Heat Treatment Of Steels
 9789339221058

Table of contents :
Title
Contents
1. Introduction to Steel and Scope for its Heat Treatment
1.1 Steel and the Role of Heat Treatment
1.2 Structure of Steel: Crystal Structures, Phases and Microstructures
1.2.1 Crystal Structures and Phases in Steel
1.2.2 Iron-carbon Phase Diagram
1.2.3 Microstructures in Steel
1.3 Steel Types and Grades of Steels
1.3.1 Steel Types
1.3.2 Classification of Steels and their Grades
1.3.3 Importance of Composition of Steel for Heat Treatment
1.4 Characteristics and Properties of Steels
1.4.1 Characteristics of Steel
1.4.2 Properties of Steels
1.4.3 Influence of Grain Size and Inclusions on the Properties of Steels
Influence of Grain Size
Influence of Inclusions
1.5 Structure-Property Relationship in Steels: Importance of Heat Treatment
Summary
References / Suggested Reading
Review Questions
2. Phase Transformations in Steel: The Basics of Heat Treatment Principles
2.1 Introduction
2.2 Phase Transformation and Structure Formation in Steels
2.2.1 Role of Nucleation and Growth (N&G) Process in the Structure Formations of Steel
2.3 Austenite Decomposition: Formation of Ferrite and Pearlite Structure in Steel
2.3.1 Cooling Rate Sensitivity of Austenite Decomposition
2.3.2 Decomposition of Austenite under Equilibrium Cooling: Formation of Ferrite and Pearlite in Steels
2.4 Austenite Decomposition: Formation of Bainite and Martensite Structure in Steel
2.5 Morphological Character of Microstructures in Steels
2.6 Martensitic Structure: its Uses and Utility
Summary
References / Suggested Reading
Review Questions
3. Introduction to Heat Treatment of Steels: Purpose and Processes
3.1 Introduction to Heat Treatment
3.2 Purpose and Scope of Heat Treatment of Steels
3.3 Some Basics of Heat Treatment Practices
3.4 Introduction to Thermal Heat Treatment Processes
3.4.1 Annealing
3.4.2 Normalising
3.4.3 Hardening
3.4.4 Tempering and Stress Relieving
3.4.5 Induction / Flame Hardening
3.5 Introduction to Thermo-Chemical Processes of Heat Treatment
3.5.1 Carburising
3.5.2 Nitriding
3.5.3 Carbo-nitriding and Nitro-carburising
3.6 Thermo-Mechanical Processes of Heat Treatment
Summary
References / Suggested Reading
Review Questions
4. Heat Treatability of Steels
4.1 Introduction
4.1.1 Introduction to Hardenability and its Implications
4.1.2 Mass Effect and other Factors Influencing the Hardenability
4.2 Hardenability of Steels
4.2.1 Determination of Jominy Hardenability in Steels
4.2.2 Alternative Method of Estimating Hardenability of Steel: Grossman Formula
4.3 Grain Boundary Characters and its Effects on Hardening of Steels
4.4 Effects of Inclusions and Segregation on Heat Treatability of Steels
4.5 Role of Quench Severity and other Factors Influencing the Hardening of Steels
4.5.1 Other Factors in the Hardening of Steels
Summary
References / Suggested Reading
Review Questions
5. Heat Treatment Furnaces and Furnace Atmosphere Control
5.1 Introduction
5.2 Heat Transfer and Heat Balance in Heat Treatment Furnaces
5.2.1 Heat Transfer Processes in Furnace
5.2.2 Heat Balance in a Furnace
5.3 Introduction to Furnace Types and their Features
5.4 Choice of Furnace and Relative Energy Efficiency
5.5 Heat Treatment Furnaces and Applications
5.5.1 Seal Quench Furnace
5.5.2 Vacuum Furnace
5.5.3 Pit Type Furnace
5.5.4 Salt Bath Furnace
5.5.5 Fluidised Bed Heat Treatment Furnace
5.6 Atmosphere Control in Heat Treatment
Summary
References / Suggested Reading
Review Questions
6. Quenching Technology and Characteristics of Different Quenchants
6.1 Introduction
6.2 Fundamentals of Quenching
6.2.1 Mechanisms of Heat Removal
6.3 Types of Quenchant and their Characteristics
6.3.1 Types of Quenchants
6.3.2 Characteristics of Quenchants
6.3.3 Choice of Quenchants
6.4 Oil-Quenching vis-à-vis Water and Polymer Quenching
6.5 Different Oil Grades for Quenching and their Selection
6.5.1 Classification of Quenching Oils
6.5.2 Working Characteristics of Quenching Oils
6.6 Polymer Quenching: Characteristics and Application Technique
6.6.1 Characteristics of Polymers
6.6.2 Application of Polymer Quenching
6.7 Cases of Quenching Defects
Summary
References / Suggested Reading
Review Questions
7. Thermal Heat Treatment Processes
7.1 Introduction
7.2 Annealing and Normalising
7.2.1 Annealing
Full Annealing
Sub-Critical Annealing
Spheriodising Annealing
7.2.2 Normalising
7.3 Hardening of Steels
7.3.1 Through (Bulk) Hardening of Steel
Process Features and Steps in Hardening Operations
Furnace Characteristics and Controls for Hardening
7.4 Flame / Induction Hardening
7.4.1 Flame Hardening
7.4.2 Induction Hardening
7.5 Tempering and Stress Relieving of Steels: Purpose and Practice
7.5.1 Temper Embrittlement
7.6 Martempering and Austempering of Steels
Summary
References / Suggested Reading
Review Questions
8. Thermo-Chemical Processes of Heat Treatment [Carburising, Nitriding, Carbo-nitriding and Nitro-carburising]
8.1 Introduction
8.2 Processes and Practice of Carburising and Case-Hardening
8.2.1 Mechanisms of Carburising
8.2.2 Methods of Carburising
8.3 Continuous Carburising Process
8.4 Vacuum Carburising and its Control
8.5 Nitriding
8.5.1 Nitriding Process and Principles
8.5.2 Operating Procedures for Nitriding
8.5.3 Vacuum Nitriding
8.6 Carbo-nitriding and Nitro-carburising
8.6.1 Carbo-nitriding
8.6.2 Nitro-carburising
Summary
References / Suggested Reading
Review Questions
9. Heat Treatment of High-alloy Steels [Stainless, PH-Stainless, Heat / Creep Resistant, Tool and DIE Steels]
9.1 Introduction
9.2 Heat Treatment of Stainless Steels
9.2.1 Heat Treatment of Austenitic Stainless Steels
9.2.2 Heat Treatment of Ferritic Stainless Steel
9.2.3 Heat Treatment of Martensitic Stainless Steel
9.3 Heat Treatment of Precipitation Hardening Stainless Steels
9.4 Heat Treatment of Heat Resisting / Creep Resisting Steels
9.4.1 Heat Treatment of Heat Resisting Steel
9.4.2 Heat Treatment of Creep Resisting Steel
9.5 Heat Treatment of Tool and DIE Steels
9.6 Heat Treatment of High Speed Steels (HSS)
Summary
References / Suggested Reading
Review Questions
Annexure
10. Surface Engineering of Steels
10.1 Introduction to Surface Engineering
10.2 Scope of Surface Engineering Processes
10.3 Plasma Carburising and Nitriding
10.3.1 Plasma Carburising
10.3.2 Plasma-Nitriding
10.4 Surface Hardening by Laser and Electron Beam
10.4.1 Laser Surface Hardening
10.4.2 Electron Beam Hardening
10.4.3 Ion Implantation
10.5 Vapour Phase Processing: CVD and PVD
10.5.1 Chemical Vapour Deposition (CVD)
10.5.2 Physical Vapour Deposition (PVD)
Summary
References / Suggested Reading
Review Questions
11. Industrial Heat Treatment Practices: Illustrative Cases
11.1 Issues of Industrial Heat Treatment
11.1.1 Residual Stress in Hardening of Steels
11.2 Metallurgical Highlights: Few Important Learning Points
11.3 An Overview of ‘Heat Treatment at-Work’ in the Shop Floor
11.4 An Overview of Steel Types and their Heat Treatment
11.5 Cases of Application Specific Heat Treatment of Steels
11.5.1 Heat Treatment of Front Axle Beamfor Commercial Vehicles
11.5.2 Heat Treatment of Crown Wheel / Pinion for Commercial Vehicle Power Transmission
11.5.3 Heat Treatment of Ball Bearing Races
11.5.4 Heat Treatment of High Speed Steel Tools
Summary
References / Suggested Reading
Review Questions
Annexure: Common Heat Treatment Defects: Causes and Remedies
Appendices
Glossary
Bibliography
Index

Citation preview

Heat Treatment of Steels

About the Author Smarajit Kumar Mandal, B.Sc. (Engg.) Met (London), Ph.D. (Engg.) (London), FIIM, FIE, is the former Director (Scientific Services) Tata Steel Ltd., Jamshedpur. Earlier to joining Tata Steel, he served Tata Motors Ltd., Jamshedpur, as Head of Central Metallurgy and Heat Treatment. He has over 30 years of industry experience between these two leading organizations within the Tata group of companies. Dr Mandal has also served as ‘Visiting Faculty’ and ‘Adjunct Professor’ in the Department of Metallurgical and Materials Engineering, IIT Kharagpur, India. He has published 76 technical papers in Indian and International journals and books in areas of steel technology and heat treatment of steels including a popular book Steel Metallurgy: Properties, Specifications and Applications, published by McGraw-Hill Education in India and USA. His industry and research experience in the field of steel technology and heat treatment has earned him quite a few national awards and recognition.

Heat Treatment of Steels

S K Mandal Former Director (Scientific Services) Tata Steel Ltd and Former Head of Central Metallurgy & Heat Treatment Tata Motors Ltd., Jamshedpur

McGraw Hill Education (India) Private Limited NEW DELHI McGraw Hill Education Offices New Delhi New York St Louis San Francisco Auckland Bogotá Caracas Kuala Lumpur Lisbon London Madrid Mexico City Milan Montreal San Juan Santiago Singapore Sydney Tokyo Toronto

McGraw Hill Education (India) Private Limited Published by McGraw Hill Education (India) Private Limited P-24, Green Park Extension, New Delhi 110 016 Heat Treatment of Steels Copyright © 2016, by McGraw Hill Education (India) Private Limited. No part of this publication may be reproduced or distributed in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise or stored in a database or retrieval system without the prior written permission of the publishers. The program listings (if any) may be entered, stored and executed in a computer system, but they may not be reproduced for publication. This edition can be exported from India only by the publishers, McGraw Hill Education (India) Private Limited Print Edition: ISBN (13): 978-93-392-2105-8 ISBN (10): 93-392-2105-2 Ebook Edition: ISBN (13): 978-93-392-2106-5 ISBN (10): 93-392-2106-0 Managing Director: Kaushik Bellani Head—Products (Higher Education & Professional): Vibha Mahajan Assistant Sponsoring Editor: Koyel Ghosh Development Editor: Deepika Jain Manager—Production Systems: Manohar Lal, Satinder Singh Manager (Editorial Services): Hema Razdan Publishing Manager: Shalini Jha Editorial Executive: Harsha Singh General Manager—Production: Rajender P Ghansela Manager—Production: Reji Kumar

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Preface

Heat treatment and application of steels are intricately connected; one is the means and the other is the end. If properties of steel are to be made exactly suitable for a given application, then heat treatment is the means. Non-heat treated wrought steel structure does not meet the properties required for most industrial uses and applications; instead, the structure needs to be suitably altered or re-created for meeting the requirements of end-applications. And, in this regard, heat treatment is the universal process of altering or re-creating the structure of steels for developing appropriate properties for different industrial applications. The process of heat treatment should begin with knowing the properties that are required for a given usage and application, thereafter working out the type of steel that should be used and finally the method and means to heat treat that steel for development of appropriate structure for required properties. Metallurgically, the study of heat treatment requires: (a) knowledge about the structure-property relationship in steels – to determine what structural changes are required for the given set of properties, and (b) understanding the transformation behaviour of steels and its control for development of appropriate structure. Structure-property relationship – a unique feature in steels – is, thus, at the centre of heat treatment of steels. Development and alteration of structure – appropriate for the required set of properties – is the principal task of heat treatment process, and this is primarily accomplished by controlling the process of phase transformation and structure formation in steels under controlled heating and cooling. Operationally, heat treatment is a process of controlled heating and cooling for bringing about the required change in structure and properties of steels – either by transformation of the structure or by conditioning the structure with the aid of thermal energy. The process may also appropriately use thermo-chemical or thermo-mechanical means for altering or facilitating required changes in the steel and its structure, whenever necessary. Scope of heat treatment is, thus, very wide – it is open to creative utilisation and application of physical metallurgical principles of

steel metallurgy for alteration, modification or re-creation of appropriate structure in steels for application-specific properties. The contents of Heat Treatment of Steels have been developed on these premises and discussed in a focused manner for developing a coherent knowledge and understanding of heat treatment of steels for fulfilling structure–property criterion. However, heat treatment is a process – where structure–property criterion sets the objective of the process and rules of the process, but it is not all of the process. Execution of heat treatment process requires performance of a set of activities – like selection of steel, selection of process and equipment, choice and control of heating and cooling processes, management of environment and atmosphere during heat treatment, and such other process steps that are required for obtaining ‘consistent’ and ‘acceptable’ quality results. The book, therefore, includes chapters on heat treatability of steels, heat transfer mechanisms and heat treatment furnaces, quenching technology and mechanisms, and few other related topics of heat treatment – in addition to chapters dealing with ‘heat treatment fundamentals’ and different heat treatment processes. As far as possible, different heat treatment processes have been grouped and discussed under the generic terms of ‘thermal, thermo-chemical and thermo-mechanical’ processes of heat treatment. Divided into 11 chapters, the book covers the principal aspects of heat treatment processes and technology based on the above line of approach. A road map of the book has been provided in the beginning of the book, which briefly outlines the coverage and purpose of each chapter. Aim of the road map is to provide a birds-eye view of ‘knowledge areas’ associated with the processes and practice of heat treatment of steels. The book has the distinction of taking special care to cover some topics that are not commonly elaborated in all standard books. Examples of such coverage are: quenching mechanisms and the characteristics of different quenching medium, emerging trend of polymer quenching, modern surface engineering techniques including basics of ‘Chemical and Physical Vapour Deposition’ (CVD and PVD), and heat treatment of high-alloy steels. The latter includes all types of stainless, heat resistance, creep resistance, and tool and high-speed tool steels. The concluding chapter (chapter 11) sums up the learning points of heat treatment rules and illustrates cases of heat treatment of steels from industries. This chapter focuses on illustrating how heat treatment tasks are viewed and conducted in industries. In these illustrations, both metallurgical and dimensional issues have been highlighted – providing a comprehensive view of the subject and its practice.

It is, however, neither a ‘handbook’ nor a ‘data book’; it is a textcum-reference book for students and professionals, covering the essential aspects of heat treatment of steels. The book has been structured to discuss and illustrate the common span of heat treatment of steels in sequential and illustrative manner – in order to provide a clear ‘goal – path – means’ picture of heat treatment practices for steels. The book is based on my industrial and teaching experience and understanding about what the students and fresh professionals – who are new in the industries – need to know about heat treatment of steels. With extensive illustrations from the industries the book is well-structured and crafted for easy reading and learning. Hopefully, the book with its unique approach and focus on the core areas of heat treatment will stand out amongst others and meet the requirements of engineering students as well as the professionals in steel and engineering industries. For improving the quality value of the book, Appendices with additional tables and charts for steels and their heat treatments, glossary of important terms, and a comprehensive ‘indexation’ have been added at the end. It is expected that this illustrative book will prove to be very useful to the students of engineering disciplines and professionals in the industries, for learning about ‘what – why and – how’ of heat treatment of steels. S K Mandal

DISCLAIMER It is difficult now-a-days to think, discuss or write something worthy of knowledge without reference or recourse to internet world. This book is no exception to this trend. In the course of reference to internet based articles and books, some materials and illustrative figures for this book have been drawn from the internet publications, carefully avoiding any infringement of copyrights to the best of my knowledge. In case any material resemble to copy righted publication, that is unintentional and the author should be excused and not to be held responsible for the same, especially keeping in view the widespread availability of such or similar internet materials and information in the open public domain and their intended use for spreading and enhancing knowledge in the related field.

Acknowledgements

First of all, I wish to thank all my friends and former colleagues at Tata Motors (1969–1989) and Tata Steel (1990–1999) – whose association and collaboration had enriched my ‘metallurgical’ experience and faculty to the level that enabled me to write this book. I owe a lot to all of them. During the course of writing this book, I had to frequently refer to various internet-based literature and articles for reconfirmation of subject matter, data and reference diagrams. This source of supply of illustrative information and diagrams, and knowledge, has been indeed valuable and indispensable. I wish to thankfully acknowledge the contribution of all the authors and contributors of those articles. I apologise for not naming them all individually here, as the list would be exhaustive. Finally, I wish to thank all my family members – especially to my wife: Chhanda, sons: Dhruva and Shuva, and daughter-in-law: Sahila – for their inspiration and support for my academic pursuits. S K Mandal

Publisher’s Note McGraw Hill Education (India) invites suggestions and comments from you, all of which can be sent to [email protected] (kindly mention the title and author name in the subject line). Piracy-related issues may also be reported.

Road Map

Learning Steps

Purpose/Coverage

Location in the book

1.

Basic knowledge about steel and steel grades, steel properties, structure-property relationship in steel and the importance of heat treatment of steels

Chapter-1

2.

Rules of phase transformation and structure formation in steels: Foundation of heat treatment principles and technology

Chapter-2

3.

Introduction to types of heat treatment processes for steels and their metallurgical purpose

Chapter-3

4.

Development of understanding about factors influencing the heat treatability of steels: Hardenability, Grain size, Inclusions and Segregation

Chapter-4

A. Metallurgical Fundamentals

B. Heating & Cooling Technology: Furnaces, Heat Transfer and Quenching Mechanisms 5.

Heat treatment Furnaces, heat transfer mechanisms, furnace characteristics and atmosphere control

Chapter-5

6.

Quenching theory and practice, and characteristics of quenchants – like Water, Oils and Polymers

Chapter-6

C. Processes & Practice of Heat Treatment 7.

Thermal heat treatment processes & practice – anneal- Chapter-7 ing, normalising, hardening - including flame/induction surface hardening

8.

Thermo-chemical heat treatment processes & practice: Chapter-8 Different processes of carburising and nitriding

9.

Heat treatment speciality of high-alloy steels – stainless, heat-resistance, creep resistance, and tool, die and high speed steels.

10.

Modern surface engineering processes – including Plasma Chapter-10 and Laser processes, and CVD/PVD surface coating

Chapter-9

D. Consolidation of Learning Points: Heat Treatment at-work 11.

Consolidation of learning points about heat treatment - Chapter-11 through case studies and illustrations of industrial heat treatment practices and precautions, including common heat treatment defects, their causes and remedies

Contents

Preface Acknowledgements Road Map

vii xi xiii

1. Introduction to Steel and Scope for its Heat Treatment 1.1 Steel and the Role of Heat Treatment 1 1.2 Structure of Steel: Crystal Structures, Phases and Microstructures 4 1.2.1 Crystal Structures and Phases in Steel 4 1.2.2 Iron-carbon Phase Diagram 8 1.2.3 Microstructures in Steel 11 1.3 Steel Types and Grades of Steels 15 1.3.1 Steel Types 15 1.3.2 Classification of Steels and their Grades 20 1.3.3 Importance of Composition of Steel for Heat Treatment 22 1.4 Characteristics and Properties of Steels 24 1.4.1 Characteristics of Steel 24 1.4.2 Properties of Steels 27 1.4.3 Influence of Grain Size and Inclusions on the Properties of Steels 30 Influence of Grain Size 30 Influence of Inclusions 33

1.5 Structure-Property Relationship in Steels: Importance of Heat Treatment 35 Summary 42 References / Suggested Reading 43 Review Questions 43

1

2. Phase Transformations in Steel: The Basics of Heat Treatment Principles

45

2.1 Introduction 45 2.2 Phase Transformation and Structure Formation in Steels 46 2.2.1 Role of Nucleation and Growth (N&G) Process in the Structure Formations of Steel 51 2.3 Austenite Decomposition: Formation of Ferrite and Pearlite Structure in Steel 55 2.3.1 Cooling Rate Sensitivity of Austenite Decomposition 55 2.3.2 Decomposition of Austenite under Equilibrium Cooling: Formation of Ferrite and Pearlite in Steels 56 2.4 Austenite Decomposition: Formation of Bainite and Martensite Structure in Steel 63 2.5 Morphological Character of Microstructures in Steels 70 2.6 Martensitic Structure: its Uses and Utility 78 Summary 82 References / Suggested Reading 83 Review Questions 83

3. Introduction to Heat Treatment of Steels: Purpose and Processes 3.1 3.2 3.3 3.4

85

Introduction to Heat Treatment 85 Purpose and Scope of Heat Treatment of Steels 90 Some Basics of Heat Treatment Practices 95 Introduction to Thermal Heat Treatment Processes 100 3.4.1 Annealing 101 3.4.2 Normalising 104 3.4.3 Hardening 105 3.4.4 Tempering and Stress Relieving 111 3.4.5 Induction / Flame Hardening 115 3.5 Introduction to Thermo-Chemical Processes of Heat Treatment 116 3.5.1 Carburising 118 3.5.2 Nitriding 122 3.5.3 Carbo-nitriding and Nitro-carburising 123

3.6 Thermo-Mechanical Processes of Heat Treatment 125 Summary 130 References / Suggested Reading 130 Review Questions 131

4. Heat Treatability of Steels

133

4.1 Introduction 133 4.1.1 Introduction to Hardenability and its Implications 134 4.1.2 Mass Effect and other Factors Influencing the Hardenability 136 4.2 Hardenability of Steels 139 4.2.1 Determination of Jominy Hardenability in Steels 141 4.2.2 Alternative Method of Estimating Hardenability of Steel: Grossman Formula 146 4.3 Grain Boundary Characters and its Effects on Hardening of Steels 149 4.4 Effects of Inclusions and Segregation on Heat Treatability of Steels 155 4.5 Role of Quench Severity and other Factors Influencing the Hardening of Steels 158 4.5.1 Other Factors in the Hardening of Steels 162 Summary 165 References / Suggested Reading 166 Review Questions 166

5. Heat Treatment Furnaces and Furnace Atmosphere Control

168

5.1 Introduction 168 5.2 Heat Transfer and Heat Balance in Heat Treatment Furnaces 171 5.2.1 Heat Transfer Processes in Furnace 171 5.2.2 Heat Balance in a Furnace 173 5.3 Introduction to Furnace Types and their Features 175 5.4 Choice of Furnace and Relative Energy Efficiency 179 5.5 Heat Treatment Furnaces and Applications 183 5.5.1 Seal Quench Furnace 184 5.5.2 Vacuum Furnace 187 5.5.3 Pit Type Furnace 193

5.5.4 Salt Bath Furnace 196 5.5.5 Fluidised Bed Heat Treatment Furnace 5.6 Atmosphere Control in Heat Treatment 201 Summary 207 References / Suggested Reading 208 Review Questions 208

6. Quenching Technology and Characteristics of Different Quenchants

199

210

6.1 Introduction 210 6.2 Fundamentals of Quenching 213 6.2.1 Mechanisms of Heat Removal 214 6.3 Types of Quenchant and their Characteristics 217 6.3.1 Types of Quenchants 217 6.3.2 Characteristics of Quenchants 219 6.3.3 Choice of Quenchants 222 6.4 Oil-Quenching vis-à-vis Water and Polymer Quenching 223 6.5 Different Oil Grades for Quenching and their Selection 227 6.5.1 Classification of Quenching Oils 228 6.5.2 Working Characteristics of Quenching Oils 230 6.6 Polymer Quenching: Characteristics and Application Technique 232 6.6.1 Characteristics of Polymers 233 6.6.2 Application of Polymer Quenching 235 6.7 Cases of Quenching Defects 237 Summary 242 References / Suggested Reading 243 Review Questions 244

7. Thermal Heat Treatment Processes 7.1 Introduction 245 7.2 Annealing and Normalising 249 7.2.1 Annealing 251 Full Annealing 252 Sub-Critical Annealing 255 Spheriodising Annealing 257

7.2.2 Normalising 259

245

7.3 Hardening of Steels 263 7.3.1 Through (Bulk) Hardening of Steel

264

Process Features and Steps in Hardening Operations 264 Furnace Characteristics and Controls for Hardening 268

7.4 Flame / Induction Hardening 272 7.4.1 Flame Hardening 273 7.4.2 Induction Hardening 276 7.5 Tempering and Stress Relieving of Steels: Purpose and Practice 280 7.5.1 Temper Embrittlement 287 7.6 Martempering and Austempering of Steels 289 Summary 292 References / Suggested Reading 293 Review Questions 293

8. Thermo-Chemical Processes of Heat Treatment [Carburising, Nitriding, Carbo-nitriding and Nitro-carburising]

295

8.1 Introduction 295 8.2 Processes and Practice of Carburising and Case-Hardening 298 8.2.1 Mechanisms of Carburising 305 8.2.2 Methods of Carburising 306 8.3 Continuous Carburising Process 309 8.4 Vacuum Carburising and its Control 312 8.5 Nitriding 315 8.5.1 Nitriding Process and Principles 315 8.5.2 Operating Procedures for Nitriding 319 8.5.3 Vacuum Nitriding 322 8.6 Carbo-nitriding and Nitro-carburising 325 8.6.1 Carbo-nitriding 325 8.6.2 Nitro-carburising 327 Summary 330 References / Suggested Reading 331 Review Questions 332

9. Heat Treatment of High-alloy Steels [Stainless, PH-Stainless, Heat / Creep Resistant, Tool and DIE Steels]

9.1 Introduction 333 9.2 Heat Treatment of Stainless Steels 335

333

9.3 9.4

9.5 9.6

9.2.1 Heat Treatment of Austenitic Stainless Steels 338 9.2.2 Heat Treatment of Ferritic Stainless Steel 341 9.2.3 Heat Treatment of Martensitic Stainless Steel 342 Heat Treatment of Precipitation Hardening Stainless Steels 345 Heat Treatment of Heat Resisting / Creep Resisting Steels 351 9.4.1 Heat Treatment of Heat Resisting Steel 352 9.4.2 Heat Treatment of Creep Resisting Steel 358 Heat Treatment of Tool and DIE Steels 362 Heat Treatment of High Speed Steels (HSS) 367 Summary 373 References / Suggested Reading 374 Review Questions 374 Annexure 376

10. Surface Engineering of Steels

382

10.1 Introduction to Surface Engineering 382 10.2 Scope of Surface Engineering Processes 386 10.3 Plasma Carburising and Nitriding 389 10.3.1 Plasma Carburising 390 10.3.2 Plasma-Nitriding 392 10.4 Surface Hardening by Laser and Electron Beam 395 10.4.1 Laser Surface Hardening 396 10.4.2 Electron Beam Hardening 399 10.4.3 Ion Implantation 401 10.5 Vapour Phase Processing: CVD and PVD 403 10.5.1 Chemical Vapour Deposition (CVD) 404 10.5.2 Physical Vapour Deposition (PVD) 405 Summary 409 References / Suggested Reading 410 Review Questions 410

11. Industrial Heat Treatment Practices: Illustrative Cases 11.1 Issues of Industrial Heat Treatment 411 11.1.1 Residual Stress in Hardening of Steels 413 11.2 Metallurgical Highlights: Few Important Learning Points 415

411

11.3 An Overview of ‘Heat Treatment at-Work’ in the Shop Floor 419 11.4 An Overview of Steel Types and their Heat Treatment 425 11.5 Cases of Application Specific Heat Treatment of Steels 433 11.5.1 Heat Treatment of Front Axle Beamfor Commercial Vehicles 434 11.5.2 Heat Treatment of Crown Wheel / Pinion for Commercial Vehicle Power Transmission 438 11.5.3 Heat Treatment of Ball Bearing Races 444 11.5.4 Heat Treatment of High Speed Steel Tools 447 Summary 454 References / Suggested Reading 455 Review Questions 455 Annexure: Common Heat Treatment Defects: Causes and Remedies 457 Appendices Glossary Bibliography Index

459 475 483 487

Introduction to Steel and Scope for its Heat Treatment

1.1

STEEL AND THE ROLE OF HEAT TREATMENT

Steel is primarily an alloy of Iron (Fe) and Carbon (C) to which other alloying elements – such as manganese, chromium, nickel, copper, vanadium, molybdenum, etc. – can be intentionally added, to develop certain user-specific properties. While carbon is always present in the steel, other alloying elements are optional; they are added or used for developing some special characteristics and properties of steels for specific applications. Thus, steel can be classified into (i) plain carbon steel – containing only iron and carbon – or (ii) alloy steel where in addition to carbon specific alloying elements are added, either singly or in combination, for developing required properties. Therefore, purpose of heat treatment can be described as to bring about a favourable change in the microstructure of the steel or in the internal physical state of the steel for making it property-wise fit for specific uses and applications.

Composition of steels used in industries is, therefore, determined on the basis of properties that are required and how those properties can be economically developed in the steel. To develop the desirable properties in steel heat treatment plays a decisive role. Mechanical properties of steel are dependent on its microstructure, which can be altered or developed in a given composition by appropriate heat treatment. Heat treatment is basically a thermal or thermally assisted process for altering the microstructure of steel for attaining the desired mechanical properties. [However, there are processes where no perceptible microstructural changes occur, but still they are a part of heat treatment; for example: stress relieving of steel, which is a type of heat treatment using lower level of thermal energy]. Due to the dependence of properties on structure, popularly described as ‘structure sensitivity of steel’, user-specific properties in steels can be conveniently developed, whenever necessary, by employing appropriate thermal or thermally assisted process (i.e. by heat treatment). This situation of structure dependency of steel for its mechanical properties makes heat treatment an indispensible part of steel technology. Heat treatment, by definition, is a process of controlled heating and cooling for developing appropriate metallurgical structure or change in the (internal) physical state of the material. Hardening of steels is the example of former type of heating while the example of the latter type is the stress relieving of steel after cold working. Therefore, purpose of heat treatment can be described as to bring about a favourable change in the microstructure of the steel or in the internal physical state of the steel for making it property-wise fit for specific uses and applications. The kind of heating and deformation undergone by the steel in the mill during production determines the ‘initial structure’. But, these structures are mostly not suitable for industrial applications and uses. Heat treatment comes handy in altering or conditioning these structures for making the steel suitable for wide-ranging industrial applications and uses. In practice, therefore, heat treatment turns out to be a process which is employed for softening, hardening or conditioning the steel structure for attaining certain specific level of properties, and, thereby, enhancing the uses and utility of steels. The changes in properties are generally accomplished in steels through the microstructural changes owing to structure sensitivity of their mechanical properties. Such microstructural changes are brought about in steel by using controlled heating and cooling, i.e. by the heat treatment. At times, changes of structure (and the properties) might require the assistance of auxiliary processes, involving thermo-chemical or thermomechanical means. This extends the scope of heat treatment – from

simple thermal process of ‘controlled heating and cooling’ to thermochemical process of ‘controlled heating and cooling for localised surface alloying and hardening’, or to thermo-mechanical process of ‘controlled mechanical deformation for extra refinement of microstructure’. The type of heat treatment in steel can, therefore, be of three types: (a) Simple controlled heating and cooling without any change in chemical composition (called the thermal heat treatment process); (b) Controlled heating and cooling along with alteration of surface chemical composition (called the thermo-chemical process); (c) Controlled heating and cooling in combination with controlled and regulated mechanical deformation for producing extra fine structure (called the thermo-mechanical process). The selection of heat treatment process, adopted for the change in structure, depends on the change of properties required in the steel. Due to unique feature of cooling rate sensitivity in the phase transformation of steels, vast scope of tailoring the properties of steels exists, by altering or conditioning the structure through one of the many heat treatment processes. Thus, heat treatment is a versatile process involving thermal or thermally assisted chemical and mechanical means for bringing about a desirable change in the structure of steels. As the structure changes, properties of steels also change – following the rules of structure-property relationship, which has been discussed later in this chapter. Structure-sensitive properties (especially the mechanical properties) of steel can be maneuvered on the basis of its uses and applications. There are at least two distinct parts in the application of steels: (a) first, the choice of right steel (i.e. right composition/grade of steel) which is suitable for developing the required properties, and (b) second, the choice of right heat treatment process for developing the required structure to meet the properties. Therefore, study of heat treatment of steel requires understanding of the basic nature and character of steels, as well as their classification / gradation, for facilitating the right selection of steel. While major part of this book will be devoted to discuss the heat treatment processes and parameters for developing different properties, this chapter will focus on highlighting (a) the basic nature and characteristics of steels, (b) their classification and gradation, and (c) structure-property relationship in steel and the role of heat treatment. Purpose of such structured approach is to develop a sequential understanding of the steps in heat treatment planning and execution. Central to the heat treatment processes is the development of structure by controlling the transformation process. The process of steel

transformation is basically dependent on its composition and grain size, on one hand, and the method of cooling from higher temperature, on the other. The process can be further influenced by the application of deformation process (or due to prior working on the steel), which works as stored energy and facilitates the process of change of microstructure. By controlling these influencing factors for transformation and microstructure formation, varieties of properties can be developed in steels through heat treatment processes. Hence, understanding of the transformation behaviour of steel becomes an essential part of heat treatment technology. Transformation of steel for the development of different microstructures has, therefore, been discussed separately in Chapter 2. Transformation is a phenomenon where steel changes its allotropic form from one crystallographic structure to another and produces different physical phases and structures (from its mother phase) on transformation. Microstructures are constituted by these phases or a mixture of these phases, depending on the way the steel has been cooled and transformed from higher temperature, and the phases have formed. Heat treatment is a means to induce these transformational changes in steel and, thereby, help in producing desirable microstructures by controlling the process. Since transformation of steel is a crystal structure dependent phenomenon, this chapter, therefore, first introduces to different crystal structures and phases in steel, and then goes on to discussing the steel types and grades available for heat treatment, along with their characteristics, microstructure and properties. Finally, the chapter highlights and illustrates the structure-property relationship in steels, which is at the centre of all heat treatment processes.

1.2 STRUCTURE OF STEEL: CRYSTAL STRUCTURES, PHASES AND MICROSTRUCTURES 1.2.1

Crystal Structures and Phases in Steel

Steel, in its simplest form, is an alloy of iron (Fe) and carbon (C). When C atoms are added to Fe it forms a solid solution of Fe and C – which is called the steel. Like all other materials (excepting glass and polymers) iron and steel are crystalline materials. Crystal structure of steel depends on its phase, which could be either (i) body centered cubic (BCC) structure, if the phase is ferrite (a) or (ii) face centered cubic (FCC) structure, if the phase is austenite (g ). There are other variants of these phases and corresponding structure, but these two are the main types that are concerned with heat treatment. Crystal structure consists of atoms that are grouped in an orderly manner in the crystal lattice – forming the crystal structure. Figure 1-1

illustrates the schematic arrangement of atomic position in two crystal structure types of iron – that is the a-iron (called ferrite) and g -iron (called austenite). When the atoms occupy the corner and the body centred positions of the crystal, it is called the ‘Body Centred Cubic’ (BCC) structure, such as the a-iron, and when the atoms occupy the corner and face centred positions of the crystal, it is called the ‘Face Centred Cubic’ (FCC) structure – such as the g -iron; vide Fig. 1-1(a) and (b), respectively. a

g

(a)

(b)

Fig. 1-1 The atomic arrangement of BCC (a ) and FCC ( g ) crystal structures

When carbon atoms – being very small in size compared to the size of iron atoms – are added to iron atoms, they occupy the vacant interstitial positions of the BCC crystal structure of the iron and form, what is called, a-solid solution. But, if any other alloying element is added – say for example, manganese (Mn), silicon (Si), nickel (Ni) etc. – whose atomic size is higher than the carbon (C) and nearly the size of iron (Fe) atom – then that atom cannot be accommodated in the interstitial position of the BCC crystal structure due to lack of sufficient space. In that case, the newly added atom (for example, the Mn atom, in this case) will enter the FCC crystal structure of Fe by replacing or substituting some of the iron (Fe) atoms and form g -solid solution, subject to temperature of reaction and level of Mn concentration. This substitution (or introduction) of alloying elements into the iron lattice takes place at the temperature range where mobility of these atoms are high but the iron is still in the solid state. Therefore, the product is termed as ‘solid solution’. In steel, there are primarily two types of solid solution product; one the interstitial solid solution (the a-steel, ferrite) with BCC crystal structure and the other substitutional solid solution (g -steel, austenite) with FCC crystal structure; they exist in steel structure depending on the state of temperature and amount of solute elements. Figure 1-2 illustrates how the atoms of added alloying elements (solute atoms) occupy different crystal structure positions – forming either ‘interstitial’ solid solution or ‘substitutional’ solid solution, depending upon the atomic size.

Fig. 1-2 Formation of interstitial (a) and substitutional (b) solid solution in steels. [Acknowledgement: Internet source of subtech.com]

Thus there are two main phases (i.e. ferrite and austenite) and crystal structure types (i.e. BCC and FCC, respectively) that dominate the steel metallurgy and the heat treatment of steels. There are strict thermodynamic rules for formation of ‘solid solution’, but broadly it depends on the atomic size and crystal structure of the solvent (parent atom) and solute atom. When the atomic size between solvent and solute atoms differs by less than 15%, substitutional solid solution can form, as shown in Fig. 1-2(b). When the solute atom is smaller and can fill the interstices of the solvent atoms, an interstitial solid solution forms, Fig. 1-2(a). In steel, elements with smaller atoms like carbon and nitrogen form interstitial solid solution and elements with larger atoms (but within the size limit mentioned above) like silicon (Si), manganese (Mn), chromium (Cr) and nickel (Ni), etc form substitutional solid solution. Formation of solid solution by alloying is, however, limited by the solid solubility of the elements concerned, which is influenced by the temperature of formation and the concentration. As regards temperature dependence, a-phase (ferrite) forms at temperature lower than the g -phase (austenite). Nonetheless, formation of solid solution by alloying elements requires appropriate temperature of formation for thermal activation of the process. Such information about the temperature–composition relationship for formation of phases in steel is provided by the phase diagram. One of the many popular representations of Iron-Carbon (Fe-C) phase diagram is shown in Fig. 1-3. From this Iron-carbon phase diagram, it can be observed that carbon (C), a solute interstitial element in Fe-C alloy system, can dissolve in ferrite (a – steel) upto 0.025% at 723°C. When solubility of carbon in interstitial solid solution exceeds this limit set by thermodynamical rules (vide phase diagram) then another phase forms, which could be iron

carbide (Fe3C) – an intermetallic compound – on cooling or austenite (g -steel) – a solid solution – on heating. Microstructure that is observed in slow cooled steel is the result of such phase changes under equilibrium cooling condition from higher temperature. Phase changes can take place on heating of the steel as well as on cooling, but the microstructure that is observed at room temperature in such steels is the result of cooling. Carbon steels when cooled from temperature above 910°C (i.e. from above A3 temperature) or 723°C (i.e. A1 temperature), as the case might be, give rise to characteristic microstructure that consists of ferrite and carbide, vide Figure 1-3. If carbon content is in excess of 0.025% but lower than 0.83% (the eutectoid carbon), the microstructure will contain some primary ferrite and pearlite – an intimate mixture of ferrite and carbide. Thus, microstructure of such steel composition will be, in general, (ferrite + pearlite) on slower cooling; vide Fig. 1-3. Temperature °F A 3000 2802 d+L 2800 2720 d d+g 2600 2552 2400 2200 2066 2000 1800 1670 1600 1400 1333 1200 1000

E

B

1539 1492

L

CM begins Primary to solidify austenite begins to solidify

g+L

g Austenite solid solution of carbon in gamma iron Magnetic (1414°F) point A2

Austenite in liquid

1400

L + Fe3C

2066°F

1130

Fe3C Austenite ledeburite and 910 y + Fe3C cementite Cementite Austenite to pearlite and A1,2,3 ledeburite 760 723

ACM

A2 A3 a+g A 1 a 0.025 Pearlite and Aa ferrite

Temperature °C

g = Austenite a = Ferrite d = Delta iron CM = Cementite

1333°F

Cementite, pearlite and transformed a + Fe3C ledeburite Magnetic change of Fe3C

Pearlite and cemetito

410

0.008% 0.50 0.83%1% 2% Hypo-eutectoid Hyper-eutectoid Steel

4.3 3%

210 6.67 4% 5% 6% 65%

Cast Iron

Fig. 1-3 Iron-Carbon equilibrium phase diagram depicting the presence of different phases of steels and microstructure with changing temperature and composition (%Carbon) under equilibrium cooling. [Source: Material Science and Metallurgy, 4th. Edition, Pollock]

However, if carbon in the steel exceeds the eutectoid composition (e.g. 0.83%C) the structure will be (cementite + pearlite). Cementite is same as carbide; sometime, carbide is called cementite when it is present as primary phase. The term ‘primary ferrite’ or ‘primary carbide’ denotes that the particular phase is the one that has first precipitated out from the cooling steel before the other phase – such as pearlite, in this case. While solid solution formation by alloying elements contributes to the strengthening of steel by creating local stress fields in the crystal structure lattice, intermetallic compound (e.g. Fe3C, the carbide phase) that forms due to excess interstitial element contributes to the strength of steel by precipitation hardening or dispersion strengthening, which are well-known steel strengthening mechanisms and often form the objective of heat treatment for strengthening of steels. In steel, alloying elements with smaller atomic diameter, like carbon and nitrogen, form interstitial solid solution, and they can participate in solidsolution strengthening as well as in precipitation hardening, depending on the concentration and treatment. Most other alloying elements with higher atomic diameter – like the Si, Mn, Cr, Ni, etc. – form substitutional solid solution and contribute to solid solution strengthening of steels. Solid solution so formed usually causes increase in mechanical strength, electrical resistivity and decrease in plasticity in the steel. Therefore, by taking note of these changes, steel composition (i.e. alloying elements) can be appropriately designed for maximum gain in strength, either through heat treatment or even without heat treatment. Opportunity for solid solution strengthening and precipitation hardening of steels by appropriate alloying is often used in thermo-mechanical heat treatment processes with advantage. However, solid solubility of alloying elements is not only crystal structure dependent; it is also temperature and composition dependent (as would be evident from Fig. 1-3) due to thermodynamic factors controlling the stability of phases. This situation leads to different phase changes and microstructure formation in steel, which results from change in temperature, cooling rate and composition. Iron-carbon phase diagram (Fig. 1-3) helps to study such phase changes and microstructure formation in greater details. Therefore, this diagram is extensively used for planning and execution of heat treatment processes.

1.2.2

Iron-carbon Phase Diagram

Phase changes in steel – containing Fe and C – can be studied by following the ‘Iron-carbon’ phase diagram, as depicted in Fig. 1-3. Full range of the Iron-carbon diagram is rather complex and not relevant for the study of heat treatment of steel. Hence, only the relevant portion of

the diagram, concerning the steel and cast iron, has been shown in the figure. The iron-carbon diagram provides valuable fundamental information on which the knowledge for heat treatment of both plain carbon and alloy steels can be based. Figure 1-3 is also called Fe-C equilibrium cooling diagram, because the changes in phases and microstructure depicted in this diagram occur under equilibrium cooling; it implies slower rate of cooling where the corresponding phases can react with each other for establishing a ‘reversible equilibrium’ condition, such as: Cooling

Ferrite + Cementite (Fe3C)

Austenite Heating

However, though this equilibrium reaction is reversible in cooling and heating, there is a hysteresis in heating and cooling, which is again influenced by the rate of heating or cooling. This hysteresis effect influences the ‘critical temperatures’ in steels – such as A1 (the lower critical temperature) and A3 (the upper critical temperature); vide Fig. 1-3. Critical temperature represents the temperature at which phase changes occur in steel either on heating or cooling, indicated by the arrest of temperature change at that point of heating or cooling. Since these critical temperatures in steel may vary due to hysteresis in heating or cooling, they are often symbolised differently for indicating if it refers to heating or cooling. Commonly, upper and lower critical temperatures are symbolised by Ac3 and Ac1, respectively, to denote critical temperatures on heating (The “c” is from the French word chauffage, meaning heating). Similarly, Ar3 and Ar1 are used to denote the critical temperatures on cooling (the “r” originating from the French word refroidissement, meaning cooling). Faster the heating or cooling rate, greater is the gap between the Ac and Ar points. However, this effect is more pronounced on lower critical temperature (A1) than on others. However, in this book various ‘critical temperatures’ have been uniformly referred to as: A1, A3, etc without notation for heating or cooling, in order to keep the subject matter simple. Iron-Carbon (Fe-C) diagram in Fig. 1-3 shows the presence of four ‘critical temperature’ points in steel, namely A1, A2, A3 and ACem, in order of increasing temperature. These are important points for phase changes. Amongst these critical temperatures, A1 – called the lower critical temperature – is independent of composition and nearly constant (except for hysteresis in cooling and heating); all others critical temperatures vary with the composition. Amongst these critical temperature

points, A1 (the lower critical temperature) and A3 (the upper critical temperature) are of utmost importance for the heat treatment of hypoeutectoid steels (i.e. steel containing carbon lower than the eutectoid composition). For this reason, A1 and A3 critical temperatures are also described as the ‘lower transformation temperature’ and ‘upper transformation temperature’, respectively. Thus, Fe-C equilibrium phase diagram is the source of important information about various phase changes in steel under slow or equilibrium cooling – leading to different microstructure formation like ferrite, pearlite and cementite (vide Fig. 1-3). The diagram illustrates that the phase changes are dependent on the carbon solubility in the respective phases with increase or decrease of temperature for a given composition. Figure 1-3 shows that a-iron phase field is severely restricted – with a maximum carbon solubility of 0.025% at 723°C. This leads to the formation of a new phase – the iron carbide (Fe3C) – when carbon in the steel exceeds 0.025% limit for ferrite. The field of carbon solubility in g -iron (austenite), on the other hand, is much wider than a -iron; vide the wider phase field of austenite in Fig. 1-3. Carbon solubility in austenite can go up to over 2.0% at 1147°C (Fig. 1-3). This high solubility of carbon in austenite is of extreme importance in the heat treatment of steel. Such a situation indicates that by heating (for solution treatment of steel) in the g -region, entire carbon in the steel can be taken into solid solution and then by adjusting the rate of cooling, character and nature of phase changes can be controlled and different microstructures in the steel can be produced. For example, by rapid cooling to room temperature, carbon separation from the solid solution can be suppressed and a supersaturated solid solution of carbon in iron can be formed. This will give rise to non-equilibrium structure of different type than when slow cooled (vide discussions in Section 2.4). On slow equilibrium cooling, the difference in carbon solubility between g - and a-iron will normally lead to formation of primary ferrite and enrichment of carbon in the remaining austenite. This process of carbon enrichment in austenite will continue until the temperature has reached the A1 temperature (also called the ‘eutectoid temperature’). At eutectoid temperature (723°C), the carbon-rich austenite becomes thermodynamically unstable and decomposes to form ‘pearlite’; this involves alternate precipitation of carbide and ferrite from the carbonrich austenite, giving rise to a lamellar structure. The process of carbon rejection due to fall in solid solubility of carbon in austenite with cooling starts on crossing the A3 temperature line in Fig. 1-3 and continues until temperature reaches the ‘eutectoid temperature’ (A1). The phase changes that take place at the eutectoid temperature

are by ‘eutectoid reactions’ – producing an intimately mixed ferrite and carbide in the form of lamellae, called the ‘pearlite’. The proportion of ferrite and carbide in the structure – which depends on the composition of the steel and the cooling condition – will finally determine the properties of the steel. Thus, by choice of steel composition, solution treatment of the steel at austenitic region, and then controlling the cooling rate, different types of structures can be produced, which is the central concern of heat treatment processes. More about the phase transformation and structure formation in steels have been discussed in Chapter 2 – correlating their applications in the heat treatment of steels. Thus, Fe-C diagram yields critical information about how the steel structure can be controlled. Iron-carbon diagram, however, represents phase changes under slower equilibrium cooling, but cooling condition in practical heat treatment may vary between slow cooling and very fast cooling. The non-equilibrium cooling rate/cooling condition forms a very important part of steel heat treatment for hardening the steel. Study of transformation behaviour of steels under faster non-equilibrium cooling condition requires references to the respective TTT (time-temperaturetransformation) or CCT (continuous cooling transformation) diagrams. More about these transformation diagrams and phase transformation and microstructure formation in steels under equilibrium and non-equilibrium cooling conditions have been discussed in greater detail in Chapter 2.

1.2.3

Microstructures in Steel

Development of desirable microstructure is the central task of heat treatment of steel. Microstructure is the product of phase transformations in steel and it forms when the steel of a given composition is cooled from the g -region. Cooling can be equilibrium cooling or faster nonequilibrium cooling. As per Fe-C phase diagram (Fig. 1-3), equilibrium cooling conditions give rise to three types of microstructure, namely, ferrite, pearlite and carbide. They all form from the austenite (g -phase) on slower cooling from higher temperature. Figure 1-4(a) shows the ferrite-pearlite structure in steel and Fig. 1-4(b) shows the appearance of lamellar pearlitic structure at higher magnification. But, if the austenite is cooled faster (i.e. quenched), suppressing the opportunity for carbon separation with decreasing temperature, then the structure becomes super-saturated with carbon, giving rise to what is called ‘martensite’; martensite is a super-saturated solid solution of carbon in iron with highly distorted ferritic lattice structure due to the strain associated with the process of formation of martensite. This makes martensite very hard but brittle. Therefore, to make martensite tougher a separate tempering treatment is given to steel after quenching. Toughness

(a) (x200)

Fig. 1-4 pearlite

(b) (x2000)

(a) Ferrite (light) and Pearlite structure (dark); (b) Lamellar structure of

gets induced in martensite by relieving internal stresses and inducing carbide precipitation under thermal energy (i.e. under heat). Carbide precipitation during tempering – which is carried out at relatively lower temperature – takes place by diffusion of carbon atoms (C) from the super-saturated martensitic structure followed by reaction with the iron (Fe) of the matrix to form ‘iron carbide’ (Fe3C). One such tempered martensitic structure is illustrated in Fig. 1-5(a). If the cooling rate is intermediate between slow and fast cooling, there could be another non-equilibrium cooling product in the steel – called bainite – depending on the composition of the steel and the mode of cooling. Since this product forms at the intermediate temperature range and at intermediate cooling rate, the product is also sometime referred to as ‘intermediate product’. Figure 1-5(b) illustrates a typical bainitic structure in low-alloy steel.

Fig. 1-5 (a) Lightly tempered martensite structure; (b) bainitic structure with feathery ferrite plates and carbide precipitates mostly on the plate boundaries (Upper bainite)

Bainite could be of two types; one, ‘upper bainite’ that forms at the higher temperature zone of the bainite formation temperature range and the other ‘lower bainite’ that forms at the lower end of the bainite transformation temperature range – with reference to TTT or CCT diagram of the steel. Upper bainite has a structure where carbide precipitations are mainly on the ferrite plate boundaries, whereas carbide precipitates are all over the matrix in lower bainite. Further details of martensite and bainite formation will be discussed in Chapter 2 under austenite decomposition. Thus, under differing cooling conditions, steels can have ferrite, pearlite, carbide, bainite or martensite structures. These structures form from mother austenite and take the morphological character as per the cooling conditions and the composition. These microstructures have widely differing properties, which are characteristics of the conditions of their formation. Their properties differ when their morphological character changes due to change in the condition of formation due to cooling rate. For example, pearlite formed from slowly cooled austenite will have a coarser inter-lamellar spacing than when cooled faster (e.g. air cooling), resulting in different mechanical properties. Pearlite that forms under relatively faster cooling has finer lamellar spacing, which exhibits superior strength and ductility than the slow cooled coarser pearlitic structure. Similarly, martensite formed from similar austenite composition by water quenching and oil quenching – having drastically different cooling rate – will be different in morphology and also in properties. More about the structure formation and properties have been discussed in Chapter 2. A closer examination of these structures will reveal that though they all look different, their constituent phases are same; the phases are basically some form of ferrite (a) and carbide but they form in different morphological patterns, primarily due to difference in the cooling rate and to some extent for their composition. It is this morphological nature of these structures that determines their ultimate properties. Heat treatment planning of processes and parameters for actual operation should consider these aspects of phase transformation and change in morphological character of microstructure with change in composition and cooling rate in order to tailor make the steel properties. Discussions in Chapter 2 further elucidate how heat treatment process can be manipulated for getting the right morphological character of microstructural phases for the target properties. Figure 1-6 further elucidates this point and depicts the magnified picture of microstructures of pearlite (a), acicular bainite (b) and martensite (c) in steels. These micrographs show that when closely examined in more details pearlite, bainite and tempered martensitic microstructures

Fig. 1-6 Magnified local area view of pearlite, bainite and martensite (as-quenched) in steels. (These micrographs are for illustration and have different magnification)

are observed to be essentially made up of some kind of aggregates of ferrite (a) and carbide phases (either iron carbide or alloy carbide as per composition of the steel), but with different morphological pattern. A closer examination reveals that these structures basically consist of two distinct phases: ferrite (light patches) and carbide (dark patches) – as revealed by etching. But, the nature of their forms and morphological character differ; ferrite (light) and carbide (dark) in pearlite (a) is in lamellar stringer form, in bainite (b) it is the aggregate of acicular ferrite plates and thicker carbide precipitations (dark streaks), and in martensite (c) it is strained ferrite needles (outlined by light diagonal streaks) with fine carbides all over (dark grey). Carbide in martensite structures precipitates out of the strained ferrite lattice and becomes more visible with tempering. Each of these structures has its own characteristic properties – depending on the exact morphological nature of the phases and phase distribution – and the task of heat treatment is to plan the heat treatment process and parameters for bringing out the beneficial effect of these morphological changes. Martensite structure in as-quenched state is a super-saturated distorted tetragonal structure in ferrite lattice, but on tempering, this tetragonality and distortion comes down or disappears due to carbon atoms coming out of this structure and forming carbide precipitates by interacting with the iron or alloying elements in the matrix. This turns the tempered martensitic structure to an aggregate of ferrite and carbide, where ferrite is in the form of acicular or needle type plates and carbides are uniformly dispersed fine precipitates. However, if the tempering temperature is too high, ferrite might lose its acicular character and carbide size becomes coarser. In tempering of martensite, carbide size increases with increasing temperature and becomes globular at higher tempering temperature.

Whatever may be the form or shape of phase distribution in the microstructures of steel, they all add to the strengthening effects. Thus, primary purpose of heat treatment of steel is to develop right combination and right form of microstructure by playing with the shape, size and distribution of phases in the structure. Microstructural variation can be built into the steels by variation in chemical composition and cooling rate by following the rules of ‘austenite decomposition’ (Chapter 2). Due to extreme importance of austenite decomposition in the heat treatment of steels, the subject has been separately discussed in Chapter 2 and formation of different microstructures has been elaborated. Austenite decomposition shows that steel has the unique characteristics of producing wide varieties of microstructural combination from its limited number of phases, depending upon the composition of the steel and the cooling condition. As such, the rules of austenite decomposition provide the basis for different thermal, thermo-mechanical or thermochemical heat treatment processes. Microstructures are of paramount importance in steel, because of the microstructure sensitivity of mechanical properties of steels. Heat treatment is the means to develop the required microstructural character that provides the properties. However, response of steel to different heat treatment processes is dependent on its composition and some inherent character like the grain size and inclusions. Influence of these parameters, i.e. composition, grain size and inclusions on the process of heat treatment has been discussed in detail in Chapter 4, under heat treatability of steels.

1.3

STEEL TYPES AND GRADES OF STEELS

Heat treatment of steels requires the knowledge about selection of type of steel for processing. In fact, going by the sequence of steps for executing heat treatment operation, first step is to select the right type of steel. In this regard, understanding about steel types, grades and their specialty assumes great importance for guiding to the choice of right type of steel for getting the right set of properties after heat treatment. This section will deal with such basic information about steel types and their grades.

1.3.1

Steel Types

So far, only steel containing carbon has been referred. But, steel can be of plain carbon type as well as alloy steel – which contains appropriate alloying elements, in addition to carbon, for development of required properties. Based on the carbon and alloy contents in steels, steels are

broadly grouped as ‘Plain carbon steel’ or ‘Alloy steel’. Amongst these groups, there are few sub-groups, each of which is important for different applications. Plain carbon steels are grouped under (1) low-carbon, (2) medium carbon, and (3) high carbon steels, on the basis of the carbon level in the steel. Similarly, alloy steels are grouped under (1) low-alloy steels and (2) high-alloy steels, depending on the percentage alloy content. Alloy content in low-alloy steels is generally limited to 5%; above this range, alloy steels are classified as high-alloy steel. However, this distinction is rather arbitrary and there are instances of different figures quoted for low-alloy steels, namely 4%, 5% and 8%. [Note: Considering that 5% metallic alloy limit covers majority of popular steel grades under different reputed standards of steels (namely AISI, BS and DIN), this book will refer 5% total alloying as the demarcation line between low-alloy and high-alloy steels.] Generally, carbon percentage (%C) in steels range from 0.02% to 1.0% with the exception of some wear-resistant steels where %C can go up to or little over 2.0%C in order to produce sufficient amount of ‘carbide’ in the structure, which is a wear resistant phase. If % carbon far exceeds this higher limits (≥ 2.0%), there would be possibility of excess carbon precipitating as ‘free carbon – called graphite’, changing the nomenclature of the material from steel to ‘cast iron’. This feature of presence of excess carbon in the form of graphite is a critical distinction between the steel and cast iron. In steels, excess carbon normally forms ‘carbide’ in the microstructure, but in cast iron excess carbon is generally present as ‘graphite’ in the microstructure; with the exception of ‘white cast iron’, where free carbon is present as carbide. Though plain carbon steels (PC Steels) are made with no intentional addition of alloying elements, some amount of manganese (Mn) and silicon (Si) are always present in the plain carbon steel grades. These elements come from the steelmaking process. Similarly, there could be some small amount of ‘sulphur’ (S) and ‘phosphorus’ (P) present in plain carbon steels, which are also carried forward in the steel from steelmaking process. Thus, plain carbon steel is not exactly free from any other alloying element. If the levels of these elements are below a specified limit these elements are considered as ‘incidental’ and not as intentional alloying. Permissible limits of these incidental or residual elements in a grade of steel vary from one standard to another, but generally Mn above 0.65% and Si above 0.50% are considered intentional alloying; below which they are considered ‘incidental’ to steelmaking process. Similarly, the limit for sulphur and phosphorus as incidental presence is below 0.05%.

However, respective standards of steels – governing the grade or applications – are the final guide about these permissible limits. Higher sulphur and phosphorus are not desirable in heat-treating grade steels, because sulphur can cause hardening cracks and phosphorous can cause embrittlement of the steel. Incidental or residual elements can be also present in alloy steels, but in traces only so that they do not effectively alter the property of the chosen steel. Various residuals elements, mostly in traces, come into the steel from the raw-materials and additions used for steelmaking, which contain sulphur and phosphorus as well as other residual elements. Steel standards comprehensively cover the permissible limits of these residual elements in order to draw a line between what is alloying and what is not. Hence, presence of residual elements should be ignored while defining the steel type (i.e. plain carbon or alloy steel). From this stand point, steel can be said to be an alloy of iron and carbon with or without any intentional alloy addition and with the possibility of presence of some residuals elements– like silicon, manganese, sulphur, phosphorus, etc that are unavoidable due to economic limitation of steelmaking processes. This situation means that the dividing line between plain carbon and alloy steels is not the presence or absence of certain elements but the level of those elements. Making alloy steels is more difficult than plain carbon steels, because of the intricacies of recovering and adjusting alloying elements from the ferro-alloy additions. Alloying elements are susceptible to rapid oxidation and loss at higher temperature where steelmaking is carried out. This poses problems for accurate alloy recovery for meeting the specification range given in the standards. Hence, making alloy steels require more precise control and, at times, additional ‘secondary steelmaking facilities’ – such as use of ‘ladle furnace’ or ‘vacuum degassing’ to control the composition, temperature and other steelmaking parameters. This makes alloy steelmaking process more elaborate and expensive compared to plain carbon steels. However, despite cost, alloy steels have wide demands in industries because of its ability to offer superior properties after heat treatment. Strength and toughness of alloy heat-treated steels are far superior to the heat treated plain carbon steels. Nonetheless, plain carbon steels have their own importance in industries for relatively lower cost and their ability to develop adequate properties for many applications after heat treatment. Selection of steel grade for heat treatment should be such that the required mechanical properties can be most economically and consistently developed, but without the risk of distortion and cracks.

For focus on right selection for right uses and applications, steels are again sub-grouped into number of types or grades as per their uses or properties. For example, Plain carbon steel grades can be broadly grouped as follows in order of carbon levels in the composition, namely: • Low-carbon steel, containing carbon generally limited to 0.15%, but can go upto 0.25%. • Medium carbon steel, containing carbon ranging from 0.20 to 0.60% • High-carbon steel, where carbon content ranges from 0.60 to 1.0% Under this grouping, there could be further sub-grouping, namely extra-low carbon steel, ultra-high carbon steel, etc. The other criterion of grouping of steels is as per shape or applications, irrespective of its carbon content or composition. Examples of shape-wise grouping of steels are: Sheets, Plates, Bar, Blooms, Billets, Beams, etc. Examples of form/application wise grouping of steels are: • Sheet steels generally with %C below 0.10% for sheet metal applications requiring good cold formability and weldability; the primary requirements of sheet steels. Steels requiring high formability often contain extra-low carbon (£ 0.05%), and at times, %C in steel can go even below 0.02%, made by special steelmaking process, and such steels are called ‘Interstitial-free steels’ (IF steel). These are used for extra deep drawing sheet metal applications – like the modern automobile body parts. • Wire-rod grade steels with %C generally below 0.15% for good cold drawability, but the carbon in these steels can be even higher for special applications like spring wire (%C range 0.60 to 0.85). • Structural steels with carbon above 0.10% to 0.25% are mostly used for structural applications like the reinforced steel bars for building constructions or beams for factory construction or plate steels for vessels and container manufacturing which requires some tensile strength as well as weldability. At times, %carbon in structural steels can go above 0.25% for cases where increased strength is required in the structural member for a specific application. • Constructional grade plain carbon steels (also called engineering steels) with higher carbon (ranging from about 0.15% to 0. 80%) for uses in engineering constructions – like machine parts, gears, shafts, torsion bars, etc. These steels are used mostly after some kind of heat treatment for developing the required properties. This is the general outline of different types of steels under plain carbon group; exact composition or carbon range for these steels are

covered under different ‘Standards’ and ‘Specifications’ of steels; some such specifications and standards of steels have been listed in the appendices at the end of the book. From heat treatment point of view, all classes of steels described above can be heat treated for some specific reason, such as for conditioning, softening, strengthening, hardening, toughening, etc. Alloy steels grades, where %C generally ranges between 0.10% and 2.0% depending on the grade, contain some specially chosen alloying elements like chromium, manganese (above 0.50%), nickel, molybdenum, niobium, copper, etc., for developing some end-use specific properties, such as: high strength in combination with good toughness, corrosion resistance, creep resistance, high temperature strength, wear resistance, etc. Based on alloy contents, alloy steels are grouped into: (a) Low-alloy steel – where total alloy content is limited to about 5.0% (as explained earlier), and (b) high-alloy steel – where alloy contents are higher than 5.0%. Examples of popular low-alloy steels are SAE 4140, SAE 5140, En-18, En-19, En-24 etc, and the examples of high-alloy steels are 18/8 stainless steel, 12Cr Heat-resistant steels, etc. Alloy steels can also be grouped on the basis of their properties or dominant chemical constituents. For example: (a) Hardening Grade steel, (b) Free-machining steel, (c) Stainless steel, (d) Heat resistance steel, etc., for the property-related grades, and (a) Chromium steels, (b) Manganese Steel, (c) Nickel steel, (d) Tungsten steel, etc., for indicating the dominant chemical constituent. Alloy steels are chosen with an eye to developing the special properties after heat treatment which plain carbon steels cannot offer, in general. Exception is the austenitic stainless steel which is not generally hardenable by heat treatment. Nonetheless, heat treatment is an important processing route for many stainless steels for further improving their strength and corrosion resistance (vide Chapter 9). Stainless steels are an important group of high-alloy steels that are used for corrosion and heat resistance. They are generally sub-grouped as per their physical structures. For example, austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, etc. Each of these stainless steel groups has their characteristic properties and applications. While chemical composition of stainless steels contributes to their corrosion and heat resistance properties, their mechanical properties are dependent on the structure and the heat treatment it gets. Heat treatment of high alloy steels – including the stainless steels and heat resistant steels will be discussed separately in Chapter 9.

1.3.2

Classification of Steels and their Grades

For convenience of selection and application, steels are classified and coded with a grade name. While classification of steel can be done according to their composition, application or any other end-use specific purpose, grade names are assigned based on the composition or utility. General steel classification systems are based on: (a) Chemical composition of the steel; for example, Plain carbon steel, Low-alloy steel, High-alloy steel, Micro-alloyed steel, etc. (b) Application or utility of the steel; for example, Structural steel, Engineering steel, Stainless steel, Die & Tool steel, etc. (c) Properties of the steel; for example, Mild steel, High-strength steel, HSLA steel, Stainless steel, Wear resistance steel, Heat resistance steel, Creep steel, etc. (d) Purpose of the steel; for example, Machining steel, Forging steel, Carburising steel, Nitriding steel, Deep-drawing steel, Throughhardening steel, Induction hardening steel, etc. For facilitating precise choice of steels and control over properties, steels are further grouped and given a designated grade name under different working standards of a country. The method of designating steel with recognizable name is known as ‘system for designating steels’ where steels are designated with specific grade names as per their chemistry or utility. There are number of steel designation systems that assign grade names to steels, which are used during buying, ordering or selection. Designation systems are generally country specific, but there are initiatives now to unify them under a universal numbering system (UNS) – as proposed by the USA and ISO (International Standards Organization). Steel designation system attempts to provide a standardized approach and understanding about the available steels in a country and their scope of uses and applications. Steel designation and standardization is an important activity in any industrially developed country – where the task is assigned to a recognized ‘standardization body or association’ in the country. The subject of standardization and grade designation is quite elaborate; hence discussing the steel designation system in details is beyond the scope of this book. However, due to its important role in the process of selection and heat treatment of steels, some popular methods will be highlighted here for developing some basic understanding. Amongst the international standardization bodies, system of standardization and grading followed by USA, UK and Germany are most popular. Table 1-1 gives summarised version of UK’s British Standard (BS) system for designating steels.

In this example, plain carbon steels without any alloying have been grouped with prefix numbers 01 to 09 for designating different types of plain carbon steels, e.g. low-carbon, mild steel, medium carbon, highcarbon, etc. For example, popular 0.40% carbon steel, used for many common engineering applications after heat treatment, is designated as 080M40 (old designation was EN-8). If similar carbon containing steel is with chromium alloying, it is designated as 530M40 (EN 18); and if similar steel has chromium and molybdenum as alloying, then it is designated as 708M40. American standardization agencies – like the Society of Automobile Engineers (SAE) or American Iron and Steel Institute (AISI) – follow nearly similar system, using 4-digit designation system, as shown in Table 1-2.

In SAE system, plain carbon steel of 0.40% carbon is designated as SAE 1040, but the similar carbon steel with alloy of chromium is designated as SAE 5140, and with molybdenum, it is designated as SAE 4140, etc. This is very similar in approach to the British system. Because of different designation system followed by different countries, there could be some confusion in selecting steels for applications and heat treatment. In this regard, list of equivalent steel grades are available for reference. [At the end of this book a list of equivalent grades of steels for engineering applications has been provided (Appendix)]. In sum, steels can be of plain carbon type or alloy steel type. Either of these steel types can again be grouped into several types as per composition, shape or applications, and each of these classes are then grouped (mostly as per composition) and given a designated grade name – such as 45C8, 080M40, SAE 1045, etc. The designation (grade name) is allotted by the country’s standardization authorities. Though designation system and standards may vary from country to country, its main purpose is to make steel selection easy and to facilitate right choice of steel for right purpose. The grade name and the (national) standard that covers that steel provide the necessary information about suitability of the steel for a given purpose. For example, if the steel has to be hardened for higher strength and toughness, then steel standard and grades covering the hardenable steel grades (or steels for hardening) should be referred and appropriate steel grade with right chemistry and other quality features should be selected. For example, for hardening, the steel must have a minimum level of carbon for response to hardening by martensite formation. Therefore, steel standards and steel grades covered under the specification for ‘Hardening Grade Steels’ must reflect this aspect of the steel composition and quality. Steel with lean carbon and no alloying will not respond to heat treatment for hardening. This is the reason why all engineering steels, which are given some kind of heat treatment for developing certain desirable mechanical properties, are with a minimum carbon content and often with appropriate alloying.

1.3.3

Importance of Composition of Steel for Heat Treatment

Whatever may be the type or grade of steel, composition is the most dominant factor that helps in achieving the end-use properties. As regards heat treatment of steels, composition, grain size and inclusion content in steel are the important parameters. As such, all steel standards for heat-treatable grade steels specify these parameters (i.e. composition, grain size and inclusion level) – as part of steel specification – for facilitating selection, processing and application of steels.

Amongst the compositional constituents of steel, carbon is the most important element in terms of developing mechanical properties by heat treatment. For example, by controlling the carbon content different microstructures with different carbide morphology and distribution can be obtained (e.g. coarse pearlite, fine pearlite, spherodised carbide, etc) giving rise to different mechanical properties. Mechanical properties in steels get developed due to microstructures, which are, in turn, influenced by the composition, (including carbon and other alloying elements), prior treatment (hot or cold working) and the cooling rate during heat treatment from higher temperature. Alloying elements – like Cr, Mn, Mo, Ni, V, etc. – have pronounced influence on the structure and properties of steels (vide discussions on heat treatability of steel in Chapter 4). Presence of these alloying elements influences the heat treatment process by altering the time and course of transformation. For example, presence of Cr, Mo, etc., as alloying elements, delay the transformation of austenite to ferrite and pearlite, on one hand, and, thereby, facilitate formation of bainite and martensite, on the other hand. Presence of such alloying elements also changes the nature of carbides, making them more complex in composition and character. These alloy carbides are, in general, more complex in structure than iron carbide, become more stable with the increasing temperature, and provide higher strength to the steel. More about transformation characteristics of steels has been discussed in Chapter 2. All steels do not equally respond to all heat treatment processes; there is composition limitation, especially for hardening of steels. Hence, study of heat treatability of steel is an important part of heat treatment technology, which has been covered in Chapter 4. Table 1-3 gives some examples of steel types, its response to heat treatments, and the microstructure produced by those heat treatment processes. This list is indicative only; actual microstructure will depend on exact composition and the process of heat treatment, especially the cooling condition and rate. There could be significant difference in structure between water quenching and oil quenching; water quenching producing finer structure than oil quenching but endowed with the risk of distortion / crack. The table illustrates how the type and composition of steel is related to heat treatment and the type of microstructure that can be expected out of the heat treatment process. The table is a general guide for choosing steel for a given heat treatment operation. But, an important point to note from the table is that steel composition is the single most important parameter that determines the response of steels to the development of structure by heat treatment. Since steel is a structuresensitive material for its properties, composition dependence of steel for

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response to heat treated structure will also control the properties after heat treatment.

1.4 1.4.1

CHARACTERISTICS AND PROPERTIES OF STEELS Characteristics of Steel

Uniqueness of steel lies in the fact that it can be heat treated to a wide range of strengthand toughness/ductility. Steel properties can be easily

tailor-made for an application by choice of right grade of steel and appropriate heat treatment. Heat treatment facilitates development of right structure for right properties. Because of the opportunity of tailoring microstructure in steel by heat treatment, strengthening / toughening of steel by different heat treatment process has been the most attractive feature in steel metallurgy and the application of steels. Facility to strengthen steel by heat treatment has made the steel most competitive material when it comes to comparing the strength to weight ratio vis-à-vis cost. Some unique features of steel in relation to heat treatment are: • Very high familiarity of steel, availability to exact quality, and heat treatability • Excellent response to different kinds of heat treatment (including thermo-mechanical treatment) for producing wide varieties of microstructural requirements • Excellent structure-property relationship in steel, facilitating choice of structure and heat treatment for developing right properties • Application related properties of steels – like strength, toughness, machinability, weldability, formability, corrosion resistance, fatigue resistance, creep resistance etc. – can be precisely developed by suitable choice of composition, alloying, thermal processing and heat treatment • Heat treatment / thermo-mechanical treatment can be easily employed to steel of appropriate composition for developing properties for processing / manufacturing, e.g. machinability, formability, deep drawability, etc. • Fatigue, wear and other surface related properties of steels can be easily improved by special design of surface hardening, surface engineering or selective hardening. • For application under extreme corrosive and hot environmental conditions, surface composition and properties of steels can be altered by surface engineering, using modern heating and cooling techniques (e.g. laser alloying and hardening, ion-beam nitriding, etc), and • Physical properties of steel, e.g. modulus of elasticity (E), shear modulus (G) and Poisson’s ratio (v) are nearly constant and not much affected by heat treatment. Steel stands out over other materials in these respects. Because of life-cycle cost advantage with steel and its high strength to weight ratio – both arising from the possibility of tailoring steel properties by heat treatment – steel is by far the most preferred material for mass production of engineering parts, automobiles parts and engines, car body and

frames, white goods manufacturing, containers, ship-building, railways, machine-tool manufacturing, structural construction, etc. List of steel usages through heat treatment processes can be endless, depending on the skill and creative thinking of people. Modern heat treatment processes using laser, plasma, ion-beam, etc., are opening up new avenues of heat treatment of steels and extending the utility and application of steels. Thus, much of the competitive advantage of steel is due to availability of wide varieties of steels that can be processed by wide varieties of heat treatment processes. With the emergence of better steels with improved cleanliness and development of modern heat treatment processes and technology, scope of application of steels for engineering solutions have got vastly enlarged. Because of the developments in heat treatment and surface engineering, steels are working miracle in facilitating developments in automobile technology, rocket engineering, chemical processing, and in many other trying and difficult engineering applications. Steel is characteristically a very amenable material for fabrication by shaping, machining or welding – allowing easy fabrication and manufacturing of different shapes and components. Steel loses its strength with temperature, allowing manufacturing of complicated shapes by hot-working (e.g. forging or rolling). Similarly, steels of appropriate composition can be shaped by cold forming; if necessary by giving a prior heat treatment for softening and conditioning the steel structure / properties. Further, steel parts and shapes can be easily machined to precise dimensions by using fast-production machine tools. During these steps for manufacturing or fabrication, there could be accumulation of internal stresses (residual stresses) in the steel parts, which can be easily removed or eliminated by lower-temperature heat treatment (called stress relieving) while being processed or after the process. If the steel parts require alteration or development of superior properties than as-formed, the parts can be further subjected to appropriate heat treatment for that purpose. Thus, in combination with the properties of steels for easy fabrication / manufacturing and flexible heat treatment, steel becomes a dominant player in the field of engineering and structural materials. And, much of the advantages of steels come from its amenability to different heat treatment operations, such as stress-relieving, annealing, normalising, hardening, surface engineering, etc. Moreover, steel grades are now available with more uniformity in composition, less impurity and inclusion content – offering even better properties than before. Hence, the study of heat treatment in relation to steel processing and application forms an integral part of steel metallurgy.

1.4.2

Properties of Steels

Superiority of steel is not only for its excellent mechanical properties helped by heat treatment, but also for its superior physical (intrinsic) properties and good chemical properties. Table 1-4 sums up the important properties of steels and their controlling factors.

High modulus of elasticity, high shear modulus, superior mechanical properties, and availability of excellent corrosion and oxidation resistance properties by using specially alloyed Cr-Ni-Mo steels make the steel a unique material where all combination of properties required for vast majority of industrial applications are obtainable. Additionally, steel offers high strength to weight ratio, which makes it economically very competitive in the world of materials. However, the most significant characteristics of steel are its response to varieties of heat treatment processes, developing near-exact end-use specific mechanical properties for wide ranging application of steels. Major factors that contribute or control the mechanical properties of steels are: • The chemical composition of the steel, which can influence the mechanical properties by way of facilitating development of different

microstructures upon cooling or heat treatment, e.g. ferrite and pearlite or martensite and bainite, etc. • Microstructure of the steel, which could be changed or developed as per requirement by subjecting the steel to different heat treatments. [Microstructure of steel includes the microstructural phases, grain size, and the shape, size, distribution, and type of inclusions; amongst these, microstructural phases and grain size are controllable by heat treatment, but the inclusion factor needs to be controlled by the use of proper steel grade and quality.] With regard to the development of mechanical properties, both are necessary; one (the chemical composition) is the means and the other (microstructure) is the end. And, the path to reach the end (microstructure) is the heat treatment. Thus, heat treatment plays a very critical role for the success of uses / applications of steel. For all practical purpose, change in the microstructural characteristics of steel is central to the steel properties, and this is brought about in steel by different thermally assisted heat treatment processes. Importance of microstructure in steel is applicable not only for engineering and structural applications – concerning the load bearing capacity of steel via its strength and toughness – but also for wear and heat resistance properties.The latter properties of steels are also influenced by the microstructures as produced by heat treatment. However, for wear and heat resistance, steel chemistry needs to have special alloying for wear and oxidation resistance, respectively. Steels for wear resistance require microstructure with high hardness and plenty of stable carbide precipitates in the matrix for wear resistance. Similarly, for heat resistance, steel should have appropriate alloying to make it resistant to oxidation and softening at higher temperature by the presence of chromium and complex chromium carbide precipitates in the matrix. For such purposes, use of steels containing chromium (Cr), tungsten (W), and vanadium (V) are preferred for their contribution to high temperature strength and oxidation properties. Adjusting chemical composition that can form stable and hard complex carbides, and controlling the microstructure for uniform dispersion of fine carbides by appropriate heat treatment are known to be the best solution to wear resistance. For heat resistance steel, presence of high temperature oxidation inhibiting elements – like the chromium – and simultaneous presence of alloying that make the steel resistant to softening at higher operating temperature are necessary. But, in both cases, despite having the right chemistry, the steel needs to be heat treated for development of right microstructure as mentioned earlier. In other words, steel needs to

have right microstructure in addition to the right chemistry for fulfilment of the application requirements. Many a time, development of required properties may require the help of prior working to make the structure fine or responsive to the heat treatment for a finer structure. For example, for development of right microstructure in high alloy steels containing carbide-forming elements like Cr, Mo, W, etc., the steel should be given prior hot working of sufficient degree before heat treatment – in order to make the carbide size finer and more uniformly dispersed for improved results. Thus, by combining the composition, mechanical deformation and heat treatment, steel can be made suitable for many challenging applications. Table 1-5 provides a glimpse of how different microstructural features in steels influence the various forming and application related properties that are commonly encountered. The other features of steel are the presence of grains (i.e. grain boundaries) and inclusions. These microstructural entities are always present in steel,

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and influence the mechanical properties significantly. While grain boundaries (hence, the grain sizes) can have beneficial roles in the deformation and transformation behaviour of steels, inclusions generally have adverse effect on the mechanical properties and heat treatability of steels. However, for heat treatment of steel, both grain size and inclusion level have to be controlled for optimum results. [This has been discussed in more details in Chapter 4: Heat Treatability of Steels] The table indicates broad influences of the microstructural constituents as per their individual characteristics, which may differ when these microstructural constituents are present together in different combination, proportion, and distribution. Exact effects in hardened steel will depend on: • Chemical composition and the volume fraction of different microstructural constituents in the steel • Formation, location and distribution of these phases / constituents • Nature of the bainite and quality of tempering of martensite • Size and uniformity of grains • Chemistry of the inclusions (e.g. oxides, silicates, sulphides, etc.), inclusion size and shape, and distribution. Controlling and regulating these features in the steel, therefore, depend on the quality of steel and the steel composition (grade), in one hand, and on the quality of heat treatment, on the other. In fact, for a given steel composition and its initial grain size and inclusion level, the task of producing the microstructural features that give optimum level of mechanical properties rests on the quality of heat treatment. Fortunately, steel offers high flexibility with the wide choice of composition and matching heat treatment, enabling the development of right microstructural morphology for optimum properties for wide ranging uses and application of steels.

1.4.3

Influence of Grain Size and Inclusions on the Properties of Steels

Table 1-5 includes the influence of grain size and inclusions on different forming and application specific properties of steels. Hence, a brief introduction to these features of steel microstructure is warranted.

Influence of grain size Grain is a physically distinguishable microstructural feature in steels that get revealed when a steel surface is polished and chemically etched. What we see on the polished surface under microscope is dark outline of two dimensional areas of grains, but actually grains are a three-dimensional

Fig. 1-7 Ferrite grain boundaries in low-carbon steel (x200)

geometric feature in the structure with two dimensions seen on the flat surface and the other dimension inside the body. Figure 1-7 illustrates such two dimensional grain boundaries in a microstructure as we observe them under microscope. Patches of pearlite (dark) can be seen on the ferrite grain boundaries, indicating that carbides tend to precipitate on grain boundaries and on the triple point of grain boundary junctions. Grain boundaries are the meeting points of different crystals that constitute the matrix. As such, they are discontinuities in the matrix of crystals with different orientation. Grain boundaries, therefore, act as obstacle to dislocation movement from one crystal to another, which is necessary for continuation of plastic flow of metal while under deformation. This means that presence of more number of grain boundaries (i.e. higher grain boundary area) will offer higher resistance to plastic flow (i.e. resist plastic deformation) implying higher strength in steel. Thus, more number of grain boundaries in a given volume – meaning finer grain size – would lead to higher resistance to deformation (i.e. higher strength). Hence, finer grains in steel are a source of higher strength and better ductility; the latter factor arises from the ability of fine grained steel to absorb higher deformation energy. Figure 1-8 illustrates the influence of grain size on the yield strength of steels. But, there are exceptions to the good effects of finer grained steels. For example, finer grains are not favourable for good hardening response in steels; because grain boundary areas can act as sources of ferrite and carbide nucleation during transformation of steel, if the cooling rate lags, even marginally, at any point in the steel cross-section. This may lead to some spots with softer pearlite or other structure in the steel despite quenching for hardening. Thus, presence of large grain boundary areas (i.e. fine grain size) may make 100% martensite formation somewhat

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450 375 300 225 150 75

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5 40

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20 2.5

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Fig. 1-8 Graphical relation showing the increase of yield strength in steels with decreasing grain size

difficult in steel. Hence, though fine grain size is favourable for mechanical properties, it is not helpful for hardening of steels. Similarly, fine grain size is not favourable for machining. Machining involves cutting the metal by tipped tools under force. Because of the fact that grain boundaries increase strength of the steel, finer grain size will mean higher strength and higher cutting forces during machining, which will cause higher friction and frictional heat at the tool-chip interface, leading to tool wear. Hence, for smooth machining coarser grain sizes, where grain boundary areas are less, is favoured. Figure 1-7 also indicates that grain boundaries could be a favourable source of precipitation of other phases. The figure illustrates that carbide – a second phase in the steel – has mostly precipitated on the grain boundaries and its junctions. This situation is often gainfully exploited during some heat treatment operations (other than hardening) for producing finer microstructure. But, on the other hand, grain boundaries having higher concentration of impurities may decrease the corrosion resistance of steel. Thus, decrease in grain size might lead to decrease in corrosion property – excepting for components susceptible to stress corrosion cracking where fine grained steel is preferred, because stress corrosion cracking susceptibility increases with increase in grain size. Means of controlling the grain size in steel is by controlled de-oxidation during steelmaking by aluminium or aluminium-silicon killing. Al-killed steel tends to pin the grain boundaries by aluminium nitride (AlN) particles – which form due to chemical reaction between excess Al and N (nitrogen) in the liquid steel. These AlN particles are stable at the heat treating temperature and, as such, they are effective in pinning and restraining the grain growth during austenitisation for hardening.

Influence of inclusions Inclusions in steel originate from the impurities and gas reactions during steelmaking process; however, the process of complete removal of inclusions from steel is extremely difficult and expensive. Inclusions can be of different types as per their chemistry; e.g. oxide, sulphide, nitride, etc. Oxide inclusions again can be of silicate and other types, e.g. SiO2, CaO, MgO, TiO2, etc. However, oxide and sulphide types of inclusions dominate the commercial quality steels. Figure 1-9 illustrates the appearance of these types of inclusions.

Fig. 1-9 Inclusions in steel: (a) sulphides in light grey with elongated shape; (b) oxides in dark grey with broken chains. Oxide and sulphide inclusions can co-exists as appears in (b)

Effects of inclusions in steel vary with its type, size, shape and distribution. In general, all inclusions are deleterious for mechanical properties of steels; some are more (like the oxides) and some are less (like the sulphides). They are also harmful for heat treatment of steels, acting as sources of hardening cracks. Hence, either for improved mechanical properties or for heat treatment, inclusions levels in steel must be controlled below certain level. Table 1-6 illustrates the limits of generally acceptable inclusion level in steels for different route of

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Since the origin of inclusions is the steelmaking process, inclusion content and level in finished steel depends on the steelmaking route adopted for steel making. Oxygen steelmaking process (e.g. LD steelmaking route) tend to give higher inclusion level than the secondary treated (e.g. use of ladle furnace for steel refining) steel or vacuum degassed steels. As such, the acceptable level of inclusions varies with the steelmaking process, as indicated in Table 1-6. Corollary of this is that for controlling the inclusion level in steel, steelmaking route has to be specified for steel grades. For evaluation and acceptance purpose, inclusion types are classified under a common ‘inclusion rating chart’ covered under the American Standard, ASTM E-45. ASTM E-45 categorise inclusions into four groups, namely sulphides (A type), Alumina (B type), silicate (C type) and globular oxide (D type). Latter type consists of soft oxides – like calcium oxide, magnesium oxide, etc – which generally appear globular on the polished and etched surface. Of the inclusions types, oxides of aluminium (B type) and silicon (C type) are most harmful for application of steels, as they occur in irregular and angular in shape and can act as sharp notch in the hardened steel matrix. Globular soft oxides (D type) are less harmful for most applications, but bad for contact fatigue resistance. Because of the softness of this type of inclusion, they tend to get crushed under contact fatigue load and give rise to pits, leading to pitting failure. Sulphide inclusions (A type) are least harmful, excepting that presence of sulphide inclusions can lower the transverse notch toughness of steels. However, as regards heat treatment, most inclusions are harmful as they tend to promote micro-cracks from their irregular interfaces with the steel matrix under the quenching stresses. However, inclusions in steel cannot be totally avoided; it can be brought down in steel, but cannot be totally eliminated under the common commercial steelmaking processes. Hence, for mechanical properties as well as for heat treatment, harmful nature of inclusions can be best controlled by controlling their volume fraction in the steel as well as by controlling their shape, size and distribution. Finer size, globular or rounded shape and uniform dispersion in the matrix are known to be less harmful than otherwise. This is reflected in Table 1-6, where inclusions are marked as ‘thick’ or ‘thin’ as indication of size, and the number for each size mentioned below correlates to their degree of dispersion. Origin of inclusions is in the steelmaking process. Different steelmaking processes give different inclusion level in the steel; best result (i.e. minimum inclusion level) is obtained by vacuum degassing the steel and worst result is obtained when the steel is made through normal ‘air melted’

route where plenty of oxygen is available for oxidation reactions and inclusion formation. Hence, depending upon the severity of applications and the nature of heat treatment, steelmaking route is specified for controlled inclusion level in the steel. [Influence of inclusions in the heat treatability of steels has been further discussed at length in Chapter 4: Heat Treatability of Steels.] Thus, grains and inclusions are an integral part of steel structure and exist in steel alongside other microstructures. Their effects on steel properties have to be considered along with the presence of other microstructural constituents. Table 1-5 shows the qualitative effect of all such micro constituents present in steel structure on different properties of steels, but these are the effects if and when they are present individually. But, that is not the case in practice most of the time; various microstructural constituents are present together in the structure and exert their combined or synergistic effects as discussed earlier. Understanding of steel characteristics and intricacies of heat treatment for optimum properties in steel is, therefore, of significant importance. Foregoing discussions lead to the fact that controlling steel properties by manipulating microstructure and microstructural constituents is a tricky task. For getting the optimum combination of properties, steel composition, steelmaking route and steel rolling process, as well as heat treatment process have to be carefully chosen and executed. By controlling these factors and basic internal quality of steels (i.e. their internal soundness and cleanliness), properties of steels can be very well controlled and optimised for most areas of steel applications with the help of heat treatment.

