a + b). Maximum chip thickness (t) can be conveniently written as follows: =

Maximum chip thickness (t) =

2¥v V¥N

D+d ¥ f D◊d

(6.110)

From the above equation, it is obvious that decrease in chip thickness is possible by increase in wheel speed V. Decrease in chip thickness leads to better finish and tighter geometrical tolerances due to lower grinding forces. It is with this reason that higher speeds are tried for precision grinding applications.

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6.22 FINISHING 6.22.1

Finishing Processes

It has been mentioned earlier that grinding operation is done to finish workpieces which must show a high quality and accuracy of shape and dimension by removing comparatively small amount of material, usually 0.25 to 0.5 mm in most grinding operations. However, for several applications, grinding does not meet the requirements of accuracy and surface finish. Hence, if a better finish is desired on a job for reasons of accuracy, appearance or looks or for improved resistance against wear, fatigue, corrosion and better dimensional accuracy of mating parts to get a particular fit, then one of the precision surface finishing processes (such as lapping, honing, super-finishing, buffing, etc.) should be employed. These micro-finishing processes make use of an abrasive to remove very small amount of material (for example, 0.005 to 0.01 mm in lapping operation) which is much less than that removed even by grinding. Further, these processes are used exclusively for finishing and polishing. Main finishing processes include honing, lapping, super-finishing, polishing, buffing, burnishing, etc. Honing

Honing is an abrading process mostly used for polishing bored and reamed holes and also external cylindrical surfaces using bonded abrasive stones (called hones). Honing is, in fact, a cutting operation and is used to remove material less than 0.25 mm but sometimes it is used to remove stock up to 3 mm also. The honing is mainly used to correct some out of roundness, taper and tool marks left by previous operation or axial distortions. Honing is done by means of bonded abrasive grit sticks (of aluminium oxide or silicon carbide) applied to the surface to be honed under controlled pressure and with a combination of rotary and reciprocating motions of abrasives. A typical honing tool-head for vertical honing machines is shown in Fig. 6.156 in which honing stones are loosely held in holders or caged spring type wheel spring is compressed as tool enters bore. Stones are cemented into metal shells which are clamped into holders or cemented directly in the holders. Stones are spaced at regular intervals around the holder. The metal frame Fig. 6.156 Honing tool used on holding the abrasives is called hone or honing vertical honing machines. tool-head which may be internal type or external type to perform internal and external honing, respectively. Honing tool-head provides a floating action between the work and the tool and any pressure exerted (mechanically or hydraulically) on tool is transmitted equally to all sides. Honing tool is given slow reciprocating motion as it rotates which results in rapid removal of stock and at the same time generation of a straight and round surface. Liberal supply of coolant (sulpherized base oil or lard oil with kerosene) is necessary to keep abrasives clean.

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Materials honed range from plastics, silver, aluminium, brass and cast iron to hard steel and cemented carbides. Tough non-ferrous metals cause glazing or clogging of voids of abrasive stick and are thus difficult to hone. The usual honing allowance is 0.1 to 0.25 mm generally. Examples of honing work include bores of cannons, diesel engine cylinders, cylinder head bores of automobiles and aeroplane engine cylinders, connecting rod bearings, hub holes in gears, etc. Honing can be done manually wherein the tool is rotated and the work is passed back and forth over the tool [Fig. 6.156(a)]. Honing can be done on general purpose machines such as lathe, drill press, portable drills, etc. Special honing machines are available for production work. These are horizontal and vertical type. A honing machine rotates and reciprocates the hone inside (or outside) the hole being finished. Horizontal machines are used for guns and large bore. Vertical honing machines are more common for shorter jobs. Maximum bore size that can be honed conveniently is about 1500 mm and minimum size is 1.5 mm in diameter. Several holes may be honed on multi-spindle machines.

Fig. 6.156(a)

A manual honing tool.

Lapping

Lapping is a finishing process and is done after grinding. Lapping produces geometrically true surfaces, corrects minor surface imperfections, improves dimensional accuracy and provides a very close fit between two contact surfaces. Since very thin layers of metal (0.005 to 0.01 mm) are removed in lapping, it is, therefore, unable to correct substantial errors in form and sizes of surfaces and is a low efficiency process used only when specified accuracy and surface finish are not obtainable by other methods. Most lapping is done with the help of lapping shoes, called laps, which are rubbed against the work. The lap made of soft material such as soft cast iron, brass, copper or soft steel is rubbed against the work with abrasive slurry in between. The abrasive slurry contains abrasive powder such as emery, corundum, chromium oxide, iron oxide, etc. mixed with a vehicle (i.e. lubricant used to hold abrasives which may be olive oil, lard oil or mineral oil) or special pastes with some carrier. Laps may be operated by hand or machine, the motion being rotary or reciprocating. In hand lapping [Fig. 6.156(b)], either the lap or the workpiece is held by hand or motion of other enables rubbing of two surfaces in contact. Lapping compound is spread over grey cast iron plate. Grey cast iron, being porous, retains the abrasive grains (lapping medium). Workpiece is placed over the lapping medium and rubbed over the same. Movement of the workpiece has to be along an irregular path (shape of No. 8). A lathe or dense may be used for lapping cylindrical work wherein the lap is reciprocated over the work in an ever-changing path. Flat surfaces may be lapped by holding work against

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MANUFACTURING PROCESSES

a rotating disc or work may be removed by hand in an irregular path over a stationary face plate lap. Lapping machines available for the job are (a) vertical spindle lapping machine [Fig. 6.156(c)] used for lap flat and round surfaces between two opposed laps on vertical spindles, (b) centreless lapping machine for continuous production of round parts like piston pins, bearing races, cups, molding discs, limit gauges, shafts, etc. and (c) abrasive belt lapping for lapping bearings and cams using abrasive coated clothes. Lapping may be equalizing lapping when work and lap mutually improve their shapes and surface, or form lapping when shape of lap is imparted to the work.

Fig. 6.156(b)

Fig. 6.156(c)

Hand lapping.

Vertical spindle lapping machine.

Super-finishing

Super-finishing is mostly done for obtaining extremely high quality surface finish along with an almost complete absence of defects in surface layer. It is thus primarily a finishing operation and not a dimensioning operation since only 0.002 to 0.02 mm stock is removed in this operation. It is considered most efficient in surface refining of cylindrical, flat, spherical and cone shaped part. The metals that can be finished by this process are steel, cast iron and non-ferrous alloys which have been previously ground or precision turned. Super-finishing differs in one main aspect that whereas honing involves two motions of honing tool, the super-finishing requires three to five or even more motions of the abrasive tool. The contact

METAL MACHINING—Processes and Machine Tools

483

surface in super-finishing is large and the tool maintains a rotary contact with the work while oscillating also and thus stone motion is multiple, random and very rapid. The finish obtained on the work surface depends on the time for which the stone is the contact with work. The reversal of the stroke is short as compared to long stroke of hone leading to the accumulation of a large amount of chips. Super-finishing is shown in Fig. 6.157. The work rotates and the tool (stone) maintains contact with the work while oscillating as shown. The stone is held in a holder or quill and placed on the workpiece. The quill is spring loaded to give light pressure of stone on the workpiece. The workpiece is rotated at a very slow speed. As the workpiece rotates, the stone block reciprocates forwards and backwards at a rapid rate, resulting into an oscillating motion of stone. Suitable lubricant is used. Typical stroke of super-finishing tool may be 1 to 5 mm with oscillating frequency of 2 kHz. The abrasive stick or stone used is of very fine grit (400 to 600). Super-finishing speeds used are from 10 to 40 m/min and the working pressure is kept as 0.1 to 0.3 MPa. A special lubricant, usually a mixture of kerosene and oil, is used to obtain a high quality of surface finish.

Fig. 6.157

Operation of super-finishing.

Examples of super-finishing work include automobile pistons, crank shaft journals, cam shafts, automobile valves, cylindrical shanks of valve tappets, etc. Super-finishing is sometimes done on lathe, but special purpose super-finishing machines are available for the job. Polishing

Polishing is a surface finishing operation done for the purpose of removing scratches, tool marks, pits and other defects from rough surfaces. A polishing wheel made of leather, paper or canvas, felt or wool is used with abrasive grains set-up with glue or thermo setting resins on the wheel face. The work is held against the rotating wheel to give the desired finish. Polishing is very similar to grinding and the work is applied by hand to wheels mounted on floor stand girders. Except honing, lapping and super-finishing, polishing may follow any of the machining methods. Only a very small amount of metal is removed in polishing. Components to be electroplated are usually polished prior to plating. For lustrous smooth appearance on stainless steel utensils, bright finished hand tools are finished by polishing followed by buffing. Buffing

Buffing gives much higher lustrous, reflective finish and is done after polishing. The process consists of applying a very fine abrasive with a rotating wheel made of felt, clothe and leather.

484

MANUFACTURING PROCESSES

The abrasive mixed with binder is applied on the buffing wheel or on the work. The buffing wheel rotates at a fast speed, up to 40 m/sec. The abrasive used may include iron oxide, chromium oxide, emery, etc. and binder is a paste made of wax mixed with grease, paraffin, turpentine, etc. Parts made of steel and other hard materials are usually ground, polished and buffed before electroplating. Sheets of aluminium, brass and copper are also buffed before electroplating.

6.22.2

Surface Finish in Machining

Surface finish

Surface finish or surface quality refers to smoothness of surface and is the measure of microirregularities produced on the surface of work during production. The surfaces machined by lapping or honing may appear very smooth but these still possess surface irregularities of micro size in relation to those surfaces which are produced by turning or milling. To have an absolutely smooth surface is practically impossible. The surface finish produced in any machining operation is a combination of several factors; the two predominant are: (A) Due to geometry of tool and feed rate, and (B) Vibrations of tool (chatter) and the built-up edge formation on tool edge, etc. A. Surface finish due to tool geometry and feed Turning operation: The cutting tool used in machining may have a pointed edge or reduced (or rounded) edge. Depending on the feed rate, an impression representing replica of geometry of tool will be produced on the workpiece surface. Let us take the case of a pointed edge tool [Fig. 6.158(a)] wherein: hmax = maximum height of unevenness Y = side cutting edge angle g = end cutting edge angle f = feed rate

Fig. 6.158(a)

Turning with a pointed edge tool.

It can be shown that: hmax =

f (tan y + cot g )

(6.111)

485

METAL MACHINING—Processes and Machine Tools

It shows that hmax is proportional to feed (f ). The average value of unevenness (hav )

f

(6.112)

4 ¹ (tan Z  cot H )

Now take the case of a tool with rounded nose or reduced edge tool [Fig. 6.158(b)] having nose radius r. In this case, hmax

or

hmax hav

f2 8r aÿ f 2

(6.113)

f2 18 3r

(6.114)

Fig. 6.158(b) Turning with rounded nose tool.

This shows that (a) feed has a very predominant effect on surface finish with reduced nose tool and (b) much smoother surface can be produced with a tool having nose radius than a pointed edge tool for a given value of feed rate. Milling:

with a straight teeth cutter, maximum height of unevenness,

hmax 

f2 4 ¹ D ¹ N2 ¹ Z2

(6.115)

where f = feed velocity of table D = cutter diameter N = cutter rpm Z = Number of teeth in cutter The average value of unevenness, f2

(6.116) 9¹ 3 ¹ D ¹ N2 ¹ Z2 It shows that (a) unevenness in case of milling operation is much less compared to turning operation and (b) finish in milling can be controlled by reducing feed and also by increasing the cutter rpm and the number of teeth in the cutter. The values of hav generally obtainable by various machining operations are as follows: Turning and boring—0.05 to 25 µm Planing and shaping—0.4 to 6 µm Milling—0.25 to 25 µm Drilling—0.75 to 12.5 µm Broaching—0.5 to 6 µm Grinding—0.025 to 6.25 µm Honing—0.025 to 1.5 µm Lapping—0.012 to 0.75 µm hav

486

MANUFACTURING PROCESSES

B. Surface finish due to tool vibration and built-up edge The surface produced may be very rough if tool vibrations (resulting in chatter) exist. The cutting condition should be chosen so as to avoid the presence of tool vibration. As regards the built-up edge, it is formed only at lower speeds; therefore, the surface roughness may be more at lower speed of work but can be improved considerably at higher cutting speeds, due to less tendency of formation of built-up edge on the tool.

6.23

METAL CUTTING SAWS

Cutting of blanks for various jobs from the steel structurals (rod, angle, channel, etc.) is an important operation in any machine shop. It is done with the help of metal cutting saws which are of the following types. (a) Power hacksaws or reciprocating saws (c) Circular saws

(b) Band saws (d) Abrasive cut-off saws

Power hacksaws (Fig. 6.159) consist of means for clamping the job and a reciprocating saw blade which is operated by power, such that during the cutting stroke, the cutting blade (along with its frame) bears down (or exerts load) on the job, whereas in the return or idle stroke, the blade is lifted up so as to be clear of the job. Because of non-continuous cutting, power hacksaws are considered slow metal cutting machines. Hacksaw blades are made of thin strip of high carbon steel or high speed steel with teeth cut on one edge of the blade. Blades may be all hard type or hard teeth with soft back.

Fig. 6.159

A power hacksaw machine.

Band saws may be vertical type (Fig. 6.160) or horizontal blade type. An endless toothed blade (called band) is mounted on two wheels, the centre distance of the wheels can be adjusted by raising or lowering the upper wheel and thus can vary the tension of the band. One of the wheels is the driving wheel. Unlike power hacksaw, it cuts continuously when work is fed to the handsaw.

METAL MACHINING—Processes and Machine Tools

Fig. 6.160

487

A vertical metal band saw.

Circular saws cut metal with a rotating circular blade or saw. A typical cold saw has a large diameter circular blade with inserted teeth and cuts very fast. Abrasive cut-off saws employ very thin grinding wheels or disc, called the abrasive disc saw. Abrasive cut-off machines may work by feeding the job towards the saw or by mounting the saw (with its motor) on a swinging arm when cutting is done in the downward motion of the swinging arm.

6.24 SLOTTER A slotter is mainly used for internal machining of blind holes or vertical machining of complicated shapes which are difficult to be produced on horizontal shaper. Jobs such as keyway cutting, machining of square holes, cutting of internal or external teeth on bigger gears and machining of dies, straight or curved slots are taken up on this machine. The salient features of the slotter are shown in Fig. 6.161. As regards its working principle, it is similar to shaper with the difference that the ram of the slotter reciprocates in vertical position and cuts in its downward strokes only. The job is supported on a swivel table which has a rotary feature in addition to the usual table movements in cross directions. Indexed machining of jobs like gears or splines is easily performed on this machine. Slotter ram may be crank driven or hydraulically driven. Stroke length of the slotter may be up to 900 mm. Slotter tools are of rigid shank and are different from those of the single-point tools. Slotter tools cut parallel to their shank and so their shape is modified accordingly. They have no side rake.

6.24.1

Main Parts of a Slotter

Main parts of a slotting machine are shown in Fig. 6.161. These are described in the following: 1. Base or bed is a heavy cast iron construction and supports other parts of the slotting machine such as column, ram and its driving mechanism, table, etc. The top of the base is accurately finished to provide guide ways for mounting of the saddle. The cross slide guide ways are perpendicular to the column face.

488

MANUFACTURING PROCESSES

Fig. 6.161 Main features of a slotting machine: 1. Cross feed handle, 2. Longitudinal feed handle, 3. Table circular feed handle.

2. Column is the vertical structure cast integral with the base. It houses the mechanism for driving ram and feeding mechanism. The front vertical face of the column carries guide ways for ram to reciprocate upon. 3. Saddle mounted upon guide ways (not shown in Fig. 6.161) can be moved towards or away from the column. The saddle carries guide ways for cross slide. It has manual or power feed. 4. Cross slide is mounted upon guide ways made at the top of the saddle and can be moved parallel to the front face of the column. It has manual or power feed. 5. Table is a circular rotary table mounted on the top of the cross slide. A circular feed handle for the table is provided. Rotation of table is effected by hand or by power. The table carries T-slots to help mounting of jobs on the table. 6. Ram and tool head: They reciprocate up and down on the guide ways made on the front face of the column. The ram carries a tool head at its bottom end. In some machines, special tool head is provided to relieve the tool during its return stroke. A slotter removes metal during its downward cutting stroke only and no metal is cut during its return or upward stroke. A quick return mechanism given with the machine enables the return or idle stroke to be completed faster than the cutting stroke.

METAL MACHINING—Processes and Machine Tools

6.24.2

489

Quick Return Mechanism for Ram

The common types of driving mechanisms for the ram are as follows: (a) (b) (c) (d) (e)

Whitworth quick return mechanism Slotted disc mechanism Slotted link and gear mechanism Hydraulic mechanism Variable speed reversible motor drive mechanism

1. Whitworth quick return mechanism is widely used on medium-sized slotters. The mechanism is shown in Fig. 6.162 wherein A and B are fixed centres of bull gear (7) and driving plate (8), respectively. The crank pin and slide block (4) rotate in a circular path at a constant speed, rotating in turn the plate (8) about B. The system has the arrangement such that when plate (8) rotates about fixed point (B), the pin (3) also rotates in a circular path about the point (B) resulting into upward or downward movement of ram (1). Stroke length of the ram is equal to twice the distance between pin (3) and centre B, which can be varied. When the block (4) is at position C, the ram (1) will be at the maximum upward position of the stroke and when the block (4) is at position D, the ram (1) Fig. 6.162 Line diagram of will be at its maximum downward position of the Whitworth quick return mechanism. stroke. When the bull gear (7) rotates in anticlockwise direction, the block (4) rotates through an angle CAD and thus the downward cutting stroke is performed. And when the block (4) rotates further anticlockwise beyond point D and through an angle DAC, the return stroke is completed. Since the block (4) rotates at a constant speed, its turning through angle CAD will take more time than turning through the angle DAC. Hence, quick return motion of ram is obtained. Cutting time and return time are related by the expression, angle CAD divided by angle DAC. Stroke length of the ram can be varied by altering the position of pin (3) with respect to the centre B. Position of stroke of the ram may be adjusted by releasing the stroke adjustment lever (Fig. 6.163) and then by altering the position of connecting rod clamping bolt within the slot provided in the body of the ram. The weight of the ram is counterbalanced by a weight attached to the back of the ram. 2. Slotted disc mechanism is shown in Fig. 6.163. A T-slot is cut in the disc through which crank pin can be set at different positions to vary the stroke length.

490

MANUFACTURING PROCESSES

Fig. 6.163

Slotted disc mechanism for driving slotter ram.

3. Slotted link and gear mechanism is shown in Fig. 6.164. It is used for heavier slotting machines. Rotation of the rear driving wheel results in the sliding of die up and down in the slot of bell crank lever. This makes the lever swing about its fulcrum. With the result, the connecting rod (and hence the ram) is alternatively pushed up and pulled down. This enables the reciprocating motion of the ram.

Fig. 6.164

Slotted link drive.

4. Hydraulic mechanism used on slotters is similar to the one described with shaper. 5. Variable speed reversible motor drive mechanism is used on large slotters. It has already been described with planer.

METAL MACHINING—Processes and Machine Tools

6.24.3

491

Operations on a Slotter

The operations performed on a slotter are given below: (i) Production of flat surfaces (ii) Machining cylindrical surfaces (iii) Machining irregular surfaces such as cam (iv) Cutting of slots, keyways, grooves, internal gear teeth, etc. Some of the slotter operations are illustrated in Fig. 6.165 to Fig. 6.168.

Fig. 6.165

Forming the semi-circular portion of a locomotive driving box on a slotter. The dotted line shows the size to which the job is to be machined. The circular portion is generated by applying power feed to the rotary table of the slotter.

Fig. 6.166

Cutting out an opening from the blank of a connecting rod: (a) A connecting rod in which a square opening is to be cut out by slotting, (b) The job has been set on the slotter table with proper layout of the opening (marking of the opening to be cut). Note the four holes drilled at four corners of the opening to help starting the cut.

492

MANUFACTURING PROCESSES

Fig. 6.167

Machining indexed external dovetail slots on a slotter.

Fig. 6.168 Checking the concentricity of the gear blank (with rotary table of the slotter) using a dial gauge. The set-up is being prepared for cutting internal teeth in the gear blank with a form cutter.

6.24.4

Feed Mechanism

Like a shaper or a planer, the feed movement of the table in a slotter is also intermittent and is supplied at the beginning of cutting stroke. Table feed can be given by hand or by power. A typical power feed mechanism is illustrated in Fig. 6.169 wherein a depressed cam groove (1) is cut on the face of a bull gear (not shown) such that when the bull gear rotates, a roller (2) moves up and down following the contour of the cam groove but only for a small part of revolution of the bull gear. The cam groove is so cut that movement of lever (3) takes place only at the beginning of cutting stroke. The rocking movement of lever (3) is transmitted to ratchet and pawl mechanism. The ratchet wheel is mounted on the feed shaft (4) of the table.

Fig. 6.169 Power feed mechanism of a slotter.

METAL MACHINING—Processes and Machine Tools

6.24.5

493

Types of Slotters

(a) Puncher slotter is used for removing heavy stock of metal and is of rigid and heavy construction. It is used for machining forgings, stamped parts and other rough-shaped castings. (b) General production slotter is used for general machining works. (c) Precision tool room slotter is primarily used for tool rooms for precision slotting works. (d) Key seater is exclusively used for cutting keyways in wheels and gears.

6.24.6

Slotting Tools

A slotting tool is robust in construction and is generally forged type. The slotting tool differs from a shaper or planer tool because a slotting tool cuts metal during its vertical cutting stroke. Figure 6.170 shows typical slotting tools with tool angles. Note that a slotting tool has top rake, front clearance, side clearance and no side rake. To provide clearance, the nose of the tool projects little beyond the tool shank.

Fig. 6.170

6.24.7

Slotter tool angles. g–top rake angle, a–front clearance angle.

Cutting Speed, Feed and Depth of Cut

Cutting speed is the rate with which metal is removed during downward cutting stroke and is given in m/min. Feed is the movement of the workpiece per double stroke and is expressed in mm. Depth of cut is the perpendicular distance measured between machined surface and unmachined surface and is given in mm.

6.25

BORING MACHINE

Boring is the process of enlarging (by machining) holes that may be previously drilled, punched or cored during casting. Boring corrects the location, alignment and size of the hole and imparts a good finish to the hole surface. The operation of boring is schematically shown in Fig. 6.171. With proper tooling and adjustment of job supporting devices, a boring machine can perform operations such as drilling, boring, reaming, facing, turning, milling, and forming. Boring of large cylinders of engines, pumps and compressors is the most common operation

494

MANUFACTURING PROCESSES

done on a boring machine. The boring tool is held in a boring bar (Fig. 6.171). When the cutting tool is a milling cutter type, it is mounted directly on the machine spindle.

Fig. 6.171

Showing the use of a table type horizontal boring machine.

Types of boring machines include horizontal boring mill (Fig. 6.172), vertical boring mill (Fig. 6.173) and jig boring machines. In a horizontal boring machine (Fig. 6.172), the workpiece is mounted on a table that can move horizontally in both axial and radial directions. The cutting tool is mounted on the spindle which projects out of the head stock of the machine. The spindle gets power from the motor and gear system housed in the head stock. The spindle can easily carry drill, reamer, tap and milling cutter. A vertical boring mill (Fig. 6.173) is similar to a lathe but has vertical axis of workpiece rotation. Jig borers are vertical boring machines with high precision bearings and are used in tool rooms for making jigs and fixtures and for other precision works.

Fig. 6.172 Horizontal boring mill. Note the movement of various components of the machine. The head stock contains spindle, spindle drive and feed gearing, and other mechanisms including boring, drilling and milling head. Boring mill may have more than one spindle.

METAL MACHINING—Processes and Machine Tools

Fig. 6.173

6.26

495

Vertical boring mill.

BROACHING MACHINE

Broaching is the method of removing extra metal from the external or internal surfaces of a workpiece by using an elongated tool having a series of multiple cutting teeth positioned in tandom (Fig. 6.174 and Fig. 6.175) wherein each successive tooth is slightly higher than its predecessor such that each tooth takes a cut when the broach is reciprocated past the workpiece. Sometimes machining on a workpiece may be completed in a single stroke of the broaching machine because of the typical features of the broaching tools. The amount of metal removed is a function of tooth depth and number of teeth in the broach. The block diagram of a vertical broaching machine is shown in Fig. 6.176.

Fig. 6.174 The cutting action of a broach and its features are shown at (i) whereas the terminology of a broach is given at (ii).

496

MANUFACTURING PROCESSES

Fig. 6.175

Illustrating the two types of broaches: (i) Pull type and (ii) Push type.

Fig. 6.176 Block diagram of a vertical broaching machine. The broaching tool is mounted on a reciprocating ram. The table can be moved inwards or outwards and rotated in all the directions to help cutting slots in circular jobs.

6.26.1

A Broach

A broach is an elongated tool provided with a series of multiple teeth. Few of the teeth are meant for rough cutting and others for finishing. When a broach is pulled or pushed through a workpiece, each tooth takes its own cut. Roughing and finishing operations are thus completed in a single stroke of the broach. The pull and push type broaches are shown in Fig. 6.175, whereas the cutting action of a broach is shown in Fig. 6.174. A broach may be made from high carbon steel or high speed steel.

METAL MACHINING—Processes and Machine Tools

6.26.2

497

Broaching Operations

Machining of square, rectangular and polygon holes, splines, keyways, serrations in a hole, internal gear and special odd profiles in the hole are some of the well-known applications of broaching. Other applications include finishing of spiral splines in gun barrels, involute splines, shaping of slots on the gas turbine rotor, washing machine components and other numerous jobs. Figure 6.177 shows a few of many applications commonly encountered in broaching internal and external shapes.

Fig. 6.177

6.26.3

Few examples of surfaces generated on a broaching machine.

Advantages of Broaching

Broaching is sometimes considered advantageous for mass production because of the following reasons. (i) Roughing and finishing operations in machining a job are often completed in single stroke of the broach, giving high rate of production. (ii) Any form that can be achieved on a broaching tool can be reproduced effectively on the workpiece. (iii) External or internal surface finishing can be done maintaining tolerances ±0.0075 mm needed for interchangeable production. The primary aim of a broaching machine is more output in lesser time and with fine surface finish (0.8 micron). (iv) Manufacturing sequence for obtaining varieties of shapes on a workpiece is often simplified by using multiple type broaches to work one after the other in the same stroke of the machine. (v) Operation of the broaching machine is simple and can be automated.

6.26.4

Broaching Methods

According to the method of operation, broaching methods may be classified as follows: (i) Pull broaching (ii) Push broaching (iii) Surface broaching (iv) Continuous broaching

498

MANUFACTURING PROCESSES

1. Pull broaching: The job is held stationary and the broach is pulled through the work while being held in a special head. This method is generally used for internal broaching. Longer broaches are used in pull broaching. 2. Push broaching: The job is held stationary and the broach is pushed through the job. Hand press or hydraulic press is used for push broaching. The method is popular for cutting keyways and for sizing holes. Shorter broaches are used in push broaching. 3. Surface broaching: This method is used mostly for surface finishing. Either the job or the broach moves across the other. Irregular and intricate surfaces are finished by this method using specially designed broach for each job. 4. Continuous broaching: The job is moved continuously and the broach is held stationary. Movement of the job may be either straight horizontal or circular. A number of similar workpieces can be broached at a time.

6.26.5

Types of Broaching Machines

Among all the machine tools, broaching machine is the simplest unit since it consists of a fixture to hold work, a broach, a drive mechanism for broaching and a machine frame. Broaching machines can be broadly categorized as follows: 1. Horizontal broaching machines are generally pull type (Fig. 6.178). These are used for cutting keyways, slots, splines, round holes and other internal contours.

Fig. 6.178

Horizontal broaching machine.

2. Vertical broaching machines may be either pull or push type, the latter being more popular. Block diagram of such a machine is shown in Fig. 6.176. They occupy less floor area. 3. Surface broaching machines have broaching tools fixed to a ram or rams which are moved in straight path in guide ways past the workpiece. When two rams are used, the machine is called duplex broach. 4. Continuous broaching machines are used for mass production of small parts. In rotary continuous broaching machines (Fig. 6.179), the workpieces are clamped on the table which is rotated continuously and the broach is stationary. In horizontal continuous broaching machines, the workpieces move as they are carried by an endless chain (Fig. 6.180) while the broach remains stationary.

METAL MACHINING—Processes and Machine Tools

499

Fig. 6.179 Rotary continuous broaching machine. Workpieces are mounted on a rotary table whereas broach is fixed (stationary).

Fig. 6.180 Horizontal continuous broaching machine. Workpieces are mounted on an endless chain as they pass under the stationary broach.

6.26.6

Limitations of Broaching

The following are the limitations of broaching: (i) The cost of tool is high. Making a broach is an expensive job. (ii) Very large workpiece cannot be broached. (iii) Surface to be broached cannot have any obstruction in the way of broach. (iv) Large stock of metal cannot be removed by broaching.

REVIEW QUESTIONS 1. What is machining? How does it basically differ from casting and forging? 2. What is a chip? How is it formed during machining? 3. What do you understand by the term machine tool? What are its essential elements? What is a machine shop? 4. Name various operations performed in a machine shop. 5. What are the functions served by a machine tool? 6. Differentiate between a standard machine tool and a special purpose machine tool. 7. Explain the following terms: (a) Turning (b) Boring (c) Facing (d) Taper turning

500

MANUFACTURING PROCESSES

8. 9. 10. 11. 12.

What is the working principle of a How is the size of a lathe given? Name the different types of lathes. Name the operations performed on Describe with sketch the following (a) Step turning (c) Facing (e) Knurling

lathe?

13. 14. 15. 16. 17.

What do you understand by taper? What is taper turning? How is taper turned on a lathe? Name different methods. Describe the method of taper turning by compound rest swiveling method. When will you use tail stock set over method for taper turning? Define the following with their importance: (a) Cutting speed (b) Feed (c) Depth of cut

a lathe machine. operations briefly: (b) Taper turning (d) Boring (f) Threading

18. Name the various lathe accessories. How does a four jaw chuck differ from a three jaw chuck? 19. What are mandrels? Why are they used? Show with sketch the use of mandrel on lathe. 20. What is the difference between a steady rest and a follower rest? 21. Explain the principle of metal cutting with a lathe tool. 22. What are the characteristics of a good tool material? 23. Name the various tool materials used for making lathe tools. 24. Define machinability and machinability index. 25. Define the word ‘tool life’. How does a tool fail? How can the tool life be increased? 26. Why is heat generated in metal cutting? What are various cutting fluids used in machining? 27. What types of cutting fluids are used in industry? 28. Define the operation of drilling. What is a drilling machine? 29. Name the various operations that can be performed on a drilling machine. 30. Explain different types of drilling machines with their specific features. 31. What is a shaper? How does it differ from a planer? 32. Where will you use a shaper and where a planer? 33. What is a milling machine? What are the operations performed on a milling machine? 34. Name the different types of milling machines. 35. How does grinding differ from other machining processes? 36. Name the different methods of grinding. 37. What is a grinding machine? Name different types of grinding machines in common use. 38. How does centreless grinding differ from cylindrical grinding? 39. What is the utility of a metal cutting saw in the machine shop? 40. Explain different types of metal cutting saws. 41. What for is a slotter used? 42. Describe the use and types of boring machines. 43. What is the use of a broaching machine? Discuss types of broaches. 44. What shapes can be made by broaching?

7 7.1

Electric and Gas Welding Processes

INTRODUCTION

The two methods of metal working, namely, casting and forging, are the most primitive methods of mass production. Although these processes are still in vogue, welding as a new method of fabrication has very effectively superseded both casting and forging, particularly for the production of those machines and structures which require light, compact but strong design. Besides providing simplicity, ease and speed in production, welding ensures overall reduction in production cost and, consequently, enjoys wide acceptance by the craftsman and users. Fabrication is the name given to a process (or combination of processes) of making a marketable product by cutting metal pieces to suitable sizes and later joining them together to form the product. The metal pieces are normally cut from standard structurals available in the market in the form of plates, rods, angles or channels and later assembled and jointed together by bolts, rivets or welding.

7.2

PREFERENCE FOR WELDING

Among various processes of jointing metals, welding has gained considerable popularity because of the following reasons. (i) Welding has replaced riveting because of being faster and noiseless. It gives a stronger and leakproof joint. It also results in saving in self-weight of the fabricated structure and is thus more economical. (ii) Big components of the machine (for example, lathe beds and columns of machine tools), which were previously made by casting, are now steel fabricated bringing lots of saving in the self-weight, besides speed and economy in production. Welding provides flexibility in making in-process changes in the design of the structure. (iii) Welding can be used for jointing dissimilar metals to form a composite metal with special properties. 501

502

MANUFACTURING PROCESSES

(iv) Fabrication of welded structures is much easier and cheaper than that of complicated castings since the standard readily available structurals like angles, channels and I-sections are effectively made use of by welding in making a product. (v) The number of operations involved in welding is much less in comparison to casting and riveting and hence there is saving in time, labour and overall cost. (vi) The appearance of a welded structure is better than that of a cast or riveted one. (vii) It has been possible only due to welding that some metals such as copper, stainless steels, aluminium and many others have found useful applications in various industries for their specific properties like resistance to corrosion and oxidation. Welding as an effective method of fabrication has thus led to the development of chemical, petroleum, fertilizer and steel fabrication industries.

7.3

APPLICATIONS OF WELDING

Welding is used in industry as an effective tool in the form of (a) regular method of production of metal structures and components for automobile, aircraft, railway and many other construction industries and (b) an easy and effective method of on-site repairs and maintenance or rebuilding of broken parts of a machine or structure. Industries where welding finds most extensive use are given below. (i) Automobile and transport industry wherein cars, trucks, jeeps and many other transportation machines and equipment are fabricated. (ii) Material handling equipment such as overhead cranes, jib cranes and tower cranes are manufactured by welding along with their auxiliary equipment like trolleys, lifting aids and gadgets. (iii) Rail-road industry wherein major fabrication and welding is involved in the production of locomotive underframes, boggies, trolleys, railway bridges, electrification network, signalling equipment, lighting tower, platform sheds and godowns, storage tanks and bodies and frames of railway coaches. (iv) Bridge construction industry utilizes welding as the most popular means of joining steel-bridge components or structurals, in a factory or at the construction site. Joining of steel reinforcement for cast-in place concrete bridges is also done by welding. (v) Ship building industry uses welding in the construction of ship body or structures including decks, supporting girders and framework, platforms and many other structures. (vi) Aircraft industry involves considerable use of welding for joining aircraft components of alloy steels, stainless steels and aluminium alloys. Besides the fabrication of the aircraft body, frames mounts, fuel tanks, ducts and fittings, welding is used for the production of allied equipment that help aircraft operations and maintenance like material handling systems, transport means for man and luggage, sheds, fuel storage tanks and many such structures. (vii) Building industry has great use of welding in traditional buildings for joining frames of doors and windows, reinforcement in concrete works, railings and staircases.