1.5

STRUCTURE-PROPERTY RELATIONSHIP IN STEELS: IMPORTANCE OF HEAT TREATMENT

Steel is a structure-sensitive material. This implies that if the microstructure of the steel can be changed, its properties (i.e. mechanical properties) can be also changed. This is where the role of heat treatment comes handy for changing the structure, and thereby changing the mechanical properties of steels. Experience and experiments demonstrate that slightest variation in the microstructure of steel can lead to considerable variation in mechanical properties. Microstructural variation can occur in steel due to various reasons, which are as follows: • Changing chemical composition • Mechanical or thermo-mechanical working

• Different heat treatment given to the steel (e.g. annealing, normalising, hardening, etc), and • Variation of heating and cooling cycle/rate during a given treatment or process. Most of these events – excepting change in chemical composition and cold (mechanical) working of steel – involve some kind of heating and cooling. In fact, all steels that we use or come across for uses, in practice, have been subjected to some kind of heating and cooling with or without mechanical working, and the microstructure that we observe in these steels is an outcome of that heating and cooling cycle, with or without mechanical deformation. For example, as-rolled steel that is bought from the market has the structure typical of the composition and the rolling condition and temperature that have been applied in the steel mill. Similarly, as-forged components that are procured for machining have the structure typical of chemical composition (including the way steel has been de-gassed or killed) and the finish forging temperature. In general, structure that gets developed in steel can be considered as the result of composition and heating / cooling cycle that the steel received during processing, which includes separate heat treatment operation. Perhaps, the structure–property relationship in steel can be best illustrated with the help of Fig. 1-10, which relates to the variation of properties in wrought steel (i.e. non-heat treated steel) with increasing carbon content. The figure shows that depending on the carbon content in the steel, structure can change from 100% ferrite at very low carbon level to increasing pearlite content in the structure with increasing carbon level – reaching to 100% pearlite at about 0.80% C. Beyond 0.80% C, which happens to be the ‘eutectoid carbon level’ in plain carbon steel, cementite starts appearing in the structure in addition to pearlite. The figure also shows that as the structure changes form ferrite to (ferrite + pearlite) and then to 100% pearlite with increasing carbon content in this wrought steel, the mechanical properties also changes. A study of Fig. 1-10 shows that with increasing pearlite content in the steel (due to increasing carbon), tensile strength and hardness continue to increase till the carbon level of 0.80% is reached, beyond which a separate phase – iron carbide (Fe3C) – starts showing up. With the appearance of iron carbide at 0.80%C, while tensile strength starts dropping mildly, hardness value continues to increase. Percent elongation in the steel (a measure of ductility), which is an important part of mechanical property, shows sharp decrease with increasing pearlite content till about the same level of carbon (0.80%C) where tensile strength starts dropping. With further increase of carbon in the steel, microstructure changes from 100%

600

ile

ns Te

30

20

n

tio

200

ness Hard

10

Elongation per cent

400

0 100 80 60 40 20 00

40

e str

ga

Pearlite %

th ng

800

on El

Tensile strength (MPa) Hardness Hb

1000

Cementite 0 Ferrite

0.2

Pearlite 0.4

0.6

0.8

1.0

1.2

1.4

Carbon per cent

Fig. 1-10 Graphical presentation of variation of microstructure (volume fraction of ferrite and pearlite) with carbon and changes of properties in wrought steel. [1kg/mm2 = 9.85 MPa] (Source: Metallurgy for Engineers, E.C. Rollason, 1973)

pearlite to (pearlite + carbide), and due to this structural changes, tensile strength and elongation tend to drop with increasing carbide content. The reason for change in the trend of variation between tensile strength and hardness with the appearance of carbide in the structure is the way these two parameters are tested and measured. Carbide being harder phase than pearlite, it increases the hardness, but being brittle compared to pearlite, it leads to lowering of fracture strength in tensile testing. Similar trend for change in properties with structure is also true for steel in hardened condition, i.e. after heat treatment; vide Fig. 1-11. On quenching for hardening, steel may either form 100% martensite or martensite mixed with bainite, depending on the composition of the steel and the cooling rate. Figure 1-11 shows increase of hardness in VPN scale with increase of carbon (in both plain carbon and alloy steel) and the description of corresponding microstructure. This figure provides the following information about martensitic structure and related properties in steels: • For a given % carbon in either plain carbon or alloy steel, the strength (hardness) varies with the structure; martensite has much higher hardness than bainite • As the % carbon increases, hardness of martensite or bainite also increases till about 0.70% carbon (approximately). Beyond 0.70%

Fig. 1-11 A graphical presentation of change in hardness values of martensitic and bainitic structures with increasing carbon in steels, and the relative hardness values of such structure (Source: Metallurgy for Engineers, E.C. Rollason, 1973)

carbon level, retained austenite starts forming in the quenched steel, especially if it is an alloy steel. • With change in structure due to appearance of ‘retained austenite’ (a form of untransformed austenite), hardness in martensite starts falling, because ‘retained austenite’ is a soft phase with lower hardness. As regards bainite structure in the steel, the level of hardness is much lower than martensitic structure; yet even in this structure, hardness varies with carbon content in the bainite – increasing with increasing carbon. [The reasons for change in strength (hardness) with carbon in martensitic and bainitic structure have been discussed in Chapter 2 – along with their formation, nature and morphological character.] Thus, in both non-heat treated and heat treated steels, mechanical properties of steels vary with the structure. In general, while strength of steel increases with the increasing percentage of pearlite, bainite or martensite in the structure, elongation (the measure of ductility) and toughness decreases simultaneously with the increase in strength; the effect of increase of strength is highest with martensitic structure, next comes contribution of bainite, and then the pearlite. However, with the compositional character of the steel and the cooling rate of formation

of these structures, morphological character of these microstructures – namely pearlite, bainite or martensite – changes appreciably. Thus, by changing the compositional character and cooling rate during heat treatment of these steels, their microstructural morphology and properties can be further changed. Morphological changes in these microstructures primarily occur due to: (a) nature and character of carbides – which could be plain iron carbide or complex alloy carbide depending on the steel composition, (b) shape, size (fineness) and distribution of these carbides, and (c) nature of ferrite associated with these structures. More acicular is the ferrite and finer and more uniformly dispersed is the carbide, higher is the strength of steel. Figure 1-11 depicts the variation of hardness of martensite with carbon, which is without tempering. Property wise, as-quenched martensite is very hard, but brittle too. Therefore, again by manipulating the structure of as-quenched martensite (which is a highly distorted tetragonal ferrite structure due to super-saturation of carbon atoms) by tempering, some amount of toughness in the steel can be induced at the sacrifice of some strength. How much increase of toughness and consequent lowering of strength takes place by tempering martensite structure depends on the temperature of tempering. Higher the temperature of tempering more is the carbide precipitation and coarsening of carbide. Tempering temperature is the means to induce and control the carbide precipitation from the martensite, i.e. to bring out the desirable structural change for obtaining the change in properties (toughness). Tempering temperature facilitates the diffusion of carbon atoms from distorted martensitic structure and, thereby, recovers some toughness while strength drops. Thus, here again, by tempering the martensite, morphological nature of the martensitic structure can be manipulated and desirable properties can be achieved. The foregoing observations from Figs. 1-10 and 1-11 clearly demonstrate that at every stage of steel uses and applications, its mechanical properties can be controlled by controlling the structure and structural morphology. Advantage of steel is that irrespective of the constituents of its microstructure, its morphological nature with respect to shape, size, distribution and character can be manipulated by appropriate combination of chemical composition control and right kind of heat treatment. This gives rise to varieties of combination of properties for industrial applications. For example, by using right kind of alloy steel, nature of bainite can be changed from upper bainite to lower bainitic morphology for a given heat treatment, and, thereby, the strength and toughness of the steel can be

vastly improved compared to that available with upper bainite. Primary structural difference between upper and lower bainitic structure is that carbide that precipitates during heat treatment, in case of upper bainite occurs on ferrite plate boundaries while it occurs all over the ferrite plates, including the boundaries, in case of lower bainite. The difference in strength and toughness between these two structures in a given steel composition is due to this change of precipitation characteristics. Similarly, difference in properties of pearlitic structure – produced by different heat treatment – is primarily due to the degree of fineness of inter-lamellar spacing in the microstructure; finer the spacing higher is the strength. In fact, by making pearlite finer by faster cooling (e.g. by air-cooled normalising process), both its strength and ductility can be improved. Structure–property relationship in steel, has, therefore, been a principal guide to the choice of steel composition and the heat treatment for required end-use properties. Choice of heat treatment could be based on how economically the end-use properties can be developed in the given steel. On the other hand, a structure–property relationship diagram like the one in Fig. 1-11, can be a guide on the choice of steel composition, if minimum properties to be achieved are known. For example, it is generally considered necessary to have as-quenched martensite hardness of 580 to 600 VPN for tempering and toughening the steel. If the as-quenched martensite hardness is much lower, then the scope of tempering gets limited and recovery of toughness may not be adequate, i.e. toughness of the steel will suffer. It is in this context, information provided by structure–property diagram, like the one in Fig. 1-11, can help by guiding to the correct steel composition. The figure indicates that a minimum carbon level of 0.35% would be necessary in the steel (as indicated by vertical line in the figure) to avoid such problems or for attaining the aforesaid hardness value of 580 VPN in the as-quenched martensite structure. In practice, the required carbon level may be a bit lower if the steel contains some alloying elements. Thus, given the required properties, composition of the steel can be decided with the help of structure–property data. Similarly, if the composition is known, then heat treatment parameters can be set from the specified properties in order to produce appropriate structure. Considering from the angle of structure–property relationship, considerable emphasis is given to the heat treatability of steel, especially with respect to the carbon composition. If the steel does not have adequate carbon, its ‘hardenability’– a measure of the ability of the steel to respond to proper hardening by developing a minimum level of hardness – suffers and the steel becomes bad for hardening. [More

about the heat treatability of steel has been discussed in Chapter 4.] Therefore, all hardening grade steels require sufficient level of carbon, or carbon and alloy mix, in order to get martensite structure formation of right hardness, which after tempering can give the combination of right strength and toughness required for an end-use. In sum, it is the microstructure of the steel that determines its mechanical properties, and the necessary microstructure in the steel is developed by choosing the right kind of steel composition and heat treatment. If the structure changes then the properties of the steel also change. Task of heat treatment is in ensuring the right structure for right properties for a given steel composition. However, in Figs. 1-10 and 1-11, effect of grain size – which is a structural feature in steels – has not been indicated separately. Grain size is a part of microstructural features of steel and exerts considerable influence on properties of steels, such as the yield strength and elongation (or toughness). Finer the grain size more is the contribution to strength, elongation or toughness. Grain size also considerably influences the impact strength of steels. Influence of grain size, which is always present in steel microstructure, is mostly due to its contribution to yield strength and through the uniform elongation. The relationship between yield strength and grain size is described mathematically by the Hall–Petch equation: ky __ sy = s0 + ___ ÷d Where sy is the yield stress, s0 is a materials constant for the starting stress for dislocation movement (or the resistance of the lattice to dislocation motion), ky is the strengthening coefficient (a constant unique to each material), and d is the average grain diameter. Following this relationship, a material can be made infinitely strong, in theory, if grain sizes can be made infinitely small – but the fact remains that an inverse trend is observed in this regard when grain size reaches the nano-size range. Thus, mechanical properties of steel are strongly influenced by the type of microstructures – including the grain size and inclusions. Microstructure can have different morphological character due to composition and heat treatment difference. For example, inter-lamellar spacing of pearlite changes with increasing carbon content and faster cooling rate, giving rise to higher strength in steel. That is why steel of same composition gives higher strength after normalising than annealing, which is a slower cooling process. Similarly, alloy steel of same composition may produce bainite of different structural morphology, namely upper bainite or lower bainite, when cooling rate changes (and

does not match the requirement for 100% martensite formation). Under this circumstance, relatively faster cooling in heat treatment of the given alloy steel will produce more of lower bainite in the structure than the upper bainite, which are morphologically very different from each other. Lower bainite in the structure in place of upper bainite gives rise to higher strength and toughness values. If the same steel is cooled even at faster rate than that forms lower bainite, martensite with acicular ferrite plates will form, which is even stronger and tougher (after tempering) than the lower bainite. Again, with improved composition and faster cooling, the steel can produce needle shaped acicular ferrite martensitic structure with internal twins, which is even more strong and tough than the martensite of acicular ferrite plates. Thus, steel offers wide opportunities for tailoring its structure and properties, and, thereby, helps to cater to the exacting needs of different engineering or structural applications. The structure-property relationship in steel, along with the possibility of producing different structures and structural morphology with the help of composition adjustment and heat treatment, provides umpteen numbers of opportunities to tailor the properties of steels for wide ranging applications. Relevance and importance of heat treatment of steel arise from this need of tailoring the structures for improvement of properties for different applications. Heat treatment, thus, plays a critical role in shaping the usefulness and utility of steels in modern industrial applications.

Summary 1. Understanding the characteristics and behaviour of steel is an important part of the ‘heat-treatment of steels’. Therefore, the chapter has dealt with the relevant aspects of steels, such as their crystal structure and phases, types and grades, characteristics and properties, and the importance and significance of structure– property relationship in steels. 2. Physical nature of steel, its main crystal structure forms, limitation of solid solubility restraining the composition limits in steels and influencing the microstructure formation has been discussed and illustrated with reference to heat-treatment processes. 3. Definition and distinction of plain carbon and alloy steels, their types and sub-groupings, classification and designation system, and their uses and utility have been discussed and demonstrated with examples. Importance of composition of steel for heat-treatment, their response and resultant microstructure have been discussed. 4. Uniqueness of steel regarding its properties and heat-treatment has been highlighted with examples. Importance of different property types (e.g. physical, chemical and mechanical) in steel and their controlling factors have been mentioned and illustrated. Characteristics of steels with reference to composition and heat-treatment for developing different mechanical properties have been

discussed, and qualitative effect of microstructures on different engineering properties of steels have been mentioned. 5. Finally, the structure–property relationship in steel has been highlighted, demonstrated and discussed to show versatility of steel for catering to wide ranging properties for applications.

References / Suggested Reading Avner, Sydney H. Introduction to Physical Metallurgy, Tata McGraw-Hill Education, New Delhi, 1997 Bhadeshia, H. and R. Honeycombe, Steels - Microstructures and Properties, ButterworthHeinemann, Oxford, 2006 Higgins, R. A. Engineering Metallurgy (Applied Physical Metallurgy), Viva Books, New Delhi, 2006 Leslie, W.C. The Physical Metallurgy of Steels, Hemisphere Press, McGraw-Hill, New York, 1981 Mandal, S.K. Steel Metallurgy: Properties, Specification and Applications, McGraw-Hill Education, New Delhi, 2014 Reed-Hill, Robert E. Physical Metallurgy Principles, Van No strand Reinhold Co., 1973 Rollason, E.C. Metallurgy for Engineers, The English Language Book Society & Edward Arnold (Publishers) Ltd., 1973 Smallman, R.E. Physical Metallurgy and Advance Materials, Butterworth-Heinemann, 2007

Review Questions 1. Define steel and identify few important characteristics of steel. Briefly point out how heat treatment can help improving the uses and utility of steels. 2. What is a ‘solid solution’ in steel? What types of solid solution are observed in steel and how do they form? Highlight the rules of solid solution formation in steels. 3. What is ‘equilibrium cooling’? With reference to Fe-C equilibrium diagram (Figure 1-3), how many phases could be present in slow equilibrium cooled steel? 4. What does ‘critical temperature’ signify in the phase transformation of steel? What are the important ‘critical temperatures’ in steel as per Figure 1-3? Point out the characteristics of ‘lower critical temperature’ of steel with reference to phase changes during equilibrium cooling. 5. What microstructures form in steel when the steel is (a) cooled slowly under equilibrium condition, and (b) cooled faster (by quenching)? By referring to Figure 1-4, point out the nature and character of different microstructures presented there. 6. What is the purpose of heat treatment of steel? List the factors that ultimately influence the development of microstructures in steel after heat treatment. 7. How steels are grouped? Name the important groups and their sub-groups in steel. What is the range of carbon content in plain carbon steels? What happens

8.

9. 10.

11.

if carbon content exceeds the higher range and where is it used for? Differentiate the cast iron from steel as regards their microstructure. How ‘residual elements’ arise in steel? What should be the limits of residual content in steels? Name few common alloying elements in alloy steel? What are the limits of total alloy content in low-alloy and high-alloy steels? Briefly discuss the roles of composition, grain size and inclusion content in the heat treatment of steels. Briefly discuss the importance of microstructure in steels. What should be the conditions of cooling in the heat treatment of 0.45% carbon steel for the development of: (1) finer pearlitic structure, and (2) 100% martensitic structure? Discuss – with examples – the relevance and importance of heat treatment of steel from the view point of its structure-property relationship.

Phase Transformations in Steel: The Basics of Heat Treatment Principles

2.1

INTRODUCTION

Chapter 1 discussed the steel characteristics with respect to its microstructural features and their influence on different mechanical properties (vide Table 1-5). It has been further brought out in Chapter 1 that the purpose of heat treatment of a given steel composition is to develop the required microstructure for attaining the right set of mechanical properties. Microstructure formation, which is at the centre of all heat treatment process objectives, is dependent on the phase transformation behaviour of the steel. Therefore, phase transformation rules in steel form the basis of heat treatment processes and principles. This chapter, therefore, discusses the rules of phase transformation in steel and the factors that control the formation of various microstructures and microstructural character. When steel undergoes heating to higher temperature, original structure of the steel changes, giving rise to new phase formation. The change is dependent on the characteristics of steel composition and the temperature of heating; vide the iron-carbon phase diagram in Fig. 1-3. With reference to this figure, if the steel is heated above the upper critical temperature of the steel (A3 temperature), it would be 100% ‘austenite’. Formation of this austenite is the first step in the process of all high temperature heat treatment processes – such as annealing, normalising, hardening, etc.

Austenite is a high-temperature phase of steel, having FCC structure (g -phase), and forms when heated above the ‘critical temperature’ of the steel. The formation begins when heating temperature reaches the level of lower critical temperature (A1) and ends on crossing the upper critical temperature (A3) of the steel. This high-temperature austenite is generally described as the ‘parent austenite’, having the same composition as the original steel. When the parent austenite phase (mother austenite) is cooled, it transforms to different phases, depending on the temperature of heating and cooling rate. This phenomenon of change of parent austenite on transformation during cooling is studied through the process of ‘austenite decomposition’. Austenite decomposition study provides all necessary information about what phases and structures could possibly form in a steel of given composition by following a specific cooling cycle/rate. Thus, understanding transformation behaviour of steels becomes a critical part of the study for heat treatment of steels; the aim of the process being to bring about a favourable change in microstructure through phase transformation. As such, the rules for phase transformation of steel act as the basis of heat treatment principles; guiding and laying out the basic rules for heat treatment operations. The microstructures that are observed after the heat treatment are the products of phase transformation processes in steels. By selecting an appropriate steel composition and controlling the phase transformation process, right combination of microstructures can be developed in steel for meeting the challenges of different combination of properties. Some such examples of diverse properties of steels are: combination of low strength and high ductility; high strength with high toughness; high strength at higher temperature; high fracture toughness at lower temperature; high wear resistance with high fatigue strength, etc. This chapter, therefore, discusses the metallurgical principles of austenite decomposition and different phase/structure formation in steels under different cooling conditions, and correlates the applications of those rules of phase transformation and structure development to different heat treatment processes.

2.2

PHASE TRANSFORMATION AND STRUCTURE FORMATION IN STEELS

Phase transformation and structure formation in steel are guided by the composition of the steel and cooling condition. Iron-Carbon diagram

is the starting point for such phase transformation study, representing the phase transformation under equilibrium or normal cooling. While such an Fe-C diagram has been illustrated and explained in Fig. 1-3, a simpler schematic version of the same is presented here in Fig. 2-1. Vertical and horizontal double arrow lines in this figure represent how phases can change with change in temperature and composition (% carbon) respectively. Liquid + g 1147°C 910°C

A3 Austenite g Austenite + Ferrite a A1 Ferrite

Austenite + Cementite Eutectoid 723°C Cementite

Ferrite + Cementite

0.02%

0.8%

2.0%

Fig. 2.1 Schematic representation of Fe-C diagram, illustrating stability of different phases at different temperature (Y-axis) and composition (X-axis) under equilibrium cooling

Following the vertical double arrow line – representing steel of about 0.50% carbon – it is obvious that the steel will convert to 100% austenite (g ) on heating to above A3 temperature of the steel. Composition of such austenite that forms on heating above the upper critical temperature becomes the same as original steel (on soaking for a while) and this is the starting material for all transformation processes in steel on cooling. Austenite is a high temperature phase in steel; it is not stable at temperature below the lower critical temperature (A1), unless the steel is very rich with alloying elements like Cr, Mn and Ni, as in austenitic stainless steels. Therefore, on cooling, austenite decomposes to other phases that are stable at lower temperature. Such phase changes – and the formation of resultant microstructure – on normal/equilibrium cooling can be followed through the Fe-C diagrams as in Fig. 1-3 or Fig. 2-1. IronCarbon (Fe-C) diagram represents different phase changes in steel with

changing temperature or composition, whichever is of interest for the study. If a steel of 0.50% carbon is considered for illustrating the phase changes, then the figure indicates that the steel will get converted to austenite on heating above the A3 temperature. However, the conversion to austenite would not be instantaneous; it would be a gradual process. According to Fe-C diagrams, austenite will start forming on heating the steel as soon as A1 temperature (723°C) is crossed and continue to progressively form, reaching 100% level on crossing the A3 temperature of the steel. This high temperature austenite is the starting point for all transformation processes in steel on cooling. Since the process is involved with decomposition of austenite to new phases, the process is called ‘austenite decomposition’. Different phases that form on decomposition of the parent austenite are functions of cooling rate and its control, apart from the steel composition. If cooled under equilibrium cooling condition (i.e. slowly cooled), the austenite will decompose to ferrite only, if carbon is less than 0.02%, vide Fig. 2-1; however, if carbon exceeds this limit – which is the case most of the time – austenite will decompose to ferrite and carbide. But, the structure that forms from such phase transformation would be (ferrite + pearlite), where pearlite is the product of an intimate mixture of (carbide and ferrite); vide discussions in Section 1.2. For 0.5% carbon steel, austenite decomposition under equilibrium cooling will lead to a structure as represented in Fig. 1-4(a) in Chapter 1. This micrograph shows that the structure consists of large volume fraction of pearlite (which a mixture of carbide and ferrite in lamellar form) and some primary ferrite. Primary ferrite* in this structure arises from pro-eutectoid separation from austenite in the temperature range between A3 and A1. Due to this pro-eutectoid separation of ferrite, which has very limited solubility of carbon in it, the remaining austenite becomes gradually richer in carbon with increasing ferrite formation. Finally, the composition of austenite reaches the eutectoid carbon level with decreasing temperature on equilibrium cooling, and the carbon-rich austenite then starts decomposing to ‘pearlite’ – on reaching A1 temperature (723°C of Fig. 2-1). Figure 1-4(b) provides the magnified view of a pearlitic filed in the structure, showing the intimate lamellar structure of carbide and ferrite lamellae. Lamellar structure of carbide and ferrite – leading to the formation of pearlite – forms by alternate precipitation of carbide and ferrite from the * This ferrite is termed as primary ferrite as it forms first in the decomposition process by proeutectoid separation.

decomposing austenite. Pearlite formation and its control is an important step in the heat treatment of steels. Therefore, mechanism of formation of pearlite has been further discussed and illustrated in Section 2.3. However, decomposition of austenite to ferrite and pearlite is governed by diffusion of carbon atom in the austenite matrix, which is a slow and time-dependent process. Therefore, if the cooling rate is sufficiently fast – without allowing any time to carbon atoms for diffusion – the austenite will be forced to transform to ‘martensite’, a highly distorted structure with super saturated carbon, through a diffusionless process. Martensite structure formed by this diffusionless process is a highly distorted structure with super saturated carbon. This distorted structure of martensite arises from fast cooling rate when carbon atoms from austenite do not get time to diffuse out and, at the same time, austenite is no longer stable at the temperature it has reached. As a result, carbon-rich austenite with carbon atoms trapped inside its lattice structure is forced to transform – giving rise to a distorted a-lattice structure with super-saturated carbon in it. This makes martensite structure very hard and brittle, requiring tempering to reduce brittleness and improve toughness. Figure 2-2 illustrates a lightly tempered martensitic structure.

Fig. 2-2 A lightly tempered martensitic structure in 0.35%C File steel (x500) [Note the orientation of martensitic needles, which illustrates that formation of martensite is confined within the prior austenite grains]

Formation of martensite structure – for strength and toughness in the steel – is central to the process of steel hardening. Hence, austenite decomposition under non-equilibrium cooling for formation of martensite has been discussed in more details in Section 2.4. However, if the cooling rate is intermediate between equilibrium cooling and fast cooling that produces martensite, the austenite may transform to an intermediate product – called ‘bainite’ – if the cooling is isothermal or the steel composition contains some alloying elements, like chromium, molybdenum, etc. Bainite structure involves diffusion

(unlike martensite); however, due to slower diffusion rate at the temperature where bainite forms, carbon from decomposing austenite can move only short distance and precipitates as carbide in the matrix – where the matrix is feathery type plates of ferrite. Thus, bainite structure that forms on austenite decomposition at intermediate cooling rate consists of feathery ferrite plate on which carbides have precipitated either on the plate boundaries or with carbide precipitation all over the matrix. Depending on the carbide precipitation nature, bainite is classified as ‘upper bainite’ and ‘lower bainite’; upper bainite having feathery carbide precipitation only on the plate boundaries and lower bainite have acicular ferrite with carbide precipitation all over the matrix. Formation of bainite structure from austenite decomposition has been discussed in details in Section 2.4 under isothermal and continuous cooling conditions. Figure 2-3 shows a typical upper bainite structure in medium carbon low-alloy steel produced by oil quenching.

Fig. 2-3

Upper bainitic structure in low-alloy steel – produced by oil quenching

Thus, different structures form during austenite decomposition in steel, depending on the composition of the steel, rate of cooling and control over the cooling. These resultant structures, under different cooling rates have different mechanical properties. Martensite structure is the strongest and ferrite is the softest structure. In general, strength of a structure depends on its temperature of formation. In that order, ferrite is the softest structure, followed by pearlite; next comes bainite, and finally the martensite – which forms at the lowest end of temperature axis for decomposition and transformation of austenite. However, the exact level of strength and other mechanical properties of a structure are dependent on the composition of the steel and its exact cooling rate, which influences the morphological character of the product. Controlling the morphological character of structures is an important premise of heat treatment of steels. Hence, while discussing austenite decomposition and structure formation in the subsequent sections of this chapter, factors

and parameters controlling the morphological nature of microstructures have been emphasised.

2.2.1

Role of Nucleation and Growth (N&G) Process in the Structure Formations of Steel

Phase changes take place in steel by nucleation and growth (N&G) process. The manner in which the phases nucleate and grow influences the morphology of structure that forms on phase transformation in steel. The N&G process in phase transformation is strongly influenced by the cooling rate. Because of the influence of cooling rate on the pattern of nucleation and growth, several possibilities of structure formation by controlling the cooling rate exist in steel. For example, a single phase steel structure - ferrite (a ) - can be obtained in extra-low carbon steel by slow cooling rate, whereas structure with multi-phase – like the pearlite and bainite – can be obtained by increasing the cooling rate in steels with relatively higher carbon or with suitable alloying elements. Structure that forms from the phase transformation takes characteristic morphological pattern based on N&G characteristics. The N&G process plays a critical role in the transformation process of steel, influencing the nature and character of different microstructures that form from the phase transformation during austenite decomposition. It is this morphological character of structures that determines their mechanical properties in steel. Figure 2-4 illustrates the characteristics of N&G process in steel for structures like pearlite, upper bainite and lower bainite. Be it in slow equilibrium cooling or faster cooling, phase formation during time-dependent austenite decomposition takes place by ‘nucleation and growth’ (N&G) process. The N&G process in a given steel composition (i.e. the austenite composition), in turn, is influenced by the cooling rate. While a component is being cooled during heat treatment from higher temperature, the upper temperature phase becomes unstable on cooling below a particular temperature and starts seeding (i.e. nucleating) another phase, which is thermodynamically stable at that lower temperature. As soon as the nucleation starts, arrest of cooling sets in due to latent heat of transformation (phase changes) under slow cooling condition. However, if the cooling rate is faster, then there would be some transitory ‘undercooling’ at that point of initiation of nucleation. This under-cooling situation helps to form more number of nuclei – characteristic of the lower temperature attained due to under-cooling – and supports the fine nuclei to become energetically stable and ready for growth. Such a condition is an important factor for growth, because nuclei must acquire a ‘critical

(a) N. small G. fast

700 Nucleation N °C

Austenite grains

Pearlite

Growth G

600

N. large N° G. slow Rate of nucleation and growth

Pearlite

(b) Fe3C

Fe3C μ

Bainite

Nucleus

Side nucleation

Edge growth Austenite Fe3C stringers C-free ferrite

(c)

Upper

C enriched a

Fe3C nucleated

(d)

Platelets Fe3C, Fe2.4 C

Ferrite with little C

Carbide ppt. ~55°

Lower

Bainite

Ferrite nucleus

Coherent Supersaturated ferrite

Fig. 2-4 Schematic depiction of the nucleation and growth (N&G) phenomenon in steel. [Note the interrelationship between nucleation and growth phenomenon; less number of nucleation means larger growth and more nucleation means less growth. (Source: E.C. Rollason, Metallurgy for Engineers, 1973)]

mass’ for growth. Faster the cooling rate higher is the under-cooling effect – thereby contributing to more number of stable nuclei in a matrix. More is the number of stable nuclei for growth less will be the growth requirement for them in the matrix, giving rise to finer phase distribution, i.e. finer structure. This situation has been illustrated in Fig. 2-4. Nucleation of a new phase from the cooling austenite is the beginning of microstructure formation in steel. For example, on slow or equilibrium

cooling of hypo-eutectoid steel, austenite will start decomposing to ferrite (a) on cooling below the A3 temperature, giving rise to phase balance of (ferrite + austenite) between A3 and A1; vide Fig. 2-1. With the formation of ferrite, which has very low solid-solubility of carbon, austenite will become progressively richer in carbon with cooling till the temperature has reached A1 (lower critical temperature). On reaching the A1 temperature, carbon-rich austenite – which got enriched to ‘eutectoid’ carbon level by then – will undergo eutectoid decomposition of austenite, transforming to ‘pearlite’ – a mixture of ferrite and carbide (Fe3C) in alternate lamellar formation. This lamellar morphology of pearlite structure arises due to the intervening ‘nucleation and growth’ process involving alternate precipitation of carbide and ferrite in close proximity during austenite decomposition on cooling. Figure 2-4 illustrates the way N&G process influences the microstructure formation in steel. The figure demonstrates that if the cooling rate is high, nucleation rate will be also high and consequent requirement of growth will be lower. This will help producing a finer structure. Alternately, if the cooling rate is low, growth requirement will be higher, producing coarser resultant structure. This is the reason for getting finer pearlitic structure by air cooling (normalising) compared to slower annealing process (furnace cooling). In sum, influence of cooling rate on N&G process leads to different morphological structure of steel arising from the nature of austenite decomposition. As indicated in Fig. 2-4, the N&G process can influence the structure of pearlite, upper bainite and lower bainite, which are the important products of austenite decomposition. As such, N&G process and its control play a significant role in the design of heat treatment operations involving ferrite, pearlite and bainite formation. In addition to cooling rate, nucleation process also gets influenced by the presence of prior austenite grain boundaries, inclusion interfaces, and areas of dense dislocation density – which are sites of higher energy. These sites being areas of high energy, act as preferred sites for nucleation by providing that extra energy required for N&G. An example of this is the structure of cold-worked and annealed steel, which is highly deformed with high dislocation density. The observed microstructure is finer than the normally annealed steel as high dislocation density facilitates large number of fine carbide nucleation leading to finer pearlite. A summary of the effect/influence of N&G process (vide Fig. 2-4) on microstructure formation is given below: • The type of structure that will form from the phase changes due to austenite decomposition depends on the characteristics of N&G process

• Nucleation type depends on the composition of the austenite and nucleation rate depends on the cooling rate of austenite • When the phase formation occurs at relatively higher temperature, numbers of nucleation are small requiring higher growth – leading to coarser structure • At lower temperature of phase formation, numbers of nucleation are large, requiring less growth – leading to finer structure • Carbide (Fe3C) is the nucleation phase for pearlite formation, i.e. pearlite formation starts with nuclei of carbide. • Under relatively faster cooling, i.e. at an intermediate cooling rate, formation of bainite starts from the nuclei of ferrite, if held isothermally at the bainite transformation range of temperature (vide Fig. 2-8). • Both upper bainite and lower bainite forms beginning with ‘ferrite’ as nuclei. Morphological nature of ferrite changes for bainite transformation with temperature where it nucleates and grow; if the temperature is relatively higher, ferrite plate becomes feathery and when ferrite nucleates at relatively lower range of bainite formation temperature, the ferrite may become acicular in nature (i.e. finer than feathery ferrite of upper bainite). The latter is observed for lower bainite structure, especially if the steel is alloy-containing steel. Figure 2-4 is silent about the N&G process in martensite transformation. It is believed now that martensite is also N&G process where due to very high super-cooling effect, numerous nuclei can instantly form and lead to instantaneous growth. Instantaneous nuclei formation is facilitated not only by high super-cooling effect but also by high dislocation density that gets generated due to quenching of austenite with super-saturated carbon. Due to high strain in the quenched matrix and availability of numerous fine nucleation sites, growth of martensite is very fast – almost instantaneous once the nuclei have formed. This makes martensite transformation process independent of time – unlike other processes such as ferrite, pearlite or bainite formation – where both temperature and time is important for completion of the transformation process. More about martensite formation mechanism has been discussed in the relevant section of austenite decomposition (Section 2.4). Therefore, formation of different structures in steel during austenite decomposition is dependent on (a) composition (b) rate of cooling, and (c) presence of such locations in the matrix as grain boundaries, inclusions, heterogeneity or high dislocation density – influencing the N&G process under a given cooling rate. Phase diagram, as shown in

Figs. 1-3 or 2-1, indicates the way phases form on equilibrium cooling from austenite and the resultant microstructure that forms in the steel. Equilibrium cooling structure in hypo-eutectoid steels (i.e. steel with carbon less than the eutectoid carbon) is generally ferrite and pearlite. But, with increasing cooling rate, morphology of these structures change, becoming finer and stronger in strength. With further increase in cooling rate, the nature of phase formation and resultant microstructure drastically changes, forming bainite and martensitic structures. Formation of these structures can be followed by using the time-temperature-transformation diagram – called the TTT diagram or its variant continuous cooling transformation diagram – called CCT-diagram – which has been discussed in Section 2.4. Thus, in general, microstructures that can form on cooling a hypoeutectoid steel (i.e. steel with carbon less than the eutectoid composition) are: ferrite, pearlite, bainite and martensite. But, due to variation of cooling rate and composition, their morphological character will change – and it is this morphological character of a structure that determines its mechanical properties. Therefore, from heat treatment point of view, it is important to understand the factors and conditions that can give rise to different morphological nature of a microstructural product influencing the properties of steels.

2.3 2.3.1

AUSTENITE DECOMPOSITION: FORMATION OF FERRITE AND PEARLITE STRUCTURE IN STEEL Cooling Rate Sensitivity of Austenite Decomposition

Austenite decomposition is the principal source of microstructure formation during heat treatment of steels. The process of austenite decomposition can take place under three different cooling conditions, namely: (1) isothermal cooling, (2) faster continuous cooling and (3) slower equilibrium cooling. Isothermal cooling means holding the job at a fixed temperature over a time before final cooling to room temperature. The process involves first cooling the austenite at faster rate from higher temperature for avoiding any prior transformation and then isothermally holding the austenite at a predetermined temperature for austenite decomposition to take place and then cooled down further. Continuous cooling, on the other hand, refers to cooling where the austenite temperature gets continually lowered till the decomposition process is complete and products have reached the normal temperature. Most industrial cooling – like air cooling or quench cooling – are continuous cooling. The problem with continuous cooling is

that the cooling rate is not uniform all through the cross section; surface cools faster than the core due to thermal lag arising from poor thermal conductivity in steels. Austenite decomposition under slower cooling can be followed through the Fe-C diagram (as has been discussed earlier), but for following the austenite decomposition under isothermal or continuous cooling conditions, TTT (time-temperature transformation) or CCT (continuous cooling transformation) diagrams are used, respectively. Uses of ironcarbon diagram, as depicted in Fig. 1-3 or 2-1, are straight-forward as per rules discussed earlier in this chapter; because it is valid for the entire carbon range of steels (i.e. it is not composition-specific). But, TTT or CCT diagram is strictly composition-specific; the nature of transformation curves in the diagrams changes with change in carbon and other composition. More about the interpretation of TTT/CCT diagrams has been discussed along with bainite and martensite transformation in the next section (Section 2.4). Irrespective of whether cooling is equilibrium or continuous type, nature and character of microstructure (or mix of microstructures) produced from austenite decomposition changes with variation of cooling rate. This is true even when the cooling is equilibrium type. For example, slower equilibrium cooling (as in annealing) produces more uniform structure than faster air cooled steel (as in normalising). But, as regards uniformity of structure is concerned, isothermal cooling gives most uniform structure in a section, because the structure forms at the same temperature of isothermal holding where the surface and centre transform at the same temperature. Continuous cooling can produce maximum non-uniform mixed structure in the cross-section, because under continuous cooling all points in the cross-section do not get time for temperature equalisation, unless the section is very thin. Thus, cooling conditions have an important role for producing structural uniformity in a steel body – in addition to its influence for change in transformation behaviour. This is an important point for heat treatment of steel where aim of such process is to produce as much uniform structure over a cross-section as possible. More about cooling rate sensitivity and uniformity of structure has been discussed under the subject ‘heat treatability of steels’ in Chapter 4.

2.3.2

Decomposition of Austenite under Equilibrium Cooling: Formation of Ferrite and Pearlite in Steels

Austenite decomposition under equilibrium cooling is reversible. This means that phases present in the initial structure could be reproduced during heating and cooling. Figure 2-5 step by step illustrates one such

situation where the initial structure of ferrite and pearlite, marked (a) in the figure, has been reproduced by heating and subsequent cooling to finer ferrite and pearlite, marked (d) in the figure. Closer examination of illustrated microstructures will reveal that though qualitatively the phases (ferrite and carbide, in this case) have been reproduced but the structure formed by these phases after cooling from austenite is different from the initial structure. Grain size and structure of initial steel was coarser, and grain size and structure (ferrite and pearlite) after cooling from austenite is finer. This effect arises due to (a) difference in cooling rate that the initial structure had undergone during casting and rolling and the final cooling rate from the austenitic state, and (b) respective austenitisation temperature and cycle of heating. This brings out the importance of austenitisation time and temperature, and the cooling rate for heat treatment of steels – even by following equilibrium cooling conditions – for effecting some favourable change in microstructure. Figure 2-5 schematically illustrates the microstructure changes with heating and cooling of 0.20% carbon steel by following the equilibrium cooling condition. Figure 2-5 first illustrates how the initial structure of the steel changes with heating; vide Fig. 2-5(a) corresponding to 0.20% carbon steel. On heating above A1, this structure starts changing by formation of austenite from the pearlitic colony of the structure; vide Fig. 2-5(b). This illustrates that on heating, austenite starts forming from pearlitic areas where carbon concentration is high – i.e. reversing the situation when pearlite had formed on cooling from carbon-rich austenite, as discussed and pointed out in Section 2.2. On further heating, when temperature exceeds A3 temperature, 100% austenite forms; vide Fig. 2-5(c). This is parent austenite which when cooled appropriately decomposes to ferrite and pearlite structure, but with difference from the initial structure (a); vide Fig. 2-5(d). The figure also schematically illustrates that if austenite grain size is fine and uniform, the resultant grain size of ferrite and pearlite will be also finer and uniform. This is because prior austenite grain boundaries act as sites for nucleation of ferrite and carbide, thereby, changing the size and distribution of these phases into the structure that forms on cooling; vide the difference between Figs. 2-5(a) and 2-5(d). This aspect of phase transformation and structure formation involving austenite grain size is an important feature of heat treatment of steels. For better appreciation of phase formation under Fe-C equilibrium diagram, it is necessary to understand why phases change on heating and cooling. Change of phase from ferrite to austenite (or viceversa) is associated with change in their crystal structure, as discussed in Chapter 1.

Fig. 2-5 A schematic representation of microstructure changes in a 0.20%C steel on heating to and from austenitising temperature (indicated by dotted vertical line) under equilibrium cooling. [Conversion of °C from °F is: (°F – 32) x 5/9]

Crystallographic structure of austenite is FCC (face centred cubic) while the structure of ferrite is BCC (body centred cubic), and the solubility of carbon in this FCC structure is much higher than that in BCC structure. For example, austenite can hold up to 2.0% carbon at higher temperature in solid solution compared to 0.02% carbon(max.) in ferrite at the A1 temperature (vide Fig. 2-1). These are thermodynamic solid solubility limits of carbon in austenite and ferrite at the corresponding temperatures. These limits change with temperature, and as the temperature changes – either during heating or cooling – the extra carbon in solid solubility comes out and forms a new phase which can accommodate that extra carbon – which then reacts with iron of the matrix and forms iron carbide. This is a simplistic view of ferrite and carbide phase formation on cooling of austenite, which will be further elaborated in the next few paragraphs with reference to Fig. 2-6. Austenite forms on heating the steel above A3 temperature and gets chemically homogenised if time is allowed for diffusion to homogenise the composition. Homogenisation of austenite is necessary for consistent

response to phase transformation because of inhomogeneity in initial structure that is often observed in rolled or forged steels. For heat treatment and phase transformation chemically homogeneous austenite is used for consistent response. The process of austenite formation and its decomposition under the equilibrium cooling for 0.40% carbon steel has been depicted in Fig. 2-6. This figure represents the phase changes and structure formation in 0.40% carbon steel under cooling only; vide the dotted line on composition axis. The decomposition of austenite starts just after crossing A3 temperature, because solubility of carbon in austenite increases with lowering of temperature below 910°C or A3 temperature of the steel. This thermodynamic situation initiates formation of another phase that has lower carbon solubility in that phase – which is ferrite in this case – in order to balance the distribution of carbon between separating ferrite phase and residual austenite phase. This means that ferrite formation is necessary for supply of more carbon to austenite needed for increased solubility of carbon in austenite with decreasing temperature. Due to ferrite formation, which has lower than 0.02% carbon in it, austenite becomes continually richer

0.4% C

0.8% C

AUSTENITE FERRITE CEMENTITE

°C 1000

(v)

(i) 900

C

U2 Austenite

(ii)

U1

800 B 700

T°C

Cementite + Austenite

Ferrite + Austenite L1

723°c

E

Ferrite Ferrite + Cementite

(iii) 600

(iv)

500 A 0

Cementite + Pearlite

Ferrite + Pearlite (vi) 0.2

0.4

0.6

0.8

1.0

1.2

1.4

%Carbon

Fig. 2-6 Illustration of structure formation on slow cooling of austenite containing 0.40% and 0.80% carbon in steel

in carbon as it cools. Ferrite that forms on cooling below the A3 temperature precipitates on the grain boundaries of prior austenite grains due to grain boundary acting as preferred nucleation sites; vide the second circle (ii) in Fig. 2-6. This process of ferrite formation from mother austenite and consequent enrichment of austenite with carbon will continue till temperature reaches A1 or 723°C as marked up in Fig. 2-6. On reaching A1 temperature line pearlite will start forming from remaining carbon-rich austenite in areas free from prior ferrite formation. Like ferrite, pearlite formation is also controlled by ‘nucleation and growth’ process, but nucleus for pearlite growth is ‘carbide’ (Fe3C). On reaching A1 temperature (723°C line), austenite, which has by then reached the eutectoid carbon level due to ferrite separation, will momentarily precipitate carbides from carbon-rich austenite and this will act as nuclei for pearlite formation, because carbide acts as nuclei for pearlite formation. Carbide precipitation, in turn, will momentarily reduce carbon from the surrounding austenite, and this will induce ferrite precipitation adjacent to carbides, giving rise to formation of alternate carbide and ferrite layers. This structural pattern of ‘lamellar formation’ – as schematically indicated in Fig. 2-6 – is called the pearlite. Thus, pearlite that will form on decomposition of parent austenite will be alternate lamellae of carbide and ferrite, as shown in this figure and in Fig. 1-4(b). While Fig. 2-6 depicts a schematic presentation of pearlite formation, Fig. 1-4 shows the actual micrographs of lamellar pearlite in steel. On further cooling below A1 temperature, there would not be any significant change in ferrite content in the structure, due to very limited solid-solubility of carbon in iron below A1 temperature. However, this depiction represents cooling at or near equilibrium cooling rate. If austenite is cooled faster through this range of transformation, there would be more volume of pearlite in the structure and denser would be the lamellar spacing, due to the influence of N&G, which has been discussed earlier with reference to Fig. 2-4. As a result, relatively faster cooling will produce relatively finer pearlite spacing i.e. finer pearlite structure. In practice, advantage of this situation is taken in the heat treatment of steel. For example, air cooling is used for normalising of steel in order to produce finer pearlite and finer ferrite grain size. Therefore, the fact stands that the nature of ferrite and pearlite formed during austenite decomposition will be characteristic of the nucleation and growth (N&G) process under a specific cooling condition. Thus, under equilibrium cooling, austenite decomposes to ferrite and carbide phases – giving rise to structures like (ferrite + pearlite), pearlite only, or (pearlite + cementite) if the steel is hyper-eutectoid. Morphology of these structures will depend on the prevailing N&G process in the

steel during cooling; faster cooling in a given steel composition tends to produce finer structure. Cooling rate sensitivity of austenite decomposition has been briefly discussed in Section 2.3.1. By changing the cooling rate, austenite can be made to transform to any or a combination of following structures: Ferrite, Pearlite, Bainite, or Martensite – depending on the chemical composition of the steel; the latter two structures are the products of non-equilibrium fast cooling system. For 100% ferrite formation, steel should be of very low carbon (£ 0.02%); for martensite formation, steel should have sufficient carbon in it for overcoming the chance of any other phase changes during fast cooling; for bainite formation – especially the lower bainite – steel should be of appropriate chemical composition containing favourable alloying elements. Thus, austenite on decomposition during cooling can form any or a combination of the following structures, depending on the composition of the steel and cooling rate: • Ferrite: A crystallographic phase with grains. Grain size in ferrite will depend on the rate of cooling during decomposition; if cooled faster, as in air cooling (normalising); ferrite grain size will be finer. If cooled slowly (annealed), grain size will be coarser and steel will be softer. • Pearlite: A lamellar product of ferrite and carbide phases forming a closely interspaced lamellar structure. Pearlitic lamellar spacing would be coarser if cooled slowly (e.g. furnace cooling) or finer if cooled faster. Faster cooling tends to produce more pearlite and finer pearlite due to N&G effect. • Bainite: A fine but feathery ferrite structure with fine carbide precipitates over the ferrite plates. Ferrite structure in bainite can change with temperature of formation; changing from feathery to acicular ferrite (more strained) with lower temperature of formation. Bainite will form when cooling rate is faster to suppress austenitic decomposition to ferrite and pearlite, but not fast enough to form martensite. Depending on the temperature of formation, bainite could be ‘upper bainite’ – where ferrite plates are feathery and carbides precipitate only on the grain boundaries – or ‘lower bainite’ where ferrite could be more acicular and carbides precipitate all over the matrix. • Martensite: A heavily strained distorted ferritic structure (a-crystal structure) with super-saturated carbon. Ferrite in martensite can take the shape of acicular platelets or needles of highly distorted ferrite, supersaturated with carbon, based on the cooling rate and steel composition. Martensite will form when the steel is cooled from

austenitic temperature at very high speed, like water or oil-quenching, suppressing the possibility of transformation to any other product. Nucleation and growth (N&G) process not only plays a critical role in determining the morphology of ferrite and pearlite, but also for bainite and martensite – the latter two structures are products of faster non-equilibrium cooling during austenite decomposition. Effect of high supercooling (due to quenching) and its role in N&G process of martensite (if it can be assumed that way) can, perhaps, be appreciated from the difference in martensitic structure produced by water quenching and oil quenching of same steel; water quenched structure could have finer martensitic needles than oil-quenched structure, subject to exact steel composition and cooling condition. Figure 2-7 illustrates the points.

Fig. 2-7 Illustrations showing finer martensitic structure (a) after water quenching and coarser martensitic structure (b) after oil quenching. [Notes: Chemical compositions of these two steels are not exactly similar; they are exhibited for illustration only. Fig. (b) also contains some spots of bainite]

Formation of these phases by austenite decomposition holds the key to all heat treatment processes of steels. Tailoring the austenite decomposition process by designing appropriate heating and cooling method, properties of steels can be suitably modified for many applications. For example, if a softer structure is required for the given steel, austenite can be slow-cooled inside a furnace, called annealing heat treatment, for producing coarser ferrite grain size and coarser pearlitic structure, which is soft. If a stronger structure is required of the same steel, the steel can be subjected to air-cooling to produce finer ferrite grains and denser pearlite, called normalising in heat treatment. If the strength of the steel has to be further improved, the same can be quenched (hardening) to form martensite or (martensite + bainite), as the case may be, by appropriate quenching. However, for hardening to martensite, steel composition has to be appropriate as per section size of the steel and required properties. Quenching means fast cooling of the steel in a manner that suppresses the formation of higher temperature phases like ferrite or pearlite in the structure.

Grain size – the other microstructural feature of steel – also influences the decomposition process of austenite by influencing the nucleation sites; vide Figs. 2-4 and 2-5. Grain boundaries can act as preferred nucleation site due to higher surface energy available with them and thereby influencing the process. Finer grain size will provide more sites for nucleation of higher temperature phases like ferrite and carbide at the grain boundaries, leading to the formation of non-martensitic structures. This is the reason why coarser grained steel is preferred for hardening. If grain size is fine, non-martensitic products (e.g. pearlite or bainite) may form in the structure due to the possibility of favourable grain boundary nucleation of ferrite and carbide, despite faster quenching. Such a situation may give rise to faulty hardened structure, especially in high carbon steels. For example, if carbon level exceeds 0.80% (eutectoid level) in fine-grained steel, there is some possibility of formation of some pools of ‘toorstite’ arising from the fine carbide precipitation on the grain boundaries prior to martensitic transformation. Toorstite is a dense globular pearlitic structure originates from carbide nuclei and it is sometime found along with martensite after case-hardening of steel, especially when the carbon level on the surface area exceeds the eutectoid level.

2.4

AUSTENITE DECOMPOSITION: FORMATION OF BAINITE AND MARTENSITE STRUCTURE IN STEEL

It has been observed that transformation of phases in steel not only requires ‘nucleation’ as starting point for growth, but also ‘incubation time’ for growth to start. The process of N&G is not instantaneous; it takes time to start, ranging between few seconds to few minutes, depending largely on the temperature and composition, and partly on the type of phase. This delay time for growth to begin is termed as ‘incubation time’ during which nuclei assume a critical mass for stability and growth. Below a critical mass, nuclei are not stable in the matrix, especially at higher temperature due to thermal fluctuations. This phenomenon gives rise to “C” shaped time–temperature–transformation curves, as depicted in Fig. 2-8. This type of diagram is called time–temperature–transformation diagram, briefly TTT diagram. Figure 2-8 shows ‘C’ type transformation curves, one for the start and the other for the finish of austenite transformation. “C” type transformation curve reflects that kinetics of phase transformation has two components in it; one the driving force for change and other is the diffusion kinetics. At higher temperature of transformation diffusion is fast, but the driving force is low. This leads to relatively longer transformation time at higher temperature. Again, at lower transformation temperature,

% Amount transfomed

the driving force is high but diffusion coefficient is low. Hence, at lower temperature also transformation process gets prolonged. At temperature intermediate between higher and lower range, combined effects of driving force as well as diffusion coefficient are high, resulting in faster transformation and giving rise to ‘nose’ of the C-curve. These features of TTT curve are illustrated in Fig. 2-8.

100

P

Coarse Pearlite Fine Pearlite

F+B

400

MS

Lower Bainite

300 200 100

Martensite + Bainite

50%

MF Martensite 1 sec

1 min

Bainitic

Upper Bainite (Feathery)

A +B

Martensitic

100% completed

Pearlitic

5

10 seconds

Fine more Acicular Rapid Etching

500

4

10

Acicular Slow Etching

600

3

10

Austenite stable A+F A+F+P

Ae3 Ae1 Austenite unstable

Temperature of Transformation, °C

700

2

10

Finer Lamellac

10

Austenite

0 V.P.H.

210 320 450

700

750

1 hr

Transformation Time (Log. Scale)

Fig. 2-8 A TTT-curve for 0.6%C steel. [Diagram depicts the time interval required for formation of different structure when held isothermally at different temperatures. (Notations: A = Austenite, F = ferrite, P = Pearlite, B = Bainite)]

TTT-diagram in Fig. 2-8 indicates the stages of austenite decomposition and different structure formation in a given steel composition – which is 0.60% carbon steel, in this case. The complete diagram – with different time–temperature–transformation zone – has been developed by cooling and isothermally holding the austenite at different temperatures. The diagram covers the entire spectrum of phase transformation starting with decomposition of parent austenite (g -phase) to (austenite + ferrite) when cooled between A1 and A3 temperature (indicated by Ae3

and Ae1 respectively in this diagram). The diagram divides the temperature of phase transformations into five zones, including phase changes in between the upper and lower critical temperatures. These zones are where parent austenite decomposes to (1) ferrite + austenite, (2) pearlite (coarse and fine), (3) ferrite + bainite (upper and lower bainite), (4) martensite + bainite, and (5) martensite, as marked up in the diagram. Stages of austenite decomposition can be followed by referring to such time-temperature cooling diagrams for a given composition of the steel. Four thin horizontal lines have been superimposed on this diagram in order to schematically represent the isothermal cooling conditions and illustrate how the decomposition process proceeds and completes, giving rise to different structures. Referring to upper thin line, schematically representing a cooling pattern, it can be observed that if the austenite is sharply cooled and allowed to transform by holding at the upper temperature (around 600°C, in this case), then after the incubation time, the transformation process will initiate with ferrite separation first and then proceed to complete the transformation by formation of pearlite. But, this pearlitic structure will be very fine with hardness level of about 400VPN as against the hardness of about 250 VPN for coarse pearlite (vide the right hand axis giving the hardness level of products forming at different temperatures). If the austenite is sharply cooled below the ‘nose of the C curve’ and held for transformation of austenite (vide the second thin lines from top) at that temperature (about 510°C, in this case), the austenite is likely to transform to bainite. But, due to relatively higher temperature of transformation and the nucleation characteristics, the structure will be, what is called, ‘upper bainitic’ – where the ferrite will be feathery plates and carbide that precipitates will be mostly on the ferrite boundaries. If the austenite is sharply cooled (i.e. without allowing any prior phase formation) to further below this upper bainitic temperature range, say about 410°C, then the bainitic structure that forms from austenite decomposition will be ‘lower bainitic’ structure in character – where the ferrite is more acicular, finer and strained, and the carbides precipitates all over the matrix i.e. both on ferrite plates and boundaries. Figure 2-9(a) and (b) illustrates the difference in upper and lower bainitic structures. Lower bainite is much harder (stronger) than upper bainite; its hardness level can reach very close to martensitic structure hardness level; vide Fig. 2-9(b). If the austenite can be very sharply cooled, without allowing any prior precipitation or transformation, to a temperature below the MS temperature (martensite start temperature), then austenite will start transforming to ‘martensitic’ structure; vide the thin lines below the MS temperature

Fig. 2-9 (a) Upper bainite structure with clearly visible ferrite plates and plate boundary precipitates of carbide (dark etched phase), (b) predominantly lower bainite structure, showing carbide precipitations on finer and acicular ferrite structure; produced by isothermal cooling. (x400)

(300°C, in this case) in Fig. 2-8. The cooling has to be sharp enough to avoid touching the nose of the ‘C’ curve for avoiding any prior precipitation before transformation to martensite. In case the cooling rate is bit sluggish before entering the zone of martensite formation by crossing the MS temperature and the steel is an alloy steel, there could be chance of austenite transforming first: to some ‘lower bainitic’ structure and then remaining austenite transforming to martensite. Because of very fast cooling of austenite from higher temperature to temperature level below MS temperature during hardening, carbon atoms from austenite does not get time to diffuse out and form carbide, despite sharp drop in solid solubility of carbon in austenite with decreasing temperature. As a result, martensite that forms on transformation of this carbon rich austenite is super-saturated with carbon, having highly distorted tetragonal ferritic lattice structure. Appearance of martensitic structure depends on the cooling rate and the composition; faster cooling tends to produce needle-shaped martensite; vide Fig. 2-7. Below the MS temperature, martensite formation is not time dependent, but temperature dependent; vide Fig. 2-8. This is believed to be due to exceedingly fast growth of martensite once it has nucleated. Nuclei of martensite could be strain-induced embryos associated with areas of high dislocation density and similar lattice defects, whose population is considered plenty in the matrix due to severe quenching stresses and shear mechanism involved in the martensitic transformation. Such a condition of nucleation and growth makes the transformation of martensite very fast once it has nucleated. However, continuation of cooling the

untransformed austenite even below the MS temperature is necessary for fresh supply of nuclei so that more and more martensite can form. Thus, with continued cooling below the MS temperature, more martensite will form, finally completing and finishing 100% transformation of austenite to martensite at the MF temperature. If cooling is disrupted and held in between the MS and MF temperature, remaining austenite will stop transforming, i.e. transformation of austenite to martensite will remain incomplete. The remaining austenite will transform to martensite only if cooling is resumed. This means that (a) for 100% martensite formation, austenite has to be cooled below the MF temperature of the steel, and (b) if necessary, cooling process below MS temperature can be modified for reducing thermal/quenching shock by briefly holding at an intermediate temperature between MS and MF temperature, but must be followed by further cooling where cooling rate is not any more important. This advantage of holding between MS and MF temperature is often availed in designing the heat treatment cycle for hardening of high alloy steels or for controlling distortion due to quenching stresses in low-alloy steels. Thus, the study of decomposition of austenite by using TTT diagram can provide information about different phase/structure formation at different cooling rate and temperature level. In general, TTT diagram can provide information about: • What phases will form from the transforming austenite if the holding temperature is changed? • How much holding time is required for transformation of a particular phase? • How fast the steel should be cooled to transform to 100% martensite? • How fast and to what temperature the steel must be cooled to avoid any bainite separation when martensite is the desired phase? • To what temperature the steel must be cooled for completion of 100% martensite (e.g. the MF – martensite finish temperature)? • What could be the approximate strength (or hardness) of different phases that form at different temperatures and their overall nature (as indicated on the right side of Fig. 2-8, which have been measured separately after the transformation to respective phases)? Transformation diagrams – such as TTT (or CCT) – are different for different composition. By studying the TTT diagrams of various compositions of steels (e.g. plain carbon steels and alloy steels), the above-mentioned information about the respective steel can be obtained – which helps in planning a fool-proof heat treatment cycle.

However, most industrial cooling is continuous in nature (i.e. cools continuously from higher temperature to lower temperature); giving rise to transformation of phases as per transformation field the austenite passes through during cooling. Figure 2-10 depicts one such continuous cooling transformation (CCT) diagram for low-alloy steel. While thermodynamics of phase changes in CCT diagram remains more or less similar as in TTT diagram, but the kinetics of phase changes and growth differ due to heterogeneous structure that gets developed inside the steel body during such cooling process.

Fig. 2-10 A typical CCT-transformation diagram of an alloy-steel (nickel-chrome steel) under different cooling conditions [The diagram depicts the condition of formation of different phases in steels on heat treatment following different cooling rate (the cooling rate is assumed to be at the centre of the bar)].

Continuous cooling not only produces continually differing morphology of a phase in the matrix due to sharp and continuous change in temperature, but also produces different phases across the section of a steel part. Because, different points across the section of the steel piece will cool at different rates due to thermal lag arising from thermal conductivity problem (e.g. centre cools more slowly than the surface due to thermal conductivity lag). Considering the practical situation of heat treatment, where mass effect on cooling cannot be avoided, CCT diagrams represent more realistic situation of what structure could be expected from the decomposition of austenite of a given steel composition under continuous cooling conditions. However, due to continuous cooling, different phases corresponding to different cooling rate may form and end point of transformation of a

particular phase may not be distinctly detectable. Thus, CCT diagram is an open diagram where start of a phase transformation is detected but not necessarily the end; vide Fig. 2-10. However, martensitic transformation is the exception, because it is not time dependent but temperature dependent. The start and finish of martensite transformation can be observed in both TTT and CCT diagrams on the temperature scale. In practice, CCT diagram of the steel is used for following up the transformation of austenite to martensite in practical heat treatment situation involving continuous cooling. With reference to CCT diagram in Fig. 2-10 and the cooling curves therein, formation of different microstructures can be interpreted as follows: • Slow cooling (the cooling curve furthest on the right) will produce essentially (ferrite plus pearlite) structure. The curve indicates that transformation reaction will be complete within the ferrite-pearlite temperature region. However, the ferrite-pearlite structure so produced will be finer than that is produced by equilibrium cooling, due to reasons explained earlier based on N&G process. • An intermediate cooling rate (marked moderate in the figure at the middle) will produce 100% bainite structure, if the transformation can get completed within the bainite formation zone. However, due to faster cooling in this lower temperature region, the bainite transformation might not get 100% completed, and, thereby, will leave scope for producing some martensite(or leaving some ‘retained austenite’ – depending on the steel composition and exact cooling condition) along with bainite in the structure. The transformed bainite will be partly upper bainite and partly lower bainite, because the cooling curve enters the bainite formation region above the nose of C-curve for bainite. Had the cooling rate been little faster, perhaps all bainite would have been lower bainite in this steel. • Faster cooling rate (the first curve in the figure marked ‘fast cooled’) will produce 100% martensite, as it enters the martensite start temperature without intersecting any other field. Martensite formation will continue with the cooling till temperature reaches martensite finish temperature (MF), which is generally about 150°C below the MS temperature (this is only a ball-path figure). However, under continuous cooling conditions, very often martensitic transformation is not 100% complete, especially in alloy steels, due to kinetic barriers. This may give rise to small percentage of ‘retained austenite’ in the structure, which generally increases in steel with increasing carbon and alloy content. Many a time, small amount of retained austenite is preferred in hardened steel for its effect on increasing the toughness in

the structure. Because, small and dispersed ‘retained austenite’ spots can cause higher absorption of energy during fracture propagation i.e. can act as barriers to fracture propagation. But, for many applications, especially involving compressive strength, softer retained austenite in the structure may lead to lower pitting resistance. In sum, CCT diagram of steel provides the conditions of the formation of different structures, including the bainite and martensite under the practical industrial cooling condition. Since martensite exhibits higher strength with superior toughness (upon tempering the martensite) than the ferrite-pearlite structure or the bainitic structure, principal aim of heat treatment operation for hardening is to produce as much of martensite as possible from the steel. Cooling conditions for producing different microstructures in steel have been illustrated in Fig. 2-11, including upper-bainite, lower-bainite and martensite. The diagram shows the transformation ‘start’ and ‘finish’ curves – in the form of C-curves – whereupon different cooling conditions have been superimposed. Nature of microstructures obtainable from such cooling conditions has been also indicated in the diagram. Heat treatment processes are designed based on these transformation rules, corresponding to specific cooling condition. Thus, the phase diagram under equilibrium cooling condition and time-temperature-transformation diagram under isothermal or continuous cooling transformation condition are important aids for controlling the austenite decomposition of steels in order to develop required microstructure and properties in the steel. Thus, microstructure and properties of steels are dependent on the nature of cooling, in addition to the composition of the steel. Therefore, consideration for cooling type and condition is very important for designing heat treatment processes. Variation in cooling rate has strong influence on the morphology of the structure developed during heat treatment. Hence, further examination of microstructural variation and their morphological nature may be helpful for the study of heat treatment, because it is the nature and character of structure (i.e. their morphology) that determines the properties in steels.

2.5

MORPHOLOGICAL CHARACTER OF MICROSTRUCTURES IN STEELS

Foregoing discussions on austenite decomposition demonstrate that nature and character of microstructure can be changed or tailored by varying the composition and cooling conditions. Accordingly, the process

Transformation start Transformation end

Austenite

800 700

Coarse pearlite Furnace cool Fine pearlite

Temperature (°C)

600

Air cool

500

Upper bainite 400

Austemper Austenite

300 200

Lower bainite

Martensite

Martemper

100

Martensite start

Oil quench Water quench –3

10

–2

10

0.1

1

10

100

3

10

4

10

5

10

Time (seconds)

Fig. 2-11 An illustrative time–temperature–transformation diagram with superimposed cooling curves of eutectoid steel composition [Diagram illustrates different cooling conditions (vide the cooling lines) required for development of different structure – as well as temperature region where they form. The figure also demonstrates the cooling cycles for ‘austempering’ and ‘martempering’ process of heat treatment, which are now extensively used for heat treatment of alloy steels for enhanced properties.]

of austenite decomposition forms the basis of heat treatment processes for developing the right microstructure for a given set of mechanical properties, such as hardness, strength, toughness, etc. Study of austenite decomposition further reveals that steel has basically only three distinct phases – namely: austenite, ferrite and carbide, and by combining among themselves these few phases can form a number of microstructures. The nature and character of these microstructures are based on different cooling rate, N&G characteristic and compositional character of the steel. The structures that form due to austenitic decomposition are ferrite, pearlite, bainite and martensite, but there could be appreciable morphological differences within each of these structures due to changes in the conditions of formation. With the change of morphology of the structure, their properties also change. Purpose of this section is to highlight such changes and associated difference in properties so that necessary adjustment in the heat treatment conditions can be made for achieving the right microstructural character for right set of properties. Table 2-1 shows the cooling conditions of formation of different phases and structures in steel and nature of their structural morphology.

Martensitic structure may at time resemble lower bainite, but it is generally finer than lower bainite due to presence of more dislocation

substructure in a highly strained quenched matrix. Figure 2-12(a) and (b) illustrates these two structures and demonstrates their structural difference. As has been mentioned earlier, microstructures in steel can be a single phase or a combination of them with different morphological character, depending on the composition and exact cooling condition. For example, in hardening grade steels, microstructure can have morphological character of equiaxed ferrite when cooled slowly, feathery or acicular ferrite plate with fine carbide precipitation as in bainite when cooled at an intermediate rate or martensite where the structure is even more acicular with finer ferrite plate (or needle) and finer precipitated carbides (seen when lightly tempered) when cooled very fast. Depending on their morphological character, properties of these structures vary considerably; vide Fig. 2-8 where hardness values have been indicated alongside the formation temperature of those structures. In general, finer the structure better is the strength and toughness of the steel in heat treated conditions. For example, finer ferrite grain size, finer pearlitic lamellar spacing, finer ferrite plates and finer carbide precipitation in bainite and finer martensitic needles with fine precipitated carbides are all known to contribute to increased strength and toughness. And, finer structure is generally associated with faster cooling rate due to its effect on the N&G process of phase formation (discussed in Section 2.2.1). Two important products of phase transformation are the lower bainite and martensite – both requiring faster cooling rate for formation. Their structure is also apparently near similar when examined by normal optical microscopy at lower magnification; vide Fig. 2-12. Due to similarity in structure, their mechanical properties are also in the over-lapping region; vide Table 2-2. One distinction between these two structures on optical

Fig. 2-12 Illustrations showing the optical micrographs of (a) lower bainite and (b) tempered martensite, respectively (x400)

microscopy examination could be with regard to retained austenite content; in general, lower bainite structures are often associated with higher amount of retained austenite in the structure than martensite for similar steel composition. This is due to austenite stability in the lower bainite transformation temperature region.

Fig. 2-13 High-magnification micrographs of Lower bainite and Martensite plates (un-tempered); Lower bainite shows the elongated acicular bainite plates with fine carbide precipitation all over; Martensite plates show dense dislocation tangles and dislocation density within the plates (dark spots and areas).

On high magnification checking, however, differences in structure between lower bainite and martensite can be identified; vide Fig. 2-13, which illustrates high magnification scanning electron micrograph of lower bainite and thin foil transformation micrograph of martensite. While lower bainite has pancake type ferrite plates where carbide precipitations are all over, martensite plates have dense dislocation tangles and density, and carbides are finer and randomly precipitated. Because of this difference, their detailed properties are also different; martensite has, in general, superior strength than lower bainite. But, lower bainite tends to temper more slowly than martensite – because of less stain in the as-quenched bainitic ferrite structure. Structurally, steel could have a softer (ferrite + pearlite) structure which gets produced under slower cooling or harder bainite or martensitic structure which gets produced by faster cooling. In ferrite–pearlite structure proportion of pearlite increases with increasing carbon content (or carbide forming alloying elements) and with increasing cooling rate. Nature of pearlite – which is an intimate mixture of ferrite and carbide in lamellar form – becomes increasingly finer (i.e. with closer inter-lamellar spacing) when the cooling rate is increased. Such pearlitic structure with

denser lamellar spacing increases the strength and elongation value of steels – which is an objective of the normalising process of heat treatment by air cooling. If the cooling rate is faster than air cooling during phase transformation or heat treatment, the steel might have a combination of (bainite + martensite) – provided the steel composition is appropriate for formation of these structures, requiring sufficient carbon and/ or alloying. This is particularly so under the condition of continuous cooling condition where cooling and temperature conditions differ from surface to the centre due to thermal lag arising from lower thermal conductivity in steel. Therefore, for steels under continuous cooling – as experienced in practical heat treatment cases – there are chances of bainite formation in the cross section of the steel, even when fast cooled for martensitic transformation, unless the section is too thin. The nature of this bainite could be upper bainite in plain carbon steel and lower bainite in alloy steels. Therefore, for ensuring 100% martensite formation in a section of steel, there is a concept of ‘ruling section’ data in the heat treatment of steels, which dictates maximum section thickness in the steel that can form 100% martensite (or a minimum level of hardness) upon quenching in a given medium – such as water or oil. If 100% martensitic structure (or minimum hardness) is desired in any steel section, then either the steel composition should be adequately improved to match the required ruling section or the cooling rate has to be appropriately increased. The latter may give rise to distortion in the steel parts due to high thermal shock / quenching stresses; hence a balance in cooling cycle is required. In practice, however, presence of some amount of bainite could be expected in alloy steels below the surface level under continuous cooling condition. Table 2-2 qualitatively illustrates the relative properties of bainite

and martensite.(Compare these with hardness value of fine pearlite, which is about 400 VPN). Considering the overall aspects of relative properties, martensite with small amount of lower-bainite stands out to be the preferred microstructure for applications requiring high toughness in high tensile steels. Lower bainite, especially if it is formed from Cr-Ni-Mo bearing alloy steel, is likely to contain some amount of ‘retained austenite’ spots, which is considered as beneficial for resistance to crack propagation in the steel matrix. Thus, small amount of retained austenite can make the steel tougher with good impact resistance. The tolerable limit of retained austenite in engineering steels depends on the end applications. While larger proportion can be tolerated for fatigue related applications, amount of retained austenite must be limited to very minimum for wear resistance applications. In general, retained austenite present in the structure should not adversely affect the strength property. Differences in properties between martensite, lower-bainite and upperbainite arise due to their difference in structural morphology (vide Fig. 1-6), which, in turn, arise from their temperature of formation, cooling rate of formation and composition of the steel. Even their properties can differ within each of these structures due to change in composition or condition of formation. For example, martensite formed from steel containing higher nickel is found tougher than martensite formed in plain carbon steel. Similarly, lower bainite of alloy steel containing Cr-Mo is tougher than the one containing Cr-Mn. Also, mode of cooling is a factor in controlling the morphology of martensite, e.g. water quenched martensite has finer structure than the oil quenched martensite of similar steel. Main features of structural morphology of upper bainite, lower bainite and martensite are: Upper-bainite forms at relatively higher temperature, but below the pearlite formation temperature (vide Fig. 2-11). When bainite forms at temperature range close to C-curve nose, it is ‘upper-bainite’. At this temperature range, mobility of carbon atoms is good and the extra carbon that comes out from strained ferrite plates can travel and get precipitated on the ferrite plate boundaries, plate boundary being the site of extra energy and preferred precipitation site. Thus, the upper-bainite structure is agglomerates of coarse feathery ferrite plates with carbides precipitated mostly on the plate boundaries; (vide Fig. 2-9a). The presence of brittle carbides at the plate boundaries renders upper bainite structure less ductile and tough compared to lower bainite or tempered martensite. Lower bainite forms below the upper bainite range. It forms at temperature significantly below the nose of C-curve (vide Fig. 2-11) and continues to form until the cooling curve crosses the MS temperature.

Because of relatively lower temperature of formation of lower bainite, carbon mobility in the matrix at the bainite formation temperature is limited. This causes the carbon atoms coming out of the ferrite lattice to precipitate to the nearest vicinity, i.e. on the ferrite plate itself as well as on the boundaries whichever is nearer. As such, carbide precipitates in lower bainite are all over the matrix; on the bainite plate and on the boundaries (vide Fig. 2-9b). This low magnification structure of lower bainite closely resembles to tempered martensite. But, there is significant difference between lower bainite and as-quenched martensite which gets revealed under higher magnifications (seen under Scanning or Electron microscope; vide Fig. 2-13. Carbides in lower bainite are more orderly and tend to precipitate on certain orientation of plane, whereas carbides in martensite are random and finer is size. Martensite forms on faster cooling of steel (quenching) when the cooling condition is such that the austenite does not get time to transform to any other phases like pearlite or bainite. This can happen when cooling curve misses the nose of C-curves for bainite formation. If the cooling rate is somewhat slow during quenching (at any part of the steel), some amount of lower bainite formation is possible in those regions, especially under continuous cooling of alloy steels. Since the cooling rate for martensite transformation is very high, carbon in the austenite being cooled does not get time to diffuse out to keep pace with lower solubility of carbon in austenite with lowering of temperature. Hence, entire carbon in the austenite gets trapped inside and transform on further cooling to below MS temperature to highlystrained ‘acicular or needle-shaped ferrite plates’, retaining all the carbon that parent austenite had. Under high strain in the lattice, some shear mechanisms take place for accommodating the super-saturated carbon atoms in the lattice, leading to highly distorted tetragonal ferrite lattice structure with high dislocation network. This makes the structure finer and stronger than others, but due to high internal strain the structure is brittle. It is generally accepted that high strength of martensite is due to the presence of high dislocation density so formed on quenching and transformation. Such an as-quenched martensitic structure, if held at room temperature, might induce some fine cracks in the matrix arising from the tendency of relieving internal stresses of strained ferrite lattice. Hence, as-quenched martensite requires immediate tempering to relieve some strain of the matrix. This takes place by heating the steel at appropriate tempering temperature, which induces some mobility of carbon atoms allowing diffusion out of the strained ferrite lattice. These carbon atoms coming out of the strained ferrite lattice react with iron in the matrix and precipitate

as carbides. Thus, tempered martensite structure becomes characteristically an aggregate of acicular ferrite and fine carbides that precipitates out from supersaturated ferrite on tempering. With increase in tempering temperature, fine carbide precipitates coalesce and become coarser. Tempering or similar type of treatment by heating below the lower critical temperature of steel can be given to other structures (e.g. bainite or pearlite) of steel, if necessary, for softening the structure for any specific uses, but their response to tempering will be different from martensite tempering. However, the term tempering is commonly used for martensitic tempering. Tempering of martensite – which is an important part of heat treatment for hardening – aims to (a) relieve some lattice strain by thermally activated diffusion of carbon atoms from the strained lattice, and (b) cause precipitation of carbide particles, and, thereby introduce toughness in the steel structure. Improvement of toughness will depend on the temperature and time of tempering. More about the stages of changes in tempering have been discussed in Sections 2.6, 3.4.4 and 7.5. Thus, type of microstructure and its morphological character – arising from austenite decomposition – differ with the rate of cooling during austenite decomposition, composition of the steel (because of its influence on the time-temperature-transformation diagram/transformation characteristics), and subsequent tempering, if any. Heat treatment process parameters – which are concerned with the development of suitable microstructure for a set of properties – therefore revolve around the factors that influence the microstructure formation in steel. This makes understanding of austenite decomposition process an essential part of heat treatment study. Amongst the steel structures, martensite is the most important of all for its wide spread uses and utility in the industries. Therefore, some more features and character of martensite structure and their uses and utility have been discussed in the following section for facilitating further understanding of how heat treatment process should be controlled.

2.6

MARTENSITIC STRUCTURE: ITS USES AND UTILITY

Martensite is the hardest structure in steel (excepting carbides), but it is brittle too in as-quenched condition. However, characteristically, martensite structure quickly improves upon its brittleness and gains toughness with tempering – which can be carried out over a range of temperatures below the A1 temperature. Martensite after tempering is called ‘tempered martensitic structure’. Higher the tempering temperature of martensite lower is the strength but higher is the toughness – a property where

strength and ductility is optimally combined to resist fracture initiation and propagation in steel. It is this tempered martensitic structure that is used in industries for its combined strength and toughness property in order to control the fracture and failure of industrial components. Therefore, understanding the tempering behaviour of martensite is very important for industrial applications. Tempering of martensite needs to be carried out as early as possible after hardening; otherwise under the pressure of internal stresses in as-quenched martensite fine micro-cracks can get generated in the steel, leading to cracks in uses and applications. Purpose of tempering – an integral part of steel hardening – can be (a) stress relieving and (b) toughening. These two processes take place as par the temperature and time of exposure to the tempering. At relatively lower temperature of tempering, carbon that comes out of martensite plates is very fine (called epsilon carbide Fe2.4C), but with increasing tempering temperature more carbon diffuses out of martensite plates and coalesce together to from coarser precipitates of carbide (Fe3C). Nature of carbide precipitation and their size contributes to the strength and toughness of martensite; alloy carbide provides more strength and toughness than plain iron carbide. Finer the carbide precipitation higher is the strength. Details of tempering of martensite have been discussed in Chapter 7. Strength of martensite can also vary with the composition of steel; it increases with increasing %C and alloying. Variation of martensitic hardness (in as-quenched state) with carbon content in plain carbon and alloy steels has been illustrated in Fig. 1-11 in Chapter 1. It can be observed from the Fig. 1-11 that: • Martensite hardness in plain carbon steel steadily increases up to 0.83%C (eutectoid carbon), but the rate of increase drops thereafter, because of the possibility of carbide precipitation in higher carbon steel while being cooled and thereby allowing pearlite spot formation in steel structure (carbide being nucleus for pearlite formation). This type of pearlite that forms in high carbon steel takes the form of dense globular particle and called ‘toorstite’. • Martensite hardness in alloy steels peaks at about 0.70%C and thereafter decreases because of the tendency of alloy steels to retain some untransformed austenite (retained austenite) upon fast quenching. However, this retained austenite in the martensitic structure mostly gets transformed to martensite upon tempering by the process of ‘diffusion of carbon atoms’ out of the austenite lattice and thereby making the austenite leaner in carbon and unstable. However, small amount of finely dispersed retained cannot be ruled out in alloy steel martensitic structure, but that is not harmful. Retained austenite in

small percentage in the structure further improves the toughness of the steel – a property that makes steel highly effective in applications requiring a combination of high strength and fracture resistance. This is because pools of retained austenite can act as ‘arrester of crack growth’ in the matrix. • Figure 1-11 also reveals that steels with %C less than 0.30% cannot be effectively hardened as the martensite of such steels are not hard enough. For martensite (as-quenched) to be considered hard enough for further processing or tempering, a hardness value of 580–600 VPN is considered minimum; and to reach to this value, a minimum carbon of about 0.35% is necessary in plain carbon steel. Hence, all hardening grade steels require sufficient level of carbon, or carbon and alloy mix, in order to get right type of martensite with right hardness level. Because of this fact, heat treatability of steels is related to the factor called ‘Hardenability’, which implies the ability of the steel to effectively harden to a specified hardness value up to a required depth. It is, therefore, obvious that by choice of carbon and alloy in the steel composition, martensitic hardness can be manipulated so that upon tempering, the steel provides the desired combination of strength and toughness. Figures 2-14(a), (b) and (c) show the nature of as-quenched martensite and martensite after tempering at different temperatures. As-quenched martensite is associated with high internal stresses due to quenching and shear mechanism associated with martensitic transformation. When the quenching rate is too fast, the process can give rise to high ‘internal stresses’ in the steel body on top of the transformation stresses and, thereby, may cause distortion or cracking (usually called quench cracking) of the steel. Hence, it is necessary to control the process of quenching as per the ruling section of the steel and susceptibility of the steel part to quench cracking. Quenching for the production of martensitic structure in the steel is a critical operation and forms an integral part of hardening operation in heat treatment. Quenching process needs to be controlled for minimising the chance of distortion/cracking, in one hand, and avoiding development of softer phases due to slack quenching at any part of the steel, on the other. Therefore, more about the quenching technology and quenching processes has been discussed in Chapter 6. Uniqueness of martensite comes from its flexibility in quenching and tempering in order to get the required combination of strength and toughness. Various combinations of chemical composition and cooling conditions can be adopted for getting the exact type of martensitic structure, which after tempering can fit to the specified properties for a given

Fig. 2-14 (a) As-quenched martensite in 0.5%C steel;. (b) and (c) tempered martensite structure of 0.5%C steel at tempering temperature of 500°C and 600°C, respectively

end-application. Some such conditions of cooling have been shown in Fig. 2-11. Other uniqueness of martensite is that it can be produced in steels selectively on the surface and surface-adjacent areas – by selecting the right composition of steel and choosing the right type of surface hardening process. Surface hardening can be carried out either by ‘induction or flame hardening’ of through-hardening steels or by case-carburising and case hardening process, involving carbon enrichment of surface area by thermo-chemical heat treatment process. Both processes bank upon the transformation of austenite to martensite in the selected areas. Martensite that is produced by surface hardening/case hardening is generally stress-relieved/tempered at relatively lower temperature, e.g. between 180 and 220°C. This is because, steels so treated are mostly used for wear resistance where high surface hardness is essential. Martensite is the most sought after microstructure in all hardening grade steels requiring high strength and toughness (or high hardness for wear resistance). But, it is not so for forming or bending applications, because of its limited ductility. Martensite is not easily workable because of insufficient ductility (vide Table 1-1). In sum, important features of martensite formation and its uses involve: • Selection of steel with right chemical composition so that right type of martensite can be obtained after quenching • Planning the right method of cooling (quenching) so that the cooling rate is just appropriate for martensite formation in the given steel but does not lead to excessive quenching stresses that can cause distortion or cracking in the steel parts. • Tempering of the martensite at appropriate temperature soon after the hardening so that there is no chance of internal crack due to

pressure from the quenched-in stresses. Tempering at 450°C or above for 45 minutes per sq. inch area has been found necessary for alloy steel martensite to get good toughness; for plain carbon steels, this temperature would be somewhat lower. Heat treatment for martensitic hardening is at the centre of hardening and toughening the steel. Hence, it is the most dominant process in the industry. The rules of austenite decomposition under time-temperature transformation conditions provide the ground rules for planning and designing the heat treatment cycle for martensitic hardening. However, as-quenched martensite being a very hard but brittle structure needs to be tempered appropriately for its final applications. Opportunity for tempering the martensitic structure offers wide flexibility for adjusting the steel properties. Through the combination of adjustment of (a) chemical composition, (b) quenching mechanism and (c) tempering treatment, a wide range of structure and structure-related properties in steels can be obtained by following the rules of phase transformation and microstructure formation in steels discussed in this chapter.

Summary 1. The chapter describes the basic rules of phase transformation in steel and formation of different microstructures, which forms the basis of all heat-treatment principles. Basic tenant of the chapter has been to discuss the decomposition of austenite under different cooling conditions and the mechanisms of formation of phases as a result of such heating and cooling conditions. 2. Based on equilibrium cooling diagrams of Fe-C system, and the theory of nucleation and growth (N&G), formation of ferrite and pearlite and their morphological changes based on cooling rate have been discussed and elaborated. 3. Conditions for formation of bainite and martensite with reference to TTT and CCT diagrams have been explained and illustrated. Simultaneously, the influence of cooling rates and alloy content in steels on the morphological characteristic of these two important phases have been highlighted throughout the discussions and their applications in the designing of hardening process of steels have been pointed out. 4. With regard to phase transformation and heat-treatment, importance of TTT and CCT diagrams has been pointed out and their respective uses have been highlighted. 5. Because of the structure sensitivity of steel properties, details of conditions of forming and morphological nature of different microstructural phases in steels have been listed out and illustrated through micrographs. 6. The chapter also highlighted the comparative difference in morphology and properties of upper bainite, lower bainite and martensite, and their applications have been pointed out. 7. Finally, uniqueness and utility of martensite – the all-important microstructure in steels – have been discussed. In this regard, influence of some special alloying elements, like Cr, Mo, and Ni have been pointed out.

References / Suggested Reading ASM Handbook, Vol. 4, Heat Treating, ASM International, USA, 1991 Leslie, W.C., The Physical Metallurgy of Steels, Hemisphere Press, McGraw-Hill, New York, 1981 Mandal, S. K. Steel Metallurgy: Properties, Specification and Applications, McGraw-Hill Education, New Delhi, 2014 Reed-Hill, Robert E., Physical Metallurgy Principles, Van Nostrand Reinhold Co., New York, 1973 Smallman, R.E., Physical Metallurgy and Advance Materials, Butterworth-Heinemann, 4th Edition, 1995 Totten, George E. (Ed), Steel Heat Treatment Handbook, 2nd edition, CRC Press, New York, 2006 Unterweiser, P.M., H.E. Boyer and J.J. Kubbs, (Eds) Heat Treater’s Guide: Standard Practice and Procedure for Steel, Edited by ASM, Metals Park, Ohio, USA, 1982

Review Questions 1. Why the rules of phase transformation in steel form the basis of its heat treatment? Name the microstructural phases and other constituents that influence the properties of steels. 2. With reference to Fig. 2-1, what would be the final microstructure of steel with (a) 0.80% carbon, (b) 0.40% carbon and (c) 0.90% carbon under equilibrium cooling? 3. What is the role of ‘nucleation’ (N) in the phase formation in steel? What are factors that influence the nucleation (N) and their growth (G) during phase transformation? How the N&G process characteristics influence the structure formation in steel? 4. With reference to Fig. 2-6, discuss the stages of austenite decomposition of 0.40% carbon steel under slow equilibrium cooling. What will happen in the state of decomposition products if the cooling rate is made faster than the equilibrium cooling? 5. With reference to Fig. 2-8, discuss the conditions under which austenite decomposes to the following microstructural products: Ferrite, Pearlite, Bainite and Martensite. Highlight how ‘grain size’ can influence the transformation process in steel. 6. Why phase transformation in steel is associated with ‘incubation period’ observed under faster cooling? Why does ‘incubation period’ vary with temperature of transformation? How does this influence the shape of time-temperature phase transformation diagram – like the TTT or CCT diagram? 7. Why CCT diagram is more relevant for heat treatment practices of steel than the TTT diagram? What information a TTT-diagram can provide for helping to design a heat treatment process? 8. With reference to CCT diagram type in Fig. 2-8, what phase transformation and microstructure formation can be expected by cooling the austenite (a) slower rate, (b) at an intermediate rate, and (c) at faster rate? When will you expect the formation of lower bainite in steel?

9. What all information could be gathered from the TTT and CCT diagram of steel for designing the heat treatment process and expected microstructures? 10. Describe the nature and morphology of following microstructures under the cooling conditions mentioned against each case: • Pearlite – under air cooling • Bainite – under faster cooling (by mild oil quenching) • Martensite – under faster cooling by water quenching and oil quenching Why morphology of the microstructural products change with faster cooling rate? 11. Why martensitic structure is the primary aim of all hardening operations? What is the uniqueness of martensitic structure vis-a-vis other structures in steel?

Introduction to Heat Treatment of Steels: Purpose and Processes

3.1

INTRODUCTION TO HEAT TREATMENT

Heat treatment is a process involving heating of a solid material to a predetermined temperature, followed by controlled cooling for obtaining a set of predetermined physical and mechanical properties in the material, but without bringing about any change in shape or physical dimensions. Required properties in the material get developed during this heating and cooling cycle by changes in the physical state of the material, where physical state refers to the state of internal stresses (if any) and structure of the material. Thus, objective of heat treatment processes is to bring about a favourable change in the structure (or state of internal stresses) of the material, without harmful distortion or cracking. The structure that will form on heat treatment is dependent on the phase transformation characteristics of steels – which have been discussed in Chapter 2 under ‘decomposition of austenite’. Hence, the rules that govern the heat treatment processes in steel are the rules for phase transformation and structure formation, discussed in Chapter 2. However, heating and cooling for structure formation may also give rise to harmful internal / residual stresses, which arise from the sequence of phase transformation and the condition of cooling. This is

also the concern of heat treatment operation. Thus, the process of heat treatment should not only be capable of producing a favourable structure in the steel, but also of minimising distortion, cracking or development of harmful residual stresses in the component. This necessitates control of heat treatment process with regard to (a) quality and correctness of steel, and (b) appropriateness of process parameters, including rate of heating and cooling. By controlling the rate of heating and cooling, and taking process precautions for avoiding distortion or harmful stresses, heat treatment as a process can fully meet the challenges of producing favourable structure and state of stress that are required for meeting the diverse range of mechanical properties for industrial application of steels. This can be illustrated by analysing the process steps and their effect in the tempering treatment of martensite. Like hardening, tempering is another heating and cooling process where quenched and hardened steel (i.e. as-quenched martensite) is heated for change in the physical state, including microstructure. In this process, following stage-by-stage changes take place as the heating temperature is gradually increased: • If the hardened steel part is heated and held to lower temperature (say, less than 200°C), the change will be mostly related to relieving of internal stresses of the steel that got accumulated in the component due to quenching and martensite formation. Such heat treatment process is called ‘stress relieving’, and is used for relieving stress of steel parts after hardening or for relieving accumulated stresses due to other reasons. • If the hardened steel is heated above this temperature (say, about 300°C) then first the stresses will be relieved and then some microstructural changes will occur by fine carbide precipitation from the hardened steel (i.e. martensite structure); thus lowering the strength of steel marginally but improving its toughness. This happens because the temperature used can provide more mobility to small carbon atoms to come out of the hardened martensitic structure and precipitate out in the matrix as carbide by combining with iron atoms in the matrix. • If the hardened steel is heated up further (say, above 450°C), more of carbide precipitates will form and finer precipitates will grow (i.e. carbide size will change); thus lowering the strength of the steel but further improving the toughness of the steel. This is due to even higher mobility of carbon atoms in the steel at higher temperature which helps fine carbides to coalesce together and form larger carbide particles, altering the size and distribution of carbide particles.

Thus, heating and cooling of a hardened steel can bring about a change in the internal stresses as well as change in microstructure, and, thereby, fulfil the objective of a specific heat treatment process. For bringing about a desired change in the physical state or structure of steel – which is the objective of any heat treatment process – following steps are essential: • Input of some thermal energy by way of heating, • Holding time at the heat, and • Cooling – as necessary for the process objective. Figure 3-1 illustrates the primary process cycle – the thermal cycle of heat treatment – for effecting a change in the structure and state of the steel.

Fig. 3-1

An illustrative heat treatment cycle

Heating, holding and cooling rate of the process are determined by the purpose of heating or the heat treatment process. For example: 1. Required heating temperature for tempering of hardened steel (as per earlier example) will be low (i.e. below A1 temperature). This is because the purpose of tempering is only to eliminate internal stresses and to toughen the steel by some carbide precipitations, which can take place at relatively lower temperature. But, if the purpose of heating is to convert the wrought steel structure to a single phase of austenite prior to phase transformation / hardening, then the heating has to be at sufficiently higher temperature (i.e. above the A3 temperature; vide Fig. 1-3). 2. Holding time is similarly influenced by the purpose. For tempering, time of holding to tempering temperature should be just enough to get adequate precipitation of carbides, and control their growth, for attaining the desired level of strength and toughness. But, for austenitising, the time of holding is determined by time taken for 100% austenitisation by the process of chemical diffusion and making

the austenite homogeneous. Homogenisation of austenite is an important step for uniform response to phase transformation / hardening by cooling, because of its influence on nucleation and growth of phases (as discussed in Chapter 2). Again, if the steel is alloy steel, holding time for austenitisation will be longer due to slower diffusion of alloying elements. 3. Cooling rate from the holding temperature is also the function of purpose of heat treatment. For example, air cooling after holding for tempering can be slow, fast or normal, depending on the purpose. It is generally standard air cooling, because cooling from such lower temperature is unlikely to introduce any thermal stress or change in the structure produced by tempering. Despite this fact, faster cooling after tempering is resorted to, in some types of steels, where there is chance of ‘temper embrittlement effect’. Faster cooling suppresses the precipitation or segregation of harmful residual elements that can cause the embrittlement of steel. Thus, cooling rate from tempering temperature – which is lower than the lower critical temperature – can be also driven by the purpose of the process. 4. Role of cooling rate in the heat treatment carried out above the upper critical temperature (A3) is more significant and itmust be controlled as per the respective process objectives. If it is for hardening, it should be fast cooling (quenching) for getting maximum hard product (i.e. martensite structure); if it is for annealing, the cooling should be very slow for getting the softest structure; and if it is for normalising, the austenite should be cooled by normal air cooling for getting finer grain size and some additional strength from the effect of fine grain size. These are few examples and elementary explanation of primary heat treating cycle, involving the purpose of heating, holding and cooling. More about these features of heat treatment cycles will be discussed at greater length later in this book. But, in sum, it stands that heat treatment is a process of controlled heating and cooling with specific purpose and care for developing required mechanical properties in steels for given uses or applications, but without causing distortion or cracking of the steel parts. While Fig. 3-1 schematically illustrates the heating and cooling cycle, Fig. 1-3 provides the actual temperature necessary for bringing about the phase changes in steel for change in the structure under slow cooling. Referring to Fig. 1-3, the Fe-C equilibrium diagram, it can be visualised that a steel is to be heated above A3 temperature to form 100% austenite phase before beginning to cool under equilibrium cooling condition for producing ferrite + pearlite structure of the steel; vide the vertical line

drawn on 0.40% carbon in Fig. 1-3. Thus, the process of heat treatment involves heating, holding (for a definite time) and cooling under controlled conditions, as depicted in Fig. 3-1, and necessary temperature of heating and rate of cooling for phase transformation or structure formation can be worked out from phase diagrams like Fe-C diagram (Fig. 1-3) or from the time-temperature-transformation diagrams (Figs. 2-5 and 2-9). Heat treatment is, thus, primarily a time–temperature process for effecting the change in the physical state of the material; either with respect to some physical phases and microstructure formation or in respect of changes in the physical state of the steel, such as internal stress, electrical resistivity, etc. The process of heat treatment is central to bring about a favourable change in the physical state or structure of the steel and thereby produce the desirable physical and mechanical properties. Processes of heat treatment that are used for effecting such changes in the physical state and structure of steel can be grouped into the following three generic categories, depending on their premises and physical-chemical characteristics of the processes: 1. Thermal Processes – refer to the processes involving simple heating and controlled cooling for change in the structure or state of internal stresses, without any change in composition or use of force (deformation) to modify the structure. Examples are: annealing, normalising, bulk hardening, surface hardening by induction or flame heating, etc. 2. Thermo-chemical processes – refer to processes where the changes in the structure are brought about by the combined action of heat and diffusion of another chemical element, whereby composition of the surface (or some selected areas of the part) gets altered. Examples of such processes are: case-carburising, nitriding, carbo-nitriding, boronising, etc., where attempt is made to increase the surface hardness for imparting some special surface properties (e.g. fatigue, wear and scuffing resistance) by enriching the surface with chemical elements like carbon, nitrogen and boron. 3. Thermo-mechanical processes – refer to the process where the changes in the physical structure are brought about by the combined action of heat and deformation under force, whereby the changed structure becomes very fine and may also give rise to some special precipitation of carbides, nitrides, etc., for strengthening the steel. Examples are: Plain thermo-mechanical rolling (TMT-rolling) for grain refinement and TMCP (Thermo-mechanical controlled process) for grain refinement, precipitation and developing grain orientations (textures).

Thermo-mechanical process is generally regarded as the special rolling process of steels, but the process functionally discharges the duty of heat treatment by changing the structure and properties of the steel. Hence, this type of treatment could be regarded as a part of heat treatment technology. A brief description of thermo-mechanical heat treatment has been included in this chapter for understanding of the scope of heat treatment. Each of these processes has its characteristic features, and needs strict control over the operational process parameters – like heating, holding and cooling, or any other process feature – for getting the right metallurgical structure and mechanical properties. However, amongst these processes, thermal and thermo-chemical processes of heat treatment dominate the field of heat treatment. Hence, these two processes will be further discussed in greater details in Chapters 7, 8 and 9 to highlight their practices and controlling parameters.

3.2

PURPOSE AND SCOPE OF HEAT TREATMENT OF STEELS

From the view point of heat treatment of steel, purpose of heat treatment can be broadly described as enhancing the uses and utility of steels by appropriately altering or improving the microstructure and its properties. Microstructure is the source of all mechanical properties of steels, and, thereby, controls the uses and utility of steels. [This, however, excludes the effect of internal or residual stresses in steel – which resides within the lattice structure but may not perceptibly alter the microstructure. Internal or residual stresses, in general, adversely affect the uses and utility of steel]. Therefore, the primary scope of heat treatment is to favourably alter the microstructure in steels for attaining the mechanical properties required for given uses and application. For example: • If good machining property is required for processing of the steel, the microstructure has to be soft and suitable for machining. In that case, annealing as heat treatment is used for high carbon steels for softening of the structure in order to make the steel better machinable. • If the steel is of lower / medium carbon grade, the steel is usually given the normalising heat treatment for producing a structure which is good for machining in these grades of steels. This is because annealing of lower / medium carbon steel can cause the structure to become too soft for good machining – whereas machining requires some hardness in the material for easy chip breaking.

• If the application of the same steel demands good fatigue strength and toughness of the finished part, the steel part is quenched and tempered (i.e. hardened) after finish machining to produce as much fine martensitic structure as permitted by the composition of the steel. Thus, for viable use and application, microstructure of steel needs to be made appropriate where heat treatment plays its unique role. Heat treatment is necessary even if the steel part is straightened after hardening to correct the distortion developed during the heat treatment. Correction of distortion by mechanical force can cause residual stress, which needs to be removed by stress relieving. Stress relieving heat treatment allows the harmful residual stresses, developed during straightening, to be relieved without any appreciable change in microstructure. Thus, scope of heat treatment extends from machining of a wrought steel part to hardening of the machined part and, if necessary, further improving the physical state of the part by relieving it from harmful residual stresses. All steels inherit a structure that arises from the solidification process or from the subsequent shaping / forming operations. Therefore, steels that are received from the steel-mills have their own initial structure as per the heating and rolling cycle these undergo during shaping. The initial structure undergoes further change during industrial shaping by processes like forging, pressing, welding, etc. But, not all such structure, or properties arising from such structure, may be adequate or acceptable for subsequent uses and applications. Hence, these initial structures need to be customised in order to get the right set of properties. Scope of heat treatment arises from this need of changing or altering the structure and, thereby, promoting right set of properties for uses and applications. Thus, the purpose of heat treatment can be restated as to ‘re-create or modify the structure and internal state of the material in most appropriate manner to suit the user-specific needs for making the selected steel suitable for processing and/or end application’. Such a purpose of heat treatment sets the scope of heat treatment processes of steels. Scope of heat treatment is, therefore, making the steel structurally appropriate and free of harmful residual stresses for effective utilisation / end application. Heat treatment process planning, therefore, starts from the point of understanding of what structure is appropriate for a given end-use / application in relation to required properties and how to accomplish that structure without producing any undesirable residual stresses in the components. All heat treatment processes involve controlled cycle of heating and cooling; thus giving an impression that heat treatment processes are purely ‘thermal processes’. However, alteration of structure may require, at times,

altering the composition of the area where structure needs to be changed. Hence, heat treatment may also be involved with ‘thermo-chemical’ process to change the local composition to facilitate structure changes, e.g. case carburising or nitriding of steel surface for high surface hardness, required for specific applications like wear and fatigue resistance. Similarly, application of steel may call for generating very fine and strong structure, which might require the help of controlled deformation and heat treatment at relatively lower temperature. In such cases, simple thermal heat treatment may not be able to induce the required change in the structure. Hence, in such cases recourse to initial deformation is taken before heat treatment to provide sufficient ‘stored energy’ in the steel for effecting the change to finer structure, or the steel is deformed appropriately during rolling and followed by controlled cooling to produce a very fine structure of appropriate nature. Example of the former heat treatment type is the prior cold-working and controlled recrystallisation, where stored energy due to cold-working induces recovery and recrystallisation at relatively lower temperature. This lower temperature recovery and partial recrystallisation produces fine grains and structure in the steel, making it stronger and tougher. Example of the other approach, which is fast gaining ground due to superior results and cost saving, is the controlled rolling of steel by giving calculated amount of hot deformation to a specially designed steel chemistry and cooling at controlled rate from the rolling temperature for producing fine-grained structure with special precipitation of hard phases, like carbides and nitrides. This type of process, involving controlled deformation and heat treatment, is known as ‘thermo-mechanical’ heat treatment, where mechanical deformation plays a critical role for enabling the required change in structure by providing necessary ‘activation energy’ at the relatively lower temperature of treatment. Therefore, heat treatment processes can involve number of combination techniques, other than simple heating and cooling, for producing different structure and properties in steels for making them suitable for wide varieties of end-uses and applications. Thus, the purpose of heat treatment is to beneficially alter or modify the structure (or stresses in the structure) of the steel – primarily with the help of thermal treatment, which can be further aided by chemical or mechanical processes, as and when required, for attaining the required change. As such, scope of heat treatment is very wide – covering the change or control of internal stresses and structures, structure types and their morphology, and surface engineering by modification of surface chemistry and alloying. This book aims to cover these scopes of heat treatment with process description and illustrations.

Thermo-mechanical rolling / heat treatment is an example of the versatility of heat treatment process for bringing about change in the structure and properties of steel by inducing recrystallisation and precipitation even at lower temperature than what thermodynamic phase diagrams would predict. By controlled amount of hot deformation at an intermediate temperature, where recovery of stresses caused by deformation is not fast enough, a situation can be created where the structure will recrystallise with simultaneous precipitation of alloy carbides (like titanium carbide, niobium carbide, vanadium carbide, etc) providing the steel with a strong and tough structure without having to harden by quenching. Such thermo-mechanical treatment can produce very fine recrystallised or partially recrystallised grains and precipitation of very fine carbides and nitrides in the steel; giving rise to high yield strength and uniform elongation required for critical forming of steel plates and sheets. However, a part of the scope of thermo-mechanical treatment is covered by thermo-mechanical rolling practices where the scope of structural changes can be combined with the rolling process – leading to energy-saving and lower cost. Similarly, steels can be subjected to selective surface hardening by thermo-chemical processes, giving rise to very high surface hardness in most economical way, for wear and fatigue-resistance applications. Table 3-1 illustrates the scope of heat treatment processes, in general, with reference to their applications.

It sums up the scope of common heat treatment processes; more about operations and control of these processes will be discussed in subsequent chapters. However, the foregoing discussions point to the fact that by manipulating the heating level, cooling rate and by taking help of chemical diffusion or mechanical deformation, the scope of heat treatment of steels can be made very wide, enabling wider application of steels for varieties of industrial purposes. These heat treatment processes by and

large follow the rules of phase transformations and structure formation in steel as discussed in Chapter 2 under austenite decomposition.

3.3

SOME BASICS OF HEAT TREATMENT PRACTICES

Heat treatment principles have been discussed in Chapter 2, which form the core of all heat treatment practices. In practice, heat treatments are carried out for bringing about a change in structure and state of material by applying heat (thermal energy) and controlling the dissipation of heat (i.e. cooling rate). The primary structural changes occur due to transformation of steel on cooling after austenitisation. Figure 3-2 illustrates a representative Fe-C diagram showing the practical heating levels of some popular heat treatment processes. The diagram depicts the temperature setting for annealing, normalising and hardening with respect to change in carbon in the steel. The diagram also covers the temperature for process-annealing and soft-annealing – also called spheriodising annealing – which is a popular industrial practice.

Fig. 3-2 Illustrates the typical temperature range of heating for different traditional heat treatment processes of steel with reference Fe-C diagram

For data interpretation from this diagram, steels can be grouped into (a) hypo-eutectoid steel (%C less than eutectoid carbon level) and (b) hyper-eutectoid steel (%C above the eutectoid level), and it can be observed that:

• If a hypo-eutectoid steel is to be softened by full annealing, then temperature of the process will have to be above the A3 (upper critical temperature) temperature of the steel for full austenitisation before furnace cooling. But, for hyper-eutectoid steel, the annealing temperature is just above the A1 (lower critical temperature) temperature. • For spheroidising anneal, the temperature of heating is just below the A1 temperature, and for ‘Process annealing’, the temperature is well below A1 temperature. • If the steel is to be normalised, again it is required to be heated for austenitisation above the A3 temperature of the steel composition, if it is hypo-eutectoid steel. And, for hyper-eutectoid steel, the heating temperature is just above ACm temperature. However, hyper-eutectoid steels are generally annealed for softening and not normalised. • For hardening, hypo-eutectoid steels need to be austenitised at temperature just above the A3 temperature, but if the steel is of hyper-eutectoid composition then heating just above A1 temperature is sufficient. Such a heating will be able to take sufficient carbon into the austenite solid solution for hardening the steel, leaving the excess carbon in the form of small spheroids of carbides. However, the process of change of steel to austenite (g -phase) due to heating is not instantaneous; the process requires the job to be held at that temperature for a time, as per its cross section / mass. This is called the ‘soaking time’ for austenitisation in heat treatment. Soaking is required for producing homogeneous austenite for avoiding non-uniform structure after heat treatment in a given section – be that in normalising or in hardening. Not all heat treatment processes are involved with high temperature heating for austenitisation or phase transformation like ferrite, pearlite, martensite, etc. There are heat treatment processes where there is no phase changes involved. For example, soft annealing which can be carried out at temperature below A1 does not involve any phase changes, but shape changes of carbide from lamellar structure to spheriodised structure. Holding time at the heating temperature for such sub-critical operation is determined by the requirement of structural changes and degree of softness. Cooling from such sub-critical operation also need not be controlled, but for softer structure and lower hardness, slow cooling is desirable for taking advantage of slow heat dissipation so that more spheriodisation effect could be attained. Formation of austenite is controlled by diffusion process, which is a thermally activated process. Diffusion rate increases with increasing temperature, but decreases with complexity of alloy content in the steel. Hence, alloy steels generally take longer holding time than plain carbon

steels for producing homogeneous austenite. Though, theoretically, diffusion rate can be made faster for shortening the holding (or soaking) time by using higher austenitisation temperature, but it is not desirable. Higher austenitising temperature will lead to larger austenite grains, which is not good for toughness of the steel. Moreover, higher austenitisation temperature may cause incipient fusions of impurities that segregate at the grain boundaries in steel, and, thereby, may lead to quench cracking or embrittlement. Grains and grain boundaries – a constant feature of steel microstructure – influence steel properties as well as heat treatability in many ways, as discussed earlier in Chapters 1 and 2. Hence, care is necessary for restraining grin growth during heat treatment of steels, which is controlled by heating the steel just above the A3 temperature of the steel. Foregoing discussions of heat treatment processes with reference to Fig. 3-2 pertain to equilibrium cooling condition. But, large part of heat treatment processes involve faster cooling than the equilibrium cooling, such as all heat treatment processes aiming for hardening of steels. Phase transformation and structure formation under non-equilibrium cooling conditions is guided by the time-temperature-transformation diagrams – like the TTT / CCT diagram, which has been discussed in Chapter 2. Transformation under faster cooling or continuous cooling is characterised by a delay time to start the transformation – called the incubation time for phase transformation; vide Fig. 3-3. For practical heat treatment, this delay-time window must be used to adjust the cooling rate (i.e. quenching) accordingly – in order to get desired phase changes and resultant microstructure. For example, in the

Temperature / °C

Ae3

Ferrite Pearlite

WS

Widmanstatten ferrite

BS

Upper bainite

Bainite

MS

Lower bainite

Martensite Log {time}

Fig. 3-3 A typical CCT-diagram of Cr-Mo alloy steel with superimposed continuous cooling curves of different cooling rate starting from Ae3 temperature (Ae3 denotes the A3 temperature on cooling). [Cooling curves in this figure represent the cooling at the centre of the bar]

given steel for this CCT-diagram, formation of martensite will require control of cooling rate for avoiding touching of the nose of the lower ‘C’-curve (indicated by an arrow). This condition is fulfilled by the first cooling curve on the time axis. This type of cooling will ensure martensitic transformation without interference from any other phase / structure formation. Similarly, if production of bainitic structure is the aim of the hardening, then cooling rate can be relaxed to the extent as represented by the second cooling curve. If for any reason, cooling rate becomes slack (as the third curve), then the structure may contain traces of ferrite, some pearlite and some widmanstatten ferrite as well. Widmanstatten ferrite may occur, generally at higher temperature, when cooling rate is relatively fast for not allowing conventional ferrite separation at the austenite grain boundaries. This leads to a situation when some ferrite precipitates on the grains along certain crystal planes and form a characteristic structural pattern, called widmanstatten structure. This structure is not desirable in steel because it reduces impact strength; hence needs to be avoided. Accordingly, cooling of steels that has the tendency of widmanstatten structure formation (such as higher carbon steel and some alloy steels) needs to be controlled during heat treatment. Figure 3-3 depicts a CCT-diagram that is typical of alloy steels, like Cr-Mo type steel, where the two regions of phase transformation – belonging to (ferrite-pearlite) and bainite structure – get separated and move to the right on the time axis. This implies that the transformation start time to (ferrite-pearlite) separation or bainite transformation will be further delayed in such steels and, thereby, ensure easy and uninterrupted transformation of austenite to martensite. For practical hardening operation, this is a desirable situation requiring less severe quenching. The figure also indicates that in this type of alloy steels, bainite formation is highly probable if the cooling rate is somewhat slower in any portion of the steel under quenching–such as towards the centre of steel sections. However, bainite so formed is likely to be in the form of lower bainite, which is structurally and property-wise closer to martensite. Hence, presence of such structure inside deeper into the section may not be harmful or objectionable for normal application of steels. An important part of heat treating steel using faster cooling is the severity of cooling or quenching rate. More severe the quenching more is the chance of distortion / cracking in hardening – though that type of cooling might be required for producing sufficient martensite in the steel. Hence, control of quenching for hardening of steel is a critical area of heat treatment practices. There are many cooling / quenching media available for heat treaters, which can be judiciously used for getting the correct result without causing unacceptable distortion or cracking of steel parts.

More about different quenching medium and quenching technology has been discussed in Chapter 6 for better insight into this problem and their solutions. Referring to Figure 3-3, austenite needs to be cooled below the MS temperature for formation of martensite. Discussions on martensite formation in Chapter 2 pointed out that martensitic transformation is an ‘athermal’ process, i.e. the progress of martensite formation depends on the temperature of cooling and not on time of holding. This means that it is not enough to quench the austenite to or below MS temperature, it has to be continuously cooled to temperature where martensite transformation is finished (MF). If the cooling of austenite is stopped in between the MS and MF, transformation of remaining austenite will also stop. For completing the transformation of all austenite to martensite, temperature must be cooled below the MF and time of cooling (i.e. cooling rate) at this stage is of no consequence. In heat treatment practices, this situation is sometime used with the advantage for interrupted quenching which helps in reducing thermal shock or quenching stress. By rapidly cooling below the MS temperature and then holding at that temperature for a short while will allow temperature equalisation before final cooling and also less thermal stress. This type of quenching technique helps reducing residual stresses and distortion in high hardenability steels, like high carbon or high alloy steels. However, TTT or CCT diagrams of steel are composition-specific, i.e. these diagrams change with the change in chemistry of the steel. Therefore, for the design of heat treatment process cycle, relevant transformation diagram with respect to the steel composition must be used or referred. There are number of atlas of transformation diagrams in steels (vide the reference of books at the end of the chapter), which can be of help in this regard. Thus, between Fe-C phase diagram and CCTdiagram of the steel (which differ with differing composition) phase changes and microstructure formation of steel under different cooling conditions can be worked out and used for designing the heat treatment parameters of different processes. Microstructure formation in steel is greatly influenced by the nucleation and growth (N&G) process; vide discussions in Chapter 2. Grain boundaries (i.e. the parent austenite grain size), chemical heterogeneity, and high dislocation density (such as that arise from cold-working of steels) act as preferred sources of nucleation of different phases on cooling. Higher rate of nucleation arising from such sources leads to lower growth requirement for new phases, and thereby producing finer structure, influencing the morphological character of the resultant structure.

In planning of heat treatment process parameters, these aspects of phase nucleation and growth need to be considered in practice. For example, for martensitic transformation, control of austenite grain size within a limit (ASTM size 5 to 8; vide Chapter 4) is necessary for avoiding the chance of prior nucleation and formation of other phases and structures. Similarly, softening of a ferrite-pearlite structure can be planned at relatively lower temperature, if the steel has been cold-worked earlier, because cold-worked structure will have numerous sites for nucleation and lower recystallisation temperature due to ‘stored energy’. As such, while critical temperatures relating to phase transformation are used as the reference points for deciding the time – temperature cycle of different heat treatment processes, presence or absence of factors influencing N&G of phases should have to be considered.

3.4 INTRODUCTION TO THERMAL HEAT TREATMENT PROCESSES Thermal heat treatment refers to the heat treatment processes where the desirable structural changes or stresses in the steel are accomplished by only using heat (i.e. thermal energy); vide Section 3.1 for classification of heat treatment processes. Commonly used thermal heat treatment processes are: annealing (full), sub-critical annealing, normalising, hardening, tempering and stress-relieving. Of these, annealing and sub-critical annealing processes are slow cooling equilibrium processes – as their objective is to soften the steel; vide Fig. 3-2. Normalising, the other equilibrium process, is carried out at relatively faster cooling than annealing to take advantage of higher N&G process for producing finer structure. Normalising is, therefore, carried out by air cooling in order to strengthen low and medium carbon steels or to soften higher carbon steels, as the case may be. Normalising is also used after hot working of the steel to make the wrought steel structure more uniform and fine grained. These equilibrium thermal heat treatment processes are carried out in reference to Fe-C diagram. The other thermal heat treatment process is the hardening of steels, which is a non-equilibrium process. Hardening of steel is principally involved with quenching for production of martensitic structure. For non-equilibrium cooling process, reference to TTT or CCT diagram of the steel is made for designing the appropriate cooling rate and technique for quenching. By the process of faster cooling, bainite – an intermediate product that forms at intermediate temperature range between pearlite and martensite – may also be produced, if the steel is alloy steel. Bainite is a harder phase than pearlite, but softer than martensite. Bainite may form along with the martensite during hardening operation at locations where

cooling rate lagged due to thermal conductivity. However, sometimes bainite (especially lower bainitic structure) is produced in the structure by plan by appropriately adjusting alloy composition in the steel and the cooling rate. Hardening is a fast cooling process with an aim to basically produce as much martensite as possible in the structure without giving rise to cracks or unacceptable distortion due to severe quenching. This condition limits the scope of using too much faster quenching of steel parts. In order to moderate the quenching rate, steel with adequate hardenability corresponding to the composition and section size (i.e. ruling section) has been discussed in the next chapter (Chapter 4). Martensite that forms under quenching is strong but brittle. Hence, a special heat treatment is given to the as-quenched martensite structure, which is called ‘tempering’. Aim of tempering is to improve toughness in the structure by sacrificing some strength through precipitation of carbides from the distorted as-quenched martensitic structure. Stress relieving is another special operation which is used to relieve internal stresses in the steel body, coming either from quenching effect or due to some cold-working on the steel. Stress relieving is carried out at relatively lower temperature than tempering. These processes put-together covers a very wide spectrum of thermal heat treatment operations with the aim of either improving the microstructure for processing and manufacturing or for changing the structures for strength and toughness for end-applications. Salient features of these processes – which are based on the heat treatment principles and parameters as discussed earlier in this and earlier chapters – are highlighted below. Principles of these heat treatment processes have not been discussed here for avoiding the repetition; discussions in Chapter 2 regarding the applicable rules and principles of different heat treatment processes may be referred when necessary. Only the salient points of their process features and practice have been mentioned in this introductory chapter. Additional features for their heat treatment practices on the shop floor will be discussed in Chapter 7.

3.4.1

Annealing

Annealing is a softening process carried out for hypo-eutectoid steel by heating above the upper critical temperature (A3), holding (soaking) it at that temperature for a time period (as per cross-section), and then slowly cooling (inside the furnace or in a closed area) for changes in structure and properties. If the steel is of hyper-eutectiod composition, the heating temperature is just above A1 temperature. Annealing lowers hardness and increases elongation, i.e. ductility. Figure 3-4 shows the range of annealing temperatures vis-à-vis steel composition.

Fig. 3-4 An illustrative part of Fe-C diagram showing the temperature range of standard annealing process (full annealing)

For lower and medium carbon steels, annealing temperature is guided by the upper critical temperature (A3), but for higher carbon steel (exceeding eutectoid composition of 0.80% C) it is guided by the lower critical temperature (A1). This is because both the temperature lines merge together for steel of eutectoid composition or over. If annealing of steel is carried out below the critical temperature A1, it is termed as ‘sub-critical annealing’ – as indicated by the band of thin lines in the figure. Purpose of these two annealing processes is similar, i.e. to soften the steel structure, but full annealing achieves this goal by producing coarse grains of ferrite and coarser pearlite spacing, whereas sub-critical annealing achieves the goal by spheriodising the pearlitic structure; respective microstructures are shown in Fig. 3-5(a) and (b). Full annealing takes place in three steps: • Recovery of any remaining or left over stresses in the steel from prior working • Recrystallisation of the steel, and • Grain growth Full softening occurs when temperature and time allowed are sufficient for completion of these three steps. Full annealing is used for softening the steel and for homogenising the structure. However, for sub-critical annealing, which is widely practiced in industries, steels are heated below the lower critical temperature (A1) where the aim is to spherodise the pearlitic carbide and achieve softening of the structure. However, if the steel has undergone prior working, the structure may also attain partialrecrystallisation due to stored energy, and add to the softening of the steel structure.

Fig. 3-5 (a) Fully annealed soft structure of low carbon sheet steel with coarser ferrite grains and pearlite in bands (due to prior working); (b) Sub-critically annealed 0.85% carbon steel for softening, showing carbides in spheroid form due to prolonged holding.

Sub-critical annealing is applied to soften cold-worked steel parts, castings and high carbon wire rods by heating between 550 and 700°C (i.e. below the lower critical temperature) for several hours in order to produce recrystallised soft structure and spheriodisation of carbides. Recrystallisation of steel is made possible at this lower temperature by the ‘stored energy’ coming from strained structure due to prior cold-working. Since the process uses lower temperature, the process is cost effective and saves energy. There is another softening process – called the ‘Process Annealing – which is used to soften work hardened in-process steel for recovering ductility for further working. Process annealing can be considered as a part of this sub-critical annealing. The process is carried out by heating to temperature just below A1 and the holding long enough for recrystallisation of ferrite phase in the steel structure and then cooled to room temperature in still air. As such, the process only changes the shape, size and distribution of ferrite, and thereby restores the near original ductility in the steel. The process is much cheaper than full annealing, but achieves similar purpose as full annealing for low carbon steels. Precautions are necessary in annealing of steels for avoiding over-heating, burning (by incipient fusion of grain boundary areas), and excessive scaling of the surface. This necessitates care in controlling temperature and time of heating at the annealing temperature. Because of the scale that forms while in the furnace or during slow cooling, annealed steels are either shot-blast cleaned or pickled before further uses and working.

3.4.2

Normalising

Normalising also consists of heating the job above the upper critical temperatures of the steel (for steels up to eutectoid composition), vide Fig. 3-2. Thermodynamic consideration for normalising is same as annealing, but with the difference of cooling. Normalising uses air cooling technique, which is faster than the slow furnace cooling for annealing. Faster air cooling influences the N&G process (discussed earlier) in favour of producing fine-grained structure, which exhibits higher strength and ductility than an annealed structure. Though many a time heating temperature for normalising and annealing is quoted as similar, but in modern industry practice, temperature of normalising is slightly lower than the full annealing temperature of the steel. If annealing temperature is (A3 + 50°C), normalising temperature is (A3 + 30°C) of the steel. This is due to (a) uses of Al-killed steel for most engineering components where annealing requires higher heating temperature for obtaining coarse grains, and (b) normalising aims for finer grains than annealing; hence lower heating temperature is preferred. However, where normalising is used for homogenisation of structure, higher soaking temperature in the range of annealing could be used. Most engineering applications use steels corresponding to hypoeutectoid composition. For such steels, the normalising process involves heating to above A3 temperature of the steel, holding for temperature equalisation (soaking) for a specific time, and then cooled in the air (as against furnace cooling in annealing). This treatment is especially given for homogenising the composition and for producing fine grained uniform structure in steels of medium carbon, such as steel grades SAE 1035, 1045, etc. Figure 3-6 shows a normalised structure of AISI 1045 grade steel, depicting fine grain nature of such structure due to faster cooling (air cooling) than annealing. The main difference of normalising with full annealing is that normalising is carried out by faster cooling (air cooling) than annealing. Such cooling process gives rise to finer grain size and finer pearlite structure, due to N&G process associated with faster cooling. However, there could be variation of structure between surface and the centre in normalised steel if the section size is large. This is because of slower cooling of centre during air cooling due to thermal lag between surface and centre when the steel bar is of larger diameter. Therefore, properties of normalised steel might vary with the section size; higher the section lower is the strength properties of the given section. Normalising produces, in general, higher tensile strength, yield strength, reduction in area, and impact value than the annealed steel. Higher strength of normalised structure comes from finer ferrite grain size and finer pearlitic spacing compared to annealing.

Fig. 3-6

A normalised structure of AISI 1045 grade steel (x200)

Primary purpose of normalising is to obtain fine and uniform grain size in the steel, but the process also serves to homogenise the structure for better machining and hardening. Normalised structure gives better response to hardening of steels by removing the effects of non-homogeneous deformation on the steel during prior shaping, such as by rolling and forging. Due to higher temperature of heating and holding, normalising can refine the existing non-uniform composition and structures into a more homogeneous composition and structure, but with the limitation of section size. Homogenisation of larger section by normalising may not be satisfactory due to faster cooling of the process, where temperature gradient between cooling surface and core might cause structural heterogeneity. However, for all practical purpose, the structure produced by normalising a low or medium carbon steel enhances its machining and forming behaviour by inducing a fine grained structure with finer pearlite. Prior normalising of forged components also improves subsequent hardening of the steel. However, there is a risk of surface degradation by oxidation and scaling in both annealing and normalising. Hence, the heat treated parts require shot blasting or chemical pickling for cleaning of surface before put to use for further operations e.g. machining, forming, welding, etc.

3.4.3

Hardening

Hardening of steel, being the most important and critical of all heat treatment processes, occupies the centre stage of steel heat treatment. Since most engineering steel composition corresponds to hypo-eutectoid steel, hardening in this section will be primarily discussed with reference to this type of steels only. Hardening operation of hypo-eutectoid steel is carried out with reference to corresponding TT-T / CCT diagram of the steel (vide

schematic representation in Fig. 3-7) by heating the steel just above the upper critical temperature (A3) which is followed by quenching (i.e. fast cooling). The thermal cycle involved in hardening operation is as follows: heating the steel gradually to just above A3 temperature of the steel and holding it for temperature equalisation and full austenitisation (called soaking time) at that temperature, followed by quenching for production of as much martensite as possible.

Fig. 3-7 An illustration showing the cooling difference between the surface and centre of a 95mm alloy steel bar when quenched in oil for 100% martensite (on the surface) with reference to its CCT-diagram. Faster the cooling rate, wider is the gap between surface and centre

Quenching is the most critical part of hardening operation. Quenching media and condition are so selected as to produce 100% or maximum volume of martensite in the structure, but without causing any crack or unacceptable distortion of the steel parts. There are number of quenching media available with different quenching characteristics to fulfil the demands of such cooling conditions, which have been discussed in Chapter 6 on ‘Quenching technology’. Success of all hardening processes depends on selection of quenching medium and technique as appropriate for the steel composition and quality. Formation of martensite in steel requires rapid cooling of austenite at high speed without allowing time for carbon atoms to diffuse out of the crystal structure of austenite till the temperature reaches the martensite start temperature (MS). But, once the martensite transformation has started, cooling rate could be cut down to relatively lower rate, which helps in avoiding cracks or distortion in the jobs. Cracks and distortion may occur in quenched parts due to high thermal stresses arising

from the quenching, and this tendency of steel goes up with increasing hardenability of the steel. However, as explained in Chapter 2, all martensite does not form instantaneously on cooling to MS temperature; it forms progressively with decreasing temperature below MS and finishing on reaching MF. In martensitic transformation, it is not time but the temperature of cooling or holding determines how much martensite has transformed. Austenite must be cooled below the martensite finish temperature for full transformation of austenite to martensite. As such, hardening process requires maintaining the temperature of quench bath below the MF temperature of the steel and holding of the steel in the quench bath till temperature of steel has reached below MF temperature. Jobs can then be taken out of the quench bath or cooling system and allowed to cool normally. A conventional quenching and tempering process is illustrated in Fig. 3-8.

Fig. 3-8 Schematically illustrates the conventional quenching and tempering process of steel, taking care that both surface and centre cools fast enough to produce 100% martensite

On fast cooling (quenching), austenite becomes super-saturated with carbon – as no time is allowed for carbon atoms to diffuse out of g -lattice (austenite). This super saturated austenite is not stable at the lower temperature and transforms to, what is called, ‘martensite’. Martensite, therefore, takes up the structure of a super-saturated solid solution of carbon in highly distorted tetragonal body centred cubic lattice structure. Figure 3-9 exhibits an enlarged view of martensite structure where nature of martensitic plates and their orientation within the prior austenite grains can be observed.

Fig. 3-9 Illustrates an enlarged view of very lightly tempered martensitic structure (self tempered) in steel.

This structure of martensite in as-quenched state is composed of highly strained ferrite plates (or needles, as the case may be) with high dislocation density in the structure where supersaturated carbon atoms are frozen in. This makes the martensite structure metastable; having high internal strain and the structure becomes strong but brittle. Because of high internal strain in martensite structure, stress relieving of the structure is necessary which should be done as early as possible after quenching. Otherwise, this structure may develop fine micro-cracks as part of self-relieving of stresses on longer holding and storage – rendering the part rejected for any further application. Once micro-crack has developed, the process is not reversible, i.e. not rectifiable. Hence, stressrelieving or tempering of as-quenched martensite is an integral part of steel hardening facility required in a heat treating shop. Stress relieving, which is carried out at lower temperature (e.g. between 180 and 220°C), only relieves the stresses without any appreciable change in microstructure. But, tempering is carried out at relatively higher temperature (e.g. from 300 to 650°C) in order to bring out the trapped carbon atoms from inside the distorted lattice of martensitic structure to form of ‘carbide’ precipitate. Temperature of tempering provides the thermal activation energy required for mobility of carbon atoms and diffusion out of the distorted structure, and, thereby, facilitate the process of carbide precipitation in the matrix. As a consequence, strain in the matrix is lowered, strength of the structure decreases and the toughness of the structure increases. Degree of drop in strength and increase in toughness

depends on the tempering temperature; higher the temperature higher is the strength loss and better is the toughness. Details of tempering have been discussed in Section 3.4.4 and in Chapter 7. However, due to variation of cooling rate from surface to centre, because of relatively lower thermal conductivity of steel, 100% martensite across a section might not be possible in many cases, unless section is too thin for the adopted quenching process or the cooling rate is too fast. The difference in cooling between centre and surface of an alloy steel bar has been illustrated in Fig. 3-7 – where while surface transforms to martensite, core passes through the bainite formation zone producing some percentage of bainite in the structure, if the steel composition is favourable for bainite formation e.g. low-alloy steel or carbon-manganese grade steel. Figure 3-10 shows a bainitic structure so produced in the inner section of a low-alloy steel bar quenched in oil for martensite.

Fig. 3-10 Bainite structure in the inner section of a steel bar quenched for martensite (x 250). (Structure contains some amount of martensite as well)

The diagram in Fig. 3-7 makes it clear that 100% martensite at the core might not be possible in many cases, unless the steel is made rich with alloy content in order to make the CCT curves move further to the right from the temperature axis. Alternatively, quenching rate could be more severe to ensure that core also cools fast enough for martensite transformation despite thermal lag. However, too high quenching rate is undesirable, because of chances of cracking and distortion of the steel parts under quenching. This situation makes it necessary to optimise the quenching rate by selecting deeper hardening steel for getting martensitic structure.

Many a time, producing 100% martensite structure in a steel part may not be possible without the risk of high distortion or crack, despite optimisation of steel composition and quenching rate. In such cases, some bainite at the core of a martensitic structure might have to be allowed, subject to functional suitability of the steel. The soaking time for hardening – for homogenising the austenite composition – is a function of steel composition and temperature of heating. The process is controlled by chemical diffusion process in the system. Generally, alloy steels take longer time for homogenisation than plain carbon steels, because of slower diffusion rate. In practice, soaking time is guided by thumb-rule in industries, which is generally about 30 minutes for a section of 30mm dia. plain carbon steel bar. This may increase a bit (in the range of 30 to 45 minutes) if the steel contains complex alloying elements like Cr and Mo together. Factor that limits the soaking time and temperature is the probability of austenite grain growth and incipient melting of grain boundary due to impurity segregation at the grain boundaries. Scaling due to oxidation and decarburisation of steel surface are other factors that need care during heating for austenitisation for hardening. Both can interfere with the hardening process; scaling by affecting the rate of heat extraction from surface and decarburisation by leading to lower surface hardness due to loss of surface carbon, which is necessary for martensitic hardening. Therefore, atmosphere control in the heating furnace may be necessary for protecting the steel from oxidation and decarburisation reactions at the austenitisation temperature. Decarburisation, if present, after hardening and tempering must be removed or neutralised by shotblasting / shot-peening or by some appropriate means. Atmosphere control in hardening furnaces has been discussed in Chapter 5. Hardening of steel is synonymous with the process of ‘quenching and tempering’. Quenching is the most critical of all steps in hardening. It involves quick transfer of the heated job to the cooling tank (quenching bath) containing appropriate medium in sufficient volume. Sufficient volume of the quenching media is required for ensuring efficient transfer of heat from the quenched jobs without abnormal rise in bath temperature. Any appreciable rise in quench bath temperature will slow down the heat transfer (i.e. cooling rate) and, thereby, may induce formation of non-martensitic product in the structure. After allowing short time for temperature equalisation of quenched jobs in the bath, the jobs are taken out and tempered. Common quenching media are water, oil and molten salt-bath, with or without agitation; each having its characteristic cooling curve and cooling rate. Water has higher quench severity than oil. Hence, water

quenching is used with discretion in order to avoid cracking / distortion of the steel parts. However, for larger cross-section of steel, water quenching becomes unavoidable in order to get the right microstructure. Because of this problem with water quenching, very often oil quenching is used for general hardening operations for medium carbon and low-alloy steels. Gentle agitation of quench bath further improves quenching by making heat removal faster and more uniform. Because of its importance in hardening of steels, quenching technology and means have been discussed in details in Chapter 6. Not all steels respond to hardening or respond equally; response to hardening is based on some intrinsic quality of the steel, including the composition. Steel must have a minimum level of carbon or (carbon + alloying) for enabling the steel to form martensite. Ability of the steel to form martensite up to a certain depth is referred to as ‘hardenability’, which is an important consideration for choosing the steel and its hardening process parameters. In general, carbon and alloying elements increase the hardenability of steels. In terms of phase transformation, this means that these elements push the transformation curves (C-shaped curves in Figs. 3-2 and 3-7) of ferrite, pearlite, and bainite transformation further to the right on the temperature time axis; thereby enhancing the possibility of martensite formation by default, if cooling rate is fast. This is the central point of heat treatability of steels, which has been discussed in Chapter 4. Thus, hardening process requires: • A minimum level of hardenability of the steel for producing martensite to the required level or depth • Good heating and quenching facilities to match the required control over the heating and cooling cycle • Facility for tempering the jobs within a close range of temperature • Facility for surface cleaning by non-chemical processes, like shot-blasting • Testing / Inspection for cracks and distortion in the parts after finish heat-treatment, because quenching can induce fine cracks, if cooling rate is too severe. Formation of different microstructural products during hardening is governed by the principles of austenite decomposition and the cooling conditions, which have been discussed in the previous chapter (Chapter 2).

3.4.4

Tempering and Stress Relieving

All steels are either stress-relieved or tempered after hardening, i.e. quenching. Tempering and stress-relieving are both lower temperature

operation, but their objectives are different. Tempering is carried out for restoring some toughness at the cost of strength in the as-quenched structures, which are strong but brittle. Figure 3-11 depicts how hardness drops and toughness improves in the tempering of martensitic-quenched structure. However, tempering is also a time-dependent process – due to its diffusion rate sensitivity. Higher the tempering temperature and time, more is the tempering effect and lower is the strength. But, higher tempering improves toughness of the structure; vide Fig. 3-11. Figure 3-12 illustrates the general trend of drop of hardness with time and temperature of tempering for as-quenched martensitic structure. Tempering is carried out at temperature much below the lower critical temperature (A1) of steels, because objective of tempering is not changing of allotropic structure of steel, but improving the structure for toughness. In the process of heating to tempering temperature, martensite first relieve the locked-in quenching stresses (internal stresses) and then with increasing mobility of carbon atoms in iron lattice, fine epsilon carbide (Fe2.4C) forms, which is a transitional phase. With further increase of temperature, more carbon moves out of strained martensite structure and forms full-fledged regular carbides (Fe3C), giving rise to typical tempered

Fig. 3-11 An illustration showing the drop of hardness and trend in the improvement of toughness with increasing tempering temperature

Fig. 3-12 The graph schematically showing the drop in hardness of martensite with tempering temperature and time

structure of martensite. This tempered structure generally contains acicular ferrite plate or needles and fine carbide precipitates all over it; vide micrograph in Fig. 3-13. Tempering of as-quenched martensite takes place in four distinct but overlapping stages: (1) Up to 250°C, relieve of internal as-quenched stress and precipitation of epsilon carbide, resulting in partial loss of tetragonality in martensite structure (2) Between 200 and 300°C, decomposition of retained austenite, if any due to incomplete transformation of austenite to martensite (3) Between 200 and 350°C, replacement of epsilon carbide by cementite (Fe3C); martensite looses tetragonality at this stage (4) Above 350°C, cementite coarsens; higher the tempering temperature more coarsening of cementite particles. At temperature over 400°C, martensite structure of plain carbon steel starts softening appreciably by coalescence of precipitated fine carbides. However, for alloy steel the required tempering temperature is higher, because of relatively higher temperature and / or longer time required for formation of alloy carbide. Alloy carbide forms by the diffusion of carbon atoms from strained martensite structure and chemically combining with alloying elements which have affinity for carbide formation, such as Cr, V. Mo, etc.

Fig. 3-13

Tempered martensitic structure of AISI 5140 steel (x400)

Figure 3-13 illustrates a tempered martensitic structure of alloy steel with 0.45% carbon, showing martensite plates and fine carbides on plates and plate boundaries; etched darker due to chemical reaction. An important feature of tempering operation is that tempering should be carried out as early as possible after hardening in order to avoid the cracking and distortion of jobs due to internal stresses produced during quenching. Hence, very often, hardening and tempering facilities are built alongside each other in the shop floor. Generally, tempering of steel is conducted in the temperature range of 200 to 650°C, depending on the type of steel.However, care is necessary in tempering for avoiding, what is known as ‘temper brittleness’. There are two forms of this brittleness; one is known as temper embrittlement that affects both carbon and low alloy steels when they are either cooled too slowly (from above 575°C) or held for excessive times in the range of 375 to 575°C. The temper embrittlement effect can be reversed by heating to above 575°C and rapidly cooling. The other is known as ‘blue brittleness’ that affects carbon and some alloy steels after tempering in the range 230 to 370°C. This effect is not reversible. Therefore, tempering for longer time in this temperature range should be avoided. Steels suspected to have induced blue brittleness should not be used in applications in which they face shock loads. Stand-alone stress relieving is generally carried out at temperature even lower than the tempering temperature with two purposes: one to relieve quenched-in stresses in steels, especially the case-hardened steels; and the other for stress-relieving of steel parts that have undergone cold deformation, resulting in harmful residual stresses, e.g. straightening of a shaft after hardening. For relieving of quenched-in stresses, the job is heated at or below 220°C and for removal of residual stresses

after cold-work, heating can go up to 400°C but avoiding ‘temper embrittlement’ and softening of the job.

3.4.5

Induction / Flame Hardening

These are surface hardening processes for applications where high surface hardness is required for wear and fatigue resistance. Hence, these processes aim to harden a small or thin portion of the surface area of the steel, leaving the core structure undisturbed. By this technique, a hard surface and tough core structure can be produced in steel parts for meeting the end-applications involving wear and fatigue. The process is termed ‘induction hardening’ or ‘flame hardening’ as per the source of heating used. Since, induction heating is more controllable than flame heating, induction hardening process is preferred for precision applications. In flame or induction hardening processes, a portion or the surface of the job is heated by induction or flame heating to temperature above A3 and then spot cooled by using a suitable media like forced air, water spray or by oil. Approach of flame hardening and induction hardening is same, but their heating method and control of processes are different. Flame hardening is carried out by heating the selected surface area by appropriate nature of flame (generally reducing flame) and then cooling fast to produce martensitic structure in the heated zone. Chemical nature of the flame is important as it controls the decarburisation of steel in the heated zone. Flame for heating is generally created by controlled oxy-acetylene gas burning in a specially designed torch burner. In order to avoid decarburisation or oxidation of the surface, the job needs to be quickly heated to the hardening temperature, which is just above the A3 temperature of the steel. Although flame and induction hardening is mainly used to develop high levels of hardness for wear resistance, the process also improves bending and torsional strength and fatigue life. One of the major advantages of flame or induction hardening is the ability to satisfy stringent engineering requirements using carbon steels. However, flame hardening lacks precision and sophistication compared to induction hardening process. Induction hardening is carried out by heating the job under a suitable inductor where heat input can be controlled by controlling the power frequency. Figure 3-14 illustrates a typical set up of induction hardening of a shaft. In induction heating, a magnetic field is passed through the coil, producing an electromagnetic induction and the resultant induced current (eddy current) leads to heating of the work piece. The depth of heating

Inductor

Fig. 3-14 Practical induction hardening set-up of a shaft hardening where inductor encases the job and the bottom wider ring has fine pores for forced air cooling. (The inductor travels upward or the job travels downward for progressive hardening)

is, however, very strongly dependent on the power frequency; higher frequency produces lower depth of heating and viceversa for a material of given resistivity. Thus, power frequency required for the induction heating is higher for lower depth of hardening, decreasing with increasing depth (for further details vide Chapter 7). The most critical part of induction hardening process is the design of the coil used for heating. The coil has to be so designed as to give proper heating pattern at very high efficiency. Once the coil is right for a pattern of heating and power source set-up is alright, induction heating and hardening can be very well adopted for mass production. Some examples of induction hardened jobs are round shafts, journals of crankshafts, gear teeth, pinion arms, rocker-arm tips, etc. Between flame and induction hardening, the latter can produce parts with more precise control of depth and surface hardness. Induction hardening is also adoptable for mass production in a high productivity shop, like automobile manufacturing.

3.5

INTRODUCTION TO THERMO-CHEMICAL PROCESSES OF HEAT TREATMENT

Thermo-chemical processes are involved with thermally assisted diffusion of alloying elements (e.g. carbon, nitrogen or a combination of them) to the surface of a job, which upon diffusing into the steel body changes the steel composition of the applied area. Purpose of enriching the surface by diffusion of additional alloying element is to cause either preferential formation of hard martensite on the surface on quenching

or formation of hard nitride type chemical compound on the surface. Both the techniques are used for applications relating to fatigue and wear resistance. Elements that are commonly used for diffusing into the steel body are carbon and nitrogen. Example of the process involving diffusion of carbon is the process of ‘case carburising’ and example of use of nitrogen is ‘nitriding’ process. There are processes where both carbon and nitrogen are simultaneously used for diffusion; they are called ‘carbo-nitriding’ or ‘nitro-carburising’ depending on the dominance of these elements. Carbon and nitrogen forms interstitial solid solution on diffusing into the base iron lattice. The carbon-enriched steel has to be hardened by quenching to form martensite on the surface, but nitrogen-enriched steel does not require any such additional hardening operation. Nitrogen forms nitride at the temperature of operation, which is a very hard chemical compound. Type of nitride that forms by nitriding could be iron nitride (e.g. Fe2N) or alloy nitride (e.g. TiN, VN, etc.), depending on the type of steel. The diffusing elements are supplied by external environment around the job being heat treated. The environment inside the furnace is enriched with diffusion elements using either gaseous or liquid medium carrying these elements in atomic stage. These elements in the atomic form then diffuse onto the surface of the steel under the thermal effect at the temperature of operation, which is above A3 temperature of the steel for carburising and below the A1 temperature for nitriding. Due to diffusion of extra alloying elements, surface becomes radically different in composition from the bulk with desirable chemical characteristics, such as that observed in carburising or nitriding processes. While carburised surface of steel requires hardening for martensite formation to give required hardness of the surface, nitriding does not require any such hardening. Because, nitriding process is not involved with any phase changes other than nitride formation, which can take place at the temperature of nitriding operation. Other than gaseous or liquid methods, the process of carburising can be carried out by ‘pack carburising’ where the jobs are packed in charcoal powder, which acts as source of carbon atoms at higher temperature. Similar solid surface diffusion process can be also used for ‘boronising’ – a process that produces hard boron carbide on the surface on heating the steel parts at higher temperature covered under boron powder. Boronised steel part has very high surface hardness and can be used for highly wearing parts in engineering assemblies. Commonly used thermo-chemical processes for surface hardening are carburising and nitriding or their combination. These processes are

widely used in engineering industries for critical applications. Therefore, understanding their basic process characteristics is an integral part of steel heat treatment.

3.5.1

Carburising

Carburising requires steels of initially leaner carbon content in order to ensure that (a) there is sufficient chemical potential for carbon diffusion from the carbon-rich environment to carbon-lean steel surface, and (b) the core, where no changes in composition take place, remain ductile while hardening the carbon-rich surface layers by retaining more or less the original structure type. The aim of carburising is to produce hard surface with a tough core. The process of carburising is carried out above A3 temperature in the range of about 900 to 1000°C for a length of time commensurate with the temperature, required depth of hardening and method of carburising. Figure 3-15 illustrates a schematic of carbon diffusion into iron lattice for enriching the surface area with carbon in carburising process. It can be observed that carbon atoms move from surface towards the centre and preferentially occupy interstitial positions in the iron lattice on diffusion. Concentration of carbon atoms vary from surface towards the centre; surface having maximum concentration of carbon atoms and with virtually no carbon atom after certain depth from the surface. Diffusion of carbon into the steel surface can be achieved by availing either gaseous, liquid or solid (powder) medium, and accordingly

Fig. 3-15 lattice

A schematic showing the process of carbon atom diffusion into the iron

carburising processes are differentiated as gas carburising, liquid (or salt bath) carburising and pack carburising. ‘Pack carburising’ process uses solid carbon-rich powders as medium of carbon source, and the process is carried out at temperature higher than A3 temperature, generally at about 950°C, for a time period of 12 hours and above, depending on the case depth to be produced. Gas carburising process uses carbon-rich gaseous atmosphere for carbon diffusion, and is carried out at temperature above the A3 temperature of the steel, but lower than the pack carburising temperature, for a time period of 4 hours and above, depending on the case depth. Shortest time is taken by liquid salt-bath carburising (about 2 to 3 hours) at the temperature similar to gas carburising. But, depth of carbon-rich layer obtained at the end of the process is lower in salt bath carburising than gas carburising. Figure 3-16 shows the general appearance of carburised steel surface and Fig. 3-17 shows the trend of case depth obtainable by different carburising processes. In practice, solid (pack) carburising is used for large components requiring very high case depth; gas carburising, which is the most popular and widely used in industries, is used for engineering components of different sizes (e.g. gears. shafts, levers, cams, etc.) for medium to high case depth but requiring precise control of the case depth and microstructure; and salt bath (liquid) carburising is given to assorted engineering and structural components requiring high surface hardness but

Fig. 3-16 An illustration showing a section of 0.18% carbon steel after gas carburising and light etching – showing the dark surface area (top) which got carbon enriched due to carbon diffusion

Fig. 3-17 An illustration showing the time taken for producing a finite case depth by different carburising process for 0.15% carbon steel at 900°C

with limited case depth. Choice of processes, especially between gas and liquid carburising, is not very sacrosanct; they can be used as per discretion of the designer or heat treater based on actual service requirements and restrictions over the microstructural constituents. For example, if there is restriction of free carbide (i.e. carbide formed by carbon reaction and not precipitation from martensite) in the structure, salt bath carburising should be avoided and gas carburising followed by hardening should be the preferred route. Due to longer time, high energy consumption, and environmental problem in the shop-floor, pack carburising is avoided as far as possible. Salt bath carburising also suffers from environmental problem due to use of cyanide containing salt and the issue of salt bath residue disposal. Gas carburising is the cleanest of all; free from environmental problem, and with automation, the process can be operated in closed and clean chambers. Advantage of gas carburising process is that it can be made fully automatic and controllable for precision control of case depth and microstructure. The process can be carried out in pit furnace (using retort for holding the jobs), seal quench furnace along with integrated quenching facility, semi-continuous or continuous rotary or mesh-belt or conveyor type furnace, and vacuum furnace. Of these processes, seal quench furnace is very popular for precision automobile and engineering industries – not only for controllability of the process, but also for in-built facility for easy material handling. In a general manual set-up of carburising process, jobs or charges are taken out of the furnace after the process is over and need to be rapidly cooled to stop further diffusion of carbon inside the job. This is to minimise case dilution with continued carbon diffusion while being hot. The jobs or charges are then re-heated in another furnace, with control of atmosphere for avoiding decarburisation, heated to above A3 temperature corresponding to surface carbon (which is generally about 830 to 860°C, but lower than the carburising temperature) for austenetisation. Thereafter, the charge is sharply quenched in oil or salt bath for hardening. Case carburised jobs are never quenched in water due to high surface carbon level which can lead to cracking and distortion. The quenching produces hard martensite at the carbon-rich area of the surface and a relatively softer and tougher microstructure at the core, where there is no composition change. Since as-quenched martensite is hard but brittle, the jobs are then lightly tempered (at about 180 to 220°C) to relieve stresses without losing much of its hardness. Generally, surface hardness of 58 to 62 HRC is aimed for the carburised surface after light tempering and core strength of 35 to 45 HRC, depending upon the application. Common starting point for carburising process is

to start with normalised and finished machined parts, where finished final dimensions can be generated after carburising and hardening by light grinding, whenever necessary. Carburising is a popular process for many engineering applications, involving wear and fatigue resistance. The process not only imparts high surface hardness beneficial for wear resistance, but also can produce favourable surface compressive residual stresses which is very beneficial for countering fatigue load in service (vide Section 11.1 for further details). Due to higher thermal contraction of the steel body being quenched than the volume expansion due to martensite formation on the surface, a well-controlled carburising process can end up producing compressive residual stresses on the surface – which is beneficial for fatigue resistance. Effective tensile stress, which causes the fatigue crack to initiate and propagate, gets lowered by the presence of such residual compressive stress on the surface of a carburised part. However, for producing favourable compressive residual stress on the carburised surface, close control of case-depth and core strength is necessary in the corresponding steel. From the practice point of view, most critical factors in carburising process (including hardening after carburising) are as follows: • Control of carburising atmosphere in the furnace to a constant level of carbon potential in order to avoid over or under carburising and formation of undesirable microstructure (e.g. angular carbides on the surface, which can induce fatigue crack on the hard surface in service) • Avoidance of shoot formation of carbon on the jobs due to lack of temperature control for proper carbon reactions and dissociation • Avoidance of scaling and pitting on the surface of heated jobs (adverse for wear as well for fatigue applications) • Avoidance of decarburisation of surface area (adverse for wear and fatigue due to softer structure at those spots) • Avoidance of distortion of the heated or quenched jobs, which is detrimental for most applications due to causing mismatch of load-bearing contact points amongst the matching parts in service • Careful handling of heat-treated jobs to avoid any dents and cuts on the surface, which can act as point for fatigue crack initiation from those spots in service. Typical areas of application of carburising are automotive gears and shaft pinions where high wear resistance along with high fatigue strength is required. Carburising of steel parts is ideal for applications involving wear, scuffing, pitting and fatigue.

3.5.2

Nitriding

Nitriding is also a diffusion-controlled process like carburising, but involves lower temperature (below A1 temperature) than carburising. In this process, nitrogen in atomic state is diffused onto the steel surface at temperature of about 550 +/– 50°C. Dissociated ammonia serves as a source for atomic nitrogen, which diffuses into the steel and occupies interstitial positions in the iron lattice in the same way as the carbon atoms. The job being nitrided does not require quenching and hardening like carburising, because the effect is not due to formation of martensite but due to nitride formation. Nitride is a chemical compound and forms at the temperature of holding; it is hard by itself. Since the temperature is low, rate of diffusion is also slow and growth of nitrided layer is, therefore, low. Figure 3-18 illustratesa gas nitrided steel section with distinct intermediate layer of nitrogen diffusion, and Fig. 3-19 shows the trend of nitrided case depth with time at the nitriding temperature of 500°C, using aluminium bearing Cr-Mo alloyed nitriding steel. Nitriding process could be gas nitriding or salt-bath nitriding, the latter process generally takes shorter time, but gives more diffused layer with lower hardness than gas nitrided layers. Gas nitriding is the popular process for engineering applications with distinct demarcation between diffusion layer and alloy layer formation in the nitrided depth. Dissociated ammonia is generally used for gas nitriding process. Only about 30% of ammonia dissociates, producing atomic nitrogen as per the reversible

Time of treatment (h)

Nutride layer

Compound layer Diffusion zone

100

80

Standard ‘Nitralloy’ steel treated at 500°C

60 40 20

50 mm

Fig. 3-18 An illustration showing the cross-section of a nitrided steel sample – showing the light nitrided compound layer on the top along with a thick diffusion layer below the compound layer

0

0

0.2 0.4 0.6 Depth of case (mm)

0.8

Fig. 3-19 A graphical presentation of the progress of gas nitrided case depth with time at 500°C in ‘nitralloy’ steel, containing 1% Al plus Cr and Mo

equation: (NH3 ¤ 3H + |N|). A part of this atomic nitrogen is absorbed by the surface layer of the steel. Plain carbon steel forms iron nitride (Fe4N) and penetrates deeper, but the case is lower in hardness and brittle. Alloy nitride – which forms when nitride forming alloying element is present, e.g. steel containing Al, Cr, V, etc. – may give shallower depth but the layer is harder and tougher. In general, diffusion rate of nitrogen is much slower at the temperature where the process is carried out; hence the process produces rather thin layer of nitride. Because of lower temperature of operations, nitriding process does not cause any distortion of the job. However, required depth of nitrided layer for most applications is quite thin compared to carburised layer; nitride layer depth generally ranges between few microns to about 200 micron, depending on the end-usage requirements. Though all steels can be nitrided, but for obtaining harder and tougher layer, steel containing nitriding forming elements like Al, Cr and Mo is preferred. Therefore, for successful nitriding with high hardness for wear and fatigue-resistant surface, the steel should have nitride-forming alloying element, like aluminium (Al) and titanium (Ti). Engineering steels for nitriding applications is, therefore, specially designed with Al content (e.g. EN 40 type steels), which is a strong nitride former. Typical obtainable hardness by nitriding of different steel containing nitride forming alloys is indicated in Table 3.2.

More about different nitriding processes and their features and applications have been further discussed in Chapter 8, under thermochemical heat treatment processes.

3.5.3

Carbo-nitriding and Nitro-carburising

Because of the obvious advantages of carburising and nitriding, there is an intermediate process known as ‘Carbo-nitriding’ which is carried out at an intermediate range of temperature, but generally above the transformation temperature of the steel; e.g. 800 to 880°C. This treatment is given to alloy steels to form a hard and wear-resistant case of very low depth. In

carbo-nitriding, not only carbon is diffused into the steel but also some nitrogen is diffused into the steel for nitride formation on surface areas. The job requires faster cooling (e.g. oil quenching) for producing desired hard case and core properties. Effect of martensitic hardening is more if the temperature of operation is higher, but the effect of nitrogen will be less. However, in general, the carbo-nitrided layer is thinner than the carburised layer obtained by standard process. The combined effect of carbon and nitrogen diffusion produces a thin hard layer which is low in co-efficient of friction, very hard, and useful for some special applications where distortion control of the job is very critical. A typical example of carbo-nitriding application is automotive crankshaft where the hardened job is carbo-nitrided in a gaseous atmosphere for a thin layer of very hard surface for wear resistance of bearing journal. Similarly there is another modified process called ‘nitrocarburising’ which is carried out at temperature similar to nitriding. But, in nitro-carburising process, not only nitrogen is diffused into the steel, some amount of carbon (using carbon monoxide or hydrocarbon as source) is also introduced. Temperature used is about the same as nitriding, i.e. 560 to 570°C and in this temperature zone, carbon diffusion rate is very slow, concentrating only on surface region while nitrogen penetrates deeper. Table 3-3 summarises the processes and process characteristics of thermo-chemical processes in practice.

However, selection of steel has to be appropriate for the process being adopted. Table 3-4 shows the typical processes and suitable grades of steels for designated applications.

3.6 THERMO-MECHANICAL PROCESSES OF HEAT TREATMENT Thermo-mechanical processes refer to where effect of heating is combined with the effect of deformation (carried out by mechanical working)

for producing a characteristic structure which exhibits higher strength and toughness than what it would have attained independently. It is a combined process of heating, deformation and cooling in different cycles. Thermo-mechanical process can be executed with or without recrystallisation of austenite, but in majority of cases, it is designed for suppressing the recrystallisation in order to utilise that energy for producing very fine structure and sub-structure in the matrix. Such extra fine structure gives rise to high strength and toughness. An older form of this treatment is the process of deforming steel outside and then giving a lower temperature recrystallisation treatment for obtaining the fine structure. But, that form of structure is not as strong and tough as demanded by many modern engineering applications. Hence, the process has been modified by giving in-situ higher temperature deformation and controlled cooling simultaneously so that the structure does not get time for full recovery. This type of thermo-mechanical treatment of steel can be carried out over a range of temperatures; starting from rolling at higher temperature where recrystallisation of austenite can take place to an intermediate temperature where recrystallisation is suppressed or partially accomplished. Figure 3-20 illustrates a typical TM rolling cycle. Though thermo-mechanical treatment can be applied at higher temperature above A3, best results are obtained by deforming and transforming the structure at an intermediate temperature below the A1 temperature, where recrystallisation gets suppressed. Such lower temperature thermo-mechanical rolling of steel produces very fine structure and sub-structure in the steel, imparting not only high strength and toughness, but also exhibiting high impact strength and resistance to brittle fracture. Heating and simultaneous deformation of steel at intermediate temperature is designed to have a heavily worked structure without complete recrystallisation, making such un-recrystallised structure prone to transformation to extra fine structures and sub-structures. Since the process involves heating and transformation under deformed condition and aims to change the microstructure, it may be considered as a part of heat treatment processes.The change in structure is brought about by rolling at appropriate temperature and controlled cooling, without having to carry out separate heat treatment operation off the rolling line. Thus, the process is highly cost effective while producing a finer structure with higher strength and toughness. A common example of original thermo-mechanical processing is the production of high strength TMT bars where the rolled bar is control quenched on-line for the surface area to get quenched, producing

Fig. 3-20

Illustrating schematic stages of TM rolling

compressive stresses on the surface region. This compressive stress on the surface then acts on the intermediate layers below the surface and deforms the same due to prevailing lower yield strength of steel at that higher core temperature. Due to deformation of sub-surface area, the steel becomes prone to transformation on cooling to fine microstructures with higher strength and elongation. Simultaneously, the heat from inside the bar helps to temper down the surface structure which contains martensite produced by control quenching of the surface. Thus, the overall result is a strong and tougher bar, eminently suitable for reinforcing the concrete or for use as reinforcing bars in the industry. However, now-a-days, thermo-mechanical rolling and processing is widely used for rolling of HSLA steel in order to producing very fine ferrite grain size and extra-fine precipitates of carbides and nitrides of some micro-alloying elements, e.g. Nb, V, and Ti. The process is followed by accelerated cooling for precipitation control. This type of TM rolling, generally described as TMCP (thermo-mechanical and controlled processing), can be applied to the rolling of plate, strip, bars or section, and the process involves hot-rolling the steel at an intermediate temperature (below A3) in a controlled manner so that any, or a combination, of the following characteristics are developed in the steel: • Fine and stable grain sizes in the steel, the formation of which would have otherwise taken a separate and expensive normalising heat treatment, and • Fine and stable precipitation in the steel, by appropriate micro-alloying of the steel with elements like Niobium (Nb), Vanadium (V),

Molybdenum (Mo), Titanium (Ti), etc., giving rise to fine precipitates of carbides and carbo-nitrides – along with very fine ferrite grain size. Degree of deformation by mechanical working is an important aspect of thermo-mechanical processing. If the steel is heated above upper critical temperature and lightly deformed, the steel will recrystallise at that temperature and the refinement in structure will be marginal. High-temperature TMT-bar rolling is an example of this. But if the steel is cooled to an intermediate temperature after austenitisation and roughrolled and then deformed by further rolling, taking advantage of lower yield strength at that temperature, the recrystallisation of austenite can be made to lag behind the effect of hot work. Such a partially recrystallised structure will produce fine ferrite grain with consequent higher yield strength and elongation. However, if the steel contains special micro-alloying of V, Nb or Ti, austenite recrystallisation will be further inhibited and heavy deformation followed by controlled cooling of this structure can produce a very fine structure with fine precipitates of carbides and nitrides. Such structures have high strength and uniform elongation and toughness; it also has high impact strength. However, thermo-mechanical rolling of HSLA steel is one such area of TMT or TMCP application; the process can be also beneficially applied to low-alloy / micro-alloyed steels for the production of fine and tough microstructures. Microstructure control in TMCP steels begins at the slab reheating stage where temperature should be controlled for controlling the prior austenite grain size, which is the starting point of TMCP exercise. When the steel is further hot rolled in the non-recrystallisation region, fine and partially worked austenite grains are formed. These fine and partially worked austenite is then transformed to fine acicular ferrite or upper bainite by accelerated cooling. Figure 3-21 presents two different microstructures; one conventionally rolled and other TMCP rolled steel plate, illustrating the fine nature of controlled rolled steel structure.

Fig. 3-21 Microstructure of TMCP rolled plate compared to conventional rolling

Various combination of thermo-mechanical working (or heat treating) depends on the purpose and composition of the steel. Depending on the temperature of hot work, thermo-mechanical processes can thus be grouped under: A. Super-critical TM treatment involving rolling above A3 with or without recrystallisation. TMT re-bar rolling is an example of this type B. Inter-critical TM treatment involving rolling between A1 and A3, as used for many HSLA rolling C. Sub-critical TM treatment involving rolling below A1, which can be again (a) prior to transformation, (b) during transformation, and (c) after transformation – used for low-alloy steels with special requirements of high strength, toughness and impact strength Thus, thermo-mechanical treatment can be designed to produce a combination of structures and sub-structures which are very fine, strong and tough. Such structures are also more resistant to brittle fracture than conventionally heat-treated structure of the same steel. Mechanisms of obtaining such refined structure by thermo-mechanical treatment are also based on the same principles of metallurgy, which guide the other traditional heat treatment processes. The process might not be included as mainstream heat treatment processes by many, however, its understanding is very important for the study of heat treatment processes for exploring various possibilities in the scope of heat treating steels. In sum, the chapter points out the classification of heat treatment processes and their purpose, correlates the principles of phase transformation in steel with structure formation that occurs in steel under normal and accelerated cooling condition, and briefly outlines the processes and principles of different heat treatment processes carried out under thermal, thermo-chemical and thermo-mechanical processes. Thermo-mechanical processes of treating steel have been included in this book as part of heat treatment, because the process is involved with heating and transformation under deformed condition and aims at changing the microstructure–which is also the aim of most heat treatment processes. This process will not be further elaborated in this book, because of its special nature of treatment requiring mill facilities. However, other two areas of heat treatment, namely, thermal and thermo-chemical processes of heat treatment, will be elaborated further in Chapters 7, 8 and 9, illustrating their processes and practices.

Summary 1. The chapter defines heat-treatment as ‘a process involving heating of a solid material to a predetermined temperature, followed by controlled cooling at appropriate rates to obtain certain physical and mechanical properties in the material, brought about by changes in the physical state of the material but without any change in shape of the part’. Purpose and processes of heat-treatment have been explained and illustrated in this chapter by reference to this definition. 2. Based on the process features, heat-treatment processes have been grouped under three types: thermal heat treatment processes, thermo-chemical processes and thermo-mechanical processes. Examples of processes under each type have been listed, illustrated and their applications have been highlighted. 3. Purpose of heat-treatment has been set as to re-create or modify the structure and internal state of the material in most appropriate manner to suit the user-specific needs for either the processing, application or both. This concept has then been expanded to justify various process approaches of heat-treatment operations and their scope. Scope of different heat-treatment processes have been mentioned in Table 3-1. 4. Fundamentals of heat-treatment processes have been outlined with reference to the phase changes as per Fe-C equilibrium diagram as well as CCT-diagram for continuous cooling under industrial heat-treatment situations. With reference to Fe-C diagram, ranges of different heat-treatment temperatures have been indicated and significance of ‘critical temperatures’ relating to phase transformation have been explained. 5. The chapter further briefly discusses different heat-treatment methods and their purpose for (a) thermal heat-treatment processes, like annealing, normalising, hardening and induction surface hardening processes, (b) thermo-chemical processes, like case carburising, nitriding and their varieties, and (c) thermo-mechanical processes. Applications and utility of these processes and their relative merits have been mentioned and highlighted.

References / Suggested Reading ASM Handbook, Vol. 4, Heat Treating, ASM International, USA, 1991 Atlas of Isothermal Transformation and Cooling Transformation Diagrams, ASM, 1977 Atlas of Isothermal Transformation Diagrams, United States Steel Corporation, Pittsburgh, 2nd revised edition, 1991 Avner, Sydney H., Introduction to Physical Metallurgy, Tata McGraw-Hill Education, New Delhi, 1997 Brooks, Charlie R., Principles of Heat-Treatment of Plain Carbon and Low-Alloy Steels, ASM International, USA, 1996 Heat Treaters Guide: Standard Practice and Procedure for Steel, Edited by P.M. Unterweiser, H.E. Boyer and J.J. Kubbs, ASM, Metals Park, Ohio, 1982 Leslie,William C., Physical Metallurgy of Steel, Hampshire Publication Corporation, US, 1981

Mandal, S.K., Steel Metallurgy: Properties, Specification and Application, McGrawHill Education, New Delhi, 2013 Rollason, E.C., Metallurgy for Engineers, Edward Arnold (publishers) Ltd., UK, 1973 Sharma, Ramesh C., Principles of Heat-Treatment of Steels, New Age International, Delhi, 2007 Totten, George E.(Ed.), Steel Heat-Treatment Handbook, 2nd edition, CRC Press, 2006

Review Questions 1. Define the process of heat treatment. Discuss with illustration the stages of a standard heat treatment process cycle – highlighting the importance of each step. 2. How heat treatment processes are categorised? Briefly describe different generic heat treatment process categories available for steel along with their purpose. Name few standard heat treatment processes under each category of heat treatment. 3. What is the purpose of heat treatment of steels? Discuss the scope of heat treating steels – with illustration – for accomplishing the purpose. 4. Briefly discuss the following and outline their applications: • Process features of sub-critical annealing vis-a-vis full annealing • Normalising vis-a-vis annealing • Bulk hardening vis-a-vis surface hardening • Nitriding vis-a-vis carbo-nitriding 5. With reference to Figs. 1-3, 2-3 and 2-4, identify and highlight the importance of different temperature lines that control different phase separation and microstructure formation in steels. Why nucleation and growth (N&G) process in steel is important for heat treatment? 6. What are the different ‘thermal heat treatment’ processes in vogue and their major purpose? Briefly identify the following: • Steps in full annealing process vis-a-vis sub-critical annealing • Purpose of normalising vis-a-vis annealing • Difference in isothermal cooling vis-a-vis continuous cooling for hardening of steels • Process steps for ideal hardening of steel 7. Why tempering of martensite is important after quench hardening? Discuss the steps in tempering of steel with reference to temperature of heating. 8. What is temper embrittlement in steel? How to take care of it in the heat treatment of steel? 9. Discuss the process of flame and induction hardening and their merits and applications.

10. Discuss the process of carbon diffusion into the steel during case carburising. What are the critical factors for carburising of steels in practice? 11. Outline the temperature range, diffusion elements and main process characteristics of different surface hardening processes. 12. Outline the main process features of ‘thermo-mechanical heat treatment’ processes. Briefly discuss the importance of the TMT/ TMCP processes and their applications.

Heat Treatability of Steels

4.1

INTRODUCTION

Heat treatability may be defined as the ability of the steel to response to a given heat treatment process for development of desirable microstructure and required mechanical properties. The factors that can influence heat treatability of steel are its chemical composition, grain size, and inclusion shape, size and distribution. Chemical segregation may also adversely affect the heat treatability of steel – through non-uniform structure and properties. The concept of heat treatability may be applicable to all heat treatment processes (vide discussions in Section 1.3.3), but its importance increases with the heat treatment processes involved with increasing cooling rate or cooling rate sensitivity. Faster cooling – like quenching – is designed to produce martensitic structure of sufficient strength in the steel, but quenching also increases the chance of cracking and distortion in the steel – unless its heat treatability factors are well controlled. Control of

heat treatability factors can control the development of martensite with sufficient strength, in one hand, and help avoiding cracks and distortion during hardening, on the other. Thus, understanding heat treatability of steel is of considerable importance for hardening of steels – where the aspects of heat treatability are studied under the term ‘hardenability’ and ‘hardenability factor’.

4.1.1

Introduction to Hardenability and its Implications

Of the factors controlling the heat treatability of steel, effects of chemical composition and grain size are combined into the ‘hardenability’ of steel – which refers to ‘how deep from the surface the steel can develop a minimum specified structure and hardness upon quenching’. In other words, hardenability is the measure of depth of a specified hardness (or structure) on quenching, and not how much the hardness is. Hardenability value of steel primarily depends on its chemical composition and grain size. Quenching is a heat treatment operational technique for fast cooling of steels; the process or cooling rate during a quenching operation, in practice, can vary. This necessitates that if hardenability is to be used as standard for reference or comparison of heat treatability of steels, then the quenching process for determination of ‘hardenability’ would require to follow a standardised test conditions and evaluation method. Such a standardised method of determining hardenability is the ‘Jominy Hardenability Test’, which has been described and discussed in Section 4-2. Jominy testing uses a forced water jet to cool the steel face for hardening – followed by determining how deep the steel has hardened to a specific hardness value. But, industrial heat treatment (i.e. hardening) cannot adopt such severe quenching system without the risk of cracking and distortion of steel parts being hardened. Industrial cooling and quenching practice is less severe than what Jominy test adopts. Hence, there is a need for providing a correlation of Jominy hardenability value of the steel with that of industrial hardening process – where different quenching systems are used. This is done by (a) classifying the industrial quenching system as per ‘quench severity’ (H-value), and (b) then comparing and correlating the ‘quench severity’ (called H-value) of a quenching process with that of standard Jominy test. Quenching condition in Jominy test is assumed as the ‘ideal quench’ with quench severity value of infinity (•) – as against the quench severity of finite value of all possible industrial cooling. Table 4-1 lists the H-value of different quenching medium and illustrates this point.

The method of testing and correlating Jominy value of steel with what could be practical under a given industrial quenching or cooling condition has been, therefore, discussed and illustrated in this chapter – with reference to quench severity factor and corresponding ‘critical diameter’ of the steel that can be successfully hardened. Critical diameter refers to diameter of the steel bar that can be hardened by following an industrial quenching practice using quenching medium of finite H-value. Critical diameter of the steel (D) can be derived from the ‘ideal critical diameter’ of the steel (DI), which is determined by Jominy test. The method of correlating D to DI has been illustrated in Section 4.2.1. Hardenability value (or band) of steel – determined by Jominy hardenability test – is a critical consideration in the hardening of steel, which occupies the centrestage of heat treatment processes. For correct hardening of steel, its Jominy value, quench severity of the cooling medium for determining ‘critical diameter’ that could be successfully hardened, inclusion level (i.e. the degree of cleanliness of steel) and freedom from any harmful segregation are of critical importance for development of desirable structure without the risk of distortion or crack. Attempt has been made in this chapter to illustrate how to use hardenability data and other factors that influence the hardening of steels for defect-free heat treatment. When it comes to hardening process of steels, heat treatability further implies that properties should be attained without causing any distortion or cracking of the treated parts. In this respect, selection of quenching medium with right quench severity (H-value) is necessary. H-values of different cooling medium have been provided in the discussions.

Heat treatability of steels primarily differs with differing composition – making it necessary to examine if the steel can be subjected to the proposed heat treatment. For example, low-carbon unalloyed steels (like AISI C-1010, C1015 etc.) can be annealed, normalised or stress-relieved, but cannot be hardened by quenching for martensitic structure. For steels to be hardened by quenching, it requires a minimum level of ‘hardenability’, which is a direct function of composition (carbon and other alloying elements) and grain size of the steel. All steels can be subjected to stress-relieving, annealing or normalising heat treatment, in principle, but for hardening, it must have sufficient ‘hardenability’ for adequate response, where hardenability is a combined factor of composition and grain size. Hardenability, in practice, is determined with reference to a pre-defined hardness level (e.g. 50 Rockwell in C scale) or criteria set by % martensite in the structure (e.g. 50% martensite). Hardenability is related to depth of hardness below the surface and must not be confused with attainable hardness on the surface. This can be illustrated as below: Consider that surface hardness of the steel after hardening has reached 60 HRC and hardness value progressively decreases on measuring below the surface, reaching 50 HRC at a depth of 12 mm. In this case, 12 mm would be accepted as the depth of hardening – which is the measure of hardenability – based on 50% martensite or 50 HRC criterion set earlier. Thus, hardenability is not the measure of surface hardness, which is 60 HRC in the example, but it is the depth (12mm) where pre-specified hardness (50 HRC) has reached. Hardenability is the most important factor for determining heat treatability of steels by hardening where aim is to produce as much martensite as possible in the structure for required mechanical properties. It determines to what depth of cross-section of a bar the steel can produce a specified percentage of martensite or hardness level on hardening. Jominy hardenability value – which is used as standard for reference and comparison among hardening grade steels – changes with exact chemical composition and grain size of the steel. Hence, such hardenability data is valid for a given composition and not the grade of steel by which steels are generally selected.

4.1.2

Mass Effect and other Factors Influencing the Hardenability

Mass of steel is an important factor in the hardening of steel – as it influences the rate of heat extraction (i.e. cooling rate) due to relatively lower thermal conductivity in the steel. Cooling rate drops from surface towards the centre; setting up thermal gradient and different cooling

rate in a section of steel. Therefore, cross-section becomes a factor for consideration for hardening of steel because mass of steel being quenched in a medium will influence the heat transfer during cooling and, thereby, influence the outcome of results. This factor is expressed in steel specification or hardening data table as the ‘limiting ruling section’ – which determines the maximum diameter or cross-section of a steel bar that can develop the specified mechanical properties on hardening by a specified medium of quenching. These values are generally derived from Jominy hardenability data by correlating with H-value of specified cooling medium. Limiting ruling section is closely related to hardenability of the steel where composition plays the primary role, but they are not exactly the same. Qualitatively, hardenability and limiting ruling section data provide an index for the steel, used for heat treatment (hardening) or comparing the heat treatability, but they are derived differently. Hardenability always refers to a special quenching system as specified in the standard for the Jominy test, but limiting ruling section refers to a specific quenching medium, e.g. water quenching or oil quenching. Limiting ruling section data is used for specifying steels of hardenable grade and for helping the designer and heat treaters to understand what properties can be achieved in what section of the steel by following a particular quenching medium. Basically, limiting ruling section attempts to relate what mechanical properties can be achieved in different sizes of bar of the specified steel in a given quenching process. Thus, hardenability and limiting ruling section – which are referred to in the standards of heat treatable grade steels – are closely related. Hardenability of steel can be increased by enriching the composition. Carbon is the basic element in all steels; excepting interstitial free steel – which does not fall under common heat treatment processes in practice. But, increasing carbon for increased hardenability has limitation in the bulk hardening of steel. This is because, (a) carbon has a milder effect on improving hardenability compared to many alloying elements, and (b) with increasing carbon beyond a level, steel may develop brittleness on quenching, making it prone to distortion and cracking during hardening. For example, a steel with 0.45% carbon (e.g. SAE 1045 steel) will generally require water quenching when section size to be hardened is more than, say, 20mm, but an alloy steel containing 0.40% carbon and with modest alloy content (e.g. SAE 5140 steel), will require milder oil quenching for similar section size producing even better mechanical properties. Oil quenched SAE 5140 steel containing similar level of carbon will have lesser distortion and very low chance of cracking during oil hardening compared to water quenched SAE 1045 steel.

Moreover, this alloy steel martensite can be tempered at comparatively higher tempering temperature than plain carbon martensite, giving rise to better toughness. In addition to better toughness, heat treatment of precision engineering parts call for lower quench severity (e.g. oil quenching) for reduced chance of distortion or cracking due to higher thermal stresses. Hardenability of the steel could guide in this regard as to what should be composition of the steel vis-a-vis its quenching process. Understanding of the characteristics of quenching media and their selection is an important part of heat treatment exercise – which has been discussed at length in Chapter 6. Grain size is another factor that influences the hardenability or heat treatability of steels, which has been briefly mentioned in Chapters 2 and 3. It has been pointed out there that coarser the grain size of parent austenite, better is the chance of producing martensite in the structure. The effect is due to the influence of grain boundaries on the nucleation and growth process that controls the phase transformation or structure formation. Size of parent austenite grains is, in turn, dependent on (a) de-oxidation process during steelmaking and generally described as Al-killed steels, semi-killed steel etc. and (b) the temperature of austenitisation. Other factors that influence heat treatability of steel by hardening are the inclusion and chemical segregation in the steel. Segregation may lead to different structure at the spot of segregation, and this may cause inhomogeneity in the structure. Inclusions, on the other hand, may lead to cracking of steel under severe quenching condition where crack initiation starts from the weak inclusion-metal interfaces. Thus, in sum, heat treatability of steel for hardening depends on: • Composition of the steel • Grain size, and • Inclusion content and segregation severity And, based on these factors, quenching medium and quenching technique has to be selected for correct response to hardening without harmful distortion or cracking. This chapter, therefore, discusses the heat treatability of steel with regard to (1) hardenability, (2) effect of grain size, (3) effect of inclusions and segregations in steel and (4) the role of quench severity for hardening. Though hardenability of steel reflects the effect of both composition and grain size, the chapter separately discusses the effect of grain size for providing insight into the role of grain boundaries in phase transformation and hardening of steels.

Similar rules of heat treatability apply for surface hardening of steels. But, surface hardening is limited to a relatively shallow region of the surface where development of surface hardness rules the process. Therefore, surface hardening process depends more on the control of composition for developing high surface hardness rather than the depth of hardness. For this reason, surface hardening by carburising process banks on carbon enrichment of surface region – because for development of high surface hardness, carbon is more effective than any other alloying element.

4.2

HARDENABILITY OF STEELS

Hardenability of steel relates to how deep the steel can be hardened to a specified hardness level or martensite content in the structure. It is not the attainable hardness of the steel; it is about the depth and distribution of hardness attained in the steel. Purpose of hardenability is to guide the heat treaters for selecting steel and suitable quenching medium for attaining a minimum level of martensitic structure and hardness in the steel section after hardening, but without giving rise to distortion or crack. This implies that quenching severity (i.e. the cooling rate) for attaining the martensitic structure in the steel should be just right and not more severe. Since hardening of steel in industrial practice is carried out by referring to continuous cooling transformation diagram (CCT-diagram) of the steel, understanding the implication of hardenability with reference to CCT-diagrams (vide Fig. 3-3 or 3-6) would be useful. Based on the purpose of hardenability and referring to the process of hardening by continuous cooling transformation (as discussed in Chapter 2), hardenability can be described as an inverse measure of the severity of cooling rate necessary to produce a martensitic structure in a previously austenitised steel, avoiding transformation to other non-martensitic structure, such as pearlite and bainite. The lower the cooling rate required by a steel to form martensite, avoiding these other non-martensitic structure, higher is the hardenability. The cooling rate at which such transformation condition can be satisfied in the steel is called the critical cooling rate for the formation of martensite. Critical cooling rate for martensite formation, therefore, reflects the hardenability of the steel and likewise, it is largely a function of composition and grain size of the steel (vide discussion in Chapter 2). Hardenability – being widely referred data for guidance of heat treatment as well for comparison of steels – is required to be determined by

a standardised test, which can form the basis for common understanding between users and producers of steels. Therefore, hardenability testing is a standardised test procedure. Figure 4-1 illustrates hardenability curves for some plain carbon and alloy steels, determined by standard Jominy tests as per BS 4437. A line drawn through 500VPN (equal to 50 HR C) illustrates hardenability difference among different steels, especially between plain carbon and alloy steels. Plain carbon steels exhibit lower hardenability than alloy steels, despite increase of carbon level from 0.40% to 0.95% carbon; vide bottom two Jominy curves. This clearly illustrates that increasing hardenability with carbon has serious limitation. However, as discussed in the structure – property correlation in steels, carbon has the major contribution in the development of hardness in steels. This necessitates requirement of a minimum level of carbon in the steel for attaining a minimum specified hardness in martensitic structure. For example, a minimum carbon of 0.30% would be required for attaining 500VPN in martensitic (hardened) structure of plain carbon steel; vide Fig. 1-11. This implies that a minimum level of carbon is necessary for heat treatability of steels by hardening. In practice, for meeting the hardenability criteria of steel, carbon is used for producing higher hardness, and alloying elements like Mn, Cr, Ni and Mo are used for increasing the depth of hardening in a given steel composition for hardening. Various Jominy hardenability curves shown in Fig. 4-1 illustrate this point.

Fig. 4-1 Hardenability curves showing relative depth of hardening of different steels in Jominy test (Source: E. C. Rollason, 1973)

The figure shows actual Jominy hardenability curves for different steels, ranging from carbon content between 0.40%C and 0.95%C, and alloys containing C-Mn, C-Mn-Mo, C-Cr-Mo, C-Cr-Ni-Mo, etc. Steel with 0.40%C shows the lowest depth of hardening (4-5 mm) with reference to hardness level of 500HV (approximately 50 HRC), whereas depth of hardness for other alloy steels are much higher; vide the fine horizontal lines drawn through the 500HV mark in the diagram and corresponding vertical lines. The figure clearly demonstrates that best way to increase hardenability is by alloying the steel appropriately, but maintaining a minimum carbon level for attaining the required hardness. Drastic increase of carbon for attaining higher hardenability does not necessarily work, as evident from the Jominy curve of 0.95% carbon steel in Fig. 4-1; although this steel gives the highest surface hardness. Moreover, increase of hardenability by increasing carbon is not always desirable, because steel with higher carbon suffers from lack of toughness compared to alloy steels at a given strength level after hardening and tempering. Also, hardened structure of plain carbon steel loses its strength fast above the tempering temperature of 450°C, making it necessary to use alloy steels for retaining sufficient strength and toughness after good tempering. Higher carbon steel also increases chances of cracking and distortion during hardening, because of high internal stresses generated during martensitic transformation of high carbon steel. Therefore, general practice for adjusting the composition of heat treating steel is to obtain high hardenability (i.e. depth of hardening) by adding suitable alloying elements and using adequate carbon for achieving the hardness level in that structure. For example, if surface hardness level required is over 60 HRC, a carbon level of 0.70 or near eutectoid composition might be required, but if the hardness level required is 55 HRC or less then a carbon level of 0.40 / 0.45% would be adequate, as per Fig. 1-11. Hence, judicious choices of carbon plus alloying elements are necessary for attaining the desired hardness as well as depth of hardening in steels. Metallurgical factors that contribute to hardenability are chemical composition and grain size, and they behave in the same way as they influence martensite formation as originally proposed by M.A. Grossman; vide Table 4-2, discussed later in this section.

4.2.1

Determination of Jominy Hardenability in Steels

Hardenability can be determined by two methods: (a) the quantitative method by Jominy testing and (b) the calculation method by using Grossman formula. ASTM A-255-10: Standard Test Method for determining hardenability covers both the methods. For quantitative

Fig. 4-2

The Jominy Hardenability test set up – as per BS 4437

Jominy test method, most countries have their own national standards for testing. However, British Standard BS 4437, which is a popular and widely accepted standard, has been referred here. Figure 4-2(a) illustrates the Jominy hardenability test set up and 4-2(b) illustrates the method of testing the test bar for hardness checking. The figures are self-explanatory. Hardenability testing set up for ‘Jominy Hardenability Test’ as per BS-4437 involves heating a 25 mm cylindrical test bar of length 100 mm to normal hardening temperature (i.e. above its A3 temperature) and then the bar is quickly placed on the Jominy fixture and quenched at one end by controlled water jet spray at high velocity. The free height between the end to be quenched and water jet is 62.5 mm. Thereafter, the specimen is cooled, a portion of the surface is flattened by slow grinding, and hardness in Rockwell-C is measured at regular interval from the quenched end, and plotted. The distance up to the point where the steel shows hardness value corresponding to 50% martensite (or 50 HRC, whatever is the criterion) is taken as the measure of hardenability of the steel. Generally, 50 HRC hardness value is taken as corresponding to 50% martensite in the hardened structure. Examples of Jominy hardenability curves generated by plotting hardness against distance from the quenched end are shown in Fig. 4-3. The figure also illustrates the steel microstructure obtained in the surface of a Jominy end-quenched sample, where micro (a) on the left shows the as-quenched martensite in the front face of Jominy bar and

Fig. 4-3 Plot of Jominy hardenability curve (at the centre) for two steels (a) and (b) of different composition – steel (a) showing shallow hardenability and steel (b) showing deep hardenability with regard to 50 HRC criterion. [The figure also shows the microstructure of the hardened face and the original steel at a distance from the hardened face (a)]

one on the right (b) shows the original structure of the 0.40% carbon steel, left unhardened at a distance from the face. Since cooling rate progressively decreases backward from the quenched face (where the rate of cooling is the fastest), structure progressively changes as well as hardness values, which decreases with distance from the front end (vide the top axis in Fig. 4-1). Figure 4-3 shows that while both the steel attain above 60 HRC on the surface (front face), values drop very sharply in steel (a) reaching 50 HRC cut-off mark at 2.0mm distance, whereas steel (b) reaches the same value of hardness at 6mm. This indicates that steel (b) is much more hardenable than steel (a), which is characterised by flatter Jominy curve. Further, if thin slices are cut from the test piece at a fixed distance, then that could reveal what microstructure has been produced by corresponding cooling rate at that point. Cooling from one end in the Jominy tests set up a cooling rate gradient along the length, which produces characteristic structure corresponding to different cooling rates and different hardness. The cooling rate along the Jominy test piece varies approximately from about 270°C per second in the front end to about 2°C per second at the back of the bar; vide Fig. 4-4 where cooling rate has been mentioned on the top. Figure 4-4 illustrates how Jominy hardenability value changes with alloy content in steels having 0.40% carbon in all of them. The figure confirms that plain carbon AISI 1040 grade steel has the lowest Jominy hardenability despite having similar carbon content as others quoted in the figure, but their maximum hardness attained on the surface is nearly

Fig. 4-4

Jominy hardenability curves of 0.40% plain carbon and alloy steels

Note: Corresponding 50, 80 and 100% martensite hardness line in these steels shown on the right hand vertical axis

the same for all steels. This graph is a testimony of fact that maximum hardness of hardened steel is primarily the function of its carbon content, though hardenability value is the function of its alloy contents. Jominy hardenability measurements, represented in Fig. 4-4, show that plain carbon steels have lower hardenability than the alloy steels of similar carbon content. This is because effect of carbon is mainly limited to producing maximum hardness whereas effect of alloying elements is on producing the depth of hardness, i.e. depth to which that hardness can be extended. However, this should not be taken as meaning that alloying elements have no role in increasing hardness in steels. Examination of Fig. 1-11 will reveal that for carbon level in steel of 0.40% and beyond, martensite in alloy steel has nearly a band of 100VPN more hardness than carbon steel of similar carbon content. This is also borne out by Fig. 4-4, which shows that alloy steels have about 5 to 6 HRC (approx. 50 to 60 VPN) more hardness than corresponding carbon level in carbon steel for corresponding 50% martensite in the Jominy bar (vide right hand side axis of Fig. 4-4). Jominy hardenability value is typically expressed by ‘ideal critical diameter’ (DI) to which the steel can be effectively hardened to 50%

martensite at the centre when quenched under an ideal condition (i.e. forced water jet – where the quenching intensity is considered infinite with (H = •)). Hardenability value obtained by Jominy test or the ideal critical diameter is comparative and not an evaluation of what it would be in actual practice, where quenching condition may vary from the simulated quenching condition in the Jominy test. Different quenching medium has different quench severity, represented by H-value. Therefore, the actual performance of steel will depend on the quenching efficiency and effectiveness of the quenching medium used in the industry. Table 4-1 gives the quench severity of different quenching media and conditions, which is expressed by ‘H-value’. Higher the ‘H-value’ higher is the quench severity of the media. The table shows that oil quenching with strong agitation has the H-value of 0.70 and water quenching with strong agitation has H-value of 1.5, in the relative scale. The table also demonstrates that agitation is an influencing factor for quenching effectiveness. Hence, there is a need for correlating the Jominy hardenability value with that of hardenability value obtainable by industrial cooling rate, having different H-value. This correlation can be established by referring to what is known as ‘Ideal critical diameter’ (DI) vis-à-vis corresponding ‘Critical diameter’ (D) to a particular quenching medium with its characteristics H-value. Ideal critical diameter – determined by Jominy Hardenability Test – is defined as the hardened diameter that has 50% martensite at the centre under an ideal quenching condition, i.e. when the surface is cooled at an infinite rate* (this is what Jominy test attempts to simulate by cooling the test face by high pressure water jet cooling). Fortunately, there is good correlation between critical diameter of hardening by Jominy test – referred to as ‘ideal critical diameter’ (DI) – and the ‘critical diameter’ (D) obtainable in actual heat treatment practice by following different industrial cooling practices. Different industrial cooling practices use different cooling media having different ‘quench severity’ (H-value), as shown in Table 4-1. Therefore in industrial practice, the ideal critical diameter indicated by Jominy test of the steel – where H-value attained by the water jet cooling is nearly infinity – will be compromised because of lower quench severity value of cooling media used in industry. But, there is a good correlation between the experimental value of D (critical diameter) and the DI (ideal critical diameter). Figure 4-5 illustrates the correlation between the ‘Ideal critical diameter’ (DI) and ‘Critical diameter’ (D) obtainable by following a quenching system of definite H-value. Such a correlation of DI with D value of a given steel composition helps the heat treaters to evaluate

the suitability of steel for an intended application first by determining or obtaining its Jominy hardenability value (DI) and then finding out the corresponding D value with respect to cooling medium to be used in the processing. In this respect, DI value acts as reference point for comparison of D value by following a particular quenching system.