ELECTRIC AND GAS WELDING PROCESSES

(viii)

(ix)

(x) (xi)

(xii)

(xiii)

7.4

503

When the building is a steel frame construction comprising steel roofing frames covered with asbestos sheets or galvanized iron sheets, welding has still greater role to play in joining the structural components for making building frames and trusses. Chemical and petroleum industry makes good use of welding for the fabrication of plant and machinery, stainless steel vessels and storage tanks, besides many other structures. Pressure vessels and tanks are used in various industries for storing fuel and other liquids. These are made by welding together the bent steel plates. Oil, gas and water storage tanks are also steel fabricated. Pipings and pipelines for oil, gas and gasoline involve welding for their fabrication and assembly. Construction equipment manufacturing largely depends on welding for the fabrication of earth moving machineries like bulldozers, loaders, trenchers, and drilling rigs for oil exploration and water tubewells and other such machineries. Manufacturing of machine tools and production tools include mass production of machine tool frames, columns, beds and other auxiliary supports, press and die equipment for cold and hot forming of steel and non-ferrous metals. These involve welding as a major means of fabrication. Other fabrication industries involving the use of welding include (a) industries engaged in the manufacturing of steel furniture like beds, tables, chairs, almirahs, and many other frames and gadgets for household and office usage, (b) farm machinery manufacturing units engaged in the production of tractors, trolleys, wheat thrashers, farming implements and tools, and (c) repairing of components of automobiles and two wheelers, hard facing and rebuilding of worn out machinery parts, fabrication of jigs and fixtures and many other aids and gadgets.

WELDING—A METAL WORKING PROCESS

Welding may be defined as a metal working process in which two similar or dissimilar metal pieces are joined together by heating them to molten state and allowing their molten portions to flow together to develop coalescence (or intimate fusion) which, on cooling and solidifying, forms a firm joint. The ‘fusion’ refers to intimate intergranular mixing of two metals to be joined. The operation of welding may be carried out with or without the application of pressure on the mating components and also with or without the use of a filler metal (often called electrode). The heat required to melt the metal pieces is obtained either from electric arc, oxy-acetylene (or oxy-hydrogen) gas flame or from the exothermic chemical reaction (as in the case of thermit welding).

7.5

BROAD CLASSIFICATION OF WELDING PROCESSES

Welding processes when categorized based on the material of filler rod used during welding include (a) autogenous welding in which no filler rod is used (such as resistance welding or

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cold welding processes), (b) homogeneous welding wherein the filler rod used is of the same material and composition as that of the base metals being welded (such as arc welding processes), and (c) heterogeneous welding in which the filler rod used is of different material than the base metal welded (such as in soldering or brazing). However, a more widely accepted classification of welding processes is given in the following. (a) Fusion welding processes (or non-pressure welding processes) (b) Pressure welding processes (resistance welding processes and solid-state welding processes) (c) Thermochemical welding processes (d) Radiant energy welding processes (e) Underwater welding processes

7.5.1 Fusion (or Non-pressure) Welding Processes Fusion welding processes involve heating the workpieces to be joined to molten state and allowing their molten portions to fuse and flow together to develop coalescence, which on cooling results into a strong joint. No pressure is exerted on the workpieces to make a joint. Further, the welded joint may be obtained with or without the use of a filler rod (electrode). Fusion welding processes are further classified as below: (a) Arc welding processes such as shielded metal-arc welding, submerged arc welding, gas metal-arc welding, electroslag welding, gas tungsten-arc welding, plasma-arc welding, etc. (b) Gas welding processes include oxy-fuel gas welding such as oxy-acetylene welding, oxy-hydrogen welding, etc. (c) Brazing and soldering

7.5.2 Pressure Welding Processes Pressure welding processes involve heating of workpieces to the temperature range in which the base metal of the workpieces becomes plastic, and then the two workpieces are joined together by applying pressure on them. The workpieces are heated only along the edges where the joint is to be formed. Heating may be sometimes concentrated only at a spot (or number of spots) on the edges of the joint. No additional filler metal (or electrode) is used in forming the weld. Pressure welding processes are further divided as below: (a) Resistance welding processes such as spot welding, seam welding, projection welding, resistance (upset) butt welding, flash butt welding and percussion welding. (b) Solid-state welding processes A solid-state welding process produces coalescence at temperatures below the melting point of the base metals being joined, without the addition of a filler metal but with the application of pressure only. Various types of solid-state welding processes include: (i) Cold welding (iii) Explosive welding (v) Ultrasonic welding

(ii) Diffusion welding (iv) Friction welding and inertia welding (vi) Forge welding

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7.5.3 Thermochemical Welding Processes The two main thermochemical welding processes are (a) thermit welding and (b) atomic hydrogen welding. Thermochemical welding is, in a way, a fusion welding process in which no outside heat source is required for melting the workpieces to be joined, for example, the exothermic reaction of the burning thermit mixture in thermit welding provides heat required for melting the joint edges of the workpieces. Similarly, atomic hydrogen welding possesses the features of both arc and flame welding processes. Arc is struck between two non-consumable tungsten electrodes in an atmosphere of hydrogen where dissociation of hydrogen results in an exothermic reaction providing heat for welding.

7.5.4 Radiant Energy Welding Processes Radiant energy welding processes involve focusing an energy beam on the mating edges (or surfaces) of the two workpieces to be joined. Heat is generated as a consequence of the energy beam striking the workpieces. Radiant energy welding processes include: (a) Electron beam welding (b) Laser beam welding

7.5.5 Underwater Welding Processes The development of off-shore gas and oil fields, mining in sea bottom and repairing of structures under water has called for the development of underwater welding. It is done in two ways: (i) Wet welding and (ii) Dry welding.

7.6

ARC-WELDING PROCESSES

Arc-welding includes those welding processes wherein heat required for welding is derived from an arc powered by electrical energy, maybe AC or DC. Very high temperatures (up to 30,000°C and more) are obtained in the welding arc developed between the tip of electrode and the workpiece. In arc-welding, the electrodes used may be consumable type or non-consumable type and accordingly the arc-welding processes may be categorized in two groups. They are as follows. (a) Arc-welding processes (consumable electrode type): In these processes, the electrode used is in the form of thin small rods or sticks (bare or coated with flux) or coil of bare round wire, which during welding is fed continuously to the welding zone wherein due to high heat, the electrode melts and is consumed, forming the part of deposited weld metal. Examples of arc-welding processes (consumable electrode type) include: carbon-arc welding, shielded metal-arc welding (SMAW), submerged arc welding (SAW), gas metal-arc welding (GMAW), electroslag welding (ESW) and electrogas welding (EGW). (b) Arc-welding processes (non-consumable electrode type): In these processes, the electrode used is not consumed during welding forming the part of weld deposit.

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Typically, a non-consumable tungsten electrode is used which, as one pole of the arc, generates high heat for welding. The electrode and the welding zone are protected against oxidation using some shielding gas such as argon or helium. Examples of arc-welding processes (non-consumable electrode type) include: gas tungsten arc-welding (GTAW), atomic hydrogen welding (AHW) and plasma-arc welding (PAW). The salient features of the welding processes discussed before will be taken up in brief elsewhere. However, in the following is given the detailed description of the shielded metal-arc welding, because this process is the oldest, simplest and most versatile and hence highly popular among the welders. Use of this process alone accounts for over 50% of all industrial welding jobs related to fabrication, production and maintenance.

7.7

SHIELDED METAL-ARC WELDING (SMAW)

Shielded metal-arc welding (SMAW) (or manual metal-arc welding or stick welding or simply manual arc welding) is the most commonly used welding process in fabrication and maintenance jobs. Arc is developed between a consumable coated metal electrode and a workpiece (Fig. 7.1). Under the intense heat of arc (temp. about 5000oC), a small part of the base metal of the workpiece melts. At the same time, the end of metal electrode also melts, giving tiny globules or drops of molten metal which pass through the arc and reach the joint where they develop coalescence with the molten part of the workpiece metal after cooling. Burning of flux coating on the electrode produces protective gaseous shield for the weld bead and the molten flux forms a slag (after cooling) that protects the weld bead from oxidation. This slag is later chipped off to get the clean weld.

Fig. 7.1

Essentials of shielded metal-arc welding process. The crucible or cup formed at the lower end of the flux coated electrode helps in concentrating the arc heat right on the weld zone and also ensures a consistent arc length.

Equipment consists of AC transformer welding set or DC welding set (motor-generator type, AC-DC transformer rectifier type) having constant current characteristics and coated electrodes covered with a flux coating. The set-up for shielded metal-arc welding is shown in Fig. 7.2, which shows the essentials of this welding process.

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All common metals and alloys can be welded by shielded metal-arc welding process. It is the most popular and simplest method of welding used extensively for fabrication and maintenance jobs. It may be slower than CO2 process for welding mild steels because it cannot be mechanized due to the use of short electrodes. Slag inclusion in weld due to coated electrodes is also a problem in making long joints. Shielded metal-arc welding may not be suitable for welding very thin jobs.

Fig. 7.2

7.8

Essentials of an electric arc welding process. The mating edges of the two workpieces (plates A and B) are first prepared for welding by grinding them slant such that when placed parallel and touching to each other along the mating line PK, a V-groove is formed to receive molten metal. The electrode is usually inclined to about 20o from the vertical position towards the direction of weld travel.

MAIN FEATURES OF SHIELDED METAL-ARC WELDING

The following make a welding circuit for the shielded metal-arc welding process. (i) Welding plant or machine (iii) Two electric leads (v) Electrode holder

(ii) Electrode (iv) Workpieces (vi) Earth clamp

The essentials of shielded metal-arc welding set-up are shown in Fig. 7.2 wherein two plates A and B are to be joined along the edge PK (or mating line). An electric arc is established between the bottom end of the electrode and the workpieces when the electrode is brought down, touched with the workpieces (at any point along line PK) and withdrawn quickly upwards to a distance sufficient to maintain the arc. The arc thus established melts the base metal of two workpieces as also the electrode. During welding, while the electrode is moved gradually along the joint PK from its one end to the other, it is also continuously fed into the molten weld puddle or pool as the welding operation proceeds. When one electrode is consumed, another is picked up and used. The flux coating on the electrode melts (and burns) and generates shielding gas which envelops the weld area and protects the weld from the oxygen of the atmosphere.

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7.8.1 Electric Arc The phenomenon of jumping of current (or electric charge) between two terminals with a small air gap is termed as electrical discharge, which appears in the form of sparking. Under suitable conditions of the power source supplying power to the circuit, it is possible to have a continuous electrical discharge (or flow of current through air gap) between the two terminals. The electrical discharge will then appear in the form of an arc. An electric arc (which is an electric discharge in air) is formed when two conductors carrying current are brought closer to make electric contact and then separated such that a small air gap is maintained between them. In an electric circuit having its two terminals arranged close to each other with a very small air gap between them, a current (or charge) may jump across the air gap when either a sufficient high voltage or a very heavy current is applied to the circuit. Air or any gas at normal temperature is a bad conductor of electricity but during the phenomenon of electrical discharge, the air between the two terminals becomes ionized (which is a state in which air temperature attains a very high value, over 5000oC). The high temperature causes the electrons to emerge from the negative electrode. These electrons collide with the molecules and atoms of the air (between electrode and workpiece) breaking it up into free electrons and ions and thereby rendering the air gap good conductor of electricity due to ionization. Thus, heated air becomes a good conductor of electricity. Electric arc or welding arc is thus a sustained electrical discharge through an ionized gas (called plasma) between the two terminals (electrode and workpieces) of the welding circuit. The electric arc used for welding purposes is essentially a high current, low voltage electrical discharge and can be considered as a flexible conductor carrying current through the plasma. The current in the welding arc may vary from 100 amperes to 2000 amperes and voltage of the arc may be as low as 10 to 50 volts. The arc contains very high amount of heat at high temperature over 5000oC and thus can be effectively employed for fusion welding of metals.

7.8.2 Arc Initiation Arc initiation refers to generating or starting an arc between the electrode and the workpiece. During welding, the electrode is kept a little away from the workpiece maintaining more or less a constant gap sufficient enough to maintain the arc. But the arc can only be ignited or initiated by first providing a conducting (or ionized) media in the air gap between the electrode and the workpiece. The initial conducting media (i.e. the ionized air gap in the beginning) for arc initiation is obtained by the following methods: (a) Touch start for arc initiation (b) High voltage discharge for arc initiation (a) Touch start for arc initiation: In touch start, the electrode is first touched with the workpiece (Fig. 7.3) and later immediately withdrawn apart. The electrode is touched with the workpiece for a very short time, creating short-circuiting conditions, i.e. resistance becoming zero and thus the current increasing to a very high order. With the rise of exceedingly high current, temperature increases at the point of contact of the electrode with the workpiece, causing local melting and vaporization

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of the electrode tip and the workpiece metal besides emission of electrons from the cathode (negative electrode). These electrons collide with the molecules and atoms of the air (existing between the gap of electrode and workpiece) breaking them up into free electrons and ions and thereby rendering the air gap to become ionized and hence good conductor of electricity. The metal vapours are also produced (due to melting) and these also get easily ionized. Under these circumstances, even a small amount of electron emission is enough to initiate a transitory arc as the electrode is pulled apart. The elements present in the flux coating of electrode (titanium dioxide, calcium carbonate and potassium compounds) are all arc stabilizers, which on melting help maintaining the arc properly. After the initiation of a transitory arc, regular welding arc may then be established if a suitable power circuit is available.

Fig. 7.3

Touch start or short-circuiting method of arc initiation. 1. Electrode being lowered towards workpiece, 2. Electrode touching the workpiece, 3. Electrode withdrawn to maintain a gap for arc formation.

(b) High voltage discharge for arc initiation: Touch start method of arc initiation may contaminate the electrode where non-consumable electrode is used (as in TIG welding), and hence under such circumstances, arc is initiated by incorporating a high frequency unit which superimposes a high frequency voltage in the welding circuit, thereby producing an electric field of very high strength of the order of 106 to 107 volts per centimetre (of air gap). The high frequency, high voltage oscillators supply a pulse of high voltage to initiate the arc.

7.8.3 Arc Stability A stable arc is uniform and steady and gives good weld bead and defect-free weld nugget. An unstable arc results in slag entrapment, porosity, blow holes and lack of fusion of the weld. For the stable arc, the welding plant should be such that a little variation in arc length (i.e. voltage) should not extinguish the arc. Continuous and proper emission of electrons from the electrode (cathode) and thermal ionization in the arc column should be ensured. Proper flux coating on the electrode with arc stabilizing elements can help making the arc stable. Arc length is the distance between the tip of the electrode and the workpiece surface upon which the molten globules (of electrode material) are deposited (Fig. 7.1). The short arc deposits more weld metal with advantages such as maximum penetration, maximum strength, slight overlay, minimum porosity, maximum ductility and better behaviour of alloy steel electrodes.

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The length of arc is an important variable in welding. The globules of molten metal should not be allowed to come in contact with air for a longer period because oxygen has an adverse effect on the weld properties (oxide formation and their inclusion in weld). If the arc is longer than normal, the arc flame gases (due to melting of flux) can no longer protect the arc stream from atmosphere. The metal will form oxide and nitrides, resulting into a weak and brittle weld. A short arc gives little chance for air to come into contact with molten metal, whereas in a long arc, the time of contact of molten metal with air is longer. This way, a long arc gives a wide shallow bead and enough heat is wasted into air and the penetration is poor and more spattering of metal results (Fig. 7.4). Although electrode may be speedily consumed with long arc, the effective weld production is low and inefficient. Similarly, too high a voltage gives a longer arc resulting into spattering of metal.

Fig. 7.4

Judging the arc length by (a) the shape and size of arc and by (b) the type of weld bead. With short arc the molten metal is well protected all around by the neutral flame (of gas envelop generated from the burning of flux coating on the electrode). In case of long arc, the neutral flame (of gas envelop) whirls around exposing first one side and later the other side of the molten metal (molten electrode) which results in the oxidation of the metal being deposited and finally in a porous and burnt weld bead, poor penetration and overlap.

7.8.4 Polarity The terms electrode positive and electrode negative are called polarity. Polarity indicates direction of current flow. The positive terminal in DC liberates more heat. When the electrode is on the positive pole, it is called reverse polarity, and when the electrode is on the negative pole, it is called straight polarity. Polarity consideration is applicable to DC power only (and not to AC power). Polarity in DC welding: One of the big advantages of a DC welding set is that either straight or reverse polarity can be used. Polarity indicates the direction of current flow.

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In straight polarity (DCSP), the electrode is negative and the work is positive. In reverse polarity (DCRP), the electrode is positive and the work is negative. Polarity can be changed by a switch on the welding machine or by changing the cable connections. Most of the heat is liberated in the positive side of the arc. DCSP:

Refer Fig. 7.5. Note the deep penetration, a special feature of DCSP.

Fig. 7.5

DC straight polarity (DCSP).

(i) Current up to 1000 amperes can be used with 6 mm electrode. (ii) 66.66% heat is generated at job and 33.33% at electrode and hence usually bare and medium coated electrodes are used in DCSP. (iii) There is deep penetration. (iv) Average arc voltage in argon atmosphere is 12 volts. (v) Electrode runs colder than AC or DCRP. (vi) There is no arc cleaning of base metal (job). (vii) It is used for most welding jobs, particularly for thicker sections. DCRP:

Refer Fig. 7.6. Note the shallow penetration, a special feature of DCRP.

Fig. 7.6

DC reverse polarity (DCRP).

(i) Currents used are usually lower than 125 amperes. for up to 6 mm dia electrodes to avoid overheating. (ii) 66.66% heat is generated at electrode and 33.33% at job. (iii) There is least penetration. (iv) Average arc voltage in argon atmosphere is 19 volts.

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(v) There are more chances of electrode overheating and melting and hence heavy coated electrodes are used with DCRP. (vi) There is better arc cleaning of job surface. (vii) It is used for welding thin jobs and where oxide removal is required, e.g. for welding aluminium. Also it is used for overhead and vertical welding where it is desirable to have the weld pool solidify rather quickly. Penetration: Penetration is the depth from the surface of the workpiece to the bottom of the molten metal (Fig. 7.1). Crater: During welding, the arc forces the metal out of the molten metal pool, and this metal sometimes piles up around the edges of a small depression which gets formed in the weld bead. This depression on cooling appears as a small cavity and is called a crater (Fig. 7.7). The crater can appear anywhere along the weld bead and is a weld defect.

Fig. 7.7

Showing good weld bead ripples. The presence of a crater at the end of weld bead is, however, not desirable. It should be properly filled up by the welder.

Arc blow: Arc blow is a phenomenon in which a strong magnetic field sets up around the electrode and it tends to deflect the arc as though a strong wind were blowing, hence the term ‘arc blow’. The arc may be deflected to the side but it is usually deflected either forwards or backwards along the direction of travel. Arc blow is specially noted when the electrode is used in a corner or towards the end of a joint (Fig. 7.8). Arc blow is encountered in DC welding machines since AC prevents formation of a strong magnetic field.

Fig. 7.8

The phenomenon of arc blow (deflection of arc). The effect of arc blow is more pronounced at the two ends of the joint.

Under arc blow, the arc may distort, deflect or rotate. The factors affecting the arc blow include magnetic fields produced in the workpiece adjacent to welding arc due to current flow through arc, presence of bus bars in the neighbourhood of welding place, operating multiple welding heads affecting each other, and magnetic field produced in the workpiece around the earth connection.

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Increased arc blow results in unstable arc, poor weld appearance, under-cutting, erratic weld deposition, spattering, slag entrapment and porosity of weld. Arc blow can be avoided by changing the position of earth clamp, avoiding presence of magnetic materials around the workpiece, lowering arc current, putting earth clamp away from the joint to be welded, using short arc, decreasing arc travel speed or pre-heating the workpiece before welding.

7.9

A FILLET WELD

A fillet weld is a fusion weld of triangular cross-section and is deposited in a tee joint, inside corner joint or lap joint. A strong fillet weld bead has usually a concave shape. Making of a typical fillet weld is shown in Fig. 7.9 wherein two overlapping plates are first tack welded (to keep them intact in a set position) at few places along the joint and later the joint is welded full length. The angle of electrode (work angle) should be 30 to 45o to the horizontal.

Fig. 7.9

7.10

Making a fillet weld. The work angle between the electrode and the workpiece taken in a plane normal to the plane of joint being welded, should be limited to 30 to 45o (as shown).

BUILDING A WELD PAD (OR BUILDING UP)

Building up involves making a series of parallel and overlapping weld beads and layer upon layer until the required thickness or size of the built-up pad is obtained (Fig. 7.10). The technique is used for repair jobs wherein worn-out parts are brought back to the original size by building up weld pad.

Fig. 7.10

Building a weld pad.

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7.11

EFFECT OF WELDING VARIABLES ON WELD QUALITY

Welding variables include: arc voltage, current, length of arc and welding speed. Optimum welding variables ensure best quality welds. Effect of welding variables on the weld quality is given in Table 7.1. TABLE 7.1

Effect of welding variables on weld quality

S. No.

Condition of welding variables

Effects on weld quality

1.

Optimum voltage, current, arc length and welding speed

Stable arc, smooth and even weld bead with fine ripples, little or no spatter, no under-cutting or overlapping

2.

Voltage in excess of optimum

Fierce wandering of arc, porosity in weld, more spatter, weld deposit flat and irregular spattering

3.

Voltage less than optimum

Frequent arc extinction with spattering sound, sticking of electrode, little penetration and irregular piling of weld bead metal

4.

Current in excess of optimum

Overheating of electrode, red hot electrode, excessive spatter, flat wide bead with deep crater and arc fierce with loud crackle

5.

Current less than optimum

Piling up of weld metal, poor bead shape, poor penetration, unstable arc and difficult slag control

6.

Faster welding speed

Under-cutting and narrow thin weld bead

7.

Slower welding speed

Wide thick weld metal deposit and difficult slag control

7.12

ARC WELDING MACHINES

The important arc welding processes discussed in the following include: shielded metal arc welding, carbon arc welding, submerged arc welding, tungsten inert gas welding and metal inert gas welding. All these processes need welding plants with different V.I. characteristics. The description in the following will, however, be confined only to the welding plants or machines used in shielded metal-arc welding using both AC and DC.

7.12.1

Need for a Welding Machine

The regular electric power supply (hydel power or thermoelectric) uses high voltage and low current. It cannot be used directly for welding purposes without a welding machine, which is specially designed and constructed to change the high voltage low ampere current into a safe low voltage (which is usually between 50 and 100 volts) and a heavy current supply

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(150 to 500 amperes or more). The welding machine delivers controllable current at a voltage required to the demand of a particular welding process. The equipment is so designed that it compensates for any change in the arc column voltage (due to the changes in arc length, as usually happens in manual arc welding), thus ensuring a steady current flow and a stabilized arc. The arc length between the electrode and the job determines the arc resistance (and hence potential drop across the arc). In other words, the arc length determines the arc voltage and this voltage allows a certain current to flow through the arc as determined by the V.I. characteristics of the welding plant used. Besides the above considerations of safety of welder and steady current flow requirements (at low voltage), the electric arc must give sufficient heat to melt the workpiece metal and hence the heating effect of current comes into consideration. A current (I amps) flowing for time (t seconds) through a conductor of resistance (R ohm, here arc resistance) requires some energy (I2 Rt joules or 0.24 I2 Rt calories) to overcome the resistance offered by the conductor. This appears as heat energy in the conductor (here arc) and the transformation of electrical energy into heat energy is called heating effect of current. It is obvious that heat in the arc is proportional to the square of the current (I) and hence the current becomes an important controlling parameter. It is due to this reason that a welding machine is primarily designed on the basis of steady current output and is also rated on the current basis. The open circuit voltage (OCV) is the voltage at the electrode when no arc is formed but the welding machine is in the switched on condition. The open circuit voltage in welding plants should not preferably exceed 70 to 80 volts in view of the safety of welder. At initial stage, the electrode requires up to 80 volts or so for striking and establishing the arc but once the regular arc is established, the welder needs only about 45 volts to maintain the arc and thus for the welding operation. Arc voltage is the voltage on electrode (or across the arc) when regular arc is maintained during welding and it is also called closed circuit voltage (CCV).

7.12.2

Welding Machines for Shielded Metal-arc Welding

Arc welding machines may be divided into the following three categories: (i) AC arc welding machines: giving AC power for welding. (ii) DC arc welding machines: giving DC power for welding with the facility of change of polarity. (iii) AC/DC arc welding machines: giving both AC and DC power for welding but only one type of power at a time. 1. AC arc welding machines: (b) AC generator set.

These are of two types: (a) AC transformer set and

An AC transformer set utilizes a transformer (both single phase and multiphase type) that changes high voltage low amperage AC power to low voltage high amperage AC power for welding. The input voltage to transformer may be 440 volts or 220 volts. The open circuit voltage at the output side of the set is usually between 70 and 100 volts and the

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output current may be up to 600 amperes. The welding set has no rotating part (except the fan in air cooled sets). It is either air cooled or oil cooled. High amperage sets are oil cooled. An AC transformer welding set is least expensive to buy and maintain. It is the lightest and smallest welding machine. Generally, for small repair or fabrication jobs, a single phase (220 volts input) transformer set is used but for higher working loads and for multi-operator’s welding, a three-phase (440 volts input) transformer welding set is used. The welding sets are properly earthed to protect the welder in the event of a breakdown in the transformer and causing main supply voltage (440 volts or 220 volts) to come into contact with welding side circuit of the transformer. The welding plant provides current setting range, i.e. output current (for welding) in the machine can be set depending on the electrode size or job thickness. An AC generator set comprises an AC generator (alternator) to produce AC power for welding and a prime mover to run the generator. The prime mover may be either an AC motor or more often a petrol or diesel engine. AC generator can deliver current at normal frequency (60 Hz) or high frequency. The electric motor driven generator sets are those which employ a motor to move at normal frequency (60 Hz) and drive a generator which produces AC power at some higher frequency for special welding applications. Engine driven AC generator sets are used for welding in remote situations where no electric power is available. 2. DC arc welding machines: These are further divided as (i) DC generator set, motor driven or engine driven and (ii) Transformer rectifier set. One big advantage of a DC welding set is that either straight or reverse polarity can be used. DC generator sets consist of a DC generator powered by either an AC electric motor or an engine (petrol or diesel). The generator set provides DC power in either straight polarity or reverse polarity using a polarity switch given on the machine. The generator supplies voltage usually between 15 and 45 volts across the arc and the open circuit voltage is between 45 and 70 volts and current supply up to 600 amperes. DC rectifier sets consist of a transformer to lower down the input AC voltage and a silicon or selenium rectifier to convert AC into DC. The machine has no moving part except a moving fan to cool the transformer. The DC rectifier set gives either straight or reverse polarity. The machine is quite in operation with low maintenance. 3. AC/DC arc welding machines: These are basically AC/DC transformers—rectifier type welding machines which give either AC or DC power for welding (only one type of power at a time). Rectifier units are designed to provide a choice of low voltages for MIG welding or submerged arc welding and a high open circuit voltage with drooping voltage characteristics (constant current type) for TIG welding, and manual arc welding. The open circuit voltage may be up to 75 volts and arc voltage 25 to 50 volts with current ratings from 200 to 1000 amperes.

7.12.3

Comparison between AC and DC Arc Welding Processes

Both AC and DC are used for arc welding processes; the preference for any one is based on the following information contained in Table 7.2.

ELECTRIC AND GAS WELDING PROCESSES

TABLE 7.2

517

Comparison between AC and DC arc welding

S. No.

AC arc welding

DC arc welding

1.

Striking of arc with electrode is relatively difficult. Maintenance of a short arc is also difficult except with iron powder electrode.

Developing an arc is easier. Maintenance of short arc is also easier.

2.

There is no problem of arc blow. Workpieces do not get magnetized as in DC.

Arc blow is a severe problem and is minimized with the use of proper corrective measures. Workpieces may get magnetized due to current flow in one direction.

3.

Arc is never stable.

Arc is more stable.

4.

No polarity change is possible and hence is not very suitable for welding all metals. It is mostly used for welding ferrous metals.

Polarity (DCRP or DCSP) can be changed at will and hence is suitable for welding both ferrous and non-ferrous metals equally efficiently.

5.

It is mostly suitable for higher current values. It is less suitable for use at low current values with small diameter electrode.

It is most suited with lower current values also, for example, at low amperage with small diameter electrode.

6.

Bare electrodes are not used. Only flux coated electrodes with arc stabilizing elements contained in flux are used.

Both bare and coated electrodes are used.

7.

Generally it is not preferred for welding thin sheets or sheet metal work due to difficulty in starting the arc.

It is more preferred for welding sheet metal as striking arc is easier and arc remains steady.

8.

Arc cleaning of job surface is there, as it gives a more forceful arc.

It is possible only with DCRP.

9.

Distribution of heat in arc is equal at electrode and job.

Most of the heat (up to 66.66%) is liberated in the positive side of the arc, i.e. when electrode is positive (DCRP).

10.

It can be used for all positions of welding but calls for proper selection of electrode and skill of the operator.

Welding can be carried out in all positions. DC is most universal in application as it can be used in practically all welding processes except the TIG welding of aluminium and magnesium where AC is used.

11.

Voltage drop in welding is less and hence welding can be done at much longer distances from the welding plant using long leads.

Voltage drop is relatively higher and hence short cables are used to weld only close to the welding plant.

(Contd.)

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TABLE 7.2

Comparison between AC and DC arc welding (Contd.)

S. No.

AC arc welding

DC arc welding

12.

An AC transformer welding set is cheaper, lighter, compact and simpler in operation. Maintenance cost is also low.

A DC generator set is costlier, difficult to operate being cumbersome, heavier and involves higher maintenance cost.

13.

Transformer plant has no moving part and working is silent.

A DC generator set has moving parts and operation is noisy.

14.

It is most preferred when input is from the AC main supply.

It is easily used with AC main supply or DC supply from the generator.

7.13

V-I CHARACTERISTICS OF ARC WELDING PLANTS

Volt-ampere or V.I. characteristics of a welding plant have great effect on the process of welding as different welding processes use welding plants with different V.I. characteristics. The V.I. characteristics indicate the relationship of the arc voltage and the arc current. During welding, the arc length between the electrode tip and the workpiece determines the arc resistance and consequently the potential drop across the arc. In other words, the arc length determines the arc voltage. Longer the arc, higher the arc voltage. And it is this voltage which permits a certain flow of current according to the characteristics of the welding plant. The three main types of welding plant characteristics are: (i) Drooping (or Constant current) type, (ii) Flat (or Constant voltage) type and (iii) Rising voltage type. Our discussion will be confined to the constant current or drooping type characteristic, which is used in most commonly used shielded metal-arc welding (or manual arc welding) plants, both AC and DC types. Drooping type volt-ampere characteristic is used on constant current type welding machines (Fig. 7.11). In manual metal-arc welding, when arc is struck, the electrode is essentially in

Fig. 7.11

Showing the constant current or drooping V-I characteristics of welding plants used in shielded metal-arc welding.

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519

short-circuit which would immediately require a sudden surge of current unless the machine is designed to prevent this. Also, as the globules of metal travel across the arc stream, they tend to cause short-circuiting. At constant current, machine is designed to minimize these sudden surges. Drooping volt-ampere characteristic is applicable for both AC and DC plants. The manual metal-arc welding plants have a drooping volt-ampere characteristic wherein drooping means the terminal voltage of the machine decreases as the welding current increases. In manual metal-arc welding, arc length (gap between the electrode and job) varies frequently (due to manual operation) causing changes in the voltage. With change in arc length, from a shorter arc B to a longer arc A, there is a marked variation (K) in the voltage but the corresponding variation (C) in the current is very small.

7.14

SIZE OF A WELDING MACHINE

The size of welding machines used in shielded metal-arc welding is designed according to their output rating. The rated capacity of the machine is in terms of maximum current available, for example, the capacity in which constant current welding machines are available include 100, 150, 200, 300, 500, 750 and 1000 amperes. These values are the output amperes at 40 volts (which is considered as average arc voltage during welding) and at 60% duty cycle. It is recommended that the open-circuit voltage (on which arc initiation depends) should not exceed 75 to 80 volts in view of safety. The 60% duty cycle means that the power supply can deliver its rated load output for six minutes out of every ten minutes. In manual welding machines (constant current machines), a power source is not usually required to deliver the current continuously as in other welding plants, for example, in fully automatic plants where duty cycle is 100%. For light and medium works, and maintenance works, a 150 to 200 ampere machine is suitable while for average production work, a 250 to 300 ampere machine is used. For large and heavy duty work, machines with capacity of 400 to 1000 amperes are employed.

7.15

ARC WELDING STATION

An arc welding station (for shielded metal arc-welding) consists of the following (Fig. 7.12 and Fig. 7.13): ● ● ● ● ● ● ● ●

Welding plant Welding cables and connectors Electrode holder Ground or earth clamp Welding electrodes Welding helmet and hand shield Protective clothings and hand gloves Chipping hammer, wire brush and goggles

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Fig. 7.12

Arc welding set-up: (1) Switch box, (2) Secondary terminals, (3) Welding machine, (4) Current reading scale, (5) Current regulating hand wheel, (6) Leather apron, (7) Leather hand gloves, (8) Protective glasses strap, (9) Electrode holder, (10) Hand shield, (11) Channel for cable protection, (12) Welding cable, (13) Chipping hammer, (14) Wire brush, (15) Earth clamp, (16) Welding table (metallic) and (17) Job.

Welding plant:

These could be AC or DC type. These have already been discussed.

Welding cables and connectors: Welding cables are of two types: (i) Electrode cable or welding cable and (ii) Earthing cable or ground cable. Electrode cable connects the electrode holder to the welding machine. Generally, these cables are of single-core tough rubber sheathed. The core wire may be of aluminium or copper. These are available in different ratings, 250, 400, 600 amperes. The welding cable should be flexible enough and adequately insulated with tough abrasion resisting insulation against dragging during use. The core wire size should be adequate to carry rated current without overheating of the cable. The ground cable is also of the same type as the electrode cable, although it need not have the same flexibility as that of the electrode cable because the ground cable is not handled by the operator during welding. The electrode cable is connected at one end with the welding machine while its other end is connected with the electrode holder. One end of the earth cable is also connected with the welding machine while the other end is connected with the work table (or connected directly with the workpiece) through either bolting or clamps. Cable connectors [Fig. 7.13(a)] are used to join two or more lengths of cables. Lugs made of copper alloy and covered with insulation are used to connect cable with the welding machine. The lugs are attached to the welding cable by soldering or by crimping. Electrode holder: It is used to hold electrodes during welding. The welding cable passes through the hollow insulated handle [Fig. 7.13(b)]. The electrode is held between two spring loaded jaws. Electrode holders are also rated in terms of current carrying capacity, for example 400 amperes and 600 amperes. Ground or earth clamp: It connects the earthing cable to workpiece either directly or through the welding table on which the workpiece is placed. Earth clamps [Fig. 7.13(c)] are made of gunmetal.

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Fig. 7.13

521

Various equipment or devices forming parts of an arc welding set-up.