Fig. 4-5 Graphical relationship of the ‘Ideal critical diameter’ (DI) with ‘critical diameter’ (D) of different rates of cooling (H-values)

4.2.2

Alternative Method of Estimating Hardenability of Steel: Grossman Formula

Determination of Jominy hardenability requires standardised laboratory testing facility and human skill. If not carried with strict conformance to the standard, the result may vary from test to test. It is also time-consuming – requires sampling, test specimen preparation, testing and evaluation of test result. Hence, an alternative method of estimating Jominy hardenability from the chemical analysis of the steel was developed by Grossman. Based on experimental results, Grossman developed a well-correlated calculation method of finding out hardenability from the steel composition, combined with grain size effect. This formula is widely used in industries for prediction of hardenability of killed steels made to standard specifications. Table 4-2 lists the effect of carbon and various common alloying elements on hardenability of steels, along with multiplication factors for different elements as per Grossman formula.

By using the multiplication factors for alloying elements (present in the steel), hardenability of the steel can be calculated. Effect of carbon content in the steel is combined with the grain size of the steel. For consistency of this calculation, steel needs to be made through Al-killing process for stable and uniform grain size. These empirical factors were derived from the actual Jominy tests carried out under close control of test conditions, involving large number of testing to cover the range of compositions in the steels and improved accuracy of results. Therefore, the calculated hardenability result is also expressed as DI in line with Jominy tests. Table 4-3 illustrates how the hardenability is calculated by using the Grossman formula.

Calculated value of hardenability from Grossman formula is only an assessment and not a guarantee, like Jominy value. Hence, it is not used as acceptance standard for steel from the steel mill, but can be used for relative comparison between different steels. The assessment gives fair estimate of hardenability of steel with controlled grain size of ASTM no. 5 to 8. As such, calculation of hardenability by using Grossman formula from the chemistry of the steel can be very useful for quick assessment of hardenability.' After calculating DI value from the Grossman formula, reference can be made to Fig. 4-5 for obtaining the corresponding D value for given quenching medium with known ‘Quench severity’ (H). The Grossman table shows the strong effect of chromium and molybdenum on hardenability. In this table, effect of molybdenum (Mo) has been shown at lower level than others, because Mo addition to steels is limited within the range of 0.05% to 0.40% in most engineering steels, except stainless steels. However, all alloying elements, excepting cobalt, increase hardenability to a varying degree and the corresponding DI value can be calculated by using Grossman formula developed as an alternative to Jominy Hardenability Test, which is time consuming. Boron is another element that has very strong effect on hardenability (though not shown in Table 4-3). Effect of Boron is appreciable even in the range of 0.0005 to 0.003%. Because, of this splendid effect of boron (B) on hardenability, boron containing low-carbon steels are extensively used for high tensile ‘fasteners’ after heat-treatment, e.g. 8 K and 10K grade ‘nuts & bolts’ for engineering applications. Study of hardenability provides information about the suitability of the selected steel to produce sufficient martensite in the structure by heat treatment. Hardenability data is an important part of selecting steels for hardening, especially for (a) selecting steel composition vis-a-vis ruling section to be hardened and (b) steel components requiring sufficient strength and toughness for end-uses. For example, referring to Fig. 4-4, it would be obvious that choice of AISI 1040 (a plain carbon steel of 0.40% average carbon) will be sufficient for a thin section hardening or a medium section where strength and toughness requirement might not be high. But, for hardening higher section with higher strength and toughness, steel containing alloy steels – giving higher hardenability – would be required, where grades like SAE 5140 or SAE 4140 could be more appropriate. Thus, either Jominy hardenability value or calculated hardenability value from Grossman formula becomes an essential part of heat treatment technology for right planning of the process.

However, balance is required in the approach for selection of steel with right hardenability for right microstructure. Right approach to good heat treatment by using hardenability data is to select the steel with right hardenability, but not excessive hardenability. Excessive hardenability than that is required for a section might lead to distortion, crack or harmful residual stress problem. For example, if the section to be hardened is within the reach of SAE 4140 or 5140 steels, choosing alloy steel with higher alloying (like SAE 4340 containing Cr-Ni-Mo and having much higher hardenability than the formers) will be unnecessary, problematic and costly. Though hardenability data is primarily meant for uses relating to through hardening of steel, the same is equally applicable in the surface hardening processes by martensitic transformation – such as induction or flame hardening. Steels for these processing require consideration for high surface hardness along with gradual drop of hardness below the surface. A sharp hardness gradient in the case region is not desirable for fatigue resistance. Similarly, such hardening processes also require control of grain size; coarser austenitic grains before transformation may lead to lower toughness of the martensitic case, despite stress relieving. With respect to grain size, uses of killed steel where grain sizes are relatively uniform are preferred. More about grain size effect in the hardening of steel is discussed in Section 4.3.

4.3

GRAIN BOUNDARY CHARACTERS AND ITS EFFECTS ON HARDENING OF STEELS

Aim of hardening of steels is to maximise formation of martensite under a given cooling condition, but without distortion and cracking. Grain size is a factor that influences the martensite formation under a given condition by influencing the N&G process. Fine grains in the steel favour formation of non-martensitic structure, as discussed earlier in Chapter 2. Grains are natural products of solidification of metal following the principle of nucleation and growth. Most solids such as metals and alloys have a crystalline structure where atoms are arranged in orderly manner in the crystal lattices in three-dimensional periodic manner to form crystals (i.e. grains), and this arrangement continues till they meet each other at inter-crystalline interfaces. The orderly arrangement of atoms in the crystal lattice gets broken down at the inter-crystalline interfaces, forming the crystal boundary (i.e. grain boundary) where atoms lose their orderliness of arrangement, and the interface (boundary) structure becomes full of defects with atomic voids and lattice mismatch. A schematic picture of grain and grain boundary formation in solid is shown in Fig. 4-6.

Fig. 4-6 A schematic presentation of two-dimensional picture of atomic arrangement within the crystals (grains)-forming grain boundaries at the meeting points

At atomic level, boundaries among crystals (grains) are jagged interfaces with ledges (micro-steps in zigzag mode) and atomic defects. This makes the grain boundary structure defective, having higher interfacial energy. Therefore, boundaries having higher interfacial energy can act as preferred sites for ‘nucleation’ of second phase during transformation of steel and may also act as points for impurities to segregate and precipitates while heated at higher temperature – such as during austenitisation at higher temperature. The crystal interfaces (i.e. the boundaries) are, however, generally planar, having two-dimensional periodic atomic structure. This situation implies that in a polycrystalline cube of size 1 cm, with grains of 0.0001 cm in planar diameter, there would be 1012 crystals or grains inside that cube, resulting in grain boundaries of several sq. meters. This illustrates the effect of reducing grain size and consequent increase of grain boundary areas in steel, which increases at the exponential rate. Hence, any influence of grain boundaries on any property – either physical phase transformation on heat treatment or mechanical properties or electrical resistivity – would significantly change with grain diameters, i.e. with grain sizes. Grains form during solidification of metals, but undergo changes with the history of heating and mechanical working. For example, grains of solidified steel could be columnar and dendritic in nature, but when the same steel is rolled and annealed, it is more equiaxed and even in size. A typical picture of ferritic grains in wrought steel is shown in Fig. 4-7.

Fig. 4-7 An illustration showing ferrite grains in low-carbon steel with specs of pearlite (dark phase) at the grain boundaries in wrought hot-worked steel

Figure 4-7, which represents two-dimensional grains, shows uneven distribution of grain sizes and also grain boundaries acting as sites for carbide nucleation leading to pearlite formation, especially at the corners of grain boundaries. This is because corners – the triple points in grain boundary structure – have even higher interfacial energy than the other areas of grain boundaries. Hence, such triple points in grain boundaries are the first amongst areas where precipitation or nucleation can take place. With re-heating and hot-working (such as forging, extrusion or heat treatment) these grain boundary positions and sizes will change, because of the influence of such hot working and heating on the recrystallisation behaviour of steels. Recrystallisation of steel produces a new set of grains – where grain size depends on the recrystallisation temperature and degree of deformation prior to recrystallisation. Heavy working generally lowers temperature of recrystallisation, producing finer recrystallised grains. Since finer grain sizes give rise to exponential increase of grain boundary areas, they may be desirable for strength and ductility in the steel, but not for martensite transformation. For martensite formation, relatively coarse grains are required – which limits the grain boundary sites where nucleation for pearlite or bainite can take place, i.e. in order to suppress pearlitic or bainitic transformation. Therefore, finer grain sizes reduce the hardenability of steels by increasing the chances of pearlite or bainite formation by nucleation at the grain boundaries and triple points. Figure 4-8 illustrates how grain sizes influence the hardenability of steels for similar carbon content. Grains that participate in the hardening process of steel are ‘parent austenite grains’, i.e. the grains that form during high-temperature heating and holding for austenitisation of the steel before hardening. Size of parent austenite grains depends on the heating temperature and the type of steels, i.e. the type of killing of the steel. Killing of steel means degassing of the steel before solidification

by using some oxidizing elements which preferentially reacts with excess oxygen of the bath, which is responsible for boiling of the bath. Oxidising elements like aluminium (Al), silicon (Si), etc., are used for degassing. For controlling grain size, the steel is finally killed by using Al or Al-Si compound. Al-killed steel is preferred for heat treatment because Al also reacts with nitrogen in the liquid steel and forms aluminium nitride (AlN) particles which is stable at high temperature and can pin the grain boundaries from growth at the austenitisation temperature. If there is no pinning particle present in the steel, controlling of austenite grain size becomes difficult. Stable austenite grains ensure reproducibility of the hardening process with reference to its CCT or TTT-diagrams.

Fig. 4-8 An illustration showing the variation of hardenability with carbon and prior grain size in steels [Source: [Reed-Hill,1973]

Because of the importance of grains and grain sizes for different physical, mechanical and chemical properties of steels, ASTM (American Society for Testing of Metals) has devised a standard method of assigning grain sizes to wrought steels. Considering that grain boundary areas exponentially increase with number of grains in a given field of examination, ASTM system assigned numbers as per grains per sq. inch

area. Lower the number of grains per sq. inch area lower is the ASTM grain size number (n). Relationship between ASTM grain size number and number of grains per unit area is given in Table 4-4. The ASTM method measures ‘average grain diameters’ and converts that to ASTM numbers as reported in Table 4-4.

ASTM grain size is generally measured in wrought steel and refers to as ferrite grain size in the steel. In fully aluminium-killed steel there is good correlation between ferrite grain size and austenite grain size – which is an influencing factor in hardening of steels. ASTM grain size number and the numbering system is universally applied for determination and denoting the grain size in steels for heat treatment. It provides a standardised platform for acceptance and comparison of steels. The effect of variation of ASTM grain size on hardenability of steels for a given carbon composition has been illustrated in Fig. 4-8. For heat treatment, especially for hardening, Al-killed steel is preferred. This is because there is good correlation between ferrite grain size and the austenite grain size that gets produced during austenitisation of Al-killed steel. Aluminum nitride (AlN) particles in Al-killed steel control the growth of austenite grains by pinning the grain boundaries and not allowing them to grow. Ferrite grains that form from such austenite grains pinned by AlN particles also get restrained in size by the size of austenite grains due to N&G process; vide discussions about N&G in Chapter 2.

Austenite grains can abnormally grow only when the steel has been heated to a too high temperature, which helps the pinned boundary to unlock with the help of thermal energy. Thus, measurement of ferrite grain size in wrought steel can give clear idea about the prospective austenite grain size on heating and hardening. Therefore, for consistency in heat treatability, steels are required with stable grains, which are achieved by Al-killing or Al-Si-killing during steelmaking. The conflict comes with the fact that while coarser grain is preferred for ease of martensite formation, the same could be the cause of lower toughness of steel and may cause hardening crack. Hence, there is a compromise in choosing the grain size of heat treatable steels, which generally ranges between ASTM sizes 5 and 8. Most international steel standards specify ASTM grain size 5 to 8 in heat treatable grade steels. Figure 4-9 presents a comparative chart of ASTM grain sizes, which is used as reference chart for assigning ASTM grain size number in a structure. This is a comparative chart used by examining the grain size under microscope, and does not require actual measurement as indicated in Table 4-4.

Fig. 4-9 Some representative photographs of ASTM grain size chart. [Note the exponential increase of grain numbers per sq. inch with ASTM number. This is not the full chart of ASTM grain sizes ranging from 1 to 8. For full chart, vide ASTM standard E-112]

4.4 EFFECTS OF INCLUSIONS AND SEGREGATION ON HEAT TREATABILITY OF STEELS Like grain boundaries, inclusions are also an integral part of steel microstructure. No matter how much care one takes during steel making, some amount of inclusions will be present in the steel. Inclusions are non-metallic chemical compounds that are present in steel by virtue of their formation by chemical reactions or contamination during high-temperature steelmaking process. There are many secondary metallurgical operations for reducing inclusion content in steels, yet getting rid of all inclusions is impossible. This is because of the fact that even presence of 1-ppm oxygen or sulphur gas in liquid steel can cause to contain 109 – 1012 non-metallic inclusions count per ton of steel. Thus, theoretically, from the viewpoint of ‘cleanness’, no steel is absolutely ‘clean’, i.e. free from inclusion. Inclusions could be endogenous which form mostly by reaction of gases present in liquid steel or exogenous, which form due to external erosion and entrapment of materials during steelmaking. The latter inclusions are generally large and can be floated-off by special steelmaking steps. But, the former, the endogenous inclusions, are finer and difficult to completely remove from liquid steel. Figure 4-10 shows some typical inclusions in rolled steel; these inclusions are alumina on the left and silicate on the right. These are endogenous inclusions and they got elongated during high-temperature rolling, subsequent to solidification of steel. Alumina is a hard and brittle inclusion; it gets fragmented due to its brittleness at the rolling temperature. Silicate inclusions have some ductility at the rolling temperature; hence it can get elongated. Because of the uses of Al and Si for de-oxidation of steel, presence of such inclusions is common in steel. Secondary metallurgy treatment like ladle-furnace treatment or vacuum treatment can lower the inclusion level and their sizes and distribution, but cannot fully eliminate them. Inclusions have deleterious effect on most mechanical properties and on the processing of steels, excepting the role of sulphide inclusions in machining. As regards hardening of steel, inclusions can give rise to quench cracking, especially if the inclusions are larger in size and nearer to the surface. Interfaces of inclusions with the steel matrix are incoherent in nature and act as points of stress concentration. Hence, during quenching, when the steel experiences thermal stresses, such stress raising interfaces can facilitate formation of micro-cracks. These micro-cracks can then open up when the steel has transformed to martensite producing lots

Fig. 4-10 General appearance of Alumina (left) and Silicate (right) inclusions in rolled steel

of internal strain due to high tetragonality of the crystal structure that results from freezing of carbon atoms in the lattice due to fast quenching. Hence, inclusions of all kinds, if not small in size and globular in shape, can cause cracking during hardening. As such, all steel specifications used for hardening limit the inclusions contents in steel. All steel standards specify the maximum acceptance limit of different inclusions types, which constitute an important point for selection of steel. Inclusions are classified under ASTM E-45 into different types, which are based on their chemical composition and harmfulness. Type A refers to sulphide inclusions, which is relatively less harmful than type B, the aluminates (oxides of aluminium) inclusions. Type C inclusions are silicate inclusions of elongated form, while type D inclusions are soft globular oxides, which are comparatively less harmful for general applications than type B. However, as regards heat treatability of steels, it is not the chemistry of inclusion that is so important as the size, distribution and their interfacial character. Larger the size and more segregated the inclusions are, more is the adverse effect of inclusions during hardening. For further details on inclusions and their effect, books under reference 1 & 2 may be helpful. Technically, steel cannot be made totally free from inclusions by commercial steelmaking processes. At the same time, inclusions of all types are harmful for most heat treatment applications. However, globular and fine inclusions are less harmful for hardening. Therefore, task of heat treater or designer is to choose steel with controlled level of inclusions and controlling their shape, size and distribution. Inclusion chart as per ASTM standard E-45 helps in selecting and specifying steels with permissible inclusion level for a given process or purpose. Table 4-5 illustrates how inclusions are classified and reported in the standards of steels. The table provides generally acceptable inclusion level for steels made through different process route. These are micro-inclusions and grouped not only by their types but also by their size (e.g. thick or thin). Such quality specifications for steels are provided in the respective steel standards as per the uses, treatment and applications.

Æ Ø

Fig. Not Clear

Inclusion levels estimated by ASTM E-45 are expressed in the form of a chart, which is also referred to as JK-chart for inclusions in steel. The ASTM chart deals with four types of inclusions, namely A (Sulphide), B (Alumina), C (Silicate) and D (Globular oxides), sub-dividing them into ‘Thick’ and ‘Thin’ categories. The globular oxides (type D) often originate from special treatment of steel by rare-earth metal compound or calcium treatment for some special applications. Figure 4-11 exhibits the standard ASTM inclusion chart.

Fig. 4-11 An illustration showing the standard comparative chart for determination of inclusion content in air-melted steels. [Classification and specification of inclusions are made in terms of A, B, C and D types in grade thin or thick by referring to chart visa-a-vis the field being examined from the steel sample]

However, the table refers to micro-inclusions that are detectable under normal optical microscopy. Very fine inclusions often escape the chance of detection by normal microscopy, because of the limitation of resolution and detection by traditional microscopic methods. Microscopic method generally detects inclusions of size 5 to 20 micron; below this level electron beam inclusion analyser is applied. For large macro-inclusions – which are considered very harmful – ultrasonic detection technique has been developed which can detect large inclusions of size 20 micron and above. Inclusions of all sizes are somewhat harmful for steel properties, but larger sized inclusions are harmful for heat treatment as well – especially the harder type and angular in shape inclusions, which tend to promote micro-cracks during quenching and hardening. Segregation is another phenomenon in steel that occurs during solidification and might not get fully cured during subsequent heating and hot-working. Segregation is a chemical phenomenon where impurities and solute elements concentrate under thermal gradient and solidification constraints. Thus, in effect, segregated areas could be made up of different chemistry affecting the response to heat treatments. While most segregation relates to segregation of impurities in steels, there are instances when solute elements, like manganese, segregate and cause microstructural defects in high-manganese steels, leading to ‘rock-candy’ type fracture. Therefore, for uniform response to heat treatment, steels should be free from harmful segregation. Impurity segregation in steel can be very well detected by ‘sulphur print’ examination.

4.5

ROLE OF QUENCH SEVERITY AND OTHER FACTORS INFLUENCING THE HARDENING OF STEELS

Hardenability is the most important criterion deciding the heat treatability of steel for martensitic hardening. Ultimate aim of hardenability study is to ensure that the steel can be successfully hardened by quenching in order to form martensitic structure in a given cross-section of the steel without harmful distortion or cracking. Probability of distortion and cracking in hardening goes up with severity of quenching. But, there is another factor that also influences the tendency of cracking or distortion – and that is cross-section of the steel part. Thicker section has lesser tendency of distortion and cracking; vice versa thin parts have higher tendency for cracking or distortion in a given quenching medium of similar severity (H-value). Therefore, in dealing with heat treatability for hardening of steels, two more factors come in; one is the effectiveness of quenching and

quench-severity, and other is the cross-section up to which required martensitic structure can be produced by the respective quenching medium. Purpose of examining these additional factors is to adjust between steel composition and quenching method for obtaining the required structure – property characteristics in a given steel section, but without risking distortion or cracking. Steel standards attempt to provide such information for hardening grade steels by specifying ‘ruling section’ in terms of bar diameter under a given cooling condition. Along with ruling section limit, steel standards also provide the expected mechanical properties from that section by following the specified cooling condition. Most heat treaters and designers follow such specification and recommendation for choice of steel and quenching method. But, quenching methods referred in such standard practices are simply water, oil or air, and not with respect to specific quenching severity. Quenching severity of such media can be changed – reduced or increased – in practice by agitation or adjusting the quench bath temperature (especially for oil). For example, quench severity can be increased by increased agitation of the bath or decreased by warm oil quenching. Thus, quench severity control during quenching offers scope for fine-tuning the process of hardening or heat treatability. More about this has been discussed in Chapter 6. Use of hardenability data, in practice, requires correlation of ‘ideal critical diameter’ (DI) with ‘critical diameter’ (D) for a given quenching medium and H-value; vide Fig. 4-5. Quench severity of different quenching medium (H-value) and conditions of quenching has been indicated in Table 4-1. Purpose of such correlation is necessary to understand how the given steel should be quenched without any harmful effect that can arise from severity of quenching. Such harmful effects are residual stress, distortion or cracking. Aim of this section is to further discuss and elucidate how steels can be hardened for maximum ruling section with optimum structure and properties without any harmful effect of residual stress or distortion / cracking. At the route of getting right set of properties in steel is getting the right structure. This can be determined by series of testing of sample of different cross-section, but accuracy and reproducibility of determination might suffer. In this regard, correlation between composition – cooling rate – and structure can be also carried out by the study of transformation diagram (as discussed in Chapter 2) e.g. CCT diagram of the steel. To facilitate direct reading of ruling section (i.e. minimum section where a specific structure can be developed for required properties) from CCT-diagram, there is a method of plotting CCT-diagram of steel

with time-axis indicating hardenable bar diameters for different cooling conditions. Figure 4-12 illustrates one such CCT-diagram of low-alloy steel – drawn with reference to corresponding bar diameter under different cooling. 900

Water quench

800 Ac3 700

Air cool Oil quench

Austenite

Start

Ac1

Ferrite Pearlite

Finish

600 Bainite 500 400 300

Ms M10 M50 M90

Martensite

200 Mr 100 0

0.1 0.2 5 10

0.3

10

1

2

5

20 20

10

20

50 50

50

100 200

500

300 100 150 200 300

Bar diameter-mm

Fig. 4-12 Illustration showing a typical CCT-diagram of low-alloy steel where Y-axis shows the temperature in °C and X-axis shows bar diameters corresponding to structure for cooling by air (top), oil (middle) and water (bottom)

This type of CCT plot allows direct reading of bar diameter corresponding to microstructure that gets developed under water, oil and air-cooling, yielding information about depth up to which the steel can be hardened to a specific type of microstructure under a given cooling condition. The plot also provides the scope of estimating probable structure for cooling rate in between water and oil or oil and air. Such information is helpful for optimising the heat treatment parameter (i.e. heat treatability of the steel) by adjusting the quench severity and quenching efficiency by one of the many means discussed in Chapter 6. Figure 4-12 aims to show what structure will form at what section of the steel when cooled differently, i.e. quenching by water, oil or air. For

example, with reference to Fig. 4-12, if the steel is water quenched, 100% martensite will form up to bar diameter of 12mm (vide arrow mark). If the same steel is oil quenched, 100% martensite will form up to around 8mm, and if air cooled, only a shallow skin of martensite with 0.2mm maximum depth might be produced. If the bar diameter is over this limit or cooling rate is in-between, the structure will be mixed one with other structures – as depicted in Fig. 4-12. Means of improving upon marginal deficit of ruling section can be either by improving hardenability of the steel (i.e. adjusting the composition) or by improving quenching efficiency / severity of the quenching process. Improving hardenability by alloying will cost more for the steel, which may not be the optimal solution. Instead, better results can be achieved by improving or adjusting quenching efficiency. Techniques for improving quenching severity or quenching efficiency (i.e. achieving consistent quenching results without producing defectives) are, therefore, important part of heat treatability or hardening of steels. As such, more about the quenching technology and quenching mechanisms have been discussed in Chapter 6. Quench severity (H-value) of different media – with or without agitation – has been indicated in Table 4-1. This table is a guide, because actual result will depend on the efficiency of agitation and quality of oil. For example, the table shows that even within oil quenching, quench severity can be doubled by adapting to better quality oil with proper agitation of the bath and thereby ruling section of hardening can be nearly doubled. There are different types of oils or other type of cooling medium with different quench severity which can fit to the exacting needs of hardening; vide Fig. 6-3 in Chapter 6. If hardenability level of the steel warrants control of distortion and cracking during heat treating, then quench severity and its control must be a part of the heat treatment process design. Hence, study of quenching mechanism and quenching efficiency in terms of quench severity is important for the study of heat treatability (this has been discussed in more details in Chapter 6). Improving quenching rate might not be the answer in all cases of hardening problem. For example, water quenching being faster than oil quenching, can produce wider gap in cooling rate between surface and the centre of a job. This temperature gradient not only produces chance of different structure in surface and centre region, but also increases the thermal stress and increased chance of distortion or cracking. Moreover, faster quenching rate can also lead to the development of harmful left over residual stresses. Thus, consideration for residual stress, distortion and microstructural difference between surface and centre prompts what precautions are

necessary for the selection of quenching medium and technique with reference to heat treatability (i.e. hardenability) of the steel.

4.5.1

Other Factors in the Hardening of Steels

Despite clear role of hardenability and quench severity (H-value) in the heat treatment of steels, there might arise few other problems in actual practice of heat treatment. Some such issues relating to hardening of steels are briefly highlighted here. One such problem is with regard to water quenching and control of distortion or cracking. Many steels – especially in plain carbon grades – will require water quenching due to lower hardenability. However, if water quenching cannot be used due to risk of distortion or cracking in such steel parts, alternative quenching method or measures to control distortion / cracking should be looked for. Alternative to water quenching could be polymer quenching – where low percentage of polymer is mixed with water for reducing quench severity, which helps in controlling distortion / cracks. (Polymer quenching characteristics have been discussed in Chapter 6). Other alternative is to quench in fixture in warm water. These are few examples only, and not the full list of possible measures – which should have to be developed, case by case in the actual shop floor situation. Good thing about quenching medium is that there are many types of quenchants available and their quench severity can be controlled in practice by different ways – such as by controlling bath temperature, bath composition, bath agitation, viscosity and quality of oil, etc. More about quenching techniques have been discussed in Chapter 6. Various application specific measures and techniques for heat treating steels developed in recent time (see ASM Handbook, Vol. 4, Heat Treating) have paved the ways for effectively solving many such tricky problems. Other problem which confronts heat treaters is how much of martensite should a structure have, i.e. how deep the steel is to be hardened to 100% martensitic structure. Producing 100% martensite in larger cross-section (cross-section of over 25 mm) calls for very deep hardening steel, i.e. steel with high hardenability band. However, measure for achieving such high hardenability in the steel needs careful consideration from different heat treatability angle. Increasing hardenability by increased carbon will enhance chances of higher distortion and cracking. On the other hand, improving hardenability by additional alloying will increase steel cost. Also, change of quenching medium from water to special oil will entail extra cost. Hence, heat treatment of steel requires judicious approach for choice of steel, its quenching method and other precautions / improvements in heat treatment in order to optimise cost and

performance. In this context, necessity of 100% martensitic structure all through the section should be critically examined. In general, steel section after hardening and tempering should meet the specified properties as per design of the components, but that does not mean that structure all through the cross-section must be 100% martensite. Some non-critical applications may not always demand all through martensitic structure – as long as mechanical property requirement has been met. However, for high dynamic load bearing components, structure should be of as much martensite as possible – though some amount of lower bainite and retained austenite in the structure is generally considered acceptable. Thus, there is no hard and fast rule about how much martensite to have in structure – as long as the structure over the section meets the specified mechanical properties. For critical components, sometime location of sample for testing is indicated in the drawing for ensuring appropriate structure and properties in the steel. Whatever could be the case, maximising martensite structure in a section may not always give commensurate benefit – especially if care is not taken to make the steel surface free from structural blemishes and surface defects – like weaker decarburised layer or surface with pits and dents or small cracks. Once a crack has opened up from such surface defects and propagated inside, stress concentration at the tip of the propagating crack could be so high as to force the final fracture through the remaining cross section, even if the structure is martensitic. Figure 4-13 illustrates one such situation where the component has failed despite having martensitic structure all through. In this case, fatigue crack has started from a surface defect of a shaft and propagated inside – leading to forced fracture at the end. Solution for this is not to go for better and tougher steel, but to ensure that heat treatment process does not introduce any surface defect. This, however, does not diminish the importance for proper hardening of steels and realising enough martensitic structure for strength and toughness; it only highlights the fact that another task of heat treatment is to ensure that the surface is free from such blemishes. Hence, prevention of crack opening from surface – by ensuring defect free surface and surface adjacent structure – assume critical importance in heat treatment of engineering parts. As regards heat treatability of steel is concerned, steel quality should have limited inclusion content for minimising such defects after heat treatment, as inclusions near the surface region tend to open up small cracks at those points under quenching stress. Thus, challenge to heat treaters is to optimise the steel character and different process parameters for obtaining right balance of structure for the specified properties, and freedom from surface defects. Heat treating

Fig. 4-13 An illustration showing failure of an automobile Axle shaft arising from surface defect

parts with least residual stress, free from decarburised weak layer, uniform and fine martensitic structure in the case region, and core region having a combination of strong and tough semi-martensitic structure might prove better cost-effective answer to many application related problems. Despite having steel with adequate hardenability, heat treatment results might not be satisfactory if quenching is not effective and surface defects like decarburisation or pits, scabs, etc., get generated during heating. Practical heat treatment is a balancing act between what best is obtainable from the steel (as regards its different heat treatability factors) and how to get there without jeopardising the functionality of the component by the extraneous factors like development of harmful residual stress, excess distortion, crack, decarburisation or any other surface defect. Foregoing discussions points to the fact that planning of heat treatment process demands consideration to number of factors, namely: • Hardenability of the steel • Grain size and de-oxidation treatment of the steel (i.e. if the steel is fully Al-killed or Al-Si killed or otherwise) • Section size to be hardened (ruling section) • Possibility of segregation and presence of macro-inclusions, and • How quenching can be made effective in practice without giving rise to distortion / harmful residual stresses or any type of surface defect. Quenching is an important part of heat treatment technology, requiring critical attention for satisfactory hardening results. It is not a direct factor in the determination of heat treatability of steel, but indirectly controls the outcome of heat treatment. As such, more details about quenching mechanisms and technology have been discussed in Chapter 6.

Summary 1. For hardening, steel must have sufficient ‘hardenability’, where hardenability is a combined factor of composition and grain size. Hardenability refers to ability of the steel to be hardened to a specified depth below the surface to a defined hardness level, which could be a pre-defined hardness value (e.g. 50 Rockwell in C scale) or criteria set by % martensite in the structure (e.g. 50% martensite). The term is related to depth of hardness below the surface and must not be confused with maximum hardness, which is measured on the surface. Hardenability is the most important factor for heat-treatability of steels; it determines if the steel can successfully respond to a hardening operation and to what depth (or crosssection) the steel can be hardened. 2. Other than composition and grain size (i.e. hardenability), heat-treatability of steels in practice also depends on (a) quenching media and method of quenching, and (b) section thickness or diameter of the steel parts (i.e. ruling section). The latter is due to ‘mass effect’ arising from the interference of low thermal conductivity of steel, resulting in progressively slower cooling from outside to the inner of a steel piece. Faster the cooling, larger is the difference between surface cooling and centre cooling. Thus, if cross section is higher, steel with higher hardenability would be required, if 100% martensite is to be produced. Thus, actual heat treatability of steel in heat treating practices depends on: (a) Composition of the steel, (b) Grain size, (c) Inclusion level in the steel and presence of segregation, (d) Quenching media and method of quenching, and (e) Section thickness or diameter of the steel parts 3. Metallurgically, hardenability can be seen as an inverse measure of the severity of cooling rate necessary to produce a martensitic structure in a previously austenitised steel, avoiding transformation to pearlite and bainite. The lower the cooling rate required by a steel to avoid these other non-martensitic structure, higher is the hardenability. The cooling rate at which such transformation condition can be reproduced in the steel is called the ‘critical cooling rate’. Critical cooling rate is largely the function of composition of the steel and grain size. 4. As regards compositional constituents of steel, influence of carbon on hardenability is due to its ability to economically produce higher hardness than the depth of hardness. In practice, carbon is used for producing higher hardness, whereas alloying elements like Mn, Cr, Ni and Mo are used for increasing the depth of hardening in a given ruling section. Therefore, best way to increase hardenability in steel is to alloy the steel appropriately. But, alloying with expensive elements increases cost of the steel. Hence, should be used with prudence and justification. Effect of different alloying elements of hardenability have been mentioned and discussed in the chapter. 5. The chapter briefly described the Jominy hardenability testing methods and various information the tests can provide for heat-treatment of steels. Since Jominy hardenability refers to ideal quenching expressed as ‘Ideal critical diameter’ (DI), its correlation to practical heat-treatment processes with respect to differing quench severity (H-value) has been presented in terms of critical diameter (D) for the respective cooling rate. 6. Considering grain size is an important factor contributing to successful hardening, nature and character of grain boundaries and their influence on hardening process has been mentioned. It has been emphasised that for stable and consistent

austenite grain size, the mother steel needs to be properly killed (degassed) by either full aluminium or by aluminium-silicon combination for avoiding abnormal grain growth during heating. Influence of grain size on hardenability has been illustrated. 7. Role of inclusions and their effect on hardening of steel has been discussed. It has been emphasised that inclusion interfaces with steel matrix can act as points of stress raisers during quenching and be the sources of micro-crack formation. In view of this potential harmful nature of inclusions, limitation of micro-inclusion content in steel for good heat-treatability has been mentioned. 8. Finally, importance of quenching and relative ruling section in the process of hardening of steel has been illustrated. It has been shown that no matter how adequate is the hardenability value of the steel, success of its heat treatment will depend on the effectiveness of quenching and quench severity of quenching media. Many a time, hardened microstructure of steel can be favourably influenced by improving the quenching system, but by avoiding distortion or cracking due to excess quenching.

References / Suggested Reading ASM Handbook, Vol. 4, Heat Treating, ASM International, USA, 1991 Brooks, Charlie R. Principles of Heat-Treatment of Plain Carbon and Low-Alloy Steels, ASM International, USA, 1996 Mandal, S.K. Steel Metallurgy: Properties, Specification and Applications, McGraw Hill Education (India), New Delhi, 2014 Reed-Hill, Robert E. Physical Metallurgy Principles, Van Nostrand Reinhold Co., 1973 Totten, George E. (Ed), Steel Heat-Treatment Handbook, 2nd edition, CRC Press, 2006 Unterweiser, P.M., H.E. Boyer and J.J. Kubbs (eds), Heat Treater’s Guide: Standard Practice and Procedure for Steel, ASM, Metals Park, Ohio, 1982

Review Questions 1. What is meant by ‘heat treatability of steels’? Define ‘hardenability’ and identify the factors that influence the hardenability of steels. Elaborate the role of hardenability of steel in its heat treatment. 2. What is meant by ‘critical cooling rate’ in the heat treatment of steel? What are the factors that control the critical cooling rate in steel? Why increasing critical cooling rate by increasing carbon in the steels is not generally recommended where the end application of the steel calls for good strength with high toughness? 3. List the following steel compositions in order of increasing ‘hardenability’: Steel containing: (a) 0.45% carbon, (b) 0.25 carbon + 1.50% manganese, (c) 0.40% carbon + 1.0% chromium, and (d) 0.40% carbon + 1.0% chromium + 0.012% molydebnum 4. Why steel with increased carbon content beyond ‘eutectoid’ composition fails to give increased hardenability? What is the practical recourse to overcome such problems for obtaining high hardenability in the steel?

5. How hardenability of steel is experimentally determined? Describe the process and discuss what all information the process can provide for understanding the behaviour of the steel in heat treatment. 6. Why experimentally determined hardenability value may vary from that observed in the actual performance during heat treatment? What do you understand by ‘Quench severity’ of a cooling medium? Arrange in order of increasing H-value (quench severity) of the following medium: Water without agitation, oil with strong agitation and water with high agitation. 7. How the difference between ‘Jominy hardenability’ value and the actual hardenability obtainable by using a specific cooling medium is reconciled for heat treatment practices? Discuss the uses and utility of ‘Ideal critical diameter’ and ‘Critical diameter’ of steel. 8. How grain boundaries influence the hardening process of steel? What are the general measures for restraining grain growth during heat treatment of steels? For hardening of steels, what type of grain size is desired and why? 9. How inclusions affect the heat treatability of steels? Does inclusion of all types and size equally affect the heat treatment? How to ensure freedom from heat treating problem due to inclusions in steel? 10. What do you understand by the term ‘ruling section’ of steel for heat treatment? Why the consideration of ruling section is important in the heat treatment of steel? How depth of hardening can be improved without change in composition or grain size in steel?

Heat Treatment Furnaces and Furnace Atmosphere Control

5.1

INTRODUCTION

Equipment are as important in heat treatment as the processes. Amongst the equipment used in heat treatment, furnace is the single-most important equipment. A good furnace helps running the heat treatment process smoothly, consistently and with precise control for reproducible results. Functionally, the role of a heat treatment furnace is to allow transfer of heat from the furnace to the jobs in most uniform, efficient and economic manner – along with the facility for control of temperature and atmosphere inside the furnace. Therefore, furnace should have the following characteristics: • Source of heating and control of heat in the furnace • Adequate heat capacity for heating up the charges • Capacity for efficient heat transfer from furnace body to the charges • Facility for uniform temperature distribution inside the furnace, and • Facility for atmosphere control for protecting the jobs from higher temperature oxidation reactions, whenever necessary.

There are many types of heat treatment furnaces and they can be classified on the basis of different criteria: • Sources of heating; electrically heated or oil fired furnace • Mode of operation; batch or continuous type furnace • Mechanism of functioning; walking beam, pusher hearth, etc., • Applications based; carburising furnace, nitriding furnace, etc., • Construction based; mesh-belt, seal-quench type, etc., Whatever be the furnace type, for heat treatment it should fulfil the specific requirements of a specified heat treatment process, which include not only meeting the quality specification of product but also productivity, energy efficiency and economic operation. On the basis of these indicators the furnace types can be classified under: • Source of heating and control of furnace; electric heat treatment furnace, oil / gas fired furnace, salt bath heat treatment furnace, atmosphere controlled heat treatment furnace, vacuum heat treatment furnace, etc. • Heat treatment process or application objectives; carburising furnace, hardening furnace, nitriding furnace, etc. • Operational mode; such as batch type or continuous type furnace. Each of these furnace types has their own specific characteristics, based on which they are selected for specific purpose. Discussing all such varieties of furnaces is beyond the scope of this book. Hence, only some popular furnaces and their characteristics have been discussed in this chapter. Characteristics of furnace type to be used for heat treatment depend on what process is to be carried out and what are the critical requirements of the process: For example: for annealing or normalising of steel, a simple batch type box type furnace with or without atmosphere control can be used, but for hardening the steel parts, such as gears and shafts for mass production, furnace should be with easy charging and discharging mechanism, should have facility for precise control of heating temperature, and facility for controlling atmosphere inside the furnace. These characteristics apply to both batch and continuous furnaces. It is important to note that all heat treatment furnaces must follow the thermal cycle of heating, holding and controlled cooling – as depicted in Fig. 3.1. To accomplish this, furnaces must have certain minimum constructional and operational features. These features are: • Heat insulated chamber • Heating mechanism (e.g. heating element, heating burner, etc.) – appropriate with respect to fuel / energy being available or used • Controller for temperature control

• Charging–Discharging facilities • Mechanism for atmosphere circulation within for uniformity of temperature, and • Atmosphere control mechanism – as per process requirement These features are to be built into the furnace during construction. Cooling of heat-treated jobs is an additional operation, which is generally carried outside the furnace but can be integrated with the main furnace, if necessary. Example of such integration is the construction of seal-quench furnace, where both heating and cooling for hardening is carried out inside the furnace chambers by design. For common heat treatment operation, focus of selection of furnace is on process and process control, productivity (i.e. throughput per unit time), and heating efficiency (i.e. cost of energy per unit of production). Heating type and heat transfer efficiency of a furnace are of primary consideration for its selection and operation – due to cost and available source of energy. Heating type can be electrical heating, oil-fired, or any other source like molten salt, induction, etc., but should be able to heat the job efficiently and effectively for ensuring output quality. Hence, heat capacity of the furnace is another consideration for choice of furnace type. However, whatever could be the furnace type, heat transfer inside the furnace – which is responsible for energy efficiency and effective heating – takes place by (1) radiation, (2) convection, and (3) conduction. These are the three fundamental mechanisms by which heat gets transferred from one body to another – such as from furnace to the steel stock being heated. In the process of heat transfer, furnace and furnace parts are first heated by external input of energy and then the hot furnace transfers some heat to the steel parts by a combination of conduction, convection and radiation. Though some heat gets transferred to the steel body for heating, part of the heat also gets absorbed in the furnace body and ultimately gets released to the surrounding. This necessitates working out of heat balance in a furnace to direct maximum available heat for heating the component being heat treated. Heating efficiency of the process largely depends on the mechanism of heat transfer that is dominant inside the furnace at the operating condition / temperature. Amongst the heat transfer processes, radiation is dominant in the region of higher working temperature (e.g. hardening furnace) and convectional heat transfer is dominant at the lower temperature range (e.g. tempering furnace). Conduction heat transfer becomes a dominant factor for the mass to acquire uniform temperature during heating or cooling of the steel during heat treatment (e.g. temperature equalisation during soaking in heating or during quenching in cooling).

This chapter, therefore, aims to discuss the essential features and characteristics of heat transfer in heat treatment furnaces first, and then illustrate those points and their control from operational point of view with reference to some popular heat treatment furnaces. [Details of process heat transfer mechanisms are not within the scope of this chapter; more information about process heat transfer can be obtained from the books and URLs under suggested readings (see ASM Handbook, Vol. 4, Heat Treating, 2004; Smith and Holcraft, 2004; Kern, 1997)]. Other than heating and cooling, heat treatment furnace has another important control point – and that is the atmosphere control inside the furnace. Therefore, means and methods of atmosphere control have been discussed wherever necessary along with the discussions about a furnace, and additionally further discussed and illustrated in Section 5.6.

5.2 HEAT TRANSFER AND HEAT BALANCE IN HEAT TREATMENT FURNACES Efficiency and economy of heat treatment process depend on the heat transfer efficiency and control of heat balance in the furnace for heating the workload. The furnace productivity and efficiency are maximised by ensuring maximum heat transfer and optimum heat balance in the furnace during heat treatment. Main requirement of furnace for heat treatment is to provide maximum heat input to the charged load for uniformly attaining the required temperature. This effective heat input inside a furnace takes place by heat transfer from the hot furnace to the load / work pieces placed inside the furnace chamber. For efficiency of heating, maximum heat should be directed to the jobs, and for doing so heat loss by the furnace to the surrounding or elsewhere must be cut down. While the former factor is the function of heat transfer process, the latter is to be controlled by planned heat balance in the furnace. Therefore, this section will highlight the principal points about heat transfer mechanisms in heating furnaces and the sources of heat loss and method of heat balance for optimum heating efficiency.

5.2.1

Heat Transfer Processes in Furnace

Heat transfer in heat treatment furnace primarily takes place by three heat transfer mechanisms; namely: Conduction, Convection and Radiation. Heat transfer mechanism under each of them is different; they follow different laws of physics. Laws governing their characteristics are highlighted below:

Conduction heat transfer is caused by conduction of heat through the material molecular structure – is governed by Fourier Equation: Q/A = – k. dT/dX, where Q/A is the heat transfer per unit area in Watts per meter square (W/m2), dT/dX is the temperature gradient in the system, and k is the constant of proportionality – called the material thermal conductivity. Convection heat transfer is governed by the equation established by Newton, considering the energy transport through bulk fluid motion. It states that: Q/A = h (Ts – TF), where Q/A is the heat transfer per area exposed, (Ts – TF) is the fluid–solid temperature difference, and h is the proportionality constant – called the heat transfer coefficient. Heat transfer coefficient value depends on the fluid velocity (e.g. air in the furnace) in contact. Thus, depending on the fluid velocity or state of the fluid, convection involved in heating or cooling can be of different types: natural convection, forced convection, boiling and condensation. Of these, natural and forced convection are of critical importance in heat treatment operation. Radiation heat transfer is energy transport due to emission of electromagnetic waves or photons from a surface or volume, which is hot. Radiation heat transfer medium can occur in vacuum or through a transparent media (solid or fluid). Heat transfer in radiation is given by the equation: Q/A = e. d. T 4, where e is the emissivity of the material (a material constant), d is the Stefan-Boltzmann constant (it equals to: 5.67 × 10– 8 W/m2 K4), and T is the temperature in absolute scale. Q/A is the heat transfer per unit area exposed. Thus, heat transfer by radiation is proportional to the fourth power of the absolute temperature of the hot body. Hence, radiation is the most powerful heat transfer mechanism, especially at the higher temperature. These three processes of heat transfer are operative during the heat treatment cycle of ‘heating, soaking and cooling’. Effect or contribution of each of these heat transfer processes depend on the stages of heating or cooling. All these processes of heat transfer may work simultaneously, but depending on the state of heating or cooling, a particular mode becomes prominent. For example, radiation is the dominant mode of heat transfer when the temperature is high – such as in the higher heating and soaking cycle for austenitisation during heat treatment. At the lower end of heating (e.g. tempering operation) it is the conduction and convection that prevails in the process of heat transfer. Similarly, during the quenching, convectional heat transfer is the prominent mode for cooling, which can be further increased by providing agitation or forced circulation to the cooling medium (more about this has been discussed in Chapter 6).

Conduction mode of heat transfer depends on thermal conductivity of the material being heated or cooled, and it is very critical for the process where workload should attain uniformity in temperature throughout the mass – which is an important process requirement for most heat treatment operations. Whatever could be the furnace type, heat transfer to the job takes place by a combination of these mechanisms – namely conduction, convection and radiation. For high temperature heat treatment furnaces (e.g. hardening furnace), furnace heating elements, therefore, require to be designed for optimum radiation heating. Similarly, for tempering furnace, which operates at lower range of temperature, furnace should be designed for maximum convectional heating. This is the reason for using air circulating furnace for all lower temperature tempering furnace. Even in high temperature furnace – where radiation mode of heat transfer is dominant – provision is made of ‘circulating fan’ inside the furnace for increasing convectional heating and for attaining uniformity in temperature over the section of steel piece being heated. In sum, judicious provisions for these heat transfer processes in a furnace in most optimal manner largely determine its heating efficiency. Simultaneously, furnace design requires careful planning of the furnace construction for minimising heat loss from the furnace body.

5.2.2

Heat Balance in a Furnace

Furnace is a multi-component composite body consisting of furnace shell, walls, chamber, floors, etc. – along with holding a specified amount of charge (load) for heating. Heat that gets transferred from the heating elements or the firing system is absorbed by all components in the furnace system. But, for energy efficiency, it is important that optimum amount of thermal energy is directed towards the heating of charge / workload. This can be achieved by analysing the sources of heat and heat loss in the furnace system – followed by actions for heat balance. Figure 5.1 illustrates the sources of heat and heat loss in a furnace, and components of heat balance involved in a furnace system. Considering various sources of heat storage and heat loss from this figure, the heat balance in the system can be expressed as: QStorage = Qht – QId – QLoss – QAir + QFan, where: Qht is the effective heat input, QId is the heat to work pieces, QLoss is the heat lost from the system, QAir is the heat absorbed by the atmosphere in the furnace, QFan is the heat input effect created due to increase of convection by fan circulation, and QStorage is the heat absorbed by the insulating refractory and structural components of the furnace for raising them to operating temperature.

Heat loss by cooling water

Heat loss from furnace walls

Heat input from fan

Cooling water

Fan

Heat absorbed by fixture

Heat stored in the furnace

Heat absorbed by work pieces

Heat input by furnace

Fig. 5.1 An illustration showing the sources of heat loss and components of heat balance in a batch-type furnace [Source: https://www.ferronova.com/International/Web/LG/FER/likelgfer.nsf/repositorybyalias/lit_atmocontrol/$file/Close%20atmosphere%20control%20gives%20high%20quality%20 automotive%20parts.pdf]

These illustrative analyses indicate that heating efficiency of a furnace can be improved by the following measures: (a) Reducing heat losses from the walls, roof, floor, etc., by conduction, due to temperature difference between inside and outside. This calls for better insulation of the furnace walls. (b) Reducing heat losses by radiation from the furnace to outside. This can be controlled by controlling the heat of furnace shell, which can get heated due to heat losses from inside through insulating bricks and construction material. (c) Reducing heat losses through the openings in the walls and doors. This is controlled by putting door seals and provision of air curtains in such openings during operations. (d) Improving the heat transfer to work pieces by promoting better convection and radiation. Provision of circulating fan on the furnace roof is one such measure for improved convection. Purpose of heat balance is to improve the ratio of ‘available heat energy’ to total amount of heat supplied for heating the furnace, where ‘available

heat’ includes the heat remaining in the furnace after subtraction of all heat losses from total heat supplied to the furnace. Available heat, therefore, is the useful heat, left in the furnace that can be used for heating the workload. Heating efficiency of furnace is the ultimate test of how much heat reaches the products, i.e. workload in the furnace for heating. Efficient and effective heating of charge load is critical for all heat treatment operations. This is an important economic consideration for choice of furnace type for a given heat treatment. An important way of improving heating efficiency of a furnace is to improve upon its thermal insulation by using right grade of thermal insulators. Thermal insulators are materials that specifically reduce the flow of heat out by limiting conduction and convection. The effectiveness of insulator is indicated by R-value or resistance value. R-value of an insulating material is the inverse of the ‘conduction co-efficient’ of the material (k) multiplied by the thickness (d ) of the insulator. In furnaces, generally, glass wools and refractory linings are used for insulation. Therefore, these materials should be of appropriate quality and thickness for effective insulation. Porous but strong refractory lining provide better insulation than dense refractory.

5.3 INTRODUCTION TO FURNACE TYPES AND THEIR FEATURES From common heat treatment practices point of view, furnace types can be grouped into: furnaces for higher temperature operations, e.g. full annealing, normalising, hardening, carburising, etc., and furnaces for lower temperature operations, e.g. tempering, nitriding, stress-relieving, spheriodising annealing, etc. Furnace characteristics and their heat transfer mechanism are different for these applications, because of difference in temperature of operation. High temperature furnaces can be further grouped into hardening furnace, annealing furnace or any other special purpose furnace, like carburising furnace. Each of these types has its own specific requirements. For example, hardening furnace must have atmosphere control mechanism for preventing decarburisation of steel before hardening, which might not be the required for annealing or normalising, in normal cases. Similarly, lower temperature furnaces could be of different types based on its specific requirement, such as tempering furnace, nitriding furnace, etc.; each having their specific needs. For example, tempering furnace requires air circulation facility for improved convectional heat

transfer whereas nitriding furnace requires circulation of nitriding gas in a manner that works for both increased convection as well intimate contact with charges. Oxygen-rich atmosphere, created by air circulation in tempering furnace, causes no harm as oxidation / decarburisation reaction is not significant at lower operating temperature. Thus, these requirements of heat treatment processes have to be built into the furnace types for heat treatment of steels. Furnaces are commonly classified in industries as per mode of operation (e.g. batch or continuous type), heating temperature (e.g. high temperature or low temperature), method of heating (e.g. electrical or gas fired), method of atmosphere control (e.g. vacuum or atmosphere controlled), etc. Table 5-1 illustrates one such possible approach for classification of different furnace types.

Under each of these classes, there are many types of furnace – based on application, construction or control, which will be discussed in Section 5.5 of this chapter. Furnaces are also popularly described as per their operational / construction characteristics. To name a few: • Walking Beam Furnace – used mainly for annealing / heavy section jobs • Pusher typefurnace –where charges are periodically pushed from one heating zone to other – used for many hardening jobs • Mesh-belt type continuous furnace – used for continuous carburising • Rotary hearth furnace – used for hardening, and • Seal-quench furnace – used for hardening / carburising and direct hardening. These furnaces are quite popular in the arena of heat treatment of steel. Furnace must have some basic characteristics for fulfilling its functional objectives. These characteristics can perhaps be best illustrated with reference to a simple laboratory furnace shown in Fig. 5-2.

Figure 5-2 illustrates the inside of an electrically heated laboratory muffle furnace, which should basically have all the characteristics of a good heating system. This type of furnace is used for hardening trials and heating the steel specimen for Jominy Hardenability tests in laboratory, requiring closer control of temperature for consistency of results. The furnace construction illustrates that: • Working chamber of the furnace must be insulated for sealing heat loss with appropriate refractory lining, which is, generally, of basic type refractory brick / block lining. • Furnace should have the provision of housing ‘heating elements’ or ‘heating source’ (e.g. burners for gas heating) as per source of heating. Source of heating can be electrical coils or radiant tube, or appropriately designed burners for gas heating. • Furnace should have appropriate temperature control mechanism for uniform heating and control of temperature. • For uniformity of temperature inside the furnace, there should be provision for atmosphere circulation inside for improved convection. This can be accomplished by using heat-sealed circulating fan for circulation of air inside. • Furnace should have atmosphere protection system built into it for protecting the jobs from heavy scaling or decarburisation. (This can be need based as per process objective) • Furnace chamber doors / exit points – for charge and discharge of loads – should be adequately insulated from heat loss by appropriate cover and mechanised door design.

Fig. 5-2 An illustration showing electrically heated muffle furnace, showing refractory lined chamber, grooves for heating elements, controls and mechanised door

These are essential features of a furnace construction for meeting various heat treatment process needs. Details about these features and their applications will be discussed in Section 5.5 where some popular types of furnaces for hardening and carburising have been described. Carburising is a high-temperature operation, involving two separate processes – carburising and hardening. Though these two processes are generally carried out separately by using process-specific furnaces, the latest trend seeks to integrate these processes into one operation for economy and productivity. Seal quench furnace is an example where both processes can be integrated. Such application requires furnace with separate heating zones with elaborate mechanism for atmosphere generation and control inside, as applicable to the operating zone of the furnace, and facility for quenching in the sealed chamber for hardening. As such, the furnace type is described as ‘seal-quench furnace’. All furnaces can be, in principle, batch type or continuous. But, use of continuous furnaces for heat treatment of steel is mainly confined to processes like carburising (by gas) and hardening of small and symmetrical parts. In general, batch type gas carburising furnace is used for large and odd shaped components and continuous furnaces are used for small symmetrical parts, like small gears, washers, pins, nuts, bolts, etc. Carburising process, which is a two-stage process, is commonly carried out separately in batch type furnaces. But, if demands of productivity and component dimensions justify, these two processes can be combined in continuous furnace operation. Popular furnace types for gas carburising are: (1) Batch type: Pit furnace and Seal-quench furnace – the latter type combines both carburising and hardening in one integrated furnace system, which can be also converted to semi-continuous operation. (2) Continuous type: Pusher type, Rotary hearth type, Mesh-belt type, etc. Some of these furnaces and their operations have been discussed in more details in the next section. These furnaces are named as per their operational characteristics. The other types of important heat treatment furnaces in use for hardening are: Salt Bath Furnace and Fluidised Bed Furnace. These furnaces use denser fluid medium of appropriate nature for higher heat efficiency through convectional and conduction heating. These are batch type furnaces; though attempts are on to make the fluidised bed furnace system more flexible and semi-continuous. These furnaces are suitable for hardening, especially of high alloy steels, where convectional heating is desirable. Convectional heating produces lower thermal gradient across a section, and lower thermal shock, due to more even heating rate.

These are the general features and requirements of heat treatment furnaces for different processes.

5.4

CHOICE OF FURNACE AND RELATIVE ENERGY EFFICIENCY

From operational point of view, furnaces could be grouped under: (a) Batch type: For example, bogie furnace, box furnace, chamber furnace, pit furnace, seal quench furnace, vacuum furnace, etc. (b) Continuous furnace type: For example, rotary hearth furnace, mesh-belt type conveyor furnace, etc. There are different types of furnaces amongst these groups; some have been mentioned in the examples and others have been listed in the previous section. Choice of furnace calls for ascertaining the task of heat treatment first, followed by considerations like source of energy available readily, cost of energy, required productivity and frequency of operation, floor space available, etc., to match the task and required economy of operation. Heat treatment tasks are different for different processes. For example, for finish heat treatment, such as through hardening, case-hardening, etc., furnace should have precise control of mechanical movement, temperature and environmental atmosphere for accurate results. Such furnaces – which operate at higher temperature – should also have mechanisms for maximizing radiation heat, and a circulating fan for optimising convectional heating. But, if the furnace is needed for a lower temperature operation, such as tempering or spheriodising annealing – emphasis should be on maximising convectional heat by air circulation – along with standard temperature control. Atmosphere control mechanism for such application might not be essential, in all cases. Thus, furnace types and facilities for such different application would be different. Furthermore, choice of furnace types also depends on the load – its shape, size and volume on one hand, and quantity and continuity of production, such as in a mass production shop, on the other. Very often, it is the load type that leads to the choice of continuous or semi-continuous furnaces in the shop. Small and medium size engineering components, which are prevalent in automobile industries and are used in large numbers, call for continuous or semi-continuous furnace. Continuous or semi-continuous furnaces are relatively more energy-efficient than batch type furnaces where substantial amount of heat gets wasted in between the charging and cooling cycle. But, batch type furnace is flexible in application and more compact than continuous furnace.

Heating sources of heat treatment furnaces could be electric, oil, gas or solid fuel. But for standard heat treatment practice, they are mostly electric, oil or gas fired. Of these energy sources, electric heating is more efficient, followed by gas heating. Heating efficiency of furnaces not only depends on the heating source, but also on the heating element design or mechanism of fuel burning nozzles. In general, electric heating is cleaner and more efficient but cost of electricity is high. Gas fired furnaces may be less efficient than electric furnace and require proper burner design for improved combustion efficiency. Oil fired furnaces might be cheaper to run, but it is prone to clog the burner, causing uneven heating. Oil fired furnaces tend to pollute the environment more than other sources of energy. Choice of furnace requires understanding of how the furnaces work. In this regard, two examples are illustrated below; one batch type and the other continuous furnace with regard to their uses and relative advantage. Figure 5-3 depicts a typical bogie hearth furnace that is commonly used for annealing or normalising. This is a batch type furnace, and can be heated by electricity or by gas or by oil. Oil fired furnaces are cheaper to run but environmentally messy; only used for batch type operations of common heating and cooling purpose, such as for forging or rolling. Electrical heating is cleaner for the shop floor, but energy cost might be more than gas fired furnaces. Gases have the requirement of right burner design and right combustion; else it can produce atmosphere of varying composition inside the furnace, ranging from highly oxidising to highly reducing atmosphere – thereby adversely affecting the heat treatment process outcome.

Fig. 5-3 An illustration showing a typical box-type gas fired furnace for annealing of heavy section steels

In batch type furnaces, jobs are generally manually loaded and unloaded into and out of furnace. This could be a source of extra heat loss – calling for more careful heat balance in the furnace. Batch type furnaces are, however, flexible and can be built or bought for different capacities. Hence, these are popular for job shops, manufacturing large to small engineering parts, requiring annealing of different descriptions and normalising. Despite the advent of modern batch type (e.g. seal quench furnace) and continuous furnace (e.g. mesh-belt type furnace), box type batch furnace is still used in plenty in industries for common and general purpose heat treatment. Continuous furnaces have automatic material conveying system that carries the material through different cycles of operation inside the furnace (e.g. heating and soaking) and delivers the material at the other end after completion of all heat treatment cycles. In continuous furnace, materials move through mechanised conveying system – delivering the output at the other end of the furnace. Thus, continuous furnace provides constant flow of heat-treated material out-put. A typical continuous furnace lay out is shown in Fig. 5-4. Continuous furnace can be roller hearth type – where rolling hearth moves the job from one point to another through the pre-set temperature zone or pusher type where the jobs are mechanically pushed for forward movement or mesh-belt conveyor type where the jobs are placed on a conveyor belt to move along with the moving / rotating conveyor (as shown in Fig. 5-3). Entry and exit doors of the furnace are protected with door curtains (either mechanical or air curtain) for minimising loss of heat. Working out heat balance in continuous furnace is less complicated and can be maintained at a uniform level. Continuous furnace is commonly used for hardening of small steel components, e.g. small gears, nuts and bolts, etc. This type of furnace is quite popular for mass production of similar sized small items, such as small gears, crosses, yokes, pins, links, chains, etc., for hardening or continuous carburising. Quenching system of the furnace is located opposite to the input load area and has arrangement of conveying the quenched items through conveyor belt. It is a highly productive unit and can be fully automated for loading and controlling the furnace temperatures and speed of conveyor travel. Continuous furnace can be heated by either electricity or natural gas for clean heat. These two illustrations (Figs. 5-3 and 5-4) briefly highlight the typical characteristics of batch and continuous type furnaces in use. These illustrations show that the purpose of proposed heat treatment and their relative cost of operation are of critical importance in the choice of furnaces.

USE • Annealing of stainless steel • Brazing • Soldering • Annealing • Sintering

Fig. 5-4 General lay-out of a continuous furnace with mesh-belt conveyor [Figure mentions its typical uses as per sales plan of the manufacturer, but not limited to these applications only]

In general, furnace selection is based on certain check points based on preferred source of energy, load type, process objectives, energy efficiency and economy of operations. Technically, following points need to be examined for making a choice of a specific type of furnace: • Heating methods – electrical, gas, oil or other mode – vis-a-vis available energy and cost • Heating efficiency in the system – radiation, convection, conduction, etc. • Type of heating elements – electric heating or fuel burning. Additional check for burning equipment: design of burners, nozzles, flow control, etc. • Energy consumption per unit volume of feed, and process efficiency • Adoptability to heat treatment process objectives, and control system– Check for adoptability to statistical process control (SPC) system on-line • Robustness of construction and refractory types – For reduced heat loss, durability and maintainability • Yield (productivity) requirement and scope for yield improvement for future expansion • Environment friendliness and environmental aspect of a heat treatment shop for conformance to safety and legal standards Energy efficiency and cost of operation is a critical consideration for choice of furnace type. In this regard, continuous operating furnace is more energy efficient than batch furnaces. But, many a time, neither the volume of production nor the size and shape of the jobs may fit to continuous process. In fact, batch type operation still dominates the heat treatment processes in industries in general due to flexibility of operation and scope of heat treatment.

Energy efficiency and cost of heat treatment are closely related; cost of energy consumed is the largest component of heat treatment cost. Electric heat might be very efficient, but its high cost stands on its way of uses. Relative efficiency of electric heating could be as high as 80%, compared to 30 to 50% of gas, but electricity cost is substantially higher than fuels such as natural gas. As such, gas fired furnaces are effectively more economical than electrically heated furnaces at the end, though its energy efficiency can be 10 to 20% lesser than that of electrically heated furnace. Lower energy efficiency of gases arises mostly from incomplete combustion and release of hot flue gases, leading to loss of sensible heat. Source of heating is given considerable importance for choice of heat treatment furnaces types, based on their availability, cost and efficiency of heating. Areas where electricity supply is erratic or natural gas availability is better, it is preferable to go for gas fired furnace with improved combustion mechanism and flue gas heat recovery system. In all heating method, radiation, convection and conduction are three main sources of heat utilisation. Of these, radiation heating is the most efficient and easily controllable in electrical heating. Since radiation heat exponentially increases with temperature, electric heating is suitable and more efficient for high temperature heat treatment furnaces. By improving the radiation heat and convection, heating efficiency of electric furnace can be vastly improved. This is the reason why all high temperature heating processes pay considerable attention to increasing the radiation effect, such as by using radiant tube technology. Other than controlling the mode of heating or heat transfer, gas fired furnaces call for steps to improve heating efficiency by combustion control and waste heat recovery. Considerable developments have taken place in gas heating with improved radiation and convection effect, on one hand, and by improving the combustion efficiency and waste gas recovery, on the other. Choice of energy sources largely depends on the availability of sources, cost of operation and consistency of supplies. In general, for higher heat treating operation, electric heat is more efficient and at lower range of temperatures or for heat treatment of longer duration (e.g. annealing, spheriodising etc.), gas fired furnace is economical and efficient. However, with advances in the technology of gas fired heating, gas is becoming more competitive with increased heating efficiency for all ranges of temperature.

5.5

HEAT TREATMENT FURNACES AND APPLICATIONS

Foregoing sections of this chapter highlight the mechanisms of heat transfer, need for heat balance in a furnace, essential features of heat

treatment furnaces, different characteristics of different types of furnaces, and their relative merits and energy efficiency. It can be noted from these discussions that every furnace type has its own characteristics and accordingly the furnace type has to be selected to fit to the requirement of a specific process of heat treatment. Heat treatment is a universal process for treating steels for different properties and application, and coverage of heat treatment application is very wide. To cater to this wide spectrum of applications, there are large verities (i.e. type) of furnaces – some of which are batch type, some are continuous type and some can be made to fit special purpose (e.g. semi-continuous). This section has outlined such approach and highlighted their merits in applications. Amongst batch type furnaces, popular types are: Muffle Furnace, Bogie Hearth Furnace, Seal Quench Furnace, Pit type Furnace, Bell Furnace, and Salt Bath Furnace with immersion electrodes. And, amongst continuous furnaces, popular ones are: Shaker Hearth Furnace, Rotary Drum Furnace, Pusher type Furnace, Mesh-belt furnace, and Rotary Hearth Furnace. In addition to these popular types of furnaces, other types are: Vacuum Furnaces, Fluidised Bed Furnace and air circulating Tempering Furnaces, which are popularly used for heat treatment of different engineering components. Each of these furnace types are used with discretion for cost, volume of production, size and shape of job, required heat treatment process and properties, and preferred heating system. For specification and functional details of these furnaces, best sources are the manufacturers’ catalogue and the guidelines provided by the ASM International, USA, through their various publications (some of which have been listed in the reference book list). Discussing all of them is beyond the scope of this chapter; hence salient operating principles and features of some of these furnaces are described below for understanding the purpose, operating principles and mechanism. Two modern (seal quench and vacuum furnace) and two traditional furnace types (pit type and salt bath) have been illustrated in order to highlight their industrial importance and utility – in addition to describing the features of fluidised bed furnace.

5.5.1

Seal Quench Furnace

Seal quench furnace is the most important equipment of modern heat treatment shops. It is a batch type controlled atmosphere furnace suitable for varieties of heat treatment processes, involving small, medium and symmetrical components, such as small gears and ball bearing parts. Principally, a seal quench furnace has a heating chamber and an attached quenching chamber. Both the chambers are integrated and sealed into

one unit, so that there is no leakage of air from outside and the work load is constantly under controlled atmosphere. As such, components coming out of seal quench furnace after heat treatment are clean and bright, free from oxidised surface. Figure 5-5 shows a popular brand of seal quench furnace on the shop floor.

Fig. 5-5

A general view of high-temperature seal quench furnace in the shop floor

The seal quench furnace can operate in electric heating or gas heating system. Radiant tube heating is becoming a popular design for this type of furnace. The furnace works as integrated unit with charging, heating, temperature control, atmosphere control and discharge system through number of auxiliary units and mechanical functions. Seal quench furnace is ideally a high-temperature furnace with high heating efficiency; which gets improved with use of radiant tube technology of heating. The furnace can be successfully used in a shop for different heat treatment processes, like hardening, normalising, carburising etc. with controlled atmosphere. Due to uniqueness of atmosphere control system in seal quench furnace, it is also used for gas carburising and hardening. Controlled atmosphere can protect the surface from atmospheric attack during processing as well as can facilitate carburising by controlling / regulating the carbon potential in the atmosphere for supply of carbon atoms during carburising. Control of atmosphere inside the furnace is vital for getting desired results, because atmospheric reactions with workload inside a furnace are of considerable importance for heat treatment outcome. For different heat treatments, different controlled atmosphere is required, such as carbon monoxide and hydrogen control for hardening,

methane or endothermic-gas for carburising, and cracked ammonia for carbo-nitriding. However, such gas mixture could be highly explosive and toxic, requiring careful handling. In this respect, seal quench furnace offers additional safety because it is fully sealed, air-tight and leak-proof. Seal quench furnace being a batch type furnace, handles charges lot wise, each lot is finishing before the next lot is charged. Both doors, at the entry and exit side, are hydraulically operated for firm closing. Charging can be done to the hot furnace by an automatic loader, which places the charge on the correct position of the hearth for internal transfer. Figure 5-6 shows a side view of the furnace construction, showing the charges, chambers and the quench chamber along with mechanical accessories.

Fig. 5-6 Schematic side view of a seal quench furnace

Once the front door is shut, the set temperature and controlled atmosphere are re-established quickly so that operation of the furnace continues without any break. To facilitate sealing and quick restoration of operation after charging, the furnace has highly insulated heating chamber and one or more doors to the heating chamber for alternate uses. With fully automatic control system, including loader and un-loader, the furnace is a highly efficient unit requiring minimum manual handling. The furnace capacity can range from 300 kg/batch to 1000 kg/batch and operating temperature range from 800 to 1050°C; making it suitable for precision hardening of small engineering parts, but not big (lengthier) parts that can heat / cool unevenly and wrap to obstruct charge

movement. Seal quench furnace can be used for hardening, carburising, carbo-nitriding and case hardening with high heating efficiency. By changing the atmosphere-generating system, different atmospheres like endothermic-gas, propane, ammonia, etc., can be maintained inside as per process requirement. The entire chain of activities in the system can be made fully automatic with high degree of temperature uniformity. Seal quench is a two-chamber operation; one for heating in the front and the other for cooling / quenching in the rear. Two chambers are separated inside the furnace by a refractory lined door which can be opened for hot charge to enter the quenching chamber, but in a manner that does not cause drastic drop of temperature or atmospheric variation in the first chamber. Generally, front loading and rear unloading is used, though there could be alternate system of charging and discharging. Once charged, the charge is surrounded by the controlled atmosphere to protect the jobs from oxidation or decarburisation, making the system suitable for precision hardening of machined steels or carburised parts. Provision for additional atmosphere (e.g. methane or ammonia) makes the furnace also suitable for carburising and carbo-nitriding. A critical requirement of seal quench furnace is thorough cleaning of parts from oil and grease before charging, because burning of oil, grease, etc., can cause serious imbalance in the control of atmosphere. Cleaning is generally carried out by alkaline solution followed by final washing by water for complete removal of grease and oil and drying. Drying before charging is necessary because even small amount of water will disturb the inside atmosphere on vaporisation. After charging, the load is held at the required temperature either for soaking for hardening or for development of case depth for carburising / nitriding. On completion of the first treatment, load gets automatically transferred to the quenching chamber or for cooling under controlled atmosphere. Mechanism wise, once the charge has reached the quenching / cooling chamber middle door closes and charge can be held for cooling under atmosphere or lowered down to the oil tank for quenching. As per heat treatment principle, charge remains in the oil for duration enough to ensure martensitic transformation. After quenching, charge can be unloaded by automatic un-loader and sent for washing again. Figure 5-7 shows the layout of quench / cooling chamber along with oil-cooling mechanism. However, attempts are now being made to make seal quench furnace with gas-quenching system and, thereby making it more flexible and economical in applications.

5.5.2

Vacuum Furnace

Vacuum heat treatment furnace is fast gaining popularity for excellent quality of output and high heat efficiency. In vacuum furnace, charge

Sealed quench furnace rear view

Water cooling traces Oil propeller drive motor Immersion heater elements External oil cooler

Insulated quench tank

Fig. 5-7 Rear view of a seal quench furnace showing the charge position and oil chamber

material is surrounded by vacuum, creating an atmosphere devoid of air or other gases that may slow down heat transfer through convection within the furnace chamber. Vacuum also removes sources of contamination (such as water vapour, oxygen, oil and other impurities) from the work pieces, making the work piece surface virgin and reactive. As such, vacuum furnace works with high efficiency of radiation and convection heat transfer. Energy efficiency of vacuum furnace is high and jobs come out very clean with virgin surface. Vacuum heat treatment is used because of its clean surface quality, free from any oxidation and decarburisation, uniform metallurgical properties, and ability to heat treat difficult and crack-prone high-alloy steels. A vacuum furnace is capable of heating material to high temperature quickly and enables processes like hardening, carburising, brazing, etc., to be carried out with high degree of efficiency. At high temperature, steels tend to rapidly oxidise, creating serious problem in heat transfer efficiency, uniformity of heating and surface quality. Vacuum removes the oxygen and prevent materials from oxidising at higher temperature, which is one of the primary reason for using vacuum furnace. Clean work piece surface helps in better heat removal and more uniform cooling during quenching. A shop floor model vacuum furnace is shown in Fig. 5-8.

Fig. 5-8 shop

Depicts the horizontal type vacuum furnace set up in an experimental

Vacuum furnace could be of horizontal and vertical type with heating capacity going up to 1500°C at the heating rate of 10 to 20°C per minute. Working furnace in the system is enclosed in a cylindrical chamber maintained at a low pressure. Required pressure within the working chamber is maintained with the vacuum pump attached to the furnace. Some advantages of vacuum furnace are: • Uniform temperature in the range of 1000 to 1500°C, controllable to +/– 1°C • Low contamination of product by carbon, oxygen or other harmful reacting gases • Quick cooling under vacuum, and • Amenable to precise control for high reproducibility of results Figure 5-9 illustrates the full set up of a vertical vacuum furnace with all auxiliary support and accessories. The figure denotes the functions of each part. The furnace displayed in Fig. 5-9 has a replaceable water cooled jacket and hearth with front door charging and top cooling, attachment of an elaborate diffusion pump system, vacuum valves, supply line for gases – which can be also used for cooling, pressure gauge for partial pressure, and thermo-couple for temperature control. For hardening, gas cooling of charges helps improving heat extraction by additional convectional heat transfer.

Fig. 5-9

A vertical type vacuum furnace with full auxiliary support

Vacuum furnace is eminently suitable for heat treatment like carburising and hardening of high-alloy steels (e.g. tools and die steels), but not limited to those applications only. The furnace configuration and material handling system can be integrated to heat treatment facility required for an operation. Vacuum heat treatment provides high productivity and excellent quality. In general, vacuum heat treatment can be used wherever it is necessary to: • prevent reactions on the surface of the job, such as oxidation or decarburisation during hardening or heat treatment; • remove contaminants from the surface of the machined or fabricated jobs, which arises from residual cutting oil or lubricant and interfere with the heat treatment or heat transfer process; and • add another element on the surface layer for diffusion and change in local surface composition, such as carbon for carburising or nitrogen for nitriding or carbo-nitriding. Thus, in the heat treatment of steel, vacuum furnace can be gainfully used for hardening of difficult steels, and carburising and nitriding of precision engineering parts. Vacuum hardened steels are known to give better fatigue resistance properties of steels. Figure 5-10 depicts a typical shop floor vacuum heat treatment furnace along with the vacuum system and control panel. Most elaborate part of vacuum furnace is its vacuum pump system and control. Operationally, the furnace is first evacuated with a vacuum pump to produce inside pressure of the working chamber which should be lower than atmospheric pressure or to any desirable vacuum level. The furnace

Fig. 5-10 floor

A general view of production vacuum furnace unit set-up on the shop

is not theoretically at perfect vacuum stage at any moment, but operates at pressure lower than the atmospheric pressure. Table 5.2 gives the pressure level generally used for heat treatment of special steels.

This level of vacuum is considered necessary for removing all oxygen, other gases and impurities from the furnace chamber. Impurities in the chamber emanate from the oil and water from the charge, despite cleaning and drying. The pressure drop (vacuum) helps creating a neutral atmosphere inside and such condition is good enough for conventional hardening and tempering. However, for special applications such as for carburising or ion nitriding, another gas of appropriate nature could be injected into the chamber, which aids the heat transfer by increased convection as well as provide atmosphere for the selected process. Charge materials are first placed into the furnace, then doors are closed and pumping-out is carried out. With pumping out of air from the chamber, the vessel becomes airtight. Under this condition, any surface contamination such as oxide films and residuals of oil and lubricants get removed, and jobs start heating up by more efficient radiation and convectional heat. Controlled atmosphere is then introduced to the chamber as per process requirements.

Heating in vacuum furnace being largely by radiation, existence of large thermal lag between the surface and the centre of jobs may not be ruled out, especially of larger size jobs, unless heating steps are regulated. Generally, stepwise heating is recommended; thermal soaking by holding for about 15 minutes at 650°C and 870°C and then stepping up to the soaking temperature. Soaking is similar to standard heat treatment process, e.g. soaking of 30 to 45 minutes for 30-mm cross-section. Source of heating of vacuum furnace is generally electricity, but new developments in areas of gas (natural gas) fired vacuum furnace are changing the scene. Gas vacuum furnace has several advantages over electric, such as lower operating cost with natural gas and faster cycle time. However, gas fired vacuum furnace has a temperature limitation to about 1050°C compared to electrical heating to 1350°C. Vacuum heat treatment is costlier compared to any other controlled atmosphere batch process. Hence, there must be a need for higher quality results to justify higher cost of adopting vacuum heat treatment. There are three basic modes of cooling in vacuum furnace after heat treating; they are: (1) vacuum cooling, (2) static cooling, and (3) forced cooling. Under vacuum cooling, the furnace is switched off and load is allowed to cool at the same rate as the furnace. Hence, it is a slow cooling process and fits to annealing or similar applications. Static cooling means filling the furnace with dry gas after the heating cycle, which provides convective heat transfer from heated jobs to water cooled shell of the furnace. This cooling is faster than the earlier one, but still might not be good enough for hardening. Hence, forced cooling is often resorted for faster cooling by backfilling the furnace with an inert gas (e.g. argon) to pressure less than atmospheric pressure (760 torr). A circulating fan moves the gas over the hot jobs and over the water-cooled coils of a heat exchanger, which helps in increased convective cooling and fast cooling. Cooling rate can be further increased or controlled by (a) increasing the flow of gas velocity and (b) increasing gas pressure. For example, for faster cooling (quenching), inert gas (e.g. argon) can be introduced at higher than the atmospheric pressure, generally at least two times, which significantly increases the cooling rate. Inert gas under high pressure gets circulated through hot zones to pick up heat before passing through the heat exchanger to remove heat, all by high-speed convection. The re-circulation process continues till desired temperature of cooling has reached. Vacuum furnace technology can be used for all heat treatment processes that are concerned with heat treatment of steels, but it is best used for hardening of high-alloy steels (e.g. tool and die steels, steel with complex

alloying, etc.), heat treatment of stainless steels, bright annealing of coldrolled steels, carburising of critical gears, and ion-nitriding.

5.5.3

Pit Type Furnace

It is a highly versatile furnace type that finds applications in host of areas of traditional heat treatment shop. Pit furnaces are widely used for processes like, gas carburising, nitriding, tempering, stress-relieving or bright annealing. Pit furnace is widely used in industry for its flexibility of operation and adoptability for differently shaped jobs. It is especially suitable for long and slender parts that have chances of distortion and wrapping, if heated flat. Typical load types in pit furnace could be: • Automotive industry: crankshafts, camshafts, pistons, pins, pumps & shafts, etc. • Machine building industry: shafts, sleeves, pins, tooth wheels, link chains, etc. • Mining industry: drilling rods, mono-blocks, drill bits • Bearing industry: bulky bearing elements • Marine industry: large power transmission components as: sun pinions, pinions, gear wheels, etc. The furnace consists of cylindrical retort of large diameter (e.g. size of 1000 mm × 1500 mm, and 1250 mm × 1800 mm) made of heat-resistant steel. The retort is hung in a pit of appropriate depth and diameter either below the ground level or in a raised platform. The pit is generally refractory lined over steel shell with groves for holding electrical heating elements. Though heating could be by electricity or gas fired, electrical heating is generally used, and heat transfer is by radiation, convection and conduction. The heating of the charge inside is indirect; the pot is heated to the temperature and heating to the charge takes place by radiation and convection of heat from the pot. Hence, heating efficiency is relatively low compared to seal quench furnace. Still, it is popular in industries because of its versatility and flexibility. Figure 5-11 shows the general view of pit furnace along with the retort head, lifting mechanism, and its atmosphere and temperature control panel (distance corner). The furnace retort could be with close bottom, open bottom or on fixed hearth, but close bottom retort pot is the popular and regular one. Jobs to be heat treated are hung vertically down inside the retort in a suitable fixture. Fixture design must fulfil two things: jobs must not touch each other, and intermediate space between jobs and the fixture stands should not obstruct the gas flow. The latter is necessary not only for uniform surface reaction but also for high convectional heating (or for cooling during cooling cycle) during heat treatment cycle. Also, jobs

Fig. 5-11 A pit type furnace in the shop floor – with retort head (upper) that can swivel and seat on the furnace top at the bottom for sealing the furnace system [The picture also shows the mechanism (special purpose EOT crane) for lifting and swivel of the retort head]

should seat or hung from the fixture in a way that does not lead to distortion while the jobs are hot. The furnace has a swivel head, which fits firmly over the retort top in the assigned groove with sealing and clamping facility. The charge material can be loaded into the pit furnace with the help of EOT crane or suitable loading mechanism. After lowering the jobs inside the cylindrical pot, top is sealed with the swivel retort head and firmly clamped either manually or hydraulically. For discharging the load after heat treatment, the swivel head is released and the workload is taken out with the help of same EOT crane. The furnace is fitted with inlet tubes for flow of atmospheric liquid (drip feed) or gas with flow control valves for atmosphere generation and control. After charging the retort inside the pit, the furnace is gradually brought to the operating temperature. For carburising, atmosphere or drip feed liquid is fed to the sealed retort only after reaching a temperature of about 700 to 800°C in order to prevent carbon soot formation inside, due to incomplete burning of gases or liquid. If jobs get coated with carbon soot early, then that layer acts as barrier for subsequent carbon diffusion on to the surface. Pit furnace is generally electrically heated where resistance coil (wire) or strips are located on the side refractory wall. Heating elements are suitably divided into number of heating zones and each zone temperature is suitably set and controlled through thermocouple or thyristor based control mechanism. The furnace is also provided with recirculation fans for temperature equalisation and atmosphere circulation. Temperature

uniformity of +/– 10°C can be achieved in pit furnace. The furnace can be designed with complete control and instrumentation system through PLC based or hardware based logic controls. The pit furnace is available now-a-days in fully instrumented condition with carbon potential and atmosphere control based on oxygen probe, and highly efficient atmosphere circulation system for temperature uniformity of +/– 5°C in the working space. In general, a typical pit furnace has a capacity range between 300 kg and 3000 kg gross load per charge, with heating power of 80 kW to 260 kW and operating temperature of 750 to 950°C. The furnaces can be firmly sealed on the top for gas leakage or air ingress, allowing good temperature uniformity. This type of furnace can be easily fitted with either liquid flow line or gas flow line for supplying carbon or nitrogen-rich atmosphere inside from an external source of atmosphere generation and control system. Examples of these are controlled flow of carbo-fluids, endothermic gas or nitrogen-methanol atmosphere for surface hardening of steel. After the heating and treating cycle is over, the charge is cooled with inert gas for a while and then taken out for cooling / quenching as appropriate for the process. For hardening (if used), a fixture with the charge has to be taken out quickly from the retort and quenched in oil tank of high oil volume capacity for preventing excessive oil temperature rise and fire hazard. For carburising, jobs are preferably allowed to cool inside under mild flow of inert gas for protection from surface oxidation and decarburisation. For nitriding, retort can be cooled by forced air circulation to lower temperature and taken out for normal cooling. Advantages of pit type furnaces are: • Flexibility in operation and schedule of production • Easy service and maintenance • Economical on capital cost and operation, requiring small amount of auxiliary facilities • High repeatability of processes • Can be run with automatic process control system, thus improving control over the process parameters • Suitable for long and large jobs, which many other processes cannot handle easily. Hence, most commercial heat treatment shops have some type of pit furnace in use for hardening (e.g. automotive axle shaft), case-carburising (e.g. for gears and pinions) and even nitriding (e.g. automobile crank shaft) or stress relieving of long heat-treated shafts. Perhaps, pit furnace is the only answer for hardening of long and slender jobs without much distortion. Pit furnace is economical and flexible in running compared

to seal quench furnace, but quality wise, especially for surface quality of heat treated jobs, it might not be as good as seal quenched jobs. Another disadvantage is its charging and discharging system, which is generally done manually by overhead crane of suitable capacity. Maintenance of pit furnace and its auxiliary equipment also require frequent monitoring and attention compared to seal quench furnace.

5.5.4

Salt Bath Furnace

Seal quench is a modern method of heat treatment, but salt bath is an age-old process, which is considered very flexible for different types of jobs and production schedule. Salt bath furnace is a widely used heat treatment method for hardening and light carburising of small and medium size components. The process is not a sophisticated one like seal quench furnace, but very useful for its flexibility. Only objection about salt bath furnace is its environmental problem; its objectionable fumes and solid salt residues and wastes, which need to be controlled and regularly disposed of. Salt bath furnace comprises a pot type furnace, wherein mixture of salt with a low melting point is placed and heated by using a set of electrode for melting the salt. This produces a molten bath of salt, which can be neutral for normal hardening or made of cyanide containing salt for supply of atomic carbon for carburising as per following reactions: 2NaCN + O2 Æ 2NaCNO 2NaCNO + O2 Æ Na2CO3 + CO + N2 2CO Æ CO2 + C Salt bath furnace uses 20 to 50% sodium cyanide mixed with as much as 40% sodium carbonate salt for carburising, and temperature of operation is in the range of 870 to 950°C. Carburising takes place by decomposition of sodium cyanide on the surface of the steel as per abovementioned reactions. Interestingly, nitrogen is also released along with carbon atoms, resulting in some degree of nitriding (which is very limited due to shorter time) along with carburising. This process produces a thin and hard shell (between 0.25 and 0.75 mm thick) that is harder than the one produced by gas carburizing. The process can be completed in 20 to 30 minutes compared to several hours taken in gas carburizing process. As such, this process produces less distortion compared to other high temperature carburizing process. It is typically used on small parts such as bolts, nuts, screws and small gears. The major drawback of cyaniding is that cyanide salts are poisonous. Figure 5-12 depicts one typical salt bath furnace arrangement.

Fig. 5-12 An illustration showing a salt bath furnace in operation

The furnace consists of an insulated pot of appropriate capacity, electrodes for heating – which could be immersion type or sub-merged type, thermostat for temperature control, and stirrer for improved convection and salt flow. Being a fluid salt medium, heat content of the bath is high, which heats up the jobs very efficiently and uniformly. It is a pot type furnace by construction where jobs are hung, in batches, dipped into the molten salt, which has fast and uniform heating capacity. A mixture of salt (e.g. sodium and barium chloride) can be used with adjustable melting point as per operating range. Table 5-3 shows various possible mixtures of salts for different operating temperatures. The bath should be adequately fluid at the operating temperature for good heat transfer. By changing the salt mixture, bath can be made suitable for even higher temperature of operation up to 1250°C. Salt bath is a batch type furnace and can operate round the clock. Electricity is the primary source of heating, however, gas fired salt bath is also available. Jobs of similar shape are charged as a lot; soaked and quenched in water, oil or martempering bath for hardening, as per hardenability of the steel. Heating is mostly by convection of molten salt, which can be aided by a stirrer when required. By changing the salt composition, bath can be made suitable for neutral hardening, carburising, nitriding, marquenching or austempering. Table 5-3 gives the salt mixture ratio vis-a-vis melting and operating temperature of salt bath operation. Being highly corrosive, two precautions are necessary for salt bath operation; first, the pot material must be of heat and corrosion resistant nickel-chrome containing steel or of ceramic construction, and the other is washing requirement of jobs after heat treating in salt bath. Through washing in flowing water is necessary, because of salt adherence to jobs. As such, the process is not so environment friendly. Effluent treatment

should be a part of disposal system for salt bath waste. Other problem of salt bath furnace is its fumes. Fumes generated due to heat and salt reactions must be extracted by a hood exhaust system and discharged away from the workplace. To control heat loss and fumes from oxidised salt from the top, added precaution should be taken by covering the bath by activated charcoal powder. Advantages of salt bath are: (a) faster and uniform heating, (b) easily adjustable operating bath temperature and composition by altering the salt mixture, (c) no surface oxidation or decarburisation, and (d) flexible cooling in water, oil or martempering bath. However, discharge from the cooling bath might require treatment to meet environmental regulation. By choosing the salt mixture appropriately, salt bath can be used for a range of heat treatments; starting from low temperature tempering and martempering to high temperature hardening, and carburising. It does not require any atmosphere generator, but needs adjustment of salt

composition only. Salt bath furnace is, thus, a very flexible furnace and finds applications in many heat treatment shops. It is not as productive as a seal quench furnace, but flexible and an essential furnace for all jobbing heat treatment shop. Salt bath is also widely used for high hardenability steel hardening, such as tool and die steels, because of its uniform heating and flexibility of quenching in a martempering bath.