Head screen and hand screen: These are used to protect eyes and face from spark and light rays emerging from the arc. The head screen [Fig. 7.13(d)] is worn on the head and kept in position by a band passing round the back of head. The hand screen [Fig. 7.13(e)] is kept in hand. A dark glass is provided with the screen to protect the eyes from ultraviolet and infrared radiations from the arc. Hand gloves: These are made of chrome leather or cloth and asbestos. They protect the hand (with which welding is done) from spark and spattering of metal. Chipping goggles protect eyes when slag or flux is removed from over the weld bead. Chipping hammer: The chipping hammer [Fig. 7.13(f)] is used to chip off the slag that solidifies along the weld bead.

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Wire brush:

It is used for cleaning the weld bead after chipping off the slag.

Protective clothings: These include apron, jacket, shoes, etc. Apron made of chrome leather protects the clothes of the welder. For overhead welding, a jacket is used to protect shoulders and arms. The welder should wear high topped shoes that go over the ankle to protect the legs from spatter, etc. Welding electrodes:

7.16

These have been discussed in the following.

ELECTRODES USED IN SHIELDED METAL-ARC WELDING

An electrode (or a filler rod) is a piece of metallic wire (called core wire) which may or may not have a flux coating on it. The electrodes are available in different standard lengths (250 mm, 300 mm, 350 mm and 450 mm). The core wire is made from different metals. The electrode is preferred to have its core wire made of the same metal as that of the workpiece as far as possible. In the flux covered electrodes, about 20 mm length at its one end is kept without the flux so that this end may be held securely in the grips of electrode holder [Fig. 7.13(b)] because the flux coating on electrode is a bad conductor of electricity. The other end of the electrode is used to establish arc between the workpiece and the electrode. The size of electrode is measured and designated by the diameter of the core wire. According to BIS specifications, electrode sizes are: 1.6 mm, 2 mm, 2.5 mm, 3.2 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm and 9 mm. Electrode size depends on the thickness of workpiece (Fig. 7.14).

Fig. 7.14

7.16.1

Selecting a flux coated electrode based on the thickness of workpiece for general purpose welding.

Current Carrying Capacity of Electrodes

The current used in welding depends on the thickness of workpiece and the size of electrode. The diameter of electrode is selected as per the job thickness. Typical data of current carrying capacity of medium-coated electrodes is given in Table 7.3.

ELECTRIC AND GAS WELDING PROCESSES

TABLE 7.3

Electrode size (mm)

523

Current carrying capacity of medium-coated electrodes

Current for light work (amp.)

6 5 4 3.2

220 180 140 90

Current for normal work (amp.) 260 210 160 110

As already stated, the electrode size and the corresponding current setting depends on the thickness of workpieces to be joined. Accordingly, the number of weld runs required to complete the joint is also dependent on the thickness of workpiece. In multi-run welds, the number of runs should be kept to the minimum by using the largest possible size of electrode, least angle of bevel (say 30o usually) and the minimum root gap.

7.16.2

Types of Electrodes

Consumable type electrodes used in shielded metal-arc welding are metallic electrodes and these are consumed during welding, forming part of molten metal which later solidifies as weld bead. The consumable electrodes may be of two types—(i) Bare electrodes, i.e. without any flux coating and (ii) Flux coated electrodes having a coating of flux on the core wire. The flux melts during welding and provides a cover on the newly made weld bead, thus protecting it against oxidation. In case of a consumable electrode, no additional filler metal is required to complete the weld as the electrode itself melts and works like a filler metal. But in case of non-consumable electrodes, an additional filler metal may be required to complete the weld. The composition of the core wire of the electrode (whether bare or flux coated) should be selected according to the base metal of the workpiece.

7.16.3

Electrode Core Wire Materials

The core wires of flux coated electrodes are made from a variety of metals to suit welding of different metals and alloys under varying welding conditions and requirements. Metals used for making core wire include: (i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix) (x)

Mild steel (semi-killed or killed) Low alloy steel Nickel steel Chromium-molybdenum steel Manganese-molybdenum steel Nickel-manganese-molybdenum steel Nickel-molybdenum-vanadium steel Aluminium Lead-bronze Phosphor-bronze

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MANUFACTURING PROCESSES

Constituents and Functions of Electrode Flux Coatings

The flux coatings on the core wire of coated electrodes comprise a number of ingredients that are used to serve some specific functions as discussed in the following. Slag forming ingredients: These are silicates of sodium, potassium, magnesium, titanium dioxide, iron oxide, china clay, silica and mica. On melting due to heat of arc, these light weight ingredients form a layer of slag on the molten metal and weld bead, protecting them from atmospheric contamination. Deoxidizing elements: These help saving the molten weld pool against oxidation. Ferro-manganese and ferro-silicon are used for deoxidizing and refining the molten metal. Gas shielding ingredients: These include cellulose, wood, wood flour, starch, calcium carbonate, etc. After burning, they form a protective gas shield (CO2) around the electrode end, arc and the weld pool. Arc stabilizing ingredients: These provide ease in initiating the arc and later stabilizing it during welding. The ingredients are calcium carbonate, potassium silicate and titanium dioxide (titania or rutile). Alloying elements: Certain alloying elements such as ferroalloys of manganese, and molybdenum present in the flux, impart improved strength and properties to the weld metal while making good the loss of important elements lost through vaporization during welding. Iron powder added to the flux improves bead appearance, besides imparting high metal deposition rates. Other functions: The flux coating on melting forms slag which protects weld bead and improves depth of penetration, bead finish and limits metal spattering (and hence a quieter arc). Flux coating improves resistance to hot and cold cracking of weld bead and makes vertical and overhead welding possible by quick freezing of the slag (molten flux) over the weld bead.

7.16.5

Bare vs Coated Electrodes

Bare electrodes could provide weld joints with good tensile strength but the welds were poor in ductility, impact resistance and fatigue strength, besides poor appearance of the weld bead. The poor performance of bare electrodes is attributed to (a) the vaporization of important elements of the weld metal during welding and (b) the presence of oxides and nitrides (as inclusions in the weld bead) resulting from the atmospheric contamination of weld metal. Because of these reasons, the bare electrodes are very rarely used. However, in coil form, these are used in metal inert gas (MIG) welding under protection with some inert gases like argon or helium. During welding of steels with bare electrodes, molten metal absorbs oxygen and nitrogen (from air) and forms oxides and iron nitrides which find their way into the weld metal as inclusions. These inclusions make the weld bead brittle which can crack easily. Electrodes with flux coatings behave differently. The flux coating vaporizes in the heat of arc and forms a protective gas (carbon dioxide) which keeps away nitrogen and oxygen from molten metal. Besides having cellulosic materials which give CO2 after burning, the flux also contains arc stabilizing materials like titanium dioxide (titania or often called rutile), calcium carbonate

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and potassium compounds. Iron oxides, manganese dioxide, mica, china clay, etc. present in flux help producing slag which, because of its light weight, forms a protective layer on the molten metal and later solidifies over the weld. It is finally removed by chipping hammer. Flux also contains deoxidizing materials like ferro-silicon, ferro-manganese and ferro-titanium which deoxidize the newly made weld bead.

7.16.6

Types of Flux Coating

Cellulosic coatings: Cellulosic coating is essentially a mixture of some binders with cellulosic materials such as wood pulp, sawdust or cotton. On burning, these materials generate large volume of gas (CO2). The electrode coating burns just a bit more slowly than the core wire of the electrode which results in forming a crucible at the end of the electrode that makes it easier to direct the arc (Fig. 7.1). These electrodes give deeply penetrating arc and can be used in any position. Weld finish is coarser and slag layer is thin and hence easy to remove. Such electrodes are mostly used with DC positive. These are suitable for welding where the job involves changes in welding position (like pipe welding). Mineral coatings (or rutile electrodes): With mineral coatings made from natural silicates such as asbestos and clay and by adding oxide of certain refractory metals such as titanium dioxide (called rutile), the harsh digging action of the arc is modified to produce one that is softer and less penetrating, with negligible spatter and easily removable slag. This type of electrode is used on both AC and DC to advantage and on sheet metals where shallow penetration is desired. A large amount of slag produced protects and controls the chemistry of the deposited metal as it cools. It also retards the cooling rate of the deposited metal, thus allowing any entrapped gas (in the molten metal) to escape and slag particles to rise to the top. A more homogeneous microstructure of the weld results. The mineral-coated electrodes are used most advantageously on downhand welding in making flat welds and those inclined up to 45o. With addition of other ingredients, the slag can be quite viscous or fluid. With very high titania contents, a large amount of fluid slag is produced and electrodes are suitable for vertical and overhead positions. Slag is easily removable. Iron-powder coatings: The use of iron powder in coatings gives high metal deposition rates and a consistent arc. The electrode may be just slowly dragged over the workpiece because the coating maintains the proper arc length. The slag is often self-removing and the appearance of the bead is improved. Iron-powder-coated electrodes give more weld length per electrode due to the iron in the coating going into the weld bead. Iron oxide coatings: Electrodes containing iron oxides give very fluid slag and are used for flat positions only. These are used for welding deep groove thick plates in flat position. Fluxes having high manganese oxides and/or silicates are also considered in this category of giving inflated slag. Low-hydrogen coatings: These are all-mineral coatings, containing no material that will form hydrogen. Low-hydrogen electrodes have coverings of those materials which will provide minimum or no hydrogen deposits in weld, such as asbestos, iron powder, clays, titania and lime. Hydrogen adversely affects alloy steels causing inter-granular under bead cracks, which finally results in hydrogen embrittlement and sudden failures. It is with this reason that basic mineral-coated electrodes, especially those used in welding stainless steel, manganese, and

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MANUFACTURING PROCESSES

molybdenum, are referred to as low-hydrogen electrodes. The low-hydrogen electrodes are especially formulated and packaged to keep the hydrogen content at a minimum. With these electrodes, porosity is also eliminated in welding steels of high sulphur content. A shorter arc should be used for better results. The low-hydrogen electrodes need to be stored and handled carefully avoiding the attack of moisture on these electrodes. It is better to redry them at 120oC before use.

7.16.7

Selection of Electrodes

The selection of a right type and size of electrode is important to have weld quality of choice. Following factors are considered important in selecting the electrode: (i) Base metal and its composition. The electrode metal should have its chemical composition more or less similar to the base metal to be welded. This ensures homogeneity of the deposited weld bead. (ii) Job thickness is the basis for selecting the right size (diameter of core) of electrode. (iii) Type of flux coating, whether cellulosic, rutile or low-hydrogen, is considered important from the point of view of arc behaviour, loss of metal elements due to vaporization, spatter and bead appearance, etc. (iv) Position of welding, flat, horizontal, vertical or overhead. (v) Type of joint, lap, butt, fillet and the number of weld-runs required to make proper size joint. (vi) Metal deposition rate is important from consideration of productivity or output of welding operation in mass production. (vii) Bead geometry and finish are sometimes important criteria for welds subjected to fatigue loadings. (viii) Type of power source available and polarity dictate terms many times in the selection of electrodes. Certain electrodes work well with DC and others with AC. So, also the polarity, DCSP or DCRP is considered in view of thickness of workpieces. (ix) Cost of electrode and easy availability are also important considerations in the selection of electrodes. Electrodes need to be handled, used and stored carefully, keeping in view that the dampness entrapped in the flux coating of electrodes results in violent arc, weld porosity and cracks. Damaged flux coating gives a weld joint of poor mechanical properties. Before use, the electrodes need to be dried as per manufacturers’ advice. The electrode should be stored in dry and well-ventilated place. These should not be allowed to bend. Special care is to be taken with low-hydrogen-type electrodes in regard to pre-heating, etc. (if required) before use. For correct identification, electrodes need to be retained in the original manufacturer’s packet. Electrodes are costly pieces and hence should not be used only half length or so, rather they should be used till they are left only about 50 mm or so.

7.16.8

Classification and Coding of Electrodes

Electrode classification gives information regarding the constituents of flux coating, nature of slag, type of current (AC/DC), polarity, arc behaviour, welding position, weld bead appearance,

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527

etc. According to the Bureau of Indian Standards (BIS), the classification and coding of electrodes is based on IS: 815-1956 and IS: 814-1963. An example of the code designation is given in the following: Let an electrode be defined as: Lf 123456LI wherein: Lf is the first letter which can be E or R, where E means solid extruded electrode and R means electrode extruded with reinforcement. 1st digit gives class of flux covering. It can be 1, 2, 3, 4, 5, 6 or 9. 2nd digit gives welding position. 3rd digit gives an idea of current, polarity and open circuit voltage of power source. 4th digit indicates tensile strength of weld metal in kg/mm2. 5th digit indicates percentage elongation of the deposited metal. 6th digit gives impact value in kg◊fm. The last letter LI can be P, H, J, K or L wherein P is for deep penetration electrode, H, for hydrogen controlled electrode, and J, K and L indicate electrode with iron powder coating and metal recovery 110 to 130%, 130 to 150% and above 150%, respectively. Digits from 1st to 6th are elaborated below. 1st digit gives class of flux covering as 1, 2, 3, 4, 5, 6 or 9 where: 1—High cellulosic content 2—High titania content (giving fairly viscous slag) 3—Appreciable titania content (giving fairly viscous slag) 4—High iron and/or manganese oxide and/or silicate contents giving inflated slag 5—High iron oxide and/or silicate contents giving heavy solid slag 6—High calcium carbonate and fluoride contents 9—Any other type of covering not mentioned above 2nd digit gives welding position for which a particular electrode would be most suitable. These can be 0, 1, 2, 3, 4 or 9. 0 and 1—All welding positions including flat, horizontal, inclined, vertical and overhead 2—Flat and horizontal 3—Flat only 4—Flat and horizontal fillet positions 9—Not classified above 3rd digit gives current, polarity and open-circuit voltage (OCV). Any number like 0, 1, 2, 3, 4, 5, 6, 7 or 9 can be the third digit. 0—D+, 1—D+, 2—D–, 3—D–, 4—D+, 5—D+, 6—D+, 7—D+, 9—Not

i.e. DCRP A 90, i.e. DCRP, or AC with OCV over 90 volts A 70, i.e. DCSP, or AC with OCV over 70 volts A 50, i.e. DCSP, or AC with OCV over 50 volts A 70, i.e. DCRP, or AC with OCV over 70 volts A 90, i.e. DCSP, DCRP, AC with OCV over 90 volts A 70, i.e. DCSP, DCRP, AC with OCV over 70 volts A 50, i.e. DCSP, DCRP, AC with OCV over 50 volts classified above

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MANUFACTURING PROCESSES

4th digit gives tensile strength as below: Digit 1 2 3 4 5 6 7 8 9

Minimum tensile strength (kg/mm2) 31 41 44 48 52 56 60 67 Any other not given above

5th digit gives percentage elongation of deposited weld metal as below: Digit 1 2 3 4 5 6

Elongation per cent, min. 14 18 22 26 30 Any other not given above

6th digit gives percentage elongation of deposited weld metal as below: Digit 1 2 3 4 5 6

Elongation per cent, min. 4.10 5.70 7.00 8.90 10.40 Any other not given above

Let us take an example. Suppose an electrode is available with the following specification on the electrode packet. Electrode Coding: E 307 421 It is explained as below. ● ● ● ● ● ● ●

7.17

It is a solid extruded electrode Titania covering, fluid slag All position electrode Operated on DCSP, DCRP, AC with OCV over 50 volts Average tensile strength is 48 kg/mm2 Elongation is 18% Impact value is 4.10 kg◊fm

WELDING POSITIONS

Welding positions refer to the situation created due to the placement of workpieces in a particular fashion. Various common welding positions are explained in the following. These are shown in Fig. 7.15. These are equally applicable to both electric arc welding and gas welding.

ELECTRIC AND GAS WELDING PROCESSES

Fig. 7.15

529

Welding positions.

1. Flat position: It is the easiest position of welding. It is also called down hand or downward position. In this position, the two workpieces (to be joined) lie flat such that their upper faces are in horizontal plane or they are held such that the welding axis remains in horizontal plane [Fig. 7.15(a)]. 2. Horizontal position: It involves making of a groove weld or a fillet weld. In case of groove weld, the two workpieces rest on edge one over the other having their flat faces in vertical plane [Fig. 7.15(b)]. The weld axis lies in horizontal position. After making weld tacks at a couple of locations along the joint, welding is completed starting from left or right (in case of gas welding, the filler rod preceding the weld torch). In case of fillet weld, the two workpieces are positioned at right angle to each other [Fig. 7.15(c)]. The axis of weld is again kept horizontal. 3. Vertical position: It is shown in Fig. 7.15(d). In this position, the weld axis is either in vertical plane or at an angle of less than 45° with the vertical plane. Welding is started from the bottom end of the joint. 4. Overhead position: It involves performing welding from the underside of the joint [Fig. 7.15(e)] such that the workpieces to be joined remain over the head of the welder. Both the workpieces and the weld axis remain nearly in the horizontal plane. Overhead welding is the most difficult position of welding and calls for greater skill on the part of the welder.

530 7.18

MANUFACTURING PROCESSES

WELDING JOINTS—TYPES AND SELECTION

The design and selection of a proper welding joint is essential to ensure adequate strength and other mechanical properties of the joint, controlled distortion of welded parts, minimum residual stresses and greater reliability and reduction in welding costs. The most important factors considered in the selection and use of a particular joint design include service, quality, safety and economy. A joint gives the position where two or more members of a machine or structure are met (or assembled) and later joined there in position by welding. The choice of making a joint by electric welding, gas welding or brazing very much depends on the base metal, plate thickness, intricacy of design, etc. Different types of weld joints are discussed in the following. Before welding any two pieces of metal, their edges (to be welded) are first properly prepared and this operation is called edge preparation. Edge preparation is also called bavelling or veeing or grooving. The edge can be prepared by machining or cutting by oxyacetylene flame or by grinding. The purpose of edge preparation is to get proper fusion of the weld metal through the entire thickness of the mating edges of the workpieces. A ‘gap’ of 1 to 3 mm is kept between two edges of the butt joint to get full penetration of the weld metal (Fig. 7.16). Various types of edge preparation for butt joint for different thickness of workpieces are shown in Fig. 7.17(b).

Fig. 7.16

Fig. 7.17(a)

Edge preparation prior to welding.

Different types of welding joints.

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531

Different types of weld joints, shown in Fig. 7.17(a) to Fig. 7.20, are preferred for some specific applications as discussed in the following. 1. Butt joints: Different types of butt joints with their edge preparations are shown in Fig. 7.17(b). Square butt joints with or without gap (between the edges of two workpieces) are used for plate thickness up to 8 mm and for smaller loads. No edge preparation is needed for this joint.

Fig. 7.17(b)

Types of butt joint with different edge preparations.

Vee joints find extensive use because edge preparation with vee is easy, although V-joints suffer from more distortion as compared to J and U joints. Bevel joints take medium loading effectively. Edge preparation is simple but full penetration poses problems. U joints are preferred to Vee joints as high quality joints and are often

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MANUFACTURING PROCESSES

used for pressure vessels having plate thickness 10 to 20 mm. These joints ensure better penetration, although edge preparation is difficult and costly. J joints are used for normal loading in pressure vessels where they are comparatively cheaper but full root penetration is difficult. Single joints (vee, bevel, J and U) are used for plates thickness 6 mm to 25 mm (single vee, 6 to 18 mm; single bevel, 10 to 25 mm; single J, 12 to 35 mm; single U, 10 to 25 mm). Double joints (double vee, double bevel, double J and double U) are stronger than single joints and are used for thicker sections of the plates. Double joints can stand better against different types of loadings such as impact, bending and fatigue. Longitudinal and circumferential butt joints of pressure vessels are often made with double joints by welding both sides of the joint. Double vee butt joints are strongest and are preferred for plate thickness 10 to 50 mm. Double bevel joints are used for 10 to 35 mm double; J joints for 15 to 50 mm and double U joints are used for 12 mm and above. Butt joints may also have straps to increase the strength of joint. Single and double strap joints are used. 2. Lap joints: Different types of lap joints are shown in Fig. 7.18. Making of lap joints needs no edge preparation. These make use of fillet welds which are the cheapest type of weld deposits as no edge preparation is needed. Since the fillet welds are not very suitable for carrying high loads (dynamic type), this generally limits the use of lap joints to applications of secondary importance.

Fig. 7.18

Different types of lap joints.

Lap joints may be single fillet, double fillet or plug weld type. Single fillet lap joints are not used in situations where bending, impact and fatigue loads are involved. Double fillet weld is stronger than single fillet weld. A plug weld is used where the bottom plate (second plate) is not accessible for fillet weld. The joint may be made with or without a tapered or straight sided hole in the upper member. 3. T-joints: T-joints with different edge preparations are shown in Fig. 7.19. It will be seen that single fillet T-joint is used for plates of smaller thickness and the joint should not be subjected to bending. Double T-joint is a strong joint and used for severe loading conditions.

ELECTRIC AND GAS WELDING PROCESSES

Fig. 7.19

533

T-joints with different edge preparations.

4. Corner joint and edge joint: Different types of corner joints and edge joints are shown in Fig. 7.20. Full open corner joints are recommended for any thickness of the plate, whereas half open corner joints and close corner joints are used for plates of smaller thickness.

Fig. 7.20

Corner joints and edge joint.

A fillet weld deposit is often used for making a variety of lap joints having either a triangular shape, concave shape or convex shape (Fig. 7.21). A fillet weld with flat face is often used as a cheaper weld as no edge preparation is needed for making it. The weld bead has approximately triangular section and is used for lap joint, Tee joint or corner joint or for joining two surfaces approximately at right angle. The flat face fillet weld, however, does not give a joint strong enough to withstand dynamic loads for which either convex or concave welds are used as per the designer’s recommendations.

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MANUFACTURING PROCESSES

Fig. 7.21

7.19

A fillet weld.

OTHER ARC WELDING PROCESSES

Important arc welding processes include: carbon arc welding, shielded metal-arc welding, submerged arc welding, gas metal arc welding, electroslag and electrogas welding, gas tungsten arc welding, atomic hydrogen welding and plasma arc welding. The shielded metal-arc welding process has already been discussed at length. The remaining processes will now be described in brief in the following.

7.19.1

Carbon Arc Welding

Carbon arc welding is an arc welding process wherein coalescence (fusion of base metals and their growing into each other) is produced by heating the workpiece with an electric arc struck between a carbon electrode and the workpiece (Fig. 7.22). Welding may be done in air or in the inert gas atmosphere (argon or helium). Filler material may or may not be used. Equipment include high voltage DC generator or rectifier set giving DC up to 600 amps, airor water-cooled electrode holder, and copper coated carbon or graphite electrode.

Fig. 7.22

Principle of carbon arc welding. Copper coated carbon (or graphite) electrodes are used for improved electrical conductivity. DCSP is preferred to restrict electrode disintegration and amount of carbon going into the weld metal.

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535

Carbon arc welding is used for welding of aluminium, copper, brass and other non-ferrous metals, repairing of castings, and brazing, pre-heating and post-heating of weldments. It is most suitable for butt welding of thinner metal (up to 2 mm) as workpiece distortions are negligible.

7.19.2

Submerged Arc Welding (SAW)

Submerged arc welding is an arc welding process wherein coalescence is produced by heating the workpiece with an electric arc (or arcs) set-up between a bare or copper coated metal electrode (or electrodes) and the workpiece. In this, the arc end of electrode, the molten pool and the arc are always invisible being submerged under the blanket of a granular flux material (Fig. 7.23). During welding, the continuously fed bare metal electrode melts and acts as a filler rod. No pressure is applied and weld is completed without usual sparks, spatter and smoke.

Fig. 7.23

Showing the working principle of submerged-arc welding process. The granular flux consisting of silica, lime, manganese oxide, calcium fluoride, etc. is fed to weld area (in the prepared Vee-groove) just ahead of the electrode tube (or gun). After melting, the flux forms a slag which on solidification covers and protects the newly made weld bead from atmospheric contamination.

The arc is struck by either touching electrode with the workpiece or placing some steel wool between the electrode and the workpiece before switching on the current. Arc is always struck under the flux. The flux (which is a bad conductor when cold), after melting, becomes a good conductor of electricity. AC or DC welding plants are used to give current up to 4000 amps. DCRP gives deeper penetration and DCSP gives flatter weld bead with faster speeds. AC is preferred above 1000 amps. Submerged arc welding is used for flat horizontal welding. Backing strips or bars are used for making butt welds in one pass. These strips are essentially required for butt welds in materials that oxidize quickly. The strip is placed on the underside of the weld joint and the joint groove (prepared edge) is flooded with flux (or shielding gas as in TIG welding) to

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protect the weld joint from atmospheric contamination. The process is effective for long stretches of welds. Faster speeds in welding thicker section of steels are possible, for example, 18 mm steel plate can be welded at a speed of 40 cm/minute in one pass. The process gives several advantages, for example, high quality weld, faster welding, deep penetration, adaptability to full mechanization, no sparks, no glare, no spatter, no smoke and no extra heat radiation. All these make this process highly suitable for mass production of steel sections.

7.19.3

Gas Metal-Arc Welding (GMAW)

Gas metal-arc welding, formerly called metal inert gas (MIG) welding, employs shielding gases, namely argon, helium, carbon dioxide and their mixtures. In this process, a consumable metallic electrode is fed automatically during welding which gives much faster speeds of welding. More common methods of gas metal-arc welding process may be divided into the following two distinct groups. (a) Metal inert gas (MIG) welding (b) CO2 welding or metal active gas (MAG) welding (a) Metal inert gas (MIG) welding: Arc is established between a continuously fed consumable metal electrode or wire and the workpiece (Fig. 7.24). Arc is shielded by inert gas like argon, helium, carbon dioxide or mixture of these gases. Electrode wire is fed from a coil rotated with a constant speed motor and the arc length is maintained constant by using (a) self-adjusted arc for semi-automatic plants or (b) self-controlled arc in fully automatic MIG plants.

Fig. 7.24

Set-up for metal inert gas (MIG) welding.

MIG welding employs power sources with flat or drooping characteristics. Mostly, DC generator or AC transformer with rectifier is used. AC is not generally preferred because of unequal/burn off rates in positive and negative half cycles. DCRP gives deeper penetration and is used for thicker jobs. Shielding gases used include argon, helium, CO2 or nitrogen and the mixtures thereof. Argon or helium is used for welding aluminium, magnesium and copper, and carbon dioxide for mild steel and nitrogen for copper. Argon plus CO2 are used for welding mild steel, low alloy steel and stainless steels, whereas a mixture of argon, helium and CO2 is used for welding austenitic stainless steel.

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537

MIG, with short-circuiting metal transfer mode, is popular for welding thin sheets and sections (less than 6 mm) of carbon steels, low alloy steels, stainless steels, aluminium, magnesium, copper, nickel and their alloys and also for welding tool steels and die steels. MIG with globular transfer mode gives deeper penetration and finds applications in aircraft, pressure vessels and ship building industry. Under similar conditions, MIG welding is a much faster process compared to TIG welding with advantages such as no flux needed, high welding speeds possible, difficult to weld metals like aluminium, and stainless steels are welded effectively. The process can be automated. (b) CO2 welding or metal active gas (MAG) welding: Gas metal-arc welding with CO2 gives smooth and high speed welds. The shielding gas is a carbon dioxide gas (CO2) which dissociates at temperatures about 7760oC during welding and by using deoxidants (like silicon and aluminium), the free oxygen (produced during dissociation of CO2) combines with these deoxidants and forms minute islands of slag that float out to the surface of the molten metal and protect the weld bead. In addition, CO2 is cheaper (one-ninth the cost of argon gas) with non-turbulent flow and it covers well over weld area. The CO2 welding is done in the similar way as MIG. The process is very popular and economical in welding light gauge steels. Because of good penetration capability, CO2 welding is also effectively used for fillet and butt welds in metals 0.63 to 38.10 mm thick. It is extensively used for welding carbon steels and alloy steels. It is also faster and a competitor to shielded metal arc welding for steels.

7.19.4

Electroslag Welding (ESW) and Electrogas Welding (EGW)

Electroslag welding is quite similar to vertical submerged arc welding. The essentials of electroslag welding are shown in Fig. 7.25 wherein two plates (A) and (B) having thickness (C) are welded in one pass. This joins the workpieces by casting the filler metal between the butted edges of the workpieces being joined and the joint is made in one pass. A granular flux is placed in the joint gap between the two plates being welded. During welding, the molten metal and slag are retained in position in the joint with the help of a pair of copper shoes (water cooled), which move automatically upwards as welding progresses. In the first instance, the arc is started between the electrode tip and the bottom portion of workpiece. The flux is added which gets melted by arc heat. With the melting of flux, a blanket of slag is formed (and covers the bottom tip of electrode) and the arc (between electrode and workpieces) goes out. The current is later conducted directly from electrode wire through the slag and thus the high resistance offered by the slag is responsible for causing most of the heating for the remainder of the welding process. Electroslag welding is used for welding hot rolled carbon steels, low alloy high strength steels and other thicker sections (50 to 900 mm thickness) with welding speeds up to 36 mm/ minute. The process is used for fabrication and building of heavy machines and nuclearreactor vessels and other structures. Current used may be up to 600 amperes. The process is quite automatic as once started, it will keep going till the welding is completed full length. Warping is minimum as heating is uniform. No joint preparation is required. The process of electrogas welding is similar in principle to electroslag welding with the difference that an inert gas is used for shielding purposes. The process is used for welding

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in one pass the edges of structural sections as also the circumferential welds with plate thickness varying from 12 to 75 mm. Inert gases used for shielding include carbon dioxide, argon or helium based on the type of material to be welded. Single or multiple flux-cored electrodes are fed to welding zone where a continuous arc is maintained during welding. The shielding gas may be provided from an external source or from flux-cored electrodes.

Fig. 7.25

Electroslag welding set-up.

Electrogas welding finds application for welding steels, aluminium alloys, titanium, etc. for the construction of pressure vessels, bridges, large diameter thick walled pipes, storage tanks and for building ships.

7.19.5 Gas Tungsten Arc Welding (GTAW) or Tungsten Inert Gas (TIG) Welding Electric arc is struck between the non-consumable tungsten electrode and the workpiece and shielding is provided by argon, helium or carbon dioxide or mixture of any two (Fig. 7.26). Filler rod may or may not be used (as in close fit joints). It is an all-position welding technique. Electrodes used in TIG welding are non-consumable type tungsten electrodes which vary in size from 0.13 to 9.52 mm and are of two main types: (a) Thoriated or (b) Zirconiated tungsten electrode which gives better electron emission, easy start of arc, cool running, better arc stability and restart of arc even at 15 amps. Pure tungsten electrode is seldom used.

ELECTRIC AND GAS WELDING PROCESSES

Fig. 7.26

539

Tungsten inert gas (TIG) welding.

Arc initiation is by (a) Touch start or by (b) High frequency power (AC). Both DC and AC are used. DC with straight polarity (DCSP) is more common for longer life of electrode. DCRP is used for welding aluminium, magnesium, nickel, molybdenum and copper where oxides are removed by blasting action of +ive ions of shielding gas striking the workpiece. In general, an AC power source is best for TIG welding of non-ferrous alloys. For ferrous alloys, DCSP is better for reducing volumetric loss from tungsten electrode. TIG was originally developed for welding magnesium and its alloys but it is now used for many alloys, particularly adopted to welding dissimilar metals and for hard facing worn or damaged dies. It is used for welding thin metal workpieces, up to 6 mm thickness and for welding of stainless steels, nickel and cobalt alloys, magnesium and its alloys, aluminium and copper alloys and titanium or highly alloyed metals where purity is essential. TIG welds are clean, smooth and do not need grinding or finishing. Backing strips are used for welding metals that oxidize rapidly, for example, aluminium, magnesium and titanium. The process is, however, slow in operation.

7.19.6

Atomic Hydrogen Welding (AHW)

In atomic hydrogen welding, heat for welding is produced by the combined effect of the electric arc and the chemical reaction resulting from the use of hydrogen gas for shielding purposes. Arc is maintained between the two non-consumable tungsten electrodes (inclined at an angle) in an atmosphere of hydrogen (Fig. 7.27). Filler rod may or may not be used. The chemical reaction occurring due to the dissociation of molecular hydrogen (H2) under high heat of arc results into an exothermic reaction yielding a lot of heat, which is in addition to the heat of arc. This combined heat helps a lot in effective welding of plain carbon steels, alloy steels, stainless steels, aluminium, copper and nickel, besides the hard facing of dies and tools and welding of heat-resisting alloys.

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Fig. 7.27

7.19.7

Atomic hydrogen welding set-up. The workpiece does not form the part of electric circuit.

Plasma Arc Welding (PAW)

Plasma is a partially ionized gas and is produced by the passage of gas through an electric field which separates it into electrons and positive ions. In plasma arc welding, a non-consumable tungsten electrode is used for initiating arc which can be developed (a) between the workpiece (+) and the electrode (–) called transferred arc, or (b) between the electrode (–) and the tip of the water-cooled constricting nozzle (+) called non-transferred arc (Fig. 7.28). In plasma arc welding, coalescence is produced by the heat obtained from a constricted arc set-up between tungsten electrode and workpiece giving temperatures between 8000 to 25,000oC (and more) depending on the plasma gas used. Such high temperatures are attained by the constriction of the arc, i.e. by forcing the arc through a water-cooled constricting nozzle at the bottom end of the inner gas cup (called constricting nozzle), thereby reducing the arc diameter and consequently increasing current density which results in increase in pressure, temperature and heat intensity of the arc. The process employs two inert gases, one for making arc plasma (orifice or plasma gas) and the other shielding gas for shielding the welding arc or welding zone. Gas lens are used to reduce turbulence in the flow of shielding gas. In plasma arc welding, a DC power source (generator or rectifier) having open-circuit voltage of about 70 volt is used. Currents may vary from 50 to 350 amps and voltage 27 to 30 volts. Normally, DCSP is used except for welding aluminium where DCRP is used. Argon is often used as the plasma gas and shielding gas. Helium or a mixture of argon and helium is also used. Both ferrous and non-ferrous metals are welded with plasma arc welding with thickness of parts usually up to 6 mm. The low current needle arc is well suited to weld light weight thin walled workpieces, thickness up to 0.8 mm, for use in radiators, exhaust manifolds, air ducting, metal mesh, fine wires, etc. High current plasma arc welding is mostly used for butt welding of joints in metals, 3 to 6 mm thick. Penetration and weld speed in plasma arc welding is much more than in the TIG welding.

ELECTRIC AND GAS WELDING PROCESSES

Fig. 7.28

7.20

541

Illustrating the principle of plasma arc welding using a transferred arc and a non-transferred arc shown at (a). The plasma arc welding set-up is shown at (b). For initiating plasma flow, a high frequency spark is first generated using high frequency current from the generator. After the start-up, DC from main power supply takes over to maintain ionization of gas.

RESISTANCE WELDING

Resistance welding is a process of joining two metal pieces based on heat (I 2R heating) and squeeze (pressure or forge) principle. It involves heating of the metal pieces at their weld junction (place of joint, to be made) due to the local resistance offered by the junction to the flow of current. The metal pieces to be welded are held pressed at their junction between two conductors or electrodes. A heavy current passed through the electrodes causes local heating of the joining edges of the workpieces to such a temperature which makes them to become plastic. The weld is later completed with the application of pressure that results in fusing and forging together of the joining edges of the workpieces into a sound joint. The local resistance at the weld junction comprises resistance of workpiece metals, contact resistance between electrode and workpieces, and contact resistance between mating surfaces of the workpieces at the weld junction. Current supply may be AC or DC, AC being more common.