5.5.5

Fluidised Bed Heat Treatment Furnace

Fluidised bed furnace is an emerging technique for heat treatment of steels. But, it is fast gaining ground since 1980s, due to its high heat transfer rate (even better than vacuum furnace), temperature uniformity, and clean surface quality of treated jobs. Fluidised bed furnace is economic in operation, nearly free from pollution with low maintenance. Furnace operation is relatively easy and safe – requiring only setting of desired temperature and gas flow for fluidising the bed. Once the bed is fluidised, charges on a fixture can be immersed into the fluidised bath from the top with the help of an overhead crane. In this regard, fluidised bath behaves like a salt bath once heated and fluidised. The bed is made up of dry sand-like alumina particles of fine size, and the top of the furnace can be closed by a swivel head. Figure 5-13 shows the setup of a fluidised bed in operation. The figure illustrates step-by-step fluidisation of the bath.

Fig. 5-13 A schematic fluidised bed furnace set up showing the stages in the process of fluidisation

Structurally, fluidised bed furnace employs a heat-resisting steel retort filled with alumina particles, called fluidising sand. Sand is placed over a porous plate fitted towards the bottom of the retort, which acts as distributor of gas or air. When a controlled stream of gas (or air) is

passed under pressure upward through the distributor at the bottom of the retort, the sand particles float, making the bed as if boiling. This state of the furnace is called fluidisation of the bath. The furnace retort containing the sand can be heated externally or internally; external heating is done generally through gas firing and internal heating is by injecting pre-heated exothermic gas from the bottom. Sands, or sandy refractory particles, quickly absorb heat from the gas and transfer the heat immediately on to the job / charged component by convection and conduction. Since the bath takes the physical form of boiling bath, top of the retort needs to be closed by a swivel head for stopping sands to flash out. For this purpose, retort cover is fitted with a lift and swing facility as well as exhaust gas outlet duct. The duct should preferably be provided with a burner for burning off the exhaust gases to prevent pollution. The furnace is provided with an electrical control panel for regulating external heating of the retort, a gas manifold for controlling gas flow to the distributor, and a gas mixing panel for atmosphere control. Because of the positive pressure on the top of the fluidised bath, there is little chance of air ingress and oxidation of the charge material at higher temperature. Also, due to constant friction with abrasive alumina / sand particles, job surface is very clean. Heat transfer is through intimate contact of fine alumina / sand particles with the jobs, involving convection and conduction mechanism. Fluidised bed furnace can be used for heat treating up to 1200°C. Hence, it can be used for heat treating die and tool steel also. Advantages of fluidised bed furnace are as follows: • Higher heat transfer rate and shorter soaking time compared to most batch furnaces, including vacuum furnace • Ease of operation and flexibility of maintaining gas environment inside the fluidised bath • Same gas or gas mixture can be used for fluidisation and simultaneously act for providing required atmosphere for heat treatment • Rapid and uniform heating up rate due to high velocity of contact between jobs and fluidised sand particles – finer the particle size better is the heat transfer due to higher surface contact • Low maintenance, because of few moving parts (like fans, conveyors, etc.) in the furnace set up • High temperature uniformity (typically +/– 3°C at 1050°C) • Nearly pollution free Figure 5-14 shows comparative heating efficiency of fluidised bed furnace and vacuum furnace.

Fluidised bed furnace can be used for thorough hardening, carburising, carbo-nitriding, normalising, annealing or stress-relieving. In fact, it may be used as a substitute for salt bath furnace, which is considered neither clean nor environment friendly. It is a highly flexible furnace for temperature up to 1200°C and any atmosphere can be created by feeding as fluidised gas. Hence, Fig. 5-14 Relative heating efficiency many consider this furnace as one- of fluidised bed and vacuum furnace stop heat treatment furnace with quick start up and shut down possibility, flexible atmosphere exchange, high temperature uniformity, clean work piece surface and no toxic effluents. However, the process is yet to reach a popular mark in most heat treatment shops world-wide. Considering the emerging importance of this heat treatment technique, especially for high-alloy and tool steels, further reference can be made to Reynoldson, 1993, for more details and operating advantages and disadvantages.

5.6

ATMOSPHERE CONTROL IN HEAT TREATMENT

Steel always reacts with the oxygen in the air and forms surface oxide film, layer or scale, depending on the temperature and time of reaction. Higher is the time and temperature of reaction thicker and faster is the oxide formation, causing deterioration of surface quality, which is of primary concern in heat treatment of steel for the following reasons: (a) Surface oxide film interferes with the heat transfer efficiency from furnace to the job; thus bringing down the heating efficiency of the process. (b) Oxygen also reacts with the surface carbon of the steel and leads to decarburised layer, which is softer than the acceptable level of hardness or strength of the steel parts. Decarburised surface layer (vide Fig. 5-15) is a source of weakness for load bearing applications as well as for metal working applications. Decarburisation could be partial or full, but it is harmful for all applications of hardened steel. Decarburised surfaceacts as weakness in the structure and leads to failure by wear and fatigue fracture initiating from such weak points. This problem is even higher with case carburised steel parts, where surface structure fails to transform to martensite on

Fig. 5-15 A magnified view of fully decarburised layer in medium carbon file steel after hardening (x100)

hardening due to depleted carbon level. As such, furnace atmosphere control is a critical necessity for: • Providing protective environment at higher temperatures against scaling, oxidation and decarburisation (results of oxidising reaction of carbon) • Improving heat transfer to charges within the furnace • Providing source for transfer of another element for surface enrichment, such as in carburising and nitriding, as a part of heat treatment process, and • Improving surface quality as regards cleanliness and making the surface free from pits and scales. While atmosphere for carburising, nitriding, etc., will be discussed in the relevant chapter, protective atmosphere for protecting the steel surface from high-temperature external reactions, such as scaling and oxidation – including decarburisation – will be discussed in this chapter under atmosphere control. Furnace atmosphere and its control play an important role in meeting the quality requirements for heat-treated steel parts for engineering applications, which are often subjected to dynamic loading. Amongst

the common heat treatment processes, atmosphere control is critical for success of (a) hardening operation, in order to avoid decarburisation and oxidation of surface, (b) carburising for maintaining a carbon-rich atmosphere for case depth control, (c) nitriding for supplying nascent nitrogen for diffusion and compound layer control, and (d) any other variant of heat treatment processes, like vacuum hardening, carbo-nitriding, ion nitriding, etc. Table 5-4 gives some examples of common heat treating processes and atmosphere control necessary for those processes.

For hardening of steels, furnace atmosphere can be classified into two types: (a) neutral or inert, and (b) reactive type atmosphere. Most hardening operations can be carried out in neutral atmosphere, such as by using high-purity nitrogen, which is a non-reactive gas. But, for moisture control and keeping the working atmosphere free of oxygen, nitrogen can be used in conjunction with small amount (5 to 10%) of natural gas or by methanol cracking (hydro-carbon) for ‘dew point’ control. Dew point is the temperature at which water vapour in the air (or atmosphere in the furnace) condenses. Oxygen in the furnace readily reacts with hydrogen of the hydro-carbon gas (natural gas) and forms water vapour. Dew point measurement and control is a means to monitor the level of oxygen in the furnace. Oxygen in hardening furnace could be present due to air leakage, moist and oily charges, and such other sources.

In principle, hardening of steel parts can be performed in an atmosphere that is neutral (or inert) to the basic carbon of the steel and is free from oxygen. Such a condition can be achieved by injecting nitrogen with small amount of hydrocarbon (5 to 10%) into the furnace. The level of hydrocarbon mix required for such controlled atmosphere is limited to control carburisation or decarburisation of steel surfaces. However, while hardening previously carburised parts, the atmosphere should be controlled at the same carbon potential level as used for carburising. Atmosphere control is critical for freedom from decarburisation and oxidation of steel surface during annealing (which takes long heat treatment cycle time), and hardening. For surface hardening by carburising and nitriding, role of atmosphere control is to provide atmosphere of higher carbon (or nitrogen) potential than the steel surface by appropriate gas generation and injection into the furnace. This is termed as ‘reactive atmosphere control’. Reactive atmosphere control is used for changing the surface chemistry of the steel, such as by carburising and nitriding. Reactive atmosphere can be generated outside the furnace and introduced through a flow control mechanism. Examples of commonly used reactive atmospheres are: (1) Endo-gas: Generated by using mixture of natural gas, propane and air passed through a hot nickel catalyst at about 1200°C. The name is derived from the fact that overall reactions is endothermic (which absorbs heat). Endo-gas quality may be a bit inconsistent due to propane quality. (2) Nitrogen plus methanol mixture: Produced by dissociating methanol and mixing with high purity (PSA grade) nitrogen. Methanol cracking process is critical, and must be designed for complete dissociation and uniform distribution. Many shops attempt to crack methanol by positioning the liquid methanol drip directly behind the circulating fan inside the furnace so that atomisation of methanol occurs readily. Atmosphere generated with nitrogen and methanol need not be of endothermic composition. As such, there proportion requires suitable adjustment based on carbon potential, i.e. change in carbon monoxide content. The carbon monoxide content of an atmosphere may vary considerably during a heat treatment cycle when using nitrogen / methanol mixtures. Oxygen probe monitoring gives better picture of carbon potential fluctuation and allows closer control of atmosphere inside the furnace. One of the disturbances of atmosphere uniformity inside the furnace comes from the door opening for loading or unloading. Hence, whatever be the atmosphere uses, inert or active, the furnace must have ‘door curtain’ flame, protecting ingress of air and moisture.

Atmosphere control process can be static flow control or dynamic control system that senses disturbances and shortfall of controlling gases inside the working zones and automatically adjusts the gas flow into the furnace according to set points. Atmosphere disturbances inside the furnace occur due to leaks during furnace loading and unloading, leakage in the furnace, moisture with the new charge (load), adhering oils and greases, etc. Dynamic system can meet the fluctuations and provide required atmosphere quantity during the span of heat treatment, vide Fig. 5-16.

Fig. 5-16 Graphical presentation showing (a) how constant flow gas control system can cause disturbances and variation of atmosphere with time inside the furnace, and (b) how dynamic and active flow control system can intervene and bring the atmosphere to more uniform level inside the furnace

Dynamic flow control system of atmosphere not only improves quality of control and the heat treated products, but also saves gas by cutting down unnecessary flow; vide blue portion of Fig. 5.16 (b) which illustrates potential gas saving scope. Steps necessary for dynamic flow control are: good system for (a) purging procedure; (b) control of furnace atmosphere composition by quality and volume; (c) process alarm for failure or disturbances; and (d) provision for reliable analysing instruments and their maintenance. The system can be further backed by data logging and remote supervision. Thus, more and more dynamic control system of atmosphere control inside the furnace is being adopted in heat treatment industries with the benefits of: gas saving, improved productivity and process capability, safety in handling inflammable gases, savings in down-stream operations like surface cleaning and shot-blasting time, and better and uniform heat treated properties. In conclusion, it can be observed that whatever be the furnace type, it must have provision for protecting the jobs from getting oxidised, and if the process calls for diffusion of some elements on to the surface, then required reactive atmosphere for the process needs to be generated and provided. Thus, heat treatment is not only a process of controlled heating and cooling, but it is also about protecting the surface from undesirable reactions at the higher temperatures and providing necessary reactive atmosphere, when necessary, to facilitate thermo-chemical processes. Furnaces like salt bath and fluidised bath furnaces are unique with regard to protecting jobs or to providing reactive atmosphere without external injection of atmosphere. These tasks are accomplished in these furnaces by change of salt or fluidising media, which act as sources of extra elements. Vacuum furnace is another example where state of vacuum itself removes the oxygen and prevents materials from oxidising at higher temperature. Other hardening furnaces, both batch type and continuous type, need to have atmosphere control for quality heat treatment. In these furnaces either neutral or reactive atmosphere are injected for atmosphere control. For example, seal quench furnace should have neutral or inert atmosphere for feeding into the furnace for protecting from oxidation during hardening and providing a suitable reactive atmosphere for carburising. Decision on generating a protective atmosphere is one part of the task; the other part is working of the atmosphere inside the furnace and its control. This can be done by fixed flow control or dynamic flow control system, as mentioned earlier. Although, fixed flow control is still prevalent in industrial practices, dynamic flow control system is fast emerging in the market. The chapter has briefly discussed these points with reference

to heat treating practices, operational features of some popular furnaces, and means and methods of atmosphere generation and control.

Summary 1. Equipments are as important in heat-treatment as the processes. Amongst the equipment, furnaces – along with its atmosphere control system – play the most crucial role in success of heat-treatment operations. Characteristics of furnace types depend on its operational characteristics, such as batch or continuous type, methods of heating, mode of heat transfer, mechanisms of operation (e.g. rotary hearth, walking beam, mesh-belt, seal quench, vacuum, salt-bath, etc.), load factor and loading pattern, quality requirements, environmental factors, and safety. 2. Technologically, furnaces can be grouped as per the operational system, e.g. hardening furnace, tempering furnace, carburising furnace, etc. or mechanisms of material movement inside the furnace. For example, rotary hearth furnace where the furnace bed rotates at a pre-fixed speed through different temperature zones for heating and then jobs can move to a separate chamber or tank for hardening. Pusher type furnace indicates that jobs are pushed from one zone of the furnace hearth to other for heating and finally for hardening or cooling. Furnaces can also be described as per its operating principles, like vacuum furnace, fluidised bed furnace, salt-bath furnace, etc. However, whatever could be the furnace types, their methods of heat transfer depend on the efficiency of radiation, convection and conduction, depending on the temperature of operation, design of the furnace and atmosphere inside for protection from oxidation. 3. Role of furnace atmosphere can be: (a) protecting from oxidation and decarburisation of steel surface, (b) providing required reactive environment for surface engineering, such as carburising and nitriding, by maintaining higher potential of a chosen medium, and (c) improving quality and cleanliness of parts being heat-treated. During discussions in the chapter, these roles of furnace atmosphere have been elaborated either while discussing different furnace characteristics or separately. 4. Types of furnaces, their general characteristics and operational features have been mentioned, followed by detail discussions on few representative furnaces that find popular applications in industries. These furnace types are: Seal quench furnace, pit-type furnace, salt-bath furnace, vacuum furnace and fluidised bed furnace. Factors like flexibility, economy in operation, heat transfer efficiency and energy efficiency in the choice of heat-treatment furnace have been highlighted. In view of this, merits of seal quench furnace and fluidised bed furnace have been mentioned. Due to emergence of more and more flexible machining centres and batch type production with high degree of efficiency and flexibility, seal quench furnace seems to be getting more popular; replacing many continuous heat-treatment operations. 5. Finally, basics of atmosphere control in heat-treatment furnaces have been discussed and merits of dynamic flow control vis-a-vis static flow control of atmosphere have been discussed and highlighted.

References / Suggested Reading Anderson, Rolf, Atmosphere Control Translate to High Quality Heat-Treated Automotive Components, and others, Linde Gas, Lidingo, Sweden. (Ref: https://www. ferronova.com/International/Web/LG/FER/likelgfer.nsf/repositorybyalias/ lit_atmocontrol/$file/Close%20atmosphere%20control%20gives%20high%20 quality%20automotive%20parts.pdf. As on 09-04-2013) ASM Handbook, Vol. 4, Heat Treating, ASM International, 2004, USA Heat Treatment in Fluidized Bed Furnaces, Ray W. Reynoldson, ASM International, USA, 1993 Kern, Donald Q., Process Heat Transfer, Tata McGraw-Hill Education, New Delhi, 1997 Prabhudev, K.H., Handbook of Heat-Treatment of Steels, Tata McGraw-Hill Education, New Delhi, 1988 Purushothaman, Radhakrishnan, Evaluation and Improvement of Heat Treat Furnace Model, Ph.D. Thesis, 2008, Worcester Polytechnic Institute, UK (Ref:http://www.wpi.edu/Pubs/ETD/Available/etd-082208-114851/unrestricted/Purushothaman.pdf. As on 25-09-2013) Smith, John W. Holcroft, Types of Heat-treating Furnaces, A Division of Thermo Process Systems Inc., (Vide: ASM International Heat Treating Handbook, Vol.4, ASM, USA), 2004

Review Questions 1. State the role of furnace in heat treating operations. What are the factors that are considered for choice of furnace types in a shop? How heat treatment furnaces are classified? 2. Briefly describe the general characteristics of a heat treatment furnace. Also highlight the different roles of atmosphere control during heat treatment. 3. What are the mechanisms of heat transfer in furnaces? Briefly discuss the laws of heat transfer in heating furnaces and point out their efficiency and merits at different stages of heating. 4. Referring to Fig. 5-2, critically discuss the sources of heat losses in a heating furnace. How different heat losses can be minimised in the furnace during heat treatment? 5. Discuss the factors that help in choosing the right type of furnace for a specific heat treatment. What should be your choice of heat treating furnace for: (a) Normalising of a steel shaft (b) Hardening of a steel gear, and (c) Case hardening of carburised parts requiring strict control of distortion. 6. Under what circumstances you would recommend using of: (a) Slat bath hardening furnace (b) Seal quench furnace, and (c) Vacuum hardening furnace 7. Discuss the operations of: (a) vacuum carburising furnace, and (b) fluidised bed hardening furnace. What are the advantages of fluidised bed furnace over the conventional hardening furnace?

8. Why atmosphere control is necessary for heat treatment of steels? Discuss the process of atmosphere control for hardening operations of a fatigue sensitive engineering component like automotive gears. 9. Briefly discuss the atmosphere types that are used for: (a) Annealing of stainless steel (b) Carburising of automotive gears (c) Nitriding of steel parts, and (d) Hardening of carburised gears 10. Discuss the methods of flow control of gases during atmosphere controlled heat treatment operation. Point out the steps necessary for effective flow control of gases in a furnace.

Quenching Technology and Characteristics of Different Quenchants

6.1

INTRODUCTION

Quenching is a mechanism of rapid heat extraction from the hot body of material. It is carried out with the help of fluid media of high quench severity (H-value) e.g. water, oil, forced air, etc, and the mode of heat extraction is mainly by conduction and convection. Quenching, as involved in the heat treatment of steels, means rapid cooling from the austenitisation temperature (typically above A3 temperature of the steel) to room temperature for production of hard and strong martensitic structure. Quenching can be also arrested in the mid-way or at a predetermined temperature, if necessary, for facilitating certain type of phase transformation and structure formation (e.g. lower bainitic structure) or for controlling the quenched-in stresses in the steel. But, in general, quenching of steel implies rapid cooling to room temperature for development of martensitic structure in the steel.

Metallurgically, the purpose of quenching is to suppress the transformation of the steel (i.e. austenite) to softer phases like ferrite and carbide and ensure that all austenite transforms to martensite. There could be exception to this rule – such as transformation to lower bainite by ‘austempering’ process – but, in general, quenching denotes fast cooling to form martensitic structure in the steel. In reality, however, 100% transformation to martensite over a cross-section of steel (especially if it is larger than 25 mm) might not be possible for different practical reasons, as discussed in Chapter 4. What structure will form inside the section of steel depends on the quench severity and the section thickness – leading to variation of structure inside. Hence, in practice, purpose of quenching of steel is referred to as achieving the required properties in steel (such as: hardness and strength) through changes in microstructures, but without promoting distortion or cracking. Thus, quenching of steel, as per the above definition, has to fulfill the task of (a) cooling the steel fast enough to form the desirable structure, (b) the cooling should be as uniform as possible in practice for ensuring more uniform structure over a ruling section, and (c) must not cause harmful distortion or cracking due to excessive rate of cooling (i.e. quench severity). These conditions necessitate that cooling rate in quenching should have to be commensurate with the hardenability of the steel and should correspond to ‘critical cooling rate’ (i.e. the cooling rate that ensures martensite formation) as per the CCT-diagram of the steel. However, it should not exceed the required critical rate, which might produce distortion and cracking. Controlling and regulating quenching rate to fulfill such conditions is the trick to successful hardening. Since quenching is involved with high rate of cooling (e.g. water or oil quenching) the steel under quenching from high temperature austenite state undergoes the following two phenomena: • Thermal contraction of the steel; which is again not uniform throughout the section due to difference of temperature inside the steel body, arising from lag in thermal conductivity in the steel, and • Simultaneous volume expansion that occurs due to new phase formation, especially the martensite, which has more specific volume than the parent austenite. Interactions of these two physical phenomena make the process of quenching rather complex; often leaving some harmful residual tensile stresses in the steel, if the process is not properly controlled. Amount of residual stresses can be so high at times, especially if the part is thin or having intricate shape, that it can induce cracking in the hardened job. Hence, the other aim of quenching technology has to be minimising the development of harmful residual stresses in the component.

Tensile residual stress, if left over in the heat-treated component, will join and increase the applied tensile load in service and affect the service performance. More severe the quenching more likely the steel will be affected by residual stresses, because of increasing thermal lag between the surface and the centre of the job. Though tensile residual stress can be finally neutralized during tempering after quenching, its development during quenching is considered undesirable and harmful – contributing to the development of distortion. Hence, quenching process should be enabled to extract heat at a more uniform rate from the surface so that temperature gradient between surface and centre is minimised. Therefore, selection of quenchant and its application in practice are important parts of steel hardening. Generally, selection of quenching medium is based on its ‘quenching severity’ value (H-value) matching with hardenability of the steel, maximum and minimum section thickness, and the shape of the job. Any stress raiser in the steel parts – like sharp radius, steps, threads, etc. – which can lead to high stress concentration also needs to be considered while planning the quenchant, because their presence can aggravate the problem of cracking during hardening, if cooling rate is too high. Therefore, quenching media and their cooling characteristics must be carefully assessed and steps to be taken to reduce or mask the stress-raising factors or points during hardening. In the ultimate test, quenching medium should be able to produce the desired structure and properties without unacceptable distortion / cracking and harmful tensile residual stress in the component. There are many options for quenching medium that can be used, each having its own characteristic H-value (vide Table 4-1, Chapter 4). However, quench severity (H-value) is not the sole criterion for selecting the quenchant; the stages of cooling performed by the quenchant through different temperature ranges the steel cools from are also important. Different media have different characteristic of cooling through the temperature range (the steel cools from) which make them different. Quenching media can be grouped into: liquid or gas. Water and oil are the examples of liquid quenchant, and argon and nitrogen cooling are examples of gaseous media used in industries. However, there are other special quenching media that can be chosen for quenching rate in between water and air, namely, polymer quench. Polymer has special characteristics of ‘inverse solubility’, by which the polymer is completely soluble in water at room temperature, but insoluble at elevated temperature. This inverse solubility occurs at around 75°C. This phenomenon provides the mechanism of cooling the hot steel by surrounding it with a polymer-rich coating that controls the rate of heat extraction in the martensitic transformation range where the steel is vulnerable to cracking

or distortion. But, once the job has been cooled below the inverse point (i.e. 75°C), the polymer goes back to water and job becomes free of polymer. Quenching effectiveness is, thus, dependent on number of factors – the steel composition and hardenability, type of quenchant and its cooling characteristics,operating condition of the quench shop, design constraint of the component, and required quality of the treated products. Other factors that influence the quenching results are the design of the quenching system (e.g. volume and shape of the quench tank, agitation, circulation, etc.) and the thoroughness with which the system is maintained. The chapter proposes to discuss these technical and application-related issues of quenching to facilitate better and consistent heat treatment results.

6.2

FUNDAMENTALS OF QUENCHING

Heat extraction during quenching is not uniform throughout the temperature range of cooling the steel body. It varies from one range of temperature to another – due to factors like vapour blanket formation, change in convectional heat extraction, role of conduction cooling, etc. The way the heat is extracted from a hot body is, therefore, controlled by different heat extraction mechanisms – which are fundamental to the quenching process. Fundamentals of quenching process can be best illustrated with respect to oil quenching process. During oil quenching, when a steel piece is quenched from high temperature, the body cools in three different stages. These three stages of quenching are illustrated in Fig. 6-1. This temperature-time diagram of cooling also indicates the cooling rate of respective stages – as given on the top axis. Immediately on quenching, the job gets enveloped with a ‘vapour blanket’, which is formed due to intense heat that vaporises the contacting fluid with the hot steel body. This vapour blanket insulates the job surface and slows down the cooling rate; vide the ‘bold line’ representing the cooling rate. This is characteristic (an attribute) of stage-I cooling. Next, when the vapour blanket gets broken, the contacting fluid nucleates numerous bubbles and when these bubbles rise or break down, it allows the fluid to come in direct contact with job surface, thus increasing the rate of heat removal by conduction. This stage is called ‘bubble boiling’ (or vapour transportation stage) as it creates condition as if the bath is boiling. Finally, when the temperature drops to a level lower than the boiling point of the fluid, cooling takes place by convection and heat

Fig. 6-1 A graphical depiction of three stages of cooling, namely (I) vapour blanket stage, (II) boiling stage, and (III) convection stage. [The curve with spike and nose is the‘ cooling rate curve’ of the experimental specimen during cooling. (Source: Houghton on quenching, Houghton International, USA]

removal becomes slow. Based on these cooling stages, mechanisms of heat extraction by a liquid fluid medium are discussed here.

6.2.1

Mechanisms of Heat Removal

Mechanisms of heat removal take place in three different stages – as illustrated in Fig. 6-1. The mechanisms of cooling corresponding to these stages are: • First stage is the vapour blanket (also termed as ‘film boiling’) stage, which refers to formation of unbroken vapour blanket surrounding and insulating the job from direct contact of the cooling fluid. It is said to form when the supply of heat from the steel body surface exceeds the amount of heat that fluid can carry away. Hence, fluid gets vaporised in contact with the hot steel body, forming vapour blanket around the job surface. This vapour blanket causes slowdown

of cooling rate at this stage. Heat removal at this stage, is the function of conduction through the vapour envelope formed. Therefore, sooner the vapour blanket is broken faster will be the cooling rate. • Second stage of cooling is known as bubble boiling stage or ‘vapour transport’ stage. This stage sets on when surface temperature of the job has cooled enough for vapour envelope formed in Stage 1 to start collapsing by bubble formation and collapsing on the rise. With the onset of this stage of cooling, bath appears as if boiling, accompanied by rapid rate of cooling, largely due to heat of vaporisation. This portion of the cooling cycle produces highest rate of heat transfer, i.e. rate of cooling. For effective quenching, higher the cooling rate at this stage better it is. • Third stage of cooling is called the ‘convection cooling’ stage. This stage begins when the temperature of steel surface drops to the level of boiling point of the liquid (quenchant). At this lower temperature, boiling stops and slower cooling by convection and conduction takes over. Factors that regulate cooling at this stage are: (a) viscosity of the fluid, and (b) difference in temperature between the boiling point of the fluid and the bath temperature. Ideally, rate of cooling at this stage should be slow and start as soon as cooling curve enters the martensite formation temperature or below. Oil has a distinct advantage at this stage of cooling over water due to higher viscosity and higher boiling point than water. Due to higher boiling point of oil, stage III cooling can start early and due to higher viscosity, conduction and convectional heat transfer can be better controlled. Analysis of these stages of cooling and mechanism of heat extraction indicates: • Need for uniform and controlled agitation of fluid over the job surface to facilitate mechanical disruption of vapour blanket stage; thereby cutting down the vapour blanket stage duration and improving the quenching. • Preference for smaller and easily detachable vapour bubbles during vapour transport stage of cooling (Stage II) for improved heat transfer, and • Selection of quenching medium having higher boiling point commensurate with the martensite start temperature (MS) temperature of the steel and capacity to wet the steel surface for uniform heat removal. Uniform agitation of the fluid bath is of critical importance in quenching so that cooler liquid can be constantly supplied to the job surface, resulting

in higher and uniform rate of heat extraction. The stages of cooling may not occur at all points of the job at the same time; because if there is section changes involved in the job, internal heat movement from core to the surface will vary, leading to localised difference in temperature and differences in the stages of cooling. However, uniform agitation will improve the situation. Agitation provides constant supply of cooler liquid to the surface, allowing improved heat extraction. Referring to Fig. 6-1, Stage I cooling starts as soon as the quenching starts and ends up at around 700°C; then Stage II takes over when steel cools very fast till the temperature of about 300 to 325°C is reached; finally, Stage III sets in with slower convectional cooling, which coincides with the boiling temperature of the oil. This stage of cooling lasts till about 150 to 180°C. With reference to the CCT-diagram of the steel, quenching condition for the given steel should ideally be so controlled as to give faster cooling during Stages I and II, and slower cooling in Stage III when steel temperature has reached close or below (MS). Faster cooling through stages I and II will help faster cooling of steel body at higher temperature, enabling the steel to cool to martensite formation zone without encroaching into the pearlite or bainite formation zones. Slower cooling thereafter in Stage III will help in reducing thermal shock – without any risk of getting into the bainite formation zone. It can, therefore, be concluded that for ensuring cooling curve not to cross C-curve of the CCT-diagram for the steel, Stage I cooling time has to be shortened and Stage II cooling rate has to be accelerated. At Stage III, cooling rate has to be slow to reduce thermal stress in the transforming martensite. All quenching fluids have to fulfill these cooling characteristics in some degrees; if not fully. An important consideration for heat treatment planning is that Stage III cooling rate has to be made as slow as possible for minimising the development of tensile residual stresses during martensite formation. Slow cooling during this stage minimises thermal shock and reduces residual stress accumulation in the steel. Major portion of residual stress arises due to high volume change of martensite structure compared to austenite from which it forms. High stresses in the martensitic transformation stage will cause higher distortion of the job, and might even lead to cracking. Oil quenching helps in reducing chances of high residual stress development and cracking due to martensite transformation by (a) taking over Stage III cooling early (i.e. before martensite has started forming), (b) slowing down cooling rate due to higher viscosity of oil, (c) allowing time for heat to move out from core to surface due to lower cooling rate and, thereby, reducing thermal

gradient between surface and core, and (c) providing better wetting of the steel surface for uniform heat removal.

6.3 6.3.1

TYPES OF QUENCHANT AND THEIR CHARACTERISTICS Types of Quenchants

Quenchant for hardening can be air and other gases (like inert gases – argon, helium, nitrogen etc.), water and their mixes with brine or salt, oils of different types, and polymers of different mixes and proportion. However, the rules of quenching stages as discussed in Section 6.2 are equally applicable to all. Air, gases or liquids proposed to be used as quenchants have to obey these rules; however, the degree of their duration and effectiveness changes with change in quenchant type. Gas quenching is a specialised process and is not generally practiced in industries; their applications are in vacuum quenching or for precision cooling of intricate shape of instrument and surgical parts. In common industrial practices, water, oil and polymer quenching are used based on needs and facility in the shop. Air quenching is also used in industry, but that too for special purpose (s), such as to harden air-hardening grade tool steels or for precision martensitic stainless steel instrument parts. Air has the lowest cooling rate amongst various fluids available for quenching, but cooling rate can be increased by using forced jet of air. Choice of quenchant primarily depends on hardenability of the steel, precision of the job being heat treated, type of heat treatment shop (i.e. jobbing type or for mass production shop like automobile parts manufacturing) and facility and resources available. Critical performance parameters for quenchants are: (a) ensuring development of required strength and toughness in the steel of a given hardenability, and (b) avoidance of crack and distortion during hardening. Hardening is mostly end-of-the-chain process for manufacturing, with very little scope for correction. Therefore, selection of quenching method and medium is as important as the selection of steel quality and its heat treatment parameters. The ideal quenching medium is the one that would exhibit high initial quenching speed in the critical hardening range of Stages I and II cooling, and a slow final quenching speed in Stage III cooling range – where martensitic transformation in hardening takes place. Therefore, the ideal quenchant should exhibit little or no vapour blanket stage, a rapid cooling in the vapour transport stage, and a slow rate during convective cooling; vide Fig. 6-1.

The high initial cooling rate is required for getting the steel past the nose of C-curve of the CCT-diagram (or the isothermal transformation diagram) during cooling. This criterion, in fact, determines if the steel will transform to full martensitic structure or be mixed with other nonmartensitic product, such as pearlite or bainite. This means quenching media must initially cool the steel at a rate faster than the critical cooling rate required for producing martensite in the steel. Slower cooling in the final stages of cooling is required for allowing better temperature equalisation and, thereby, reducing the chance of cracking and distortion. These conditions of ideal quenching mechanism must be fulfilled by the chosen quenching medium for successful steel hardening. From these considerations of ideal quenching mechanism, uses of water and oil as quenching media can be compared. In this regard, significant point is water has faster cooling rate in all three stages of cooling compared to oil where cooling rate in stage III considerably slows down and, thereby, reduces the chance of development of high internal stresses during martensitic transformation. This reduces the chance of distortion and cracking. Major advantage of oil over water is its higher boiling point than water. While water boils at 100°C, boiling point of oils can range from 230 to 480°C. Because of higher boiling point of oils, Stage III cooling by convection can start early. This can then slow down the cooling rate and favour release of transformation stresses during martensite formation, thus lowering chances of distortion. Hence, oil quenching is preferred for steels with higher hardenability and intricate shapes that are distortion prone. On the other hand, water exhibits high initial cooling rates due to rapid break-up of vapour film and increased vapour transport due to direct contact of water with the hot body. But, because of low boiling point of water, this fast cooling persists until the steel is cooled to temperature below 150°C or lower. As most steels have already formed or are in the process of forming martensite by this point, transformation stresses have little time to get released. As a result, water quenching can cause higher distortion or cracking if the steel is of higher hardenability or intricate in shape. Therefore, use of water quenching is typically limited to low hardenability steels, like the plain carbon steels, and to simple shapes or heavy sections where oil quenching will be ineffective. Figure 6-2 shows a similar, but elaborate, experimental quenching curve as that in Fig. 6-1. The experimental curves in the figure belong to a fast-quenching oil grade. Corresponding cooling rate observed in this oil has been indicated in the lower axis shown (as time/rate of cooling). The curve indicates that cooling rate of oil is relatively slow in the first stage, high in the Stage II and again slow in Stage III. Such cooling

mechanism is preferable for most alloy steels. Disadvantages with slow cooling in Stage-I can be overcome by uniform agitation of the bath which helps in breaking down the vapour blanket around the job. Hence, this type of fast-quenching oil is preferred for quenching and hardening of alloy steel.

Fig. 6-2 A graph showing the quenching stages and the corresponding cooling rate curve for a high speed quenching oil [Conversion of °F to °C use the formula: °C = (°F – 32) × 5/9] [Source: http://www.heat-treat-doctor.com/documents/Vacuum%20 Oil%20Quenching%20Tech.pdf – last accessed on 02-11-2014]

6.3.2

Characteristics of Quenchants

Each quenchant has its own characteristics due to its chemical and physical nature, contributing to different quenching mechanisms. Air is a clean quenching medium, but it has very limited industrial use – excepting for hardening of fine and precision jobs made of alloy steels. Cooling mechanism of air is mostly through convectional cooling. Two most popular type of quenchants used in large scale in industries are water and oil. Therefore, general characteristics of these two medium have been outlined in Table 6-1. As regards quenching mechanism, quenchingoil serves two primary purposes: (a) it facilitates hardening of steel by controlling heat transfer during different stages of quenching, and (b) enhances wetting of steel during quenching to minimise the formation of undesirable thermal and

transformational gradients which may lead to increased distortion and cracking. Table 6-1 indicates relative merits of water and conventional oil quenching of steels. Water has no effect in enhancing wetting ability of the steel surface during cooling, and it cools equally fast during the Stage-III cooling. Thus, for high hardenable steel, water quenching may not be desirable; instead oil quenching would be better, if section size (ruling section) is within the oil hardening scope. In general, stages of quenching outlined in Table 6-1 apply to all fluids that are used for quenching, and relative merits of a quenching medium can be evaluated with reference to these stages of cooling and ‘ideal cooling condition’ mentioned therein.

It is obvious from the table that quenching power of oil is far less drastic than water, and immensely suitable for heat treating low-alloy steels of higher hardenability without the risk of cracking or unacceptable distortion. But, oil is costlier and to an extent messy on the shop floor due to inevitable oil spillage and soaking.

Added advantage of oil is its viscosity. A good quenching medium should have higher viscosity at lower temperature of slow-cooling region, but lower viscosity at higher fast-cooling region. Oil viscosity proportionally decreases with increasing temperature, which facilitate fulfilment of former condition of good quenching medium. Further, due to decrease of viscosity with rising temperature in oil, viscosity drag on the steel body decreases with temperature rise of the oil quenching bath. This allows oil to move more freely over the steel surface, increasing its effectiveness to break the ‘vapour blanket’ layer. But, vapour transport stage (Stage II) is not much affected by temperature rise of oil bath, whereas conduction cooling involved in Stage III cooling becomes slower with increasing bath temperature.This has distinct advantage for moderating cooling during stage III. Latter is a definite advantage for slowing down the cooling rate when austenite starts transforming to martensite. Thus, with increasing bath temperature, oil quenching becomes more efficient by increasing cooling rate of Stage I and slowing down cooling rate of Stage III. But, there is a limit to the temperature to which oil quenching bath should be operated; too high a bath temperature is not desirable, because it may lead to oxidation or burning of oil and change its wetting characteristics and quenching efficiency. Oils can have different additives to improve their quenching speed as well as wetting characteristics for better heat removal. Similarly, water can have dissolved salts (brine quenching) or caustic additives for conditioning the quenching mechanism. Where a combined effect of water and oil is required, polymer quenching of different dilution can be used. Polymer quenching produces a cooling rate in between the water and oil, and the rate can be varied over a range by adjusting the proportion of polymer and water. However, cooling by using polymer can still be sharper than oil. More about polymer quenching has been discussed later in this chapter. Other quenching media that are sometimes used are salt water and cryogenic quenching. Salt water (also called Brine quenching) is a more rapid quenching medium than plain water, because the bubbles are broken easily and allow for rapid cooling of the part. However, salt water is more corrosive than plain water, and hence must be rinsed off immediately. Cryogenic quenching refers to sub-zero temperature quenching (generally ranges between – 40°C and below, as per requirement) for special purpose quenching, which makes use of suitable cooling medium like solid carbon dioxide, liquid nitrogen, etc.

6.3.3

Choice of Quenchants

Sensitivity band of common quenchant with respect to hardenability and quench severity is shown in Fig. 6-3.

Fig. 6-3 An illustration showing the sensitivity band diagram of common quenchant, including polymer (This is an indicative diagram only)

The figure sums up uses of different quenching media (in term of quench severity – H) for steel grades having low to high hardenability, including influence of section sensitivity. While salt solution in water gives highest quench severity, hot oil gives slowest cooling rate. Different medium mentioned in Fig. 6-3 has to be used with discretion as per hardenability and section size of the job to be treated as well as its cooling characteristics with reference to Fig. 6-1. The sensitivity band for different quenching medium provides preliminary selection guide based on quench severity and grade of steel and its section size. Thereafter, cooling characteristics of the medium should be examined with respect to required properties and control over dimensions. For oil quenching, such fine-tuning of quenching quality can be done by choosing the right type of oil and their special character like the viscosity, flash pint, etc – as discussed in Section 6.5. In general, water can be used for lean hardenability plain carbon steel of heavy forging / section, whereas normal or slow speed oil can be used for high hardenability alloy steel of complicated section profile. But, if the steel is of very high hardenability (e.g. tool or die steels), hot oil quenching or marquenching is recommended. Polymers can be used as substitute for intermediate quenching rate between water and oil of medium to high speed grade. More about polymer quenching has been discussed in Section 6.6. In addition to polymer quenching, gas quenching is another field of heat treatment which is getting developed over the years due to its process

cleanliness, environment friendliness and for clean surface of quenched jobs. Both oil and polymer quenching requires immediate washing, creating necessity for treating rinsed water and waste disposal. Another advantage of gas quenching is that heat transfer co-efficient of gas is nearly constant, which facilitates better control of cooling rate by regulating flow control. Gas quenching is a popular method of cooling in vacuum heat treatment process. Gas quenching speed is controlled by three factors: physical property of the gas, its pressure and velocity past the job. However, gas quenching can be expensive unless the pressure for regulating gas flow is critically controlled to the most optimum level. For example, though gas pressure of 20 bars may give better cooling effect, but pressure above 6 to 10 bar may not be economical. Velocity of gas passed through the jobs can be controlled by proper design of gas jet and furnace being used for heat treatment. Amongst gases, helium has the highest relative cooling rate, and then comes nitrogen and argon; their relative cooling rate is in the ratio of 2:1:0.6, respectively. Gas quenching, based on foregoing considerations, can be successfully used for heat treatment of precision jobs. Cryogenic quenching or deep freezing is another type of cooling, which is commonly used for eliminating retained austenite after quenching. Retained austenite is frequently associated with martensitic or bainitic transformation, and its presence leads to drop in strength of steel. Retained austenite is especially a problem in higher alloy steels containing Cr-Ni-and Mo. In high-alloy steels, there could be certain amount of untransformed austenite, generally present in spots. This untransformed austenite, if left as it is, can cause loss of strength or hardness, dimensional instability, or cracking at a later stage due to their transformation to martensite at normal temperature under internal pressure, if held too long without tempering. While tempering can relieve internal stresses and reduce retained austenite level, deep freezing or cryogenic treatment is necessary for total elimination of retained austenite, especially in high alloy steels. Martensite so transformed from retained austenite will, however, require further tempering (light tempering) for restoring toughness in the steel.

6.4 OIL-QUENCHING VIS-À-VIS WATER AND POLYMER QUENCHING Traditionally, water and oil has been used as common industrial quenchant. Polymer quenching, which is relatively new development, had been developed to overcome the shortcomings of oil and water quenching.

But, the process is still not fully accepted for use in regular heat treatment shops due to consistency and reproducibility problems. Thus, water and oil are still popular mode of quenching in industries. Between water and oil, handling of water and controlling the water bath temperature is easier and cost effective. Water is also cleaner and environment friendly; does not require much of waste disposal facility. But, main drawback of water quenching is its high speed of quenching in all stages of cooling, extending deep into the field of martensite transformation temperature. This causes distortion and cracking, unless the steel is of leaner hardenability and simple in shape. Water quenching is effective for plain carbon steels of leaner chemistry and for heavy section where distortion is limited and, if any, can be corrected (e.g. by straightening or grinding). For engineering parts, which are made of higher carbon steels (e.g. spring steel) or low-alloy steels for high strength and toughness, oil quenching is used with proper agitation of the bath in order to get desirable quenching characteristics. But, oil quenching has some disadvantages; it is prone to fire hazard, foaming, oxidation and spilling. Fire safety of heat treatment shop using oil-quenching system is of paramount importance. This is an additional burden to the shop that goes against the uses of oil quenching. As regards foaming and oxidation of oil bath, oils are now available with additives which can increase the ‘flash point’ of the oil and can make it less foaming and oxidising. Yet, fire and oil spill on the shop floor is a problem with oil quenching. Also, oil-quenching system is not very environment friendly and requires elaborate arrangement for waste (sludge) disposal. Technically, oil quenching serves two primary purposes; one, to facilitate appropriate critical cooling rate for hardening of steels, and the other to minimise undesirable thermal and transformational gradients in the steel by enhancing wetting of steel surface and, thereby reducing the thermal lag between surface and core. Thermal and transformation gradient across steel section is often the cause of distortion or cracking. Wetting properties of oil provides more intimate contact of quenchant and the steel, facilitating more uniform heat removal from the surface. In this respect, oil has superior characteristics than water. Water being a thin fluid of low viscosity, can easily get evaporated in contact with the hot steel surface and there is no wetting effect. As a result, thermal and transformation gradient in the steel increases due to higher cooling rate and, thereby, increases the chance of higher distortion and cracking. However, water is a clean medium with no fire hazard or environmental problem. Oil, on the other hand, is subjected to degradation due to constant contact with hot steel and the oxidation that follows, forming sludge. Oxidation and sludge formation may cause slow down

of cooling rate of stages I and II due to change of chemical character of oil with oxidation. Hence, oil requires elaborate cooling system of the bath (through heat exchangers) to control rise of temperature of the bath. Another problem with oil quenching is the possibility of contamination of bath, especially of water. Water and oil are not fully mixable; it forms pockets of water. Water contamination with oil may, therefore, lead to non-uniform surface cooling and spots with high cooling rate, causing micro-cracks. Though there are varieties of oil grades and quality available now, where oxidation and change of viscosity can be controlled, understanding of the effects of contamination of oil bath is important for practical operation of a quench shop. Other contaminants of oil bath which can affect cooling characteristics of oil are soot (burnt carbon from oil) and sludge. Viscosity is another important criterion of quenching media. Viscosity influences the temperature of the bath and rate of heat extraction. However, viscosity of a medium is not constant; it decreases with increasing temperature. This phenomenon is especially helpful in oil. If viscosity of a quenching medium is high, it can affect the maximum cooling rate by changing the character of Stage II (vapour transport or nuclear boiling stage) cooling. This is because, if the oil is viscous, it does not wet the steel surface that well, which is required for uniform cooling. This affects heat removal. Fortunately, viscosity comes down with increasing temperature of oil. Water lacks this advantage that comes from viscosity difference with temperature. However, with decreased viscosity at higher temperature, bath temperature of oil bath may increase, leading to higher oxidation, creating more sludge and non-uniform cooling. Thus, an optimisation is necessary for striking the right balance of bath temperature by use of adequate volume of oil for quenching and proper re-circulation and cooling to maintain a set level of bath temperature for minimizing oxidation and degradation of oil. Oxidation, in turn, increases the viscosity of the quenching oil which may cause decrease in wetting properties, affecting uniformity of cooling. In fact, oxidation of oil bath decreases maximum cooling rate and also the temperature of maximum cooling (vide Fig. 6-2), seriously affecting the quenching efficiency. Increase in viscosity due to oxidation can also cause bubble formation difficult, reducing the maximum cooling rate attainable by the use of oil in Stage II cooling. Based on these considerations, quenching facilities – which include quenchant quality, quenching tank capacity (i.e. volume of quenchant used for cooling), agitation and circulation of bath, control of bath temperature, volume of jobs to be quenched (or frequency of quenching)

and mechanism of withdrawal of jobs from quench bath to auxiliary bath – are required to be planned. Adequate quenchant volume helps in keeping the bath temperature under control. Because of the problem of oxidation, excessive sludge formation, and fire hazards associated with oil quenching, ‘Polymer quenching’ has been developed, which has average cooling rate in between water and oil. Commonly used polymer for quenching is an organic water-soluble compound of glycol, where the cooling rate can be adjusted by adjusting the proportion of polymer to water. Polymer content in water can range from 5% to 30% , depending on the purpose and cooling rate required. As the polymer content increases in the water, its viscosity also increases, improving the rate of heat extraction. Polymer quenching has been successfully used as alternative to oil quenching in many shops, but requires regular monitoring of bath composition for consistent results. This is because polymers have ‘inverse solubility’, which causes polymer compound to precipitate on the steel surface while hot in the quench tank and all of which may not get fully reversed on cooling at lower temperature, leading to some loss of polymer compound from the bath. This inverse solubility is the cause of reduced Stage III cooling, but it can also lead to loss of polymer concentration by drag with the components. Since viscosity and polymer content has direct relationship, viscositymeter is generally used to monitor polymer concentration of bath in the shop. Compared to oil quenching, polymer quenching is fire proof, clean and highly biodegradable. Though polymer quenching has been successfully used for plain carbon and lean alloy steels, it has problem with high hardenability crack-prone grades like tool steels; vide sensitivity band in Fig. 6-3. This is because polymer quenching rate sharply increases with decreasing polymer content in the water, and maintaining constant level of polymer in the bath is difficult, especially when polymer content of the bath is high to replicate oil quenching rate – such as that used for hardening of tool steels. At 25% polymer mix, the quenchant can act as good as or even better than fast-quenching oil with superior fire safety and environment friendliness, but requires constant monitoring of polymer content for consistent results. Polymer quenchant is non-flammable, better corrosion resistant and low foaming, and is capable of providing desirable cooling rate economically. The only disadvantage is that there could be high ‘drag losses’ of polymer (due to inverse solubility of polymer in water above 75°C) and needs for constant monitoring of bath temperature and composition; the latter can be controlled by using viscosity-meter. Advantages of polymer quenching over fast quenching oils are:

• Safety; no fire hazard or oil fumes • Capable of producing desirable cooling rate • Completely water soluble and no foaming of the bath • No environmental impact; fully degradable • Easy to wash with normal water • Economical Where water quenching is likely to produce high distortion or crack and standard oil quenching might fall short of target cooling rate, polymer quenching – with adjustment of % polymer – can be used for better results, but with continuous control of bath composition and temperature. Thus, understanding and control of quenching and quenchants is of critical importance in the field of heat treatment of steels. Howsoever good the steel could be, it cannot be hardened to best of its properties if quenching is not correct. Lack of understanding of the principles and mechanisms of quenching can lead to sub-optimal choice of quenchant and can result in inappropriate microstructure (i.e. inadequate hardness / strength), excessive distortion and even cracking of jobs, forcing scrapping of costly engineering steel parts. And, if that quenchant is a poor quality oil, it can futher add to environmental problems of smoke, fume and fire in the heat treatment shop. These considerations necessitate careful examination of which oil should be selected and how to make the quenching process effective and environment friendly.

6.5 DIFFERENT OIL GRADES FOR QUENCHING AND THEIR SELECTION Despite fire hazard and not so environment friendliness, oil quenching is the most popular method of industrial heat treatment. A single type of oil, in as it is condition, may, however, not be highly efficient and effective for cooling, but its cooling characteristics and other bath behaviour can be modified by blending and by using appropriate additive additions. Hence, most oils used for industrial quenching are mixed grades and added with additives for modifying the characteristics for better and consistent results. In addition to conforming to cooling characteristics as discussed in Sections 6.2 and 6.3, efficient and effective quenching oils should have the following additional characteristics: • Thermal stability, i.e. low degradation rate of oil molecules as a result of heat • High-speed quenching

• Excellent oxidation resistance, even at higher temperature. • High flash point (the lowest temperature at which oil vapour is ignitable) for minimising fire risk • Low viscosity at higher temperature for faster rate of cooling, and higher viscosity at the lower temperature for slower Stage III cooling, • Good surface wetting property and penetration, • Non-corrosive, and • High specific heat of the oil. Cooling rate obtained in oil quenching and the final hardness of the steel are function of its specific heat as well; greater the specific heat, greater is the hardness. Therefore, oil selected should fulfil the above criteria for efficient heat transfer and effective hardening. Most widely used quenching oil falls in the category of ‘mineral oils’, which, as it is, have mild quenching behaviour, but can be improved by blending. Mineral oils are generally classified under ‘aromatic’, ‘naphthenic’ or ‘paraffinic’. These terms refer to the structural arrangement of carbon and hydrogen atoms in the oil molecules. Blending of mineral oils is done within these groups for selectively improving the required properties as per chart in Table 6-1. In general, quenching oils are a mixture of either mainly paraffinic or predominantly naphthenic oils. Aromatic oils have poor ageing property, poor resistance to oxidation and poor viscosity–temperature characteristics, but have good wetting properties. Hence, its uses as quenching oil are very limited. Naphthenic quenching oils have moderately better ageing property and viscosity-temperature relationship compared to aromatic oils. Naphthenic oil has also poor volatility. Compared to these, paraffinic oils have more favourable cooling characteristics, much more oxidation resistance and better viscosity-temperature relationship. As such, paraffinic oils and its mixes form the basis of most industrially used quenching oils.

6.5.1

Classification of Quenching Oils

From practice point of view, quenching oils are classified under three groups for application purpose: (1) Non-accelerated Cold Quench Oils: These are straight quenching oils in simplest form and available in economical rate. Most common types are refined paraffinic oils with viscosity of around 20 centistokes (mm2/second) at 40°C. Viscosity control is necessary to make these grades efficient in cooling, but too low

viscosity is a danger to decreasing flash point of the oil, increasing the risk of fire. At time, these quenching oils are also treated with certain additives to increase oxidation and corrosion resistance, and to make it water washable. But, these oils do not contain additives for increasing cooling rate. These oils are popular for quenching high-carbon and alloy steels with good hardenability characteristics. (2) Accelerated Cold Quenching Oils: These oils are produced from specially selected base oil composition and treated with additives for accelerating the cooling rate. Hence, these oils are also called ‘accelerated quench oils’. By choice of additives and blending treatment levels, these varieties of oils can be formulated to give cooling rates ranging from moderate to very fast. Basic characteristic of accelerated quenching oils is that it can give increased cooling rate at higher temperatures, but retain the slow cooling rate behaviour at the martensitic transformation range, where distortion and cracking can occur. These oils reduce the vapour blanket stage, increase maximum rate of cooling and temperature of maximum rate (i.e. nose tip of the cooling rate curve), and retain slower cooling below MS temperature. The additives used could be animal fats, polymer and high molecular weight hydrocarbons, which can suppress vapour blanket stage reducing its duration and can increase the rate of heat removal in the boiling phase, i.e. vapour transport stage. Accelerated quenching oils are available with low viscosity, which are used mainly for plain carbon and alloy steels, and for tempering from higher temperature (above 550°C). The low viscosity oils give good penetration of heat extraction and may allow through hardening even with larger components. (3) Hot Quench Oils: This oil is necessary for quenching high hardenability alloy steels which are prone to high distortion and cracking due to mismatch of surface cooling (leading to martensite formation) and the core temperature. This mismatch is the source of tensile residual stress in the quenched jobs of high hardenability steels. To avoid formation of tensile residual stresses and distortion in such steels, marquenching operation is often adopted where the steel parts are quenched to and held just above martensite start temperature for equilibrium cooling. If temperature can equilibrate between surface and core, then slow cooling like air will allow the austenite to martensite transformation to take place without such residual stress build up or distortion. Hot quench oils are designed

to use for such elevated bath temperatures where bath temperatures can range from 150 to 230°C. Because of limitation of flash point of mineral oils to 230°C, bath temperature cannot be further increased without risk of fire. Despite this limitation of hot quench oil, they are very good for controlling distortion and cracking in high alloy steels. However, these hot quench oils are with higher viscosity at room temperature, which decreases with increasing temperature to give effect to required cooling rate at the holding bath temperatures. Since these oils are used at higher bath temperature, there will be higher degradation of oil. Hence, these oils contain special inhibitors that slow down oxidation and thermal degradation at the bath temperature, thereby extending the useful service life of oils. Other grades of quenching oils are: Synthetic oils, Water-washable quenching oil and Oil-in-water Emulsion. While synthetic oils are formulated from organic and inorganic alkaline and used in low concentration (5 to 15%), water washable oils contain some emulsifying agent which enables the quench oil to be easily washed off. The oil-in-water emulsion is an emulsified mixture of oil and water, which was once used for getting cooling rate intermediate between water and oil. All these grades have limited uses, because of cost for the synthetic varieties, and water contamination problem and cooling rate variation in the emulsified grades. Due to water contamination, cooling rate becomes erratic and can reach closer to water, giving rise to the problem of distortion or soft spots. Hence, amongst oil grades, straight cold quench oil, accelerated cold quench oil and hot quench oils generally find their applications in industries.

6.5.2

Working Characteristics of Quenching Oils

To achieve the required quenching properties, all these grades of oils should have to fulfil certain working characteristics for efficient heat removal and satisfactory end results. These characteristics are: viscosity, flash point, volatility, oxidation stability and thermal stability. An outline of these properties / characteristics in oil and their role in quenching is given below: Viscosity is the thickness of the oil, commonly measured in centistokes (cSt). It has direct influence on rate of cooling. Heat transfer during the convective stage is exponentially dependent on the oil’s viscosity. Low viscosity will allow more turbulence around the quenched job and minimise drag-out loss, but will lower flash point and increase volatility, putting a limit on how far viscosity can be lowered. While low viscosity

oil is preferred for accelerated quenching, high-viscosity oils are generally used for hot-quenching oils where viscosity-temperature relationship allows proper turbulence of the bath due to lowering of viscosity with temperature. Flash point of oil is the lowest temperature where vapour above the oil bath can get easily ignited. A high flash point is recommended for (a) reducing fire risks, and (b) ensuring that the oil does not contain high volatile fraction to cause prolong vapour blanket stage, slowing down Stage I cooling rate. For commercial application, selected quenching oil should have minimum flash point of 90°C. Volatility refers to the tendency of oil to evaporate. Low volatile oil is preferred because of (a) ensuring sharp Stage I cooling, (b) control over flash point, and (c) ensuring that oil fraction does not contain high volatile fraction to impair quenching efficiency and also cause contamination. Oxidation resistance is required for longer oil life without degradation and excessive sludge formation in the bath. Oxidation results from reaction between oil molecules and atmospheric oxygen, the rate increasing with rising temperature. Products of oxidation are generally acidic; hence it also makes the oil corrosive. Good quenching oil must have additives for controlling oxidation of the bath oil. Thermal stability refers to molecular disintegration of the oil due to intense localised heat generated during working with the oil. Thermal degradation of the bath causes excessive soot deposition (carbon deposit from molecular split) and increased volatility. Soot formation will cause maintenance problem of the bath and poor maintenance will lead to erratic cooling. Additives are used in the oil to control the thermal stability. Because of oxidation and thermal disintegration of oil molecules, oils should be sent through filtration and recirculation process for keeping contaminant and sludge level low in the working bath. With recurrent uses quenching oil gets decomposed, forming sludge and varnish affecting viscosity value of the oil, which increases with time and degradation. This increase in viscosity due to degradation of oil will lower the cooling rate, unless care is taken to filter out sludge and varnish from the oil. Figure 6-4 shows viscosity change over time of marquenching grade oil which had higher viscosity to start with but increasing further with degradation of the oil. Very high viscosity makes the fluid thick and unsuitable for effective quenching applications. Sludge content is the other problem of oil quenching. The sludge in the quenching fluid is produced as a result of thermal and oxidative degradation. High sludge formation in the bath results in non-uniform heat transfer, increased thermal gradients, cracking, and distortion.

Fig. 6-4 A graph showing the viscosity increase of a hot quenching oil with time. [Source: http://www.globalspec.com/learnmore/materials_chemicals_adhesives/ industrial_oils_fluids/quenc-hing_oils_heat_treatment_fluids – last accessed on 02-11-2014

Sludge may also plug filters and foul heat-exchanger surfaces, causing overheating, excessive foaming, and fires. Other problem of oil quenching is contamination, especially with water. Presence of small amount of water in the oil might cause problem of increased cooling rate of a medium speed oil bath below MS temperature where martensite transformation takes place, leading to higher distortion and cracking. Similar water contamination in a high-speed oil quenching bath, on the other hand, may cause lowering of critical cooling rate at spots and lead to soft spot formation. There are tests for detecting water contamination of a bath, but practical shop floor method is to notice the ‘crackling’ sound from the bath indicating the presence of water spots in the quenching bath.

6.6 POLYMER QUENCHING: CHARACTERISTICS AND APPLICATION TECHNIQUE Polymer quenching practice has grown over the years and it is one of the most important bulk quenching fluids today. Some basic information about polymer quenching has been already mentioned in earlier sections of this chapter, but because of its growing importance, some more discussions will be helpful. At present, polymer quenching accounts for nearly 25% of market by volume.

Referring to sensitivity band of quenchant in Fig. 6-3, polymer quenching is commonly used between water and medium speed oil in the range of H-value of 0.5 to 2.0. These polymers are ‘Polyalkylene Glycols’. There are other polymer types, but polyalkaline glycol is by far the most popular. Polyalkaline Glycol type polymers (PAG) exhibit ‘inverse solubility’ by which the polymer is completely soluble in water at room temperature, but insoluble at elevated temperature. This inverse solubility occurs at around 75°C, but can range from 60 to 85°C. This phenomenon provides the mechanism of cooling the hot steel by surrounding it with a polymer-rich coating that controls the rate of heat extraction at higher temperature. But, once the job has been cooled below the inverse point (i.e. 75°C), the polymer goes back to water. Typical polymer quenching cooling curves of different polymer content are shown in Fig. 6-5. This figure illustrates that faster cooling rate is obtained with 10% polymer in this case, gradually slowing down with increase of polymer in the solution. Furthermore, polymer cooling curves are different from oil cooling

Fig. 6-5 An illustration showing cooling curves using different polymer percentage in water solution. (Curves represent 10%, 20%, 30% and 40% polymer solution, respectively, from the bottom. Top curve shows typical cooling stages of conventional oil quenching) [Source: http://www.iasj.net/iasj?func=fulltext&aId=50586 (last accessed on 02-11-2014)

curves as their Stages I and II cooling is sharper (a typical oil cooling curve has been superimposed in the figure for comparison). However, now-adays, new generation water based polymer quenchant with quenching properties like oil is available, which are being widely used. In polymer quenching, there are three principal parameters that control the cooling rate, namely: (1) quenchant concentration, (2) quenchant temperature, and (3) quenchant agitation. Of these, quenchant concentration (i.e. polymer concentration) is the dominant factor. Figure 6-6 shows the effect of polymer concentration on cooling rate. The cooling rate of polymer quenchant can thus be changed simply by changing the concentration in the bath. As regards variation of cooling rate with concentration is concerned, following guidelines generally hold: (a) Low concentration upto 5% should be used where polymer quenching is intended to replace water quenching. Low concentration of polymer improves wet-ability of steel surface, and thereby improves the chances of uniform heat extraction (i.e. quenching) thus minimising chances of soft spots that one may encounter with water quenching. (b) Concentration of 10 to 20% can achieve quenching rate that is comparable to fast quenching oil. Therefore, polymer at this level is used for low-hardenability applications or plain carbon steel with intricate shape. (c) Polymer concentration of 20 to 30% offer cooling rates of medium speed quenching oil used for wide range of through hardening and case hardening steels.

Fig. 6-6

Effect of polymer concentration on the cooling rates

6.6.2

Application of Polymer Quenching

Principally, polymer quenchants are a series of non-flammable aqueous solutions of water and liquid organic polymer, aided by corrosion inhibitors for minimising corrosion of heat treated parts. Cooling rate of polymers can be additionally improved by agitation and flow control of polymer bath; horizontal flow of fluids has been reported to be better in this respect than vertical flow. A flow speed of 0.1 m/s to 0.4 m/s in the vicinity of work piece is said to give optimum result. It is further claimed that phenomenon of ‘inverse solubility’ modifies the conventional threestage cooling mechanism and provides greater flexibility and control over of cooling rates. Similar to water, with rising bath temperature, there is decrease of cooling rate with temperature of polymer bath. However, due to better wetting of steel surface in polymer, rising temperature of bath does not lead to prolonged vapour blanket stage as in water. Agitation is helpful for breaking down the vapour blanket stage and increasing the rate of heat extraction. Polymer bath temperature should be controlled at the desirable level (based on inverse solubility temperature) for consistent results. In this regard, it should be considered that specific heat of water soluble polymer is nearly double the specific heat of oil. Hence, temperature rise of bath, quenching similar mass of steel, will be one-half of oil. Though conventional polymer quenchants eliminate many of the disadvantages of oil quenching (especially the fire hazard and oil spilled shop floor), they have the following limitations: • Risk of drag-loss and change in bath composition • Risk of higher distortion of quenched parts • High concentration is required for quenching oil quenching grades, involving higher cost of consumption of polymer • Higher risk of cracking, making high hardenability steels (e.g. die and tool steels) not suitable for polymer quenching, in general • Necessity for close monitoring and adjustment of bath, as well as good maintenance for consistent results. To overcome some of these disadvantages of conventional (PAG type) polymer, new types of polymers (Polyacrylates) have been developed which does not show inverse solubility but exhibit oil like cooling characteristics. In contrast to conventional polymer cooling curve, this advanced type polymer cooling curve shows similar cooling behaviour as oil with three-stage cooling, but with different mechanism. This type of polymer modifies the stages of cooling and cooling rate by producing high viscosity, polymer rich layer (but not with inverse solubility) around the jobs. 10 to 20% polymer concentration of this advanced polymer is generally used for substituting medium quenching oil. It generates a

much thicker and more effective insulating film of polymer on the job which gets produced by evaporation of water near the job surface. This film is stable and slows down the lower temperature cooling rate like oil. It is claimed that this type of polymer can be used for high hardenability steels, like the die and tool steels or alloy steels like SAE 4140, SAE 4340, EN-24 etc., which are generally oil quenched with medium speed quenching oil or hot oil. Table 6.2 shows relative cooling rate of water, oil, PAG polymer and advanced polyacrylates quenching bath.

Though quenching behaviour of advanced polymer is close to oil, its cost and availability is prohibiting large scale uses of this quenchant. Instead, Polyalkaline Glycol (PAG) is finding better acceptance for applications relating to forging and casting heat treatment. This type of quenching is better suited for hardening forgings and castings where water quenching causes cracks and distortion. Thus, PAG quenching is getting popular for quenching of plain carbon and low-alloy steel forged parts. However, limitations of polymer quenching in the shop floor still appear to be stability of bath composition and drag loss during quenching. As regards use of polymer quenching for finish machined engineering and automobile components are concerned, the technique is still under development in the shop floor level. In this area of hardening and quenching, oil quenching, with due precautions, continues to be the preferred process. However, limited trials on hardening of machined and case carburised gears have indicated superior metallurgical results of polymer (glycol based) quenching vis-a-vis oil quenching, as indicated in Table 6-3.

The trial was carried out by using SAE 8620 grade carburising steel, on similar gear, with surface carbon level of 1.60%C after carburising. The result clearly demonstrated the superiority of polymer quenching with regard to metallurgical structure formation in such high carbon surface. But, it must be factored in the use of polymer quenching that polymer degrades and molecular weight decreases over time, leading to increase of cooling rate and simultaneous loss of insulating layer that controls the rate of heat transfer. Hence, polymer quenching requires very precise control of bath concentration for consistency in results.

6.7

CASES OF QUENCHING DEFECTS

Hardening of steel involves three essential steps: reheating for austenitisation, cooling (quenching) for martensite transformation and tempering for toughening. Of these steps, quenching is the most critical, because it is the means of transformation and at the same time source of generation of many defects. Common quenching defects are: • Cracking during quenching • Distortion during quenching • Improper microstructure Quenching may not be able to make a bad steel good, but bad quenching can make a good steel bad. Since quenching is carried out after processing – which adds substantial cost to the product – rejection of steel parts after quenching entails enormous loss. Hence, an understanding of sources of quenching defects is necessary for the heat treaters. Figure 6-7 illustrates a schematic hardening cycle – showing three possible variations in quenching. Task of a heat treater is to control each of these stages of hardening and quenching – adopting the right cooling curve. If all other controls are right but not the choice of cooling rate, then there are possibilities of generation of any of the quenching defects mentioned above. Some common quenching defects and their causes and remedies are listed in Table 6-3: The list pertains to quenching defects only and not the other hardening defects that are commonly noticed in the heat treatment of steels. A more detailed list of heat treatment defects has been provided at the Annexure of Chapter 11. However, it should be appreciated that quenching is not a stand-alone process; it is intricately connected with austenitisation discipline prior to quenching for hardening. If austenitisation temperature is high or low, it will certainly manifest in quenching defects like: (a) coarse martensite structure and incipient grain boundary

cracks, if austenitisation temperature is too high or (b) the structure will be soft, if austenitisation temperature is low. In discussing quenching defects, these types of defects have been kept outside the scope; only direct quenching related defects have been described.

Fig. 6-7 A schematic showing hardening – quenching – and tempering diagram for steel showing different options for cooling

Table 6-4 describes few such quenching related defects, their causes and remedies. [For more detailed study of quenching defects, refer to ASM Metals Handbook, Vol. 4, Heat Treating, ASM International, USA, 1991]

Amongst the quenching defects, cracking during quenching and distortion due to faster cooling at the martensitic transformation temperature region are quite common. Figures 6-8 and 6-9 depicts two quench cracked steel samples. The figures indicate that the quench cracks have originated from the surface and moved relatively straight into the bulk. In Fig. 6-9, crack has started from the radius of the shaft, which acts as stress raiser and facilitates easy crack initiation. For avoiding such defects, heat treaters from shop floor often take the measure of masking such areas by coating so that quenching rate at those spots is partly lowered. Distortion due to quenching is the outcome of interactions between stress, temperature and distortion in the atomic lattice structure, which takes place mostly in the martensite transformation temperature region i.e. during Stage III cooling. It does not manifest into any obvious microstructural defects, but it leads to dimensional discrepancies. Being

a dimensional issue, the degree of distortion is generally checked by inspection gauges and tools or in metrological laboratory.

Fig. 6-8 Quench crack in SAE 4340 shaft due to severe quenching

Fig. 6-9 Depicts a quench crack originating from the radius of a SAE 4142 steel shaft × 100

Another quenching defect is the retained austenite (vide Fig. 6-10), which is a type of microstructural defect. It can originate due to inadequate holding at the quenching bath or higher austenitisation temperature,

Fig. 6-10 As-quenched structure of SAE 5160 spring steel samples quenched from 890°C in oil (x500)

especially for alloy steel hardening. Retained austenite can exist along with lower bainitic or martensitic structure. But, its presence beyond a certain percentage (e.g. 5% RA) in martensitic structure is often considered defective. Remedy for excess retained austenite is double tempering the steel. There are high alloy steel grades (e.g. tool steels) where high volume of retained austenite is inherent in the quenched structure due to higher alloy content in the steel, making the austenite stable at the quench bath temperature despite holding the jobs in quench bath for longer duration. As such, these high alloy steels are always subjected to double tempering and/or sub-zero temperature treatment for minimising the retained austenite content in the structure. For most industrial applications, higher volume of retained austenite is not desirable – though presence of small amount of retained austenite (about 5% or less) in the hardened structure is often considered beneficial for fatigue related applications, but not for wear. For eliminating or reducing the retained austenite in steels that are prone to this problem, double tempering or sub-zero treatment after quenching is a must, especially for wear related applications. Foregoing discussions on quenching process and quenchant show the merits and demerits of different quenching media with respect to ideal quenching behaviour. While, the chosen quenching fluid should conform to the basic rules of ideal cooling rates, their selection must also consider the following factors: • Economics / cost (initial investment, maintenance, upkeep, life) • Performance (cooling rate/quench severity) • Stability over a broad range of operating conditions • Minimisation of distortion (quench system) • Variability (controllable cooling rates), • Safety (Fire hazards), and • Environmental concerns (recycling, waste disposal, etc.) Use of polymer quenching, which is gaining considerable ground, is considered from the point of view of safety and environment, but its cost might be high and performance could be less consistent than oil. Advanced polymer that exhibits near equivalent to oil cooling behaviour works out to be more costly. Hence, oil quenching still remains the most dominant quenching medium being used in the industry, especially for the alloy steels and precision engineering component and machinery manufacturing.

Summary 1. Quenching is a process by which steel is rapidly cooled from the austenitisation temperature (typically above A3 temperature of the steel that generally ranges between 830 and 890°C) for hardening, and its purpose is to achieving required hardness, strength or toughness in steels through changes in microstructures, but without distortion or cracking of the steel parts. Metallurgically, purpose of quenching can be said to suppress the formation or transformation of highertemperature phases in steel (like the ferrite, pearlite or bainite) and ensure as much transformation of austenite to martensite as possible without causing distortion or cracking. 2. The chapter analyses different stages of cooling (Stages I, II and III) and related heat transfer mechanisms and show that quenching condition should ideally be so controlled as to give faster cooling during Stages I and II, and slower cooling in Stage III when steel temperature has reached close or below the martensite start temperature (MS), in order to reduce quench-in stress in the martensite. For ensuring cooling curve (representing cooling rate) not to cross C-curve of the CCT-diagram, Stage I cooling time has to be shortened and Stage II cooling rate has to be accelerated. 3. It has been illustrated and emphasised that Stage III cooling rate has to be made as slow as possible for minimising the chances of developing residual stresses due to martensite formation. Primary reason of residual stress during quenching is high volume change of martensite compared to austenite from which it forms. Hence, slow cooling in Stage III when martensite starts forming is a critical factor for controlling distortion and cracking of jobs. It has been shown that oil quenching helps in reducing or eliminating chances of residual stress development due to martensite transformation by (a) taking over Stage III cooling early, (b) slowing down cooling rate due to higher viscosity that produces higher specific heat capacity of the oil bath, (c) allowing time for heat to move out from core to surface and, thereby, reducing thermal gradient between surface and core, and (c) providing better wetting of the steel surface for uniform heat removal. 4. It has been pointed out that the ideal quenching medium is the one that would exhibit high initial quenching speed in the critical hardening range in Stages I & II, and a slow final quenching speed through the lower temperature range (Stage III). Therefore, the ideal quenchant should exhibit little or no vapour blanket stage, a rapid cooling in the vapour transport stage, and a slow rate during convective cooling. Based on this observed ideal cooling behavior for quenching, characteristics of different quenchant and their relative merits and limitations have been discussed. A sensitivity band of different quenchant and their thumbrule applications with respect to hardenability of the steel, complexity of shape and size, and conventional quench severity (H-value) has been shown. 5. The chapter discusses at length pros and cons of oil quenching vis-à-vis water and polymer quenching for better understanding of quenching characteristics of oil, which has retained its unique position as most preferred quenchant till date. It has been pointed out that technically, oil quenching serves two primary purposes; one to facilitate appropriate critical cooling rate for hardening of steels, and the other to minimise undesirable thermal and transformational gradients in the steel by enhancing wetting of steel surface. Thermal and transformation gradient across steel section is often the cause of distortion or cracking. Wetting properties of oil

provides more intimate contact of quenchant and the steel, facilitating uniform heat removal from the surface. Water being a thin fluid and low viscosity, can easily get evaporated in contact with the hot surface and there is no wetting effect. Water increases maximum cooling rate and extend the maximum cooling filed well below the martensite start temperature. As a result, thermal and transformation gradient in the steel increases, thereby, increasing the chance of higher distortion and cracking. Polymer quenching though improves the situation over straight water quenching, but its closeness to water in quenching behaviour, especially at lower % concentration of polymer, and consistency of bath behaviour are its limitation. 6. The chapter also discusses basic properties of different generic oil grades and factors influencing the choice of oil grades for effective quenching of steels with different hardenability. It has been pointed out that for efficient and effective quenching oils should have the following characteristics: • Thermally stability i.e. low degradation rate of oil molecules as a result of heat, • High speed quenching, • Excellent oxidation resistance, even at higher temperature, • High flash point (the lowest temperature at which oil vapour is ignitable), • Low viscosity for faster rate of cooling, • Good wetting and penetration, • Non-corrosive, and • High specific heat of the oil. 7. The chapter throws some light onto the development of polymer quenching practices, which is growing with the development of fresh understanding of the technology and development of new types of polymer. In this context, quenching characteristics of Polyalkylene and Polyacrylates types polymer quenching have been mentioned and their cooling characteristics and behaviour have been pointed out. Merits of polymer quenching and their advantages and limitations have been highlighted. 8. Finally, the chapter highlights, briefly, some quenching defects and their causes and remedies. Purpose of this highlight has been to make the heat treaters aware of the problems of quenching and make them refer to the theories and techniques of quenching discussed earlier.

References / Suggested Reading ASM Metals Handbook, Vol. 4, Heat Treating, ASM International, USA, 1991 Boyer, Howard E. and Philip R. Cary, Quenching and Control of Distortion, ASM International, 1988 http://www.heat-treat-doctor.com/documents/Vacuum%20Oil%20Quenching%20 Tech.pdf –accessed on 02-11-2014 http://www.iasj.net/iasj?func=fulltext&aId=50586 (accessed on 02-11-2014) http://www.globalspec.com/learnmore/materials_chemicals_adhesives/industrial_oils_ fluids/quenc-hing_oils_heat_treatment_fluids – accessed on 02-11-2014 http://www.wpi.edu/Pubs/ETD/Available/etd-0429102-153911/unrestricted/ 8APPENDIX - accessed on 16-04-2015

Liscic B., and W. Luty, Theory and Technology of Quenching: A Handbook, Edited by Hans M. Tensi, Springer-Verlag, 1992 Marsh, E.C.J., Oil Quenching of Steel: An Analysis of the Properties of various Oils, MCB UP Ltd. (http://www.emeraldinsight.com/journals. htm?articleid=1676899&show=html ) “New Developments in the Technology and Application of Polymer Quenchants”, Heat Treatment of Metals, 1984. 1, Vol. 11, Wolfson Heat Treatment Centre, Birmingham, UK Polymer Solutions, ‘Heat-treating’, Metals Handbook, Vol. 4, 9th Edition, ASM, 1981 Totten, George E., Charles E. Bates, & N.A. Clinton, The Handbook of Quenchants and Quenching Technology, ASM International, 1993

Review Questions 1. What is the purpose of quenching in a heat treating operation? Why ‘residual stresses’ get generated in steel parts during quenching? How oil quenching helps in reducing development of tensile residual stresses in a quenched part compared to water quenching. 2. Discuss the stages of cooling during quenching of steel parts. Point out what should be the ideal cooling conditions at different stages of cooling for effective hardening with minimum distortion of the parts. 3. Discuss the merits and demerits of water quenching versus oil quenching. How agitation of bath can help in improving the quenching efficiency of oils? 4. With reference to ‘quench severity’ (H) of different cooling media, discuss the choice of cooling medium for: (a) Low-alloy steel parts with complex geometry (b) Medium carbon heavy section steel shaft (c) High-alloy tool steel, and (d) Case carburised steel gears 5. Critically discuss the merits and demerits of quenching steel parts using water, oil and polymer. Why attempts are being made now to move to polymer quenching from oil quenching in a job shop? 6. What are the selection criteria for choice of oil grade for fast quenching operations? Discuss the broad groupings of different quenching oils available in the market. 7. Define and discuss the role of following characteristics of oils in the quenching mechanisms: (a) Viscosity, (b) Flash point, (c) Volatility, (d) Thermal stability, and (e) Oxidation resistance 8. Discuss the principal features of polymer quenching and its advantage. How the cooling stages of polymer quenching differ from that of oil cooling stages? 9. How the cooling rate of polymer quenching can be made to vary to suit a particular quenching need? How the Stage III cooling rate in polymer quenching can be further improved? Discuss the disadvantages/limitations of conventional polymer quenching vis-a-vis oil quenching. 10. What are the recent trends of development in polymer quenching for overcoming the present limitations? Discuss the ultimate factors that should lead to the choice of quenching fluid for a mass producing heat treatment shop producing medium size automobile gears.

Thermal Heat Treatment Processes

7.1

INTRODUCTION

Heat treatment is an integral part of industrial and metalworking processes, used primarily for altering the mechanical properties of steels through changes in microstructure. As outlined in Chapter 3, heat treatment processes can be grouped under thermal, thermo-chemical and thermomechanical processes. Of these processes, thermal and thermo-chemical processes dominate the regular industrial heat-treating practices. Thermomechanical processes are carried out along with steel rolling for technical and economic reasons – which have been explained in Chapter 3. Each of these processes has its specific domain and purpose. This chapter and the next discuss the thermal and thermo-chemical processes, respectively. Thermo-mechanical processes have been covered to the extent necessary for this book in Chapter 3. Heat treatment processes where alteration or modification of structure, and the properties, are achieved with the help of thermal energy only – without any intentional change in chemical composition – are

termed as ‘thermal heat treatment processes’. For example, softening of steel achieved by annealing of steel is a thermally assisted process. Annealing process uses only thermal energy for heating and softening of steel by producing larger ferrite grains and coarser pearlitic lamellae. As a result of this structural change, steel becomes softer (i.e. lower in strength) and more ductile. Similarly, hardening of steel achieved by quenching from higher temperature is a thermal heat treatment process where heat is used to convert the steel to austenite and then quenched (a thermal process of heat extraction) for formation of hard martensitic structure. Both these processes are assisted only by thermal energy (such as controlled heating and cooling) for bringing about the change in the structure and properties. There is no intentional change in composition involved in these processes; decarburisation of steel surface by oxidation or scaling is an unintentional act, and prevention of same is a part of operational strategy in thermal heat treatment processes. Difference between thermal and thermo-chemical processes of heat treatment can be further illustrated by referring to induction surface hardening (a pure thermal process) and case-carburising and surface hardening of steels (a thermo-chemical process). Induction surface hardening does not attempt to alter the composition of the steel; it simply attempts to harden the surface area of the steel by heating and quenching to produce hard martensitic structure. As against this practice, case hardening by carburising and quenching attempts to alter the chemical composition of the surface area of the steel by heating and chemical diffusion of extra carbon to the surface – which is required for the steel to response for producing hard martensite on quenching. In practice, purpose of thermal heat treatment takes two major directions: (1) To soften the steel with increased ductility for intermediate shaping, working, forming or uses where softness / ductility is required, and (2) To harden or strengthen the steel with increased toughness for uses and applications where strength and / or toughness is required. The former objective of heat treatment is aimed at to improve processability of steel for industrial applications where strength is not a critical criterion, and latter objective of the process is to improve the strength and toughness of steel for engineering and structural applications, involving load-bearing capability under static and dynamic loading. Towards fulfilment of these objectives, principal thermal heat treatment processes are annealing, normalising, hardening (including surface hardening) and tempering. There are other thermal processes which

are intermediate between these principal processes; namely sub-critical annealing, process annealing, spheriodising, stress-relieving, etc. Stressrelieving – a popular process for metal working – can be an independent process or can be a part of tempering process – which has to be invariably carried out after quenching the steel to martensitic structure. Therefore, tempering and stress-relieving has been discussed together in this chapter. Outline of processes, purpose and principles of these thermal heat treatment processes have been described in Chapter 3 with an aim of introducing the subject soon after discussing the rules for phase transformation and structure formation in steel (Chapter 2), governing the principles of heat treatment processes. This chapter (Chapter 7) now further extends the scope of discussions initiated in Chapter 3 on thermal heat treatment processes. Discussions in the chapter rest on knowledge and information described so far in Chapters 2 to 6, which include the role of phase transformation, heat treatability, furnace technology and quenching mechanisms in the heat treatment of steels. Changes in the structure during heat treatment is not instantaneous; it takes some time for change to initiate (nucleate) and shape up (grow). Therefore, heat treatment process has a cycle of heating, holding and cooling for allowing changes to occur, as discussed in Chapter 3 (Fig. 3-1). Controlling this cycle to the desirable limits and conditions is the very core of thermal heat treatment process. Thermal heat treatment processes require control of: • Rate of heating and cooling, • Temperature to which the part is heated, • The time duration for which the part is held at that temperature, • The measures to prevent any surface reactions of steel at that temperature of heating – in order to avoid any perceptible change in composition in the steel, and • Regulated cooling (after heating and holding) for desirable change in structure. In principle or in practice, it is the structure that is to be altered or controlled in the steel for achieving the prescribed properties, as per structure – property relationship in steel discussed in Chapter 2 and emphasised all through this book. Chapter 3 has discussed and illustrated how the temperature of heating and cooling for heat treatment is decided with reference to Fe-C diagram or TTT / CCT-diagram of the steel, as appropriate. Time to hold at that temperature depends on the character of the steel (e.g. alloy content and character), section size of the part and load in a charge for heating.

Higher the section size or load, higher should be the time of holding. This is because, purpose of holding time in practice is to homogenise the temperature and structure of steel attainable at that temperature. Similarly, more complex is the alloying in steel longer would be the holding time. Theoretically, time is required for equalisation of temperature in the section and then time for metallurgical reactions / transformation for making the steel homogeneous in structure so that transformation of the steel on cooling can start from a uniform platform. For example, when a steel piece is heated for normalising or hardening, purpose of allowing some soaking time is to allow (a) temperature equalisation in the section, and (b) 100% austenite formation before the start of cooling. This step is necessary for conditioning the austenite for subsequent transformation by cooling, as appropriate for the process. If austenite is not homogeneous to start with, the process of heat treatment may not respond correctly or consistently due to variation in the condition for nucleation and growth (N&G) of new phases. Most thermal heat treatment processes of steels involve heating above the upper critical temperature (A3), excepting stress-relieving, sub-critical annealing, tempering and aging of precipitation hardening type steels. Sub-critical annealing is carried out just below the lower critical temperature (A1) for spheriodising and softening the steel structure. However, whether the heat treatment process is below the critical temperatures or above, time has to be provided for accomplishing the objectives of a heat treatment cycle. Time of holding for lower temperature operations (e.g. stress-relieving, tempering or sub-critical annealing) is determined by change in properties required in the steel, and time of holding for higher temperature operations are determined by time to equalise temperature over a section, control of austenite grain growth, and conversion to 100% austenite – a diffusion-controlled process of change. Grains are an integral part of steel structure, which participate in the process of heat treatment. Grains and grain sizes have somewhat opposing influence on hardening of steel. In hardening, fine grain size may tend to make 100% martensitic transformation difficult by promoting grain boundary nucleation of carbide or ferrite before the start of martensite transformation – leading to formation of some pearlite or bainite in the structure along with martensite. This is not desirable from the point of view of strength and toughness of the steel. Though fine-grained steel is preferred for higher mechanical properties, the same may pose problem for effective hardening of the steel (vide discussions in Chapter 4). Therefore, relatively coarser grain steel is preferred for hardening. To

strike a balance between these two opposing factors, steel with ASTM grain size of 5 to 8 is generally used for hardening grade steels. Whatever could be the initial grain size, grains have the tendency to grow at higher temperature – leading to austenitic grain growth. Coarser austenite grains will, in turn, produce coarser ferrite grains and coarser structure under standard cooling – which are not good for strength and ductility in the steel. Similarly, coarse austenite grains will also produce coarser martensitic structure, which is not the best choice. Martensitic structure should be fine for better strength and toughness after tempering. As such, most heat treatment processes require steel with stable grains which do not easily grow at higher temperature during heating and soaking. Al-killed or Al-Si killed steels are used for such cases of heat treatment, where grain growth is restricted due to pinning effect of aluminium nitride particles present in such killed steels (vide discussions in Chapter 4). Based on these basic tenets, premises and rules of thermal heat treatment, common thermal heat treatment processes have been discussed in this chapter with reference to their industrial practice and applications. The concerned heat treatment processes are: • Annealing, which are described variously as per their process objectives, such as: full annealing, sub-critical annealing, process annealing, inter-critical annealing, recystallisation annealing, recovery annealing, isothermal annealing, etc. • Normalising, a popular heat treatment for grain refinement and homogenisation of medium carbon steels • Hardening (including surface hardening) – under water, oil and gas quenching • Tempering and stress-relieving, which is carried out for promoting toughness in martensitic structure or to relieve internal stresses, and • Martempering and Austempering – which are special types of tempering for crack prone grade steels Latter two processes are carried out by improvising the quenching process and used for hardening of special grade steels and alloys.

7.2

ANNEALING AND NORMALISING

Annealing and normalising are in the same class of heat treatment; heated to similar temperature range for full conversion to austenite and then transformed to fully recrystallised (ferrite + pearlite) structure by following a process-specific cooling condition. It is this cooling condition

that distinguishes these two processes of heat treatment; annealing uses slow cooling, preferably inside a furnace, and normalising involves relatively faster air cooling, exposing the parts to air outside the furnace. Due to cooling rate difference, these two processes produce different types of ferrite-pearlite structure. Normalising produces finer ferrite grain size and finer pearlitic structure whereas annealing produces relatively coarse ferrite grains and pearlite structure due to slower cooling rate. The other difference between annealing and normalising is in holding time; annealing requires longer soaking time compared to normalising – this is because full annealing aims at attaining complete homogenisation and recrystallisation of structure for producing desired level of softness after treatment. Normalising, on the other hand, aims at structural refinement rather than softening the structure. Due to slower cooling, structure after annealing is more uniform than in air-cooled normalising process. Air-cooling might not always be able to off-set the effect of temperature gradient from surface to centre in a thicker section. This is the reason why heavy section steels are mostly annealed for uniformity of structure and properties. Annealing and normalising are two widely practised heat treatment processes in industries. In practice, normalising process is popular for low and medium carbon steels (e.g. steels with carbon content between 0.15%C and 0.50%C) and annealing is popular process for high carbon engineering steels (e.g. steels with carbon content of 0.50%C and above); the exact choice of treatment for a given carbon content in the steel may also be influenced by the section size. However, annealing is also a popular process for lower carbon steels (below 0.15%C), especially sheet steels, for improving ductility and formability. Annealing has different sub-categories due to its uses for different purpose and economy of operation. Types of annealing could be: full annealing, sub-critical annealing, spheriodising annealing, process annealing, etc. Amongst these, full annealing is carried out by heating at temperature above A3 temperature of the steel; all other annealing processes are carried out below A1 temperature. Normalising, on the other hand, is a process, which has no variant; it is always carried out by heating above the upper critical temperature (A3) followed by air cooling. Figure 7-1 illustrates different temperature ranges involved with different types of annealing and Figure 7-7 illustrates the temperature range used for normalising. Process details of annealing and normalising – with reference to Figure 3-2 and these two figures – have been discussed below, along with the logic for choice of different process parameters for different variants of annealing.

Fig. 7-1 An illustration showing general practices of different annealing processes with reference to critical temperatures of steel. [Note: Fig. 3-2 may be referred for more details]

7.2.1

Annealing

Figure 7-1 illustrates that other than full annealing all other annealing processes are carried out below the lower critical temperature (A1). Opportunity for carrying annealing process below the lower critical temperature comes from the fact that: (a) steel can be recrystallised even at a lower temperature (as low as 550°C), if the initial structure is heavily cold-worked, i.e. strained, (b) consequent to recrystallisation, ferrite grains in the prior cold-worked matrix can grow even at this lower temperature, making possible softening of steels, and (c) because of strained cold-worked structure, pearlite content in the steel can be made to ‘ball-up’ (i.e. spheriodised) by heating and holding at temperature below the lower critical temperature (A1). Spheriodised structure where carbides conglomerate and ball-up are softer, and the process is widely used for softening high carbon steels or high alloy steels like tool steels. Thus, annealing processes and their purpose can be divided into two groups, namely, full annealing and soft annealing. Thumb rule of their industrial applications is that full annealing is to be applied to steels in forged, cast, rolled or machined conditions for homogenising, softening, recystallisation or grain size conditioning (e.g. for electrical steels). While soft annealing is to be applied to prior cold-worked steels for softening and improving ductility and workability by taking advantage of ‘stored energy’ in the steel structure arising from prior cold-work. Spheriodising annealing – which is also a softening process – is, however, used for softening high carbon and high alloy steels with or without prior working.

Full Annealing Full annealing is the primary annealing process carried out by heating above the upper critical temperature (A3) of the steel. Generally, steel is heated about 30 to 50°C above the A3 temperature for ensuring (a) the austenite is homogenised, and (b) the steel is not exposed to temperature where austenite grains can rapidly grow and coarsen; vide temperature band indicated in Fig. 7-1. Steps in full annealing process are: • Placing the job at pre-heated furnace – in order to avoid thermal shock • Slowly increase the temperature to the full annealing temperature, e.g. A3 + 50°C for hypo-eutectoid steel composition and A1 + 50°C for hyper-eutectoid steel • Soak the job for temperature equalisation and full austenitisation; soaking time may vary with section size of jobs, steel composition and furnace characteristics. Generally, a soaking time of 1 to 1.5 hour for 30-mm section is commonly used • Control the furnace atmosphere to slightly reducing or minimally oxidising state for avoiding heavy scaling of the job surface • On completion of soaking, shut the furnace down and allow the jobs to be slow cooled inside – till temperature has reached well below 400°C for ease of handing • Finally, take the job out and air-cool to room temperature, and • Sand-blast clean the surface for removing oxide scales formed during heating Annealing is a common heat treatment process for plain carbon steels, especially for high carbon grades, which find wide industrial applications for various structural and engineering parts. Few examples of such components are shafts of various sizes, wheels, blades, hammers, shovels, common gears and drives, etc. Annealing is applied to these steel parts for softening by grain coarsening to improve machining, drawing or further working. But, if the treatment leads to fall in strength below the required specification level, then the steel needs to be hardened after machining or processing and followed by tempering to required strength level. Jobs for annealing can vary in sizes and weight. Therefore, to cope with such fluctuation in load pattern, the process is mostly carried out in batches, using batch-type furnaces, such as walking beam, bogie hearth or box furnaces. Figure 7-2 illustrates one such box type annealing furnace used for full annealing in the shop floor. However, for large scale production, continuous annealing processes are also in practice, especially for

Fig. 7-2 furnace

An illustration showing a set-up of an industrial batch-type annealing

low carbon cold rolled sheet steels for softening and texture development for better formability. Softening of steel is not the only purpose of full annealing as applied to engineering steels; other purposes may include homogenising, recrystallisation, removal and refinement of coarse cast structure, and grain coarsening (for improved machinability or for electrical applications of silicon-alloyed transformer grade steels). Full annealing can be applied to forgings, rolled sections, and castings of all steels, including stainless steels for conditioning the structure and properties. Stainless steels get easily work-hardened during cold-working / processing and require intermediate annealing at higher temperature for improved cold workability. Because of austenitic structure of stainless steel, full annealing is required for any recovery of structure for further processing. Temperature for full annealing of steels can range between 750 and 1050°C (or even higher for grain coarsening of Si-steels), depending on the carbon composition, section size and purpose. Metallurgically, the charge must be taken to sufficiently high temperature for complete austenitisation, but without incipient melting of grain boundaries or burning. Grain boundaries, being areas where impurities segregate can have lower melting point than the steel matrix, are susceptible to incipient fusion or melting if the temperature is not correctly controlled. Hence, care has to be taken during full annealing of steel to avoid any probability of grain boundary melting or burning.

Figure 7-3 schematically illustrates the nature of grain boundary fusion and film formation, causing embrittlement of steel. Overheating of steel leading to grain boundary fusion and film formation must be avoided in all heat treatment of steels. This could be a special problem in annealing where higher heating temperature is maintained for longer time than other processes. Incipient fusion of grain boundaries is generally observed near the surface area where heat exposure is more. Annealing of steels not only requires higher soaking temperature, but also requires longer holding time (soaking time), which is about 1 to 1½ hour per 30-mm section of jobs or even more, if it is an alloy steel. If load is high, some additional time should be allowed for temperature equalisation. Full annealing being a high-temperature operation, may require atmospheric protection of jobs inside the furnace against oxidation and scaling of surface. If atmosphere is provided that must continue during cooling cycle as well – till the temperature is sufficiently down to about 400°C. As such, many a time, industrial annealing cycle could be very long, ranging from 24 hours to 3-4 days.

Fig. 7-3

A schematic showing the grain boundary fusion in over-heated steel

Though full annealing, requiring higher temperature of heating, is a popular method of softening steels, but not all steels or all softening processes will require high temperature of heating. Many of the industrial purpose of annealing can be accomplished by heating the steel to temperature below the lower critical temperature (A1). These processes are: sub-critical annealing, process annealing, recrystallisation annealing, inter-critical annealing, etc. These processes are used after cold-working, which helps in early recrystallisation due to ‘stored energy’ from coldworking. Figure 7-4 illustrates the state of recrystallisation of cold-worked mild steel with annealing temperature. The figure also indicates how the mechanical properties, like ultimate tensile strength (UTS), yield point,

Fig. 7-4 An illustration showing the effect of annealing temperature on the structure and properties of cold worked mild steel. (Source: Rollason, 1973)

elongation and reduction in area, change with annealing temperature. The figure illustrates that annealing of cold-worked mild steel at around 580°C can set on rapid softening of the steel, and allow recovery of ductility. Such a situation clearly demonstrates that in many cases heating to higher temperature for annealing can be avoided, thereby saving fuel and energy cost, on one hand, and necessity of atmosphere control, on the other. Based on such observations, soft annealing processes can be devised and operated. Accordingly, some of the soft annealing processes are discussed below.

Sub-critical Annealing Sub-critical annealing is the most important of all lower temperature annealing processes. This process is carried out below the lower critical temperature of steel (i.e. below A1 temperature) for softening and restoring ductility of steels. This treatment is given to steel during its processing for restoring softness and ductility for further working. In this process, the steel is generally heated in the range of 600 to 650°C for several hours. Since the starting structure of steel is cold-worked strained structure, on heating above 500°C, it starts getting recrystallised and

pearlite lamellae in the structure start getting softened by partial spheriodisation. Actual microstructure resulting from sub-critical annealing will depend on composition of the steel, original starting structure, and the temperature of heating and holding. Steels subjected to sub-critical annealing should have a ‘critical amount of prior cold-work’ for facilitating faster recrystallisation and ferrite grain growth for softening. A typical sub-critical annealing cycle of medium carbon steel for heavy forging would be heating in a batch furnace to about 670°C with soaking of about 12 to 15 hours followed by slow cooling inside the furnace till room temperature is achieved. The furnace should, however, be thoroughly purged with inert nitrogen gas before charging to avoid excessive scaling. Though sub-critical annealing of bulk items is carried out in batch type furnace, continuous annealing furnace or atmosphere protected ‘bell-type’ furnace is used for bright annealing of steel strips. For example, heavily cold-rolled thin steel strips can be continuously annealed (sub-critically) in a suitable furnace at a high speed for brightening and tempering the hardness of the strip in order to making it suitable for manufacturing of cans and containers or for applications in panelling. For such annealing, the strip is heated to about 650°C and passed through an atmosphere protected strip furnace at high speed (about 400 meter/min.) and briefly soaked and cooled quickly to avoid any ferritic grain growth, leaving the strip bright and hard. One problem of sub-critical annealing is that if heating time is prolonged and slow cooled, some steels might show ‘embrittlement’ effect due to thin carbide film enveloping the ferrite grain boundaries due to long holding or slow cooling. This type of embrittlement may lead to cracking during subsequent forming, despite the steel being soft. Hence, steel strips for forming need to be quickly cooled from the treatment temperature for avoiding embrittlement due to film formation at grain boundaries. Process annealing can be considered as a part of this sub-critical annealing, but generally applied to low carbon steels (carbon less than 0.20%) for softening. Process annealing is also used as steps for improving in-process ductility for further cold-working of the steel. The process is carried out by heating to temperature well below the A1 temperature (vide Fig. 3-2) and then holding long enough for recrystallisation of ferrite phase in the steel structure – followed by cooling to room temperature in still air. As such, the process only changes the shape, size and distribution of ferrite (referring to low-carbon steel), and thereby restores the near original ductility in the steel. The process is much cheaper than full annealing, but achieves similar purpose as full annealing for low-carbon steels.

Spheriodising Annealing Spheriodising annealing is another important annealing process carried out to modify the structure of higher carbon steels for forming and machining. Figure 7-6 shows the relative temperature (relative to lower critical temperature) range of spheriodising annealing. The process is carried out for essentially pearlitic steels (i.e. steels with 0.60% carbon or higher) and purpose is to soften the steel by spheriodising the carbide lamellae of pearlite. Since this process is not confined to prior cold-worked steels, the process may take longer time for spheriodisation. Typical application of spheriodising annealing is for improved formability and machining of high-carbon steels. At times, inter-critical annealing is also applied for spheriodising the steel structure, which may take shorter time than conventional spheriodising annealing. In inter-critical annealing, the steel is heated and held at a temperature between A3 and A1 temperature of the steel to obtain partial austenitisation and then the temperature is quickly brought down to below A1 temperature and held for spheriodisation process to complete. Spheriodisation treatment leads to evenly distributed spheroids of carbide in a ferrite matrix (Fig. 7-5)

Fig. 7-5 An illustration of the spheriodised annealed medium carbon (0.45%C) steel structure, spheriodised at 670°C for 3hrs (x400)

Spheriodising annealing is widely used for tool and alloy steels for improving its machining and forming behaviour. Figure 7-6 illustrates two different spheriodising cycles used in industries for this purpose. The modified cycle on the right show that the steel is taken to temperature above the lower critical temperature (LCT) for a short time before cooling. This step aims to better spheriodise the structure with shorter cycle time. Other important annealing processes are: recrystallisation annealing, recovery annealing, and isothermal annealing.

Fig. 7-6 An illustration showing two modes of spheriodising annealing process, though both achieve the same result. Note that the latter cycle is shorter than the former

Recrystallisation annealing is applied to cold-worked steel by heating in the temperature range of 500°C to 780°C (depending on the state of cold-work) in order to produce fresh recrystallised grains without going through the phase changes in the steel. Stored energy from prior coldworking of the steel provides the required activation energy for recrystallisation of steel at this lower temperature level and produces new grains and structure. By going to higher end of this temperature range, certain degree of grain growth can also be induced in the steel for producing softer structure. Recovery annealing is applied to cold-worked steels for reducing the level of residual stresses and to recover ductility as much as possible by heating to temperature lower than the recrystallisation annealing temperature. Recovery anneal temperature could be as low as 150°C and can go up to 550°C, but always below the recrystallisation temperature of the cold-worked steel. There is no appreciable structural change due to this treatment, but only recovery of some properties. Lower temperature range is used for relieving residual stresses, going up further when some recovery of ductility is necessary. Recovery of ductility results from partial breaking up of pearlitic carbide stringers. Isothermal annealing is the other form of annealing which is applied to steels for producing coarse pearlitic structure. In this process, steel is heated in the range of 700 to 900°C for taking it partially or fully into the austenitic stage and then slowly cooled to below the lower critical temperature and held for isothermal transformation at the temperature

of choice. Temperature of holding (with reference to the TTT-diagram of the steel) must be in the pearlite transformation region; otherwise the structure might develop some bainite due to lower holding temperature. The process produces coarse pearlitic structure (vide effect of N&G for austenite decomposition to ferrite and pearlite), which is soft and ductile. The process is used for making the steel structure suitable for easy drawing at high speed, e.g. wire-drawing of high carbon steels.

7.2.2

Normalising

Normalising is a process that involves thermal cycle very similar to full annealing, but with relatively faster air-cooling from austenitisation temperature – as against furnace cooling for annealing. In contrast to annealing, normalising process is used for improving strength of the steel along with ductility (i.e. elongation). This is made possible by producing a structure after normalising that has finer and more uniform grains than annealing. Finer grains mean more grain boundary areas – which makes steel stronger by resisting the flow of dislocation glide through them (which is necessary for plastic deformation to progress in the steel matrix). Finer grains can also absorb more strain in it – thus allowing higher elongation before fracture. Therefore, aim of normalising is to produce finer and more uniform grain size – whereby strength and ductility of the steel gets improved, as discussed in Section 1.5 of Chapter 1. Normalising is generally applied to rolled and forged steels where initial grains are relatively coarser and mixed in size – due to inhomogeneity in working and non-uniform cooling. Normalising not only produces finer ferrite grains, but also produces finer pearlitic structure due to the effect of relatively faster air-cooling on nucleation and growth (N&G) process (discussed in Chapter 2). Thus, while the aim of full annealing process is to homogenise and coarsen the structure for getting softer and more ductile properties, the aim of normalising is to create condition whereby a given structure (generally a mixed structure) gets refined and produces finer and more uniform ferrite grain size and finer pearlitic structure. Differences in the conditions for heat treatment in these two processes are: (a) Annealing requires longer holding time at the austenitisation temperature for homogenisation and grain growth, as against normalising – where holding time is less; just for austenitisation and homogenisation. (b) Annealing requires slow furnace cooling as against relatively faster air-cooling in normalising. This means, jobs in annealing remain in higher temperature for much longer time than when normalised.

These differences make the normalising cycle much shorter in time – may be 25% of annealing time cycle. As such, there is no demand for atmosphere control in the furnace, excepting nitrogen purging of furnace before the start of cycle to expel air and reduce oxygen level in the furnace for minimising surface oxidation and scaling. Annealing – especially of high carbon steels – often requires some atmosphere control in the furnace (e.g. nitrogen purging and nitrogen feeding into the furnace while jobs are at higher temperature of the cycle) during the long process cycle for avoiding scaling as well as decarburisation of steel surface.

Fig. 7-7 A graphical presentation of the normalising temperatures of steel for different carbon content

The prescribed temperature is always above the corresponding A3 / ACm temperature of the steel – though very rarely steel of hyper-eutectoid composition is normalised in practice. For the comparison purpose, it may be further noted from Fig. 3-2 that corresponding full annealing or hardening temperature of the steel is above A3 temperature for hypoeutectoid composition and above A1 temperature for hyper-eutectoid composition. As a thumb rule, normalising is carried out for steel that has maximum pearlite possibility of 60% for the best result; if the steel composition has the possibility of producing more than 60% pearlite in the structure, then the steel should preferably be annealed for softening This implies that normalising is not recommended for steel containing higher carbon. However, this is a guideline for applications where normalising is the end heat treatment – such as the use of normalised AISI 1045 steel shaft for agricultural tractor; vide Fig. 7-8(a). But, if normalising is used for homogenising or refining structure before another heat treatment (e.g. hardening), then this thumb rule does not apply. In the latter

Fig. 7-8 (a) Shows normalised structure of AISI 1045 steel (x100) used for agricultural tractor axle shaft and (b) shows the normalised structure of AISI 5120 – a lowcarbon alloy steel – for improved machining (x200)

cases, adoption of normalising will depend on the purpose; even lower carbon steel can be normalised when necessary (vide Fig. 7-8b). For normalising of steel, austenitisation temperature of about 30°C above the upper critical temperature (i.e. A3 + 30°C) of the steel is used (unless it is for homogenisation purpose when a holding temperature of A3 + 50°C can be used). Jobs are held at that temperature for sufficient time required for full austenitisation, and not more. (In this regard, the process again varies from annealing, where in addition to austenitisation time is allowed for controlled grain growth for softening). After austenitisation, the jobs are air-cooled outside the furnace. Precautions for heating are similar to annealing process, i.e. the heating should be gradual in order to avoid any thermal shock or thermal gradient which can lead to distortion during heating – especially if the parts are long and slender. Similarly, during cooling hot jobs need to be handled carefully for avoiding bend and distortion. For this reason, normalising is often carried out in ‘bogie hearth furnace’ – where the jobs can be handled without disturbing while hot. Alternatively, jobs can be placed on pre-designed fixtures and fixture along with the jobs can be taken out from the furnace and allowed to cool in still air on a cooling platform. Because of the difference in purpose from annealing, normalising is generally applied to steels of medium carbon grades with carbon tentatively ranging from 0.20 to 0.50%C. Purpose of normalising these steels is to make the ferrite–pearlite structure finer and uniform, improving the strength and ductility (coming from finer grains size) or for better machining and processing. For optimum result after normalising, it is often recommended that the steel should have carbon above 0.30% or if carbon is less, then steel should have some alloy content like Cr, Mn, etc – in order to produce ferrite–pearlite structure somewhere near in the

ratio of 40:60. Normalising produces finer ferrite grains and finer pearlite lamellae, which contribute to improved strength and elongation, but too much pearlite content in the structure might increase hardness and pose problems in uses and applications, such as for machinability. Normalising is often used for improved machinability of steel structure by controlling the ferrite–pearlite balance, which induces better chip breaking characteristic. Ferrite grain boundary or ferrite-pearlite boundary acts as point for chip-breaking during high-speed machining. But for steel composition of higher carbon where percentage pearlite on normalising might exceed 60%, it is better to anneal the steel for getting better forming / machinability properties. This is the reason why higher carbon steels or cast irons are often annealed. Within the composition limitation, normalising is widely used for making the structure of as-forged or as-rolled steel more uniform. In as forged or rolled structure, steels will generally have non-uniform structure due to non-uniform working over the section; more so near the surface region where cooling rate is also faster. Such non-uniform structure can lead to non-uniform hardness, causing problems for fast machining. Normalising is used with advantage to make the structure as uniform as possible, especially near the surface where machining operation is generally carried out. As such, normalising of forged or rolled steel sections are widely used in industries for (a) better machining – which is a common manufacturing process for all engineering industries, and (b) better response to hardening operation, due to starting structure being more uniform. However, normalised structure may not be as uniform over the entire cross-section as in the annealed structure. This is because of faster cooling in normalising – which sets up some temperature gradient and cooling rate difference between surface and the centre. But, any such non-uniformity of the structure after normalising is limited deep inside the core of the section and does not pose any problem for machining. Machine shop is the most important customer of normalised jobs; hence, application of this process is often planned and designed to suit that end. Normalising is not, however, as good as annealing for homogenising the structure, because of shorter cycle time and faster cooling. Common areas of applications of normalising are: forgings and castings, heavy section hot-rolled plates, wheels and axles, and such other component which require structural integrity, more uniformity in structure, better machinability and better response to hardening. Traditionally, forged automobile components are normalised for better machining and response to quench hardening. However, now-a-days, attempts are being made to control cool the forged parts from finish forging temperature for

simulating structure in the steel similar to normalising. If such process modification can be successfully implemented, it can save considerable energy and cost.

7.3

HARDENING OF STEELS

Hardening of steel under thermal heat treatment processes comes under through hardening and surface hardening of steels. In through hardening process, steel parts are heated for austenitisation in a controlled atmosphere furnace, held for a short time and then quenched in a liquid bath for hardening throughout the cross section. But, in surface hardening process, only the job surface is heated locally by using flame heat or induction heat for austenitisation and immediately cooled by suitable quenching method – such as by spray cooling or by dipping in a fluid or under forced air jet for limiting the hardened depth to a specified minimum. In surface hardening, structural changes are only in the surface region and core of the steel is not affected, but in through hardening process changes in structure take place all through. In both types of hardening, the process is involved with austenitising the steel at higher temperature and then cooling rapidly for transformation to martensite, avoiding formation of any softer structure due to slackness of cooling. Cooling is the key to success for hardening – both for through and surface hardening – which should be just right for developing martensitic structure, but not too fast for producing distortion or crack in the components. The means, methods and mechanisms for achieving this task of hardening depend on number of factors, such as control of: • Steel hardenability factors (Chapter 4) • Quenching mechanisms (Chpater 6), and • Heating efficiency and effectiveness (Chapter 5). These controls are necessary in addition to correct process plans as per metallurgical transformation characteristics of the steel, discussed in Chapter 2. Process plans for hardening of steel are based on its transformation characteristics, which have been discussed in Chapter 2. Austenite decomposition under fast cooling non-equilibrium condition lays the foundation and principles of martensitic hardening of steels. Quality attributes of steel for correct response to hardening have been highlighted in Chapter 4 under heat treatability of steels. Care is necessary to select steel grade fulfilling the condition of hardenability for attaining desired structure profile in the steel section after hardening.

Hardening of steel is also very sensitive to how the steel has been heated in a furnace (e.g. rate of heating, control of temperature, control of atmosphere, etc.) and has been cooled from the austenitisation temperature (i.e. the quenching technique). Control of quenching mechanism – which might vary with change in steel composition, shape of the job, and precision in quality required – is of critical importance. Based on aforesaid controls, different types of martensitic hardening operations (excepting thermo-chemical processes) have been discussed in the following sections. For convenience, hardening processes have been divided into two groups for discussions; one ‘Through Hardening’ processes and the other ‘Surface Hardening’ processes – although their metallurgical principles are same, i.e. creating conditions for martensitic transformation of the steel.

7.3.1

Through (Bulk) Hardening of Steel

Process Features and Steps in Hardening Operations Through hardening is the most widely used heat treatment process amongst all. It is carried out to alter the microstructure by temperatureinduced transformation from softer ferrite–pearlite based structure to harder martensite or martensite–bainite structure. The process involves heating the steel to about 30 above upper critical temperature (A3), soaking for time adequate for converting the original structure to 100% austenite, followed by fast cooling (quenching). Figure 7-9 illustrates the schematic hardening cycle, involving heating and controlled cooling with reference to critical temperatures of steel. This is the cycle that is commonly used for quench hardening of steels. As such, the process steps of hardening will be first discussed with reference to this diagram and then illustrated with the help of some industrial furnaces and practices. Since hardening is the finishing operation, care has to be taken in heating and cooling so that the components do not distort either during heating or cooling from the austenitisation temperature. Hence, heating rate has to be gradual and controlled in hardening for preventing distortion or warping of components, especially if they are long and slender. This is done by placing the charge in a pre-heating zone or lower temperature zone of the furnace and then gradually taking up the temperature as the charge moves in. Temperature control within the furnace should be as close to target temperature as possible, preferably within +/– 5°C for hardening.

[Changes: (Ferrite + Pearlite) Æ Austenite Æ Martensite / (Martensite + Bainite)]

Fig. 7-9 A graphical view of heating, soaking and fast cooling cycle of hardening process. [UCT and LCT are upper critical and lower critical temperatures, respectively]

Referring to Figure 7-9, next control is the soaking temperature and time. Soaking temperature for steels is generally kept limited at about 30°C above the A3 temperature of the steel. Higher soaking temperature may lead to coarser austenite grains, leading to coarser martensitic structure. Figure 7-10 shows the effect of increasing austenitisation temperature on the hardened structure of steels.

Fig. 7-10 Micrographs of low-alloy steel samples austenitised at (a) 900°C, (b) 1000°C, and (c) 1100°C and quenched – indicating relative coarsening of parent austenite grain size with increasing austenitisation temperature and its effect on the coarseness of martensite. [Micrographs are at same magnification (x200)]

Soaking time for austenitisation is generally 30-mm cross-section; highest section thickness the time. Generally, lower soaking time (e.g. 30 carbon steel, while longer soaking time (e.g. 45

30 to 45 minutes per in a piece determines min.) is used for plain min.) is used for steel

with complex carbide forming alloying elements (e.g. containing Cr and Mo). Most industrial hardening furnaces operate with 2-door facilities; one for charging and the other for discharging. Hence, charges move with time from charging door to discharging door through graduated temperature zones for ensuring effective hardening. Hardening of steel takes place by quenching in a suitable bath of quenchant, e.g. water or oil – as per hardenability of the steel. There should not be much time lag between charges coming out of the furnace and going into the quench tank – because of the chance of initial separation of ferrite or pearlite if held in the inter-critical zone of temperature. Therefore, quench bath should be close to the furnace discharge door and jobs should dip (or dropped) into the quench bath quickly, but without impacting upon each other. Other aspect of heating for hardening of steel is the control of atmosphere within the furnace so that there is no chance of surface oxidation or decarburisation – which may interfere with attaining right surface hardness. Means and methods of atmosphere control in hardening have been discussed in Chapter 5. However, for hardening, atmosphere has to be either exactly neutral or slightly reducing for avoiding oxidation and decarburisation – which is unlike thermo-chemical hardening where atmosphere should be rich and appropriate for ensuring carbon or nitrogen diffusion. Hardening can be carried out under inert gas atmosphere (e.g. high purity nitrogen gas), inert gas plus a small amount of hydrocarbon gas (e.g. natural gas) or in vacuum. These aspects of hardening have been discussed in Chapter 5 under furnaces and heat treatment atmosphere. Next step is making the quenching effective for removal of heat as per process / quenching plan. As discussed in Chapter 6 for quenching, the quench tank controls are: • Control of fluid volume vis-a-vis charge volume – so that the bath can quickly extract heat without heating up beyond a level. Bath temperature for oil-quenching should be controlled preferably below 60°C. • The bath (oil bath) should have mild agitation for breaking down the vapour blanket stage of cooling and improving heat extraction. If water quenched, flow of water to and from tank should be controlled for improved cooling. Mild horizontal flow of water is preferred for efficient heat extraction. • Jobs should stay inside the quench tank till temperature is well below the martensite transformation temperature of the steel. Generally, when oil quenched, oil bath temperature of about 55 to 60°C is maintained for efficient quenching. The jobs can then be air-cooled and washed in a flowing water tank, if necessary.

Quenching is a critical step in the hardening of steels. As such, details of quenching mechanisms and quenching characteristics have been discussed and illustrated in Chapter 6. Important point to note with regard to oil-quenching is that there should be control over the oil bath temperature for controlling viscosity of the oil, but without allowing further rise in bath temperature which may decrease cooling rate and risk higher rate of oxidation of oil bath – increasing sludge formation and inconsistency in cooling. Other than quenching for cooling to produce as much martensite as possible in the structure, quenching process should be controlled for possible distortion / cracking of jobs. These factors – including chances of warping and distortion during heating – call for certain features in the heat treatment furnace for efficient hardening. They are: • Furnace should have a door shield (e.g. flame curtain type) for controlling oxygen ingress during charging • Furnace should have a pre-heating zone at the charging end for gradual heating of the parts • Should have a soaking zone for attaining uniform temperature, which depend on time, section size of jobs, and charge load • Should purge the furnace with inert gas in the beginning for removing oxygen from the furnace and then ensure flow of appropriate ‘atmosphere’ at regulated manner for protecting the surfaces from any oxidation reaction; oxygen generally comes from the furnace leakages and moist charges with traces of oils and lubricants • Install door flame curtain or shield at the discharge end of the furnace to avoid temperature drop inside the furnace, leading to temperature fluctuation, and ingress of oxygen from outside. Construction and operational features of all hardening furnaces must provide these facilities for fault-free operations. As such, understanding of these operational features of hardening is necessary for understanding the process of hardening. Principles of hardening by martensitic transformation have been discussed and illustrated in Chapter 2 under austenitic decomposition under non-equilibrium cooling – referring to CCT diagrams of steels. It has been pointed out there that TTT or CCT diagram of steels are composition specific – unlike Fe-C diagram under equilibrium cooling. This means that for deciding the hardening parameters, only the composition specific CCT diagram of the steel should be referred, which represents the transformation behaviour of the steel under continuous cooling condition that prevails in shop floor practices. Respective CCT diagrams of the steel (vide Chapter 2) provide (a) temperature of austenitisation,

and (b) critical cooling rate that must be ensured during cooling for martensitic transformation of the steel. However, while these parameters can be derived theoretically by referring to correct CCT diagram for industrial heat treatment, success of the process depends much on how the steel has been heated, protected from undesirable surface reactions, and quenched for martensitic hardening fulfilling the critical cooling rate. Thus, in practice, hardening of steel additionally requires knowledge of furnaces and heating technology, quenching techniques for effective cooling, and control of temperature, atmosphere, and component handling mechanisms for good heat treatment. Based on these specific requirements and steps for hardening of steel, the process of hardening has been illustrated in this chapter with reference to different types of furnaces and furnace features used for the process. Understanding of furnace features along with the process logic is necessary, because furnace type and furnace control are of critical importance for getting the right results in hardening.

Furnace Characteristics and Controls for Hardening Perhaps, the process of hardening and its control can be best illustrated by referring to furnaces and their operational controls. Hence, purpose of this section is to illustrate the hardening operation and its control with reference to a popular furnace and its characteristics and control. Since, most of the discussions and description of furnaces in Chapter 5 have been with respect to hardening operation (e.g. seal-quench furnace, vacuum furnace, salt bath furnace, fluidised-bed furnace, etc); this section will not reiterate those furnaces and their operation here. Instead, this section will illustrate the hardening operation by referring to another very important furnace, the ‘Rotary hearth furnace’ – a popular semi-continuous or continuous furnace type used widely for hardening of critical components requiring distortion control. Hardening furnaces can be batch type or continuous hardening furnace; choice depends on the size of the job, production volume and tolerance of variation in heat treated properties. Unless the jobs are small and production volume is large (e.g. hardening of spring clips, springs, washers, pins, small gears, etc.), most other jobs are hardened from batch-type furnace, where yield can be substantially improved by improving heating efficiency and atmosphere control. For example, ‘seal quench furnace’ – discussed in Chapter 5 – is a very popular batch-type hardening furnace where productivity of symmetrical jobs (e.g. gears) can be considerably improved and used as semi-continuous furnace with auxiliary support system. Rotary hearth furnace is the example of a popular continuous

furnace for hardening with many advantages, especially for components where distortion could be a problem. Distortion control in hardening operation is a critical function, and this should be taken care of either by designing proper furnace bed for continuous operation or fixture design in batch type furnace to hold the jobs in a manner that minimises distortion (vide hardening in pit furnace in Chapter 5). There is no bar in using any type of furnace for hardening – as long as the furnace ensures efficient heating, atmosphere control and efficient material handling system for easy charging and discharging for quenching. Yet, there are few types of furnaces which are popular for hardening operations. Some of these furnaces have been discussed in Chapter 5 – such as seal quench furnace, vacuum furnaces, pit type furnace and salt bath furnace. Present chapter, therefore, focuses illustration of hardening process with reference to ‘rotary hearth furnace’ – a popular furnace for hardening of distortion sensitive automobile components, like crownwheel and axle transmission gears after case carburising. This illustration will sufficiently highlight the factors required to be controlled in the through hardening of steels. Figure 7-11 gives an over view of a ‘rotary hearth furnace’, which finds uses in heat treatment shop floors for hardening. The furnace is provided with two doors with flame curtains to protect ingress of air within. The hearth is a flat bed, made up of heat-resistant alloy steel plates or silicon carbide plates, if operating temperature needs to be higher than normal. From operation point of view, rotary hearth furnace for hardening can be electrically heated or gas-fired. As the name implies, the hearth containing charges rotates on a rack-and-pinion system. The hearth is made of heat-resistant high-alloy steels or silicon carbide Fig 7-11 An illustration showing hearth plates and care is taken that popular brand of ‘Rotary Hearth no impingement of flame on the job Furnace’ with charging and discharging takes place inside. The charge on the doors – shown on the left and right rotating hearth passes through the gradually increasing temperature zones (i.e. gradually heated), reaching the soaking temperature towards the middle of the hearth. The next zone in the furnace is the soaking zone.

Rotary hearth furnace is capable of heating up to 1250°C, though most hardening of steel is carried out in between 830 and 930°C, depending on composition and alloy content. In principle, jobs should be heated to as low temperature as possible above A3 temperature of the steel for minimising distortion, and decarburisation. Hence, thumb rule of (A3 + 20 to 30°C, max) temperature norm is used for hardening of steels. The rotation cycle can be set as per heating requirement and the furnace can be provided with continuous feed of protective atmosphere, e.g. endothermic gas generated outside and fed through a mechanised system into the furnace. After soaking cycle is over, the charge can be taken out from the exit door end and directly quenched in the adjacent facility of quenching press / die or a quench tank, as necessary for distortion control. Quenching media could be water or oil depending on the steel hardenability, but for most machined components for engineering applications high-speed quenching oil is used for quenching. Quenching oil needs to be optimally cooled (by circulation through heat-exchanger) for higher quenching efficiency and viscosity control (vide Chapter 6 on Quenching and Quenchant). Oil temperature is continuously monitored and kept within 55°C. Thus, rotary hearth furnace fits well to the stages of quench hardening, involving: • Gradual heating to the soaking temperature • Heating and soaking cycle carried out under protective atmosphere • Control of soaking time by control of hearth rotation speed • Flat bed resting of components for least warping and distortion during heating • Facility and flexibility for atmosphere control within the furnace • No flame impingement on the charges • Temperature stability and uniformity within the furnace due to sealed door • Facility to build necessary quenching facility next to the exit door, so that quenching can be carried out without delay time. Because of atmosphere control inside, jobs come out clean. The process fits well for hardening of critical components like automobile crown wheels, ball bearing races, spring steel disks and blades, and many such components. In fact, rotary hearth furnace is suitable for continuous hardening of all gears and precision engineering parts that are required to be heated flat on a flat bed to control distortion. Heating cycle for different job configuration can be adjusted by varying the rotation speed of the hearth.

Other than furnace and furnace atmosphere, hardening of steels demands consideration of the following factors, which have been pointed out earlier but reiterated once more for consolidating the factors for successful hardening. • Hardenability of the steel being hardened • Quenching rate and medium to be used • Control of oil / quenching bath temperature • Loading and fixture for holding the jobs under heat treatment to avoid distortion • Control over rate of heating to avoid distortion of components • Control over soaking time as per cross-section of jobs (generally 45-min soaking per 30-mm section is recommended), and • No time delay for quenching. Theoretically, hardening can be carried out in all types of furnaces, starting from laboratory muffle furnace to modern fully automated vacuum furnace. But, quality of out-put will vary and might not fit to jobs requiring precise control of heat treated properties. Popular continuous furnaces for hardening are rotary hearth for medium size jobs and meshbelt furnace for small jobs (like springs, clips, discs, washers, etc). For large size jobs, hardening is generally carried out in pit type or box type furnace with necessary control of atmosphere and with proper arrangement for hot charge handling and quenching. However, quenching of large jobs in oil carry the risk of oil fire – due to oil temperature in contact with the surface can exceed the flash point of the oil and catch fire. Heat treating shop floor carrying out such oil quenching for hardening must have suitable fire protection system at hand for dealing with the emergency. Now-a-days, high flash point oils are available for reducing the chance of catching fire – though cost of the oil will be much higher. The other method of hardening of precision engineering components is vacuum hardening, where surface quality with regard to freedom form oxidation and decarburisation is far superior to other processes, heating and cooling is more clean, efficient and uniform, and the resultant structure is far superior. Essential aspects of vacuum heat treatment have been already discussed in Chapter 5, which hold good for hardening of steel as well. But, vacuum hardening is very expensive and may not be amenable to handling large jobs. The vacuum process is also popular for hardening of martensitic stainless steels and tool steels – resulting in higher service life, which might off-set the added cost of vacuum treatment. For more about vacuum furnace characteristics and features for operation, vide Section 5.5.2 in Chapter 5.

7.4

FLAME/INDUCTION HARDENING

Processes of flame and induction surface hardening have been briefly introduced in Chapter 3, and discussions in this chapter are extension of those, but with highlights of process control. Discussions in this chapter aim to highlight practical aspects of flame and induction hardening process from practice point of view. Flame and induction hardening are another set of thermal heat treatment process, which is carried out for controlled hardening of surface area upto a specified depth. Unlike case-carburising where surface area is further alloyed with carbon for attaining the specified hardness after hardening, in flame or induction hardening process, steel composition is so selected as to ensure right response to the process. When the heating mode is flame, it called ‘flame hardening’ and when the heating mode is ‘induction heating’, the process is called induction hardening. Figure 7-12 shows illustrative profiles of surface hardening, which are aimed at by using induction/flame hardening process. If controlled adequately, these processes can produce quite uniform and controlled case depth of martensitic structure in steels.

Fig. 7-12 A schematic showing hardening profile of case and core in gears. Such uniform case depth is obtainable by appropriately controlled induction hardening process

Both flame and induction hardening processes consist of heating the surface to be hardened above the A3 temperature of the steel, followed by rapid cooling. Heating produced by flame and induction system is

generally very intense and can be confined to locality. Due to high heat, there is hardly any holding (soaking) time required for heating. Therefore, the job is cooled soon after heating is over. Cooling can be by conventional quenching using oil or water, as required by the steel grade or by forced air or gas, depending on the hardenability of the steel and profile of the component. Steel which is to be surface hardened must contain sufficient carbon and/or alloying for hardenability – so that the steel responds to quenching for production of martensitic structure. Both the processes require light tempering or stress relieving of the hardened surface for reducing quenching stresses and improving toughness to some degree for handling and impact resistance. Amongst these two processes, induction hardening is amenable to better control of process and capable of producing more precise depth of hardening and hardening pattern than flame hardening. Figure 7-12 illustrates the uniformity of hardened depth possible by induction hardening. But, in practice, induction hardened depth is rather limited compared to flame hardening – which is often applied to large parts where thicker hardened depth is required. Because of the possibility for precise control, induction hardening is very popular for applications in automobile and engineering industries for hardening of pins, cams, journals, and many other small as well as medium size parts (e.g. crankshaft pins and journals). Induction hardening of such small parts is often carried out in the machining line itself for flow of production process.

7.4.1

Flame Hardening

Flame hardening is carried out by heating a specified steel surface to above A3 temperature by using high-temperature oxy-acetylene flame produced by using oxygen and acetylene mixture. Flame could also be made up of natural gas, propane, etc., mixed with oxygen in a manner that produces either neutral or slightly carburising flame. Flame directly impinges onto a specific surface area to be hardened; hence, the flame tip temperature has to be controlled so that it does not cause melting or incipient fusion of the surface. Process of heating can progress either by movement of the flame torch or by movement of the job with respect to a stationary torch. The hardening result is controlled by five factors: • Design of the flame head and flame character • Flame to work surface distance • Duration of heating • Temperature to be heated up to • Composition of the steel and the quenching method

The process alters the structure of the surface area only, leaving the core structure intact. Depth of flame hardening is limited to about 0.8 mm to 6.25 mm, though it can be increased if necessary. But, this type of hardened structure often leaves harmful tensile residual stress on the surface if the hardened depth is too high compared to diameter of the job. Figure 7-13 illustrates a flame hardening process in progress.

Fig. 7-13 A schematic showing a flame hardening process in progress, showing quenching arrangement of the process

Depth of the hardened zone may be controlled by adjustment of flame intensity, heating time or speed of travel. Process needs to be carefully controlled so that job is not over heated, which may result in cracks after quenching. After hardening, the job should be stress relieved by heating in the range of 180 to 220°C and air-cooled. Stress relieving should not appreciably reduce the surface hardness. The process is used mostly for plain carbon steels, carbon content ranging from 0.35 to 0.60 percent, but alloy steels can also be hardened with more controlled heating; because austenitisation of alloy steels take longer time than plain carbon steels and this longer time of heating may result diffused surface hardened zone. For improved result, jobs can be given prior normalising treatment for uniform (ferrite–pearlite) structure of the steel, which responses better in flame hardening. Large load bearing jobs, where flame hardening is applied for wear resistance of the surface, are always given prior normalising for uniform response of hardening. Advantages of flame hardening are: • A useful, flexible and economical method of surface hardening with attainable surface hardness depth higher than case-carburising • Large machined parts can be surface hardened economically for wear resistance e.g. machine bed

• Surfaces can be selectively hardened with minimum warping and freedom from quench cracks, if not over-heated • Due to short heating cycle, there is little scaling or decarburisation. However, the process is difficult to control for thinner depth of hardening and needs trial and error method to establish time of heating for precise control of hardened depth. Application wise, though the process can be applied to plates, shafts, gears and others, it is popularly used for shafts requiring surface hardness for wear resistance or as bearing seat. Figure 7-14 shows one such illustrative case of flame hardening of plates and gears. Heating torch

DOT : Direction of Travel Quenching water

DOT Flame

Hardened surface

Quenching jet Blow pipe

Heated surface Workpiece (a) Principle of flame hardening

Gear (b) Flame hardening of gear teeth

Fig. 7-14 An illustration showing flame hardening process of (a) plate surface and (b) gear tooth

Flame hardening of gear teeth requires control over heating zone, which should not penetrate the gear root and radius. Failure of control can result into non-uniform heating of tooth radius and may cause brittleness of root areas, which experience high bending load under fluctuating stresses (i.e. fatigue loading). Hence gear teeth hardening by flame process needs very strict control, which is difficult at time for longer heating and heat penetration by conduction. Because of its simplicity of set up and low fuel cost compared to electrical heating, attempts are being made to make flame hardening process more productive and versatile. Figure 7-15 shows one such set-up of flame hardening of a large diameter shaft where the heating gas flow can be controlled through flow meters and distributed through ring-type burner for even distribution of heat while the job uniformly rotates. In general, flame hardening is a simpler operation than induction hardening, but cannot handle difficult profile for hardening. Also, flame hardening is slower than induction hardening and less productive; it cannot

Fig. 7-15 An illustration showing the set-up of a sophisticated flame hardening arrangement of large diameter shaft

match the quality or productivity requirement of mass production shop floor.

7.4.2

Induction Hardening

Induction hardening makes use of electro-magnetic field to generate induced eddy current to heat up jobs. In this process a magnetic field is passed through the coil (called inductor), inducing current for heating. This phenomenon is called ‘electro-magnetic induction’. The current intensity can be controlled by varying the frequency of power supply, and this is the mechanism by which heating depth (or the hardened depth) of a job can be controlled. As such, the process can be highly automated for precision control of heating and hardening; in contrast to flame hardening where the process is not so amenable to automation. Figure 7-16 shows a simple laboratory based induction hardening set-up, demonstrating the essential features of induction hardening process. The figure illustrates that the process requires: • A water cooled copper tube which acts as inductor • Current sources with frequency control, and • Cooling system for quenching the jobs. In actual production set-up, all these features can be integrated closely and controlled from a single point. Some of the features of induction hardening have been already highlighted in Chapter 3; this chapter is a further elucidation of the same.

Fig. 7-16 An illustration showing the laboratory induction hardening arrangement of a steel pin shaft

Following this basic approach to induction heating, the process been well developed for adaptation in the industry, fitting to the need of hardening from simple round pins to complex collar of crankshaft journals. Most critical part of induction hardening is the design of inductor, especially for profile hardening. If suitable inductor can be designed for a profile hardening, then the scope of induction hardening is unlimited. Most important part of any surface hardening process is the control of case depth (i.e. hardened depth). In this regard, induction hardening has clear advantage over the flame hardening, because the depth of heating is strongly dependent on the frequency of the alternating current used to induce eddy current. Higher frequency produces lower depth of heating and vice-versa for a material of given resistivity. Thus, power frequency required for the induction heating is higher for lower depth of hardening, decreasing with increasing depth. Table 7.1 shows the recommended frequency range for different case depth in steel.

There are two alternative methods of induction hardening: conventional ‘scanning hardening’ (or progressive hardening) and the ‘single-shot hardening’. Scanning or progressive hardening involves relative movement between the job and the induction coil, heating the required area over a time controlled by the speed of travel. Generally, the coil is fixed and the job travels through the coil, continually exposing the areas of heating. Scanning induction hardening process can again be divided into vertical and horizontal hardening. In the vertical method, the job is held stationary in a fixture and the coil moves along the length. But, if need be, arrangement can be changed to stationary coil with the job progressively moving through the coil. The heating is followed by quenching for hardening. Quenching of induction heated job has to be carefully designed so that the job does not distort or bend due to stiff temperature gradient set up by part quenching or progressive quenching, especially if the job is long, such as automobile axle shaft. Induction hardening of automobile axle shaft is an important example of its application. Automobile axle shaft is a long round bar with flanged end. This requires protection of flange radius from hardening i.e. controlling the hardened surface only on the shaft diameter and stopping some distance away from the radius. However, there are some exceptions to this design of hardening where hardened zone is extended upto the flange radius. There could be two ways of induction hardening the axle shaft; one vertical method, and the other is the horizontal method of scanning and heating the surface. In vertical scanning method, the job is held vertically in a fixture and gently rotated while the inductor coil moves up to heat the required length of the job. Problem with such vertical holding is the chance of buckling and distortion due to longitudinal thermal expansion of the job while being heated gradually. To overcome this problem, jobs can be heated horizontally in a process where the coil is stationary and the job is passed through the coil for heating, followed by quenching. Horizontal holding of long jobs apparently causes less distortion than vertical holding. In single shot hardening, the respective area to be heated in a job is first heated in one go and then quenched. Hardening of shaft pin as shown in Fig. 7-15 is an example of this process. This method uses multi-turn coils that encircles the area to be hardened, which is heated in one go. However, for job lacking rotational symmetry, an inductor that follows the contour of the job could be used. This requires skill for designing proper inductor coil matching the job profile as well as hardening profile. Such type of inductors are also required to be designed to push extra heat

into areas such as fillets of a flanged shaft or a radius, which otherwise would be left under hardened or with improper hardening profile. Single shot hardening minimises distortion because the entire job is heated at a time. This offers improved results for jobs with complex contour / profile or sudden change of section / diameter, e.g. gears or pins. Hence, single shot method is typically used for components of shorter length with steps and radius (as in pins) or irregular / complex shape (as gears). Scanning process of hardening is fast and simple, consumes less energy than single shot. Single shot heats up the whole area together, requiring more power, and the inductor design is often more complicated due to necessity to match the job and hardening contour. However, the choice of process is not by the availability or economy, but by the characteristics of the jobs to be hardened. A long and larger diameter shaft, for instance, would preferably be hardened by scanning process, because power needed for single shot to heat up the whole area at a time will be too high, though distortion will be less. But, jobs with complex profile (such as gears) will have no alternative but single shot heating with carefully designed inductor of matching profile. For small gears, induction hardening can be done by using solenoid coil inductor that encircle the entire perimeter of the gear and get hardened at one shot. But, for large gears, such hardening will require huge induction generator for power supply. Therefore, for large gears, tooth by tooth hardening or segment wise hardening can be resorted to. Figure 7-17 illustrates a progressive induction hardening set-up for pinion shaft – where pinion teeth are heated encased in an inductor.

Fig. 7-17 An illustration showing progressive induction hardening set-up of a pinion shaft with fixed inductor and moving job

Regardless of induction hardening method being used, inductor design is the most critical part of success. This is because inductor coils are to be designed for a specific task of heating, producing the right pattern of heating and right profile of depth of hardening. Some notable examples of induction hardening applications are: hardening of automobile crankshaft pins and journals, axle shafts, pinions, gears, and rail heads, in addition to numerous other small and cylindrical parts, such as small parts for automobile manufacturing, that are regularly used in industries. Popularity of induction hardening of small parts is due to its high productivity, cleanliness of environment and consistency in the hardening. If the steel composition is within close range of the specification limits, surface hardness with close tolerance of (+/– 2 HRC) can be achieved. However, high-sulphur fast machining steels are generally not recommended for induction hardening (or flame hardening) because of sulphur stringers than can cause cracks on quenching from high heat. The induction hardening process is also being increasingly used for hardening of small and large gears by high frequency single shot induction heating process where thin case depth is required. High frequency induction hardening has been found to be excellent for tooth hardening of large gears, giving thin but uniform case depth in the tooth flank as well as at the tooth root by using profile inductor. If the gear is too large, hardening can be done by tooth to tooth heating, progressively passing the inductor between the roots of adjacent tooth, without allowing softening of the core. Induction heating and hardening is highly flexible process, which can be used for any type of surface hardening by adopting right inductor design and frequency for heating. Induction heating is also used for selective softening of steel parts where existing hardness is required to be lowered for reduced notch sensitivity. An example of this application is softening of key holes or ring clip seat grooves in shafts and pinions.

7.5

TEMPERING AND STRESS RELIEVING OF STEELS: PURPOSE AND PRACTICE

Steels hardened to martensitic structure are strong but brittle – due to highly strained structure of martensite arising from super-saturation of carbon, in one hand, and martensitic volume expansion during austenite transformation, on the other. Therefore, if quenched jobs are held for a time without stress relieving or tempering, they can develop fine cracks and distortion. Cracking is a natural tendency of a highly stressed body – in order to lower its internal stress (i.e. energy) level. Cracking releases

locked-in stresses. As quenched martensitic structure, which has high locked-in stresses, therefore, can develop cracks – either during quenching (if stress level is too high) or with time, if left without stress relieving. The problem of cracking of untempered martensitic structure with time is more with higher carbon steels or higher alloy steels (e.g. tool steel) – where the locked-in stresses is higher. Therefore, all steels after quenching requires either stress relieving or tempering to relieve locked-in stresses, and for high carbon or high alloy steels, this stress-relieving must be carried out as soon as possible after the quench hardening. Stress relieving of as-quenched martensite can be carried out either by separate stress-relieving treatment at around 180 to 220°C or integrated with the conventional tempering treatment at temperature of 300°C and above (but below the A1 temperature). For example, simple stress relieving treatment at temperature between 180 and 220°C is given to case hardened steels after case-carburising or induction / flame hardening, whereas full tempering at 450°C or above is given to engineering steel parts after quenching for improved toughness. Case hardened jobs are stress relieved only at lower temperature – because of the necessity to retain as much surface hardness as possible, but without any tensile residual stress. Hence, such jobs are stress relieved at the temperature range of 180 to 220°C – which ensures sufficient stress reduction for stability of the structure but without much reduction of original surface hardness. This is one part of stress relieving treatment; the other part is the stress relieving of any steel part, with or without heat treatment, where some internal stresses have been allowed to build-up due to cold working or cold processing (vide recovery annealing discussed earlier in this chapter). Purpose of such stress relieving treatment is to recover the original forming properties of the steel which got affected due to coldworking or processing. Such stress relieving treatment can be carried out at a slightly higher temperature, generally in the range of 200 to 350°C. Purpose of such limited stress relieving for such cold worked or processes steels is to relieve internal stresses accumulated from the working, but without appreciable change in the structure. If structural change is required, such cold-worked steels are subjected to one of the annealing processes where heating is below the lower critical temperature and involves partial recrystallisation. Full tempering of as-quenched martensite is, however, carried out at temperature higher than stress relieving temperature – because purpose of such treatment is to restore as much toughness in the steel as possible within the specified strength level. Tempering of the structure is done at temperature higher than the stress relieving temperature; generally above

350°C. Theoretically, minimum tempering temperature should be above the MS temperature of the steel. Metallurgical process change during tempering has been discussed in Section 3.4.4 in Chapter 3. Thus, selection of process between stress relieving and tempering depends on the purpose. If maintaining as high hardness as possible for simple wear resistance of the part is the requirement, then the part should be stress relieved for maintaining a surface hardness level of 60 +/– 2 HRC. On the other hand, if purpose is to induce or restore as much toughness as possible within a specified strength range for high fatigue resistance, then tempering is carried out; tempering temperature and time being the function of final strength required and steel type e.g. plain carbon steel or alloy steel. Alloy steels, especially the one containing Cr, Mo and V require higher tempering temperature for a given combination of strength and toughness than plain carbon steels. Exact tempering temperature and time will depend on (a) composition of the steel and (b) required degree of toughness (combination of strength and ductility, as a measure of resistance to crack propagation through the steel). Figure 7-18 shows an illustrative hardening and tempering cycle of steels.

Fig. 7-18 An illustration of quenching and tempering cycle of steels

Both stress-relieving and tempering require closely controlled heating and standard cooling in the air after the treatment (except in cases of steel that can show temper brittleness – which has been discussed later). Since the temperature range is low, air circulating furnace is used for these treatment where the rate of heating is by convection mode and more uniform. Typical furnaces for tempering are the box-type or muffle type air circulating furnace heated by electrical heating. Due to lower

combustion, gas fired furnaces are not generally used for tempering or stress relieving. However, salt bath furnace can be used, if necessary. Stress-relieving process is not involved with any significant change in structure; if any change occurs that is the formation of epsilon-carbide (Fe2.4C) – a transient carbide phase, which does not alter the hardness significantly. But, tempering is a process where change in both structure and strength are desired. Tempering process parameters are planned for optimising the strength with toughness by changing the structure. However, changes in structure on tempering occur in stages – as per heating, which has been discussed in Section 3.4.4. Martensite structure produced on quenching can be of two different types; one is the lath or plate type martensite which forms from plain carbon steels or steels with lower in carbon content, and the other is the needle type martensite which forms from higher carbon steel or some alloy steels containing Cr, Ni, Mo type alloying. Therefore, change in martensite structure on tempering has been illustrated with respect to these two types of martensite in Fig. 7-19.

Fig. 7-19 A schematic presentation of stages of microstructural changes with tempering temperature in lath (plate type) and needle type martensite, present in some low carbon plain steel and high carbon plain steel or alloy steels, respectively. (Source: E.C. Rollason, 1963)

The figure illustratively shows the stages of martensite tempering in these two major types of martensite. Upper set of sketches illustrate the stages of tempering of lath type martensite which are generally present in lower carbon steels, and the lower set illustrates the change in needle type martensite, present in alloy or high carbon steels.

Upper set of the figure (marked a-d) illustrates that the following: • Lath martensite has dense dislocation tangles associated with as-quenched state (a) • On tempering at around 300°C, some carbides precipitates out, thickening the plate boundaries of martensitic lath (b) • On tempering at about 500°C, the dislocation structure recovers by more carbide precipitation, forming thick cell-like structure within the lath plate (c) • On tempering at higher temperature – say 600°C or more, carbides coalescence and re-nucleated equiaxed ferrite starts forming (d). Higher the temperature of tempering more is the carbide coalescence and spheriodisation. Carbide precipitation on tempering takes place in stages – first by precipitation of transient iron-carbide (Fe2.4C) and then changing to normal carbide (Fe3C). Carbide prefers to precipitate on the plate and cell boundaries (vide sketches b and d). Carbide precipitation from lower carbon steel – where lath type martensite is common – further lowers the carbon level and allows nucleation and formation of small equiaxed ferrite in the structure when tempering temperature is high. With carbide precipitation internal stress in the martensitic lath comes down, strength reduces and toughness increases. Coarser the carbide precipitates grow with temperature more is loss of strength and gain in ductility. Time of tempering is also important; longer the time more is the time for carbon to diffuse and coalesce with each other. But, temperature has the sharpest effect in bringing about change in structure by tempering. Lower set of photographs illustrate the tempering stages of needle-type martensite generally observed in low-alloy steels. The figure illustrates the changes in needle type of martensite with internal twin. • Figure (e) illustrates the needle type martensite with internal twins produced on quenching • When this structure is tempered at lower temperature (between 100 and 200°C), very fine transitional carbides (Fe2.4C) precipitate across twin boundaries (f) • Tempering at around 200 to 300°C causes the fine carbides to dissolve and re-precipitate as normal carbides (either cementite or alloy carbide) along the twin plate boundaries (g), and • When tempered at around 400 to 650°C, twin structure more or less breaks down and the carbides spherodise to prominent globular form, known as spherodised structure (g). Carbide precipitates are generally present either on prior needle boundaries or twin boundaries.

Fig. 7-20 An illustration showing the variation of impact toughness with tempering temperature of AISI 4140 Cr-Mo steel

These structural changes in martensite with tempering temperatures allow the adjustment of steel properties as per specification or for an end application. Tempering is, therefore, an integral part of quench hardening. Time is another dimension of tempering, but temperature of heating is the dominant factor controlling the structure and strength of the steel. Figure 7-20 illustrates the trend in change of hardness with temperature of tempering of a Cr-Mo alloy steel. Tempering or stress relieving of quenched structure has to be carried out as early as possible after quench hardening. Because, in the absence of thermally assisted process of stress relieving, stresses within the steel body start relieving itself – even in room temperature – by time-dependent dislocation re-orientation and micro-cracking. The problem of micro-cracking is more in steels with ‘retained austenite’ in the martensitic structure or in high carbon martensite. Retained austenite, if not very stable due to high alloy content, tend to transform to martensite at room temperature with time under the strain of the matrix, resulting in localised volume expansion due to martensite formation. This volume expansion in an untempered brittle martensite structure can lead to micro-cracking. Retained austenite is often present in martensite or bainitic structure of alloy steels or high carbon steel, such as case carburised parts. Some amount of retained austenite can be beneficial for acting as spots to arrest

propagating cracks in the matrix in service; thereby delaying failure or avoiding sudden fracture. But, the retained austenite should be sufficiently stable to remain untransformed with time and service conditions. If retained austenite transforms to martensite under service, the effect is opposite i.e. there will be chances of micro-crack generating from the those spots which create a state of extra localised stress due to volume expansion associated with local martensite formation. Therefore, another objective of tempering is to eliminate the chance of having retained austenite that can transform easily over a time. In general, retained austenite, if present, gets transformed during tempering on reaching the temperature corresponding to martensite start formation temperature of the steel and, thereafter, gets automatically tempered with further rise of temperature or with time of holding. However, many higher alloy steels that exhibit larger volume of stable retained austenite needs double tempering or even one more tempering. This step is necessary for rendering all retained austenite unstable (due to diffusion of carbon and alloy atoms out of it), induce transformation to martensite and getting all freshly transformed martensite fully tempered. High alloy special steels – like the high speed tool steels – may require cryogenic treatment for conversion of retained austenite before final tempering (vide Chapter 9). Stress relieving is the substitute process of tempering after surface hardening. The process is applied to all types of surface hardened jobs, namely after case carburising, flame hardening or induction hardening. Stress relieving involves heating at lower temperature – generally ranging between 180 and 220°C for about 40 to 60 minutes – for relieving the part from any residual or internal locked-in stresses caused by the applied surface hardening process. The process is not aimed at to change any structural feature but to relieve the stresses only. Like tempering, stress relieving should also be carried out soon after the hardening. General range of temperature for tempering is between 250 and 650°C, depending on the type of steel and required level of hardness (i.e. strength). Since reaction rate for change in microstructure is rather slow at these lower temperatures, time required for tempering vary with required hardness or microstructural changes. Alloy steels take longer time than plain carbon steels for tempering due to slower diffusion rate of alloying elements relative to carbon at the same temperature. Generally, jobs are air cooled from the tempering temperature, but for certain grades of steels faster cooling from tempering temperature is recommended for avoiding ‘temper embrittlement’. Temper embrittlement can occur in some grade of steel while heated and held for tempering in the temperature range of 450 to 650°C or slow

cooled through this range. Temper embrittlement of steel causes loss of impact strength, as shown in Fig. 7-20. The figure shows the trend of variation of impact toughness with tempering temperature in SAE 4140 steel (equivalent to popular 42 CrMo4 or EN-19 steel). While decrease of hardness with tempering is smooth, increase of toughness with increase of tempering temperature shows drop in the temperature range of 450 to 650°C, beyond which toughness again rises sharply. Most important purpose of tempering is improving the toughness of steel, but if toughness is affected by tempering in certain temperature zone, then not only the purpose is defeated but also can endanger the application of that steel. Therefore, an understanding of the phenomena of temper embrittlement is important for heat treaters.

7.5.1

Temper Embrittlement

While tempering is a straightforward process of heating and holding to a temperature for pre-determined time, the process may have the associated problem of ‘temper brittleness’ due to holding in certain temperature region or cooling too slowly through the temperature range. Figure 7-21 illustrates the temperature ranges where temper embrittlement phenomenon is observed (as evaluated by drop in impact toughness value).

Fig. 7-21 An illustration showing the temperature range where steels are prone to ‘temper brittleness’ as observed by drop in impact energy value

The figure shows that temper embrittlement can occur in the temperature range of 250 to 400°C and again in the higher temperature range of 450 to 650°C (this figure is also quoted as 375 to 575°C by some researchers). Therefore, temper brittleness can affect steels – both plain carbon and alloy steels – when either they are cooled slowly from above 650°C or are held for excessive time in the range of 450 to 650°C for tempering (assuming that heating and holding for long in the temperature range of 250 to 400°C is avoided during tempering). Temper embrittleness that occurs in the higher temperature range is reversible, but the one that occurs at lower temperature range is not reversible i.e. once formed it renders the steel relatively brittle for any application. Temper embrittlement that occurs at higher temperature (450 to 650°C) can be reversed by heating above the 600°C and rapidly cooling thereafter. However, one positive aspect of such brittleness is that it has practically no effect on other mechanical properties at room temperature other than lower impact energy absorption value. Nonetheless, this is undesirable for most engineering applications of steels where components encounter sudden change in load or fluctuation of operating temperature and load. The reason for lower temperature brittleness (also known as blue brittleness) is said to be precipitation of carbide on decomposition of martensite in the form of films at the grain boundaries. Once formed, this is irreversible. Entering the dangerous temperature zone for embrittlement from either below (on heating and holding at that temperature) or from above (on slow cooling) can produce the same result. While blue brittleness cannot be reversed, other type of brittleness can be removed – by heating above the range and cooling fast. Reason for the embrittlement at temperature 450 to 650°C is believed to be grain boundary precipitation/ segregation of some impurity elements, especially phosphorus. In practice, it has been observed that silicon is beneficial for protecting steel from lower temperature brittleness, and carbon steels with lower manganese (%Mn less than 0.50) are not prone to higher temperature reversible brittleness. However, alloy steel, notably steels with popular alloying with Cr, Ni, and Mn, are susceptible to such temper embrittlement. Small addition of Mo in steel is believed to reduce temper embrittlement, but higher percentage of Mo may have opposite effect i.e. increasing the tendency. Other factors are that impurities in alloy steels, especially phosphorus, tin, antimony and arsenic, promotes this embrittlement effect, and, in this respect, commercially available alloy steels made largely from scrap melting in electric arc furnaces, are more susceptible. The measures to prevent temper embrittlement are, therefore, restricting the contents of harmful impurities in steels and promoting fast cooling

from the higher temperature of tempering, as and when required. Small addition of Mo in the steel (0.08 to 0.15%) can be helpful in avoiding high temperature temper embrittlement. Purpose of Fig. 7-21 is to highlight this side effect of tempering process and provide a guideline for avoiding this embrittlement phenomenon in steels during tempering or prolonged stress relieving at temperature around 200°C.

7.6

MARTEMPERING AND AUSTEMPERING OF STEELS

Martempering and austempering are two special types of heat treatment operations where specially designed tempering processes are integrated with the quenching / cooling cycle. These tempering processes are very effective in minimising distortion and cracking in high hardenability steels - because of temperature equalisation while the steel is in austenitic state and yet to form martensite. Figure 7-22 illustrates the martempering process cycle. The process is carried out in four integral steps: • Heating to above A3 temperature of the steel for full austenitisation • Quenching rapidly in a liquid salt bath maintained just above MS temperature of the steel • Holding in the salt bath for a period of time for temperature equalisation between surface and centre

Fig. 7-22 An illustration showing temperature-time cycle of martempering operation, arresting the quenching above MS temperature

• Air cooling to room temperature or below MF temperature for completion of the transformation. If required, the steel can be given another tempering operation for improved toughness by heating to about 450°C. Because of holding at just above martensite transformation start temperature and before transformation, there will hardly be any temperature gradient inside the steel during martensite transformation and martensite will start forming across the steel body nearly at the same time. Further, due to slower air cooling than quenching after martensite has stared forming, there will be no thermal shock or residual stress in the transformed martensite. This will eliminate any chance of distortion or cracking, which is otherwise a threat in higher hardenable steels. This process is widely practised in industries for case hardened jobs after case carburising. High carbon in the case makes such steel parts prone to distortion or having high residual stress after conventional quenching. Only treatment that follows martempering of case hardened jobs is the stress relieving at lower temperature (e.g. 180°C) for elimination of any chance of tensile residual stress. However, if the treatment is for toughening of steel, a separate tempering operation – appropriate for required strength and toughness combination – may be carried out. Martempering is a popular heat treatment method for high carbon steels and tool steels where distortion and cracking is a serious problem. Hence, the process is widely used for hardening of pre-carburised distortion prone parts and for tool steels. Austempering cycle, on the other hand, is designed to transform steel to 100% lower bainite in order to get higher toughness with increased ductility (as measured by % Reduction in Area) and improved impact strength. Toughness of steel after austempering is much higher than conventionally hardened and tempered steel of similar hardness level. The heat treatment cycle of austempering is shown in Fig. 7-23. The process is carried out as an integrated cycle, involving: • Heating to above A3 temperature of the steel for full austenitisation • Quenching rapidly in an ‘isothermal bath’ maintained just above MS temperature of the steel • Holding isothermally in the bath for sufficient period of time for 100% transformation to bainite, followed by • Air cooling to room temperature. Isothermal bath is generally a salt bath of appropriate salt composition, which is held at about 350 to 425°C, corresponding to bainite transformation range of the steel. Objective of the process is to transform to

Fig. 7-23 An illustration showing the austempering cycle for 100% transformation to bainite

bainite and not martensitic hardening. The process is complete by itself and no separate tempering is required. Austempering is superior to conventional hardening with respect to properties like RA (reduction in area), impact strength and bending, especially when the steel is hardened to 45 to 55 HRC range. However, for austempering, steel composition and section size should be such that it can be fast quenched to lower bainitic range of temperature without producing any prior transformation to pearlite. Very often, this limits the uses of austempering heat treatment. The process has been successfully used for hardening of springs and similar products requiring high toughness at moderate hardness. Another area where austempering is increasingly used is in the heat treatment of ductile cast iron components, and the product is called ADI – austempered ductile iron. ADI is applied to such critical items as automobile crank shafts of lower diameters. The process of ADI is similar to conventional austempering, but the effect is a bit different. Austempering of ductile iron promotes formation of acicular ferrite (not exactly bainite), accompanied by rejection of carbon into the austenite, making the austenite stable. This structure is called ‘ausferrite’, which gives special properties to ADI. Ausferrite exhibits about twice the strength of a pearlitic ductile iron. The properties of ADI are due to its unique matrix of acicular ferrite and carbon stabilised austenite. ADI, produced through austempering route, does not have any internal stress arising from conventional quenching, and this enhances the bulk properties of ductile iron, especially the ductility at a given hardness level.

Summary 1. Heat treatment is an integral part of industrial and metalworking processes, used for altering the physical properties of steels for either softening or hardening or conditioning the structure for a specific purpose/end application. In practice, purpose of heat-treatment takes two major directions; one to soften the steel with increased ductility for intermediate shaping, working or forming, and the other to harden or strengthen the steel with increased toughness for load-bearing applications. Examples of former are annealing and normalising, and the examples of the latter are hardening and tempering. Therefore, this chapter further elaborates the processes of annealing, normalising, hardening and tempering along with different forms and variants of the processes. 2. Metallurgical process differences between full annealing and other sub-critical annealing processes (i.e. processes which are carried out below the lower critical temperature of steel) have been discussed in details, and reasons for adaptation of different annealing processes and their applications have been highlighted. It has been pointed out that the enabler for the success of all sub-critical annealing processes is the availability of cold-worked energy in the steel, which helps in early recrystallisation of such cold-worked structure and produce the desired level of softness. 3. Similarity and dissimilarity between full annealing and normalising have been pointed out, and the process of normalising has been discussed with reference to annealing operation but with faster air cooling. It has been pointed out that for the normalising action to be effective, the steel should have carbon above 0.30% or alloy content like Cr, Mn, etc. which helps to produce about 40:60 ratio of ferrite and pearlite in the structure. Normalising produces finer ferrite grains and finer pearlite lamellae, which contribute to improved strength and elongation of the steel. In addition, normalising makes the steel better machinable due to better chip-breaking characteristics in steel that is not very soft. Normalising is not, however, as good as annealing as regards homogenising the structure, because of shorter cycle time than annealing and faster cooling. 4. Hardening of steels have been discussed more from the practical point of controlling uniform heating and soaking, and efficiency of quenching in order to not only produce the desired microstructure or hardness, but also to minimise chances of distortion and cracking. In this regard, principal features of hardening furnace have been illustrated with reference to the operation of ‘rotary hearth furnace’ – a continuous hardening furnace. Furnace atmosphere and importance of furnace atmosphere control in hardening of steels have been emphasised and critically discussed. Finally, uses of different furnaces for hardening of steels have been briefly mentioned and concluded that choice of furnace depends as much on the shape and size of components as much as on the steel types and properties to be achieved. 5. The chapter has also discussed the practice of flame and induction hardening processes, which fall under the category of ‘thermal heat-treatment’ processes. Merits and advantages of flame and induction hardening have been discussed. Methods and measures for controlling the results of flame and induction hardening have been pointed out.

6. Stress-relieving and tempering of hardened steels have been discussed together; because they are processes of the same chain of thermal activities with the objective of minimising the effects of quenched-in and locked-in residual stresses, on one hand, and improving the toughness of the steel, on the other. In discussing these two topics, emphasis of timeliness of these treatment have been emphasised, and the problem of ‘temper embrittlement’ during tempering through certain region of temperature has been highlighted. Reasons for temper embrittlement and their prevention and cure have been also discussed. 7. Finally, the chapter highlighted the process features of martempering and austempering. Merits of these processes in minimising the distortion and cracking of steels, especially for the alloy steels and tool steels have been emphasised. Applications of these two special heat-treatment processes in industries have been mentioned.

References/Suggested Reading ASM Handbook, Vol. 4, Heat Treating, ASM International, USA, 1991 ASM International, Practical Heat Treating, 2nd Edition, Metals Park, OH, USA, 2006 Heat Treater’s Guide: Practices and Procedures for Iron and Steels, ASM International, 2nd Edition, Metals Park, Ohio, 1995 Indian Institute of Metals and National Metallurgical Laboratory, Heat Treatment and Surface Engineering of Iron and Steels (HTIS – 94), Jamshedpur, May 11-13, 1994, Jamshedpur Rudnev, V., D. Loveless, R. Cook. and M. Black, Induction Hardening of Gears: A Review, Heat Treatment of Metals, Quarterly Journal of the Wolfson Heat Treatment Centre, vol. 2003.4 and 2004.1, University of Aston, Birmingham, UK Rudnev, V., D. Loveless, R. Cook. and M. Black, Handbook of Induction Hardening, CRC Press, 2002 SECO/Warwick Corporation Heat Treating Data Book, Meadville, PA, USA, (10th Edition e-book) Sharma, R.C., Principles of Heat Treatment of Steels, New Age International (P) Ltd., Publishers, New Delhi, 2003 Sharma, C.P., T.V. Rajan, and Ashok Sharma, Heat Treatment: Principles and Techniques, PHI Learning, New Delhi, 2nd Edition, 2010 Totten, George E., (Ed) Steel Heat-Treatment Handbook, 2nd edition, CRC Press, 2006

Review Questions 1. Highlight the stages of heat treatment cycle and discuss the role of each step for successful heat treatment. Based on these basic rules of heat treatment, list the popular heat treatment processes for steels. Why the ‘critical temperatures’ are important for heat treating steels? 2. Compare and contrast the ‘annealing’ and ‘normalising’ processes of heat treatment with reference to 0.45% carbon steel. What do you understand by ‘Process annealing’ and where it is applied?

3. Outline the changes of structure in mild steel with change in annealing temperature, and discuss the reason for changes. What is ‘grain growth’ and ‘grain boundary’ fusion? What precautions should you take to avoid grain growth and fusion during full annealing? 4. Outline the processes and application of following annealing processes: (a) Spheriodising anneal (b) Recovery anneal (c) Recrystallisation anneal and (d) Isothermal anneal. 5. What are the thermal stages and temperature of hardening of steels? List out the precautions to be taken for successful hardening of steel parts. Discuss the importance of ‘quenching’ in hardening operations. 6. Based on different precautions necessary in hardening operations, list out the features of hardening furnaces for efficient and effective hardening. Discuss how ‘Rotary hearth furnace’ fits to the requirement of precision hardening operations. 7. What is the difference between ‘flame’ and ‘induction’ hardening? For critical automobile shaft, what method of surface hardening would you adopt and why? What frequency levels are used for achieving different case depth in a round shaft? 8. Discuss the purpose of ‘tempering’ after hardening of steel. How tempering differs from stress relieving after hardening? Discuss the precautions necessary for tempering of steel and how to avoid temper embrittlement. What are effects of temper embrittlement on steels? 9. Compare and contrast the process of ‘martempering and ‘austempering’ of steels. Critically discuss their applications. 10. Discuss the following: (a) Limitation of normalising and when annealing of steel is preferred over the normalising (b) Atmosphere control in hardening furnace (c) Stages of tempering of high carbon martensite, and (d) Different types of temper embrittlement in steels and how to avoid them.

Thermo-Chemical Processes of Heat Treatment [Carburising, Nitriding, Carbo-nitriding and Nitro-carburising]

8.1

INTRODUCTION

Thermo-chemical processes of heat treatment refer to processes in which chemistry of the steel surface is changed by further alloying with the help of temperature-assisted chemical diffusion method. Popular examples of thermo-chemical processes are case carburising, nitriding, boronising, etc., wherein steel surface is enriched with diffusion of carbon (C), nitrogen (N) and boron (B) atom, respectively. Purpose of such alloy enrichment by thermo-chemical process is to impart special properties to the surface – such as higher hardness for wear and fatigue resistance. Since the process is involved with surface diffusion of chemical elements, its practice is limited to those elements which can diffuse easily into the existing steel matrix. Carbon and nitrogen diffusion are popular because of their small atomic diameter and ease of diffusion into the steel matrix; carbon, however, takes higher temperature for diffusion than nitrogen. Diffusion of boron to steel surface for producing hard boron carbide is also used sometime, but the process requires higher temperature than carburising and has the limitation in applications because of inherent brittleness of boron carbide layer.

For diffusion of a selected element, an atmosphere rich in that specific element is created (or injected) inside the furnace where appropriate temperature is maintained for facilitating diffusion. Because of strong temperature dependence of diffusion process, setting and control of appropriate temperature are very critical in the thermo-chemical heat treatment process. By controlling the temperature and environment mix, diffusing elements, such as nitrogen and carbon, can be introduced into the steel surface for their special effects. By controlling the temperature and type of diffusing elements, the process can be tailored for pure carburising, pure nitriding or for combined action of carbo-nitriding or nitro-carburising. The processes produce either carbon-rich or nitrogenrich surface or a combination of them as per control of temperature and diffusing elements. As per heat treatment principles, carbon-rich surface requires hardening by quenching for producing martensite by transformation of carbon-rich surface composition, whereas nitrogen-rich surface does not require such additional quench-hardening treatment. This is because nitrogen forms nitride by combining with iron atoms or atoms of alloying elements present, which are hard enough for various applications. Thus, carburising process must finish with an additional hardening process – which is not required for nitriding. However, there is marked difference in temperature of operation of carburising and nitriding. Diffusion of nitrogen in steel can take place below the lower critical temperature (A1). Hence, the process of nitriding is carried out at sub-critical temperatures (i.e. below A1 temperature) to form appropriate iron or alloy nitride layers which are very hard and immensely suitable for wear resistance. This process does not require quenching for hardening as no martensitic transformation is involved. But, for carbon diffusion, steel has to be heated above the upper critical temperature (A3) for workable rate of carbon diffusion and carburising result. The carbon-rich layer of steel has to be then transformed to hard martensite by appropriate quenching for getting the strength. Thus, the process of carburising involves diffusion of carbon atoms on the surface areas and then hardening the carbon-rich surface for strength or hardness. Hence, the process is generally called ‘carburising and case hardening’. In this process, only the carbon-rich surface and surface adjacent area develops hard martensitic structure on quenching, and the core, where there is no change in composition, remains, more or less, unchanged, i.e. non-martensitic. Like all as-quenched martensite, martensite formed by quenching the carburised surface requires stress-relieving or tempering in order to adjust the strength and toughness level required for specific applications.

Carburising and case hardening is a versatile process for simultaneously developing superior fatigue and wear resistance properties in steels. Wear resistance property comes from higher hardness that the steel surface achieves due to martensitic structure and fatigue property comes from the higher strength and toughness of martensitic structure on the surface, on one hand, and ductile core on the other. For optimum fatigue strength, the process of carburising and case hardening requires the following: (a) Adjustment of right ratio of case depth to core; (b) Development of right case and core microstructure; and (c) Control of transformation sequence between case and core area in a manner that helps developing compressive residual stresses on the surface. These conditions require use of relatively lower carbon steels for carburising, control over case depth of carbon-rich layer, and right hardening process. Nitriding process, on the other hand, requires right choice of the alloy content in the steel for facilitating the process and choice of the right temperature level for economic operation. Operationally, nitriding is a simpler operation than case carburising and hardening, but the process could be much longer due to slower rate of diffusion of nitrogen atoms at the lower operating temperature that is generally followed for the process. Carburising and nitriding are the two popular processes of heat treatment based on the application of thermo-chemical principles. These two processes can be further divided into number of process types as per process objectives and process characteristics. These are: pack carburising, gas carburising, salt-bath carburising, vacuum carburising, etc., within the carburising discipline, and gas nitriding, carbo-nitriding, salt-bath nitriding, plasma-nitriding, etc., within the nitriding discipline. However, principles of chemical reactions involved within each discipline are similar. It involves production of (free) atomic carbon or nitrogen (for carburising and nitriding, respectively) and injecting them into the furnace chamber for thermally assisted diffusion of respective atoms as per the process specifications. In carburising, carbon-rich surface requires additional hardening operation by quenching for production of hard martensite, but in nitriding no quench hardening is involved as the nitride produced in the process is hard by itself. The distinction between different types of these surface hardening processes lies in their process features and characteristics. In this chapter, therefore, different processes of carburising and nitriding will be discussed with reference to their process features, characteristics and operating procedures.

8.2

PROCESSES AND PRACTICE OF CARBURISING AND CASE-HARDENING

‘Carburising’ is a process where austenitised steel parts are exposed to carbon-rich environment of sufficiently high carbon potential to cause chemical absorption and diffusion into the steel, while ‘case hardening’ refers to quench hardening of the case to martensitic structure by sharp cooling. These two operations can be carried out separately or by combining in one operation, as being done now-a-days by using: (a) Al-killed steel for grain size control, and (b) better controllable furnaces where both carburising and hardening can be combined without making any compromise in metallurgical quality – such as by using seal-quench furnace, vacuum furnace or continuous carburising and hardening furnace, wherein both operations are combined for economic and metallurgical advantages. Now-a-days, limiting factor for the choice of twostage or single-stage process is shop floor requirement as regards batch of manufacturing and productivity; there are quality furnaces and furnace technology available (vide Chapter 5 on Furnaces) for adopting to singlestage process if shop floor production programmes permit. As regards steel quality for carburising, carbon level of the original steel must be sufficiently low for efficient carbon diffusion – in order to facilitate faster transfer of carbon from the atmosphere with higher carbon potential to the surface with lower carbon. Hence, starting material for carburising is always steels of lower carbon – generally in the range of 0.12 to 0.25%C. Such low-carbon steel surface is then carburised to surface-carbon level nearly of eutectoid composition (i.e. around 0.83%) level or near to this value. Carburising temperature is high, generally ranging between 900 and 10500C, and time is also long–ranging between 8 and 12 hours for gas carburising and between 12 and 40 hours for pack carburising. As such, the process runs the risk of grain coarsening and, thereby, lowering the toughness of hardened steel. For this reason, Al-killed steel that prohibits grain growth at higher temperature is used for case carburising. At carburising temperature, carbon gets deposited on the surface and thereafter diffuses inside due to difference of carbon potential between the surface and the core. Up to the limit of eutectoid carbon level, carbon forms solid solution in austenite at the carburising temperature. However, due to diffusion boundary, carbon content progressively decreases below the surface, confining the carbon enrichment only on the surface and surface adjacent area. Carbon gradient from surface, produced by diffusion boundary, needs to be controlled – so that there is adequate carbon below the surface to form martensitic structure on

quenching, on one hand and simultaneously, on the other hand, the depth of such hardened layer is controlled to meet the specified case depth limit – in addition to controlling the case to core ratio so that case-core interface is free from unnecessary strain and stress. The depth up to which the carbon gradient is controlled for attaining martensitic structure is termed as ‘case depth’. Figure 8-1(a) illustrates the method of estimating case depth in case-carburised parts and Fig. 8-1(b) illustrates the pictorial view of case and core area of carburised gear tooth after hardening.

Fig. 8-1 (a&b) (a) A graphical method of estimating case depth based on 550HV hardness; (b) A pictorial view of case-core area of case hardened gear tooth where the hardness measured (in HRC) from surface towards the core have been shown.

Generally, a martensitic hardness level of 550 or 580 HV (about 55 or 58 HRC) is considered as the termination point for case depth; thereafter core structure starts after a short transition area. Considering HV550 criterion for termination of case, Fig. 8-1(a) shows attainment of 0.9 mm case depth in the illustrated figure. Therefore, control of carbon diffusion in the process of carburising is a critical one and requires close control of carbon potential in the furnace atmosphere and strict control of temperature and time cycle. For making the carbon distribution more uniform in the case area, a special ‘diffusion cycle’ is used wherever possible for allowing carbon from surface to get diffused into the case area more uniformly. Steps in the practice of carburising process can, perhaps, be best described with reference to ‘gas carburising’ process, which follows the following sequence of operations: 1. Cleaning of machined parts from oils and grease and drying the components before charging in the furnace. This helps in reducing the contamination of carburising atmosphere by the burnt oxidised gases generated by the burning of oils and grease inside the furnace. 2. Purging the furnace by inert gas (e.g. Nitrogen). This is meant for expelling out air and other harmful gaseous elements that can interfere with carburising process. 3. Charging the furnace with components in a manner that allows free circulation of gases inside. This is generally done with the help of fixtures and tackles for stacking up the jobs. 4. Gradually heating the furnace for attaining the operating temperature. The optimum temperature is about 900°C or nearby. 5. Starting the injection of carburising gases (or fluids) inside the furnace. This is started at around 750°C (i.e. around the A1 temperature) to facilitate adequate combustion of the injected gas. Idea is to avoid soot formation of un-burnt carbon particles, which hinder the carburising process. 6. Taking the temperature to the operating temperature level and controlling it steadily for the whole time cycle for carburising. Steady rate of carburising gas flow or fluid flow should be maintained for carbon enrichment of the surface and simultaneous diffusion of carbon inside. 7. Applying a final ‘diffusion cycle’ at the end of time cycle by withdrawing the carburising gas flow or considerably reducing the flow. This step is necessary for ensuring that the carbon that has deposited on the surface gets the chance to diffuse-in and promote more uniform carbon distribution in the case.

[Any excess of carbon on the surface might lead to formation of grain boundary carbide, which can lead to embrittlement of case structure, despite subsequent hardening and stress-relieving.] 8. Cooling from the final gas-carburising temperature, which can take two directions: (1) Slow cooling of the charge in some protective gas medium till it has cooled below 400°C and then cooled normally to handling temperature or (2) Direct quenching the jobs from carburising temperature after temperature adjustment and then followed by tempering. The process steps indicate that there are few operational precautions in carburising steels in addition to metallurgical factors for obtaining the quality results. A typical gas carburising cycle is illustrated in Figure 8-2.

Fig. 8-2 A typical gas carburising cycle, involving direct quenching and tempering

It should be noted from Fig. 8-2 that while carburising temperature is above 900°C, which is to be brought down to much below 900°C before quenching in the oil. This step is necessary for (a) avoiding quenching stress involved with quenching from higher temperature, (b) grain refinement, and (c) for avoiding excessive retained austenite in the martensitic structure. Higher holding temperature before quenching leads to higher carbon in the austenitic case, which can then lead to higher retained austenite in the case region after quenching, (vide Chapter 2). There are quite a few ways (i.e. gas carburising cycle) by which carburising can be effected in the steel, but the one in Fig. 8-2 represents a simple but common one. If the parts being carburised are prone to

Fig. 8.3 An illustration of gas-carburising cycle with pre-heating and step cooling for quenching

distortion during heating, it would be necessary to ensure gradual heating in steps – instead of straight heating as shown in Fig. 8-2. A typical cycle with step heating and cooling is shown in Fig. 8-3. In either case, cooling to about 850°C or below before closing the carburising process is followed to reduce the chance of unacceptable retained austenite on the surface structure. Hardening after carburising can be a separate process after slow cooling the jobs or can be made integral with the carburising process (i.e. direct quench) as in the Seal Quench process discussed in Chapter 5. However, for direct quenching process, care is necessary for selecting Al-killed steel which is resistant to grain growth at the carburising and hardening temperature. Purpose of hardening the case – either in separate operation or in an integral process – is to ensure formation of martensitic structure in the surface area – with the aim for attaining (a) right surface hardness and (b) sufficient case depth of right hardness as per process specification. Hardening of case follows the same rule for austenitising and hardening as discussed in Chapter 2, in terms of austenitic decomposition under continuous cooling. However, carbon in case-carburised steel, being near the eutectoid composition level, has relatively lower required temperature for austenitising (called hardening temperature), i.e. around 810 to 840°C, which is still above the A3 temperature of the steel surface composition. Failing this control, case structure might show up higher volume of retained austenite than that is tolerable for fatigue-sensitive applications. Another critical part of case-hardening process is the control of quenching process; quenching rate must not be very high which can give rise to cracking or distortion of high carbon surface. On the other hand, too slow a cooling rate can lead to formation of non-martensitic structure on the surface, which is unacceptable. Generally, oil-quenching of

suitable H-value is used (with adequate rate of agitation) for case hardening of steels or a marquenching type cooling process is used. An important part of case hardening is to produce required ‘case depth’ with high hardness, but without drastic fall or transition of hardness between case and the core. This requires redistribution of carbon that primarily deposits on the surface during carburising process. This is accomplished by incorporating a ‘diffusion cycle’ in the carburising process programme. For this reason, wherever possible, carburising process cycle is designed with ‘diffusion cycle’ for diffusing carbon deeper into the surface and create a carbon profile balance for obtaining required ‘case depth’ of right hardness, but without sharp transition. If diffusion cycle is not provided (or is not correct / adequate), case depth may be shallow and sharp, which is not desirable from the viewpoint of applications. Much of the properties of case-hardened parts depend on the ‘case depth’ and ‘case hardness’; hence controlling the carbon gradient by diffusion control is an important process feature. This is where the distinction or advantage of different carburising processes comes into force. For example, earlier practiced ‘pack carburising’ where jobs are packed into a box containing solid carburising agent, like charcoal powder, is not amenable to carbon gradient control by additional diffusion cycle. This is because carbon-rich environment inside the box or furnace cannot be changed. Therefore, processes where atmosphere cannot be changed and controlled for diffusion cycle may often develop high surface carbon that sharply terminates below the surface. This is objectionable for two reasons: (1) high surface carbon above eutectoid carbon level is prone to develop harmful grain boundary carbide in the surface region, making the hardened case rather brittle, and (2) sharp transition of case may give rise to notch effect at the case-core junction. Controlling such situation in pack carburising is rather difficult; hence the process is generally not favoured in the industries, unless the job is too big and no other process can accommodate the job. Salt bath carburising – another industrial carburising process – also suffers from lack of carbon gradient control, because bath composition is constant and cannot be changed quickly. Hence, this process is recommended only for non-critical components and for thin case depth where only wear resistance property is called for. On the contrary, gas carburising – where the carbon potential of the carburising atmosphere could be precisely controlled – is more flexible. In this, both carbon potential and carbon gradient inside the job can be accurately controlled in the process by manual or automatic intervention, as well as by giving additional diffusion cycle. Thus, gas carburising is popular in the industry – because the process can be controlled easily

with the help of instrumentation. Pack carburising practice is now nearly obsolete, and slat bath carburising being a flexible process for production of small batches is used mainly for thin case depth requirement, such as in spacers, flanges, washers, etc. Carburising processes are generally termed as per their method of operation or application of carburising atmosphere. When a process uses a pack of carbon-bearing material in solid state for giving effect of carburising (e.g. charcoal powder or cake), the process is termed as ‘pack carburising’. Similarly, when the carburising medium is gas, containing atmosphere which can produce free carbon for diffusion, the process is called ‘gas carburising’. This is by far the most popular and widely used carburising process in industries. Gas carburising process may resort to varieties of furnace type for the operation – some are batch type and some are continuous type. Amongst the notable gas carburising processes, sealquench process and vacuum process are becoming increasingly popular for their productivity, quality and flexibility. Vacuum carburising is a process of gas carburising, where the process of reactions between the steel surface and free carbon is not hindered by the presence of any oxygen or hydrogen – which is the limitation of non-vacuum processes. Non-vacuum processes, including seal-quench process, cannot totally eliminate oxygen from the furnace atmosphere. The presence of oxygen inside the furnace limits the diffusion rate. Therefore, in this regard, diffusion rate in vacuum furnace is higher than others non-vacuum furnaces. Similarly, when carburising is carried out in molten salt bath, liquid salt completely protects the steel surface from oxygen or other atmospheric attack and diffusion rate remains good due to freshly etched surface of steel, which is free of scales or oil/grease. But, control over carbon gradient in the case area is not good in salt bath process. Moreover, salt bath requires use of cyanide salt for generating free carbon, which is poisonous and poses environmental hazards. Table 5-1 (Chapter 5) provides a list of salt that can be used for hardening and carburising. Thus, case carburising and hardening can be carried out by choosing an appropriate available method. There is no fundamental difference in principles of carburising among these methods, but there are differences in practice and operating procedures. From practice point of view, the process of carburising should be examined with regard to (1) mechanisms of carburising, (2) furnace to be used, and (3) atmosphere control method. Basics of carburising process have been highlighted in Chapter 3, furnaces and atmosphere control used for carburising and hardening has been discussed in Chapter 5, and this chapter proposes to further discuss the mechanisms and methods of carburising with illustrations. Therefore,

approach followed in this chapter has been to highlight the carburising mechanisms and processes, and then illustrate them with reference to two very important processes, namely continuous carburising and vacuum carburising process.

8.2.1

Mechanisms of Carburising

The carburising process requires continuous supply of ‘free carbon’ (atomic carbon) that can get absorbed on to the steel surface – setting up a carbon concentration gradient. After setting up the carbon gradient on the surface, carbon atoms move – by thermally assisted diffusion process – to a position with lower carbon content, i.e. from high-carbon area of the surface to low-carbon area of the steel below the surface. Free carbon can be derived from solid, liquid or a gaseous medium by characteristic chemical reaction, catalysed by the presence of metal (such as Fe or Ni). Free carbon from solid can be generated by burning charcoal – a carbon-rich burnt wood – which is used for pack carburising. In salt bath, free carbon can be produced by using cyanide salt, which breaks down to give free carbon and some nitrogen in the bath. Principal chemical reaction involved in gas carburising is production of carbon by the reaction: 2CO Æ CO2 + C (free carbon), which then gets absorbed on the steel surface. Various chemical reactions involved with providing free carbon from the uses of different medium used in gas carburising are summarised as follows:

where Fe(C) represents carbon going into solution in the iron (which is in the form of austenite at the operating temperature). Thus, the rate of carburising in gas-carburising process is controlled by the availability of free carbon produced by these reactions and also the diffusion rate of the carbon within the austenite. Diffusion rate is very strongly dependent on the temperature, rapidly increasing with increasing temperature. Hence, recourse to higher heating temperature than the A3 temperature can be taken for carburising in order to achieve reasonable rate of carburisation for process economy. But, keeping practical difficulties of furnace load and life (as well as variation of steel quality) in mind, gas carburising temperature is generally kept limited in the range of 890–930°C. However, salt bath carburising uses somewhat lower carburising temperature because of high heating efficiency and faster process rate. A temperature higher than this is not

generally resorted to for avoiding: (a) excessive grain coarsening, and (b) heat effect and deterioration of furnace parts. Since diffusion rate is also influenced by the carbon gradient, base carbon of steel grade for case carburising is generally limited to about 0.23 or 0.25%C max. Depth of carburisation (i.e. the distance below the surface to a depth where carbon penetration of desired level by diffusion has occurred) is, thus, dependent on: • Temperature of carburising • Time of carburising • Carbon potential of the atmosphere, and • Original composition of the steel Carbon potential of atmosphere can be measured by various means. A common method is use of thin low-carbon steel foil, which is exposed to the atmosphere for carbon penetration till equilibrium condition has been established between the carbon content of the carburising medium and the carbon content of the steel foil. Generally, a carbon potential of 0.80 to 0.90 is recommended to be maintained in the atmosphere; beyond 0.85/0.90% carbon, angular grain boundary carbide may precipitate on the steel surface during carburising. Also, the steel after carburising and hardening might develop appreciable amount of ‘retained austenite’. While angular carbide makes the hardened structure prone to microcracking, retained austenite either produces soft spot on the surface or produces untempered martensite by time-dependent transformation due to high internal stresses, thus making the steel brittle.