7.20.1

Principle of Resistance Welding

As metals impede the flow of current, heat is generated. Resistance welding involves heating of metal at the weld junction because of giving local resistance to the passage of electric

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current. The metal is raised to a temperature which causes it to become plastic, and under pressure, it is fused or forged together as any blacksmith-forged joint. The amount of heat (H) generated depends on the amount of current (I), the length of time (T) for which current flows and the total resistance (R) of the circuit. The heat (H) generated would thus be proportional to I2RT (i.e. H μ I2RT). In resistance welding, usually large AC current of the order of 3000 to 1,00,000 amperes with a voltage between 0.5 and 10 volts is passed through the two metal pieces touching each other at the point of joining. A schematic of resistance welding process (spot welding) is shown in Fig. 7.29.

Fig. 7.29

7.20.2

Principle of resistance welding process (spot welding). In order that very high current may be obtained for welding, the primary circuit of transformer has many turns while the secondary winding has ordinarily one turn.

Variables of Resistance Welding

The main variables of resistance welding are Current, Resistance, Time and Pressure. Other variables include: welding machine characteristics, electrode’s design and surface condition of workpieces. Current: The heating of metals (or the temperature) is regulated by controlling the current, both in magnitude and in timing. As mentioned above, a very large amount of current is needed in resistance welding as low welding currents do not give proper fusion of the workpiece metal. Also, with increased current density, the weld time can be reduced, thus saving the electrode contact surface from overheating. Current supply in resistance welding may be: AC, DC or capacitor stored energy. Majority of resistance welding machines work on single phase AC of 50 cycles/second, employing a transformer that gives current at low voltage but with high amperage. In order that very high current may be obtained for welding, the primary circuit of the transformer has many turns while the secondary winding ordinarily has only one turn (Fig. 7.29). Resistance: The resistance here refers to the total resistance of the system comprising (a) Resistance of metal workpieces (R1), (b) Contact resistance between electrode and work pieces (R2) and (c) Resistance between the two mating edges or surfaces of the workpieces (R3).

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For getting a sound weld and avoiding overheating of electrodes, the resistances R1 and R2 should be relatively much less than R3. Since R1 depends on the type and thickness of the metal, it cannot be changed but R2 can be minimized by keeping electrode tip and workpiece surface clean. The shape and size of the electrodes and pressure exerted on them also matters in reducing R2. Electrodes should be made from highly conductive materials like coppercadmium or copper-chromium alloys. Similarly for reducing R3, the mating surfaces should be very clean and without dirt or scale of oxides. Time: There are several segments (or periods) of timing set-up in a resistance welding machine, for example, in a spot welding machine these are: (i) squeeze time, (ii) weld time, (iii) hold time and (iv) off time (Fig. 7.30). In squeeze time, the upper electrode comes in contact with workpiece and presses it with full force. At the end of squeeze time, welding current is applied. In welding time, the current flows through the circuit. In hold time, the pressure on the electrode is maintained and current is turned off. Off time is the interval between the end of hold time and the beginning of squeeze time of the next cycle. The weld time is controlled using an electrode timer.

Fig. 7.30

Segments of periods or timing set-up in resistance welding (spot welding).

Pressure on electrodes: There is a built-in system in the welding machines for exerting required pressure on the workpiece. Pressure on the workpieces is exerted by the electrodes extending from the arm of the machine using hydraulic, air pressure or lever mechanisms.

7.20.3

Advantages of Resistance Welding

Following are the advantages of resistance welding. ● ● ● ● ●

No filler metal is required. The operation in faster. Less skill is needed in operation of the machine. Similar and dissimilar metals can be welded. Elimination of warping of weldments is possible.

However, the equipment is slightly costlier than the manual metal arc welding. The resistance welding is restricted only to thinner sections.

7.20.4

Application of Resistance Welding

Resistance welding is used for joining sheets, tubes, bars for making furniture, aircraft and automobile parts, fuel tanks, wire fabrics, grills and containers.

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7.21

RESISTANCE WELDING PROCESSES

Resistance welding processes include the following. (i) (iii) (v) (vi)

7.21.1

Spot welding (ii) Seam welding Projection welding (iv) High frequency resistance welding Resistance butt welding: (a) Upset butt welding and (b) Flash butt welding Percussion welding

Spot Welding

The essentials of spot welding are shown in Fig. 7.31. Spot welding involves clamping of two pieces of metal between two copper electrodes, then applying pressure and later passing sufficient current to make the weld. Pressure, weld time and hold time are the fundamental variables of spot welding as already discussed (Fig. 7.30). Weld time is the period for which current flows, for example, for two 1.6 mm thick steel sheets, 0.25 second could be alright with a 60 cps power supply. Hold time is basically a cooling period. Water-cooled electrodes transfer heat more rapidly from weld. The pressure or squeeze to bring the two pieces together is very important as resistance (R) at joint interface is inversely proportional to pressure (P), that is, R μ 1/P. Different stages in making a spot weld are shown in Fig. 7.32. As the current (3000 to 1,00,000 amp) is passed, the temperature of the weld zone (small area below the electrode) reaches over 900oC and under the effect of constantly applied pressure, a joint is forged.

Fig. 7.31

Showing essentials of spot welding set-up at (a) and weld-joint (nugget) at (b).

Fig. 7.32

Showing various stages in making a spot weld.

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Spot welding machines are of various types: (a) Rocker-arm type machines, (b) Press type machines, (c) Portable machines called guns and (d) Multi-electrode machines. The rocker-arm type machine (Fig. 7.33) is the most simple type.

Fig. 7.33

Rocker-arm type spot welding machine.

The main features of the process are as follows: ●

● ●

●

7.21.2

Spot welding is widely used for the fabrication of sheet metal parts. Attachment of braces, brackets, pads and clips to formed sheet-metal parts like covers, trays and cases are best done by spot welding. The technique finds very extensive use in automobile and aircraft industry. Different types of stainless steel articles like utensils and cutlery are spot welded. Aluminium, aluminium-magnesium alloys, copper and its alloys, nickel and its alloys and monel metals are also effectively welded. Low cost, simple, no distortion high speed in welding, less skill needed, no edge preparation, operation may be semi-automatic or automatic.

Seam Welding

Seam welding is a continuous type of spot welding wherein instead of using pointed electrodes, work is passed between copper wheels acting as electrodes (Fig. 7.34). Thyratron and ignitron tubes are used to make and break the circuit which results in giving the completed weld an appearance of a series of overlapping spot welds resembling the stitches. Rotating electrodes and a regularly interrupted current give such welds. The process produces highly efficient water-tight and gas-tight joints. The two workpieces to be seamed are cleaned, overlapped suitably and placed between the two circular electrodes that clamp the workpieces together by electrode pressure. Current impulses are applied through the rollers to the workpieces being welded. The heat thus generated makes the metal plastic, and pressure from the electrodes

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completes the weld. Most seam welding is limited to sheet metals 0.25 to 3.12 mm thick and intermittent spots can be made at the rate of 600 spots per minute, 12 mm apart. When a continuous fusion zone made up of overlapping nuggets is obtained, the process is called stitch seam welding (as shown at (a) in Fig. 7.34). But if nuggets are obtained at constant time interval, the process is called roll-spot welding (as shown at (b) in Fig. 7.34). The electrodes are cooled by refrigerant fluids that flow inside the electrodes. Seam welding machines usually operate on single phase AC power.

Fig. 7.34

The two methods of seam welding: (a) Stitch seam welding and (b) Roll-spot seam welding.

Seam welding is used for welding carbon steels, alloy steels, stainless steels, alloys of aluminium, nickel and magnesium. The method gives water and air tight joint. Seam welding is applied to lap-joint seams of cylinders and cabinets and to circular seams for welding bottoms at the end of cylindrical tanks.

7.21.3

Projection Welding

Projection welding is a variation of spot welding wherein small projections are first raised on one side of the sheet (workpiece) with the help of a punch and die. These projections help in localizing the heat of the welding circuit during the welding process. Later on, these projections collapse due to high heat and pressure, resulting in bringing the parts to be joined in close contact (Fig. 7.35) and thus finally making a weld joint.

Fig. 7.35

Illustrating the principle of projection welding. The single-spot projection welding is shown at (a) and (b). The multi-spot projection welding is shown at (c).

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Metals welded by projection welding include: low carbon steels, tin plates, galvanized sheets, naval brass, stainless steel and titanium alloys. One of the common uses of this process is for attaching fasteners, nuts, studs and such parts on the larger parts. The process finds use in automobile industry and in welding refrigerator parts, grills, etc.

7.21.4

High Frequency Resistance Welding (HFRW)

High frequency resistance welding employs high frequency current (up to 450 kHz). In this process (Fig. 7.36), the current, instead of following a direct low-resistance path (between the two contact probes), follows a long, low inductance circuitous path, as shown by arrows in Fig. 7.36 wherein the high frequency resistance welding is shown for making a butt-welded tubing. Note that the current is conducted through the sliding contact probes to the edges of roll-formed tube and the heated edges are then pressed together by passing the tube through a pair of squeeze rolls. This method allows high intensity heating in shallow zones (which gives minimum heat-affected zone). Also, the high intensity heating in localized area provides the whole length of two edges of the workpieces metal to reach welding temperature at the same rate as the roll-formed tube passes through the squeeze rolls.

Fig. 7.36

Schematic of high frequency resistance welding.

High frequency resistance welding is often used for welding very thin sheets (0.1 to 0.5 mm thick) by lap welding. It is a fast speed welding process used for welding spiral pipes and tubings, finned tubes (for heat exchangers), etc. Dissimilar metals are also welded effectively, for example, welding of high speed steel teeth to high carbon steel backing for making band saws.

7.21.5

Resistance Butt Welding

Resistance butt welding is of two types: (a) Upset butt welding and (b) Flash butt welding (Fig. 7.37). 1. Upset butt welding: In upset butt welding (Fig. 7.38), coalescence is produced simultaneously over the entire area of abutting surface by the heat obtained from the resistance offered to electric current through the area of contact of the abutting

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surfaces. The material to be welded, usually a bar stock, is clamped so that the ends of the workpieces are in contact with each other. During welding, some pressure is first applied on the workpieces; later heating is started and maintained. Pressure is then increased to give a forging squeeze upset. Upsetting takes place while the current is flowing and continues even after the current is shut off. Upsetting action results in mixing workpiece base metals homogeneously and pushing out the impurities. High density current, 300 to 800 amps per cm2, is used for welding.

Fig. 7.37

Resistance butt welding: (a) Upset and (b) Flash.

Fig. 7.38

Resistance upset butt welding.

The materials butt-welded are in the form of wires, bars, strips or tubes. Metals welded include copper alloys, low carbon steels, stainless steels, aluminium and nickel alloys. The process is used for welding small strips of ferrous and nonferrous metals, for welding longitudinal butt joints in tubes and pipes. 2. Flash butt welding or flash welding: Flash butt welding has largely replaced upset butt welding. In flash butt welding, the ends of the workpiece stocks are first clamped with a slight gap (Fig. 7.39). When the current is turned on, it jumps

ELECTRIC AND GAS WELDING PROCESSES

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through the gap creating a flash, followed with a great deal of heat. The two workpieces are moved towards each other in an accelerated motion and as they reach the proper temperature, they are forced together under high pressure and the current is cut off.

Fig. 7.39

Resistance flash butt welding. (a) Shows the flashing created due to the jumping of current through the small gap between the two ends of the workpieces, (b) Shows the completed weld with a visible fin at the joint.

The process produces a homogeneous weld between two sheets, wires or bars without overlapping and without the addition of filler metal. Metals welded by flash welding include: low carbon steels, low alloy steels, stainless steels, tool steel, aluminium alloys, copper, magnesium and nickel alloys. Flash welding is suitable for end to end or edge to edge joining of metal sheets (of similar or dissimilar metals) and plates with thickness 0.2 to 25 mm and metal bars of diameter 1 to 75 mm. In general, it is used for butt welding of sheets, tubes, bars, strips, forgings and fittings. The process is used in automobile industry, aircraft industry, refrigerators and farm implements. Following are the differences between flash butt welding and upset butt welding: ●

●

● ●

No arcing (or flashing) takes place in upset butt welding between the faces being joined and the heat is produced solely by the electrical resistance (at the abutting faces) to the passage of the current. In flash butt welding, arcing takes place during welding. The movable platen in flash butt welding keeps on moving constantly towards the stationary platen which does not happen in upset butt welding. Less current is consumed in flash butt welding but time is more. Heat application in flash welding precedes the pressure application whereas in upset butt welding, constant pressure is applied during heating processes.

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7.21.6

Advantages claimed in flash butt welding are more, for example, it gives joint strength factor of 100%, cheaper process as cost of current per weld is smaller, dissimilar metals are welded efficiently, faster process, and gives a smaller upset. Besides, flash welding can be used to weld highly alloyed steels as it avoids hardening and cracking in them.

Percussion Welding

Percussion (or stored energy) welding is a resistance welding process wherein coalescence is produced simultaneously over the entire area of abutting surfaces by the heat obtained from the arc produced by a rapid discharge of electrical energy, with pressure percussively (rapidly) applied during or immediately following the electrical discharge (Fig. 7.40). Arc between faces of workpieces is struck by applying a DC voltage, high enough to ionize the air gap, or by super-imposing an auxiliary high frequency, high voltage AC in the circuit and the welding force given by spring, elecromagnetically or using other means.

Fig. 7.40

Percussion welding set-up.

Percussion welding is mostly used for welding aluminium and its alloys, besides welding of copper alloys, low carbon steels and stainless steels and copper to molybdenum. Because of extreme brevity of arc, weld formed is clear and without any upsetting effect.

7.22

THERMOCHEMICAL WELDING PROCESSES

Two types of thermochemical welding processes are: (i) Thermit welding (ii) Atomic-hydrogen welding

7.22.1

Thermit Welding

Thermit welding is a fusion welding process in which no outside heat source is required for melting the components to be joined but still a very high temperature (3000 to 4000oC) is obtained from the exothermic chemical reaction (a reaction in which heat is released out) that takes place inside a thermit, which is employed for welding the joint. The thermit is a mixture of some metal oxide (mostly iron oxide or sometimes copper oxide) and a metal reducing agent, usually finely divided aluminium powder (although magnesium is also used sometimes).

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551

Thermit welding is employed for welding steel and cast iron. Copper, nickel and manganese are also welded with this process. The main use of the process is for joining or repair of shafts, rails of railway tracks, machinery frames and gears and also for welding lugs to copper cables. Different types of thermits are employed for welding different metals. The most commonly used thermit contains about one part by weight of aluminium and three parts of magnetic iron oxide (Fe3O4). For starting the chemical reaction, the thermit is required to be first heated to a temperature above 1206oC. A special ignition powder is employed to start the ignition of the thermit mixture by bringing it to the above mentioned temperature at which the chemical reaction starts. The ignition powder is, in turn, ignited by a burning magnesium ribbon, or with a match. During thermit reaction, the aluminium oxide (Al2O3) is formed and it floats as a slag over the molten iron that settles down in the bottom of the container (or mold prepared for the joint to be welded). The mold and crucible employed in a typical case of thermit welding are shown in Fig. 7.41. The chemical reaction is as below: 8A1 + 3Fe3O4 Æ Fe + 4Al2O3 + Heat

Fig. 7.41

Thermit welding.

By transfer of heat, which gives approximately double the melting temperature of steel, the temperature of the workpieces is raised until they reach their fusion point. Such welds are sound because the metal solidifies from inside towards the outside and all the air is excluded. Thermit welding is different from foundry casting method in the sense that the metal being poured in the mold is at a considerable higher temperature than a molten metal used in foundry casting. Another use of thermit welding technique is in the welding of lugs to copper cables. The joint to be made is housed in a graphite mold, and powdered copper oxide and powdered aluminium are placed in the joint and ignited by a spark. Process being exothermic, heat is given out which melts the ends of the cable leads and the lugs and a good joint is formed.

7.22.2

Atomic-hydrogen Welding

It is a welding process wherein coalescene (fusion) is produced by heating the job with an electric arc maintained between two non-consumable tungsten electrodes in an atmosphere of hydrogen, which also acts as a shielding gas. Filler rod and pressure may or may not be used.

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In atomic-hydrogen welding, the job does not form a part of the electrical circuit. The arc remains only between two tungsten electrodes and the edge of the arc fan is used to weld the workpieces (Fig. 7.42).

Fig. 7.42

Atomic-hydrogen welding set-up.

It is the combined energy of the arc and a chemical reaction, which is utilized for welding. The arc supplies energy for a chemical reaction to take place which is described below. The molecular hydrogen (H2) passes through the electric arc and it dissociates into atomic hydrogen (H) by absorbing the energy supplied by the electric arc. H2

H + H – 100,700 calories

The atomic-hydrogen thus formed is unstable and has a tendency to revert to molecular state. This recombination takes place as the atomic hydrogen reaches a comparatively cooler region just outside the arc boundary or as the atomic hydrogen touches relatively cold workpiece to be welded. The recombination is an exothermic reaction which liberates large amount of heat (temperature being approximately 3739°C). This heat combined with that of the arc produces temperature higher than those in oxy-acetylene or metallic arc welding processes and is utilized for welding. Moreover, owing to very high heat conductivity of hydrogen at elevated temperatures, the heat is delivered to workpieces at very fast rates. Applications

Atomic-hydrogen welding is used for welding plain carbon steels, alloy steel, stainless steel, aluminium, copper and nickel. It is also used for hard facing or surfacing of dies and tools, and welding of heat-resisting alloys. The process is most commonly used for welding of stainless steels.

7.23

RADIANT ENERGY WELDING PROCESSES

These include (a) laser beam welding and (b) electron beam welding.

7.23.1

Laser Beam Welding

LASER stands for ‘Light Amplification by Stimulated Emission of Radiation’. The schematic of a laser welding process is shown in Fig. 7.43. The operation of laser consists of firing a

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brilliant light (as capacitor discharges into a helical flash lamp or Xenon tube) and directing this light into a ruby rod by using parabolic reflecting mirrors placed on either end of ruby rod. Ruby comprises aluminium oxide with chromium dispersed throughout it.

Fig. 7.43

Schematic of laser beam welding.

When the reflected light beams fall on the ruby rod, it stimulates the chromium atoms of ruby rod to a level higher than normal energy levels resulting into emitting a quantum of light energy in the form of red fluorescent light. As the red light emitted by one excited chromium atom hits another chromium atom (of ruby rod), the second atom also gives off red light which is in phase with the colliding red light wave. The red light thus produced reflects back and forth along the length of ruby rod. The chain reaction collisions between red light and chromium atoms become so numerous that finally, the total energy bursts over a threshold and escapes from a small hole in the mirror at one end of ruby rod as the laser beam. This laser beam is focussed to produce small intense light spot of high energy on the job to melt or vaporize any known material. Lasers are of following different types. (i) (ii) (iii) (iv)

CO2 (pulsed or continuous wave) Nd: YAG (neodymium: yttrium-aluminium-garnet) Nd: glass, ruby Excimer lasers

Laser welding gives a very high depth to width ratio (typically 4 to 10) with minimum shrinkage and distortion. Welding of transmission components in automobile industry is one of the most common uses of this process. It is also used for welding thin metals and for cutting metals. Microwelding applications include miniaturized components and attaching leads on to small electronic components and integrated circuitry.

7.23.2

Electron Beam Welding

Electron beam welding (Fig. 7.44) is the process in which heat required to produce fusion of workpiece metal is obtained from the impact of high velocity and high density narrow beam of electrons on the workpiece. Upon impact, the kinetic energy of electron is converted into thermal energy causing melting of metal. The process needs special equipment to focus the electron beam in a vacuum (as air molecules interfere with beam); higher the vacuum, more is the penetration of beam. The process gives a high depth to width ratio, up to 25 to 1.

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Fig. 7.44

Essentials of electron beam welding.

The electron beam welding is employed for welding special metals and alloys such as zirconium alloys for reactors, titanium and alloy steels for space industry and aircraft manufacturing. Highly reactive metals and dissimilar metals are effectively welded.

7.24

SOLID-STATE WELDING PROCESSES

A solid-state welding process produces coalescence at temperatures below the melting point of the base material being joined and without the addition of a filler metal but with the application of pressure. For satisfactory welding, at least one of the metals being joined must be highly ductile and should not exhibit extreme work-hardening. Following are the types of solid-state welding processes: (i) Cold welding (ii) Diffusion welding (iii) Ultrasonic welding (iv) Explosive welding (v) Friction welding and Inertia welding (vi) Forge welding

7.24.1

Cold Welding

Ordinary bright metallic surfaces consist of hills and valleys not seen by naked eye. A layer of metal oxide (20 to 200 molecule thick) exists on the metal surface carrying on its top a

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moisture-absorbed oxide layer. In cold-working, the surfaces are cleaned for degreasing and a part of oxide removed by wire brush. When two such partly cleaned surfaces are pressed together, the remaining thin oxide film from high spot fragments and metal behind that suffers plastic deformation under pressure. Metal-to-metal contact occurs. Pressure is applied over a narrow strip. The pressure applied is so much that the original thickness is reduced to nearly one-fourth. The ductility of the metals produces a true fusion condition. Two metal sheets are brought into overlapping contact and a punch is pressed into them (Fig. 7.45). The interface between the sheets is thereby subjected to a transverse tensile strain while it is under a high compressive stress. The tensile strain causes fragmentation of the oxide film, permitting metallic contact to bond the two sheets.

Fig. 7.45

Cold welding.

Applications

Cold welding is used to weld aluminium, copper and its alloys, aluminium to copper, nickel to iron, etc. The process finds extensive application in cladding and joining many similar or dissimilar metals. Cold welding is also used for joining metals in explosive areas.

7.24.2

Diffusion Welding

In this, coalescence of the meeting surfaces is produced by the application of pressure and elevated temperatures to carefully cleaned and mated metal surfaces so that they actually grow together by atomic diffusion. Similar or dissimilar metals can be joined without the use of filler metal. Diffusion welding involves several stages (Fig. 7.46). To achieve intimate metal-to-metal contact between the pieces of metal, pressure (350 to 700 kg/cm2) is applied that deforms high peaks of the metal surface breaking the surface oxide layers. For achieving the diffusion and grain growth to complete the weld, temperatures up to 1100°C may be used. After the metal-to-metal contact is established, the atoms are within the attractive force field of each other and hence a high strength joint is established.

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Fig. 7.46 Different stages of diffusion welding process: (a) Surfaces I and II to be welded shown with peaks and valleys (asperities) in a magnified view. (b) Deformed peaks and valleys under pressure. (c) Metal-to-metal contact at places where oxide film disrupted. (d) High strength joint developed as atoms are within attractive force field of each other after metal-to-metal contact. (e) Development of a stable metallic bonding between two surfaces being welded.

Diffusion welding is done by gas-pressure bonding and vacuum fusion bonding. In gas-pressure bonding, parts in intimate contact are heated to about 815°C and an inert gas atmosphere is used around the weld. This method is used for welding non-ferrous metals. Vacuum fusion welding is used for ferrous metals. Heating may be done up to 1150°C and pressure applied may be up to 700 kg/cm2 and process carried out in a vacuum chamber. Applications

Diffusion welding finds application in the fabrication of reactor components in atomic energy industry, rocket engines, helicopter rotor hub, missiles, bombers and space shuttle. It is also used for the fabrication of composite material, i.e. dissimilar metals. Diffusion welding is used for joining titanium alloys, zirconium alloys and nickel-base alloys.

7.24.3

Ultrasonic Welding

Ultrasonic welding is a solid state welding process wherein coalescence is produced by the local application of high frequency vibratory energy to the workpieces as they are held together under pressure. The workpieces are clamped together under modest static force normal to their interface and oscillating shear stresses of ultrasonic frequencies (10 kHz to 75 kHz) are applied approximately parallel to the plane of interface for about one second. The combined effect of pressure and vibration causes movement of the metal molecules, and brings about a sound union between the faces of materials in contact. Pieces to be welded are clamped between the welding (sonotrode) tip and an anvil (Fig. 7.47). Both sonotrode tip and anvil are faced with high speed steel, since considerable wear can occur at the contacting faces. A frequency converter converts 50 cycles line power into high frequency electrical power and a transducer changes the high frequency electrical power into ultrasonic vibratory energy which is transmitted to the welding joint through the welding sonotrode tip attached to the transducer. The tip oscillates in the plane of the joint interface. To start with, some triggering mechanism lowers the welding head, applies necessary clamping force and starts the flow of ultrasonic energy.

ELECTRIC AND GAS WELDING PROCESSES

Fig. 7.47

557

Essentials of an ultrasonic welding set-up.

Ultrasonic vibrations, combined with the static clamping force, induce dynamic shear stresses in the workpieces, then local plastic deformation of joint materials occurs at the interface. Oxide coatings and other surface films are shattered and dispersed so that intimate contact and bonding of the workpiece surfaces take place. Applications

Ultrasonic welding is especially useful for welding plastic parts, eliminating solvents, heat or adhesive. It is used for welding of aluminium sheets (up to 2 mm thickness), electrical and electronic components, hermetical sealing of volatile substances and preparing bimetallic junctions.

7.24.4

Explosive Welding

Explosive welding is the technique of using explosive charges to form a metallurgical bond between two pieces of metal and wherein coalescence or fusion is effected by high velocity impact between the two mating surfaces. The essentials of two modes of explosive welding operation, (a) parallel arrangement and (b) inclined arrangement, are shown in Fig. 7.48. The flyer plate (metal 1) and the parent plate (metal 2) are to be welded. The flyer plate is propelled by an explosive charge to impact on and unite with parent plate supported on anvil. The buffer above the flyer plate may be of rubber or cardboard and is for protection of flyer plate from the detonation of explosive. Above the buffer is a layer of explosive which is detonated from the lower edge such that under the effect of tremendous pressure generated due to detonation of explosive, the flyer plate is driven down to give a high velocity impact on the parent plate. As the explosive is ignited, detonation wave-front progresses across the surface of flyer plate in a straight-forward manner. As a result of explosive impulse, extremely high normal pressure accompanied by some shear or sliding pressure takes place between the flyer plate and the parent plate. At the point of impact, say P, a high instantaneous pressure is generated (which is much more than the shear strength of the mating plates) which causes a portion of the two mating surfaces (called jet) to become fluid and be expelled. The severe deformation of the colliding surfaces and the resulting jet breaks up any surface film, forcing the surfaces into intimate contact. The jetting phenomenon, required for bonding, causes the collision point to become plastic and flow into the space between the two plates. By this mechanism, the necessary condition for the formation of a direct metal-to-metal bond occurs. Melted zones are formed, but at discrete intervals along the bond rather than as a continuous layer (Fig. 7.49). The surface jetting contributes greatly to the strength of weld, providing a mechanical lock in addition to the metallurgical bond.

558

MANUFACTURING PROCESSES

Fig. 7.48

Two modes of explosive welding operation.

Fig. 7.49 Photomicrograph of tantalloy explosively welded to a columbium alloy. Note the “Rippled” effect at the interface, providing a mechanical lock in addition to the metalurgical bond.

Advantages of explosive welding include simplicity of process, very large surfaces can be welded, thin foils can be bonded with thick plates, even heat treated components can be welded, plates of wide range of thickness can be welded and weld joint has no porosity and hence better strength of the weld joint is ensured. Applications

Application of explosive welding is in welding and cladding of metals. A number of dissimilar metal combinations are explosive welded, for example, aluminium to steel, tungsten to steel, titanium alloys to Cr-Ni steels, etc. Metals such as zirconium, titanium, stellite, copper and nickel alloys are cladded on carbon steels and low alloy steels. The process finds use in space

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559

and nuclear application. Major application is in explosive cladding of heat exchanger tubes, pressure vessels, die-casting industry, chemical industry, ship building and cryogenic application.

7.24.5

Friction Welding and Inertia Welding

In friction welding, coalescence is produced by the heat obtained from mechanically induced sliding motion between the two rubbing surfaces. The temperatures developed are below the melting point of the metals being welded but high enough to create plastic flow and intermolecular welding. The workpieces are held together under pressure during welding. The operation is carried out on lathe machines [Fig. 7.50(a) and Fig. 7.50(b)].

Fig. 7.50

Inertia welding is similar to friction welding with the difference that the energy supply in inertia welding is from a rotating flywheel (Fig. 7.51) whereas in friction welding, energy supply is from a conventional system of electric power or hydraulic power.

Fig. 7.51

Operational steps in inertia friction welding using a flywheel.

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MANUFACTURING PROCESSES

Friction welding variables include rotational speed, contact pressure and time of welding. The relative rotational speed varies from 1500 rpm (for carbon steels) to 3800 rpm (for aluminium). The contact pressure in heating phase is between 280 kg/cm 2 (for aluminium) and 525 kg/cm2 (for steel) and in forging phase, the contact pressure may go up to 1120 kg/cm2. Welding time is up to 15 seconds. Materials welded include aluminium and alloys, brass, bronze, stainless steel, nickel alloys, carbon steels, tool steels, etc. Dissimilar metals welded are alloy steel to carbon steel, super alloys to carbon steels, copper to carbon steel, copper to aluminium, etc. Applications

Friction welding finds application in automobile industry, replaces brazing and arc welding in many situations like production of bimetallic shafts, joining of super alloys turbine wheels to steel shafts, cutting tools of high speed steel welded to carbon steel shanks, etc.

7.24.6

Forge Welding

It is the oldest known welding process, although because of some difficulties involved in carrying out this process, it has restricted use for welding wrought iron and low carbon steels with job thickness up to 30 mm. Workpieces are heated to plastic stage in a furnace fired by coal, coke or oil and gas. Ends of the workpieces to be joined are prepared by upsetting to different geometrical shapes to facilitate welding; a typical swedged joint edge preparation is shown in Fig. 7.52 for a lap scarf joint. The workpieces are heated to above 1000°C to plastic stage and later placed on the anvil end-to-end and hammered down to form the joint. Flux is also used, mostly borax with sal ammoniac.

Fig. 7.52

Forge welding for making a lap scarf weld.

Applications

Forge welding is used for welding solid parts only. Metals forge-welded are wrought iron and low carbon steels. It is a slow process and needs furnaces etc. and skilled welder. Forge welding is used in blacksmith shop for general repair works. Also it is used in rail-road shops and for making pipes from plates rolled to cylindrical form wherein long edges are butted together in the dies at high forging temperature.

7.25

UNDERWATER WELDING

Ocean covers 70% of the earth. Development of offshore gas and oil fields, fisheries, mineral resources and mining in sea bottom and repairing or salvaging of ships and other structures

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561

in the sea called for the development of underwater welding. Problems encountered in underwater welding are: (a) Chilling action of water on weld metal. (b) Higher water pressure under which welding takes place. (c) A gaseous envelop surrounding the arc due to the combustion of flux on electrode and the dissociation of water. All this makes the arc unstable. Underwater welding is done in two ways: (i) Wet welding (ii) Dry welding (a) Hyperbaric welding (b) Cavity welding

7.25.1

Wet Welding

Waterproof electrodes are used and welding is done in the most normal way as done outside the water. Since the arc and the weld are not protected, the hydrogen generated due to dissociation of water gets dissolved in weld giving consequential tendency to cracking.

7.25.2

Dry Welding

It needs a pressured enclosure around the weld so that it does not come in contact with water. In hyperbaric welding, a chamber is constructed around the joint to be welded and water expelled by providing a pressurized gaseous atmosphere in the chamber or enclosure at a pressure equal to water pressure to keep water away. In cavity welding, the conventional arrangements of feeding wire and shielding gas are surrounded by a means for introducing a cavity gas and the whole is surrounded by a trumpet-shaped nozzle through which a high velocity water jet passes (Fig. 7.53). Cavity welding avoids the need for a chamber around weld joint. The process tends to automation and remote control itself. Shielded manual arc welding, TIG and MIG are adopted for underwater welding.

Fig. 7.53

Underwater welding with cavity welding technique.

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MANUFACTURING PROCESSES

7.26

GAS WELDING

Gas welding is a method of joining two metal pieces (similar or dissimilar) together by melting and fusing their edges at the joint. It involves applying intense concentrated flame on the metal pieces (at the joint) until an area (of both pieces) under the flame becomes molten and forms a liquid puddle such that the molten puddle of one metal piece mixes up and runs together with the molten puddle of another metal piece. The welding rod (or filler rod) may or may not be added to the molten metal puddle. The molten puddle on cooling and solidification results into a strong joint. The flame for melting the metal pieces is produced by burning various fuel gases. Fuel gases used in gas welding include: acetylene, hydrogen, city gas, natural gas or liquefied petroleum gas (LPG). The gases (oxygen and fuel gas) are mixed in proper proportion in a welding torch which carries two regulators—one for controlling the quantity of oxygen and the other for controlling the quantity of fuel gas. The mixture of oxygen and acetylene is most popularly used for gas welding and produces temperature within a range of 3200–3300oC, which makes it possible to melt and weld all common metals. A filler rod (that also melts during welding) makes the joint stronger on solidification (Fig. 7.54).

Fig. 7.54

7.26.1

Illustrating the working principle of oxy-acetylene gas welding.

Types of Gas Welding Process

Gas welding includes all those welding processes in which gas flame is used as a heat source for melting metals. It is further divided into three main types: (a) Air-acetylene welding, (b) Oxy-acetylene welding and (c) Oxy-hydrogen welding. (a) Air-acetylene welding involves the use of mixture of acetylene gas and oxygen from atmospheric air. A lower temperature flame is obtained. The process is used for welding lead or for brazing and soldering operations. (b) Oxy-acetylene welding is the most popular process because of higher flame temperatures (about 3200oC). A mixture of acetylene and oxygen is burnt for making flame. Both the gases are readily available in cylinders of different capacity. The combination is used for both welding and cutting of metals. (c) Oxy-hydrogen welding involves burning of the mixture of hydrogen gas and oxygen for producing flame. The hydrogen flame, however, does not attain that high temperature

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563

as obtained by burning oxygen and acetylene. It is with this reason that oxy-hydrogen welding is used for welding metals with low melting point such as aluminium, magnesium or for brazing purposes. Since hydrogen itself is a reducing agent (antioxidation), its flame minimizes oxidation of metal during welding. Hydrogen has no odour and is available in cylinders. Hydrogen connections need to be checked regularly as hydrogen makes a powerful explosion with air or oxygen. It is used for underwater gas cutting and welding at depth greater than 5 metres to 50 metres as hydrogen can sustain high pressures than acetylene. Since the oxy-acetylene welding (and cutting) is the most popular method used in industry, this will be dealt with in detail in the following.

7.27

OXY-ACETYLENE WELDING

In oxy-acetylene welding, the flame is obtained by burning acetylene (which is a fuel gas) and oxygen which supports the compulsion of acetylene. When mixed with oxygen and ignited, acetylene burns explosively, undergoing oxidation to carbon dioxide and water and release of heat. The reaction is given below. The chemical name of acetylene is Ethyne (C2H2). 2C2H2 + 5O2 Æ 4CO2 + 2H2O + 620 calories

7.27.1 Oxygen Oxygen is a clear gas without any colour, odour or taste. Oxygen for industrial use is produced by (a) liquid air process or (b) electrolytic process. In liquid air process, the air is first cleaned of dust and other carbonic gases and later cooled to liquefy at –180oC. Nitrogen is separated from this by gradual warming. In electrolytic process, electric current is passed through water containing an electrolyte such as caustic soda for increasing conductivity. As the current is passed, oxygen is produced in the form of bubbles on the surface of the positive pole and hydrogen on the negative pole. However, this system of making oxygen is costlier than the liquid air system. Oxygen is filled in solid drawn steel (mild steel or alloy steel) cylinders of different capacity. The oxygen is filled in the cylinders to a pressure of 125 to 140 kg/cm2. The volume of oxygen in its cylinder is proportional to its pressure, for example, if during use, the pressure of oxygen cylinder drops by 5%, it shows that 1/20th of the cylinder contents have been used. Oxygen cylinders are painted black and the valve outlets are screwed right handed. A safety nut is provided at the top of the cylinder (Fig. 7.55) to allow leakage of oxygen at the time when due to increase in temperature, the gas pressure increases beyond safety load of the cylinder. Oxygen cylinders should not be stored near the combustible gas cylinders. Fig. 7.55

Oxygen cylinder.

564

MANUFACTURING PROCESSES

7.27.2

Acetylene

The acetylene gas for welding or cutting is obtained from the following sources: (a) Acetylene cylinders filled with acetylene gas from some gas producing plant. Such acetylene is called dissolved acetylene which, in fact, is the compressed acetylene filled into steel cylinders. (b) Acetylene generators which could be fabricated locally and installed in the shop. In the generator, calcium carbide and water are brought in contact to produce acetylene. CaC2

+

(calcium carbide)

2H2O (water)

Æ

C 2H 2 (acetylene)

+

Ca(OH)2 (calcium hydroxide)

The most popular and simplest way to procure acetylene is through compressed acetylene cylinders (or dissolved acetylene). The acetylene cylinders are also solid drawn steel type and filled with a porous substance soaked with acetone, which is a hydrocarbon liquid capable of dissolving large quantities of acetylene and this helps in increasing the storing capacity of acetylene. The pores of the porous spongy material (like charcoal, asbestos, pith from corn stalk or balsa wood, etc.) remain completely filled with acetone in which acetylene is dissolved under pressure. At atmospheric pressure and temperature, acetone can dissolve acetylene about 25 times of its own volume. Hence, at 15 atmospheric pressure, which is the normal charging pressure of dissolved acetylene, this is increased to about 375 times. It has been observed that compressing free acetylene to a pressure more than one atmosphere is not safe. Hence, if acetylene is compressed to a pressure of 16 to 20 kg/cm2 into ordinary cylinders (without acetone), it may explode even at 1.4 to 2 kg/cm2. The presence of porous material soaked with acetone in the cylinder divides acetylene into small globules as it enters the small pores of porous material which helps in sudden decomposition of acetylene for its safe storing. Acetylene cylinders are painted maroon and their outlet valves are screwed left handed. The cylinder is charged with acetylene to a pressure of about 16 kg/cm2. An acetylene cylinder, shown in Fig. 7.56, has a number of fusible plugs at its bottom which may melt and give way to acetylene to escape out at 104oC if the cylinder is Fig. 7.56 Acetylene cylinder. exposed to excessive heat. The spindle valve of the cylinder can be opened with a special wrench. Acetylene generators are used for local production of acetylene at the shop itself. In these, acetylene is produced by action of water on calcium carbide. Acetylene generators are of two types: (i) low pressure generator and (ii) medium pressure generator. The low pressure generator gives acetylene at a pressure of 0.10 kg/cm2 (about 0.1 bar). It is considered portable and gives acetylene at the rate of 15 litres per minute. The medium pressure generator gives gas at a pressure of up to 0.1 to 1.5 kg/cm2 and at the rate of up to 3000 litres per minute. This type of generator is stationary type and is the most commonly used one.

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Acetylene generators are of two types: (a) carbide to water type and (b) water to carbide type. The carbide to water type generator is preferred to water to carbide type as the former uses complete reaction of water and calcium carbide and is more efficient. A typical carbide to water type acetylene generator is shown in Fig. 7.57. In this, carbide is fed to the water for generating gas. The generator is partially filled with water. The calcium carbide in proper size pieces is stored in the hopper provided at the top of the generator, where calcium carbide is fed down to the water by a feed mechanism and acetylene is produced as a result of action of water on calcium carbide. No sooner is the predetermined pressure of gas attained, the feed of calcium carbide is stopped automatically. The calcium hydrate (carbide sludge) is collected at the bottom of the generator.

Fig. 7.57

7.27.3

Carbide to water type acetylene generator.

Special Precautions for Acetylene Cylinder

The following precautions are needed for safe use of acetylene cylinders. (i) Acetylene is highly inflammable, so no nacked flame should be brought close to the cylinder. (ii) Leaks can be detected by smelling or by applying soap bubbles on the cylinder body and should be attended immediately. The gland nut should be tightened. (iii) In case of fire, the spindle valve on the cylinder should be shut. (iv) The cylinders should be stored and used in upright (vertical) position to contain acetone within the cylinder as entry of acetone in blow pipe (welding torch) may result into explosion. (v) Never open or close the spindle valve with hammer. (vi) Never strike an arc on the cylinder. (vii) When cylinder is not in use or is being transported, keep the cap screwed on it. (viii) Acetylene should never be used at a pressure more than 1 kg/cm2. (ix) Before fixing regulator on the spindle valve, open the valve for an instant to clear off dust particles. After attaching the regulator, see that adjusting screw of the regulator is released and then cylinder valve should be opened. (x) Never attempt to transfer gas from one cylinder to another.

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MANUFACTURING PROCESSES

(xi) When empty cylinders are returned for refilling, make sure that the valves are closed to avoid evaporation of acetone. (xii) It must be remembered that an acetylene cylinder absorbs heat as acetylene is released from it. Hence, the rate of flow of acetylene from cylinder is somewhat limited. Acetylene should not be drawn very fast.

7.28

OXY-ACETYLENE FLAMES

Oxy-acetylene flame can be defined as a phenomenon produced at the surface of the nozzle tip (of the welding torch) where two gases (oxygen and acetylene) meet and undergo combustion with evolution of heat and light. The chemical reaction for complete combustion is as follows: 2C2H2

+

Æ

5O2

(acetylene)

(oxygen)

4CO2

+

(carbon dioxide)

2H2O (water vapour)

The structure of the flame is shown in Fig. 7.58.

Fig. 7.58

Structure of oxy-acetylene flame showing only the inner cone and outer cone. The secondary luminous cone exists only in a carburizing flame.

There are three different cones in an oxy-acetylene flame. (a) Inner or luminous cone is formed right at the front of torch tip. This is the cone where two gases burn with a brilliant light and the primary chemical reaction takes place in this zone as follows: C 2H 2 (acetylene)

+

O2 (oxygen)

Æ

2CO

+

(carbon monoxide)

H2

+

Heat

(hydrogen)

The temperature in the luminous inner cone may vary from 3200oC to 3500oC for different types of flame with maximum temperature occurring at the pale blue tip (vertex) of the cone. A neutral flame has a well-defined white inner cone. In oxidizing flame, the inner cone is purple in colour and is shorter than the inner cone of neutral flame. (b) Outer cone envelops the inner cone and provides a reducing atmosphere that helps protecting molten weld metal against oxidation during welding. Oxygen from atmosphere is derived within this zone to complete secondary chemical reaction as below: 2CO + O2 Æ 2CO2 (carbon dioxide) 2H2 + O2 Æ 2H2O (water vapour) Any free oxygen available at the welding point is also absorbed in this cone, thus giving a reducing atmosphere. The temperature in this cone may be up to 2100oC and at the tip of the cone, it is about 1280oC.

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(c) Secondary luminous cone exists only in a carburizing flame (Fig. 7.59) and surrounds the luminous inner cone while extending into the outer cone. This happens because of the excess quantity of acetylene.

Fig. 7.59

The three types of oxy-acetylene flames.

Among the three types of oxy-acetylene flames, the neutral and oxidizing flames have only two cones (inner cone and outer cone) but the carburizing flame has all the three cones as discussed above.

7.28.1

Types of Oxy-acetylene Flames

For complete combustion of acetylene, the following reaction takes place: 2C2H2 + 5O2 Æ 4CO2 + 2H2O + 620 calories of heat It shows that two volumes of acetylene combined with five volumes of oxygen produce four volumes of carbon dioxide, two volumes of water vapours and heat. In other words, for complete combustion of one volume of acetylene, two and a half volumes of oxygen are required. The primary reaction that takes place in the luminous cone consumes one volume of oxygen supplied through the torch (from cylinder) but to complete the secondary reaction (in the outer cone), the balance oxygen supply of one and a half volume is met with oxygen derived from the atmosphere. It is with this reason that more oxygen supply in the torch is needed while working in confined places.

568

MANUFACTURING PROCESSES

The following three types of oxy-acetylene flames are obtained by mixing acetylene and oxygen in various proportions. Neutral flame: The neutral flame is the result of a nearly perfect equal proportion of oxygen and acetylene. The flame has two cones—the inner cone and the outer cone as shown in Fig. 7.59(a). Different temperatures attained are shown. Among all the three types of flame, the neutral flame is said to be the correct flame as it gives neither oxidizing effects nor too much of carburizing (or reducing) effect. Neutral flame is used for most welding operations and is highly suited for welding mild steels and cast irons, stainless steel, copper and aluminium. Even during flame cutting of steels, the pre-heating flame may be a neutral flame. Oxidizing flame: It is obtained when oxygen is burnt in excess of acetylene [Fig. 7.59(b)]. The inner cone is shorter and pointed with a sharp hissing sound. It gives maximum temperature among all the three oxy-acetylene flames. Since oxygen is a rapid supporter of combustion, when oxidizing flame is fed to a red hot steel, the iron present in steel burns up rapidly. The oxidizing flame is, therefore, not used for general welding purposes (at least for welding ferrous metals, steels and cast irons). The oxidizing flame is used where maximum temperature is desired or in situations where oxidizing effect is not harmful, rather proves beneficial, for example, a slightly oxidizing flame is used in welding of non-ferrous metals particularly copper base metals as brasses and bronzes and zinc base metals, where it is desirable to have oxidizing flame giving oxide film to check vaporization of zinc and also to reduce further oxidation after oxide film is formed. This flame is also used for pre-heating purposes during flame cutting of steels. Carburizing flame: It is obtained by burning acetylene in excess of oxygen. It has three cones [Fig. 7.59(c)], wherein the secondary luminous cone is extra in comparison to the two other types of flames. The secondary luminous cone gives reducing effect in the welding area. A carburizing flame is mostly used for welding aluminium, monel metal, stainless steel, die cast metals and several other non-ferrous metals, besides the high carbon steels. The carburizing flame prevents excessive formation of oxides on non-ferrous metals which interfere with proper fusion of metal, since oxides of non-ferrous metals have very high melting points because of which they are difficult to melt by oxy-acetylene flames. Carburizing flame gives a slight case-hardening effect on certain steels. It is also used for hard-facing of steels with stellite rods.

7.29

BACKFIRE OR POPPING

Backfire or pop is a small explosion of the torch flame followed by either extinguishing of flame or continued burning of gases. It usually occurs due to pre-ignition of gases. The causes of backfire are: (a) When gases come out too slowly from the torch under low pressure due to small tip used, pre-ignition or burning of gases may take place within torch, showing that the speed of flow of gases is less than the speed of burning gases. (b) The tip may become over heated. It may be clogged also.

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(c) Carbon or hot metal particles that get deposited inside the torch tip work like ignitors when they become overheated. This results in pre-ignition of gases within the torch. Tip should be cleaned regularly. (d) Popping may also result when accidentally the inner cone of the flame is submerged in the weld puddle.

7.30 FLASHBACK A flashback occurs when the flame disappears from the tip of the torch and travels back in the hose. The flame makes a hissing sound at that time. The remedy is that both the gases should be shut off immediately to avoid combustion within the torch. The torch should be allowed to cool before re-lighting. The flashback may be caused by a clogged barrel or mixer passage and excessive pressure of oxygen. Sometimes, accumulation of organic oxides in oxygen hose may also trigger the flashback.

7.31

OXY-ACETYLENE WELDING OUTFIT

The oxy-acetylene welding outfit refers to the equipment and gadgets required to carry out gas welding operations. It comprises the following (Fig. 7.60).

Fig. 7.60

Oxy-acetylene welding outfit.

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(i) Gas supply equipment comprises oxygen cylinder and acetylene cylinder or acetylene generator. Cylinders are accommodated on a trolley for easy movement from one location to another. (ii) Regulators complete with high and low pressure gauges. Two sets of regulators with pressure gauges are required, one for oxygen and another for acetylene. (iii) Hoses, two numbers, one for oxygen and one for acetylene. The hose for oxygen supply (from pressure regulator to welding torch) is coloured blue and has right handed thread connections. The acetylene hose is coloured red and has left handed thread connections, with chambers or grooves on the nuts. The welding hose has a seamless lining made from rubber compounds and reinforced or wrapped with cotton piles. The hose is resistant to gases used in welding and can withstand high pressures. It has metal clamps or clips to attach welding hose to a nipple. The clamp squeezes the hose around the nipple to prevent it from working loose. A nut on the other end of the nipple is connected to the regulator or torch. Hose couplers are used to join two pieces of welding hoses. (iv) Welding torch (or blow pipe): The two gases, oxygen and acetylene, having been reduced in pressure by the gas regulators are fed through their hoses to the welding torch or blow pipe. The torch mixes and controls the flow of gases to the welding nozzle or tip of the torch where the gas mixture burns and produces flame (Fig. 7.61).

Fig. 7.61

Welding torch (or blow pipe).

(v) Welding nozzles or tips are fitted on the front end of the welding torch. These may be: (a) interchangeable type tip screwed on the head of the blow pipe or (b) goose neck extension type fitted directly onto the mixer portion of the blow pipe. (vi) Pressure regulators reduce the pressure from the cylinder and maintain it at correct value regardless of the pressure variations at the source. These are also helpful in adjusting the pressure of gas to torch. (vii) Goggles protect eyes from harmful heat and radiation (of infrared and ultraviolet rays) with the help of coloured glasses fitted in them. (viii) Spark lighter provides instant and convenient means for lighting the welding torch. It consists of a pointed stone and a rough surface to produce spark when rubbed together. (ix) Apron and gloves are for the protection of clothes and hands of the welder.

7.31.1

Welding Torch (or Blow Pipe)

As mentioned before, the welding torch is a tool for mixing two gases in the desired proportions and burning the mixture at the end of the torch tip. It has a handle to hold it and two inlet

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connections for two gases at one end such that each inlet has a valve to control the volume of oxygen and acetylene. The two gases from the two paths mix up in the mixer in the torch, and the gas mixture that comes out of the tip of torch is ignited to produce the flame. The welding torches are of two types: (a) High pressure (or equal pressure) type (b) Low pressure (or injector) type 1. High pressure (or equal pressure) type torch: This is the most commonly used torch (Fig. 7.62) and is used when the delivery of both gases (oxygen and acetylene) is from the cylinders. The construction of the torch is such that each gas is required to be supplied under enough pressure to the mixing chamber. The torch carries two needle valves, each for controlling the supply of individual gases. The torch may be of brass, aluminium and stainless steel, and the torch tips are made from copper. The torch tip size is given by the manufacturer for various job thicknesses. The torch handle carries packing of asbestos or impregnated leather. Depending on the thickness of the workpiece to be welded, the torch tip of proper size is selected and the gas pressure is accordingly adjusted with the help of pressure regulator. The gases in this torch are delivered at a pressure generally above 0.7 kg/cm2, the acetylene pressure is about 0.7 to 1 kg/cm2 and oxygen pressure varying up to 1.7 kg/cm2, depending on the tip size.

Fig. 7.62

A high pressure (or equal pressure) type welding torch.

2. Low pressure (or injector) type torch: This torch is shown in Fig. 7.63. It is used when acetylene is drawn from the generator. It has an injector nozzle inside its body through which high-pressure oxygen flows. The low-pressure acetylene

Fig. 7.63

A low pressure (or injector) type welding torch.

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generated in generators is not sufficient to force it through the small passage in the torch. The high-pressure oxygen draws the low-pressure acetylene with it in the mixing chamber and gives it a required velocity to keep up a steady flame. The arrangement works as a means to make acetylene flow to the mixing chamber and also helps in preventing backfiring. The acetylene pressure is less than 0.7 kg/cm2 and that of oxygen is from 0.7 to 2.7 kg/cm2, depending on the tip size. The low pressure blow pipe is costlier than the high pressure blow pipe. In both the torches, oxygen tends to draw in acetylene and mix with it due to high pressure. The gas mixer in the torch should be able to perform the following functions: (i) (ii) (iii) (iv)

7.31.2

Mixes gases thoroughly before they leave out of the nozzle. Arrests flashback that might occur through improper operation. Arrests any flame from travelling further back (towards hose pipe) than the mixer. Permits a range of tip sizes to be operated from one size of the mixer.

Pressure Regulators

Pressure regulators are fitted on the gas cylinders. They serve the following functions: 1. Reduce the inlet pressure (from the cylinder, manifold or generator) to that required at the torch. 2. Maintain a constant pressure at the torch tip regardless of any variation that might take place in the source (cylinder or manifold). 3. Adjust the delivery of gases at the desired pressure within its range. 4. Oxygen regulator reduces the cylinder pressure from 150 kg/cm2 depending on the torch and the size of the tip. A 220 kg/cm2 gauge is attached to high pressure side and a gauge reading up to 11 kg/cm2 or less is attached to low pressure side. 5. Acetylene regulator reduces the cylinder pressure from about 18 kg/cm2 to torch pressure not exceeding 0.8 kg/cm2. A 30 kg/cm2 gauge is attached to high pressure side and a 3.5 kg/cm2 gauge is attached to low pressure side. Regulators are of two types: (a) Single-stage regulator (b) Two-stage (or double-stage) regulator 1. Single-stage regulator: In single-stage regulator, reduction of pressure from the cylinder pressure to the welding torch line pressure is done in single stage. The schematic illustration of the single-stage regulator is given in Fig. 7.64. The gas pressure in the hose is controlled by regulating pressure on the spring through the spring tension adjusting screw. The spring applies pressure to a flexible rubber diaphragm which is connected to the high pressure valve. The single-stage regulator is used with cylinders only. 2. Two-stage regulator: The principle of pressure regulation in a two-stage regulator is same as in a single-stage regulator, except that the gas pressure is reduced in two stages instead of one, using two diaphragms and two control values (Fig. 7.65). A two-stage regulator is used with both cylinders and manifolds (a battery of cylinders).

ELECTRIC AND GAS WELDING PROCESSES

Fig. 7.64

Fig. 7.65

7.32

573

Working principle of a single-stage pressure regulator.

Working principle of a two-stage pressure regulator.

GAS WELDING ROD (OR FILLER ROD)

Welding rod is the material added to the molten pool of metal being welded. It assists in filling the groove (made as edge preparation) and thus finally forms the integral part of the weld joint. The filler metal is available usually in the form of a rod. Aluminium filler metal is available in coils, although flux coated aluminium rods are also available. The filler rods are made from different materials and their composition is as given below. 1. Mild steel rods are used for welding mild steel and wrought iron and steel castings of low carbon contents. 2. Mild steel copper coated rods are excellent low priced rods for general use in welding mild steels pipes, plates and other steel structurals, low carbon steel castings and wrought iron. 3. Vanadium steel rods are used for welding all types of steel requiring higher strength.

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4. Nickel (3%) steel rods (IS: 1278, type 4.4) are used for welding nickel steels and other alloys requiring higher strength welds. Also these are used for building up worn-out parts like gears, shafts, cam shafts, etc. 5. Chrome-vanadium steel rods are used for wear resisting purposes, for example, building up worn-out railway crossings, tramway tracks and stone crushing tools. 6. Stainless steel rods (IS: 1278, type 4.2) are used for welding austenitic stainless steel tubes, sheets and tanks. 7. Stellite rods are used for hard facing of tools, valves, lathe centres and drill tips. 8. Cast iron rods, also known as super silicon rods (IS: 1278, type 5.1), are used for welding machinable grey cast iron castings such as lathe beds and cylinder blocks. 9. Cast aluminium rods are used for welding aluminium castings and automobile crank cases. 10. Aluminium alloy (5% silicon) rods are used for welding pure aluminium and its sheets, tubes, extruded section, aluminium alloy castings (not having zinc). 11. Aluminium alloy (5% copper) rods are used for welding aluminium castings. 12. Nickel bronze rods (IS: 1278, type 6.4) are used for braze welding of steels, malleable cast irons, alloys of copper-zinc and nickel. 13. Drawn copper rods are used for welding copper and its castings. 14. Drawn brass rods are used for brazing and general welding of brass castings. 15. Drawn manganese bronze rods are used for building up bronze surfaces on cast iron castings and steels.

7.33

GAS WELDING FLUXES

During the process of welding, oxygen may combine with the molten metal (at the point of welding) and form oxides which, when entrapped in the weld joint, make the joint weaker. A flux is used to avoid this entrapment of oxides in the molten weld pool. Fluxes are the chemical compounds used in gas welding to prevent dissolving or to facilitate removal of oxides and other undesirable substances from the weld pool. Fluxes are used both in welding and in soldering (or brazing) and accordingly, these are categorized as welding fluxes or soldering fluxes. During welding, flux chemically reacts with the oxides and forms a slag that floats over and covers the molten metal pool saving it from atmospheric oxygen. The fluxes used in welding are in the form of either powder, paste or liquid. These may be directly applied on the surface of the base metal to be welded or by dipping the heated end of the filler rod in the flux frequently during welding. The slag formed on the weld bead is removed by chipping, filing or grinding after the welding operation is over. Borax and salt (sodium chloride) are two compounds generally used by welders as flux. No flux is used for gas welding of steels in general. Various fluxes used for welding different metals are given below. Fluxes for cast iron are composed of boric acid, soda ash and sodium chloride. Fluxes for stainless steel and alloy steels contain borax, boric acid and fluorspar. Fluxes for aluminium and its alloys are composed of lithium chloride, sodium chloride and potassium chloride.

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Fluxes for copper and its alloys are borax-based fluxes containing borax (fused), boric acid, magnesium silicate, lime, etc. Fluxes for magnesium and its alloys contain sodium chloride, potassium fluoride, magnesium chloride and barium chloride. Fluxes are used in hard facing also, for example, dehydrated borax is used in stellite depositing on carbon steel.

7.34

TECHNIQUES OF GAS WELDING

There are two general techniques of gas welding which depend on the ways in which welding rod and welding torch are used in the process of welding. 1. Leftward (or forehand) welding 2. Rightward (or backhand) welding

7.34.1

Leftward (or Forehand) Welding

Leftward welding is the oldest method practised by welders. In this method, welding is started from the right-hand side end of the weld joint (Fig. 7.66). The welding torch is held at an angle of 30 to 45o with the workpiece. The flame spreads on the workpiece joint and thus pre-heats the area ahead of the torch flame cone under which welding takes place. The flame is given a circulatory, rotational or side to side motion to obtain uniform fusion on each side of the workpiece plates. The filler rod (or welding rod) is held at about 30o to the workpiece. It is important that the cone of the flame should never go outside the ‘puddle’, which is a small pool of molten metal created due to the intense heat of the flame.

Fig. 7.66

Leftward (or forehand) welding.

The forehand welding is usually employed for thin jobs up to 5 mm thickness because when welding jobs are over 6.5 mm thick, good penetration is not obtained and hence the quality of weld decreases as job thickness increases. For plates thicker than 3 mm, bevelling of the plate edges to produce V-joint is needed. Good welding bead with nice appearance is the quality of this method. The forehand welding, however, needs careful manipulation of torch to safeguard against excessive heating of base metal, resulting into too much mixing of base metal and filler rod metal. Also, in this method, the view of the joint edge is interrupted which slows down the process.

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7.34.2

Rightward (or Backhand) Welding

The essentials of rightward welding are shown in Fig. 7.67. The torch and the welding rod are held at an angle of 30 to 45o with the workpiece. In this case, the flame is directed back on the weld portion which has just been completed. The welding rod is given a circulatory motion while the torch moves in a straight line. The method is suitable for welding thicker sections over 5 mm. No edge preparation is needed up to 8 mm thick plates. With proper edge preparation, plates up to 16 mm thickness can be welded in one pass. A larger size welding torch is used in this method. Since the welding torch is moved in a straight line, the molten pool of metal below the flame is least agitated and hence the oxidation losses on the weld metal are reduced.

Fig. 7.67

Rightward (or backhand) welding.

The benefits of backhand welding are as follows: (i) In backhand welding, the flame is directed on the just welded bead; the flame thus gives an annealing effect on the weld metal, relieving the welding stresses to a great extent. (ii) The direction of flame helps the welder in forming good bead and better penetration since the molten pool is clearly visible and better control on the weld is thus obtained. (iii) The technique is suitable for welding thicker sections, over 5 mm and up to 25 mm. (iv) Welding speeds are about 20% higher than leftward welding. (v) Gas consumption is reduced by 15 to 25%. (vi) It provides better shielding against oxidation.

7.35 PUDDLING A weld puddle is a small pool of molten metal (that is created during welding) right below the torch flame cone. The puddle is produced because of the intense heat of the flame. This weld pool or puddle is carried ahead along the seam (or joint) of the parts to be welded. Puddling is an essential part of any gas or electric arc welding. The condition and qualities of the puddle are an indication of good or bad penetration, torch adjustment and its movements, and largely affect the weld characteristics. On account of puddling, the penetration of metal appears as a ‘sag’ along the joint and under the bottom side of the workpiece being welded (Fig. 7.68). The size of a puddle has a proportion to its depth and hence the welder can adjust

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the penetration by watching and controlling the puddle size. The tip of the inner cone of the torch should always be held within boundaries of the puddle to avoid entry of atmospheric oxygen coming into contact with the molten metal.

Fig. 7.68

7.36

Illustrating the puddling procedure. Penetration sag on the underface of the workpiece is shown at A.

JOINTS USED IN GAS WELDING

Gas welding joints are of the following types. These have already been covered under shielded metal-arc welding. (i) Butt joint (iii) Corner joint (v) Edge joint

(ii) Lap joint (iv) Tee joint

Butt joint is most commonly used for gas welding. For thickness up to 2 mm, the closed square butt joint is used but without the filler metal. For thickness of plate 2 to 5 mm, the open square butt joint with filler is used. For thickness more than 5 mm, edge preparation is needed. Lap joint is used on thickness under 3 mm. Lap joint is undesirable above 3 mm because of excessive local heating and increased looked up stresses. The joint is suitable up to 3 mm. These joints are used widely to weld stiffness in aircraft and thin-walled structures. Above 3 mm thickness, the tee joint is undesirable due to overheating. Corner joint is suitable for both light and heavy gauges. In some cases, joint may be made without filler metal. Edge joint is commonly used as lap joint and is suitable for thin sheets only. Characteristics of a good weld: A good weld has the following characteristics. These apply to both electric and gas welded joints. (a) The weld width is the same all through the weld length. (b) The weld is straight. (c) The weld is crowned slightly and this crowning is consistent throughout the length of the weld. (d) The weld surface has ripple evenly spread all along the weld. (e) The weld has no colour spots, scale, pitting, cracks and spatter along the weld bead and the head is clear.

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(f) The penetration of the weld is correct and consistent along the weld length. (g) It is only necessary to produce a weld as strong as or slightly stronger than the base metal of workpieces because a joint with too extra strength is uneconomical. Therefore, the cross-sectional examination of the weld should show no signs of overspreading of weld sideways over the base metal. There should be no undercutting of the weld.

7.37

METALS WELDED BY GAS WELDING

Gas welding is used for welding ferrous and non-ferrous metals as also dissimilar metals as given below: (a) Ferrous metals (i) Carbon steels and cast steels (iii) Galvanized steels (v) Stainless steels (b) Non-ferrous metals (i) Copper and its alloys (iii) Magnesium and its alloys (v) Die cast products of white metal (vii) Lead and alloys (c) Dissimilar metals (i) Steels and cast irons

7.38

(ii) Low alloy steels (iv) Tool steels (vi) Cast irons of all types (ii) Aluminium and its alloys (iv) Hard facing and stelliting of steels and cast irons (vi) Brass and bronze

(ii) Tipping of tools

GAS WELDING—A REVIEW

The selection of a welding process for a particular operation or application depends on the workpiece material, on its thickness and size, on its shape complexity, on the type of the joints and welding position, on the strength required, and on the change in product appearance caused by welding (e.g. distortion, which may be more in arc welding than oxy-acetylene welding). All arc welding processes are high temperature processes and produce intense localized heat for fusion of the metal. Gas welding techniques are used primarily for joining thin sections, preferably less than 25 mm. The process is well applicable for welding both ferrous and non-ferrous metals, e.g. carbon steels, alloys steels, cast irons, aluminium, nickel, magnesium and its alloys, copper and its alloys. Gas welding is extensively used in sheet metal fabrication, automobile and aircraft industry and repair jobs. Being a low temperature process, it is particularly suitable for welding those metals which may lose some of their elements in arc welding (a high temperature process) and where rapid heating and cooling of metal may create harmful changes in the metal structure. The process of gas welding is a versatile process as it can be easily adapted to a wide variety of manufacturing and maintenance jobs. The welding gives relatively slow rate of heating which may be of advantage in some cases. Since the filler metal and the torch are

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separate, the weld metal deposition rate is easily controlled by applying heat to either base metal or filler rod. The welder can exercise good control over the temperature of the metal in the weld zone and thus by properly coordinating the heat input with welding speed, the size and viscosity of the weld pool are easily controlled. Gas welding equipment is portable, low cost and versatile, and besides welding, it can be used for metal cutting, brazing, soldering and for pre-heating jobs. Gas welding may not be used for joining heavy sections economically, less temperature of flame being one of the reasons. Time taken in heating by flame is more. Flux shielding is not as effective as inert gases in arc welding (TIG, MIG). Fluxes used in gas welding sometimes give irritating fumes. Refractory metals like tungsten, tantalum, molybdenum and reactive metals like titanium and zirconium cannot be welded by gas welding technique.

7.39

WELD DEFECTS

As already discussed, a good weld (or weld bead) should have uniformly rippled weld surface with even contour and even width of the bead. The bead should be straight. There should not be the presence of surface porosity, overlap, undercutting, cracks or crater in the deposited weld bead. Internal defects like blow holes or inclusions or incomplete penetration should not be present in a good weld. Weld defects can be divided into two broad categories: (a) External weld defects and (b) Internal weld defects. These are sub-divided further as given in the following. The external defects are visible at the weld surface, whereas the internal defects are seen only on cutting the section of the weld deposit. Weld defects

(a) External weld defect (i) Incorrect profile (ii) Cracks (iii) Crater (iv) Spatter (v) Edge of plate melted off (vi) Surface porosity (vii) Incomplete filled grooves (viii) Distortion

7.39.1

(b) Internal weld defect (i) Blow holes and internal porosity (ii) Cracks (iii) Inclusions (iv) Lack of fusion (v) Incomplete penetration

External Weld Defects

External weld defects (Fig. 7.69) are discussed in the following. Incorrect profile (or unspecified contour): Incorrect profile covers a list of several defects observed in the profile of a weld. Incorrect profiles are of the following types. (a) Overlap is an imperfection at the toe or root of a weld bead. It is caused due to the flow of metal on the surface of the parent metal without fusion with the latter. Slow rate of welding and incorrect size and angle of electrode may cause this defect. Too large electrode and slow welding speed should be avoided.

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Fig. 7.69

External weld defects.

(b) Undercut is a groove cut into the base metal along the toe of the weld. Excessive arc length, current and speed of welding result in undercutting. Improper manipulation of electrode and dampness in electrode and arc blow are also the factors contributing to undercutting. (c) Excessive penetration bead is mainly due to incorrect joint preparation. Other reasons are high current and larger arc length. (d) Excessive convexity and concavity are mostly due to excessive current and improper welding. Cracks: Crack in a weld is a discontinuity generated due to either tearing or fracture of the metal. Tearing happens when the metal is in a plastic stage during its solidification from molten state. The presence of more sulphur is the main cause of tearing, particularly when manganese presence is less to take care of sulphur. Fracture, on the other hand, happens due to uneven cooling of the weld besides improper welding technique. The main causes of cracks are presence of localized locked up stresses (generated in welding), poor weldability of parent metal and improper welding technique. A crack may occur in the weld bead, in the heat-affected zone, at the outer surface of the weld or within the weld bead not visible from outside. Cracking may occur at two stages during cooling of

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the weld metal. Cracking at a high temperature just below the freezing point is called hot cracking and that at room temperature or little above is called cold cracking. Crater: A crater is formed at the end of a weld run and is due to improper welding technique. When finishing a weld, the welder should not draw away the arc quickly, rather maintain the arc there (at the end of the weld) with no travel movement to give chance to the crater to fill up. Spatter: Spatter is caused due to metal particles expelled out from the molten metal pool under the effect of harsh arc during welding. These particles are not the part of weld but freeze out in small tiny balls along the weld bead. Excessive current, a longer arc and damp electrode cause spatter. Edge of plate melted off: Edge of plate melted off occurs mostly in lap joints due to the use of oversize electrode, excessive current and wrong manipulation of electrode. Surface porosity: The surface of the weld bead may be porous. This defect is caused because of using wrong electrode, longer arc, improper welding conditions and technique. Incomplete filled grooves: In butt welds, this defect occurs due to inadequate deposition of weld metal and is caused because of the use of wrong size of electrode and faster welding speeds. Distortion: Distortion is the change in the shape of the workpiece after welding. This occurs due to the temperature gradients that exist in different sections of the workpiece during welding. The area close to welding is at a higher temperature than that which is away from the welding zone. Thus, uneven cooling of the material takes place causing this defect. Use of welding fixtures and proper sequence of welding (for example, skip welding) may help in avoiding distortion.

7.39.2

Internal Weld Defects

Blow holes and internal porosity: The atmospheric gases that get dissolved in molten weld pool try to get released during solidification and cooling of the weld. These gases cause blow holes and porosity. A crowded group of gas pores in the weld is called porosity. A large gas cavity exceeding 1.5 mm in diameter is called a blow hole. Both these defects are caused due to the wrong use of electrodes, excessive current, longer arc, poor weldability of parent metal and improper welding technique. Cracks: A crack, when not visible from outside but found existing within the weld, is called an internal crack. Causes for this are the same as already discussed for external cracks in weld bead. Inclusions: Inclusions are the slag (oxides), nitrides and other foreign matter entrapped within the weld. The reasons for inclusions are inadequate cleaning of each weld bead deposited during multiple pass welding, improper and damaged flux coating on the electrode, longer arc, improper joint preparation and improper welding technique. Lack of fusion: Lack of fusion is lack of union of the deposited weld metal with the parent metal (workpiece). It occurs due to (i) failure in raising the temperature of parent metal (or previously deposited weld metal) to the melting point and (ii) failure to dissolve during

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welding the oxides and other foreign material present on the workpiece surfaces with which the deposited metal should fuse. An example of lack of root fusion is shown in Fig. 7.70(a).