8.2.2

Methods of Carburising

Carburising can be carried out by using solid, liquid or gaseous media. Solid carburising medium is used in ‘pack carburising’ process. For solid carburising medium, charcoal or petroleum coke is used along with some energiser, like barium carbonate, in small percentage (10 to 15%). Energiser helps for rapid action on the steel. On burning inside the furnace, the carburising medium decomposes to carbon monoxide (CO), which further dissociates in presence of iron to form carbon dioxide (CO2) and free carbon (C). Rest of carburising reaction follows the same route as described earlier. Gas carburising can use liquid carburising medium like (a) neutral carrier gas (e.g. nitrogen) enriched with hydrocarbon gas like propane or LPG, (b) endothermic gas mixed with carbon-rich gas, such as methane or propane, or (c) mixture of nitrogen and liquid methanol. Majority of modern furnaces use endothermic gas as carrier gas enriched with high purity propane or natural gas, mixed in proportion as required, for

generating carburising atmosphere. Some old shop floors still use vapourable liquids like methanol or alcohol, in the form of drip feed, which is pumped to a hot target plate inside the furnace. Methanol quickly vaporises on hitting the hot plate and dissociates to form free carbon. This process is, however, used with additional nitrogen gas pumping in order to increase the gas volume and circulation inside the furnace for better convective heat transfer and circulation of atmosphere, which results in uniform reaction. The process is called ‘nitrogen-methanol’ process. Most widely used carburising gas in industries is the ‘endothermic’ gas. Endothermic gas is externally generated by mixing a hydrocarbon gas (like propane or methane) with air. The air and gas mixture (generally in the ratio of 8:1 to 20:1 air to hydrocarbon gas) is passed through a heated steel chamber that is filled with catalyst (nickel). Heating of the mixed gas in presence of catalyst to about 1050°C causes it to break down to several simple gases, primarily hydrogen, nitrogen and carbon monoxide. This cracked gas has to be then quickly cooled to prevent soot formation and reversal of carbon monoxide to carbon dioxide. The cooled endothermic gas is then fed to the furnace. Both propane and methane can be used as hydrocarbon gas, but propane has higher carbon content than methane. A propane based endothermic gas will typically compose of about 24% CO, 32% H2 and 44% N2 with air to gas ratio of 8:1. Due to various constituents of gases or liquids fed to furnace, the furnace atmosphere in gas carburising process is actually a mixture of carburising and decarburising gases; gases most often present are: CO, H, CO2, water vapour (H2O), nitrogen, methane etc. However, CO and H2 are reducing gases which will shield steel surface from oxidising due to presence of water vapour and CO2. Therefore, controlling water vapour (i.e. dew point) or CO2 level in the furnace could be the process control point for gas-carburising operation. Nitrogen–methanol process is also used for its simplicity. When injected into the furnace on the hot plate, methanol immediately dissociates into CO and H2. By mixing 60% methanol and 40% nitrogen, an atmosphere equivalent to endothermic gas could be produced in the furnace with 20% CO, 40% H2 and 40% N2. In all combination of carburising gases, role of nitrogen (an inert gas) is very critical; it must be used in sufficient quantity for increasing the gas flow inside the furnace, which is necessary for circulation and uniformity in atmosphere within. Carbon potential of gases and their chemical behaviour inside the furnace is the central process control point, other than temperature

control. The gases CO and CH4 are carburising, but the presence of gases like H2, H2O and CO2 are decarburising. Therefore, to ensure sufficient carbon availability, some doses of hydrocarbon gas is always used, which reduces the H2O by dew point control. Other control is the control of CO content, assuming that carburising is taking place as a direct result of CO content. This is done by oxygen probe analysis which measures the carbon activity inside the furnace. Calculation of carbon potential inside the furnace using carbon activity requires complex mathematical calculation, but this is built into the oxy-probe analyser and controller, where CO and alloy factor of the steel being carburised are taken as ‘constant’ for a given load conditions (known as ‘process factor’). Salt bath carburising requires a mixture of cyanide salt for generating free carbon by reactions as follows:

Salt bath furnace, therefore, uses 20 to 50% sodium cyanide mixed with as much as 40% sodium carbonate salt for carburising. The bath is maintained in the temperature range of 870 to 950°C. Carburising takes place by decomposition of sodium cyanide on the surface of the steel. Interestingly, nitrogen is also released along with carbon atoms, resulting in some degree of nitriding (which is very limited due to shorter time) along with carburising. The salt bath carburising process can produce a thin and hard case (0.25 to 0.75 mm thick), which is harder than the one produced by gas carburizing. The process can be completed in 20 to 30 minutes compared to several hours in gas carburizing process. As such, this process produces less distortion compared to other high-temperature carburizing process. Due to such merits of salt bath carburising, the process is still in vogue in many industries, despite its problem of shop floor pollution and environment control. Thus, among the carburising processes, gas and salt bath carburising processes are quite popular in industries and widely practised. These processes have been described and discussed in some details in Chapters 3 and 5 earlier – where their process features and controls have been illustrated. Advantage of salt bath carburising over gas carburising is that the process does not require the help of external atmosphere control – like in the gas carburising – for protecting the carburised surface from oxidation or decarburisation. The jobs remaining immersed in the molten salt pool during carburising are self-protecting. But, there is no scope of carbon diffusion in this process; carbon potential of the bath produced by

salt mixtures cannot be easily changed. Hence, salt bath-treated jobs can have high surface carbon and presence of angular carbon due to surface carbon level in excess of eutectoid level. Nonetheless, gas-carburising is the most versatile of all carburising processes; it is widely applied in industries for critical engineering components requiring special surface properties for wear and fatigue resistance. Hence, the mechanism and methods of carburising discussed here will be further correlated and illustrated with reference to continuous carburising and vacuum-carburising processes – which are not only popular gas-carburising processes, but are increasingly gaining grounds for their merits. Batch-type gas carburising processes, which are also extensively used in the industrial shop floors for batch-type operations or for heat-treating large and irregular shaped components, have been adequately covered in Chapter 5 while discussing the furnace types used for such processes.

8.3

CONTINUOUS CARBURISING PROCESS

Furnace type used for carburising depends on the method of carburising, such as ‘pack’, ‘gas’ or ‘salt bath’. For pack-carburising, which is seldom used now-a-days, batch-type box or bogie hearth furnaces are used. For salt-bath carburising, salt-bath type furnace as described in Chapter 5 is used along with martempering bath for quenching set-up alongside the furnace. These are, however, batch type of processes. Gas-carburising, on the other hand, can be carried out in either batch or continuous type furnaces. Batch-type and continuous-type differs mainly in their method of handling of charges and in the throughput; batch-type adopts charge and discharge in batches, and in continuous furnace work pieces enter and leave furnace as units in continuous stream. Features of batch-type furnaces – like the popular seal quench furnace and pit-type furnace – have been discussed in Chapter 5; hence, this chapter will attempt to illustrate the furnace features for carburising by using continuous furnace and vacuum furnace. Vacuum furnace for carburising is an important area for precision components, requiring superior mechanical properties. Hence, vacuum carburising will be discussed separately in Section 8.4. Continuous carburising can be carried out in several types of furnaces, namely rotary hearth, roller hearth, pusher type and mesh belt type furnaces. Rotary hearth is more popular for continuous hardening after carburising as described in Chapter 5. In automotive industries, pusher and mesh-belt type furnaces are more popular. They are used for various small parts and gears for carburising and hardening. Figure 8-4 shows a mesh-belt type furnace in operation.

Fig. 8-4

An illustration showing a mesh-belt type carburising furnace in operation

Mesh belt furnace is a low-height closed chamber conveyor furnace where mesh belt is driven continuously carrying the jobs to be carburised, delivering them on to the other end. When required, the jobs can be appropriately quenched at the other end. Some features of the furnace consist of: • Pre- and post-cleaning system • Loading system, which can be automatic or manual • Air-cooled fan for atmosphere circulation • Gas or electric heating; gas heating with radiant tube is becoming popular • Speed control with tension-free drive • Purging vestibule at both ends to prevent dilution of furnace atmosphere The furnace is suitable for carburising small parts, like stampings, washers, spring clips, grease nipples, and such other precision machined jobs. However, carburising is one application of mesh belt-type furnace, other operations that can be carried out in this furnace is hardening, normalising, annealing, etc. As regards carburising, the furnace is generally operated by using endothermic gas with nitrogen purging. Since small parts generally do have lower case depth requirement, the furnace is operated with lower carbon potential than other standard carburising processes. These low case depth jobs should be finish machined prior to carburising, because there is very little scope of grinding and correcting these jobs afterwards due to lower case depth. The other popular continuous carburising furnace for automobile components is the ‘pusher type’ furnace. In pusher type furnace, work pieces are placed on trays or fixtures, and they are pushed through different zones of the furnace. On completion of the cycle, the jobs can be quenched or slow cooled as per plan. Pusher furnaces also have

purging vestibules at both ends for preventing dilution of furnace atmosphere. Figure 8-5 shows a typical pusher type furnace in a shop floor. The furnace can be of single-row or multi-row for increased productivity, depending on the job sizes and shape. This process can also run by using endothermic gas with nitrogen purging.

Fig. 8-5

An illustration showing a Pusher Furnace in a shop floor

Leaving aside mechanical engineering part of the furnace design and operation, metallurgical process of carburising is similar to seal-quench type furnace with different zones of heating, carburising and quenching or cooling. A typical, process cycle of continuous carburising process is shown in Fig. 8-6.

Fig. 8-6

A schematic of the process sequence of continuous carburising process

The figure indicates that at 930°C, jobs can pick up carbon of 1.1%, but the same is brought down in the final zone to about 0.80% by controlled diffusion cycle. Nonetheless, continuous furnaces are more delicate than the batch type furnaces. Hence, carburising temperature in continuous furnace is kept towards the lower end and case depth requirement is somewhat lower to avoid overheating the furnace parts under continuous operation. Typical continuous furnace case depth is 0.40 to 0.80 mm. For higher case depth components, batch-type pit furnace with nitrogen-methanol system or seal quench system with endothermic gas technology is prevalent.

Irrespective of furnace type, the process of gas-carburising follows the same set of metallurgical principle as discussed earlier under the carburising mechanisms. However, quality of results from these mechanisms of carburising is based on the quality of atmosphere used or present in the furnace. Most conventional carburising furnaces have some oxidising atmosphere and, as such, it is difficult to avoid any oxidation reaction with the steel surface, which not only affects the diffusion rate and surface quality but also can cause grain boundary oxidation, which is harmful for some carburised parts. Possibility of oxidation problem in conventional furnaces becomes a hurdle for better carburising results, at least for some critical components, which cannot tolerate any surface oxidation or degradation. In this situation, vacuum-carburising becomes the choice. But, vacuum-carburising – like all other vacuum heat treating processes – is a batch-type operation, for all practical purposes. Its advantage over other carburising processes is that it is a single-component atmospheric gas process, where a carbon-rich gas is introduced into the furnace as partial pressure in the vacuum system. Vacuum-carburising is a state-of-the art thermal process where carburising is carried out under very low pressure (and not exactly in total vacuum). In this process, the parts are heated above the upper critical temperature of the steel and then exposed to carbon carrying gas or gas mixtures under partial pressure for carburising.

8.4

VACUUM CARBURISING AND ITS CONTROL

Construction and operation of vacuum furnace has been described in Chapter 5. Much of the discussions there have been with reference to hardening process. As regards furnace features and characteristics, same applies to vacuum furnace for carburising also. Therefore, this section will highlight only the process features of vacuum carburising, and not the furnace features. Vacuum furnace carburising had been developed as efforts to overcome the dependence on atmosphere quality of conventional processes. Vacuum carburising uses an oxygen-free environment at very low pressure; hence, the process is also called ‘low pressure carburising’ (LPC). During heating, vacuum (or very low pressure) helps to clean any surface oxide film and other contamination; thereby helping in better carbon diffusion. Because of the vacuum, surface quality is free from any oxidation or decarburisation and contamination. Therefore, the process is capable of producing uniform metallurgical properties due to no interference from oxidation, which is a problem for all high-temperature operations.

Since carburising is carried out at higher temperature than hardening and for longer time, chances of oxidation of surface area as well as grain boundary oxidation is high in all conventional carburising processes. If surface gets pre-oxidised, rate of carburisation reaction may slow down and the case may become non-uniform. Added to this is the problem of grain boundary oxidation, rendering the part brittle. Hence, for components of critical applications, like critical automotive gears, aircraft landing gears, etc., vacuum-carburising is the preferred route where any kind of oxidation problems can be totally avoided. Vacuum process has the built-in cooling facility (i.e. quenching) that uses oil or high pressure gas quenching by nitrogen or helium. A typical vacuum-carburising furnace is shown in Fig. 8-7. The quenching takes place in the sealed chamber. In vacuum-carburising process, a carbon-rich gas (e.g. acetylene or methane or propane) is introduced as a partial pressure into the hot zone of the furnace at temperature typically between 900 and 1050°C. The gas immediately dissociates into free carbon (C) and hydrogen (H2), and this free C starts diffusing into the surface area of the components. Because of the oxygen-free atmosphere, rate of carburisation is faster due to clean surface. The metallurgical process of vacuum-carburising is similar to other conventional processes, which start with pre-heating, then carbon boost for carburising and case build up, followed by a diffusion period for ensuring right profile of the case (vide Fig. 8-6).

Fig. 8-7 An illustration of a vacuum-carburising furnace with control panel and other auxiliary facilities [For further details of vacuum furnace, vide Fig. 5-9]

Much of the success of gas carburising processes depends on the choice of carbon-rich fluid and their control. Mode of atmosphere control forms an integral part of carburising operations. There are three popular methods of controlling atmosphere: (1) Dew point control system, (2) Infrared analyser for CO2 measurement, and (3) Oxygen probe control system for measuring carbon potential. These are based on the products of various chemical reactions for producing free carbon, as mentioned earlier. Free carbon can be produced in vacuum-carburising by dissociation of carbon-rich fluid like methane, propane, ethylene or acetylene. Of these, methane and propane which are richer in carbon have the problem of soot formation and non-uniform carburising under low pressure carburising condition. Hence, acetylene system of vacuum-carburising is more popular. Vacuum-carburising should be operated at lower pressure (about 15 torr) for uniform distribution of gas and better carbon penetration. But, due to low pressure and low flow rate, system using methane often causes some carbon shadow (carbon depletion) in areas of complex shape with recesses and corners. This then might give rise to non-uniform case depth. To overcome this problem, system using methane or propane gas needs to be replenished periodically by increasing the pressure. But, that too has the problem of soot formation due to excess free carbon. Hence, propane or methane system needs to be delicately balanced and adjusted for obtaining optimum results. However, now-a-days, automatic control of gas feed with periodic pulses is available to meet such delicate demand. Other system using ethylene (C2H4) or acetylene (C2H2) can operate at lower pressure of about 15 torr. At this low pressure, carbon penetration can be increased with proper distribution of carburising gas in the furnace, improving the uniformity of carburisation and cased depth. However, vacuum-carburising process still calls for precautions to avoid carbon soot formation if rate of flow of carburising gas is not controlled. The low pressure vacuum-carburising process is more popular process because of its consistency and reproducibility, and its common applicability to wide range of components in engineering industries. Compared to conventional carburising, vacuum-carburising offers the following advantages: • Less distortion • Clean surface, free from oxidation or contamination • Freedom from grain boundary oxidation of treated parts • Better uniformity and reproducibility of case depth

• Better control of atmosphere, resulting in right carbon potential and more control over surface carbon level of the case. • Higher fatigue strength of treated parts. In sum, the main operational advantages of low-pressure vacuumcarburising are increased heat transfer resulting in reduced process times, improved layer uniformity, no internal oxidation, increased stress resistance, and better surface quality.

8.5

NITRIDING

The processes and procedures for nitriding have been introduced in Chapter 3. Purpose of discussions in this chapter is to further elucidate and illustrate the processes in detail. As such, discussions in this chapter are focused towards the principles of nitriding and practices of nitriding process in industries.

8.5.1

Nitriding Process and Principles

Nitriding is a surface-hardening process where nitrogen is diffused into the steel surface by holding the steel at a temperature below the lower critical temperature (A1) of the steel in contact with free nitrogen. Free nitrogen is generally produced by cracking ammonia (NH3). In practice, nitriding is carried out in the temperature region of 450 to 570°C. Higher range of temperature (e.g. 600 to 650°C) may be used for salt-bath nitriding, which is carried out in liquid bath containing cyanide salts. Since the process is carried out below the critical temperature of transformation, the parts are not required to be quenched for hardening the case; the case gets hardened by the formation of nitrogen rich compound of appropriate composition.

Fig. 8-8 An illustration showing the nitrided microstructure, nitrided compound layer, followed by diffusion zone in plain low-carbon steel

Nitriding layer is composed of two zones; one compound layer and the other is the diffusion zone underneath the compound layer, vide Fig. 8-8. Compound layer is about 2 to 30 micron thick in gas-nitrided process, consisting of nitride compound of variable composition of carbon and nitrogen, such as e – Fe2-3 (C, N) and/or Fe4N (g phase). The compound layer is also referred to as ‘white layer’ due to its appearance. The layer can greatly improve the wear, scuffing and corrosion resistances of steel. Figure 8-9 illustrates the characteristics of compound layer consisting of different nitride compound and diffusion zone consisting of nitrogen-rich interstitial a -iron solid solution in plain carbon steel. In alloy steel the diffusion zone will consist of alloy nitride precipitates.

Fig. 8-9 An illustration showing the characteristics of compound layer and diffusion zone in plain carbon steel (nitrided at 570°C)

Below the compound layer is the diffusion zone, which goes deeper into the steel, typically ranging from 100 to 500m (micron). Load bearing capacity and fatigue strength of nitrided parts largely depend on the hardness and depth of diffusion zone. Exact penetration of nitrogen depends on the nitrogen potential and the steel composition. Nitriding parameters can be set based on ‘Lehrer Diagram’, which was developed in 1930s for pure iron. The diagram and concept is used with the belief that phase equilibrium and nitriding behaviour of steels are similar to those of pure iron. Lehrer diagram can be plotted between ‘nitrogen potential’ (KN) and temperature or ‘nitrogen concentration’ below the surface. A typical Lehrer plot for an alloy steel is illustrated in Fig. 8-10. This type of diagram gives good guidelines for what type of compound layer to expect for the nitriding potential maintained in the furnace. By choosing the right nitriding potential, nitrogen-rich compound layers as

Fig. 8-10 An illustration showing the nitrogen profile in SAE 4140 steel at 548°C with increasing nitriding potential – derived from Lehrer Diagram. [Acknowledgement: Ref-8: Nitriding – Fundamentals, Modelling and Process Optimisation, Mei Yang, Doctoral Thesis, Worcester Polytechnic Institute, UK, 2012]

well as totally compound layer-free nitrided surface (i.e. diffusion zone) can be produced. An illustrative diagram indicating the nitrogen profile and obtainable case depth in nitriding – following the principle of Lehrer diagram – is shown in Fig. 8-11. Depth of various nitrided zones will, however, depend on the nitrogen potential maintained in the process.

Fig. 8-11 nitriding

A schematic profile of nitrogen concentration and case depth in gas

Therefore, critical part of nitriding in ammonia or diluted ammonia gas process is to control the nitriding potential. Nitrogen potential is defined as: KN = p(NH3) ÷ p(H23/2), where KN is nitrogen potential and p(---) is the partial pressure of ammonia and hydrogen produced from the dissociation of ammonia by the reaction NH3 = [N] + 3/2 H2. Main reaction point in gas nitriding is the dissociation of ammonia (NH3). Ammonia dissociates to (3H + N). About 30 to 40% of ammonia dissociates forming nascent nitrogen (i.e. free nitrogen), which then gets absorbed onto the surface layer of the steel. Therefore, control of nitrogen potential requires measurement of either ammonia content or the hydrogen content in the atmosphere inside the furnace. This control is done through infra-red or other gas analyser which measures the partial pressure of hydrogen or ammonia. Now-a-days, continuous sensor of hydrogen which can monitor the partial pressure of hydrogen is available for nitriding shops. For reliable results, accurate process control by measuring and monitoring one of the controlling parameters (namely, partial pressure of ammonia or hydrogen) is necessary. Though foregoing principles of nitriding have been discussed with reference to gas nitriding, the principles hold good for processes carried out by using gas, salt or plasma. The salt-bath process is giving way to ammonia gas nitriding process due to pollution problem with salts, which contain cyanide. Plasma process is gaining ground today due to its cleaner process environment, but yet the gas nitriding process is dominant in industries. Classical gas nitriding has been popular in automobile industries for developing increased fatigue strength and load-bearing capacity of various components without encumbering appreciable distortion of treated parts. Though the process can be applied to all steels, for fatigue and wear application special alloy steels containing Al for special nitriding effect or containing Cr-Mo alloy are preferred. Only snag of nitriding is the long duration of process cycle, ranging between ten and hundred hours for getting sufficient case depths. Table 8-1 shows different gas nitriding processes and their medium of nitriding and approximate duration. Duration of treatment is based on case depth required, nitrogen potential maintained, and the type of steel. Alloy steels with Al or Cr and Mo have higher affinity for nitrogen pick up and formation of hard nitrogen compound. Hence, they are the preferred steels for nitriding. Functional properties of nitrided steel

essentially depend on the compound layer for wear, corrosion and other tribological properties. With increasing hardness and compactness (i.e. less porous) of compound layer, all types of wear properties increases. Diffusion zone hardness, on the other hand, depends on the steel and its prior structure of the core and the core strength. Higher the core strength higher is the depth of diffusion layer. Higher hardness of diffusion zone increases resistance to surface fatigue and contact fatigue as observed in shafts and bearings. Hence, for the latter applications, attempt is made for producing less of white compound layer and more of diffusion zone by controlling the nitrogen potential. In general, both surface fatigue and wear improves with hardness, making it preferable to use suitable alloy steel of higher base strength for these applications. Table 8-2 shows the choice of properties, steel types and the influence of compound layer and diffusion zone in the functional requirements. In plain carbon steels, compound layer with different iron nitrides forms upto deeper depth than alloy steels, but they are comparatively porous, less hard and brittle. Hence, for better results in nitriding, either Al-containing nitro-alloy steel or Cr-Mo containing alloy steels are preferred. In such steels stable nitrides form, which may not diffuse readily but produce intensely hard (900 to 1100 VPN) case. Alloy steels containing Cr-Mo also has the effect of flattening the hardness gradient below the surface due higher core strength and preventing embrittlement effect that can arise from long holding at and above 500°C. Table 8-2 depicts the required functional properties and their controls through control of steel types, compound layer and diffusion zone characteristics.

8.5.2

Operating Procedures for Nitriding

Theoretically, nitriding can be done on any steel, but the process responses best to some special alloy steels and to the structure which

is already hardened and tempered. The former is to ensure sufficient nitrogen affinity for the steel and the latter for ensuring adequate diffusion zone hardness. Jobs are also required to be cleaned from oils and grease by proper cleaning method, like chemical degreasing, vapour degreasing, etc. Jobs can then be placed in the furnace either in trays (for small jobs) or in fixtures or racks, as necessary. An illustrative nitriding furnace set up is shown in Fig. 8-12 with gas stations and other accessories, as marked up in the furnace layout. Since ammonia dissociation is limited to about 30-40% in Fig. 8-12 An illustration showing gas the process, there is consider- nitriding set-up with (1) Source of process able amount of ammonia in gas, (2) Process gas control panel, (3) the exhaust gases that need to Nitriding furnace (pot or pit type), (4) Cooling system, (5) Re-circulating water be decomposed or absorbed in cooling system, (6) Job racks, and (7) water and disposed off. This is an Exhaust ammonia gas neutralising pipe and important process requirement system for safety of the operators.

Fig. 8-13 A typical production cycle of nitriding, showing the gas types used at different stages of the cycle

Actual nitriding heat treatment cycle has several stages of processing; one such recommended process cycle is shown in Fig. 8-13. Typical gases used for nitriding production cycle are nitrogen, ammonia, and hydrogen gases, where purging of the furnace is done by nitrogen, and nitriding cycle is carried out with ammonia and hydrogen combination in the ratio of 3:1, respectively. After loading and sealing the furnace, it is necessary to purge the furnace for expelling out the air so that job surfaces do not get oxidised, which retards nitriding process. Purging must be complete before the furnace has reached 150°C. All traces of air must be expelled out before ammonia can be introduced into the furnace, because of the danger of explosion. Thus, the furnace has to be conditioned and then dissociated ammonia can be introduced to the furnace while gradually heating up. For economic reason, gas flow rate during processing can be controlled into two stages; one with higher flow rate for active nitriding and then with lower flow rate for finishing. On completion of the nitriding cycle, the furnace has to be purged again with nitrogen so that unused ammonia does not get released to the working environment causing irritation to the working personnel. However, introduction of nitrogen for purging can be delayed, if necessary, till the charge has not cooled down to about 150°C or below. The process of gas-nitriding, using ammonia, can be conducted in single stage or in double stage. In the single stage process, nitriding temperature is maintained in the range of 490 to 530°C with ammonia dissociation rate of 20 to 30%. The process generally produces white nitrogen rich compound layer, which is brittle. For two stage process, first stage remains more or less same, but in the second stage higher dissociation rate of ammonia (in the region of 60 to 75%) may be used

with or without increased temperature. If temperature is required to be increased, it is in the region of 550 to 570°C. Two-stage process ensures adequate depth of diffusion zone. For higher ammonia dissociation, an external arrangement for ammonia dissociation would be required. Higher temperature during second stage can be used for increased case depth, but due to temperature effect, case hardness including diffusion zone hardness might be lower.

8.5.3

Vacuum Nitriding

Gas nitriding can be carried out by using all common types of furnaces wherever gas-carburising can be done, including vacuum furnaces. In addition to these processes, cyanide-free salt bath nitriding is also used for nitriding of many assorted automobile components. However, amongst the nitriding processes, vacuum nitriding comes second (in volume) after the conventional gas nitriding – due to its lower cycle time than the conventional nitriding process, and uniformity in case depth and control over the white layer thickness. The process of vacuum nitriding is very similar to vacuum carburising excepting that it is carried out at much lower temperature (between 500 and 550°C) and for a shorter duration. A typical vacuum nitriding cycle includes: • Charging of clean work load on to the furnace, preferably in a stainless steel basket • Closing and sealing the furnace for any possible gas leakage • Pumping down the furnace chamber to approximately 10-2 torr pressure for removing all air from the furnace chamber • Back fill furnace with nitrogen to approximately 800 torr (0.5psig) via back-fill valve provided in the vacuum furnace • Introduce partial pressure of nitrogen and start circulating fans • Heat up the furnace to the nitriding temperature (can go up to 650°C, but generally operated between 500 and 600°C) • Pump out some nitrogen, if necessary, for setting the set pressure below 800 torr • Back-fill furnace with ammonia (ammonia on dissociation provides the nitriding gas) maintaining furnace pressure of about 800 torr • Introduce partial pressure of ammonia and nitrogen continuously during the nitriding cycle at a fixed ratio with flow control. Gas flow control requires nitriding gas analyser and control system for regulating flow • Arrange for venting out of excess gas during the process through venting valve in the furnace or appropriately – without causing shop floor pollution of ammonia gas

• Ensure uniform circulation of nitriding gas during the process cycle • Cool the work load after the cycle is over inside the furnace till a handling temperature has reached and discharge. Like other gas nitriding process, ammonia (NH3) on dissociation allows the intake of atomic nitrogen for diffusion and nitrogen (N) as inert gas allows the dilution of nitriding power of the dissociated ammonia for control of the process. This low pressure nitriding process (i.e. vacuum nitriding) is adopted for precision engineering components requiring wear resistance, different transmission and friction parts, cutting and extrusion tools, plastic injection moulds etc. Deep penetration of nitriding even into the cavities along with uniformity of nitrogen penetration and less white layer makes the vacuum nitriding an attractive process of heat treatment, vide Fig. 8-14. The industrial objectives are: to provide a hard surface that is resistance to mechanical and other service stresses and also to improve the frictional and corrosion resistance properties of the surface.

Fig. 8-14 An illustration showing vacuum gas nitrided steel after quenching and tempering; note the compact and uniform nature of compound layer

In general, principal reasons for nitriding are: • To obtain high surface hardness • To improve wear resistance and resistance to scuffing and galling • To improve fatigue life, including contact load fatigue • To improve corrosion resistance, and • To obtain a surface hardness that does not easily soften due to heat upto the nitriding temperature.

One of the unique properties of nitriding is that the nitride that forms and contributes to surface hardness in alloy steels is stable upto its temperature of formation (i.e. the nitriding temperature) and does not soften till operating temperature reaches that level. Additionally, nitriding hardly produces any distortion of the parts treated, because of lower temperature of operation and not requiring fast cooling for developing hardness of the surface. Hence, for critical applications where no distortion of the component can be tolerated, nitriding is often resorted to after stress relieving of the original steel parts in order to minimise any chance of distortion. Though nitriding can be applied to most grades of steels, including low carbon steels, their response and utility differs. Carbon steel grades with carbon content from 0.2% to 0.60% are commonly used for low duty components and gears where mild to moderate wear and scuffing is involved. But, for most fatigue and high duty wear resistance applications, alloy steels are preferred. Amongst the beneficial alloying elements for nitriding, Al, Cr, Mo and V are most prominent. Since Al is a strong nitride former, for many heavy duty nitrided parts (e.g. commercial vehicle crank shafts) Al alloyed steels like EN 41B is used, containing Al between 0.90 and 1.30% Al along with 1.4 to 1.8% Cr and 0.15 to 0.25% Mo. Steel containing Al as alloying can develop very high hardness (between 900 and 1000 VPN), but the case can be somewhat brittle compared to case of Cr-Mo containing steels. Hence, for tougher cases Cr-Mo steels are preferred. Molybdenum has added advantage of protecting the steel from becoming brittle due to longer holding at temperature above 450°C. Nitriding is a versatile operation that can be applied to many steel grades for improving their performance with regard to wear, corrosion and fatigue strength. Some such grades of steels are: • Low-carbon, chromium-containing low-alloy steels of SAE 3300, 8600 and 9300 series • Medium-carbon, chromium-containing low-alloy steels of SAE 4100, 4300, 5100, 6100, 8600, 8700, and 9800 series • Hot-work die steels containing 5% chromium such as H11, H12, and H13 • Air-hardening tool steels such as A-2, A-6, D-2, D-3 and S-7 • High-speed tool steels such as M-2 and M-4 • Ferritic and martensitic stainless steels of 400 and 500 series • Austenitic stainless steels of 200 and 300 series, and • Precipitation-hardening stainless steels such as 13-8 PH, 15-5 PH, 17-4 PH, 17-7 PH, A-286, AM350 and AM355.

Other than these common carburising and nitriding processes discussed so far in the book, these processes are carried out by using plasma ionisation process for heating. Plasma carburising and nitriding are more precise process and selectively applied for high duty applications. Hence, these processes will be discussed under the modern surface engineering technology.

8.6

CARBO-NITRIDING AND NITRO-CARBURISING

Both carburising and nitriding processes offer wide-ranging improvements of engineering properties, such as load-bearing capacity, bending fatigue, contact fatigue, wear, corrosion resistance, etc. But, carburising may have some limitations in cases where no surface oxidation and distortion can be tolerated. Carburising is a high temperature operation requiring close control of atmosphere to prevent oxidation in one hand and promote carburisation on the other – while the process depends of CO2 formation for releasing free carbon and CO2 is an oxidising gas. Nitriding being a low temperature operation is not prone to oxidation, but requires very long time for developing an effective case depth. Hence, there are processes developed by appropriately combining the merits of these two versatile processes for optimising their respective strength. Such processes are: Carbo-nitriding and Nitro-carburising.

8.6.1

Carbo-nitriding

Carbo-nitriding is a gaseous surface modification technique by combining carburising with nitriding, where both carbon and nitrogen are simultaneously diffused into the steel. It is carried out by using carburising atmosphere in which small amount (about 5%) of nitriding gas (dissociated ammonia) is added. The process is operated at temperature between 800 and 880°C, where the base steel turns austenite. Unlike nitriding, carbo-nitriding is carried out at the austenite state of the steel, but by using temperature close to just austenitisation. The case depth produced is lower than carburising but the surface hardness is higher. Typically, the process is used for case depth requirement in the range of 0.10 to 0.50 mm and surface hardness of 55 HRC plus. Steel types used for carbo-nitriding are similar as for carburising. The process is extensively used for increasing the surface hardness of lowcarbon steels for wear and scuffing resistance, such as sliding plates, washers, and many other small machined components requiring only wear resistance property. Key variables in carbo-nitriding are the temperature, time and atmosphere. Temperature used varies between 800 and

880°C; higher temperature is resorted to when case depth requirement is higher. However, for distortion prone components, lower range of temperature should be used. Like carburising process, time required for the process depends on the case depth. The process atmosphere is generated by using carbon bearing gas (e.g. methane for carbon) and ammonia for nitrogen, which are fed to the furnace with carrier gas (N2). By maintaining proper ratio of methane and ammonia (process gas), carbon and nitrogen rich surface of varying case depth can be developed. After carbo-nitriding, the parts are to be quenched for achieving full case hardness through martensitic transformation of carbon rich surface. Since such low case depth steel does not allow any scope of grinding for correction of distortion, care should be taken for using as mild a quench as possible for hardening the case. Generally, oil quenching or hot oil quenching is resorted to for carbonitrided jobs. Ratio of gas mixture is important for controlling excess nitride formation on the surface. High nitrogen concentration on the surface will make the layer porous and will also interfere with hardening of the case due to austenite stability in high nitrogen steel. Therefore, shorter processing time is preferred for restricting the nitrogen pick up into the surface. Figure 8-15 shows a typical carbo-nitriding cycle in low-pressure carbonitriding (i.e. vacuum carbo-nitriding), which, in principle, also applies to standard carbo-nitriding process at natural pressure using nitrogen as carrier gas. Primary purpose of carbo-nitriding is to impart the steel with a thin hard layer for wear resistant case. Nitrogen diffused to the surface along with carbon provides superior hardenability to develop hard case, but too high a surface nitrogen level may stabilise austenite and interfere with hardening. Hence, ammonia is introduced in the process towards the end of the cycle for controlling nitrogen pick up; vide Fig. 8-15.

Fig. 8-15 nitriding

A typical time cycle and uses of gases in low-pressure (vacuum) carbo-

Surface structure with some amount of nitrogen in it also helps to retain the hardness upto higher service temperature than surface of similar hardness produced by carburising. Typical applications of carbo-nitriding are: small gears, cams, shafts, pins, piston rods, bearings, fasteners, and automotive clutch plates. Primary reasons for adopting carbo-nitriding versus full carburising are that carbo-nitrided surface has higher temperature resistance, increased hardenability, improved wear resistance, and less distortion. Carbo-nitriding can be carried out by using all types of furnaces and methods that case carburising can use, including low pressure (vacuum) carburising. However, due to lower operating temperature than carburising, thermal efficiency arising from radiation heat will be somewhat lower in the process. In low pressure carbo-nitriding, carbon bearing gases (e.g. methane or propane) are generally used in the boost phase and ammonia for nitrogen pick up is used at the diffusion phase, as indicated in Fig. 8-15.

8.6.2

Nitro-carburising

Nitro-carburising is a variant of the nitriding process where both nitrogen and carbon are simultaneously diffused into the steel, but at temperature lower than the carbo-nitriding. Nitro-carburising is carried out in the ferritic region of the steel (i.e. below the lower critical temperature) in contrast to carbo-nitriding, which is carried out in the austenitic region. In this process, both nitrogen and carbon are diffused into the steel to produce nitrogen and carbon containing compound layer. Since the process involves some diffusion of carbon, the process is carried out at temperature higher than normal nitriding, generally above 570°C. The process cycle could be similar to nitriding cycle shown in Fig. 8-13 or could be modified to ensure right balance of nitrogen and carbon in the case, vide Fig. 8-16.

Fig. 8-16 A modern nitro-carburising cycle, where N2 (nitrogen) is used for purging the furnace, followed by dissociated ammonia for nitrogen and propane for carbon source along with high purity N2 as carrier gas

In this process, both nitrogen and carbon are diffused into the steel to produce nitrogen and carbon containing compound layer. Since the process involves some diffusion of carbon, the process is carried out at temperature higher than normal nitriding, and a special diffusion cycle is used (vide shaded portion of Fig. 8-16) with carbon-rich atmosphere for obtaining a balance between nitrogen and carbon in the case depth. Gases used for the process is a mixture of ammonia and carbon carrying gas. This could be 50:50 ammonia to standard endothermic gas mixture ratio or a mixture of NH3 plus CO2 (about 5%) and nitrogen (about 45%). Nitrogen transfer will depend on the ammonia dissociation (in the same way as in nitriding) and carbon diffusion will be the function of CO and H2 as in carburising (vide equation CO + H2 [C] + H2O). To improve upon the carbon pick up, the end diffusion cycle is carried out with carbon rich gas, e.g. propane or acetylene. Since carbon diffusion in nitro-carburising temperature will be slower than carburising, the process may take longer time than standard carburising cycle. The process can be also carried out by using salt bath furnace and by plasma process. Problem of nitro-carburising process, using two gas mixtures using CO2, as discussed above and shown in Fig. 8-16, is that the nitrogen content in the compound layer cannot be adjusted independently. Because, carbon transfer increases with higher hydrogen content, but that also lowers the nitrogen potential. Thus, compound layers produced by nitro-carburising will have lower carbon if high nitrogen content is produced, and vice versa. To overcome such a situation, nitro-carburising process can be modified as shown in Fig. 8-16 where the process is split into two distinct parts; in first part, attempt is made to produce high nitrogen in the compound layer by running with ammonia plus CO2 plus nitrogen as carrier gas, and in the second part a gas mixture of ammonia and propane plus nitrogen as carrier gas is used for diffusing more carbon to the compound layer. Diffusion plays a critical role in nitro-carburising of high alloy steels and tool steels. Figure 8-17 illustrates a typical nitro-carburised section of a steel part – showing shallow compound layer and relatively deeper diffusion zone above the original steel substrate. Compound layer produced by nitro-carburising has excellent wear and corrosion resistance, and it is not brittle like layers produced by nitriding or carbonitriding. Below the compound layer shallow carbon rich case depth forms which offers good fatigue resistance. Thus, the process is suitable for all parts where case depth requirement is not high and component distortion should be low. Advantage of the process is that it can harden parts for wear and corro-

Fig. 8-17 An illustration showing a nitro-carburised steel sample where thin line of compound layer and deep diffusion zone can be observed

sion resistance without pre-hardening and producing minimum distortion as the jobs do not require quenching for hardening. The process offers: • High resistance to wear • Excellent scuffing and seizure resistance to rotating parts • Considerably improved corrosion resistance • Very low distortion, and • Relatively lower cost compared to carburising or other surface hardening process. As such, the process of nitriding is used for small gears and pinion, valve seats and stems, cylinders and plungers, sliding plates, cam surfaces, etc. and for a number of steel tools for increasing wear and scuffing resistance. Austenitic stainless steels, which cannot be otherwise hardened for increased hardness and wear resistance, can be also treated for nitriding for increased surface hardness and wear resistance. Carburising and nitriding are two very versatile processes of heat treatment that are widely used for improving the surface properties of steels by enrichment of additional alloying elements like carbon and nitrogen. Primary objectives for applying these processes are (a) to increase fatigue resistance and (b) wear resistance. However, the processes, especially the nitriding, can also bring about simultaneous improvement in corrosion resistance of steel surface by lowering the surface energy and producing an impervious thin layer of nitride. A very thin nitriding layer is known to improve the corrosion resistance of steels by many folds due to its impervious nature. For improved corrosion resistance, such diffusion layers should be very thin and firmly adherent to the original steel surface. For this reason, low-temperature gas nitriding – such by plasma or vacuum

process is preferred. The process of plasma nitriding has been discussed under ‘surface engineering’ in Chapter 10. In sum, the spectrum of thermo-chemical processes for surface modification and hardening range from conventional carburising and nitriding to surface engineering techniques using chemical and physical vapour deposition (CVD or PVD). Conventional processes have been discussed in this chapter and advanced thermo-chemical processes for surface engineering have been discussed in Chapter 10. In conventional thermo-chemical processes, there are two streams; one is thermo-chemical treatment at the austenitic temperature region and the other is the treatment in the ferritic region. Carburising and carbo-nitriding are examples of former and nitriding and nitro-carburising are the examples of latter. Important parameters for process control in these two processes are temperature, time, and gas mixture used for the process; carburising uses carbon rich gas like methane, propane or acetylene and nitriding process uses dissociated ammonia. The processes also use an inert carrier gas (like nitrogen) for sufficient flow of gas inside the furnaces and for uniformity of temperature. Nitrogen, a neutral gas, is also used for purging the furnaces of air and moisture. Demand of these processes in modern engineering and manufacturing industries is promoting rapid development of these processes by using vacuum, plasma and ion-bombarding technology. Demands for these high tech modern processes come from necessity of surface engineering, which involves exactly tailoring the surface structure and for obtaining high-performance hard and adherent surface and substrate structure. These latter processes have been discussed in Chapter 10 under ‘Surface Engineering of Steels’.

Summary 1. The chapter discusses the principles, objectives and processes of thermo-chemical heat-treatment and highlights different thermo-chemical processes in practice, namely the carburising, nitriding and the variants of these main processes like carbo-nitriding and nitro-carburising. Principal approach in thermo-chemical processes is to change the surface composition of the steel by diffusing extra carbon or nitrogen or both for producing a hard microstructure. In carburising, hard surface structure is produced by diffusing carbon into the surface and then quenching to produce hard martensite. And, in nitriding, hard surface is produced by the formation of hard nitrides as chemical compound without the help of quenching. There are few variants of these main processes which are used in industries for tailoring to some exact needs. 2. Of the thermo-chemical processes, carburising and carbo-nitriding are carried out in the austenitic temperature region followed by quenching for producing hard martensitic case, but in nitriding and nitro-carburising, the processes are carried out in the ferritic region of the steel (i.e. below the lower critical temperature) and hardness of surface is achieved by the formation of hard nitrides, which

3.

4.

5.

6.

do not necessarily require quenching. Hence, nitriding and nitro-carburising processes produce very little distortion of the treated parts. The process of carburising takes place by absorption of free carbon onto the steel surface from the carbon-rich atmosphere created inside the furnace, which is then diffused inside the steel surface by thermally activated process. Rate of diffusion is dependent on the temperature of carburising, steel composition and the carbon potential of the atmosphere, controlling the rate of transfer of carbon to the surface. Free carbon is derived from solid, liquid or gaseous medium (as per the process used) and the dissociation of the medium to produce free carbon is catalysed by the presence of Fe or Ni. Principal chemical reaction involved in carburising is production of carbon monoxide (CO) which then breaks up in presence of iron by reversible reaction to: 2CO to CO2 + C (free carbon). This free carbon then gets absorbed on the steel surface. Based on these working principles, operation of important gas carburising processes using mesh-belt, pusher and rotary hearth furnace for continuous carburising and vacuum carburising as modern batch-type process have been discussed and illustrated. Different sources of gases and gas mixture used in these furnaces have been also discussed, highlighting the merits and demerits of the gases used. Based on gases used, possible process control mechanisms have been also mentioned. Principles of nitriding and operational complexities of nitriding process have been critically discussed in view of relatively scarce literature generally referred in books on heat treatment. Importance of compound layer and diffusion zone and their properties for industrial applications have been discussed and illustrated. Processes of controlling compound layer and diffusion zone have been also discussed. The chapter also discusses suitability of different steel types for response to nitriding operation and expected compound layer and diffusion zone properties for each of these steel types. The chapter also discusses the variants of nitriding and carburising processes, namely carbo-nitriding and nitro-carburising, and highlights their process differences and resultant properties for applications. Importance of these lower temperature processes for imparting special wear and scuffing properties have been highlighted.

References / Suggested Reading Advances in Thermo-chemical Diffusion processes, Ipsen International, http://www. ipsenusa.com/processes-and-resources/processes/thermochemical-diffusion -as on 30-04-2015 ASM Handbook, Vol. 4, Heat Treating, ASM International, USA, 1991 Edenhofer, B., Advantages and Applications of Direct-feed Atmospheres for Carburising, Heat Treatment of Metals, 1995, 3, Wolfson Heat Treatment centre, Birmingham, UK Fuller, G.A., Atmosphere Carburising at less than 927°C for Distortion Control, ASTM International, 1989

Ghring, W. and C. H. Luiten, Direct Atmosphere Generation and Control in Heat Treatment Furnaces, Heat Treatment of Metals, 1980,4, Wolfson Heat Treatment Centre, Birmingham, UK Heat Treating: Equipment and Processes, Proceedings of International Heat Treating Conference – Equipment and processes, ASM International, April, 1994 (HTIS-94), Heat Treatment and Surface Engineering of Iron and Steels, National Metallurgical Laboratory, Jamshedpur, May, 1994 Totten, George E. (Ed), Steel Heat Treatment Handbook, 2nd edition, CRC Press, 2006 Yang, Mei, Nitriding–Fundamentals, Modelling and Process Optimisation, Doctoral Thesis, Worcester Polytechnic Institute, UK, 2012 [https://www.google.co.in/#q=Fundamentals%2C+Modelling+and+Process+Optimi sation%2C+Mei+Yang%2C+Doctoral+Thesis%2C+Worcester+Polytechnic+I nstitute%2C+UK%2C+2012%5D] as on 16-02-2014

Review Questions 1. What are the steps in successful thermo-chemical heat treatment processes? Critically discuss how these steps can be assured during thermo-chemical processes without giving rise to any defect. 2. Outline the difference in carburising and nitriding processes. Discuss the mechanisms of case carburising with respect to a carbonaceous medium. How the depth of carburising can be controlled? 3. Discuss the available medium of gases and their relative merits and demerits in gas carburising operations. 4. Discuss the process of gas carburising of automotive axle gears using a continuous production furnace. What are the precautions necessary for assuring defect-free production? 5. Discuss the process features of vacuum-carburising and the advantages of the process over the conventional gas-carburising. Why vacuum-carburising is also called the low-pressure carburising process? 6. Discuss the uses and utility of vacuum-carburising process. Discuss its advantages over the conventional carburising processes with respect to highly fatigue sensitive automotive gears. 7. What could be the sources of free nitrogen in nitriding processes? Discuss the method of gas-nitriding using ammonia as source of free nitrogen. 8. Can all steels be nitrided? Discuss the features of steel that are desirable for nitriding for wear and fatigue resistance components. 9. How carbo-nitriding differs from nitriding? Highlight the main process features of (a) nitriding, (b) carbo-nitriding, and (c) nitro-carburising along with their possible applications. 10. Point out the technical advantages of vacuum-nitriding over conventional gas—nitriding process. List the steps and care necessary for successful vacuum nitriding operation.

Heat Treatment of High-alloy Steels [Stainless, PH-Stainless, Heat/Creep Resistant, Tool and Die Steels]

9.1

INTRODUCTION

High-alloy steels are special steels where alloy system and alloy contents are designed for fulfilling certain special properties for applications under specific environmental or industrial conditions. Examples of environmental and industrial conditions are: corrosive atmosphere, marine environment, high temperature oxidation and degradation, creep, wear, friction, abrasion, etc. Examples of steels that are commonly used for countering such application-specific requirements are stainless steels, heat resisting steels, creep resisting steels, and tool and die steels. Tool and die steels are used specifically for metal cutting and metal-forming industries, involving high rate of wear and abrasion arising from friction and heat generated during metal cutting or forming operations. For stainless and heat-resistant steels, two-step approach could be adopted to impart the required properties; first by designing alloy system and composition to satisfy resistance to corrosion and heat or both, and then by appropriately heat-treating, whenever required, for modifying or producing desirable structure appropriate for the application. For stainless steels, conventional heat treatment may not be applicable for all grades and applications, but for heat-resisting steels, heat treatment to produce stable and fault-free structure is an essential step, in addition

to the choice of right alloy composition for high-temperature oxidation resistance. Because of commonality of requirement of oxidation resistance, both stainless and heat resisting steels have commonality in compositional requirements, containing high percentage of Cr and Ni; chromium (Cr) to give high oxidation resistance and nickel (Ni) to render the steel austenitic, known for their corrosion resistance. In fact, ferritic stainless steels, which generally contain high %Cr, are often used as heat-resisting steel after some modification of alloying with Mo for high-temperature structural stability. Amongst the high-alloy grades and class of steels, stainless steel group leads the table of bulk consumption. Other group of high-alloy steels – like heat-resistant steels, creep resistant steels, and tools and die steels – are also important as regards their specific uses and utility for applications demanding special properties, but their consumption is lower than stainless steels. Consumption of stainless steel is high because of their unique properties, and availability of large varieties of stainless steels for wide-ranging applications. Application of stainless steels spread from cutleries for domestic uses to cryogenic containers and pressure vessels for industrial applications, and medical and surgical instruments and implants. Because of wide-ranging applications, demands on quality and properties of stainless steels also vary widely. Stainless steels are basically chemical composition based steel for serving its basic purpose of corrosion resistance. But, there are varieties of stainless steel where applications call for higher mechanical properties for final applications or modification of original properties for forming and shaping, requiring heat treatment. Heat treatment of stainless steel can range from conventional annealing and bright annealing to conventional hardening, precipitation hardening or nitriding, as per requirement. Compositions of stainless steels are adjusted accordingly for response to such heat treatment, but not at the sacrifice of its corrosion resistance property. Basic composition of stainless steel or its grades must first fulfil the requirement of corrosion resistance, as per criticality of the application; heat treatment can then be applied, if necessary, for developing required properties. However, all stainless steels are not equally heat treatable, because of their basic structure type. For example, austenitic stainless steels (e.g. grade like 18-8 stainless steel) are not hardenable. Thus, heat treatment of stainless steel differs with differing composition. In general, role of heat treatment in stainless steels arises from the requirements of forming and processing of the steel or for improved strength for end usage. Corrosion resistance is the primary requirement of any stainless steel; hence, care has to be taken during heat treatment that the corrosion-

resistance property of the steel does not get affected during such heattreatment operations. Heat resistance grade of steels is required to resist not only heat but also oxidation and other thermal and environmental degradation under both static and dynamic load. Examples of uses of heat resistant steels are furnace parts and conveyor, retorts, gas turbines, boilers, etc. Heatresisting steels require three important properties: (1) resistance to oxidation and surface degradation, (2) creep strength for retaining strength at the operating temperature, and (3) structural stability at the operating temperature. While high-temperature oxidation resistance and degradation are functions of alloy content in the chemistry, structural stability and appropriateness of structure is ensured in these grades of steels by proper heat treatment, especially with regard to carbide precipitation and spheriodisation on one hand and avoidance of sigma phase formation and temper embrittlement on the other. Here again, care has to be taken not to adversely affect the corrosion or oxidation property of the steel due to heat treatment. Tool and die steels can be grouped into different varieties; namely, high-speed steels (named as per cutting characteristics), cold-working die steel, hot working die steel, shock-resisting die steel, etc. Composition, properties and heat treatment vary with the type of steel under tool and die steel grades. These steels are used only after heat treatment for developing appropriate microstructure and mechanical properties. Primarily, these steels should have structure after heat treatment which is hard, deformationresistant under shock load, resistant to wear and abrasion due to friction, and with adequate hot strength to resist frictional heat in the applications – involving high speed cutting and hot-working. For imparting such hard, shock and heat resistance properties, these steels may contain complex alloying system with Cr, Mo and W (Tungsten), making their heat treatment processes complex. Attempt has been made in this chapter to outline different types of heat treatment processes applied to these grades of high-alloy steels and explain the reasons for the same. Heat treatment of tool and die steels will be discussed separately after discussing the heat treatment of stainless and heat-resisting steels, because these grades show extremely high structure-property sensitivity due to the requirement of high hot-hardness and shock / impact resistance in applications.

9.2

HEAT TREATMENT OF STAINLESS STEELS

Stainless steels are popularly grouped into three major classes: (1) austenitic stainless steel, (2) ferritic stainless steel, and (3) martensitic stainless

steel – based on their structure. Each of these classes of steels contains number of composition-specific grades; Table I in the annexure of this chapter lists these grades and composition. There is another class of stainless steel, called precipitation hardening grades, containing high Cr-Ni and-Cu for precipitation. Austenitic stainless steels contain chromium and nickel as alloying elements with 16-26% chromium (Cr), 6-22% nickel (Ni), and low percentage of carbon. This steel is austenitic in structure which is stable at the room temperature, non-magnetic and not heat treatable by quenching and tempering. It can be hardened only by cold-working and the only heat treatment that can be applied to these steels is the recrystallisation anneal after cold work to soften the structure. However, for wear-resistance applications, surface nitriding, which is a lower-temperature operation, can be applied to these steels. Ferritic stainless steels contain 11-27% chromium and low carbon; it is free from expensive nickel as alloying. These steel are magnetic in contrast to austenitic steels. These grades of steels are also not hardenable in general due to low carbon content; though at time hardening for partial improvement of strength has been attempted after adjusting the carbon level. Common heat treatment for ferritic stainless steel is also annealing for softening or conditioning the structure for a given application. Martensitic stainless steels contain variable carbon with 10-18% Cr and some Ni and Mo, if required. As the name implies, these grades of steels are for hardening by quenching and tempering for producing martensitic structure for applications requiring strength and toughness, along with corrosion resistance. Therefore, these grades require adjustment of carbon level in the steel for response to hardening (vide Chapter 2). These steels are magnetic, strong and hard; popularly used for making knives, blades and surgical instruments. There is another type of stainless steel, called precipitation hardening stainless steel (PH-stainless steel), which is heat-treated for developing very high strength that can be retained upto 400°C or above. This type of steel is suitable for applications in areas of pumps and shafts working in nuclear, marine or hazardous environment, turbine blades for generators and many aero-space engineering parts. The steel gets its strength after special heat treatment – called ‘solution treatment’ and ‘ageing’, which is carried out at low to moderate temperatures for controlled precipitation. Heat treatment of these grades will be discussed separately after the heat treatment of martensitic stainless steel. In principle, steels should possess ‘critical temperature’ ranges for phase transformation and hardening (vide Chapter 2), making possible the

conventional heat treatment of hardening by quenching. Since austenite and ferrite stainless steels are stable down to room temperature, these steels do not exhibit A3 and A1 type critical temperature range in stainless steels, other than martensitic grades where carbon level is higher. Hence, opportunity for conventional hardening by phase transformation is absent in austenitic and ferritic stainless steels. But, there are few other methods of heat treatment which are not exactly critical temperature dependent, like stress-relieving, tempering, recrystallisation annealing after cold-working, surface hardening by nitriding, etc, which can be easily applied to all grades of stainless steels. Hence, as and when necessary, these heat treatment processes can be selectively applied to most grades of stainless steels. For example, austenitic stainless steel can be bright annealed after cold-rolling or nitrided for improving surface hardness and corrosion properties. Similarly, ferritic stainless steel can be recrystallised after cold-working and tempered for adjusting the strength (or softness). In general, purpose of heat treatment of stainless steel is to produce changes in physical condition, mechanical properties and state of residual stresses, on one hand, and restoring optimum corrosion resistance when the same has been adversely affected due to fabrication or heating during processing, on the other. Example of the latter is the bright annealing of stainless steels, which is carried out by controlled heating for taking all corrosion-resistant alloying elements into solid solution – followed by controlled cooling for avoiding any carbide precipitation during cooling, which may otherwise lock up the useful corrosion-resisting alloying elements from the matrix. Purpose of controlled cooling during this treatment is to help restoring any loss of corrosion properties.

Fig. 9-1 Typical bell-type annealing furnaces for coils and sheets

Bright annealing is a popular process of heat treatment of stainless steels for coils, wires, strips, etc. Figure 9-1 illustrates a typical ‘belltype’ annealing furnace for bright annealing of stainless steel strips. The process is carried out after strip rolling in order to improve corrosion properties. Metallurgical task for bright annealing is to take all carbides into solid solution in the steel and avoid any carbide precipitation during cooling. Therefore, bright annealing involves heating to higher temperatures (1050 to 1150°C) for taking the carbide phases into solid solution of austenite. Hence, the process is required to be conducted at higher temperature under protective atmosphere, which is generally 100% nitrogen or hydrogen in order to protect the oxidation of oxidisable elements from the surface or degradation of the surface. Major challenge in bright annealing of stainless steel is to avoid carbide precipitation from the steel. In these high-alloy steels, carbide locks-in precious alloying elements like the Cr and the nearby area, especially the grain boundary areas where tendency of precipitation is high, becomes depleted with Cr and, thereby, causes loss of corrosion resistance. Because of higher %Cr in these steels, complex Cr-carbide might form even while heating the steel. Hence, it is necessary to take the annealing temperature to higher range (typically 1050 to 1150°C) for taking all such carbides into solid solution to the austenite, but without holding for long to avoid grain growth and deterioration of mechanical properties. However, if the steel is slow-cooled from annealing temperature, carbides may re-precipitate and lead to inferior corrosion properties. Hence, in most cases, stainless steels require fast cooling after annealing – bringing down the temperature to below 350°C – for avoiding carbide precipitation. Fast cooling is best effected by convectional cooling using air pressure or by water spray quenching, if permitted. Based on this general information, practical heat treatment processes of different types of stainless steels are discussed below.

9.2.1

Heat Treatment of Austenitic Stainless Steels

Austenitic stainless steels can be divided into three categories: (1) Normal un-stabilised compositions, e.g. popular grades like SAE 304, 310, 316, etc. (2) Stabilised compositions that contain additional amount of Ti or (Nb + Ta) (Ta = Tantalum); examples are SAE 321, 347 and 348 grades. (3) Extra-low carbon semi-stabilised compositions containing Mo, e.g. SAE 316L, 317L, etc, ‘L’ indicating low carbon.

Stabilisation in austenitic stainless steel is carried out by specially adding strong carbide and nitride-forming elements so that carbon does not lock the corrosion-resistant elements like chromium by reacting and forming chromium carbide. Heat treatment and heat treatment parameters vary along this line of austenitic stainless steel type. Table 9-1 shows the type of heat treatment that can be applied to some of these grades and what should be the heating / cooling parameters.

Heating to right annealing temperature is important for austenitic steels, because carbide precipitation can take place during the process of heating. Carbide precipitation range in these steels lies between 425°C and 900°C. Hence, steels need to be heated well above this temperature range and cooled in a manner (i.e. faster cooling) to ensure that dissolved carbide remains in solution. The problem of carbide precipitation is more in un-stabilised grades where carbide precipitations take place rather rapidly, necessitating rapid cooling (e.g. water spray cooling, if distortion can be minimised) from the annealing temperature for preventing carbide precipitation. Stabilised grades, which use elements like Ti and Nb for stabilisation, form stable carbides of Ti or Nb and do not lock Cr or Mo, affecting overall corrosion resistance. Hence, stabilised grades do not require faster cooling from annealing temperature. However, an additional stabilising treatment prior to, or during the process of fabrication of these steels could be helpful.

Additional stabilisation treatment can be given by heating to about 820 to 900°C and holding for 4–5 hours. Such treatment maximises the corrosion resistance in the steel ensuring total freedom from carbide precipitation. Since annealing or stabilisation temperature is high, care is necessary for maintaining the atmosphere inside the furnace for avoiding any oxidation or decarburisation. Also, sulphur in the gas (if gas heating is used) should be kept at minimum. Austenitic stainless steel can, however, be surface-nitrided for wear and galling resistance. Surface nitriding for these can impart high surface hardness; surface hardness in these steels can reach up to 1000 VPN due to presence of Cr that helps forming chromium nitride. The process is applied to spindles and pins made of austenitic stainless steels. Figure 9-2 depicts such spindles that are nitrided before use.

Fig. 9-2 An illustration of precision machine spindles made of austenitic stainless steel that are used after nitriding for harder wear resistance surface

Nitriding of austenitic stainless steel can be carried out in salt bath for common cases and by gas or plasma nitriding for special cases, following the principles of operation as discussed in Chapter 8. When stainless steels are treated with traditional nitriding a surface layer forms that consists of a diffusion zone and sometimes also a compound layer. The process characteristically produces chromium nitride in the diffusion zone, developing very high hardness for wear resistance. But, the process might also reduce the level of corrosion resistance (due to lock up of chromium in chromium nitride), which, however, can be minimised if subjected to gas nitriding process with controlled nitriding depth. Thus, austenitic stainless steel can be generally heat-treated for stress-relieving, re-crystallisation annealing, bright annealing, stabilising annealing, and nitriding.

9.2.2

Heat Treatment of Ferritic Stainless Steel

Ferritic stainless steels contain 10 to about 27% Cr as principal alloying element with low carbon (ranging between 0.08 and 0.20%). These steels are not exactly hardenable by quenching, though it is applied at times to partially improve the strength. Regular heat treatment of ferritic stainless steels is annealing for softening and improvement of ductility by further tempering. If annealing is carried out properly (as per principles outlined earlier), it also improves corrosion resistance. Annealing of ferritic stainless steel can also be used for homogenisation of structure. Ferritic grades have the problem of, what is called, 475°C ‘embrittlement’; this is due to prolonged exposure or slow cooling within the range of 400 to 525°C. The brittleness is believed to be due to precipitation of a high-chromium ferrite, and the embrittlement effect increases with increasing Cr content in the steel. Notch impact strength of the ferritic stainless steel is most affected by this brittleness phenomenon. Hence, it is important in the heat treatment of ferritic steels to avoid this 475°C embrittlement range. However, this brittle condition can be recovered by re-heating the job above the embrittlement temperature range (required heating is in between 600 and 850°C) and fast cooling. Annealing temperature of ferritic steel and the mode of cooling are, therefore, set accordingly as shown in Table 9.2.

Holding time at temperature depends on the section thickness, and it is usually 1 to 2 hours for bulk heating and 3 to 5 minutes per 0.25 mm thickness of sheet. Ferritic stainless steel containing high Cr may also suffer from ‘sigma phase’ formation, which drastically lowers the ductility, notch toughness and corrosion resistance of the steel. Sigma phase forms due to long exposure to temperature range between 500 and 1000°C, and chances of sigma phase formation increases with the presence of Si in the steel. However, problem of sigma phase formation in stainless steel is more in service exposure than during heat treatment. If sigma phase is formed during annealing, it can be re-dissolved by heating to above 900°C, which can then be followed by fast cooling for avoiding the formation of sigma phase.

9.2.3

Heat Treatment of Martensitic Stainless Steel

Heat treatment of martensitic stainless steel is very similar to normal low-alloy steels, where maximum strength and hardness of martensite depends on the carbon content. Therefore, carbon in these hardening grades of martensitic stainless steels is higher than the other grades, varying between 0.10 and 1.20% carbon, in order to facilitate hardening and development of high hardness / strength in the steel. Main alloying element in these grades is Cr, typically in the range of 12 to 15%, and Mo of about 0.20 to 1.0%. As such, these grades of steels can be subjected to annealing, hardening and other types of heat treatments as for low-alloy steels. However, annealing for softening and hardening for strengthening are two major heat treatment operations for martensitic grade stainless steels. Figure 9-3 illustrates the obtainable hardness values in different types of martensitic stainless steel after different treatment.

Fig. 9-3 An illustration showing obtainable hardness values in martensitic stainless steels after different treatment. [Source: http://www.kvastainless.com/stainless-steel. html -as on 02-11-2014]

The figure demonstrates that similar to low-alloy steels, maximum strength and hardness in martensitic stainless steels primarily depend on the carbon content (vide annexure for composition of different grades of martensitic stainless steels). The major metallurgical difference between martensitic stainless steels and low-alloy steels is that due to high-alloy content in the martensitic stainless steel grades, martensitic transformation is very sluggish and hardenability is so high that steels get fully hardened across the volume

even by air cooling. This situation leads to heat treatment problems, like distortion and cracks. Because of higher hardenability, martensitic steels are more sensitive to heat treatment process variables than low-alloy or carbon steels. As such, rejection rates after heat treatment is comparatively high in martensitic stainless steels, leading to high cost of quality because of high raw material cost and process cost. Challenge to martensitic grade heat treatment is how to overcome the problem of scrap and rejection after heat treatment, especially due to cracking and distortion. Because of these associated problems in heat treatment, uses of martensitic stainless steel have to be carefully weighed and adopted only when the application calls for superior corrosion resistance along with high strength and toughness requirement. Steps for quality heat treatment of martensitic stainless steels are: • Prior cleaning before charging into the furnace • Pre-heating the jobs by step heating in order to avoid rapid heating, i.e. to slow down the rate of heating to austenitisation temperature • Austenitisation at the optimum temperature, and • Appropriate quenching for (a) avoiding grain boundary precipitation of carbides, and (b) avoiding excessive ‘retained austenite’ in the quenched structure due to lower martensite finish (MF) temperature of these high-alloy steels. If the steel is of very high hardenability, ‘martempering’ process of hardening should be considered for ease of hardening (vide Chapter 7) Prior cleaning is necessary for efficient heat transfer during heating by facilitating free heat contact with the surface, as well as to support the protective atmosphere used for such heat treatment. Presence of oil and grease on the surface of jobs can affect the atmosphere inside, making it carburising and thereby increasing the problem of crack and distortion. Purpose of preheating is to slow down the rate of heat pick up during heating for austenitisation in these grades of steels. Thermal conductivity of these steels is lower than the common steels and is likely to promote high thermal gradients and stresses if rate of heating is high, leading to cracks and distortion. Preheating is particularly important for (a) parts with thinner gauges and/or section changes, (b) parts with sharp corners, radius and key grooves, (c) parts with prior cold-working and (d) previously hardened jobs. Preheating temperature of the steel is between 750 and 780°C and is to be held for sufficient time for uniformity in temperature in the whole section. Higher the carbon in the steel more is the requirement of preheating; such as for grades like SAE 414, 420, 431 and 440. After preheating, jobs are taken up for austenitisation.

Austenitisation temperatures of different grades of martensitic stainless steel are shown in Table 9-3. The table also indicates the quenching condition and tempering temperatures for these steels.

When the combination of maximum corrosion and strength is required, steel should be austenitised at higher end of the temperature range in order to take all alloying and carbides into solid solution. Soaking time is adjusted in a manner such that chromium and its carbides are taken into the solution but not for too long, in order to avoid decarburisation and excessive grain growth. In general, for section thickness up to 12 mm, initial soaking time of 30 to 45 minutes is recommended and thereafter additional soaking of 30 minutes for every 30-mm section. However, soaking might have to be doubled if the structure had been annealed before hardening. Quenching can be carried out using either air-cooling or by cooling in mild oil. While air cooling might be possible for all grades of martensitic stainless steel, faster oil cooling is recommended for avoiding carbide precipitation and, thereby, ensuring higher strength and corrosion resistance. Adequate care in cooling is necessary for grades like SAE 414, 420, 431 and 440 for avoiding grain boundary precipitation of carbides - which reduces corrosion resistance of the steels. Cooling through the temperature range of 870 to 540°C is critical for grain boundary carbide precipitation in these steels. High carbon grades (e.g. SAE 431 and 440) can, however, be subjected to martempering to avoid chances of distortion and cracking as well as for controlling retained austenite. High percentage of retained austenite is a problem with higher carbon and Ni-bearing martensitic grade stainless steels, where retained austenite up to 30% can be observed. For reducing retained austenite,

the structure needs to be cryogenically treated at about – 73°C immediately after quenching. For complete elimination of retained austenite, double tempering might be helpful after sub-zero treatment. Purpose of double tempering is to ensure that all martensite formed from retained austenite are tempered in the final structure. Figure 9-4 illustrates the appearance of martensitic structure in martensitic stainless steel containing molybdenum.

Fig. 9-4 Martensitic structure in a Mo-bearing martensitic stainless steel [Austenitised at 1020°C for 30 minutes then oil-quenched]

Mechanical properties of martensitic stainless steel may significantly vary with the austenitisation temperature, its control and the way the steel is cooled. Steels containing Mo can also exhibit secondary hardening with increasing tempering temperature above 500°C. Tempering stages of martensitic stainless steel is similar to low-alloy steels; tempering to 205 to 260°C leads to stress-relieving of the quenched structure, which restores some ductility and then the structure starts softening above 315°C. Therefore, for wear resistance application, tempering temperature of the steel should be limited to about 300°C max for controlling the hardness drop. Typical mechanical properties of some martensitic stainless steels after heat treatment are given in the Annexure (Table-II) of this chapter.

9.3 HEAT TREATMENT OF PRECIPITATION HARDENING STAINLESS STEELS Precipitation hardening stainless steels – popularly called PH-stainless steel – are another type of stainless steels that are increasingly gaining importance because they can be strengthened by heat treatment after fabrication without resorting to very high temperatures or rapid cooling.

These steels possess properties that find applications for high-duty structural parts in nuclear engineering, aerospace and jet engine manufacturing. The PH-stainless steels can be grouped into three types: martensitic, semi-austenitic and austenitic – as per their structure. Desirable characteristics of these grades of steels include ease of hot working, machining, forming and welding, allowing easy fabrication of high-strength engineering parts that can retain strength up to the temperature of 400°C or more (but below the ageing temperature used for the treatment). High strength can be achieved by relatively low-elevated temperature ageing treatments or by refrigeration plus ageing, and ultrahigh strength can be produced by a combination of cold-working and ageing. Key to the properties of PH-stainless steels is their heat treatment, and the purpose of the entire chain of heat treatment is to ensure high strength and ductility along with good corrosion and oxidation resistance. Uniquely, they have high strength characteristic of martensitic stainless steel and good corrosion resistance characteristic of austenitic stainless steel. Main alloying of PH-stainless steels are chromium and nickel, along with some addition of alloying elements for precipitation hardening by ageing, e.g. Cu, Al, Ti, Nb, etc. These steels may also require good weldability for fabrication. Hence, % carbon is kept low in these grades. Typical nominal compositions of some of these steels are given in Table 9-4:

Heat treatment of A-286 type austenitic steel involves solution treatment at 980 or 900°C – as per application – followed by ageing for 12 to 16 hrs and air-cooling. Austenitic structure is retained after the solution treatment and strength gets developed after ageing that leads to precipitation of carbides and intermetallic compounds. While 980°C treatment gives highest creep rupture strength, 900°C treatment gives best combination of ductility and strength. For martensitic and semi-austenitic grades, high strength in the steels results after proper solution treatment – which conditions the martensitic or austenitic matrix, followed by ageing for precipitation. Precipitation strengthening is achieved through addition of one or more of the elements like Cu, Al, Mo, Ti or Nb. For hardening by precipitation, these grades of stainless steels are given multi-stage heat treatment; once to prepare the structure for fabrication (e.g. machining) by softening and then to develop the final properties – such as high strength, good weldability, good corrosion resistance, and elevated temperature properties – by another set of heat treatment. Typical heat treatment cycles for the popular 17-4 PH martensitic stainless steel are illustrated in Table 9-5 for general guidance about the process of heat treatment in these steels.

The table demonstrates that PH-stainless steels can be heat treated at varieties of temperatures for developing wide range of properties, but the principal mode is the solution treatment and ageing. Table 9-6 illustrates the variation of mechanical properties of 17-4 PH-stainless steel after different hardening/solution treatment temperatures.

The table demonstrates that with increasing solution temperature, strength decreases but ductility improves. Hence, depending on the strength and toughness required for an application, solution treatment temperature should be appropriately adjusted. Due to high-alloy content in PH-strengthening steel, rate of heating has to be slow and gradual. Properties of PH steel are sensitive to the temperature of heating for solution treatment. This is because variation of solution temperature leads to variation in the amount of alloying that is taken into the solid solution and, thereby, conditions the martensite formation and finish temperatures (MS and MF) of the austenite. Higher solution temperature will take higher amount of alloy into the solution and lower the MS and MF temperatures, leading to incomplete transformation to martensite and retaining some delta-ferrite and austenite. As a consequence, incomplete martensitic transformation will lower the strength, but associated retained austenite will improve the toughness. Such a trend in the result can be observed from Table 9-6. Final structure of PH-steel, however, emerges after the ageing. Ageing temperature and time is set after solution treatment for producing the

characteristic precipitates. Precipitates types in 17-4 PH steel contain Cu-rich particles and some intermetallic compounds, as per the composition of the steel. Mechanical properties can be optimised by varying the precipitation hardening treatment; exhibiting maximum hardness and strength by ageing between 450 and 510°C due to precipitation of coherent Cu-rich particles. However, for maximum ductility and toughness, the steel should be aged at temperature above 540°C when the Cu-rich precipitates become incoherent with the martensitic matrix and loose some strength.

Principally, therefore, precipitation hardened stainless steels are first softened or ‘solution-treated’ when it is soft and can be machined. Thereafter, the steel can be given the appropriate ageing treatment for improving the strength and toughness through fine precipitation of agehardening elements like Cu, Ti, Al, etc. Typical applications of 17-4 PH stainless steel are in aerospace and nuclear engineering where high strength at higher temperatures is required for safe and secured performance. In general, these steels are used for components like: landing gears, valves and other aerospace engine components, high-strength shafts, turbine blades, nuclear waste casks, moulding dies, etc. Maraging steel is yet another kind of high nickel precipitation hardening steel; one such composition is the steel containing 20% Ni and some Ti (1.3 to 2.0%). Nickel ensures initial austenitic structure and titanium reacts with nickel to form precipitation strengthening by producing fine Ni-Ti intermetallic compound. This type of steel can be first solution-treated at around 830°C for softening the steel for necessary fabrication, then aged at about 710°C which causes precipitation

of Ni3Ti type intermetallic compound. Due to precipitation of nickel compound, matrix gets depleted of Ni and stability of austenite is reduced. Therefore, on air-cooling from the ageing temperature, martensite transforms from this steel matrix, making the structure stronger. If necessary, steel can be refrigerated for some time for ensuring complete transformation – followed by low-temperature tempering. The other method of heat-treating maraging steel is cold-working the annealed steel for 25% reduction or more, and then refrigerate it for several hours for transformation of cold-worked austenite to martensite. Upon appropriate heat treatment, maraging steel can develop very high yield strength (in the range of 1400 Mpa and above) along with very high toughness. Hence, these steels have high strength along with excellent fracture toughness for applications in the critical nuclear engineering structural parts. In summing up of stainless steel heat treatment, it can be observed that stainless steels, barring martensitic grades, are not hardenable by conventional quenching and tempering. Therefore, heat treatment of austenitic and ferritic grades is limited to annealing for solution treatment and other lower temperature treatments, e.g. stress-relieving and tempering after cold-working. Hardening of these steels can be achieved only by cold-working and tempering / stress-relieving, as per hardness level required. Annealing is the main high-temperature heat treatment for the austenitic and ferritic grade steels, involving solution treatment and conditioning of austenite composition for higher corrosion resistance. As such, annealing temperature of these grades of steels is designed to take as much corrosion resisting alloys (e.g. Cr and Ni) into solution as possible and cool it down in a manner to prevent grain boundary carbides, which reduces corrosion resistance property. Heat treatment of martensitic grades is very similar to normal low-alloy steels and can be subjected to hardening and tempering, as discussed earlier. Problems of hardening of martensitic grades are its high hardenability and variability of properties with slightest variation of heat treatment parameters, especially with respect to strength and corrosion resistance due to tendency of retaining higher volume of retained austenite and formation of grain boundary carbides. Steps and precautions necessary for heat treatment of martensitic steels have been discussed in this section. Finally, Table 9-7 illustrates the popular heat treatment processes and properties of different grades of stainless steels for an overview. The table is only indicative of what type of heat treatment can be given to various types of stainless steels and the nominal values of mechanical properties that can be expected after the heat treatment.

In addition to these conventional heat treatments, austenitic stainless steels can also be subjected to surface engineering by nitriding for improved wear and corrosion resistance. Martensitic steels, on the other hand, can be subjected to martempering for reducing heat treating problems involved with high hardenability of the steels. Similarly, precipitation hardening grade stainless steel (e.g. 17-4 or 17-7 PH-stainless steel) can be double-aged or over-aged after solution treatment and hardening for further improvement in impact strength.

9.4

HEAT TREATMENT OF HEAT RESISTING/ CREEP RESISTING STEELS

This group of steels have commonality between them in properties and composition. Both types of steels require high oxidation resistance and hot-strength for operation at higher temperature. These properties are developed in these steels through alloying with higher percentage of chromium (Cr) and other alloying like Mo and V or W (Tungsten) are used for imparting higher temperature strength.

9.4.1

Heat Treatment of Heat Resisting Steel

Heat-resisting steels are a special group of high-alloy steels that are used for varieties of applications involving heat and corrosion/scale resistance. Table 9-8 exhibits some standard grades of heat-resisting steels as per BS 10095 – which are grouped under ferritic, austenitic and martensitic as per their principal structure. These steels are characterised by the presence of chromium and/or nickel in sufficient quantity for imparting heat-resisting properties; percentage of chromium/nickel increasing with change from ferritic to austenitic grades. Principal areas of application of heat resistance steels are in the areas of high-temperature furnace parts, boilers and high-temperature engine parts – like engine valves. In general, these steels are characterised by the presence of higher chromium for producing large amount of chromium carbides for higher temperature strength and stability of structure. Compared to stainless steel or the composition in Table 9-8, some heat-resisting grade of steels may have higher carbon with high chromium for higher elevated temperature strength. Higher carbon is used for some selected grades for promoting larger coalescence of carbide particles, which are known to offer less obstruction for grain growth – a phenomena which helps in improving creep strength required for resisting deformation in the higher application temperatures. Some such grades of steels and their heat treatment are shown in Table 9-9, where carbon is relatively higher and nickel is somewhat limited. Since these steels are generally used for high temperature furnace parts, care is necessary for using very high nickel steel. This is because steels with high nickel may react with gases containing sulphur dioxide or other sulphur compound present in most furnace atmosphere, making their usage difficult. At higher temperatures nickel reacts with sulphurious gases and form deleterious intercrystalline nickel sulphide films, leading to embrittlement of the steel. Hence, higher nickel bearing steels call for control of sulphurious atmosphere within the furnace where these types of steels are to be used. Compositions of steels in Table 9-9 are modified for making them suitable for applications involving high temperature, wear and creep – such as engine valves of diesel engines and furnace parts. Steels in serial number 1 to 4 are designed essentially for exhaust valves in automobile, aero and marine diesel engines. Other alloys are designed for hightemperature oxidation resistance as well as creep-resistance applications – like the one encountered in high-temperature annealing or heat-treatment furnaces. Since these steels are partly modified in chemistry, no exact grade names have been mentioned; instead structure type of these

steels after heat treatment has been indicated in column-4 of Table 9-9, which is an established method of recognising these steels for their applications. In addition to the heat treatment conditions given in the table these high-carbon high-chromium steels require, in practice, special care for homogenisation of structure and composition during heat treatment. As such, these steels are pre-heated and held above 750°C and then homogenise, soaked and quenched for heat treatment from temperatures indicated against each steels. Heat treatment cycle and stages shown for these steels are more or less valid for other grades of heat resisting steels.

Functionally, heat-resisting steels must have the basic properties of: • Resistance to oxidation and scaling at higher temperature • Retention of strength at the elevated temperature (i.e. creep strength), and • Structural stability with respect to carbide precipitation, spheriodisation, sigma formation and temper embrittlement. Chromium, silicon and aluminium in the composition provide the oxidation and scale resistance at higher temperature; and retention of strength (i.e. the creep strength) is achieved by (a) increasing the softening temperature of the steel by appropriately alloying with Cr, Ni, Mo and W (Tungsten) that form stable carbides, and (b) judicious use of

precipitation hardening, but avoiding over-ageing, as per the intended service temperature. Structural stability is ensured by alloying with elements that form stable carbides, e.g. Cr, Mo, W, etc., and appropriate heat treatment. Since furnaces may have to be shut down from time to time exposing the furnace parts to repeated cooling and heating cycles, steel parts made of heat-resisting steel should not undergo airhardening on cooling to avoid premature failure of furnace parts. Hence, right amount of Cr and Si is required in the steel for raising the critical temperature of phase changes above the point that can be reached in service. To attain high strength and heat resistance characteristics, these steels are subjected to quenching and ageing. Purpose of quenching in these cases is to take all carbides and intermetallic phases into solid solution before ageing. Holding temperature for hardening is generally higher than conventional hardening temperature of engineering steels, vide Table 9-9. This is necessary for taking all extra phases in these high-alloy steels into the solid solution and producing the required grain size for better high-temperature creep strength. Cooling rate should be appropriate for limiting carbide precipitation during cooling; excessive precipitation of carbide may cause loss of oxidation property in the steel. Ageing or tempering can be accomplished by isothermally holding at the temperature as indicated in Table 9-9 or as appropriate for a specific grade. Heat-resisting steels with higher alloy content (vide austenitic grades in Table 9-8 containing high Cr and Ni) can be best heat-treated by following a ‘double process’ where first heating is at higher temperature (1150 to 1200°C) for homogenisation of the structure and producing coarser grain size, which is beneficial for creep. The second heating is at 1000 to 1100°C for preferential and controlled carbide precipitation at the grain boundaries and formation of hardened structure as per the composition. Ageing or tempering is rendered for producing stable carbide precipitates of complex structure, e.g. M23C6, M6C, M7C3, etc, which precipitates from the high temperature solid solution during ageing. These carbides offer stable structure in the steel which can withstand higher service temperature. If the steel contains high Ni along with alloying like, Al or Ti, ageing can promote formation of intermetallic compound of Ni3Ti or Ni3Al, which are more stable at elevated temperature and offer better high temperature strength and stability of the structure. A frequently observed problem of heat-resisting steel is the sigma phase formation in service. Sigma phase formation is not a problem during heat treatment, but in service. When the steel is heated above 550°C in service for prolonged time, ferrite present in the steel may break down to complex iron-chromium-nickel-molybdenum intermetallic compound,

Fig. 9-6 Microstructure of 25Cr-12Ni heat-resisting steel after heat treatment

known as ‘sigma phase’. Figure 9-6 shows a high magnification structure of 25Cr-12Ni heat resistant steel; ideal microstructure of such steel should have been austenite plus carbide, but it has austenite plus ferrite plus carbide. The presence of ferrite in this steel is attributed to segregation in the high-alloy steel during solidification. Segregation can lead to ferrite formation in such steels which, in turn, increases the chance of sigma phase formation. Structure containing some ferrite patches is susceptible to sigma phase formation in service and can lead to failure arising from intergranular fracture due to lower impact strength of such structure. Sigma phase formation also leads to lower corrosion and oxidation resistance due to locking up of related alloying elements (Cr and Ni) in the compound. The effect of sigma phase in service is shown in Figure 9-7. The figure illustrates a service failure of furnace roller hearth due to sigma phase formation at the grain boundaries.

Fig. 9-7 Illustration (a) showing the brittle fracture face of a furnace hearth roller and (b) showing the grain boundary sigma phase precipitation of the roller in service, leading to failure. [Source: http://cdn.intechopen.com/pdfs-wm/39181.pdf as on 02-11-2014]

The failure depicted in Fig. 9-7 is caused by severe drop in impact strength with the formation of sigma phase. Avoiding this embrittlement effect due to sigma phase formation in service temperature above 650°C is difficult in ferrite containing steel, but chances can be minimised by giving a prior homogenisation treatment of the casting (for long time) followed by controlled cooling. The microstructural inhomogeneity, leading to the possibility of ferrite formation, can be minimised in the steel by long holding time at sufficiently high temperature (e.g. above 1150°C). Finished parts having started forming sigma phase in service can be given a prolonged homogenisation treatment, which can change the morphology of sigma phase from dendritic structure to globular structure. Globular structure of sigma phase is less harmful than dendritic structure. Sigma phase, if present at all, should preferably be converted to globular structure, which is known to relatively improve the impact strength and ductility of heat resisting steels.

9.4.2

Heat Treatment of Creep Resisting Steel

Creep is an elevated temperature phenomenon, involving time and temperature dependent plastic deformation of the material. When the steel is subjected to stress at elevated temperature, it can undergo plastic deformation, which is time, temperature and stress dependent. Creep deformation is plastic in nature and irreversible; once the damage has occurred, it limits the useful service life of the components. Resistance to such creep damage comes from the hot strength and structural stability of the material. There are lot of similarities in steel grades between heat-resisting and creep-resisting steels, because they all have to resist deformation under the service load at elevated temperature. As such, there is commonality in the heat treatment approach to these types of steels. Like heat-resisting steels, creep-resisting steels contain Cr as principal alloying element, over which other alloying elements are added – as per the requirements of application temperature and stress – in order to build up the necessary internal structure and assure the stability of the structure. Basic approach to heat treatment of creep-resistance steels is, therefore, to produce the right type of internal structure (e.g. tempered martensitic or austenitic structure) with precipitation hardening that is stable at the operating temperature. Precipitation type can be complex alloy carbides and nitrides, as per composition. Figure 9-8 depicts the typical creep deformation stages of steel, varying with temperature.

Fig. 9-8 A typical creep curve exhibiting stage I (Primary), II (Secondary) and III (Tertiary) of creep (curve B) at different temperature [Creep curve A pertains to lower temperature, curve C is for higher temperature and curve B is at an intermediate temperature between A and C].

The figure demonstrates that though creep of steel might go through different stages in service, the useful service life is determined by the secondary creep (or the so-called ‘steady state creep’) rate. Alloying of the steel and its heat treatment is, therefore, selected for developing the structures capable of lowering the secondary (i.e. steady state) creep rate, expressed as ‘mean creep rate’ vide Fig. 9-8. This means that for a creep application, steel has to be selected as per the service temperature and stress. For lower temperature and low-stress applications, the steel could be a simple Cr-Mn grade steel but for higher temperature and stresses, the steel has to be with appropriate alloying for producing a strong and stable structure. For this purpose, steel composition with alloying such as Cr-Mo, Cr-Mo-V or Cr-Mo-W, etc., are commonly used. Different grades of steels used for creep applications are covered by different national and international standards; for example, ASTM grade A106, A193A/M, A516, and BS (British Standards) EN 10095, 10302, etc. Some popular creep resistant steels are:

Structurally, these steels are either martensitic or austenitic along with precipitates of carbides, nature of which depends on the exact alloying. In fact, by combining different percentage of Cr and Mo, varieties of steels of different compositions can be developed–with or without addition of other alloying for precipitation hardening. Examples of latter types are: 5Cr-Mo-Ti, 5Cr-Mo-Si-Al, 5Cr-Mo-Nb, etc. Thus, creep-resistance steels are a large family of steel where composition can be developed or selected as per the applied stress and temperature, and heat treatment is carried out with the objective of producing a strong and stable structure as per the service conditions. Heat treatment of creep-resistance steels depends on the type of steel and their application. Austenitic steels containing high nickel cannot be hardened by quenching or fast cooling like the ferritic grades. Austenitic grades are, therefore, solution-treated and tempered (vide the heat treatment of austenitic heat-resistance steels) for producing carbide precipitates for hardening. Other varieties of creep steel are the plain C-Mn or low-alloy steels generally used for pipes and tubes for relatively low temperature application. These steels are generally given a softening treatment at around 650°C, followed by air cooling, for stabilising the structure. The other types of creep resistance steels are Cr-Mo grades; content of these elements may vary depending on the grade, and can be with or without other alloying for precipitation strengthening. These steels can be hardened to martensite structure and tempered for producing a strong and stable structure for the service application where higher tensile and impact strength is required. In general, hardening is carried out by quenching (either in oil or in air as per composition) from 900 to 950°C and then tempered at suitable temperature below the A1 temperature. Higher hardening temperature than conventional low-alloy steels are given for taking the complex alloying elements into solid solution and producing coarser grain size, which favours creep strength. However, for higher Cr-Mo content steels, including 12Cr steel, hardening temperature is in the range of 950 to 1100°C. Higher temperature is required for these steels for taking into solid solution all the complex carbides and other phases that can exist in such high-alloy steels before transforming to martensite by fast cooling (generally air-cooling). This treatment ensures complete utilisation of alloying elements for optimum microstructural development. Tempering of such martensitic structure at temperature below 750°C can then produce sufficient precipitation of stable carbides and nitrides or carbo-nitrides, if nitride -forming elements are present in the steel. Higher tempering temperature is used in these cases by following the cardinal rule that a structure is stable only up to or below the temperature where it forms.

Perhaps, approach to the heat treatment of creep-resistant steel can be best illustrated by referring to the heat treatment of 12% Cr steel, which is commonly used for turbine blades. Heat treatment of 12 Cr steel is generally carried out by hardening from 950 to 1100°C, followed by tempering at 650 to 730°C. Purpose of high-temperature hardening is to take all alloying and other phases present in the steel into solid solution and then cool in air for producing martensite. Tempering at 650 to 730°C is recommended for producing precipitation hardening by stable carbides and nitrides. Higher temperature of tempering may also produce some ‘free ferrite’ in this type of steel, improving the impact strength. Free ferrite in the tempered structure of 12 Cr type steel is known to improve the impact strength, making the steel tougher in the elevated temperature applications. Considering the beneficial effect of free ferrite in such structure, ferrite forming elements like Mo and Si (up to 0.50%) are added sometimes for ensuring the presence of free ferrite. Figure 9-9 shows a typical microstructure of high chrome ferritic creep resistance steel after tempering around 650°C. In these high-chrome ferritic steels, martensite can be produced by simple air cooling, exhibiting lath type martensitic structure with high dislocation density, which is introduced during cooling. These dislocations rearrange themselves during tempering into cells and sub-grains within the parent austenite grains that confine the martensite. Such a tempered martensitic structure with sub-grains and dislocation tangles provides higher resistance to deformation and, thereby, offering higher creep strength. But, with increasing tempering temperature, dislocation density decreases, relatively lowering the creep strength.

Fig. 9-9 A typical microstructure of high-chrome ferritic steel, showing smaller subgrains within large grains and patches of free ferrite

The tempered martensite structure in the high-chrome steel is characterised by the presence of typical lath type martensite along with complex (M23C6) type carbides, and sometime carbo-nitrides. The structure is very stable at elevated temperature.

9.5

HEAT TREATMENT OF TOOL AND DIE STEELS

Tool and die steels are used for varieties of metal cutting and forming applications, such as for metal cutting, drilling, machining and metal forming under dies and tools. Such applications may be at room temperature or at elevated temperatures. Based on the temperature of application, tools and die steels are divided into two categories; namely: (1) Cold work tool steels, and (2) Hot work tool steels. The other grade of tool steels is the high speed steels (HSS), which are widely used for manufacturing of metal cutting tools like taps, drills and broaches. In general, cold working tool steels are used for room temperature applications like tools for cutting, shaping, forming, bending, etc and hot working steels are used for applications involving higher temperature of operations – like forging and extrusion dies. As the name implies, high speed steels (HSS) are used for manufacturing drills, taps and other tools for metal cutting operations under higher speed of cutting; these tools do not work well at low cutting speed. Because of higher speed of cutting, the tools not only encounter high friction which promotes higher heat at the tool face, but also can face high impact due to cutting force variation or vibration in the machine. Thus, high speed steel tools are required to have high hardness and strength along with good toughness. Depending on the application, properties for tool and die steels should fulfil: • High hardness for wear resistance under the cutting force and friction, • High strength and toughness for absorbing shock and impact coming from erratic cutting forces and fluctuation of application conditions, • Hot hardness and strength for hot working tools and dies, and • Impact strength. Demands of these application specific properties call for a strong and tough structure as well as freedom from any internal stresses and flaws. Tool and die steel structures are characterised by the presence of uniformly distributed fine and stable carbides – for providing stability of the structure, strength and hot hardness. Most tool and die steel applications experience rise in temperature during cutting or working due to frictional heat, demanding good hot hardness of the steel.

There are number of steels that satisfy the application conditions of tools and dies for different purpose – such as: cold working, hot working or high speed cutting. However, their steel composition and alloy content varies over a wide range. Type of steel may vary, starting from simple carbon-manganese tool steel with higher carbon (e.g. 1.0%C-.65%Mn) or composition like 0.9%C-1.7%Mn steel for standard cold working applications and going up to complex high-alloy steels containing 18%W for high speed cutting tools and high chrome-tungsten steel, containing W-Cr-Mo-V, for die blocks. And, depending on the composition and alloy content, and the purpose, their heat treatment details differs. Table 9-10 exhibits the general classification and grades of tool steels used in the industry.