Fig. 7.70

(a) Lack of root fusion, (b) Incomplete root penetration.

Incomplete root penetration: Incomplete root penetration refers to the incapability of welding arc to reach or penetrate a narrow corner [Fig. 7.70(b)]. The defect can be overcome by using deep penetration electrodes or high currents. Other reasons of incomplete penetration are: (a) Failure of root face of butt weld to reach fusion temperature for its entire depth (b) Failure of weld metal to reach the root of a fillet weld leaving a void (c) Use of oversized electrode (d) Higher welding speed (e) Less current

7.40

INSPECTION AND TESTING OF WELDMENTS

The main object of weld inspection is to ensure the high quality of welds through careful examination of the workpiece at each stage of manufacture. Inspection is carried out at three stages, namely, (a) Before welding, (b) During welding and (c) After welding.

7.40.1

Inspection before Welding

Inspection before welding is done for ascertaining whether or not a given workpiece is convenient for welding, considering the stresses set up by welding heat, etc., whether proper welding equipment and electrode have been chosen to work with, whether the material to be welded is of weldable quality, whether the edge preparation of the workpieces is according to the standard specification, whether the current and polarity selected by the welder are correct and whether the use of suitable jigs and fixtures is necessary.

7.40.2

Inspection during Welding

Inspection during welding includes studying the proper arc characteristics, for example, arc length, current setting, speed of weld deposit, sequence of welding and examining whether proper cleaning of slag is being done, particularly in multi-run welding. Other important points to be checked are: spattering of electrode, under- or over-flushing of metal and manipulation of the electrode besides making sure that electrodes being used are of proper size and shape (flux not damaged and not moist).

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7.40.3

583

Inspection after Welding

Inspection after welding is done to check the weldment (welded component) that it is free from welding defects. To find out defects in a welded product, various methods have been developed. Visual inspection of welded joint can detect quite a few surface defects in the weld bead. For checking internal defects in a weld, non-destructive or destructive methods are available. Inspection after welding is essential because several parts after welding are required to be sent for machining or finishing and if a defect existing in the weldment is detected only at the stage of machining, then it results in the wastage of labour, time and money in bringing and setting the job on machine. Inspection after welding includes the following three types of tests: (a) Non-destructive tests (b) Semi-destructive tests (c) Destructive tests Non-destructive tests

These are the tests which do not impair the usefulness of the weldment after testing. Examples include visual inspection, hydraulic tests, ultrasonic testing and radiographic tests (X-ray tests). Semi-destructive tests

Tests covered under this category are those tests which do not destroy the weldment completely during testing and permit rewelding of the damaged part (impaired during testing). One method is to cut a slice or a portion of the weld to expose the interior of the weld. If the joint is found satisfactory, the cut portion can be rewelded. In yet another method, drilling test, drilling of a hole in the weld enables the inspector to inspect the interior weld quality. Destructive tests

These tests involve testing of the weldments to destruction. Such tests are done to find out the mechanical properties of the welded joint. One of the main objects of such tests is to assess the quality of an electrode by testing the weld made with that electrode. Tests are also carried out to assess the ability of a welder, by determining the strength of a particular weld made by him during his test. The usual destructive tests include tensile tests, bend tests and impact tests.

7.41

NON-DESTRUCTIVE TESTING OF WELDMENTS

Non-destructive testing of weldments includes the following tests:

7.41.1

Visual Inspection

Visual inspection is carried out as a general check for ascertaining the appearance and quality of the weld such as contour, cracks, straightness, overlap or undercutting, etc. Some visual defects in welds are shown in Fig. 7.69.

7.41.2

Magnetic Particle Inspection

This test is done to check surface flaws such as cracks and slag inclusions in metals which can be magnetized. When a piece of metal is placed in a magnetic field and the lines of

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magnetic flux get intersected by a discontinuity such as a crack or slag inclusion in the metal piece, magnetic poles are induced on either side of the discontinuity. In the magnetized metal piece if a crack or void interrupts the magnetic field, the latter gets distorted. The magnetic permeability of air being too low in comparison to iron, the magnetic flux spreads out to get around the void. Some of the magnetic flux lines extend outside the metal in the air over the discontinuity and the discontinuity is noticed or located distinctly because the magnetic particles collect and pile up at any discontinuity or crack. Ordinary dry magnetic powder of iron or black magnetic iron oxide is applied on the job surface in the form of a cloud or spray. In wet method, the powder is suspended in low viscosity non-corrosive fluid such as kerosene and is sprayed over the job. It is then subjected to magnetic field created either by passing current through the job or by placing a powerful magnet against it. Any lack of continuity at or near the surface creates a local north and south pole and attracts the metallic particles in the wet magnetic powder suspension. On removal of magnetic field, the flaws are detected by concentrations of magnetic particles along the flaw.

7.41.3

Liquid Penetrant Inspection

It is done to detect small surface cracks in both ferrous and non-ferrous metals. Small surface discontinuities such as cracks, shrinkage and porosity open to the surface, which tend to retain penetrants, can be checked by this method. Smooth and machined surfaces give best results. Either a liquid dye penetrant or a fluorescent liquid is applied to the surface and allowed to penetrate for some time. The liquid is later removed and surface-dried. In case of liquid dye penetrant, a developer is sprayed on the job surface which brings out the colour in the dye penetrant that has penetrated into the flaws. In case of fluorescent penetrant, a black light source (which is between visible and ultraviolet in the spectrum) is brought to the surface to show where the liquid has penetrated as the black light causes the penetrant to glow in dark.

7.41.4

Stethoscopic (Sound) Test

Defect-free weld metal gives good ringing note when struck with a hammer, whereas with defects (crack, inclusion), it gives a flat note. Physician’s stethoscope is used to magnify and identify the sound. Structural welds and pressure vessel welds have been successfully tested by this method.

7.41.5

Leakage Tests

This is applied for checking the quality of weld joints of pressure vessels. Carbon monoxide gas is pressurized in the welded tank and a soap-water solution applied on the outside of tank on the welded joints. Leaks are detected by the formation of bubbles. Sometimes vessels are pressurized and a pressure gauge is installed to indicate fall in pressure. Hydraulic test is yet another leakage test in which water pressure in the welded vessel is built up by pump, usually brought to about double the pressure the vessel will have to stand in service. Drop in pressure with lapse of time is read from the pressure gauge indicating leakage of the weld joint.

ELECTRIC AND GAS WELDING PROCESSES

7.41.6

585

Ultrasonic Test

Ultrasonic test is employed to detect and locate internal defects such as cracks, porosity, inclusions, lack of fusion and incomplete penetration. The method is applicable to ferrous and non-ferrous metals, plastics and ceramics. It makes use of acoustic waves with frequencies in the range of 20 to 20 MHz transmitted through the weldments and these get reflected by both the bottom face of the job and also from any flaw inside the job. Study of transmitted waves and reflected waves forms the basis for detection, location and size of the defect or flaw. Ultrasonic waves are generated due to the piezoelectric effect under which electrical energy is converted into mechanical energy. A quartz crystal is used for this. When a high frequency AC (of about one million cycles per second) is impressed across the faces of the quartz crystal, the expansion and contraction of crystal generates vibrations (sound waves). The principle of ultrasonic testing is shown in Fig. 7.71. A composite probe comprising transmitter probe and receiver probe is installed on the surface A of the welded job. A Cathode Ray Oscilloscope (CRO) is used to measure the time interval between the outgoing and reception of incoming signals. As the wave is sent by the transmitter probe, it strikes the upper surface (A) of the welded job and gives a sharp input pulse peak or pip (1) on the CRO screen. If the job is sound, this wave next strikes the bottom surface (B) of the job and after getting reflected from this will give a smaller peak (3) on the CRO screen. But when a defect or flaw exists between the surfaces (A) and (B), most of the beam striking the defect will get reflected from the defect and will reach the receiving probe indicating a pip (2) on the CRO screen. The distance of the defect from the surface (A) can be determined with the help of a distance scale in the form of a square wave constantly shown on the CRO screen. Quantitative assessment of defect size may be made based on the comparison standards of real and artificial defects such as drilled holes in metal blocks.

Fig. 7.71

Ultrasonic inspection.

The method of ultrasonic testing is fast and reliable. However, it is not suitable for weldments of complicated shapes and configurations.

586

7.41.7

MANUFACTURING PROCESSES

Radiography Tests

These tests have been used for inspection of metals of all types and thickness ranging from minute electronic components to plates up to half-a-metre thick. Cracks, porosity, holes, slag inclusions, lack of fusion and incomplete penetration, etc. are effectively detected. Radiography technique is based on exposing the weldments to short wavelength radiations, X-rays (wavelength less than 0.001 ¥ 10–8 to 40 ¥ 10–8 cm) and gamma (g ) rays (wavelength, 0.005 ¥ 10–8 to 3 ¥ 10–8 cm) from a suitable source such as X-ray tube. The g -rays given off by radium and radioactive isotopes such as cobalt-60, iridium-192, etc. can be used for inspecting thicker sections than what X-rays can inspect. X-ray radiography testing involves exposing the weldments to X-rays produced in an Xray tube in which a filament (cathode) provides electrons which proceed towards the target (anode), but these are obstructed by a tungsten disc where they strike; a part of their kinetic energy is converted into energy of radiations which are passed out of the tube as X-rays. When the weldment is exposed to X-rays emitted from the X-ray tube, a cassette containing X-ray film is placed behind (and in contact with) the weldment, perpendicular to the rays. During the exposure, X-rays penetrate through the weld and finally affect the X-ray film. The exposed and developed X-ray film showing light and dark areas is termed Radiograph (or Exograph) and is a negative transparency. Gamma rays have more penetrating power than X-rays and are used effectively for thicker sections. Further, with gamma rays it is possible to test a number of weldments simultaneously. These rays are not deflected by magnetic fields. Due to easy availability of radio isotopes at cheaper rates, gamma ray testing has now become more economical than X-ray testing. During testing, X-ray absorption of the test specimen depends on (i) its thickness, (ii) density, and (iii) most importantly of all, on the atomic nature of the material of the specimen. When X-rays or gamma rays strike the specimen, some of the rays pass through the material (or specimen) and the rest are absorbed by it. The fraction of rays transmitted depends on the nature of material and its thickness. For example, if the object is a steel structure with a weld joint having a void formed by gas inclusion, more radiation will pass through the section containing the void. Because the void results in a reduction of the total thickness of steel structure to be penetrated, a dark spot corresponding to the projected portion of the void will appear on the developed film. Thus radiograph is a shadow picture. Darker regions on film represent the more penetrable parts of the object and lighter regions more opaque. In X-ray testing of weldments, blow holes or honey combing, cracks and slag inclusion are detected but reading a radiograph is the job of an expert. In general, gas inclusion in the weld joint usually results in blow holes or bubbles. Their radiographic images are round dark spots with sharp corners Slag inclusions are composed of metal oxides. Since the average atomic number of metallic oxides is lower than that of the parent metal (iron), they are, therefore, less opaque to X-rays (than the parent metal) and, therefore, produce dark indications on the radiograph. Cracks and discontinuity in a radiation absorbing material (specimen) may produce a radiographic image in the form of narrow dark lines with irregular trend, i.e intermittent or continuous.

ELECTRIC AND GAS WELDING PROCESSES

7.42

587

DESTRUCTIVE TESTS

These tests involve testing of the weld piece to destruction. These are conducted to find out the mechanical strength of the weld. One of the main objects of such tests is to assess the quality of electrodes by testing the welds made by those electrodes. These tests are sometimes carried out to assess the ability of the welder by checking the quality of weld made by the welder. The usual tests are (a) Tensile test, (b) Bend test and (c) Impact test. The tests are carried out on standard size specimen of weldments made as per IS: 814-1963. Tensile tests are carried out on universal testing machine for finding out ultimate tensile strength, percentage elongation and percentage reduction in area. The impact tests are carried out on the Izod or Charpy specimens and machines.

7.42.1

Testing of Butt-welded Joints

Normal tests on butt-welded joints are tensile test and bend test, the latter being more common. The following three types of bend tests are carried on weldments. Root bend test

In this test, the root of weld is kept on the tension side by bending the specimen around a mandrel of specified diameter and through an angle of 180°. Any root defect or inadequate penetration is clearly shown up by this test. Face bend test

In this test, the face of the weld is kept under tension and bent through an angle of 180° around a mandrel of specified diameter. This test gives an index of the ductility of the weld metal. Inadequate penetration into the sides of the groove is also clearly shown. Guided bend test

This test is similar to the above two tests except that it is conducted under controlled bonding conditions. In all the above three tests, no crack or defect should occur at the outer surface of the specimen greater than 3 mm measured across the specimen or 1.5 mm measured along the specimen length.

7.43

THERMAL CUTTING OF METALS

Cutting is a process of severing or separating metals and is achieved by (a) machining or cutting by metal saws, broaches, etc., (b) chemical reaction (oxidation) or oxygen cutting and (c) arc cutting. In the following only the last wo processes of cutting metals have been described. Various oxygen cutting processes include oxy-acetylene flame cutting, oxygen-lance cutting, metal powder cutting, chemical flux (injection) cutting and oxygen-arc cutting. Arc cutting processes include manual metal arc cutting, oxygen-arc cutting, air carbon arc cutting, tungsten arc cutting and plasma arc cutting.

588 7.43.1

MANUFACTURING PROCESSES

Oxygen Cutting Processes

Oxy-acetylene flame cutting

Flame cutting is affected by means of chemical reaction of oxygen gas with the workpiece metal at elevated temperature and is thus and accelerated process of oxidation. When oxygen is united with ferrous metals at their kindling temperature (above 870°C), burning results and the metal is reduced to an oxide, called slag. A stream of oxygen under pressure, when directed at pre-heated steel, cuts or burns a hole or a kerf through it (Fig. 7.72). The product of burning in the above fashion is iron oxide which has its melting point lower than the melting point of steel. The heat generated by the burning iron is enough to melt iron oxide and converts it into molten slag. 3Fe (iron)

+

2O2 (oxygen)

Fig. 7.72

ææÆ

Fe3O4 + (iron oxide)

Heat (1120 kJ)

Oxy-acetylene flame cutting.

Oxygen-acetylene flame cutting is carried out (a) manually or (b) in a mechanized way. Manual cutting equipment include a cutting torch with different size tips, gas cylinders of oxygen and acetylene and all other tools/gadgets that form the normal kit for a normal gas welding set-up. The cutting torch is different from the welding torch. The cutting torch has an additional passage for high pressure oxygen for cutting besides the pre-heating oxygen tube and acetylene tube (Fig. 7.73). A cutting oxygen valve lever on the torch controls the flow of cutting oxygen while two separate valves are there for controlling oxygen and acetylene for pre-heating. The tip of the torch has a central hole for cutting oxygen and a number of orifices along the periphery for exit of oxygen-acetylene mixture for pre-heating flame. The cutting torches are of two types: (a) injector type in which mixing of oxygen and acetylene takes place within the tip of the torch and (b) equal pressure type in which mixing of two pre-heating gases takes place within the torch and not in the tip. With automatic cutting machines, called profiling machines, straight, round or any other shape is easily cut by oxy-acetylene flame. Machines are either stationary or portable type. Portable machines are easy to move and can be set on the large plate on which cutting is to be done. Stationary machines are of different types, for example, rectangular quardinate cuting machine or camograph cutting machine. Metals cut by oxy-acetylene flame cutting include carbon and low alloy steels, cast iron and wrought iron.

ELECTRIC AND GAS WELDING PROCESSES

Fig. 7.73

589

An oxy-acetylene cutting torch.

Oxygen-lance cutting

This process is applicable to cutting tap holes in steel furnaces and piercing of heavy steel plates. The apparatus comprises an oxygen cylinder, a regulator, a length of hoe and a lancer (a small pipe) (Fig. 7.74). When cutting larger sections of steel, even manifolding of oxygen is used. The operation of cutting is similar to that of oxy-acetylene flame cutting with the difference that the part to be cut is heated separately to white heat and then oxygen is turned on. The lance is slowly consumed during the operation and is replaced by a new pipe when needed.

Fig. 7.74

Oxygen-lance cutting.

Metal powder cutting

Metals such as stainless steels and cast irons are not easily cut by oxy-acetylene flame because of high melting point of their oxides. These oxides do not melt at lower temperatures

590

MANUFACTURING PROCESSES

and also do not allow the cutting oxygen to come in contact with the iron in the metal being cut. When finely divided iron powder is injected into the cutting flame through a special opening in the torch, the particles of iron powder get rapidly oxidized resulting into a sudden increase of heat on the metal surface. This melts the metal oxide making cutting easy. Chemical flux injection cutting

Certain powders react chemically with refractory oxide (iron oxide, etc.) and result in increasing their fluidity so that molten oxide slag is easily washed out of the reaction zone by the jet of oxygen. Mixture of iron and aluminium powder is often used as a chemical flux. Oxygen-arc cutting

The principle of cutting is saline (as flame cutting) with the difference that necessary temperature is attained by an electric arc struck between the job and the hollow electrode through which cutting oxygen can be passed. The process is used for underwater cutting besides piercing and gauging of metals (ferrous). Arc cutting processes

Arc cutting is different from the aforesaid processes in the sense that the necessary temperature for burning the metal is achieved by arc and the cutting of metal is due to the arc action. The process is based on melting rather than oxidation. AC and DC welding plants are used for the purpose. Manual metal arc cutting

In this process, cutting of metal is effected by melting with high heat of an arc established between a covered metal electrode and the job. DC or AC can be used. High current, up to 350 amperes (for a 4-mm electrode) is used. Cutting is started from the bottom edge of the job plate for effective removal of cut metal. Manual metal arc cutting is used only for small run jobs, e.g. cutting scrap, rivet cutting or piercing holes. It is not a production level method. Air carbon arc cutting

In this process, heat is generated by the arc between the job and a carbon electrode and compressed air stream is used to facilitate cutting. Tungsten arc cutting

Arc is established between a non-consumable tungsten electrode and the job with a shielding of mixture of inert gases. Stainless steels and other oxidation-resistant metals are easily cut by this process. It is used for cutting stainless steels, aluminium, magnesium, copper, silicon bronze, copper-nickel alloys, etc. Plasma arc cutting

Plasma arc cutting involves melting a localized area with a constricted arc with high heat intensity and temperatures (20,000°C or more) and removing the molten metal with a jet of ionized gas issuing from the plasma cutting torch orifice. Plasma arc cutting makes use of DCSP (with electrode negative). The principle of plasma arc constriction has already been discussed under plasma arc welding. The base metal is continuously melted by the intense heat of the constricted arc and is removed by the jet-like gas stream coming from the torch

ELECTRIC AND GAS WELDING PROCESSES

591

nozzle forming a kerf. The principal difference between the plasma welding torch and the plasma cutting torch is the absence of the shielding gas in cutting torch. The plasma flow is initiated through a pilot arc established by the high frequency generator in the welding circuit. The orifice gas (cutting gas) may be nitrogen, nitrogen-hydrogen and argon-nitrogen. For cutting aluminium and magnesium, nitrogen is used. Stainless steels thicker than 50 mm are cut by argon and hydrogen. Oxygen is used as a cutting gas by introducing it downstream in the flow orifice (Fig. 7.75).

Fig. 7.75

Plasma arc cutting. Oxygen introduced downstream in the flow orifice eliminates oxidation and erosion of the electrode.

Advantages of plasma arc cutting are as follows: (i) It can cut faster and leaves a narrow kerf (or width of cut). Mild steel is cut at a speed more than three times faster than oxy-acetylene flame cutting. (ii) Operating costs are less. (iii) Plasma arc cutting can be employed for cutting any metal. (iv) It can cut stainless steels and non-ferrous metals which are difficult to be cut by other methods. The process finds use in shipyard, chemical, nuclear and pressure vessels, cutting hot extrusions of steel to required length.

7.44 7.44.1

WELDING OF IMPORTANT METALS Welding of Wrought Iron

Wrought iron has extremely low carbon contents. It is highly ductile, high corrosion resistant and has good fatigue strength. Wrought iron welded products include furniture, railway couplings, crane hooks, chains, radiant heating system, sewer outfalls, smoke stacks, etc. Wrought iron is readily adaptable to various welding methods, namely, resistance welding, forge welding, oxy-acetylene welding, shielded metal arc welding, thermit welding and submerged arc welding, Because of high melting point of wrought iron (1539°C), welding wrought iron needs high heat input. Spot, seam, and flash butt weldings are used successfully. In forge welding, wrought iron is worked at a temperature of about 1460°C.

592 7.44.2

MANUFACTURING PROCESSES

Welding of Cast Irons

Welding of different types of cast irons is briefly described in the following: Welding of White cast iron

Welding or brazing of white cast is not usually done. Welding of grey cast iron

Grey cast iron has the lowest melting point (1178°C) in all the ferrous alloys. Because of its composition, welding grey cast iron is most difficult. However, welding is used for repair of castings in foundry shop and other cast iron parts. It may be mentioned here that most of the welding is done only on grey cast iron. Welding of malleable cast iron may turn the metal back into white cast iron or grey cast iron depending on the cooling rate. Similarly, welding of nodular cast iron is also not usually done. Hence, only the welding of grey cast iron will be discussed in the following. During welding of grey cast iron, when the weld pool solidifies, its microstructure will depend mostly on the cooling method. When cooling is fast, most of the carbon will go into the combined form (cementite) resulting into a very hard structure, brittle, unmachinable and liable to cracking. In order to get a soft, stress-free, machinable welded joint, cooling of the welded joint should be done very gradually by resorting to pre-heating the casting to 600–700°C before welding and also post-heating after welding and cooling the welded casting slowly by covering it with asbestos or burying it in the sand. Welding processes used for grey cast iron include metal arc welding, oxy-acetylene welding, braze welding, brazing and thermit welding. In metal arc welding, proper edge preparation is needed with a groove angle of 60 to 90°. Notching or studding may also be adopted (Fig. 7.76). Cast iron electrodes with high silicon and low sulphur are employed. Mild steel electrodes are also used. Other electrode materials are phosphor bronze, monel metal and nickel alloys. Pre-heating is desirable while using cast iron and mild steel electrodes. Phosphor bronze electrodes give a machinable joint. Monel and nickel alloys need no pre-heating and are preferred most. Aluminium-bronze electrodes are used for joining cast iron to non-ferrous metals. AC or DC may be used with skip welding technique preferably (Fig. 7.77).

Fig. 7.76 Different techniques used in metal arc welding of cast irons. (a) and (b): Special joint preparations using back-up plate. (c) and (d): Adopting strengthening measures using studs and notches.

Grey cast iron is welded successfully by oxy-acetylene welding. However, large amount of heat is needed for pre-heating and during welding. Cast iron filler rods having titanium and high silicon are used. Super-silicon cast iron rod (IS: 1278, type 5.1) of Indian Oxygen Ltd. is used for high grade castings where subsequent machining is required. Fluxes used are composed of borates, soda ash, ammonium sulphate, iron oxide, etc.

ELECTRIC AND GAS WELDING PROCESSES

593

Fig. 7.77 Showing the skip welding technique for welding cast iron with metal arc welding. A short length of weld metal is deposited in one part of the seam, then the next length is done some distance away, keeping the sections as far as away from each other as possible, thus localizing the effect of heat of welding.

In braze welding of grey cast iron, nickel-bronze rods are preferred. Pre-heating may not be required in light castings. Slightly oxidizing flame is used. For cleaning the casting (before welding), use of salt bath is best. In absence of that, the component may be heated with a slightly oxidizing flame to dull red and wire brushed. Brazing of cast iron is used to repair castings. After cleaning the casting (as described in braze welding) and proper edge preparation, brazing is carried out at as low temperature as possible. Filler rods of copper and copper-base alloys are not preferred as they need working at higher temperatures. Tin-bronze rods and silver-brazing rods with nickel are most commonly used. Pre-heating of job with torch between 200 and 427°C may be done during brazing, a neutral or slightly carburizing flame is used. Thermit welding of grey cast iron is done for heavy structure such as base and frame of heavy machines. Since thermit metal shrinks as much as cast iron, any weld larger than eight times the sectional thickness may develop minute hair cracks.

7.44.3

Welding of Carbon Steels

There are three broad categories of carbon steels: (i) low carbon steels (carbon up to 0.3%), (ii) medium carbon steels (carbon up to 0.5%) and (iii) high carbon steels (carbon up to about 1.5%). Welding of low carbon steels

These are easily weldable by any common method of welding, e.g. flux shielded metal-arc welding, submerged arc welding, TIG, MIG, CO2 welding, plasma arc welding, thermit welding, resistance welding processes, oxy-acetylene welding, brazing and soldering. Flux shielded manual metal-arc welding is the most common method of welding mild steels. Mild steel and low hydrogen electrodes are used. Oxy-acetylene welding is frequently used. No flux is needed. A neutral flame is employed and filler rod is of mild steel. No preheating or post-heating is needed. Welding of medium carbon steels

Welding of medium carbon steels poses problems as the weld joint tends to harden when mastensite is formed in the heat-affected zone (HAZ) due to rapid cooling. Pre-heating at 150 to 260°C is recommended to eliminate and reduce the hard and brittle areas. Post-heating is also done between 595 and 675°C. Medium carbon steels can be welded by flux shielded metal-arc welding, resistance welding, submerged arc welding, thermit welding and oxy-acetylene welding. With arc welding, low hydrogen electrodes should be used. In case

594

MANUFACTURING PROCESSES

of oxy-acetylene welding, an excess of acetylene should be used in the flame. No flux is used. High tensile steel rods are recommended. Welding of high carbon steels

Flux shielded metal-arc welding, resistance welding, thermit welding and oxy-acetylene welding are often used for welding high carbon steels. In case of oxy-acetylene welding, a carburizing flame is used. Filter metal is of high carbon steel. When flux shielded metal-arc welding is done, pre-heating up to 205°C before welding is essential and post-heating after welding is required between 700 and 790°C. Electrodes used may be mild steel electrodes, austenitic electrodes or low hydrogen electrodes. Flash and upset butt welding and spot welding are also used for welding high carbon steels.

7.44.4

Welding of Tool Steels

Tool steels include plain high carbon steels, alloy steels and high speed steels. In case of high carbon steels and high alloy steels, pre-heating and post-heating are needed. The tool steels are welded by oxy-acetylene welding, shielded metal-arc welding, submerged arc welding, atomic hydrogen welding, inert gas shielded metal-arc welding and silver brazing. In oxy-acetylene welding, a carburizing flame is used. Silver brazing is used to repair steels sensitive to cracking and for drills, saw blades, broaches, punches, etc.

7.44.5

Welding of Cast Steel

Arc welding, gas welding, braze welding and brazing are employed for welding cast steels. With metal-arc welding, low welding current and small diameter electrodes are used to minimize weld-metal shrinkage and hardening of HAZ. Pre-heating and low hydrogen electrodes are recommended to reduce under-bead cracking. Low carbon steel castings are weldable with mild steel electrodes. For medium carbon steel castings, mild steel and low alloy steel electrodes are used. High carbon steel castings are welded with low hydrogen electrodes. Pre-heat and interpass temperatures between 205 and 370°C are essential. The welded casting should be cooled very slowly or post-heat irnmediately between 595 and 680°C. Gas welding is generally done in one pass on section less than 12.5 mm thickness. A neutral flame and backhand technique are preferred. Flux is not generally used. For arc welding of low and medium alloy steel castings, electrode should be of low alloy steel or matching the composition of the casting metal. Pre-heating and post-heating are essential.

7.44.6

Welding of Alloy Steels

Alloy steels may be (a) low alloy steel (total alloying element up to 5%), (b) medium alloy steels (total alloying element 5 to 8%) and high alloy steels (total alloying element above 8%). The weldability of alloy steels depends on composition and low hardenability. Obviously, it is easy to weld alloy steels having low hardenability. Welding is done in the same way as carbon steels of equivalent carbon contents. Welding methods are also the same. Low alloy steel electrodes or mild steel electrodes are used. Pre-heating and post-

ELECTRIC AND GAS WELDING PROCESSES

595

heating are essential. For resistance welding, carbon contents should not exceed 0.12% and manganese 1.0%

7.44.7

Welding of Stainless Steel

Stainless steels are of three main types: (i) austenitic stainless steels, (ii) ferritic stainless steels and (iii) martensitic stainless steels. Austenitic stainless steels have better corrosion resistance and heat resistance and are preferred to the other two types for production of tough and ductile weld. Austenitic stainless steels (except for free machining grades) are more weldable than ferritic and martensitic stainless steels. Welding of stainless steels (austenitic) poses the following problems as compared to common carbon steels: (a) Thermal conductivity is 50% lower and hence there is a tendency of building up temperature. (b) Thermal expansion is about 50% greater. This is associated with problems of distortion and residual stresses. (c) Electrical resistance is about six times greater. (d) Melting point is lower (by about 93°C). The above factors indicate lower current requirement for welding stainless steel. When austenitic stainless steels are heated within temperature range of 427–870°C, problem of formation of chromium-rich carbide arises. Steel should have lower carbon content because as carbon increases more than 0.08%, carbide precipitation increases. Techniques used for welding stainless steels include oxy-acetylene welding, arc welding (MIG, TIG, submerged arc and plasma arc) resistance welding and brazing. The most suitable processes for welding stainless steels should produce rapid localized heat. Gas welding does not qualify that way as it produces a larger HAZ and heats the weld zone slowly. It is, therefore, used for welding thin sheets only (3 mm thick). A neutral flame should be used as oxidizing flame will oxidize chromium, although it will give more heat. Shielded metal-arc welding is most widely used because of its flexibility. Similarly, MIG and TIG are very effective methods. In fact, TIG can be applied to all weldable stainless steels in the wrought form or castings. Submerged arc welding gives fast metal deposition rates in welding austenitic stainless steels. Resistance welding (spot, seam, projection, etc.) is a very common method of welding stainless steel because of its low electrical conductivity that reduces high current requirement and welding time. Brazing of stainless steels is very common with silver brazing rods. Carbide precipitation is minimized by making a shorter thermal cycle. Straight chrome stainless steels (Cr—11.5 to 14%) tend to be martensitic. Both ferritic and martensitic stainless steels can be welded more or less in the same way as discussed above but special care is taken for pre-heating between 149 and 260°C and for post-heating between 732 and 788°C. The pre-heat will reduce the severity of thermal cycle (cracking tendencies caused by thermal hardening, grain growth over 900°C) and post-weld heating will temper the martensite that may get formed during fast cooling of weld.

596 7.44.8

MANUFACTURING PROCESSES

Welding of Aluminium and Its Alloys

Problems associated with welding aluminium and its alloys are as follows: (i) Oxide film removal. This is done by using a suitable flux in gas welding and brazing; using DCRP in MIG, oxide is removed by the cleaning action of arc when aluminium forms negative pole. In TIG using AC, cleaning of oxide is done when electrons strike the job surface in half cycle. (ii) Due to high thermal conductivity, heat dissipation is fast. Bigger nozzle is used in gas welding. Higher currents are used in arc welding. Thicker pieces are pre-heated before welding. (iii) High coefficient of linear expansion, which may result in buckling or distortion of weldments. (iv) Aluminium is weak when hot, hence welding of thinner sections needs extra care to avoid buckling. (v) Aluminium does not show colour change when heated and hence greater care and experience is required on the part of the welder. Welding processes used for welding aluminium and its alloys include oxy-gas welding, manual metal-arc welding, TIG, MIG, resistance welding, solid-state welding, carbon arc welding, atomic hydrogen welding and brazing.

7.44.9

Welding of Copper and Its Alloys

Copper alloys find extensive application in electrical industry, heat exchangers and where excellent corrosion resistance, high thermal and electrical conductivity and formability are needed. Varieties of brasses and bronzes and copper-nickel alloys, silicon bronzes and nickel silvers are used in industry. Weldability of most copper alloys is excellent with soft soldering, good to excellent for brazing, and fair to good for oxy-acetylene welding. TIG and MIG are effectively employed for welding most of the bronzes (silicon and aluminium bronzes) and brasses except for low leaded brasses, high leaded brasses and forging brass. Shielded metal-arc welding is not considered suitable for welding most of the copper alloys. Copper-base metals have high thermal conductivity and thermal expansion, susceptibility to hot cracking and high fluidity. Higher thermal conductivity needs high heat input as preheating. Greater thermal expansion gives rise to warping and residual stresses. Copper has low strength above 482°C and poses problems of cracking. Besides these, copper has a tendency to absorb oxygen in molten state. Severe oxidation happens beyond 400°C and the presence of oxygen tends to reduce the corrosion resistance and strength of the alloy. Oxygen embedded in parent metal grains avoids proper bonding of metal grain. As compared to molten steel, molten copper is very fluid and this calls for greater welding speeds. Processes used for welding copper and its alloys include TIG, MIG, gas welding, brazing and soldering. Very good results are obtained when deoxidized coppers are welded by TIG using argon shielding up to 3 mm thickness of workpieces and argon-helium mixture for thicker sections. MIG is not preferred to that extent (as TIG) because of greater heat input

ELECTRIC AND GAS WELDING PROCESSES

597

and lesser localization of heat obtained in MIG, porosity and low strength result. In gas welding of copper alloys, a larger tip size is used and flame is neutral type when flux is used; otherwise slightly oxidizing flame is preferred since oxide thus formed protects the molten metal from further oxidation. Brazing is done using filler rods of copper-silver-phosphorus, copper-phosphorus and silver-copper-zinc-cadmium. Joint design should be proper to allow capillary action of the brazing material.

7.44.10 Welding of Dissimilar Metals When two different metals are joined together by welding, it is called dissimilar metal welding. For welding such a joint, certain factors should be considered and these are given below: (i) Thermal expansion heating during welding creates compressive stresses in the metal and cooling creates tensile stresses. A danger is there that a metal with low tensile strength may fail during cooling. Proper design of joint and pre-heating before welding may help in this regard. (ii) Galvanic corrosion may occur due to the dissimilar metals in contact. (iii) Metallurgical stability includes consideration of (a) formation of alloys and properties of alloy phases in the metals being welded, and (b) dilution. Dissimilar metals are joined by fusion welding processes such as shielded metal-arc welding, resistance welding, electron beam welding, ultrasonic welding, friction welding, and brazing. As already discussed elsewhere, dilution is the reduction in the alloy contents of the weld deposit. This is checked in several ways. The joint faces of the base metals (dissimilar) can be buttered or overlaid with a weld deposit such that welding will be done later between the faces of deposited weld. Use an austenitic filler metal between chromium steels and austenitic steels; carbon steel rod buttering when welding carbon and chromium bearing metals; copper alloy for joining copper-base metals to iron-base metals; nickel alloy rod to butter nickel alloy with any dissimilar metal and vanadium buttering in welding titanium to steel. For fusion welding of dissimilar metals, the technique of using pre-fabricated joining piece is often employed. The pre-fabricated joint piece (Fig. 7.78) consists of short sections of dissimilar materials that are to be welded. These short sections are prepared (joined) in the workshop under controlled conditions and later used in making a joint at site. This way an opportunity is made available to join only the similar metals; example includes joining of aluminium with copper for manufacturing refrigerators. Resistance welding sometimes gives better results, for example, joining of copper and aluminium by flash-butt welding, spot welding or projection welding. Solid-state welding processes are often used successfully and effectively for welding dissimilar metals. Brazing is the most practised process of welding dissimilar metals of any type and without any problem. However, metals susceptible to intergranular penetration should be annealed before brazing. Zinc bearing silver solders and copper-zinc alloys are used for brazing copper alloys, all types of heat-resisting alloys, nickel alloys and tool steels.

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Fig. 7.78

Showing the use of pre-fabricated joint piece in fusion welding of dissimilar metals.

7.45 WELDABILITY Weldability denotes the relative ease of producing a weld free from defects such as cracks, porosity, non-metallic inclusion, etc. Weldability depends on one or more of the following factors: (i) Melting point: When welding low-melting point alloys (e.g. aluminium alloys), care is to be taken to avoid melting too much base metal. (ii) Thermal conductivity: Alloys with higher thermal conductivity are difficult to bring to fusion point and hence need more heat to be applied, for example, aluminium has much higher thermal conductivity as compared to steel and for a given size needs heat up to three times as much heat per unit volume as does steel. (iii) Thermal expansion: Rapid cooling of alloys with high thermal coefficient of expansion results large residual stresses and excessive distortion. (iv) Surface condition: Surface having coating of oil, dirt, oxides or paint hinders fusion and results in porosity. (v) Change in microstructure: Not only are the steels above 0.4% carbon subjected to grain growth in the heat-affected zone (HAZ) but martensite is also formed whenever the temperature exceeds 723°C for a sufficient time. The American Welding Society has defined weldability as ‘the capacity of the metal to be welded under the fabrication conditions imposed into a specific, suitably designed structure and to perform satisfactorily in the intended service’. It means that metal with good weldability should be welded readily so as to perform satisfactorily in the fabricated structure and should not require expensive and exacting procedures in producing a sound joint. The weldabiiity of any metal can be changed by changing the physical, chemical, thermal or metallurgical properties using proper welding procedure, shielding atmosphere, fluxes, filler metal and proper heat treatment before and after welding.

7.45.1

Weldability of Some Important Metals

Weldability of plain carbon and alloy steels

Steels vary widely in their weldability, especially according to their carbon content and/or alloying elements. Plain low carbon steels have excellent weldability but for steels above 0.3% carbon, special precautions are taken while welding; pre-heating and post-heating are

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needed. Weldability of alloy steels is generally good as long as carbon contents are in low range. With increase in carbon, these also need pre-heating and post-heating. Best results are obtained by arc-welding alloy steels using low-hydrogen electrodes to avoid brittleness in the welded joint. A general guide to weldability is given in Tabe 7.4. Table 7.4

General guide to weldability

Steel composition

General weldability

Pre-heating

Post-heating

Carbon steels, carbon less than 0.3% Low-alloy steel, carbon below 0.15%

Readily weldable

None

None

Carbon steels, carbon 0.30–0.50% Low-alloy steel, carbon 0.15–0.30%

Weldable with care

Preferable

Preferable

Carbon steel, carbon more than 0.5% Low-alloy steel, carbon more than 0.3% Alloy steels, total alloy above 3%

Difficult to weld

Necessary

Necessary

Weldability of stainless steels

Special care is needed in welding stainless steels. The molten pool should be, as free as possible, from oxides if stainless properties are required to be retained. TIG is best for high quality welding. Only low-carbon grades or grades with special carbide stabilizers should be preferred for welding to serve later in corrosive atmosphere. In normal standard grade stainless steels, carbide migration to grain boundaries of the heat-affected zone takes place resulting into intergranular corrosion. To minimize carbide precipitation (i) use low carbon (less than 0.03%) in stainless steel, (ii) use a columbium stabilized filler rod and (iii) heat the metal above 982°C and quench immediately after welding. Stainless steel is easily spot-welded (because of its low thermal conductivity). Weldability of aluminium alloys

Weldability of aluminium varies greatly. Low melting point, high thermal conductivity, high chemical activity and high thermal expansion make aluminium alloys difficult to weld. TIG and other resistance welding processes are adopted for welding aluminium alloys.

7.45.2

Effect of Heat on Welding and Heat-affected Zone (HAZ)

Two inherent problems in a welding process should be anticipated by the weldment designer: (a) the effect of localized heating and cooling on microstructure and properties of the base metal, and (b) the effect of residual stresses locked up in the weldment as a result of uneven cooling of the weld deposit. In general, weldments have poorer fatigue and impact resistance than correctly designed castings and forgings. In the welding process, a thermal cycle is introduced during heating and fusion of the metal. The parent metal (job metal) is heated over a range of temperatures over melting or fusion (Fig. 7.79) and followed by cooling to room temperature. Metal at a far off distance from weld will be simply warmed up but as weld area is approached, progressively higher temperatures are obtained. Figure 7.79 shows the thermal gradient during a welding process. The slope of the thermal gradient depends on the welding process, the metal being welded,

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Fig. 7.79 Showing the thermal gradient during a fusion welding process. Fusion zone is the weld metal pool zone in which metal (base metal and filler metal) are in molten state. The HAZ is the heat-affected zone which did not melt but has undergone change in microstructure.

and the way in which heat is supplied per unit volume of metal per unit time besides the thermal conductivity of the base metal. The flow of heat in the weld zone is highly directional towards the adjacent cold metal and this results on cooling in the formation of columnar grains at right angles to the fusion face (Fig. 7.80) and going towards the centre of fusion zone.

Fig. 7.80 Structure of a fusion welded joint of a low carbon steel. The heat-affected zone (HAZ) has three distinct regions (A, B and C). The weld structure may comprise coarse pearlite, fine pearlite or even martensite depending on the chemical composition and cooling.

In the weld metal zone (or fusion zone, Fig. 7.81), the parent metal and the electrode melt and later solidify as an integrated mass under the equivalent of chill casting conditions. The alloying elements and other microconstituents of the base metal and the filler rod undergo redistribution. Further, the unmelted base metal is subjected to a temperature gradient extending from the melting range to ambient temperature followed by a cooling cycle induced by neighbouring cold metal and atmosphere.

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Fig. 7.81 Regions of a welded joint with their relation to temperature (shown on a part Iron Carbon Equilibrium diagram).

There are three distinct zones in the weld area: (i) weld metal zone, (ii) heat-affected zone and (iii) unaffected base metal. Weld metal zone

This zone is formed as the weld solidifies from molten state. The ratio of base metal and filler metal in this zone depends on welding method, type of joint, thickness of workpiece, etc. The microstructure of this zone reflects the cooling rate in the weld. Based on the chemical composition and cooling rate, there may be martensite (due to fast cooling) and finite pearlite and coarse pearlite showing comparatively slower rate of cooling. It may be noted that columnar (long elongated) crystals are formed near fusion faces due to directional cooling of weld towards the centre (Fig. 7.80). Heat-affected zone (HAZ)

Heat-affected zone (HAZ) is adjacent to weld zone. This zone comprises only the base metal that did not melt but was heated to very high temperature for a sufficient period to allow grain growth in the zone. The microstructure and mechanical properties of the base metal in HAZ have been affected and altered as a result of welding heat. The thermal cycle of HAZ is complex as it is subjected to sudden heating and rapid cooling. Because of this temperature variation and method of cooling the weld, HAZ consists of a variety of microstructures, for example, in plain carbon steels, the structures may vary from small region of martensite to coarse pearlite and hence make HAZ the weakest area in a weld. The width of HAZ depends on the process of welding, for example, in arc welding, HAZ width is smaller (only a few mm from fusion boundary, the external boundary of melt

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zone), whereas in gas welding, HAZ is wider and so also in electroslag weld. This is because that in two later cases, base metal was heated more and for a longer period. In a low carbon steel specimen, welded in one run by arc welding, there may be three distinct regions in HAZ: (i) grain growth region, (ii) grain refinement region and (iii) transition region (Figs. 7.80 and 7.81). The grain growth region is immediately next to fusion zone and in this, the metal has been heated to a temperature well above the upper critical temperature AC3 resulting into coarsening of grains or grain growth. Grain size and extent of grain growth depend on the cooling rate. Slower the cooling, more the grain growth. There will be a large region of pearlite and smaller grains of ferrite. In grain refined region, which is next to grain growth region while moving towards lower temperature zones, the base metal is heated to just above the upper critical (AC3) temperature where grain refinement takes place and finest grain structure exists with finer areas of both ferrite (white) and pearlite (dark). In the transition region, a temperature range between (AC1) and upper critical temperature (AC3) exists when partial allotropic recrystallization takes place which makes pearlite grains still more finer. Beyond this zone (which is the last zone of HAZ) exists the unaffected base metal.

7.45.3

Weld Cracking

A crack is defined as a fissure resulting from the tearing action of metal. The presence of brittle microconstituents is responsible for cracks following the welding operation. If after welding, the ductility of the weld metal is retained, crack formation is avoided as ductile constituents are able to deform plastically. Besides the microconstituents having different rates of contraction as compared to parent metal, development of tensile stresses in the weld zone due to contraction of weld metal during cooling is yet another factor of cracking. It can be said that the main cause of cracking is insufficient ductility or strength of metal at relevant stage (in cooling of weld) to tolerate the welding stresses exceeding the fracture stress of the metal. Check may occur in (i) weld metal, (ii) HAZ or (iii) both. More prominent types of weld cracking are (a) hot cracking or solidification cracking occurring at elevated temperature just below freezing point of weld and (b) cold cracking which may take place in welds at room temperature or little above. Hot cracking

Hot cracking in carbon and low alloy steels is influenced by the presence of sulphur and carbon that form a low freezing point alloy, iron sulphide which segregates to form a network at grain boundaries of steel and remains in liquid state even after the rest of the metal has frozen. Since weld solidification starts from the outer face of fusion zone (coming in contact with HAZ) towards the centre of weld, alloying element or impurities (such as iron sulphide) are driven to the centre of weld. In the last stages of solidification, due to the separation of the low-freezing point liquid film of iron sulphide etc., all the metals in weld solidify except the low-freezing impurities. The solidification cracking is, therefore, caused in the weld metal itself by the tearing of grain boundaries before complete solidification has taken place and metal is still in plastic stage. Low-freezing impurities are caused by either sulphur and phosphorus in steel or by the alloying element (such as aluminium and magnesium alloys) giving a wide freezing range. High carbon contents in weld metal also promote hot cracking.

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Other factors promoting hot cracking include higher current, metal composition with wide freezing range, joint restraints, crack sensitivity of electrodes and long arc which increases crack sensitivity and dilution of weld metal. Cold cracking

This occurs at a relatively low temperature much lower than the solidification temperature of carbon and low alloy steels. Cold cracking is caused due to the brittleness of metal combined with the tensile stress in cooling exceeding the fracture stress of metal. The brittleness is caused due to the solidification of low-freezing point constituents and the phase changes (such as formation of martensite) during cooling. Other factors contributing to cold cracking are presence of impurities and hydrogen in weld metal, insufficient area of weld, high welding speeds and low current and joint restraints. Cold cracking occurs in both weld metal and the HAZ. Besides some of the reasons explained above for cracking in weld metal, the cracking in HAZ may occur due to the diffusion of hydrogen in HAZ from the weld metal.

7.45.4

Weld Decay

Weld decay is a phenomenon observed in the HAZ, particularly in welding of unstabilized stainless steel (such as 18/8 type). It is due to the precipitation of chromium carbides at the grain boundaries in an area around the weld having temperature in the range of 600–850°C (Fig. 7.82). Because of the precipitation of chromium carbide, the corrosion resistance of the said region (under 600–850°C) reduces considerably due to the depletion of chromium and the phenomenon is known as weld decay. To avoid weld decay, weld metal may be heated to 1100°C and then cooled rapidly to take carbide phase back in solution. Decrease carbon contents (to about 0.03%) to avoid formation of carbides. Adding stabilizing elements such as titanium, tantalum, niobium in metal and filler rod metal helps in forming carbides preferentially at the grain boundaries preserving chromium contents and also reducing precipitation tendency.

Fig. 7.82

Weld decay.

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7.45.5 Dilution The welding can be (a) autogeneous welding in which no filler metal is added (resistance welding, cold welding, etc.), (b) homogeneous welding wherein a filler rod is used of same composition that of the base metal (arc welding processes) and (c) heterogeneous welding, in which the filler rod composition is different from the parent metal (soldering or brazing). In both the above cases of welding, wherein filler rod is used, the weld deposit is a mixture of the base metal and the filler metal. When in welding, the composition of filler rod is little different from that of the base metal, the weld bead may exhibit a composition lying somewhere between that of the filter metal and base metal and this effect is called dilution. This happens because the filler rod metal penetrates and melts in the parent metal. Dilution (%) =

Weight of parent metal × 100 Total weight of fused metal

Dilution is of significance in joining dissimilar metals or cladding of metals. The factors affecting dilution are type of joint, welding process used, variation in composition of filler metal and base metal, number of weld runs and weaving of electrode.

7.45.6

Modes of Metal Transfer in Welding

Metal transfer across the arc from filler rod (electrode) to the weld joint may take place in two ways: (a) free flight transfer such as spray transfer and globular transfer, and (b) shortcircuiting or dip transfer. These are discussed below in reference to Fig. 7.83.

Fig. 7.83

The three different modes of metal transfer in welding.

Spray transfer

The spray transfer arc aims in achieving a high enough current density for a given wire size that causes the metal to form fine droplets and can be propelled across the arc at high speed. Several hundred minute droplets of metal per second are passed through the arc. The system is used mainly on normal metal joints and argon gas gives the best spray. Small diameter electrodes are used for lighter gauge metals. With high heat input and using larger size filler rods, plates up to 25 mm thick can be welded by this system. Globular transfer

Globular transfer is characterized by the size of the droplet having diameter larger than the electrode diameter. This type of metal transfer takes place best with DCRP when current density is relatively low as it provides a more concentrated weld pool and deeper penetration. Globular metal transfer happens in gas metal-arc welding processes (MIG) with CO2 shielding even at high current density.

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Short-circuiting transfer or dip transfer

With the introduction of system in DC plant for limiting the surge of current, short-circuiting metal transfer is used with small diameter wires on DCRP at a maximum of about 150 amperes. The limit on the surge of current (due to short-circuiting) also has a limit on the pinch effect on the molten metal at the tip of electrode which is sufficient to cause separation and the arc is again established. This rapidly reoccurring sequence is repeated more than 100 times per second. The method is most suited to welding thin sections.

7.45.7

Residual Stress and Distortion in Weldments

Residual stresses

During welding, the base metal (workpiece) is heated to the melting point below the arc and a few centimetres away, the temperature of base metal is quite lower in comparison to the weld zone. This sharp temperature differential causes non-uniform expansion, areas near the weld expanding more (being at higher temperature) than those areas which are far off from the weld zone. This causes metal displacement (may result in bulging or distortion) when the parts being joined are restrained (due to structural rigidity of the parts itself or due to clamping). When metal cools, the molten weld pool tries to contract more but it is restrained by the metal surrounding the weld pool which contracts less being at comparatively lower temperature. And thus uneven contraction takes place and internal stresses (residual stresses) are developed in the weld metal which increases the tendency to distort. Thus, the residual stresses result from the restrained expansion and contraction that occur during the localized heating and cooling in the region of weld deposit. The magnitude of such stresses depends on the weldment design, support and clamping of the components being welded, their material and the welding process used. Consider a metal bar held in clamps at either end and being heated in the centre [Fig. 7.84(a)]. No lengthwise movement can occur and hence centre must bulge [7.84(b)]. However, when bar cools again, it tries to contract but the clamps at its both ends do not allow. As a result of this, residual tensile stresses are built up in the bar as it tries to contract. The same happens in welding. The molten pool solidifies as a casting is poured into a metal mold. It is restrained from contracting by an amount which varies with the welding process. The cooling rate has a great influence on the amount and nature of residual stresses.

Fig. 7.84 Showing the thermal expansion of a part with its both ends constrained or held, as shown at (a) while the heating of the part is done at its centre. Subsequent cooling under these conditions will result in contraction and the development of residual tensile stresses in the part, as shown at (b).

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Distortion

Distortion is being out of shape or deformed. A metal piece gets bent after welding a bead over it (Fig. 7.85). During welding, heating and cooling of the metal close to the weld bead takes place from molten state to cooling down to the temperature of the surrounding. When molten weld pool cools down, it tends to shrink while pulling all other metal around it. Under these circumstances, three alternatives are possible. The metal surrounding the cooling molten

Fig. 7.85

Showing the possibilities of distortion after welding in three different situations.

Fig. 7.86 Some methods of reducing distortion in welding plates A and B. 1. Restricting heat input using water-cooled chill blocks. 2. Controlled restraint to permit some movement during heating and cooling. 3. Tack welding to provide restraint during welding. 4. Pre-straining to expected distortion level (a) distortion to be expected without pre-strain, (b) welding in pre-strained condition, (c) release of pre-strain to obtain undistorted product. 5. Bead sequence as shown, allowing opposing stresses to correct distortion obtained due to prior bead. 6. Skipping sequence of welding to prevent stress accumulation of continuous weld and allow opposing stresses to correct previous distortion. 7. Root gap to permit the weld metal to shrink transversely and reduce rotational distortion, the double V and sequence (No. 5) produce minimum distortion.

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weld pool will follow the shrinking of the weld pool (or weld bead) and hence distortion will result in which the part may bend depending on the pull due to shrinkage of weld. If the metal surrounding the weld bead is restrained or not allowed to follow shrinkage of weld by holding the part by clamping, etc., then the part will be internally stressed and will develop residual stresses which in nature are tensile stresses. The third possibility could be that when the metal may be overstressed to the extent that a crack may develop. Welding sequence is important in minimizing distortion and residual stresses. Welding fixtures can be used to reduce distortion especially on thin pieces. Some methods of reducing distortion that occur, in welding are shown schematically in Fig. 7.86. Thermal input to the workpiece may be lowered by watercooled chill blocks. Mechanical restraint permitting movement only in one direction can be used. Skipping sequence of welding is a common method in avoiding warping of weldments.

7.46

WELDING FIXTURES

The fixtures are used (i) to minimize distortion in weldment, (ii) to permit welding in convenient position, (iii) to increase welding efficiency, safety and output, and (iv) to minimize assembling problems, as with fixtures the components can be assembled and welded by placing them in proper relationship. Welding fixtures are specialized devices that enable the workpieces being welded to be easily and rapidly set up and held. They permit the changing of the position of the work during the welding operation so as to place the weld-joint in a plane convenient to the welder at all times. Rollers, clamps, wedges, turn table, indexing plate, etc. are the components of the fixture to help positioning and clamping of the job. A simple fixture for welding sheets is shown in Fig. 7.87.

Fig. 7.87

Use of a welding fixture for butt welding of two thin plates A and B.

REVIEW QUESTIONS 1. Define the following processes of welding. Give examples also. (a) Autogeneous welding process (b) Homogeneous welding process (c) Heterogeneous welding process

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2. What are the qualities of the flame used for welding? 3. What are the three different types of oxy-acetylene flames and for what application are these used? 4. How are the following defects caused during welding? (a) Poor penetration (b) Porous weld (c) Undercutting (d) Spatter (e) Cracks (f) Poor fusion 5. What is arc blow? How is it caused? What is its remedy? 6. Discuss the advantages of AC arc welding over DC arc welding. 7. What is an arc? 8. How does penetration vary for DCSP and DCRP welding? 9. What are the basic differences between metal arc welding and submerged arc welding? 10. Why are electrodes for AC arc welding coated with potassium silicate binder and those for DC arc welding with sodium silicate? [Hint: AC changes from positive to negative in every cycle giving an unstable arc. Potassium is a good arc stabilizer because its ionization potential is much lower than that of sodium.] 11. Does arc voltage increase or decrease with increase in arc length? What are the advantages of a shorter arc over a long arc? 12. What are DCRP and DCSP? Where are they used? Give examples. 13. Explain the terms (a) hot cracking and (b) cold cracking. How can these be taken care of? 14. Compare MIG and TIG welding in respect of their principle of working and field of application. 15. What are the advantages of submerged arc welding? 16. Draw a diagram showing classification of welding processes. 17. What is thermit welding? What reactions take place in thermit welding? 18. Make a neat sketch showing the structure of an oxy-acetylene flame. Give different temperatures in various cones of the flame. 19. What factors should be considered when selecting a coated electrode? Why is it necessary that the coating should have higher melting temperature than the base metal electrode? [Hint: In heavily coated electrodes, core wire melts before the coating giving rise to a cavity, hence producing arc constriction and arc heat concentration on the workpiece.] 20. What is welding? Why is it preferred to other metal joining processes? 21. Name some of the important manufacturing sectors where welding finds extensive application. 22. Discuss the principle of resistsance welding. 23. What is gas welding? 24. Differentiate between (a) upset and (b) flash butt resistance welding. 25. Discuss various methods of arc initiation in an arc welding process. 26. What are the advantages of using AC welding transformer machine? 27. What do you understand by characteristics of welding plant? Write a short note on constant current type characteristics of the welding plant.

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28. What is arc length? What is a stable arc? What are too short arc and too long arc? Discuss their effects on welding. [Hint: Too long arc is unstable, causes spatter, makes hissing sound, and is erratic. Too short arc length results in improper arcing between electrode and workpiece, sticking of electrode an improper shielding. Stable arc is silent in working, is uniform and speedy, has good penetration and no porosity, and gives strong weld joint.] 29. What different types of arc welding machines are you familiar with? Discuss AC vs DC arc welding. 30. Compare the bare and coated electrodes. What is the use of coating an electrode with flux coating? 31. What are the metals used for electrode core wire? 32. What different types of flux coatings are used on electrodes for arc welding? 33. What are the factors considered in selecting an electrode for metal arc welding? 34. What is a welding position? Discuss various welding positions with the help of neat sketches. 35. Compare manual metal arc welding with CO2 welding in reference to welding of mild steel. 36. What is the function of using shielding gases? Name the shielding gases used in TIG and MIG. 37. What is resistance welding? Discuss its principle and variables of operation. What are the advantages of resistance welding? 38. What are the characteristics of a good weld? What is radiographic testing of weldments? 39. How is oxy-acetylene cutting torch different from the welding torch? 40. Define the term weldability. On what factors the weldability of a metal depends? 41. Write short notes on: (i) Leftward welding and (ii) Rightward welding in relation to gas welding. 42. Write short notes on: (i) Laser beam welding (ii) Plasma arc welding (iii) Electron beam welding (iv) Ultrasonic welding (v) Underwater welding 43. What is explosive welding? How is it carried out? 44. What are solid-state welding processes? 45. Write short notes on: (i) Friction welding (ii) Projection welding (iii) Percussion welding (iv) Laser beam welding (v) Electron beam welding (vi) Underwater welding 46. What is tool tipping and how is it done? 47. Discuss the problems encountered in welding cast iron and stainless steel. 48. Compare metal arc cutting with plasma arc cutting. 49. What is HAZ? What is its importance in welding. 50. Define the terms (a) weld decay and (b) dilution. 51. Why do residual stresses get developed in weldments? Discuss various methods adopted for avoiding or minimizing distortion. 52. Why is it difficult to cut aluminium alloys with oxy-acetylene flame?

8 8.1

Soldering and Brazing

INTRODUCTION

Soldering and brazing are the processes of joining metal pieces, making use of heat, and a filler metal whose melting point is lower than the melting points of metals to be joined. In soldering, the melting point of solder (or filler metal) is usually less than 427oC, whereas in brazing and braze welding, the melting point of filler metal is higher than 350oC. In soldering and brazing, there is no direct melting of the base metal of workpieces being joined, rather the solder or filler metal flows between the surfaces to be joined through the capillary action. The most common use of soldering and brazing is in joining two dissimilar metals. Differences between soldering and brazing are given in the following: (a) The melting point of solder or filler metal used in soldering is lower than 427oC, whereas in brazing, the melting point of filler metal is higher than 427oC. (b) Soldered joints do not resist corrosion to the extent that brazed joints can do. (c) Brazing gives a stronger joint which can stand to higher temperature service. (d) In brazing, bonding conditions are set up to allow large amount of diffusion to take place along the surfaces being joined, whereas in soldering, diffusion is of secondary importance. Diffusion bonding refers to the metallurgical joining of metal surfaces by the application of heat or/and pressure to cause co-mingling of atoms at the joint interface. The interface surfaces should be very clean and free of contamination. Diffusion bonding in short instances is accomplished entirely in solid state.

8.2

SOLDERING

Soldering is defined as a metal joining process wherein coalescence is produced by heating the surfaces to be joined to a suitable temperature and melting the filler metal, which is a fusible alloy called solder (melting point usually less than 427oC), so that it may be distributed between properly fitted surfaces of the joint through the capillary action. Soldering operation 610

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611

is performed by bringing molten solder in contact with the pre-heated surfaces (being joined) and heating the joint area to a good wetting temperature (about 55 to 80oC above the melting point of soldering alloy). The solder is then left to cool and freeze as quickly as possible to avoid development of internal microcracks in the joint. The principle underlying soldering is that when the surfaces to be joined are cleaned off well from oxides, they can be joined together using molten solder that may adhere easily to the workpiece surfaces due to molecular attraction. The molecules of solder entwine with the parent metal molecules and form a strong bond.

8.3

METALS JOINED BY SOLDERING

The efficacy of soldering depends on the characteristics of oxides of metal being soldered and mutual solubility of solder and base metal. Therefore, all alloys are not equally wetted by solders. (i) All types of carbon steels are soldered easily but the wettability decreases with increase in carbon contents in steels. (ii) Cast irons are difficult to solder as graphite carbon flakes resist wetting. (iii) Stainless steels are difficult to solder because of the chromium oxide on stainless steels. Rougher the surface, better the steel can be soldered. (iv) Copper and its alloys (brasses, bronzes) are easily soldered. (v) Phosphor bronzes are easily soldered. (vi) Aluminium or silcon bronzes are not easy to solder due to the presence of oxide. (vii) Aluminium and its alloys are soldered with difficulty. (viii) Nickel alloys are easy to solder.

8.4

SOLDERING JOINTS

In soldering, butt joints are avoided and lap joints are preferred. The soldering joints are weaker than brazed joints and hence the former needs to be designed such that in service, the soldered joint does not take load, the base structure should take the load. Joint strength can, however, be enhanced to some extent by designing proper interlocking system at the joint. Different types of solder joint designs are given in Fig. 8.1. The simple butt joint should be avoided as it has no strength. A lap joint is most widely used where overlapping may be three times the thickness of the workpiece metal. Lap joint fits more easily and gets good strength due to extended overlap. The joint gap or clearance is necessary for the capillary action of the molten solder. It is usually kept 0.075 to 0.25 mm.

8.5

TYPES OF SOLDERS

Solders are broadly categorized as soft solders and hard solders. Soft solders melt usually below 427oC and hard solders melt at temperatures above 600oC. Soft solders are primarily the alloys of lead and tin. More lead increases melting point of solder, whereas tin increases

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flowability of solder. Soft solder melts at a temperature which is lower than the melting point of the alloying elements, for example, lead melts at 327oC, tin at 232oC but their solder melts at 205oC. Sometimes antimony, silver, bismuth and cadmium are added to the soft solders. Hard solders are basically the alloys of copper and zinc to which silver or phosphorus is sometimes added. Hard soldering is employed when a stronger joint is required than that is obtained by soft solders. Hard soldered joints can stand higher service temperatures. Hard solders are used in the brazing process wherein the melting point of filler metal is more than 427oC.

Fig. 8.1

Soldering joints.

Soft solders fall under the following categories: (i) (ii) (iii) (iv) (v) (vi)

Tin-lead solders Tin-antimony-lead solders Tin-zinc solders Lead-silver solders Cadmium-silver solders Cadmium-zinc solders

1. Tin-lead solders: These are commonly used solders for joining most metals. These are cheaper and have good resistance to corrosion. Three typical solders of this category are: (a) Plumber solder, having tin 32%, lead 68%, melting point 253oC. (b) Tin man’s solder, having tin 62%, lead 38%, melting point 183oC. (c) Ordinary solder, having tin 50%, lead 50%, melting point 217oC. 2. Tin-antimony-lead solders: Antimony, when added up to 6% of tin contents, increases mechanical properties of the soldered joint. But antimony solder should be avoided on zinc, cadmium or galvanized metals because this solder gives a brittle joint. (a) For soldering iron and steel, a solder with tin 12%, antimony 8% and lead 80% is used. (b) For general purpose, a solder with tin 43.5%, antimony 1.5% and lead 55% is used.

SOLDERING AND BRAZING

613

3. Tin-zinc solders: These are used for joining aluminium components. A typical composition of this solder may have tin 91%, zinc 9% and melting point 199oC. 4. Lead-silver solders: Lead-silver solders easily wet steel and copper components. Addition of silver up to 2.5% forms a good lead-silver solder which melts at 307oC. 5. Cadmium-silver solders: These are used for joining aluminium to itself or other metals. For high temperature application, a solder with cadmium 95% and silver 5% is used. It melts at 338oC and flows at 393oC. 6. Cadmium-zinc solders: These are used for soldering aluminium. Die-cast (zinc base) products are also soldered efficiently with a solder containing cadmium 82.5% and zinc 17.5%; the solder flows at 265oC.

8.6

FUNCTIONS OF SOLDERING FLUXES

Soldering is a delicate operation of joining two metal parts. The presence of any dirt, grease or oxide on the surfaces being soldered will adversely affect the quality of soldered joint. Some chemicals called soldering fluxes are used in soldering, depending on the metals to be soldered. The soldering fluxes are available in the form of powder, paste, liquid or solid. Flux also improves joint strength. It also protects the metal from oxidation. After the flux is applied on the joint surface (before soldering), it is heated by a soft flame or soldering iron. The flux loses its liquid, then melts and decomposes to form hydrochloric acid which dissolves oxides film from the surfaces of the workpiece. The fused flux provides a protective cover over the soldered joint.

8.7

TYPES OF SOLDERING FLUXES AND THEIR COMPOSITIONS (a) Inorganic or acid corrosive fluxes consist of zinc chloride and ammonium chloride. These are acidic in nature. A flux with zinc chloride (75%) and ammonium chloride (25%) with melting point 177oC gives a very good cleaning effect and covering properties. This flux is used in liquid form because during soldering, the water or solvent evaporates and the flux melts. Zinc chloride type fluxes are used for soldering low carbon steels, low alloy steels and copper alloys (brass). (b) Mild fluxes are composed of organic acids which are less corrosive than the inorganic acids. Although as applied, the organic fluxes are also corrosive, on heating, these become mildly corrosive. These fluxes comprise lactic acid, stearic acid, etc. Organic fluxes are useful in situations where sufficient heat can be used to fully decompose the flux so that corrosive constituents may be volatilized. (c) Non-corrosive fluxes are the rosin fluxes and these are non-corrosive, and are used for soldering of electrical and electronic joints. This flux is obtained from tars found in pine trees. Since this flux acts slowly (its spreading is slow), some hydrochloride compound is added to rosin to work as an activator. Rosin fluxes are used on copper and copper alloys.

614

MANUFACTURING PROCESSES

8.7.1 Surface Cleaning Agents Used in Soldering Although flux also serves the purpose of cleaning the surface, before starting the soldering operation, the workpiece surface to be soldered is first cleaned off dirt, grease and other foreign matter. Cleaning can be done by wire brushing, scrapping, grinding or sanding. Some chemical agents are also used for cleaning. These are (a) alkaline degreasing solvents to clean oil or grease, and (b) pickling (acid cleaning) to remove rust, scale or oxides.

8.8

STEPS IN SOLDERING OPERATION 1. The surfaces to be joined by soldering must be thoroughly cleaned off any dirt, grease or oxide by acid cleaning (pickling). Wipe out the joint clean. 2. The joint surfaces are heated with a soft oxy-acetylene flame. 3. Soldering flux is later applied on to the heated surfaces of the joint. It is heated further sometimes. 4. The solder is then applied with the help of soldering iron and it is allowed to melt only by the heat present in the heated surfaces of the workpiece. 5. Soon after the soldering operation is over, the soldering flux should be cleaned from the joint with a wire brush or by filing.

8.9

METHODS OF SOLDERING

Soldering methods are classified according to the method of applying heat to melt the solder and also for pre-heating the surfaces to be soldered. These include soldering iron method, torch method, dip bath method, resistance method, ultrasonic method, induction method, wave soldering, etc. Only the most common methods of soldering have been elaborated in the following.

8.9.1 Soldering Iron Method The soldering iron method (or soldering copper method) is one of the oldest methods used for lead-tin alloys soldering operations. The method is shown in Fig. 8.2. The soldering iron consists of a square or octagonal solid copper rod having at its front end a four-sided tapered copper point and the tool is available in different sizes. The copper point helps in quick transmission of heat from the tool to the workpiece joint. Solders are available in the form of wire or strip. The heated end of soldering copper is touched with the solder to melt a part of it, which is subsequently tinned over to the copper point and transferred to the joint. The soldering copper may be heated by gas flame, blow lamp, or an electrical resistance heating system. Electric soldering irons are available which maintain uniform heat. A 200-watt iron is commonly used for most sheet metal soldering. The tip of the soldering iron must be kept clean and tinned. Soldering iron gives concentrated heat which is most desirable for soldering. It is easier with soldering iron to spread or smoothen the solder on the joint surface.

SOLDERING AND BRAZING

Fig. 8.2

615

Soldering iron method.

8.9.2 Soldering Torch Method Soldering torch method is shown in Fig. 8.3. A soldering torch is used to provide heat and is the fast and versatile method of heating. Soldering torches are available in different designs, for example, gasoline blow torch, natural gas-oxygen torch, compressed air-acetylene torch and oxy-acetylene soldering torch. These torches ensure giving of a clean flame to heat the surfaces to be soldered. The heat of flame is easily adjustable. This method of soldering finds extensive use in refrigeration and air-conditioning plants, building up of irregular surfaces to get a finished surface. Threaded connections in plumbing are being replaced by soldering.

Fig. 8.3

Soldering torch method.

616

MANUFACTURING PROCESSES

8.9.3 Dip Bath Method In dip bath method, solder is melted in a pot and it is protected against atmospheric contamination (oxidation) by using a hood or cover on the pot or by providing a chemical coverage by powdered charcoal. The workpieces are first dipped in the flux bath and later in the solder bath. The articles soldered with this method are required to be cleaned before they are coated with flux and soldered because the moisture present on the workpiece may produce instant high temperature steam, leading to the explosion of bath.

8.9.4 Wave Soldering Wave soldering is an automatic process of soldering employed for the manufacture of printed circuit boards wherein the latter with its components pass over a wave of molten solder. All components of the circuit are bonded in one quick operation as the molten solder is pumped vertically upwards through a narrow slot forming a steady wave. This way, the dynamic movement of the solder across the work surface improves wetting and minimizes heat distortion, resulting in an oxide-free bright surface of solder.

8.10

TINNING, SWEATING AND FLOATING

Tinning is an operation of adhering a very thin layer (or film) of solder on to the metal surface. Copper wire or containers made from thin steel sheets are tinned to protect the surface from corrosion. In soldering operation, solder actually tins the joint surface. Sweating is an operation of applying sufficient heat to the workpiece to make the solder run freely. Floating is the term used to describe the process of flooding a quantity of solder along a joint on the inside of a vessel. This is done to give the inside corner a smooth rounded form and watertight joint.

8.11 BRAZING Brazing is a technique of joining two similar or dissimilar metal pieces together by heating the surfaces and by using a non-ferrous filler metal having its melting point above 427oC but below the melting points of metals to be brazed. The molten filler metal is distributed between the joint surfaces by the capillary action, which on cooling results in a sound joint. The main advantage of brazing process is the joining of dissimilar metals and thin sections. The process is mostly used for joining pipes and other fittings, carbide tipping on tool shanks, electrical parts, radiator, repair of cast iron parts and heat exchangers. In brazing, bond is produced by the formation of either solid-state solution (diffusion bonding) or intermetallic compounds of the parent metal (job) and one of the metals in the filler material (brazing alloy). The strength of the brazed joint is provided by metallic bonding.

SOLDERING AND BRAZING

8.12

617

BRAZING VS WELDING

Following are the differences between brazing and welding. (i) In brazing, the joint surfaces are not raised to fusion point (or melted) and the joint is produced by the solidification and adhesion of a thin layer of molten filler metal spread between the mating surfaces. In welding, the two surfaces to be joined are always heated to molten state for making a joint. (ii) The filler metal in brazing spreads between the joints by the capillary action. In welding, the molten filler rod solidifies at the same place where it melts. (iii) In brazing, there is no penetration of the filler metal into base metal, whereas in welding it is there.

8.13

BRAZING OPERATION

Brazing is carried out in the similar way as soldering. The surfaces to be brazed are cleaned and heated to a temperature enough to melt the flux spread over the surfaces to be joined. Since the flux melts at a temperature which is lower than the filler material, it wets the joint surfaces and removes oxide film, etc., giving a clean surface. Since the capillary attraction between the base metal and the filler metal is several times higher than that between the base metal and the flux, the molten filler metal replaces the flux and flows in between the mating surfaces of the joint by capillary attraction. Lesser the joint clearance, better will be the capillary action. Upon cooling, the joint is found filled with filler metal and the solidified flux is found on the joint periphery.

8.14

BRAZING JOINTS

There are certain requirements of brazing joint design, for example, a good joint allows enough allowance for filling of the filler metal into the joint area when the filler metal placed at one side is sucked through the joint by the capillary action. Since the brazing alloy (filled in the completed joint) should not be subjected to direct stress, the lap joint is considered best because this makes the brazing alloy subjected to shearing, which is a better situation. In case of brazing, close fit between the surfaces to be joined is essential. Therefore, setting up of proper clearance (to allow flow of filler metal) between mating surfaces is an important factor. The gap or clearance depends on the fluidity of the filler metal, its wettability and susceptibility to slag (oxides) entrapment. For different types of filler metals, joint clearance is specified as follows: (a) (b) (c) (d) (e) (f)

Aluminium-silicon alloys—0.15 to 0.25 mm (lap less than 6 mm) Copper-phosphorus alloys—0.25 to 0.62 mm (lap greater than 6 mm) Copper-zinc alloys—0.05 to 0.125 mm Silver-brazing alloys—0.05 to 0.125 mm Magnesium—0.10 to 0.25 mm Nickel—0.05 to 0.125 mm

618

MANUFACTURING PROCESSES

Brazing joints are shown in Fig. 8.4. These are lap, butt, scarf, tee, flanged bottom, butt lap and modified lap.

Fig. 8.4

8.15

Different types of brazing joints.

METALS JOINED BY BRAZING

The following metals can be joined by the process of brazing. 1. Low-carbon steels are easily brazed. Low-alloy steels tend to harden during brazing and hence these should be brazed carefully allowing them to cool down slowly. 2. Stainless steels pose problems in wetting due to chromium and need special fluxes to be used. 3. Cast irons (grey, malleable, ductile cast irons) are easily brazed with silver brazing filler rods. 4. Aluminium and its alloys with copper, silicon and magnesium are easily brazed. However, higher contents of magnesium in aluminium alloys are difficult to braze. 5. Magnesium and its alloys containing aluminium, zinc, manganese are easily brazed. 6. Copper-zinc alloys need to be brazed carefully with proper application of flux to reduce loss of zinc during brazing. Copper-silicon alloys need to be stress-relieved before brazing to avoid intergranular penetration of filler metal resulting in reduced strength of the joint. Copper-aluminium alloys call for special fluxes to take care of refractory oxides formed during brazing. Copper-nickel alloys are readily brazed with silver brazed rods. Copper-tin alloys (phosphor bronze) with low tin contents are brazed with silver, copper-phosphorus, or copper-zinc filler rods. 7. Other materials that can be brazed include high carbon steels, high speed steels, carbides, ceramics, and dissimilar metals.

8.16

FILLER METALS (OR BRAZING ALLOYS)

The following alloys are used as filler metals in brazing operations. (a) Aluminium-silicon alloys are general purpose brazing alloys, with melting point 550 to 780oC.

SOLDERING AND BRAZING

619

(b) Magnesium filler metals are used for brazing magnesium alloys. (c) Copper and copper-zinc alloys (brass) and copper tin alloy (bronzes), with melting range 850–950oC, are used for brazing ferrous alloys (steel and cast iron), nickel and copper-nickel alloys. (d) Silver-brazing alloys are used for joining most ferrous and non-ferrous metals except aluminium and magnesium. Alloys of silver and copper or silver-copper-zinc melt between 600 and 850oC and can braze all brazable metals. (e) Nickel filler alloys are used for brazing stainless steels and nickel and cobalt-base alloys. Because of better corrosion and heat resistance property, nickel filler alloy brazed joints can stand temperature over 980oC. (f) Copper-phosphorus alloys are used for brazing copper and its alloys. They melt between 700 and 750oC.

8.17

BRAZING FLUXES

A flux serves several important functions in brazing. Surface oxidation of bare metals creates hindrance in wetting and gives improper joints. Fluxes help preventing undesirable reactions during brazing by dissolving surface oxides and removing it from the joint. By spreading ahead of filler metal, fluxes protect the metal surface from oxidation. They also promote the capillary action of filler metal. Keeping these functions (of fluxes) in view, a brazing flux should melt easily and cover braze area effectively. It should not decompose and thus cause contamination of the joint area. Because of the sufficient surface tension properties, a flux should be able to maintain a film of it within the braze area. Brazing fluxes composed of fused borax (for high temperature fluxes), sodium, potassium and lithium borate compounds (used for temperature 760oC or more), and fluoborates (that are compounds of fluorine, boron, sodium and potassium) give better flowability and oxide removal. Chlorides, boric acids, sodium and potassium hydroxide are also used. Brazing fluxes for brazing different metals are given in Table 8.1. TABLE 8.1

Base metal to be brazed

Brazing fluxes

Filler metals

Flux constituents

Copper, copper-base alloys (except with aluminium), cast irons, steels (carbon and alloy), Nickel-base alloys, stainless steel

Copper-phosphorus

Basic acid, borates, fluorides, fluoborate

Aluminium and its alloys

Aluminium-silicon

Fluorides, chlorides

Magnesium alloys

Magnesium

Fluorides, chlorides

Aluminium-bronze and aluminium-brass

Silver, copper-zinc, copper-phosphorus

Borates, fluorides, chlorides

620

MANUFACTURING PROCESSES

Brazing fluxes are available in the form of powder, paste and liquid. After the completion of brazing operations, flux should be removed from the joint area to avoid corrosion. Joint may be washed in hot water or cleaned by wire brushing or chipping.

8.18

APPLICATIONS AND ADVANTAGES OF BRAZING

Brazing is a versatile method of joining metal pieces. It is used to join cast irons, wrought iron, carbon steels, alloy steels, high speed steels, stainless steels, alloys of copper, aluminium and magnesium. In certain cases brazing is preferred to welding for preserving specific metallurgical characteristics even after making the joint. Brazing is a most effective technique of joining cast metals to wrought metals, non-metals to metals, dissimilar metals, etc. Thinwalled tubes and light gauge sheet metal assemblies are better joined by brazing as welding results in burning of the base metal and joint is not formed. Complex and heavy structures as also other components can be joined by brazing in stress-free condition. Brazing maintains precision production tolerances in making the joint. It has the ability to preserve protective metal coatings or claddings. Brazing is one such technique which makes possible to join pieces of greatly different thickness. Brazed joints are pressure tight. Brazing as a production process has certain limitations in its application. It calls for a properly machined fitting in making parts to be joined for proper capillary action. Limitation on the size of components to be brazed is there as in the process outer area of the joint is to be heated. Large components, therefore, cannot be heated properly to brazing temperature. Brazed joints develop corrosion if flux is not properly removed. Brazing needs certain degree of skill and experience on the part of the welder for handling special brazing jobs.

8.18.1

Tipping of Tool Bits

Tool bits are small pieces of cutting tool materials such as high speed steel or tungsten carbide or ceramic. Bits are ground to proper shape and size. These bits are either held with the clamps or welded on a tool shank of cheaper material like medium carbon steel. A tool having this type of arrangement is called a tipped tool and the process of joining the tool bit on the steel shank is called tool tipping. The tool bits are attached on the steel shank by either welding or brazing, the brazing being more common. Thin copper strips are used as brazing material. Other brazing materials include brazing foils having a layer of copper in the middle and brazing material (tin) on the two sides of the same. Silver solder is sometimes used for joining carbide tips and is good for low temperature brazing to avoid warping or damage to the tool shank. The brazing filler metal in the form of shims is placed on the seat of the tool bit and also on the sides (Fig. 8.5). After applying flux, the assembly is clamped properly and kept in the furnace. The brazed joint is later cleaned.

8.19

BRAZING METHODS

Various brazing methods are based on the ways and means employed for heating the workpieces and melting of filler metal, for example, torch brazing, furnace brazing, dip brazing, induction brazing, infrared brazing, etc. More popular brazing methods are described below.

SOLDERING AND BRAZING

Fig. 8.5

8.19.1

621

Tipping of a lathe tool by brazing a carbide tip (or tool bit) on steel shank.

Torch Brazing

Torch brazing is the most commonly used method. Heat is provided by usual gas welding torch by burning acetylene and oxygen. Oxy-hydrogen torches are also used for brazing aluminium and other non-ferrous metals. A neutral or slightly reducing flame is applied on the thoroughly cleaned joint surfaces, heating thicker section first. A flux is then applied on the joint area to avoid oxidation during heating and also to clean the surfaces. The filler metal is then hand fed to the joint area as soon as the latter attains the brazing temperature. Torch brazing involves least investment on equipment. Heating of the metals can be done in a controlled way by adjusting the flow of gases in the torch. However, it is a slow method. Flame heating of inaccessible joints poses problems.

8.19.2

Furnace Brazing

Furnace brazing is a mass production method employed for light jobs (not heavier than 1.5 kg), and also where the number of joints to be brazed are many which need simultaneous brazing in one go. In this method, joint assembly is done prior to brazing setting proper clearance, etc. Flux and the brazing filler metal available in the form of strip, rod or ring are placed in position at the joint to be formed. The assembly so formed is placed inside the furnace. A flux, in addition to the suitable atmosphere inside the furnace, is generally not required except for those base metals whose oxides cannot be readily reduced in the furnace atmosphere at the brazing temperature. During heating in the furnace, the filler metal melts wetting the joint surface and filling the joint gap (in the assembly). The brazed components are cooled within the reducing atmosphere provided in the cooling hood which is an integral part of the furnace. Furnace brazing ensures accurate control of brazing temperature besides uniform heating that helps in reducing the residual stresses. The base metals of the two workpieces should preferably have more or less the same coefficient of expansion and the joint clearance can be adjusted taking care of this point.

622 8.19.3

MANUFACTURING PROCESSES

Dip Brazing

Dip brazing may be done in two ways: (a) dip brazing in a metal bath or (b) dip brazing in a molten salt bath. Dip brazing in a metal bath involves pre-cleaning of assemblies to be joined, placement of flux in position and later dipping the assembly into a bath of molten filler metal held at the brazing temperature. Only those brazing metals can be used which have high melting points as low-temperature brazing alloys may vaporize from the bath. The brazing time is a critical consideration to avoid excessive alloying between the filler metal and the base metal of the workpieces. Dip brazing in a molten salt bath involves holding of the joint assembly in a fixture after the former is thoroughly cleaned. The joint assembly has the pre-placed filler metal. The assembly is later dipped in a molten flux or salt bath heated by torch or immersion heating coils. The bath temperature and the salts used for the salt bath are barium chloride (955 to 1315oC), chlorides of barium, sodium and potassium (650 to 955oC), mixture of nitrites and nitrates (less than 538oC). Salt bath provides a shielding action against oxidation. The process is preferred for dip brazing of bigger size assemblies. Wet components should not be dipped in salt bath to avoid explosion.

8.19.4

Induction Brazing

Induction brazing comprises fluxing of joint assemblies, placement of filler metal in position at the joint and placing the assembly within or near an induction coil wherein the high frequency current passing through the coil heats the joint assembly. Thus, the outer surface of the workpiece assembly is heated and the inner surfaces are heated through the conduction of heat from outer to inner surface. Very rapid localized heating is possible reducing the risk of warpage. The production rate is high.

8.19.5

Silver Brazing

Silver brazing is the process of brazing using filler rods of silver alloys. It is also called silver soldering or hard soldering. The process was formerly used only for production of art and jewellery items. It is now used for fabrication of condensers and evaporators in air conditioners, fuel tanks of tractors, carbide tipping on steel shanks, etc. Silver brazing alloys contain silver, zinc, copper and cadmium. A typical alloy having silver (40 to 50%) and zinc (25 to 15%) is most commonly used. The melting points of these alloys may be over 600oC. These are available in the form of rods, rings or foils. Silver brazing can be done by torch method or any other described above.

8.20

BRAZE WELDING (OR BRONZE WELDING)

In braze welding, a groove, fillet or plug or slot weld is made with a non-ferrous filler metal having its melting point lower than that of base metal but above 427oC. The filler metal is not distributed in the joint surfaces by the capillary attraction as happens in brazing. The process is called bronze welding because bronze filler rods are used in braze welding. While brazing is used with closely fitted surfaces, the filler metal being distributed to the jointing

SOLDERING AND BRAZING

623

surfaces by capillary attraction, braze welding is employed for making a fillet weld or groove or plug welds in which the filler metal is puddled into and not distributed in the joint by capillary attraction. In braze welding, the base metal does not melt fully, only limited base metal fusion may occur. This is why braze welding is an intermediate process between true welding and brazing. Braze welding is mostly carried out with an oxy-acetylene torch. Braze welding is used primarily on steels and cast irons. It is also used for copper, nickel and nickel alloys. The braze welding filler rods are made from copper and zinc alloys with some amount of tin, for example, a filler metal having copper 60%, zinc 38%, tin 1% and iron 1% melts at 890oC.

REVIEW QUESTIONS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Define soldering and brazing processes of joining metals. What is braze welding? What is the difference between soldering and brazing? Name the metals joined by soldering. What different types of solders are used? What is a soldering flux? Why is it used? What different types of soldering fluxes are used? Why is surface cleaning of a job necessary before soldering? Name the different methods of soldering. What do you understand by the terms tinning, sweating and floating? What metals are joined by brazing? What are brazing alloys? What for these are used? What are brazing fluxes? Where is the brazing process preferred to soldering? Where is the braze welding used?

9 9.1

Metal Forming Hot- and Cold-working and Press-working

INTRODUCTION

Metals are converted into various usable products of desired shape and size using different manufacturing processes such as casting, machining, welding or metal forming (rolling, forging, extruding, etc). Metal forming of metals involves deforming a metal plastically into various desired shapes and sizes under the effect of externally applied forces. The stresses induced during the deformation process are greater than the yield strength, but less than the fracture strength of the metal. The type of loading may be tensile, compressive, bending or shearing or a combination of these. Also, the operation of deforming the metal may be carried out either in cold state or in hot state of the metal using mechanical means such as press, die and rolls. The process of metal forming is also called mechanical working of metals. The formability of a metal indicates the response and suitability of the metal for plastic deformation processes. Metals behave differently at different temperatures and also respond differently to different deforming processes. Hence, all such factors should be considered in deciding the overall formability of a metal. Metal forming operations, such as rolling, forging, drawing, extruding (Fig. 9.1), have come up as high productivity processes as compared to other metal working processes like casting, welding or machining and they also generally ensure high dimensional accuracy, good surface finish with no waste of material in the form of chips, etc. and are, therefore, economical also. These processes are capable of producing items with specified physical and metallurgical properties. A large variety of metal forming processes have been developed for various specific applications, but in view of the applied forces on the job during working, these processes can be broadly categorized as follows. (a) Direct-compression type processes such as forging and rolling, wherein the force is applied to the surface of the job and the metal flows at right angles to the direction of compression [Fig. 9.1(a)]. 624

METAL FORMING—Hot- and Cold-working and Press-working

Fig. 9.1

625

Few typical metal forming operations.

(b) Indirect-compression processes such as wire drawing, tube drawing, extrusion and deep drawing wherein primary applied forces are generally tensile but the indirect compressive forces developed by the reaction of the job with die reach high values, and the metal flows under the action of a combined stress state which includes very high compressive forces in at least one of the principal directions [Fig. 9.1(b)]. (c) Tension type processes such as stretch forming of sheet metal under the application of tensile forces when the sheet metal is wrapped to the contour of a die [Fig. 9.1(c)]. (d) Bending processes involve the application of bending moments to the sheet metal or other metal structurals such as rod, wire or angle [Fig. 9.1(d)]. (e) Shearing processes involve the application of shearing forces of adequate magnitude to rupture the metal in the plane of shear [Fig. 9.1(e)]. The process of forming a metal may take place any time it is subjected to loads (or stresses) that are greater than its ‘yield point’, or in other words, when the deformation stress moves from the elastic range to the plastic range. Metal forming may be ‘cold’ or ‘hot’; cold forming or cold-working is carried out at room temperature or below the recrystallization temperature of the metal and the hot forming or hot-working is done above the recrystallization temperature but below the melting point of the metal. The hot-working is preferred for

626

MANUFACTURING PROCESSES

primary solid-state shaping processes such as forging, rolling, extruding, etc. because the power (or forces) required in shaping the hot metals is lower and larger reductions in the size of the metal are easily and economically possible without its cracking. Hot-working gives high production rate of products of various sizes and shapes from the original ingot received from steel plants. It also generally does not bring noticeable changes in the properties of metal (i.e. hardness and ductility, etc.). The cold-working, however, causes more noticeable changes in the mechanical properties by increasing the tensile strength and yield strength of cold worked metal with a corresponding loss in the ductility of metal.

9.2

METAL FORMING

If a metal is subjected to some stress (loading), forming will take place when the deformation stress moves from the elastic to plastic range, i.e. beyond the yield point. For every material there is a limiting value of load (or stress) up to and within which the resulting strain or deformation disappears entirely on removal of load. The value of stress corresponding to this limiting load up to and within which metal shows elastic behaviour (i.e. coming to original shape on removal of load) is known as the elastic limit of metal. Whenever a metal is stressed beyond its elastic limit, the applied stress causes plastic deformation which is permanent in nature. It will be further noted that in loading beyond elastic limit, increase in strain is far more larger and rapid than the corresponding increase in stress level (i.e. metal does not obey Hooke’s Law). This situation continues till the yield point is reached at which the strain increases even without any further increase in stress and the stress at which this sudden stretching occurs is called yield point of the metal. Plasticity is the property of metal because of which it can undergo permanent deformation under load without rupture or failure and is an important property for forming purposes. Plastic deformation (or permanent deformation) occurs only when metal is stretched beyond the yield point, i.e. from the elastic to the plastic range.

9.2.1 Classification of Metal Forming Processes Metals can be plastically deformed or (worked) at room, warm or higher temperatures. Their behaviour and workability depend largely on whether the deformation takes place below or above the recrystallization temperature of the metal. Recrystallization temperature ranges between 0.3Tm and 0.5Tm, where Tm is the absolute melting temperature of the metal. Recrystallization is the process in which, at the said temperature range, new equiaxed (having equal dimensions in all directions) and strain-free grains are formed. Metal forming processes are traditionally classified as: (a) Hot forming or hot-working processes (b) Cold forming or cold-working processes (c) Warm forming processes The categorization of metal forming processes may be done in the following ways where T stands for the absolute working (or forming) temperature and Tm is the absolute melting point of the metal.

METAL FORMING—Hot- and Cold-working and Press-working

627

For cold-working, ratio of T/Tm is less than 0.3. For warm-working, ratio of T/Tm is 0.3 to 0.5. For hot-working, ratio of T/Tm is greater than 0.6.

9.3

WROUGHT PRODUCTS

The cast products (such as steel ingots or continuous steel castings) are converted (or manipulated) in the solid state to wrought products (or mechanically worked products) by various deformation processes. The choice of deformation or manipulation process (hot-working or cold-working) depends on the metal being handled, desired shape of the end product, accuracy and finish and the quantity required. The cast steel ingots or continuous steel castings are always first hot worked to reduce them to the products of various shapes and sizes primarily for the refinement of grain structure of the cast product [Fig. 9.2(a)], improved directional properties or directional control of ‘flow lines’, elimination of porosity in cast ingots, and breaking up and distribution of inclusions and impurities in the metal, thus avoiding weak points or locations in the product. Wrought products may be in the form of long-length forms such as sheets, plates, bars (black, hot rolled) or bright-drawn bars (cold drawn), rolled sections as I-beam, channels, angles, tubes, wire and other extruded sections. The other types of products coming under the category of wrought products are individual components like large forgings and forged small components such as connecting rods, crank shafts, etc.

Fig. 9.2(a) Showing hot rolling and cold rolling of metals. Hot rolling refines the grain structure whereas cold rolling distorts it. Hot rolling is an effective way to reduce grain size in metals, for improved strength, ductility and impact resistance.

9.4

HOT FORMING (OR HOT-WORKING) OF METALS

When metals are formed or worked at temperatures above their recrystallization temperature but below their absolute melting temperature, they are called being hot worked. At that stage, they behave as perfectly plastic materials. With increasing temperature, generally the yield strength and rate of strain hardening progressively reduce and ductility increases. Hot-working may also be defined as metal working at a temperature above which no strain hardening takes

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place. In other words, hot-working refers to deformation carried out under the conditions of temperature and strain rate such that recovery processes occur substantially during the deformation process, so that large strains may be achieved with essentially no strain-hardening. The strain in hot-working is large (e ª 2 to 4) compared with tension or creep test. Not only less energy is required in hot-working the metal, the blow holes and porosity are also eliminated by the welding together of these cavities. The coarse columnar grains of the cast ingots are broken down and refined into smaller equiaxed recrystallized grains. This results in increase in ductility and toughness of the hot worked metal. The four major hot-working processes are schematically shown in Fig. 9.2(b).

Fig. 9.2(b)

Schematic illustration of four major hot-working processes.

When a metal is hot worked, say by rolling, it passes through the opposing rollers [Fig. 9.2(a)] and is thus reduced in its section thickness. The original large grain structure of the hot metal being rolled is elongated while passing through the rollers and later broken up into fragments in the deformation zone, and the fragments of the crystals thus formed become the nuclei for the formation of new smaller crystals and hence a fine uniform grained structure is produced in the hot rolled portion of the metal. The hot rolling thus refines the grain structure. The temperature range (upper temperature and lower temperature) for hot-working is discussed in the following. The metals that can be hot worked include carbon steels (low, medium and high), alloy steels, stainless steel, copper and its alloys such as brasses, bronzes, aluminium and magnesium alloys and titanium alloys.

9.4.1 Temperature Range for Hot-working As already mentioned, hot-working is carried out at a temperature above the recrystallization temperature but below the absolute melting temperature of the metal. Generally for hot-working, the ratio of absolute hot-working temperature (T) to the absolute melting point (Tm) of the

METAL FORMING—Hot- and Cold-working and Press-working

629

metal being worked is kept greater than 0.6. In most cases, the metal is heated to such a temperature (below its solidus temperature) that after completion of hot working its temperature will remain a little higher than its recrystallization temperature. The following temperature range for hot-working is given only as a general guide. For ferrous metals (steels)—930 to 1370°C For copper, brasses and bronzes—590 to 930°C For aluminium and magnesium alloys—345 to 480°C The upper temperature limit for hot-working is determined by the temperature at which either melting or excessive oxidation occurs. Usually the maximum working temperature is limited to 50oC below the melting point of the metal. This is to safeguard against the segregated regions of lower-melting point material present in the metal, as only a very little amount of a grain-boundary film of a lower-melting constituent may make the metal crumble into pieces when it is deformed. Such a condition is called hot shortness or burning. The lower temperature limit for hot-working of a metal is the lowest temperature at which the rate of recrystallization is rapid enough for eliminating strain hardening in the time when the metal is at that temperature. The lower hot-working temperature will depend upon amount of deformation and the time that the metal is at temperature. A metal which is capable of being rapidly deformed and cooled rapidly from temperature, will require a higher hotworking temperature for the same degree of deformation than will metal slowly deformed and slowly cooled.

9.4.2 Advantages and Disadvantages of Hot-working Advantages

(i) Large deformations (or changes in size and shape) in the metal are easily, more rapidly and economically produced as the metal is hot and in plastic state and thus needs less power in deforming. Hot-working is mainly preferred where heavy reduction in size or heavy deformation is required and work-hardening (for increase in strength) is not the main requirement. (ii) Hot-working gives high production volumes of products of desired shapes and sizes (examples: I-beams, channels, plates, rods, etc.) from the cast ingots received from the steel plants. These hot worked or wrought products find direct use in the market, thus bringing saving in time, material and machining costs by avoiding further forming or shaping of the wrought products. (iii) Hot-working of ingots (or metal in any form) improves mechanical properties of the metal by refining the grain structure, minimizing porosity (ingots have porosity being a cast product), developing directional flow lines of grains and breaking up and distributing unavoidable inclusions or impurities in the metal. (iv) Hot rolling is an effective way to reduce grain size in metals, for improved strength, ductility and impact resistance. (v) Most hot worked products become the raw material for the secondary processes used to produce finished items by cold forming, cutting, drawing, bending, machining or welding.

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Disadvantages

(i) Rapid oxidation or scaling of hot metal occurs. (ii) Loss of carbon from steel surface during hot rolling results in loss of strength. The rolled product may thus give rise to fatigue crack during service. (iii) Close dimensional tolerances are not generally achieved in hot worked parts. (iv) Hot rolling and other hot-working operations like hot forging involve large tooling costs on plants and their maintenance, although this is compensated by the high production volumes.

9.5

MAJOR HOT-WORKING PROCESSES

All hot-working processes change the shape of metal by applying pressure (or impact force) on the hot metal, making it to flow into the dies or through rolls and thus obtain desired shapes of the products without the fracture of the metal. Some mechanical means such as pressing system, dies punches, rollers, etc. are used to help in giving the desired shape to the metal being worked. Major hot-working processes are as follows: (i) (ii) (iii) (iv) (v) (vi) (vii)

9.6

Rolling Forging Extrusion Rotary tube piercing Manufacturing of welded pipes and tubes Drawing Spinning

HOT ROLLING

The process of hot rolling consists of passing the hot metal (ingot) between two rolls revolving in opposite directions. Rolling reduces the thickness or changes the cross-section of a long metal workpiece by applying compressive forces through a set of rollers. Rolling may be (a) flat rolling and (b) shape rolling. In flat rolling [Fig. 9.3(a)], the end products of rolling have rectangular cross-sections such as slabs, plates, sheets, etc. In shape rolling [Fig. 9.3(b)], the end product may have various cross-sectional shapes such as I-beams, channels, angles, squares or hexagonal rods and tubes, etc. The initial breaking down of an ingot (or of a continuously cast slab) is done by hot rolling into the shape of a bloom. The cast ingots have typically dendritic structure with coarse and non-uniform grains and also have some porosity and are hard. Hot rolling being done above the recrystallization temperature, converts the cast structure to a wrought structure with fine grains and increased ductility. Rolling is a mass production process. Rolling (including both hot and cold rolling) accounts for about 90% of all metals produced by metal working processes. The initial breaking down of an ingot or of a continuously cast slab is always done by hot rolling. The traditional method of casting ingots is being rapidly replaced now by continuous cast slabs or other castings in steel plants. The product of first hot rolling is called a bloom, or slab or billet.

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631

Fig. 9.3(a) Flat rolling a thick flat to thinner strip (i) and showing a corresponding increase in the width (spreading) of the flat rolled strip (ii).

Fig. 9.3(b)

Shape rolling; showing various stages in shape rolling an H-section.

Bloom has a square cross-section (at least 150 mm side); a slab is usually rectangular in cross-section; and a billet is usually square with cross-sectional area smaller than bloom. Blooms are processed further by shape rolling into I-beams, rail roads, etc. Slabs are rolled into plates and sheets. Billets are rolled further into rods, bars, seamless pipes, wire rod, etc.

632

MANUFACTURING PROCESSES

(Fig. 9.4). Some of the hot rolled products are shown in Fig. 9.5. It may be emphasized here that the hot material, which is in the plastic condition, resists the change of shape (during rolling or other hot-working operations) with considerable force, although quick application of deforming force heats up the metal adding to its plastic flow. Even then, it is necessary to go through a series of stages before the final shape can be produced. Each stage requires a different set of tools or equipment (i.e. set of rollers with different profiles). Reheating of the metal is usually required between stages. The sequence of reducing a billet to a round bar stock in different stages of rolling is shown in Fig. 9.6.

Fig. 9.4

Showing the schematic outline of various flat rolling and shape rolling processes. White hot steel ingots (continuous cast products) are passed through rolls which form the plastic steel into slab, billet and bloom for further processing.

During hot rolling of blooms, billets or slabs, the surface of the hot metal is first ‘conditioned’, i.e. prepared for subsequent operations by removing heavy scales with a gas torch (called scarfing); lighter scale is removed by grinding. Also prior to further processing by cold rolling, the scale developed in hot rolling is always removed by pickling with acid or blasting with water or by grinding.

METAL FORMING—Hot- and Cold-working and Press-working

Fig. 9.5

Fig. 9.6

633

Some common rolled sections available as structurals in the market.

Showing the sequences in reducing a billet of 100 mm x 100 mm to a round bar stock.

9.6.1 Rolling Parameters and Their Effects Metals are given desired shapes in hot-working by subjecting them to forces which cause them to undergo plastic deformation when above the crystallization temperature at which metal becomes plastic and causes the growth of grains. By hot-working, grains are broken up into small more numerous crystals (refined) and at that stage the metal possesses little elasticity and low load is required to shape the metal as both the strength and hardness are decreased at elevated temperature. Hot rolling is done to roll ingots into slabs, blooms, or billets and on further hot rolling into plates, bars, rails, angle section, channel section, etc. are obtained. Because of the limitations in workability of metals and availability of equipment, rolling is done in progressive steps, i.e. a number of passes are required to get the desired configuration. Rolling operation is illustrated in Fig. 9.6(a). The rolls revolve in opposite direction to each other. They are in contact with the passing metal (or ingot) over a distance shown by arc LM which subtends at centre (O) an angle LOM (i.e. a) called angle of contact or maximum angle of bite. The friction between the ingot and rolls provides the required grip of two rolls over the ingot which draws it through them; the greater the friction, the more will be the possible reduction in thickness of ingot (in one pass). The pressure exerted by rolls over the ingot is not uniform throughout; it is minimum at extremities L and M and maximum at a point (S) called point of maximum pressure or no slip point. At this point, peripheral

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MANUFACTURING PROCESSES

speed of roll (Vr) and metal (ingot) is same; otherwise, from L to S, the metal moves slower than the rolls and frictional force acts in the direction to draw the ingot into the rolls but after crossing the neutral point (S), i.e. from S to M, the ingot moves faster than the roll surface, as if it is being extruded and the friction opposes the travel tending to hold the metal back resulting in setting up of stresses within the metal to obstruct its reduction. Between entrance and exit, metal velocity changes continuously but roll velocity (Vr) is constant. The amount by which velocity (Vf) is greater than roll velocity (Vr), is called forward slip and expressed as percentage of roll velocity as below: Forward slip =

V f - Vr Vr

¥ 100

Its value varies from 3 to 10% and increases with increase in roll diameter, coefficient of friction and reduction the thickness of strip being rolled. Backward slip =

Fig. 9.6(a)

Vr - Vi ¥ 100 Vr

Metal rolling process. The shaded area is deformation zone. Angle of bite is a which is subtended by arc LM on the roller centre (O). Neutral point is denoted by S which is also called ‘no-slip point’ or ‘point of maximum pressure’. Angle a varies from 15 to 30° for hot rolling and 2 to 10° for cold rolling of oiled sheets.

Let li, bi and ti and lf , bf and tf be the initial and the final length, breadth and thickness of the metal, respectively. Absolute draft (dt) being the difference between the initial and the final thickness of the metal being rolled, dt = (ti – tf), mm

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635

Absolute elongation (dl) being the difference between the final and the initial length of the metal being rolled, dl = (lf – li), mm Absolute spread (db) being the difference between the final and the initial width of the metal being rolled, db = (bf – bi), mm Relative draft (Rt) being the ratio of the absolute draft (dt) and the initial thickness (ti) of the metal and expressed as percentage of the same,

Rt =

dt ti

=

(ti - t f ) ti

¥ 100

Elongation coefficient (m1) being the ratio of the final length of rolled strip to its initial length,

( m1 ) =

lf li

Since there is no change in the volume of the metal by rolling, hence, tibili = tf bf lf lf ti bi = = m1 t f bf li

or

(coefficient of elongation)

Angle of contact (a) is the angle subtended at the centre of the roll by the arc of contact LM. It can be computed as: cos a = 1 –

dt D

where dt = (ti – tf ) D = diameter of the roll Forces in rolling: At the moment of bite, the metal is under action of two forces: (i) normal reaction P [Fig. 9.6(b)] and (ii) frictional force or tangential force mP, where m is the coefficient of friction between the rolls and the metal strip. Also,

Frictional force (m P ) = tan b = m, Normal reaction ( P ) where b is the angle of friction. Resolving these forces in the direction of rolling, for equilibrium, 2P sin a = 2m P cos a

Fig. 9.6(b) Forces at bite in rolling. Angle of friction is denoted by b and S is the ‘no-slip point’.

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MANUFACTURING PROCESSES

tan a =

or

mP

P The component 2mP cos a tries to pull the stock through the rolls, whereas 2P sin a tries to resist the rolling action. Tan a is also equal to the coefficient of friction (m) (as m = mP/P). Hence, 2mP cos a must be greater than 2P sin a for rolling to take place

2mP cos a > 2P sin a

or

m > tan a

or

Thus, for rolling to take place, the necessary condition is: tan a < m

or

a