Since these steels contain, in general, higher carbon and alloy than normally used for general engineering purpose, these steels require some short of softening treatment (e.g. annealing by slow cooling) for manufacturing the tool. The softening or annealing treatment of these steels is generally carried out between 730 and 760°C for C-Mn type steels and 750 to 800°C for other alloy steels. Hardness range aimed for after annealing also varies; it varies between 201 and 217 HB for C-Mn grades to 210 to 250 HB for alloy steel grades, exact value depends on the alloy type. Softening temperature and time, therefore, needs to be adjusted accordingly. Steels containing high Cr and W, which is a high-melting alloying element, may require even higher annealing temperature than that mentioned earlier for other grades – for achieving the hardness range of 210 to 250 HB. The principal method of heat treatment of these steels for developing the final end-use properties is the hardening and tempering. However, high-alloy grades, especially steel containing high Cr and W, are required to be given a pre-heating treatment while being heated to the hardening temperature in order to relieve internal stresses and avoid high thermal gradient in the steel body. Pre-heating and controlled heating to higher temperature is necessary for low thermal shock resistance of these steels. Figure 9-10 illustrates the principal points of heat treatment cycle for tool steels in general.

Fig. 9-10 steels

An illustration showing the conventional cycle of heat treatment for tool

[Source and Acknowledgement: Simply Tool Steel – http://www.simplytoolsteel.com/conventional-tool-steel-heat-treating-cycle.html as on 04-11-2014]

Critical points of tool steel heat treatment are (a) necessity of preheating, and (b) application of double tempering. Due to higher carbon and

alloy content, these steels on quenching can have substantial amount of retained austenite – which needs to be neutralised by double tempering. Otherwise, there could be soft spots in the treated tool and that might cause heavy wear. Figure 9-11 illustrates the effect of double tempering on the retained austenite.

Fig. 9-11 (a) and (b) An illustration showing effect of double tempering on a quenched tool steel sample. [Second tempering transformed the austenite patches to tempered martensite]

In these micrographs, it could be noted that prior retained austenite areas have turned more grey after second tempering. However, such structure requires another tempering to temper down the martensite formed from prior austenite by second tempering. If the steel contains high molybdenum and nickel, cryogenic treatment at sub-zero temperature might be necessary for elimination of all retained austenite. Sub-zero treatment is carried out in between the double tempering process as indicated in Fig. 9-10. An idea about the typical heat treatment cycle of different types of tool steels can be obtained from Table 9-11, which shows that a pre-heat is always necessary for heat-treating tool steels. In contrast to standard heat treatment for low-alloy steels, tools steel hardening temperature is relatively higher and the duration of holding at that temperature is shorter. The latter is to avoid any chance of grain growth or burning at higher holding temperature. As these steels are mostly higher alloy steel, they need to be tempered soon after the quenching (i.e. hardening) in order to avoid any build up of residual stresses and internal micro-cracks. Various precautions required for heat treating tool steel can be further illustrated with reference to the heat treatment of 1.5C-12Cr-1Mo cold-

working die steel (popularly known as D2 steel), which is known for its outstanding wear resistance. Some important characteristics of this steel are: 1. Due to high chromium content in the steel, the steel is susceptible to distortion or cracking during heating to hardening temperature. Hence, a pre-heating schedule would be necessary. 2. Austenitisation is to be carried out at higher temperature than others (1030°C, vide Table 9-9) due to high C, Cr and Mo content. But, despite the higher austenitisation temperature, the steel may retain 5 to 10% undissolved carbides due to higher C and Cr. Hence, without further raising the temperature of hardening, time of heating should be adjusted to make the carbide particles even in size and dispersion for better wear resistance. 3. If necessary, the steel can be tempered at 550°C for developing secondary hardening – as the steel contains high Cr and 1% Mo. 4. The steel might require refrigeration for 100% conversion of martensite, because due to high C and Cr content, the MS

temperature may go down below the room temperature, retaining some untransformed (retained) austenite. This will necessitate low temperature stabilisation treatment and/or double tempering for introducing toughness in this steel. Therefore, the heat treatment schedule for this steel, in practice, is followed as below: • Rough machining of the tool from suitably annealed stock – followed by – stress relieving at 650°C for half to one hour • Preheating to around 650°C with holding time of 1 minute per mm thickness • Austenitise at 1030°C with holding time of 0.5 min to 1 minute per mm thickness followed by quenching in air/oil • Stabilisation by sub-zero treatment at –75°C • Temper at 200°C or at 550°C for secondary hardening, if required • Grinding to final shape and dimensions with sufficient coolant and care to avoiding grinding cracks. This type of heat treatment schedule is also followed for hardening the hot work die steels. Figure 9-12 illustrates the common features of heat treatment of hot working die steels containing Cr-W and V. Figure 9-12 illustrates that: • The steel is not only preheated, but also requires controlled heating in steps to avoid any distortion or cracking, and • After quenching, tempering is carried out in two stages, where the second tempering is at lower temperature than the first one. Further, heat-treated steel can be given a nitriding treatment at the end for developing a very hard wear resistance surface. However, nitriding temperature must be controlled below the last tempering temperature – because longer nitriding cycle may otherwise cause drop of hardness and strength of final product. Generally, a gas-nitriding treatment is given at relatively lower temperature.

9.6

HEAT TREATMENT OF HIGH SPEED STEELS (HSS)

High speed steels – popularly known as HSS – are a special class of highalloy steels, with C, Cr, W and Mo, as main constituents. These steels are primarily used for manufacturing of high speed cutting tools, such as drills, taps and dies, milling cutters, reamers, broaches etc, and required to have good wear resistance, toughness and hot hardness. For improved hot hardness and wear resistance, vanadium (V) and cobalt (Co) are also used as alloying for these steels.

Fig. 9-12 An illustration of the steps in the hardening of a hot working die steel containing Cr-W-V with hardening temperature of over 850°C. [Acknowledgement and Source: Steels for Die casting Mould, Buderus Edelstahl, Germany]

Based on their composition, these steels have been grouped and designated by AISI (American Iron and Steel Institute) as per their principal alloying element; namely ‘M’ type denoting molybdenum as their base alloying and ‘T’ type, denoting tungsten as their base alloying. Table 9-12 gives the composition of some representative grades of high speed steels.

Functionally, these grades of steels require the following basic properties: • Hot hardness for retaining its hardness and cutting edge efficiency for working at higher temperature – generated at the tool tip due to cutting force and friction • Toughness for shock and impact absorption during high speed cutting, and • Wear resistance (at room temperature and higher temperature) for resisting wear arising from the friction and frictional heat at the tool tip due to applied cutting force. These properties are obtained by selecting the appropriate grade of steel, and processing and heat treating the same as per requirement.

Quality and accuracy of heat treatment exerts considerable influence on the HSS tool life. Heat treated structure of HSS tool is very sensitive to austenitising temperature and time, which vary with the type of alloy content. Therefore, an understanding of alloy types and how they behave during austenitising is necessary – for adjusting the heat treatment parameters. Carbon in high speed steels contain relatively higher for (a) increasing the hardness of the martensite, and (b) for producing sufficient amount of complex alloy carbide – by reacting with other alloying elements. Uniformly dispersed carbides are necessary for providing hardness/ strength, in general, and wear resistance in particular. In fact, about two-third of the carbon goes to the formation of complex alloy carbides in these steels. Tungsten (W) – the most important of all alloying elements in these steels – is known to greatly improve the wear and hot hardness of the steel by producing complex (M6C) type alloy carbide (M stands for metal, which should be W in this case). In addition to this, when tungsten is dissolved in the steel, it also increases hardenability, increases hot strength of the steel, and considerably retards softening on tempering. However, for taking W into solution, higher temperature of heating is required (e.g. 18-4-1 HSS requires austenitisation between 1150 to 1300°C). Molybdenum (Mo) has nearly similar effect as W, and they can be used interchangeably by substituting W by Mo in the ratio of 1:1.6 to 2 for cost. Mo produces complex M23C6 type carbide and gives rise to secondary hardening. However, Mo bearing HSS require lower austenitisation temperature than W steel and require stricter temperature control due to the tendency of grain growth. Also, tempering temperature and time of Mo bearing steel would be lower than the W steel due to the nature of softening in heating. Chromium (Cr) is another alloying which is present in all high speed steels. It is principally added to promote hardenability. Chromium also reduces chance of oxidation and scaling while hot. Another contribution of Cr is that by reacting with carbon, it can form M23C6 type carbides which add to structural stability at elevated temperature. Cobalt (Co) is added to HSS upto 12% in order to increase the hot hardness, and, thereby, improve cutting efficiency. Cobalt increases the austenitisation temperature, and on cooling mostly remains in the matrix without contributing to any carbide formation. Hence, nearly all of cobalt added to the steel goes to increasing the hot hardness. However, Co in steel might slightly reduce the toughness after heat treatment. Another alloying is the vanadium (V). Contribution of V in HSS is its ability to form stable carbides of type MC or M4C3, which almost remain

insoluble in the steel during austenitisation, and, thereby, restrict grain growth at high austenitisation temperature required for the W bearing steels. Carbide types formed by V also exhibit maximum hardness (above 2200 Knoop Hardness) compared to other similar type alloy carbides (e.g. Cr-carbide having 1800 Knoop Hardness max.). Figure 9-13 illustrates a common heat treatment schedule for high speed steel hardening.

Fig. 9-13 tempering

An illustration on heat treatment cycles of HSS for hardening and

The schedule comprises of the following steps: • Preheating • Austenitisation • Step quenching/marquenching, followed by air cooling, and • Multi-stage tempering. Starting material for heat treatment of HSS is generally the spheriodised annealed structure containing uniformly dispersed globular carbide structure in the matrix of ferrite. Annealing of this steel is carried out at around 850°C. Furnace types used for HSS hardening could be salt bath, electrically heated muffle furnace, fluidised bed or vacuum furnace. Of these, salt bath and vacuum furnaces are popular and more consistent on results. Preheating is carried out in steps, where first preheating should be at lower side of the temperature, heated in air, followed by second preheating at around 800 to 850°C in neutral medium to avoid any

scaling. Alternately, salt bath can be used for avoiding scale. Soaking during preheating should be sufficient for the job to be become thermally equilibrium at that temperature. Proper second preheating can effectively reduce the austenitisation time. Austenitisation of HSS tools is carried out at higher temperature, ranging between 1150 and 1300°C, depending on the alloy type. Higher austenitising temperature is preferred for W bearing steels, but with care to avoid grain coarsening. However, austenitising temperature should be so adjusted as to obtain right balance of dissolved and undissolved carbides, and to control the amount of C and other alloying in the composition before hardening to martensite structure. In general, there are three basic parameters that need to be maintained during austenitisation of HSS: • Temperature should be controlled as close as possible in order to minimise variation in hardness in the final product. This is due to differential in carbide dissolution with temperature, affecting the percentage alloying taken into the solution. • Time should be controlled for sufficient dissolution of carbides but not long enough for grain coarsening at that higher temperature. Time will, however, depend on the alloy type, alloy like W is sluggish to go into solution. • Neutrality of the atmosphere during heating should be maintained for avoiding scaling and decarburisation. Decarburisation of surface will hinder the attainment of high hardness on the surface. For minimising distortion and thermal stresses, high speed steels are generally quenched in high temperature salt bath at about 550°C or in martempering bath. Soon after hardening, the steel would require tempering. Purpose of tempering HSS is not only to temper the martensite, but also ensuring transformation of retained austenite – which always accompanies the martensitic structure in these high-alloy steels after quenching. Tempering is carried out in two to three stages at temperature higher than the salt bath or martempering bath. First tempering is to temper the martensite and convert the initial retained austenite to martensite; the second tempering is to temper those transformed martensite that formed from retained austenite. Third tempering might be necessary depending on the alloy content, such as high Mo that may have stabilising effect on retained austenite, and also to produce secondary hardening. Purpose of third tempering is to ensure that there is no residual retained austenite in the structure, which otherwise might transform to martensite due to heat during cutting and may cause brittleness of the cutting tip. Common rage of tempering temperature of high speed steels is 560 to 580°C.

In order to reduce the co-efficient of friction and cutting efficiency, HSS tools after hardening can be also given post-hardening surface treatment, like the gas nitriding, oxy-nitriding or tufftriding – a trade-mark process where both carbon and nitrogen atoms get deposited onto the surface.

Summary 1. The chapter discusses the heat treatment of high-alloy steels, covering stainless group of steels, heat and creep resistance steels, and tool and die steels including the high speed steels (HSS). Functional purpose of uses of these steels, desirable heat treated structure, heat treatment processes and parameters, and metallurgical reason for the choice of those parameters have been highlighted and discussed. 2. Heat treatment of stainless steels have been covered by reference to their structural types, such as austenitic, ferritic and martensitic, because of the sharp difference in the approach of their heat treatment. It has been pointed out due to the absence of ‘critical temperatures’ in austenitic and ferritic grades, options for their hardening is limited to cold working and sub-critical annealing. It has been pointed out that if high temperature annealing is carried out in these grades, then that is for solution treatment and carbide dissolution in order to ensure full availability of Cr and Ni in the steel after heat treatment for ensuring no loss of corrosion resistance. Problems associated with 475 degree temper embrittlement and sigma phase formation in ferritic steels have been mentioned and their prevention has been described. 3. Martensitic grades are, however, hardenable by quenching (generally air cooling) in the same way as ordinary low-alloy steels, but needs more care and control of the process due to very high hardenability of these steels. Due to very high hardenability of martensitic steels, they are susceptible to distortion and cracks. Because of higher hardenability, martensitic stainless steels are more sensitive to heat treatment process variables than low-alloy steels. This situation calls for certain precautions during heat treatment, which have been pointed out and explained in details. Also, need for refrigeration or double tempering the high nickel variety of steels for reducing the retained austenite content has been pointed out. 4. Heat treatment process outline of precipitation hardened stainless steels (PH-stainless steel) have been mentioned and implication of heat treatment process as regards their properties have been mentioned. Likewise, heat treatment of maraging steel has been also briefly mentioned. 5. Purpose and process of heat treatment of heat resisting steels have been discussed and outline of steel composition and their heat treatment have been presented. It has been pointed out that while heat resisting steels are quenched and aged for developing the right structure for applications, purpose of higher temperature heating and quenching is take the carbides and all other dissolved phases into solid solution before ageing so that thermally stable carbides and carbo-nitrides get produced by ageing. Steps for heat treatment of high Cr-Ni containing heat resistance steel have been described and how the precipitation types can be controlled has been explained. Further, the problem of sigma phase formation in

service in these grades has been discussed and precaution to avoid it has been mentioned. 6. Though characteristics of heat resisting and creep resisting steels have certain similarity, heat treatment of creep resisting steel have been discussed separately. Compositional and structural characteristics of creep resisting steels have been mentioned and the heat treatment steps have been elaborated with reference to the heat treatment of 12%Cr steel, which is popularly used for turbine blades. 7. Heat treatment of tool and die steels have been covered under cold work tools, hot work tools and dies, and high speed steel. Structure – property sensitivity of these steels have been discussed with reference to their functional requirements. Heat treatment parameters of different grades of tool and die steels have been mentioned and special precautions necessary for heat treating these grades have been highlighted, especially the need for pre-heating during heating and double tempering after the hardening operation. 8. Heat treatment of high speed steels have been discussed in some details in view of special properties like very high hardness, hot hardness and toughness required in their applications. Characteristics of molybdenum bearing and tungsten bearing HSS have been pointed out and their specific heat treatment parameters have been discussed. In this regard, speciality of heat treatment of W bearing high speed steel tool has been pointed out and precautions for heat treatment have been highlighted.

References / Suggested Reading ASM Handbook, Volume 4, Heat Treating, ASM International, Metals Park, Ohio, USA, 1991 ASM Metals Handbook, Volume 7, ASM International, USA, 1972 Dossett, John L. and Howard E. Boyer Practical Heat Treating, ASM International, USA, 2006 (HTIS-94), Heat Treatment and Surface Engineering of Iron and Steels, National Metallurgical Laboratory, Jamshedpur, May, 1994 http://www.kvastainless.com/stainless-steel.html -as on 02-11-2014 http://cdn.intechopen.com/pdfs-wm/39181.pdf -as on 02-11-2014 http://www.simplytoolsteel.com/conventional-tool-steel-heat-treating-cycle.html as on 04-11-2014 Maruyama, K. Strengthening Mechanisms of Creep Resistant Tempered Martensitic Steel, ISIJ International, Volume 41(2001), No. 6 Payson, Peter, The Metallurgy of Tool Steels, John Wiley, 1962 Rollason, E. C., Metallurgy for Engineers, Edward Arnold (Publishers) Ltd., 1973 Wilson, R., Metallurgy and Heat Treatment of Tool Steels, McGraw-Hill, UK, 1975

Review Questions 1. Which way heat treatment of high-alloy steels differ from heat treatment of low-alloy steels? Explain why austenitic stainless steel cannot be hardened for martensitic structure.

2. What is the purpose of ‘bright annealing’ of stainless steels? Discuss the logic of heat treatment parameters for bright annealing with regard to corrosion resistance property. 3. Outline the applicable heat treatment processes for (a) austenitic stainless steel, (b) ferritic stainless steels, and (c) martensitic stainless steels. How martensitic stainless steel is rendered responsive to quench hardening? 4. Discuss the hardening and annealing process of un-stabilised austenitic stainless steel. What are the precautions necessary for preserving the original corrosion resistance property in the steel? How does the process change for stabilised austenitic steels? 5. How martensitic stainless steel hardening differs from standard low-alloy steels? What extra precautions are necessary for hardening of martensitic stainless steel? 6. Describe and discuss the heat treatment of 17-4 PH-stainless steel. How the final structure of PH-stainless steel after applicable heat treatment differs from hardened martensitic stainless steel? 7. Discuss the heat treatment process of high Cr-Ni containing heat resistance steel. What precautions are necessary for ensuring freedom from sigma phase formation during service? 8. Discuss the process of heat treatment of 12Cr creep resistance steel used for turbine blades. How can the impact strength of this steel be improved? 9. Outline the heat treated properties required in tool and die steels. Why these steels are generally designed with higher carbon content? Critically discuss the heat treatment of Cr-Mo containing D2 type cold working die steel. 10. Discuss the heat treatment cycle required for hardening of high speed tool steel. What precautions are necessary for austenitising these grades of steels? What properties should be developed in these steels for efficient performance in service?

Surface Engineering of Steels

10.1

INTRODUCTION TO SURFACE ENGINEERING

Opening up of new frontier of surface modification by using advanced thermal and thermo-chemical processes is an emerging trend of modern heat treatment of steels. The process is broadly called ‘surface engineering’ – because it aims at engineering properties of the surface for certain application-specific purpose. ASM Metals Handbook defines surface engineering process as ‘treatment of the surface and near-surface region of a material to allow the surface to perform functions that are distinct from those demanded from the bulk of the material’. This implies that a material without the aid of surface engineering will not be able to meet the challenges of certain type of applications requiring special surface properties – like the fatigue, wear, corrosion resistance, etc. In practice, the term surface engineering is broadly used to denote the processes of alteration or modification of surface characteristics and properties of a material for an application-specific purpose – with the help of chemical, mechanical or thermo-chemical means. Therefore, the process of surface engineering can factually involve:

(1) Surface modification by altering surface chemistry of the steel; examples are case carburising, nitriding, boronising, etc., by thermo-chemical means. (2) Surface modification by adding new material on to the surface (i.e. coating), but without changing the basic composition of the material; common examples are electroplating, galvanizing, powder coating, etc., and (3) Surface modification by thermal or mechanical working process without changing the material chemistry. Example of modification by thermal process is the flame or induction hardening of steel surface, and example of surface modification by mechanical working is shotpeening of steel surface or cold-rolling of steel shaft collar / radius for increased fatigue resistance. Objective of these surface engineering processes are: • Improved fatigue resistance and fracture toughness • Improved wear resistance • Improved corrosion resistance (excepting the stainless steels) • Improved oxidation resistance / heat resistance (thermal insulation) • Reduced frictional force • Improved electronic or electrical properties, and • Improved aesthetic appearance and durability Aim of choosing a surface engineering process could be achieving any one of these or a combination of these properties. Thus, the task covers wide-ranging properties and processes for tailoring the surface properties for specific end-application. Major applications of surface engineering processes are with regard to: improving surface fatigue strength; wear resistance of the surface; and corrosion resistance. Figure 10-1 exhibits the list of various surface engineering processes used for wear and corrosion resistance – along with the thickness (or depth) of modified surface. The figure exhibits all types of surface modification processes, including mechanical working, as outlined in the previous paragraph. However, not all these processes are thermal or thermo-chemical, and, as such, they are outside the scope of a book on heat treatment. As regards heat treatment of steels, surface engineering covers the thermal or thermo-chemical processes – like carburising, nitriding, carbonitriding, nitro-carburising, plasma carburising, plasma nitriding, surface alloying by laser, ion implantation, and chemical vapour deposition (CVD) and physical vapour deposition (PVD). The latter two processes are a

Fig. 10-1 An illustration exhibiting various surface engineering processes for wear and corrosion resistance and their corresponding approximate thickness range. (Acknowledgement: Surface Engineering for Corrosion and Wear Resistance, ASM International, 2001)

sort of high-temperature coating process, but they also involve diffusion of selected elements into the substrate surface under heat. Hence, focus of this chapter is to elaborate on some of these advanced thermo-chemical surface engineering processes that are increasingly being adopted in industries for improving fatigue, wear, corrosion resistance and other application-specific properties. Some of these are being increasingly used for treating or coating tools and dies for improved service performance. Thermo-chemical processes of surface engineering involve, in general, chemical deposition and diffusion of selected elements onto the surface and substrate area of the steel. Purpose of such processes is to suitably alloy the surface of the steel and harden the surface area by metallurgical means of transformation or precipitation or formation of solid solutions. Such mechanism of surface modification is also true for the conventional surface hardening processes (like induction hardening, carburising and nitriding) discussed earlier in Chapters 7 and 8. But, modern thermochemical surface engineering processes differ from the conventional ones in terms of their precision of operation and control; flexibility and versatility; opportunity of controlling the depth of hardening or alloying to a very precise limit; and higher quality of treated surface for varieties of special purpose application of steels. Quality and metallurgical characteristics of surface layer produced by such techniques are very dense, adherent and free from any pores or surface impairment – making the product eminently suitable for precision and special / high duty applications. Process like ion implantation has been even applied to coating of stents being used for human heart angioplasty. Surface engineering processes are generally applied for improving (a) fatigue strength of steel, (b) wear resistance, (c) friction, and (d) corrosion resistance. Of these, emphasis of conventional thermo-chemical processes is on the improvement of fatigue and wear of steels, but the focus of advanced thermo-chemical processes is mostly on improvement of wear and corrosion properties of steels. Table 10-1 outlines the benefits of different surface engineering processes in the application of steels. Carburising and nitriding are generic terms and they have number of methods for execution; each having their own specific characteristics and ability to impart some added advantage. For example, vacuum carburising process is known to produce totally oxide-free surface of uniform thickness – offering superior fatigue-resistance properties than conventional gas-carburising or salt-bath carburising. Similarly, vacuum-nitriding is known to offer better control on white layer in nitrided surface and also allows uniform nitriding of cuts, grooves and corners. As such, vacuumnitriding is preferred over the normal gas-nitriding for precision and critical applications. But, if the component is complex in shape with closed

or shadow areas – such as curves, corners, holes and cuts, sharp radius, etc. – then glow discharge plasma carburising or nitriding may work better for imparting uniform depth at all points of the component, and thereby more uniform properties to the component. This distinction in quality of results among various thermo-chemical processes leads to the specialisation of surface engineering by using advanced thermo-chemical processes, viz., the plasma glow process, ion implantation, laser alloying, CVD, PVD. These processes are more precise, controllable and flexible in operation and can produce very thin case of modified structure with superior properties. This chapter will attempt to highlight in brief the process and merits of some of these advanced processes / techniques for surface engineering.

10.2

SCOPE OF SURFACE ENGINEERING PROCESSES

In principle, surface engineering processes aim at tailoring or altering the surface and substrate structures and composition of steel, as precisely as possible, without bringing about any change in the adjacent core structure. Meeting this goal of leaving the core area totally unaffected by adoption of carburising or nitriding process might be difficult – because heating the bulk material during these processes is necessary. To achieve this goal, most of the modern surface engineering processes use laser or

plasma heating / electron beam methods for precise heating of the surface for modification of surface structure and properties, leaving the base and core totally undisturbed. Added to this list of modern surface engineering techniques are vapour phase deposition processes by (a) chemical vapour deposition (CVD) and (b) physical vapour deposition (PVD). Latter two processes of vapour phase deposition are the techniques for producing thin layers of alloys on the surface, such as carbides, nitrides and borides or any other hard frozen structure, which have very high surface hardness and specific end-use properties with regard to wear- and corrosion-resistant property. Importance of surface engineering comes from the fact that: • Surface of any engineering component degrades over time in service and leads to what we call ‘premature failure’; prevention of which is very critical for some high-value components, and • Due to increasing challenges of manufacturing engineering, there are demands for improving the performance / service life of components, like tools and dies for forming, especially in mass producing shops. The degradation can be by corrosion, friction, wear, contact load, abrasion, erosion, heat or any kind of environmental interaction and degradation. If engineering components have to perform with high degree of reliability, defying premature degradation and service failure, then attention must be paid to make the surface and its substrate system strong and capable of withstanding the environment and service condition. Common application matrix of different thermo-chemical processes has been given in Table 10-1. At times, conventional processes may fall short of precision, sophistication, quality and effectiveness required by many advanced engineering applications. Role of modern surface engineering in heat treatment of steel comes in here; the process involves more challenges in operation and precise control of the process for developing specially designed surface / substrate structure for exacting performance. Examples of modern surface engineering processes are: laser or electron beam hardening, plasma/ion nitriding, vapour phase processing (CVD and PVD), ion implantation, etc. These processes are different from other thermal and thermo-chemical processes discussed so far – due to broader scope of the processes, preciseness of results and quality of performance. As such, boundary of surface engineering is not defined by whether it is thermal or thermo-chemical process, but by the challenges involved in task requiring preciseness of control and sophistication of technique for

producing quality of surface treated jobs, which conventional surface hardening processes cannot adequately cope up with. Processes using plasma, laser and electron beams for surface engineering – along with CVD and PVD processes – are fast proving to be efficient, effective and popular for their flexibility and ability to produce varieties of special purpose surface structures. This chapter, therefore, attempts to cover some of these modern thermo-chemical surface engineering processes for steels with illustration of processes and their applications. Steps in the efficient execution of surface engineering processes are: • Understanding precise properties required for the end-application (this requires an understanding of service conditions, in practice) • Identification of material requirement for structure and the surface. This may require analysis of service failure for selection of better steel • Selecting surfacing material (or alloying elements) and surface engineering process for meeting the requirement of structure and properties. (This requires knowledge of structure-property relationship in steels) • Establishing the process procedure and control points for optimum results • Shop-floor execution of the process as per plan and controls Modern surface engineering methods are by-and-large environment friendly and are being widely applied now-a-days, especially for highalloy tools and die steels, high-duty gears, wear-prone screws, blades and drives, and parts involving very high surface contact load or wear and corrosion of special nature. Modern surface engineering techniques are being used in industries like automobile, aerospace, biomedical and electronic industries to develop a wide range of functional properties; these include physical, chemical, electrical, electronic, magnetic, mechanical, wear-resistant and corrosion-resistant properties, which are required at the surface and substrate areas. A paradigm shift in surface engineering approach has been seen in recent years, due to development of processes such as vapour phase deposition and advanced diffusion processes that use heat sources like plasma / ion, laser, electron beam, microwave, solar beams, pulsed arc, pulsed combustion, spark, friction and induction. Development of these modern physical techniques has opened up a whole new field of surface engineering applications. Discussing all these techniques will be beyond the scope of the book; hence only few important and popular surface engineering techniques and their basic features and merits will be briefly referred here.

For focus in the discussions, different surface engineering methods have been broadly grouped under three heads: (a) involving plasma glow process, (b) involving the ‘Beam’ methods of heating (e.g. laser beam, electron beam, ion-beam, etc) and (c) the ‘Vapour phase processing’ (e.g. CVD and PVD). Some examples of these processes and their working features have been highlighted in the next few sections.

10.3

PLASMA CARBURISING AND NITRIDING

Plasma heating is a very clean and efficient heating process that can induce rapid carburising by controlling the electrical characteristics of plasma. There are two popular plasma processes: plasma carburising and plasma nitriding; both the processes are based on glow-discharge phenomenon in which the process gas is ionized under plasma-forming conditions and ions get discharged on the work surface, emanating characteristic glow. Figure 10-2 illustrates one such glow discharge on the work surface. In these processes, either carbon-bearing gas or nitrogen is split into ions in an electromagnetic field under high voltage and very low pressure (near vacuum). In principle, plasma-carburising and plasma-nitriding processes are very similar. They differ only in their temperature of operation and gaseous environment. However, plasma-carburising requires post-treatment quenching from higher temperature for hardening of the case, which also causes change of the core structure. Plasma-carburising and nitriding are two emerging surface hardening processes for low-carbon steels.

Fig. 10-2 An illustration showing glow discharge from parts being plasma nitrided (cathodes)

10.3.1

Plasma Carburising

Any steel that can be surface carburised by gas can be also surface carburised and hardened by plasma process, because the principle of the process is same and the aim is to increase surface carbon for formation of martensite by quenching. However, for efficient carbon diffusion, carbon content of the steel should be limited to 0.35%C max, preferably below 0.25% carbon. The process is carried out in a batch-type vacuum furnace with integrated tanks for quenching (usually oil quench) and operated under a vacuum of very low pressure (less than 5 torr pressure). Glow discharge of plasma is created by the application of high DC voltage between the job (cathode) and the furnace, which ionises the hydrocarbon gas (e.g. methane or propane) and reacts with the surface for carburisation. A typical set up for plasma carburising is shown in Fig. 10-3, which illustrates the process requirements.

Fig. 10-3

A schematic illustrating plasma-carburising process

Under the influence of pressure, temperature and voltage, the process gas gets ionised and forms plasma. Ions from plasma are attracted towards the work piece (cathode) and within the short distance from it the positively charged ions acquire electrons from the work piece and emit photons. This photon emission results in the visible glow discharge vide Fig. 10-2. The elemental species then strike the work piece, converting the kinetic energy into heat and then reacts and diffuse into the steel for carburisation. The thickness of glow envelope can be controlled by regulating pressure, temperature and voltage, and the gas mix; thereby controlling the case depth.

Important process steps for plasma carburising are: • Heating the parts to the temperature range of 950°C or less in the vacuum furnace • Hydrocarbon gases, e.g. methane or propane – along with nitrogen for dilution – is introduced at a furnace pressure of 2 to 3 torr • DC voltage of several hundred (between 400 and 800V) is then applied between the job (which acts as cathode) and the furnace, which produces glow discharge plasma surrounding the jobs. The gas ions in the plasma react with the job surface and carburising proceeds as mentioned earlier • Allow time for carburising as per case depth required • Once carburisation is complete, stop the gas flow and plasma • Hold the job at the temperature in vacuum to allow further diffusion of carbon into the substrate • Cool the jobs to a lower temperature (about 850 to 900°C) before quenching into the oil • Stress relieve as soon as possible as done in normal case. Plasma-carburising process can produce a case depth range from 0.25 mm to 1.5 mm with hardness level above 750 VPN (depending on the base steel alloy composition) at half the time of conventional process and within very close range of specification. For efficient carburising, steel parts can be cleaned within the furnace chamber prior to carburising by spattering and hydrogen reduction of any oxide films. Plasma carburising also allows selective carburising of the parts (e.g. teeth of a gear wheel only) by mechanical or chemical masking the remaining part of the work piece. Some important features of plasma-carburising are: • No grain boundary oxidation. Finished surface is very clean and glossy • Since no sooting is encountered, layer thickness is even all through the surface • Rate of carburising is faster than conventional processes • Can be applied to otherwise difficult to carburise material, e.g. stainless steel and powder-metallurgy parts • Environment-friendly pollution-free process • Jobs can be quenched in high-pressure gas instead of oil for further improvement of environment and quenching efficiency. Plasma-carburising is quite popular with automobile and engineering heat treatment shops where precision parts – like the engine valves, injector pins, high duty washers, precision gear teeth, etc., are required to be surface hardened for improved fatigue strength.

10.3.2

Plasma-Nitriding

Plasma-nitriding – which is also known as ion nitriding, plasma ion nitriding or glow discharge nitriding – is another popular industrial surface hardening treatment for precision engineering parts requiring high wear resistance and scuffing resistance. In plasma-nitriding, reactivity of nitriding media is not due to the temperature but for the ionized state of the gas. Highly active gas with ionized molecules is termed ‘plasma’, naming the technique. The gas used for plasma-nitriding is usually pure nitrogen which gets split into ions under electro-magnetic field. In plasma-nitriding process, molecular nitrogen (N2) is split into ions in an electromagnetic field. The process is carried out at a very low pressure and under high voltage at a relatively low temperature, between 450 and 600°C. Process-wise, the glow discharge system is similar as discussed under plasma-carburising; only the process parameters change. Plasma/ion-nitriding is carried out in a vacuum chamber at a pressure of 1 to 10 torr under a D.C voltage of 400 to 700 V applied between work pieces and the chamber wall. The work piece is connected to the negative terminal (cathode) and the chamber wall acts as anode (positive terminal). The wall has to be grounded. The chamber is first evacuated for air by taking down to a low pressure (0.10 torr) and then filled up with a gas mixture of nitrogen, some hydrogen containing gas (e.g. ammonia or methane) and an inert gas, which could be high purity nitrogen again. The gas mixture is continuously supplied to the furnace maintained at 1 to 10 torr pressure. Furnace pressure can be controlled by the gas flow rate and with the help of vacuum system. Electrostatic field between the cathode and anode ionises the gas, forming glow discharge plasma on the surface of work pieces. Positively charged ions of hydrogen and nitrogen are attracted by the negatively charged work piece. Ions are accelerated by the electric field. Hydrogen ions react with the job surface and cleanse it of darts and oxides. The nitrogen ions move towards the cathode and bombard its surface where they dissolve and chemically react with the steel and its chemical constituents, like iron, chromium, aluminium, vanadium, molybdenum, etc., to form respective iron nitride and alloy nitride. Hydrogen containing gas mixture is necessary for cleaning the steel surface from oxide films. Oxide-free and activated surface readily reacts with the nitrogen ions and helps to progress the process faster. In sum, important process features / steps for plasma nitriding are: • Components are placed in the reactor vessel and made the cathode of the process for plasma generation by applying high DC voltage. Chamber walls form the anode

• Pressure needs to be reduced to 1-10 torr for striking the discharge • Nitriding gas is ionized by electrons streaming out from the cathode, i.e. the component. This leads to formation of ‘dark space’ through which ions are accelerated and impinge on the components. Size of the dark space can be changed by adjusting the pressure – and control of this allow nitriding of complex shapes and blind spots, if any • Ion bombardment provides the heat for the process. Hence, plasma nitriding can take place at lower temperature than conventional gas nitriding • The process allows good control over compound layer thickness and properties. Plasma processes significantly reduces the cycle time for nitriding, which otherwise takes longer time under conventional gas process. A case depth of 0.1 to 0.40 mm can be conveniently achieved by plasmanitriding process. The case so produced is composed of different forms of iron nitrides, e.g. Fe2N, Fe3N and Fe4N, and the layer is quite compact and adherent. The process can be typically applied to gears, crankshafts, engine valves, forging dies, cold-working tools, moulding tools, etc. Figure 10-4 shows a plasma-nitrided structure – showing the compact nitrided layer on the section (right) and uniform precipitation of nitride on the surface (left).

Fig 10-4 An illustration showing plasma-nitrided structure – deposited on a carbon steel plate surface

Plasma-nitriding drastically cuts down the cycle time (2 to 3 times faster than conventional gas nitriding operation) and energy consumption is less than 50%. Because of lower operating temperature and working under near vacuum, jobs come out very clean and free from any distortion. Other advantages of plasma-nitriding are:

• Gas consumption is very low • Compound layer is with very low porosity, hard and not brittle; thereby imparting very high wear resistance and low coefficient of friction of the compound layer. • Process can be fully controlled or automated for producing precise case depth with high degree of reproducibility. • Excellent corrosion resistance of the surface due to compact nitrided layer and lower free energy of the surface, and • The process environment is free of any pollution or health hazard. Achievable surface hardness by plasma nitriding of different kind of steels is shown in Fig. 10-5. Like conventional nitriding process, plasma-nitriding also requires use of steel containing some nitride forming elements in it – like chromium, vanadium and aluminium. In this regard, dies and tool steels stand out as they contain high percentage of chromium, vanadium and molybdenum – making them eminently suitable for plasma-nitriding. Additional cost of plasma-nitriding in these components gets compensated by substantial increase of tool life in service. 1300 Cold-work tool steel 17% or stainless steel Hot-work tool steel Nitriding steel Heat-treatable steel Carburising steel Cast iron Gray iron

1200 1100 1000 900 Hardness (HV 0.2)

800 700 600 500 400 300 200 100 0

0.2 0.4 0.6 0.8 Distance from surface (mm)

Fig. 10-5 Graphical presentation of the hardness profile and case depth achievable by plasma nitriding in vacuum for different types of steels

For optimum results, starting material should be pre-hardened, preferably of alloy steels with alloy content that has higher nitride forming affinity. By closely controlling the process, case depth with or without compound layer can be produced.

Thus, plasma nitriding offers more flexibility to engineer the surface and substrate structure. When wear and corrosion-related properties are required, process can be designed and operated for appropriate compound layer, which is denser than conventional nitriding. On the other hand, where fatigue related properties are required, case can be designed with appropriate diffusion zone depth, free from compound layer. Most distinct advantages of plasma or ion nitriding are: (1) the cycle time and temperature of operation is low, allowing heat treatment of parts with negligible distortion and dimensional disturbances; (2) absence of pollution and very low gas consumption, favouring stringent environmental laws; and (3) allowing selective nitriding of parts without affecting the surface or surface properties in any way. Figure 10-6 depicts the picture of plasma glow and selective nitriding of precision gear wheel teeth with uniformity of case depth below the tooth root.

Fig. 10-6 Select area plasma-nitriding of gear wheel teeth

10.4

SURFACE HARDENING BY LASER AND ELECTRON BEAM

The process of laser and electron beam can be used for: (1) Surface hardening by solid state transformation for generating highly wear-resistant layers on complex geometry parts and shafts. Due to martensitic transformation involved in solid state transformation, it is possible to develop high residual compressive stresses in such parts for enhanced dynamic load bearing capacity. (2) Surface alloying (e.g. alloying with nitrogen, titanium, tantalum, etc)) is the other method for modifying surface properties where the process can be designed for enriching the surface layer by hard phases in order to improve wear properties of parts used under highly abrasive conditions.

(3) Surface remelting and chilling for producing a very hard phase for wear and abrasion resistance. An example is the remelting of cast iron surface for promoting hard ledeburitic layer for increased life cycle on parts subjected to high wear load, e.g. cam shaft. (4) Surface cladding for depositing almost any type of material onto different substrate with high degree of precision than that of welding or spraying process for cladding. By this method, clad layers can have metallurgical bonding with the substrate with minimum of dilution of the substrate and, thereby, controlling the surface properties as per the characteristics of deposited material. These are the common objectives of laser and beam processes, but their major applications are in the areas of surface hardening and surface alloying. With respect to these applications, surface engineering by laser and beam process have been briefly outlined.

10.4.1

Laser Surface Hardening

Laser surface hardening is widely used to harden localised areas of steel surfaces by focusing a laser beam onto the surface area to be hardened. Generally, the powerful CO2 laser is used for producing a narrow but intense laser beam. The beam diameter and the power it delivers can be controlled by controlling the laser source and focusing lens to match the surface area to be heated. It allows focus of laser on a narrow area and controlled heat in-put. The laser beam can reach to a spot which would be difficult for conventional processes to reach. The process is carried out in air. Laser beam is focused onto the work piece, which absorbs the heat and undergoes selective austenitisation. The temperature and depth of heating is controlled by controlling the residence time so that there is no fusion or burning of the surface under high heat of laser. Once heated and beam is withdrawn, the surface normally undergoes self-quenching by the conduction of heat into the bulk body. Quenching of the heated layer triggers normal martensitic transformation in the same way as in the induction hardening process. A schematic laser hardening process is illustrated in Fig. 10-7. Laser source could be placed at a distance depending on the amount of heat input required for the hardening. The process, in its simple form, has similarity with the induction hardening process. However, advantage of laser hardening over induction hardening is that the laser source could be further away from the job unlike inductor which is to be placed close by. Generally, laser hardening is limited to a case depth of 0.75 to 1.0 mm case because of the chance of surface melting in the process of attaining higher depth of heating.

Fig. 10-7 A schematic showing laser hardening process

Laser surface hardening is very useful for selective hardening of wear and fatigue prone areas of automobile and engineering components, e.g. crankshaft, cam shaft, etc. Other than laser transformation hardening, laser beam can also be used for selectively alloying the steel surfaces for developing special properties. Figure 10-8 shows the division of laser surface treatment processes, which involves two streams: one with no compositional change and the other with change in surface composition.

Fig. 10-8 Type of laser surface treatment processes as per process objectives

For alloying – involving change in surface composition – the surface area is heated to melt under intense heat and alloying elements are added / fed to the molten metal pool for alloying or cladding, involving change of the composition. Figure 10-9 illustrates a typical set-up of laser treatment for change in surface composition (i.e. alloying) or cladding.

Fig. 10-9 An illustration of a typical set-up of a laser beam treatment process for alloy cladding

Primary process control parameters of laser surface engineering techniques are control of power density and interaction time. Figure 10-10 illustrates power density and interaction time for various laser surface modification processes. The data in the figure is not absolute, but for guidance only.

10

10

Shock hardening

Power density 107 2 W/cm

Cladding & alloying

Laser glazing

Transformation hardening 3

10

–4

10

Interaction time, sec

10

1

Fig. 10-10 An illustration of various laser surface modification processes and respective power density and interaction time

As regards transformation hardening, which is carried out within close range of power density, control of interaction time is critical for precise control of hardened depth. Use of higher power density and longer interaction time can cause melting of substrate surface. Laser transformation or alloying process can be used for most ferrous material, including cast iron. However, the process is popular for

localised treatment of different profiles of critical components – like dies and tools, crankshaft, engine valves, cam shaft, etc., for improved service performance. Figure 10-11 illustrates the profile of some typical application of laser hardening techniques in industries. Uniform accessibility of laser beams into such critical but obstructive areas makes the process attractive.

Fig. 10-11 An illustration of examples of suitable geometry for the application of laser technology for surface hardening

However, due to cost involved in equipment and auxiliary facilities, laser processes have to be used with cautions and judgment where the cost justifies the gain in applications.

10.4.2

Electron Beam Hardening

Electron beam hardening is very similar to laser surface hardening, but uses a concentrated beam of high-velocity electrons as energy source

for heating the selected area. Electron emitted by a source are accelerated and formed into a direct beam by electrical lenses, before being suitably deflected by deflecting coils to fall on to marked up area of the steel surface. The process is operated under high vacuum (10–5 torr) in order to prevent scattering and oxidation. Like laser hardening, electron beam hardening also does not require any extra quenching; mass of the work piece itself should be able to sink the heat content fast enough for producing martensitic structure. In electron beam surface hardening process, surface of the steel part is rapidly heated to austenitising temperature using defocused electron beam for preventing localised melting. The beam can be manipulated using electro-magnetic coils for precise focusing and enable the process to harden selected areas very precisely, both in depth and in location. Highlights of electron beam transformation process for surface hardening are: • The process is conducted in vacuum – which prevents any chance of oxidation of surface, and, thereby, ensures high quality of surface structure. • Heat comes from the high energy impingement on components – helping precise hardening of selected area leaving other areas totally unaffected by heat. • Precise positioning of electron beam helps in reaching the exact spot for hardening. • Quenching takes place by rapid extraction of heat by conduction into the bulk material. • Depth of heating can be very precisely controlled; generally a case depth of 0.1 to 1.0 mm is obtainable within very close tolerance. • No problem of distortion or warpage due to very intense localised heat. The jobs also do not require any post operation tempering treatment. Figure 10-12 illustrates a simple electron beam process set-up. The process is generally used for improving wear resistance of critical automobile, aerospace and machine-tool components as well as for specialised medical technology. However, the process entails high capital cost. Hence, it should be applied with caution for justifying the cost versus benefit. The process can be further modified to ‘pulsed electron beam’ process for surface alloying of difficult metals – like the aluminium, which gets rapidly oxidised at the operating temperature. Aluminium surface alloying of steel is used for severe corrosion-resistance applications.

Fig. 10-12 An illustrative diagram of electron beam process

10.4.3

Ion Implantation

Ion implantation is another surface modification process which is an ion process and its operation resembles plasma-nitriding. The process is used for implanting chosen alloying element into the steel surface by direct ion bombardment. In this process, ions with very high energy are driven onto a substrate. The process, therefore, involves ionisation of a gas or alloying element under vacuum and directing the ions under electrical lens to the working spot. Nitrogen ion implantation for selective surface hardening is quite popular, though theoretically all sorts of atomic species can be implanted. Nitrogen ion implantation on steel substrate considerably improves corrosion and other tribological properties (e.g. wear and friction). By this process, reliable and precise alloying is possible, without any disturbance of finished dimensions. A simple ion implantation set-up is shown in Fig. 10-13.

Fig. 10-13 A simple ion implantation set-up with magnetic lens for controlling and focusing to the target

Ion implantation equipment typically consists of an ion source, where ions of the desired element are produced; an accelerator, where the ions are electro-statically accelerated to a high energy level; and a target chamber, where the ions impinge on a target, which is the material to be implanted. The process is carried out in vacuum in which a beam of ions is directed through electro-magnetic lens at the surface of the jobs. The ions lose energy in collision with the target, generate heat and come to rest in the surface layer of the material for penetration and implantation. The ion penetration depth depends on the ion species, ion energy and target material. The process involves very rapid quenching of implanted element. During implantation, ions come to rest beneath the surface almost instantly and produce modified near-surface region rich in implanted material. Because of this rapid process, many novel surface alloys in very controlled thickness can be produced which are otherwise unattainable by other techniques – providing unique chemical and physical properties. The process is used for semi-conductor industry, chemical industry and others. For steel, main ion-forming element used is nitrogen – which hardens the surface by formation of nitride precipitates and solid solutions. Damage introduced by the implantation process also introduces a compressive surface residual stress which additionally helps in improving the fatigue resistance. The process is quite useful for surface engineering of steel tools and tool bits where the compressive surface stress produced by the process can beneficially limit the crack propagation under shock

load. The implanted layer can also improve corrosion resistance of steel surface. Other area where it is being applied is the prosthetic medical devices where a very thin layer of suitable implanted material produces surface of very high corrosion and frictional wear resistance. As a result, reliability of such prosthetic devices substantially improves. Advantages of ion implantation process are: • It is a low temperature process • No distortion of the finish treated parts • Highly controllable • Highly versatile – can use number of alloying elements for implantation under a vacuum compatible target (the job). However, the process is limited to a very thin depth (less than 1µm) and can be applied only to line of sight job surface. Ion implantation is routinely used for expensive plastic injection moulding tools where any amount of wear is detrimental.

10.5 VAPOUR PHASE PROCESSING: CVD AND PVD These processes are generally treated as coating process, but the intricacies of the processes involved in producing a very thin, coherent and highly adherent coating of complex material like carbide, nitride, oxide, boride, distinguishes these two techniques from conventional thermal-coating process. The process involves atom by atom layer of vapour deposition and diffusion in atomic scale for the build up of reaction layer. The process is getting very popular for hard coating of carbide and nitride layers on the cutting tools and dies for engineering industry. As such, the processes form an important part of surface engineering for manufacturing industries and, therefore, are being clubbed with other thermally assisted surface engineering processes for discussions in this book on heat treatment of steels. These processes make use of vapour deposition, surface chemical reactions and solid state diffusion for achieving a measured coat of hard alloy coating, which are normally carbides, nitrides, borides, etc. The process involves appropriately heating (e.g. using plasma or electron beam or arc discharge) the metal (to be derived or deposited) in a high pressure chamber and then introducing a reactive gas for reaction, deposition and diffusion. The reaction layer thickness may go from 3 micron to more than 100 micron (0.1 mm). By this process, layers of carbides, nitrides, borides, silicides, etc., or of any reactive metals like titanium, tantalum, etc., can be produced in precise thickness. These specially deposited hard layers impart very high wear and abrasion resistance to steel surfaces, and

the process is specially used for semi-conductor industries and for coating the forming tools and dies of high speed steels or tungsten carbides.

10.5.1

Chemical Vapour Deposition (CVD)

Chemical Vapour Deposition (CVD) involves deposition of thin films or coatings through chemical reactions of gaseous reactants on and near the vicinity of a heated surface. The coating proceeds by atomic level deposition, and can produce single and multilayer coating of highly pure material or compound. The process can be controlled to produce unique structure at atomic or nanometre level. CVD can be used for coating of composites and nano-structured high-purity material, which is extremely hard. The process is extensively used in semiconductor industries for producing thin films of chosen compound. In a typical CVD process, the substrate is exposed to one or more volatile precursors (any chemical compound that participates in the chemical reactions producing another compound), which reacts and/or decomposes on the substrate surface to produce the desired deposit. Such chemical reactions might also produce other volatile by-products, which must be removed by gas flow through the reaction chamber. Figure 10-14 illustrates a plasma assisted CVD deposition set-up.

Fig. 10-14

An illustration of plasma-assisted CVD coating set-up

Chemical vapour deposition (CVD) process can use a number of means by which chemical reactions are initiated. For example, the process can be: • CVD at atmospheric pressure • CVD at low sub-atmospheric pressure, and • CVD at very low pressure (ultra-high vacuum), typically below 10-8 torr.

Generally, low-pressure or ultra low-pressure process is preferred in order to minimise un-wanted gas phase reactions and for improving film thickness uniformity across the substrate. CVD is extremely useful for atomic layer deposition – depositing extremely thin layers of chosen material. Being a gas process, it can penetrate into many blind areas and not being a ‘line of sight’ process, like the beam processes, it has the potential for application in many critical areas. Thus, CVD is a versatile process for applications in areas like blind holes, key ways, etc. Precision engineering or micro-fabrication processes (e.g. semiconductor production) widely uses CVD for depositing materials in various forms that include mono-crystalline (atomic), polycrystalline, amorphous and epitaxial. The materials could be tungsten, silicon carbide, titanium nitride and many others, depending on the end-uses. Other areas of common applications of CVD are: • Thin coating of tungsten or titanium nitride on forming tools and dies • Semiconductor and related devices like the integrated circuit, sensors etc. • Optical fibres for telecommunication • To produce ceramic matrix composites – like carbon-carbon, carbonsilicon carbide or silicon carbide-silicon carbide matrixes. In sum, CVD is proving to be a very versatile process for coating thin deposits of very high purity materials, which are otherwise very difficult to deposit or synthesise, for high-performance.

10.5.2

Physical Vapour Deposition (PVD)

In principle, PVD process is similar in some respect to CVD with the difference that CVD involves depositing a solid material (produced by reactions) from a gaseous phase, whereas in PVD process, precursors are solid and the material to be deposited is vapourised from a solid target and deposited onto the solid substrate. Basically, it is a vapourisation coating process where basic mechanism is atom by atom transfer of material from solid phase to the vapour phase and back to solid phase (vide Fig. 10-15) – gradually building a film on the surface to be coated. In case of reactive deposition, depositing material reacts with a gaseous environment (e.g. nitrogen) of co-deposited material to form a thin film of compound material, such as nitride, carbide, carbo-nitride, etc. The process is carried out under high vacuum at temperatures between 150 and 500°C. High purity solid precursors (i.e. solid materials for

coating, such as titanium, chromium, aluminium etc) are either heated or bombarded with ions. Simultaneously, a reactive gas (like nitrogen or a gas containing carbon) is introduced into the chamber, which forms the compound with the metal vapour and is deposited on the components as a thin and highly adherent film or coating. For round parts or shaped parts, the job should be rotated at uniform speed for obtaining uniform coating thickness. The PVD process may utilise any of the following methods for heating and vapourisation technique: (a) Thermal Fig. 10-15 An illustration of PVD (arc) evaporation, (b) Electron-beam Process Flow Diagram evaporation, (c) Sputtering, and (d) Ion plating. In brief, the process steps for PVD can be summarised as: 1. Vapour phase generation from coating material stock by the process of: • Evaporation • Sputtering • Arc Vapourisation • Chemical vapours and gases 2. The transfer of the vapour phase from source to substrate by: • Line-of-sight • Molecular flow • Vapour ionisation by creating a plasma 3. Deposition and film growth on the substrate These steps can be independent or superimposed on each other, depending on the desired coating characteristics. The final result of the coating/substrate composite is a function of individual material properties, the interaction of the materials, and process constraints, if any. Figure 10-16 shows an illustrative PVD coating set-up using electronbeam vapourisation. Much of the success of PVD depends on the relative merits and advantages of vapour phase generation processes. In this regard, Table 10-2 highlights the relative advantages and limitations of different vapour phase generation processes of PVD.

Fig. 10-16 An illustration of PVD coating set-up using electron-beam vapourisation technique

Thus, the critical step in the PVD process is to select the appropriate vapour phase generation process in accordance with the process objective. In general, electron beam PVD process is used for coating the steel tools and dies for industrial purpose with good success. Ability of the vapour phase processes (i.e. CVD and PVD) to coat thin films of very hard and adherent compound has made the process very popular for treating precision engineering components, in general, and for treating forming tools and dies, in particular. The latter has helped manifold increase in tool life and productivity improvement in various manufacturing industries. Figure 10-17 illustrates an example of application of PVD process for assorted high-speed cutting tools.

Fig. 10-17 An illustration showing a bunch of assorted HSS tool that have been coated with TiN (Titanium nitride) by PVD process. Titanium nitride coating produced a gold coloured coated surface

High hardness combined with very low co-efficient of friction of nanolayered PVD coating ensures considerably longer tool life at severe service conditions – such as dry machining, high temperature, and high-speed machining of soft materials – where chip breaking during machining is a serious problem and tool-chip interface can generate very high heat. PVD process can be used for coating tools with many other ceramic coatings – like Zirconium Nitride (ZrN) and Titanium Aluminium Nitride (TiAlN) – which are known to produce even higher hardness and lower co-efficient of friction than Titanium nitride. Such PVD coated tools are suitable for applications in difficult to machine materials, e.g. machining of titanium aerospace alloys and soft stainless steels at high speeds.

Summary 1. Surface engineering is a broad-based term – involving scores of processes ranging from deposition, transformation, precipitation, solid solution, coating, plating, cladding, etc – for imparting superior surface specific properties to materials in distinct difference to the bulk. 2. However, considering the basic premises and scope of heat treatment, only some advanced thermal and thermo-chemical processes for surface engineering have been discussed in this chapter. These processes are: plasma carburising and nitriding, laser beam hardening, electron beam hardening, ion implantation, and chemical and physical vapour deposition (CVD and PVD). 3. The chapter justifies the inclusion of these selected processes under the head of modern surface engineering for their: scope of applications, flexibility of operations, controllability for precise surface coating and depth, and purity and integrity of surface/substrate structure. 4. Based on these premises, modern surface engineering methods have been broadly grouped and discussed hereunder three heads: (a) involving plasma glow process, (b)involving the ‘Beam’ methods of heating (e.g. laser beam, electron beam, ion-beam, etc) and (c) the ‘Vapour phase processing’ (e.g. CVD and PVD). The latter two processes are a type of coating process involving diffusion of selected elements into the substrate for formation of extra strong, but tough and very adherent, coating layer. 5. Coming to the specifics of surface engineering processes, the chapter first discusses the plasma-carburising and nitriding processes – which are glow discharge process under vacuum – capable of producing clean and uniform depth at points of the jobs. Superiority of plasma processes has been highlighted over the other conventional carburising and nitriding processes with regard to: process efficiency, versatility and quality of the hardened layer. It has been pointed out that plasma process is much faster than conventional carburising and nitriding processes, requiring half the time for a given case depth. Other advantage of the process, especially for nitriding, is that the process can take place at lower temperature than the conventional ones. 6. Surface engineering by laser and electron beam process – which can be used for surface hardening/surface alloying/surface cladding – have been illustrated by demonstrating their scope, flexibility and applications. These processes are capable of producing very thin layer of deposited material at very fast rate with high degree of surface integrity and quality. 7. Relative merits of laser and beam hardening have been mentioned and their areas of applications have been highlighted. Intense, but fully controllable, heat in these processes allow very thin and adherent deposition for hardening. 8. Ion implantation has been discussed along with the beam process, but its operation closely resembles plasma-nitriding. The process involves implantation of chosen elements onto the surface by direct bombardment of ion species under high impact energy. The process involves ionisation of gas under vacuum and directing ions under electrical lens control to the work spot. Because of high impact energy of ions, the process also produces some surface compressive residual stress which is beneficial for fatigue strength. 9. Finally, the chapter discusses the vapour phase deposition process of CVD and PVD. Principles of operation of these two processes have been illustrated and their

relative merits have been mentioned. These gas-based processes are especially suitable for blind area treatment where other ‘line of sight’ processes cannot effectively work. Both CVD and PVD process allows layer by layer deposition and coating of very hard materials, like nitrides and carbides, making them very useful for treating tools and dies for prolonged service life.

References / Suggested Reading ASM Handbook, Surface Engineering, Volume 5, ASM International, Ohio, USA, 1994 Chattopadhyay, Ramnarayan, Advanced Thermally Assisted Surface Engineering Processes, Kulwer Academic Publishers, The Netherlands, 2004 Choy, K.L., Chemical vapour deposition of Coatings, Progress in Materials Science, Vol. 48 (2003), pp. 57-170, Pargamon Press, Oxford, UK Davies, J.R., Surface Engineering for Corrosion and Wear Resistance, ASM International, Ohio, USA, 2001 Martin, Peter, Introduction to Surface Engineering and Functionally Engineered Materials, John Wiley & Sons, New Jersey, USA, 2011 Matrox, Donald M., Handbook of Physical Vapour Deposition (PVD) Processes, Elsevier Inc., USA, 1998 Mittemeijer, Eric J. and M.A.J. Somers, (eds) Thermo-Chemical Surface Engineering of Steels, Woodhead Publishing, Cambridge, UK, 2014

Review Questions 1. Define surface engineering and outline its general objectives. What are benefits of different surface engineering processes? 2. Name few advanced surface engineering processes practiced in industries and identify the mechanisms involved for hardening or modifying the surface areas. Why surface engineering becomes necessary for industrial components in the first place? 3. What is the purpose of thermo-chemical process of surface engineering and what are the broad mechanisms followed by these processes? Highlight few beneficial changes in properties of steels following these processes. 4. Discuss the steps necessary for effectively planning and executing a surface engineering treatment for steels. 5. Highlight the working principles of plasma glow discharge processes for carburising/nitriding. Critically discuss the process of plasma carburising and nitriding and their applications in industries vis-a-vis vacuum carburising. 6. Briefly discuss the following surface engineering processes and their possible areas of applications: (1) Laser beam surface hardening (2) Electron beam surface hardening 7. Briefly discuss the process of ion implantation and its applications. What are its advantages? 8. What are the working principles of vapour phase deposition processes? Compare and contrast the CVD and PVD processes as regards their operating methods and scope of applications.

Industrial Heat Treatment Practices: Illustrative Cases

11.1

ISSUES OF INDUSTRIAL HEAT TREATMENT

Heat treatment is now a well-understood subject – based on the principles of phase transformation and rules of thermo-dynamics governing various physico-chemical phenomena that take place or intervene during heat treatment of steels. The process parameters of heat treatment in industrial practices are broadly built around for controlling these intervening physico-chemical phenomena that control the outcome or results. Results in practice are measured by conforming to different structure and properties of steels as well as physical dimensions of the components. Additionally, industrial heat treatment is concerned with cost and productivity of operation and dimensional quality. Therefore, some important tasks of industrial heat treatment are: • What type of steel to select for consistent response to a chosen heat treatment process? (Refer ‘heat treatability of steels’ in Chapter 4) • How the structure formation in the given steel can be controlled by controlling the conditions of austenite decomposition or

recrystallisation process, as the case might be? (Refer ‘Austenite decomposition and microstructure formation in steels’ in Chapter 2) • How the cost, quality and production efficiency of heat treatment can be improved by adopting the right furnace technology? (Refer ‘Furnaces and heating efficiency’ in Chapter 5) • How thermo-chemical reactions during heat treatment can be controlled for favourable product quality? (Refer ‘Furnace atmosphere control’ in Chapter 5 and discussions of surface hardening by induction hardening, carburising and nitriding in Chapters 7 and 8) • How the quenching process or cooling mechanism can be regulated and controlled for right structure and control of distortion / crack in the product? (Refer ‘Quenching and quenching technology’ in Chapter 6) • How distortion and accumulation of harmful residual stresses in the product – which are known to adversely affect the performance in application – can be controlled and minimised? (Refer discussions in Chapters 5, 6, 7, 8 and 11) These tasks are common for most heat treatment processes; and additionally, there could be other application-specific task for a particular component or process. This chapter will attempt to highlight how these issues are considered and addressed in actual practice in the shop floor. Shop floor must be equipped with necessary equipment, fixtures and auxiliary support mechanisms for fulfilling these tasks. Thus, the practice of heat treatment is much more than selection of steel and controlling heating and cooling cycle for the development of right structure for the given property specification. Controls of distortion, damage, decarburisation, surface oxidation, residual stresses, etc., are of critical importance for heat treatment of steels. All efforts of heat treating might fail if these quality aspects of heat-treated products are not taken care of. Heat treatment, in practice, is an engineering process for making the steel fit for uses or end-applications – demanding careful planning and control of the process. In practice, the process of heat treatment has to deal with structure and properties, distortion and cracking, surface quality, surface engineering, and control of residual stresses that can arise from the heating and cooling practices. In fact, control of residual stress, which is often bypassed or overlooked in the heat treatment practice, is of critical importance for many end-applications – such as for fatigue where fluctuating load is experienced in application. Since development and control of residual stress

has not been specifically dealt with earlier in the book, a brief discussion of this subject here might be helpful.

11.1.1

Residual Stress in Hardening of Steels

Residual stress in hardened components is another problem – which influences the performance of heat-treated parts. Residual stress can arise from a combination of thermal stress, developed during quenching, and / or transformation stress occurring due to differential volume expansion. Martensitic transformation and associated volume expansion during hardening is a case in-hand where residual stress gets developed in the component. Residual stress could be ‘tensile’ or ‘compressive’ in character – depending on its origin. Most hardening operation of through hardened steels produces some tensile residual stress. However, in most cases, this residual tensile stress gets released during stress-relieving or tempering (as discussed in Chapter 7). Therefore, problem with the development of tensile residual stress is not their removal but their control – so that the magnitude of such residual stress is not large enough to cause crack or distortion in the hardened component. High tensile stress developed during quenching and martensitic transformation generally lead to distortion or cracking of the quenched parts. This development takes place due to a temperature differential between the surface and the core and due to volume expansion associated with martensite transformation from austenite. Effects from both these sources can superimpose and cause the development of high tensile residual stress during the martensitic transformation – leading to distortion or cracks in the component. Steels with high-carbon and high hardenability, or when it is quenched very severely, show tendency for higher tensile residual stress level arising from higher volume of martensite formed by quenching. Tensile residual stress can be best controlled by judicious choice of steel composition (i.e. right hardenability and not more) and quenching medium having low cooling rate at stage-III cooling (vide Chapter 6). Tempering / stress-relieving, soon after quenching, is the other cure for any tensile residual stress. While tensile residual stress is harmful, development of compressive residual stress is beneficial – but requires careful planning of heat treatment process for the same. Control of tensile residual stress during hardening is a critical factor in industrial practices. Steps must be taken to reduce the level of tensile residual stress development by controlling the ‘hardenability’ factor of the steel; appropriate measure be taken in quenching – especially in the stage-III cooling – so that resultant residual stress due to quenching and transformation can get released in the process itself. Such heat treatment

processes are ‘marquenching’ or ‘martempering’, ‘austempering’, etc – where thermal stresses are reduced and transformation stresses are minimised by holding at higher temperature vide Chapter 7, Section 7.6. Tensile residual stress (+ s R) adversely affects the performance by adding up to the operating applied load in the service, but if the residual stress is compressive in nature (– s R) it will reduce the applied load by the same amount. Hence, compressive residual stress can be used with an advantage, if the process can be controlled for that purpose, especially for fatigue-sensitive applications. Compressive residual stress can be developed by two methods: (1) controlled cold-rolling of surface area after heat treatment of a through hardened steel part, or (2) by careful choice of case-core ratio in surfacehardened steel. In the latter case, the composition of the base steel and the quenching condition are so chosen that the transformation of lowcarbon core is more or less complete before the transformation of highcarbon surface to martensite (Vide Rose and Hougardy, 1967, for further details). Crank-pin collar (i.e. radius) rolling of automobile crankshaft is an example of the former and controlled carburising and hardening of case-carburised automobile gear is the example of the latter. The condition for compressive residual stress development in casecarburised part is that low-carbon core should, more or less, complete its transformation before transformation of high-carbon surface has started. Early transformation of core will induce some tensile stress on the surface enveloping the core, which will then get a chance to be neutralised because of higher temperature of surface at that point of cooling. Transformation of case to martensite thereafter – while the temperature has come down further – will accompany localised volume expansion due to martensite formation on the surface area. Martensite has higher specific volume than austenite from which it forms, which will be restrained by the rule of continuity of matrix. This pull-back condition between the core and the case will lead to the development of compressive residual stress in the case region, provided the case to core ratio is controlled. Too high case depth might create micro-cracks at the case-core interface due to high interfacial force caused by higher volume of martensite in the surface and stress arising from the martensitic volume expansion. Heat treatment of automobile crown-wheel gear has been discussed in Section 11.4.2 of this chapter, where benefits of compressive residual stress have been emphasised. Thus, the metallurgical tasks in industrial heat treatment operations are: • Selection of steel and process parameters for heat treatment as per required properties

• Development of right microstructure for the required mechanical properties by executing the process as per plan • Control of distortion or damages during heat treatment • Avoidance of surface oxidation and decarburisation – these are often the cause of fatigue failure of hardened steel parts • Elimination of tensile residual stress (if any) in through hardened parts • Development of compressive surface residual stress in case-carburised parts (wherever necessary) by appropriate control of steel composition and transformation sequence. These tasks are accomplished in industrial heat treatment by: right choice of steel grade and quality, proper planning of process parameters based on transformation behaviour of the steel, by adopting right furnace technology and control mechanism, heating with care and concern for surface protection, and by controlling and conditioning the quenching (or cooling) process for freedom from crack, distortion, damage and harmful residual stresses. In this regard, types and causes of some common heat treatment defects have been highlighted in the Annexure of this chapter; a reference to these defects, their causes and remedies may help readers to appreciate different steps and precautions necessary for industrial heat treatment of steels.

11.2 METALLURGICAL HIGHLIGHTS: FEW IMPORTANT LEARNING POINTS Various chapters of this book have illustrated the governing principles and rules for heat treatment and their practices by referring to different heat treatment processes, parameters, and their controls. For the convenience of discussions in this chapter, a summary of basic understanding (based on those principles and rules) about heat treatment processes and practices, is highlighted here: • Heat treatment is a means to appropriately altering or modifying the structure of steel for developing a set of required properties. This is possible due to high structure sensitivity of steel for its mechanical properties. Heat treatment is generally carried out by controlled heating and cooling, but at times mechanical working (e.g. cold working or controlled deformation at intermediate temperature range) can be used for facilitating the change in structure. • On controlled heating and cooling steel undergoes phase changes, and the nature and type of phases formed depends on the composition of the steel and exact heating / cooling condition. The order











of phase changes in steel, with change in carbon and temperature, under normal cooling condition can be obtained from the Fe-C phase diagrams. Study of Fe-C phase diagram provides understanding of why the structure of steel changes with the change of carbon and temperature. Such an understanding is necessary for planning of heat treatment processes. As regards heat treatment of steel, carbon is considered as principal alloying element (not by percentage but by its importance and criticality), which strongly influences the course of phase changes and structure formation in steel and has overwhelming importance on the course of heat-treating process. Presence of adequate level of carbon is necessary for proper response to martensitic hardening of steels. All phases of steels are not thermodynamically stable at all temperatures (vide austenite decomposition with reference to Fe-C diagram); they can change on heating or cooling over crossing a temperature line – called critical temperature. Critical temperature is the temperature point at which phase changes take place during heating or cooling due to thermodynamic instability of the parent phase (i.e. the original phase), and leads to the formation of new phase(s). Presence of critical temperatures in an alloy system is necessary for effecting the different phase transformation – a critical requirement of heat treatment. Barring the low-carbon high-alloy austenitic and ferritic stainless steels, where austenite and ferrite are stable even at the room temperature, all other carbon-bearing steels exhibit number of critical temperatures and, thereby, allow various possibilities of phase changes and transformation in steels, thus facilitating different microstructure formation through heat treatment processes. This includes much sought after process of martensite formation in steel by hardening. Nature and character of phases and structure formed after transformation will, however, depend on the composition, heating and cooling conditions; vide discussions on austenite decomposition based on Fe-C, TTT and CCT diagrams. Different structures can, therefore, be generated in steels (other than single-phase steels, like austenitic stainless steel) by controlling the composition and the cooling conditions. For example, medium carbon steels can undergo transformation to form (ferrite + pearlite) structure by slower cooling or to martensite structure by faster cooling. Steel is unique with regard to the production of martensitic structure, which forms due to faster cooling. Fast cooling can force the high temperature austenite phase to abort normally expected phase

changes with cooling and retain all the dissolved carbon in it down to a lower temperature. But, such a situation of over saturation of carbon in austenite is not thermodynamically sustainable in austenite lattice below a critical temperature – called the MS temperature. Hence, the same carbon-rich austenite gets forced to transform below a critical temperature – called ‘martensite transformation start temperature’ (MS) – and the product is called ‘martensite’. Because of the carbon super-saturation, martensite so-formed has highly distorted tetragonal ferritic structure with high lattice strain, making the structure very hard and brittle. • As-quenched martensite, which is hard and brittle due to lattice strain and distortion, therefore requires further treatment to moderate its strength and lower the brittleness i.e. improve toughness. This is done by a relatively lower temperature tempering operation when excess carbon comes out of strained martensite lattice and forms carbides as precipitates. If the steel is alloyed with carbide forming elements (like the Cr, Mo, V, etc) then alloy carbide forms along with iron carbide. Such precipitation of carbides – which takes place at stages depending on the tempering temperature and time – leads to the improvement of toughness. This opportunity of tempering and controlling the precipitation type and character opens up the vast possibility of producing different hardened and tempered structures in steels for different applications. • High-alloy steels – such as stainless steels, heat-resisting steels, tool and die steels, etc – are no exception to these rules of martensitic hardening by quenching if carbon content is adequate and the steel exhibits the critical temperatures pertaining to transformation, as mentioned above. But, some low-carbon high alloy steels (e.g. the austenitic stainless steels) do not exhibit ‘critical temperatures’ as per their structure and composition. Therefore, such steels cannot be hardened by martensitic transformation on quenching. They can be hardened, if required, only by cold-working and stress-relieving (or recovery annealing) at lower temperature. The other popular heat treatment of such steels is what is known as solution treatment. Purpose of solution treatment of this type of steel is to optimise the precipitation process of the steel and improve upon the corrosion properties. • For making possible the hardening of high-alloy steels through martensitic process, carbon and alloy composition of high-alloy steels have to be appropriately modified for establishing the critical temperatures and enabling phase transformation through hardening by quenching (fast cooling). In this regard, vide the difference in









composition in martensitic stainless steel grades with that of austenitic grades. Steel is a very strong structure-sensitive material, and heat treatment opens up umpteen numbers of possibilities for producing different structures either through martensite and martensite tempering route or through solution treatment and ageing route. Austenite is the starting structure for both the process route, which is produced by controlled heating to higher temperature. Aim of such controlled heating is ‘conditioning of austenite’ by taking adequate carbon and alloying elements into solid solution in order to facilitate production of right structure through right cooling, followed by controlled tempering or ageing, whichever is applicable. Because of the structure sensitivity of mechanical properties of steels, success of heat treatment largely depends on the understanding of what structure is required for what properties. This should be clearly understood before planning/starting the heat treatment. Not only the phases, but also the combination of phases in the structure, their structural morphology and the type, character and distribution of different precipitates (which are generally chemical compounds of high stability) determine the properties. Hence, heat treatment process is built around these requirements of producing the right structure for right purpose. Accordingly, the steel composition and the process of heat treatment are selected. Heat treatment of steel is not limited to bulk hardening or heat treating; steel can also be heat-treated locally, such as the full or selected area of a surface. This is called surface hardening and can be carried out with or without disturbing the structure of the bulk. Surface hardening can be through martensitic route or by production of additional phases (like the nitrides); vide Chapter 8. Surface structure modification is of great importance due to the fact that application of service load is generally through the surface, which is exposed to the hazards of wear, corrosion, fatigue, etc. The opportunity of surface hardening allows actions for developing structure appropriate for combating wear and fatigue, and at times for corrosion as well by surface engineering. Surface engineering processes using heating by electrical, gas, induction, plasma, ion-beam, laser, etc., form an important group of industrial heat treatment practices now-a-days vide discussions in Chapters 8 and 10. Finally, most heating and cooling treatments also require protective / control atmosphere for protecting the surface from harmful environmental attack or for modifying the surface chemistry. Surface protection is to ensure that no undesirable surface reactions take

place to jeopardise the quality of heat-treated structure (e.g. by deoxidation or carburisation of the surface). Control of atmosphere is necessary for surface chemistry modification, which are required for producing new phases or structure in the surface area (e.g. carbide, nitride, carbo-nitride, etc) to combat higher loads and wear. Surface chemistry modification, such as nitriding or carburising of steel, is carried out by prolonged heating under appropriate gaseous atmosphere with necessary control (vide Chapters 3 and 8). These are few important rules and understanding that govern the process of heat treatment. Most industrial heat treatment problems can be addressed by appropriately applying these rules and understanding, coupled with judicious selection of steel composition and heat treatment process parameters. Important issues (other than selection of steel and its heat treatment for developing structure and properties) in industrial heat treatment practices are: control of distortion, damage, decarburisation and surface oxidation, residual stresses, etc. All efforts of heat treating might fail if these quality aspects of heat-treated products are not taken care of. Thus, heat treatment practices in the shop floor have to be concerned not only with how to develop right structure-property relationship, but also how to deliver the heat-treated products free of distortion or dimensional discrepancies as well as freedom from oxidation, decarburisation or any other surface blemishes. Residual stress (tensile) arising from the hardening process is another factor which needs to be controlled during industrial heat treatment operations for control of distortion.

11.3 AN OVERVIEW OF ‘HEAT TREATMENT AT-WORK’ IN THE SHOP FLOOR Purpose of this section is to provide an overview of how heat treaters work in the shop floor for executing the scheduled heat treatment operations. This has been illustrated with reference to heat treatment of automobile axle shaft – a load (force) transmission component in the automobile. Nature of force acting on axle shaft in service is ‘torsional’ in nature, which requires higher strength and toughness than what it would have been if the force was pure bending. Axle shaft is a long rounded shaft with forged flange at one end and upset forged diameter for gear seat. Figure 11-1 illustrates few such axle shaft designs. Because of the length of the shaft, acting torsional stress in service is also high – requiring use of steel that can ensure very strong and tough structure after finish heat treatment. For this reason, very

often, axle shafts are ‘induction hardened’ for strong surface where the crack generally first appears in service. Alternatively, they are made from Cr-Mo type alloy steel after hardening and tempering to a strength level of 120 to 140 kgf/mm2.

Fig. 11-1 An illustration showing few axle shaft designs for uses in automobile trucks and tractors. (Flange, steps around the flange and the threaded end at the other side are weak spots for heat treatment defects)

Shop floor heat treatment after forging is, generally, required for: (1) To produce structure and properties for facilitating machining and thread rolling, and (2) Heat treatment to produce final structure and properties as per specification – along with correct dimension, straightness and crack free surface quality for end-application. Following this line of approach, axle shaft – forged from appropriate quality of steel – is subjected to: (a) Normalising after forging for producing right mix and nature of (ferrite + pearlite) structure of hardness value that is good for machining to finish size and shape, and (b) Hardening and tempering (or induction hardening the surface, if this process has been chosen by the designer) after finish machining – for strong and tough martensitic structure. (c) Dimensional check for straightness and rectification, and (d) Crack testing of the shaft surface Accordingly, heat treatment of axle shaft can be divided into two steps: one for normalising after forging and the other for hardening after machining to finish size. Steel used for axle shaft must be capable of hardening to high strength and toughness. Hence, very often, alloy steels like SAE 4140, 5140,

4340, etc., are used for heavy duty shaft. But, use of plain carbon steel of 45C8 grade for lower duty shaft is not uncommon. If the design is induction hardened shaft, then the steel must have sufficient carbon and alloy content for response to hardening. Whatever could be the steel grade, the shaft after forging is required to be heat treated for (a) homogenisation of structure, and (b) production of right type of ferrite-pearlite structure for high-speed machining. Homogenisation of structure is important for uniformity and consistency of properties after heat treatment. Steel in as-rolled or as-forged condition is not very homogeneous in structure, especially if the component is heavy or complex in shape and size. Moreover, deformation in forging and shaping might not be uniform all over; there will be areas of higher deformation (e.g. flanges of a shaft) than normal (e.g. shaft rod or stud). Hence, a component like forged beam or axle shaft does require some sort of homogenisation treatment before putting to normalising or hardening. Homogenisation of structure and production of machinable structure is produced in axle shaft by subjecting it to ‘normalising’ process with longer soaking and controlled cooling. Normalising of axle shaft is generally carried out in bogie-hearth furnace with nitrogen gas protection for reduced scaling, and controlled cooling in air for maintaining right machinable hardness and structure. Loading / stacking in the furnace must ensure that shafts do not bend or distort during heating and cooling. Axle shafts are shot-blast cleaned after normalising, straightened and stress relieved (around 400°C) for further processing by machining. Next stage of heat treatment of axle shaft is the hardening – either by conventional through hardening and tempering or by induction hardening of low-alloy steel shafts. Hardening is always carried out in industries for all critical components requiring fatigue strength and toughness. Automobile axle shaft – which transmit force to the wheels – is one such example. Automobile axle shaft hardening in the shop floor is generally carried out by through hardening (bulk hardening) or surface hardening by induction. Choice of exact heat treatment process, however, depends on the steel grade and properties required. For lower duty axle shaft, many shops tend to use ordinary plain carbon 45C8 type steel after hardening and tempering to 80 to100 kgf/mm2 strength. But, for standard automobile axle shaft for commercial vehicles, often alloy steel containing Cr-Mo or Cr-V is used either after through hardening to about 120 to135 kgf/mm2 or after induction hardening the surface to a case depth of 2 to 3 mm with surface hardness of 50 to 55 HRC. In fact, more exotic steel like the SAE 4340 (EN-24) is also used

for axle shaft application based on the duty and properties required for end application. Heat treatment-wise, general shop floor precautions and process steps for hardening this component are: A. Through-hardening Process, precautions and steps: • To avoid development of bend in this long component, jobs are charged in the furnace in pre-designed fixtures where jobs hung vertically • Furnace is gradually heated – preferably with a pre-heating zone – and brought to the austenitisation temperature • Since carbon content in the steel is of medium level, jobs are required to be protected from surface oxidation and decarburisation. Therefore, nitrogen as protective atmosphere is introduced in the furnace from the pre-heating zone time – i.e. from temperature level of about 600°C. • On completion of heating and soaking, the jobs are taken out in the fixture and immersed in an agitated oil bath of sufficient volume and depth for quenching. Quenching of this long distortion-prone job is very critical. • Being a long component, axle shaft is prone to longitudinal distortion upon quenching. Hence, oil quenching is used for through hardening the shafts, using fixtures to hold the component vertically down, preferably with some support clamps. • Due to excess heat introduced during quenching, there is chance for oil surface of quenching tank to catch fire. Hence, proper precautions are taken by using high flash point quenching oil and using sufficient volume of oil in the tank. To eliminate fire hazard, long jobs are often quenched in covered tank where oil can be isolated from air / atmosphere. Also, polymer quenching with higher polymer content (20% or more) to replicate oil quenching characteristics is used at time, but with care to monitor and maintain bath composition. • After taking out from quenching bath (generally oil bath of appropriate oil grade and quality), shafts are given a low temperature tempering (as per required strength level) – for retaining as high strength of the steel as possible, but with complete removal of stresses in the structure. • Surface after heat treatment is shot blasted or shot peened to remove any oxide layer from the surface and to neutralise the effect of decarburisation, if any.

• Since axle shaft requires straight line alignment in assembly, the components are individually checked along the length for bend and corrected by applying light pressure on those spots of bend. • After hardening and straightening, 100% jobs are further stressrelieved at about 200 to 220°C for about 45 minutes (temperature could be higher for alloy steels in the region of 400°C). • Jobs are crack tested by using suitable non-destructive technique (e.g. magnetic particle testing) for detection of surface defects (e.g. seams, cracks, pits, dents, etc). B. Induction hardening precautions and steps: • Axle shafts are induction hardened either after through hardening or normalising, but the starting material is always shot blast cleaned or machined surface without any surface decarburisation or other blemishes. • Since the axle shaft is prone to excessive bending during progressive induction heating and hardening, either single shot inductor is used or necessary measures to control the bending during inductionhardening process are taken. • Generally, heating pattern is controlled for not reaching the root radius of the flange; alternatively, heating is done to cover the entire root radius. This step is necessary for avoiding notch effect due to partial hardening of the radius. • Quenching for hardening is either by oil or by suitable polymerquenching (discussed earlier) • Jobs are shot blast cleaned after induction-hardening and straightened • Straightened jobs are stress relieved at about 180 to 200°C for about 45 minutes. • Crack-testing by magnetic particle method. Since induction hardened axle shaft is with higher surface hardness, care is necessary for avoiding any type of surface cracks, flaw, dents or defects, which is ensured by stricter norm of crack testing in induction hardened shafts. Any flaw on the hard surface will act as point of stress concentration and crack initiation under the torsional load operating on this component in service. Axle shaft is one illustration of heat treatment at-work on the shop floor; there are many others, each having its specific requirement for heat treatment to alter or develop properties. Examples of such components are crankshafts, axle-beams, wheel hubs, etc. Heat treatment of sheets and plates for high strength applications is generally left to the steel plants rolling those products, because engineering

shops are not equipped with the facility for their limited uses. However, due to high degree of precision and control available in modern rolling mills, many a time the task of shop floor heat treatment of sheets and plates is passed on to the rolling mills – where in-process treatments are carried out for developing similar properties. An example is the ‘thermo-mechanical treatment’ (TMCP rolling) in the rolling mill, which is, in principle, a type of heat treatment producing high strength steel plates and sheets with high ductility and notch toughness. These treatments produce even finer structure than conventional hardening with equal or superior properties. Controlled rolling of HSLA steel plates is an example which has now replaced the use of C-Mn steel plates with separate normalising treatment. Surface-hardening, either by carburising / nitriding or flame / inductionhardening, is also a widely practised in industries. Applications requiring resistance against wear and / or fatigue are compulsorily given one such treatment for improved surface hardness and properties. Flame / induction hardening are a line process on the shop floor, i.e. carried out on the shop floor as a part of the machining line and grinding. Many small and medium size engineering components (e.g. grease nipples, pins, small shafts, cams etc) are hardened by induction hardening due to higher productivity of the process. Case carburising and nitriding operations are carried out in separate shop equipped for these processes, such as by using gas process, saltbath base process or, to a limited extent, by using fluidised bed process. Details of such processes have been discussed and illustrated in Chapters 5, 7 and 8. For better control of the process (especially for precise control of carbon / nitrogen pick up) gas process is preferred in the shop floor for most critical engineering parts. For non-critical parts, salt bath process is adopted for shorter cycle time and flexibility. Metallurgically, shop floor controls for these processes are: • Surface hardness • Control of distortion • Control of decarburisation or depletion of any other element • Control of surface quality by ensuring freedom from scale pits, dents, cracks, etc • Control of uniformity of case depth, which can vary in the range of 0.20 mm to 2.0 mm case for carburising and between few microns and 300 microns for nitrided depth. Thus, the task of industrial heat treatment starts from (a) understanding functional requirements of the component, (b) understanding working principles of different heat treatment processes, and (c) implementation

of the process – along with precautionary measures – in order to attain the functional quality for end application with minimum variation of product quality. Success of execution and implementation of heat treatment process parameters and discipline on the shop floor depends on how well the process is handled for controlling the variations of process parameters and how fast the corrective / preventive actions are taken with urgency. The focus for such heat treatment practices is the control of the process and process parameters for the target quality. The system follows the norm that if the process is correct, the product will be correct too with consistent results. For this end, all shop floor heat treatment practices are planned and executed with an approved ‘Process Chart’ – which involves: • Understanding the purpose of the heat treatment and the level of properties required along with restrictions on dimensional tolerance change and surface quality • Process planning – including steel type and grade, process parameters, furnace and furnace control, cooling / quenching method, finishing operation (e.g. stress-relieving, shot-blast cleaning, etc), quality checks and acceptance standard. • Heat treatment scheduling – along with consideration for quantity of charge, furnace and actual furnace behaviour, loading and fixturing method, and check and control of necessary auxiliary and finishing facilities (e.g. protective gas generation, dew point control mechanism, surface cleaning, etc) • Execution of heat treatment process as per process planning sheet • Finishing and quality checks – including hardness checking, tests for strength and structure, decarburisation / oxidation level, crack testing, dimensional checking, and any other special check • Following the process for storage, stacking and delivery of components to user departments.

11.4 AN OVERVIEW OF STEEL TYPES AND THEIR HEAT TREATMENT All steels cannot be treated for all types of heat treatment. Response of steel to a given heat treatment primarily depends on its composition (vide discussions in Chapter 4: Heat treatability). There are basically two stream of heat treatment; one stream relates to annealing and normalising of steels, and the other stream relates to bulk (through) hardening and surface hardening of steels. Composition dependence of annealing and normalising of steel is more or less guided by the condition of 40:60

ratio of ferrite-pearlite fraction in the structure or the special needs of an application (vide Chapter 7) – such as spheriodisation of structure or conditioning of structure by process annealing. But, for hardening, steel composition plays more critical role. For example, as steel containing less than 0.32% carbon cannot be effectively hardened, steels containing carbon more than 0.30% or so (i.e. the hardening grade steels) cannot be effectively case hardened by carburising. This is due to (a) requirement of sufficient carbon potential difference between surface of the steel and the carbon potential of the environment inside the furnace – which can otherwise lower the carbon pick up from the environment and diffusion into the steel, and (b) higher core strength that may result from quenching after carburising due to higher carbon in the base steel – leading to distortion and cracking. Use of higher carbon steel for carburising and hardening may also end up with severe interfacial cracks at the case-core interface due to high core strength and sharper transition of case. Table 11-1 provides the overview of types of steel and heat treatment processes that are generally practised in the shop floor.

The list is illustrative only; actual heat treatment and the parameters might vary with exact requirement of properties. Barring the structural and sheets and plate grades of carbon steels, other steels are used in the shop floor with any or a combination of the following heat treatment conditions: • Normalised (vide Chapter 7): Mostly plain carbon steel with carbon level of 0.32 to 0.50 are used after normalising at temperature between 870 and 900°C with as-normalised strength varying between 50 and 80kgf/mm2 and elongation of 8 to 12% on 2 inch gauge length. Examples of such steel grades are: 35C8, 45C8, C50 • Hardened and Tempered (vide Chapter 7): Mostly medium carbon low-alloy steels containing carbon between 0.35 and 0.50 with suitable alloying of Cr, Mn, Ni, V or Mo – in some combination or other. These steel grades are always used after hardening and tempering for critical dynamic load bearing capacity (i.e. under fatigue load) requiring good toughness, including impact strength. Plain medium carbon steel grades are also hardened and tempered for general type of engineering components, e.g. small pins, shafts, keys, etc. Examples of low-alloy steel grades used after hardening and tempering are: 34Cr4 (SAE 5135), 41Cr4 (SAE5140), 42CrMo4 (SAE 4140), 50Crv4 (SAE 5152), 50CrMo4 (SAE 4147), SAE 4340 etc. General heat treatment parameters for these grades are shown in Table 11-2. • Surface hardened by Induction hardening (flame hardening is also used for general engineering parts) is another process of heat treatment of medium carbon / alloy steels (vide Chapter 7). This process is adopted where high surface hardness – along with a tough and strong core is required. Many automobile shafts, pins, levers, etc are surface hardened by induction hardening for wear resistance application. Surface-hardening and through-hardening steels are similar in composition, but surface hardening is invariably carried out after prior normalising or hardening of the steel.

• Case carburised and hardened (vide Chapter 8): This is the other end of heat treatment application where the surface hardness of the steel is enhanced by additional carbon diffusion and hardening, simultaneously optimising the core properties (for higher load bearing) through the same hardening and stress relieving cycle. Earlier, there was a practice of ‘core refining’ by subjecting the steel with a special heat treatment cycle below A1 temperature, but with the advances of steel quality and control of carburising process, this step is now merged with the main case hardening process. It is no longer practised on the shop floor as regular operation. Because of the involvement of chemical diffusion of carbon atoms, the process is termed as thermo-chemical process as against thermal process of hardening. Steel grades used for this process can be plain low-carbon killed steel or a low-carbon low-alloy killed steel for higher strength and duty. Examples of few popular steel grades used for this purpose are: C14/15, 15Cr3, 16MnCr5, 14CrNi6, 18CrNi6, 20MnCr5, SAE 4320 (Cr-NI bearing steel), etc. In addition to case-carburising, case-hardening by surface-nitriding or carbo-nitriding is also used for many small wear and scuffing resistance parts, e.g. sliding plates for cams and gears, washers between the gears, spacers, etc. For nitriding, alloy steels with nitride forming elements – like Al, Cr, V, etc – are preferred, though the process can be used with some benefit in plain carbon steels as well. Table 11-2 presents an illustrative heat treatment table for medium carbon hardening grade steels used for dynamic load bearing (fatigue loading) components in engineering and automobile industries.

The table demonstrates that: • Steel containing similar level of carbon is normalised from similar set of temperature. • Soft annealing is carried out on the machining grade steels for controlling the structure, maintaining a general level of hardness between 210 and 223 BHN max. • Hardening temperature of medium carbon low-alloy steels does not change appreciably with alloy content, and generally ranges from 830 to 850°C. But, alloy content will change the holding time for austenitisation; increasing with complexity of alloying elements. Steels containing V and Mo require higher hardening or holding temperature. • General quenching medium for low-alloy steels is oil – due to the necessity of controlling distortion and cracking; vide Chapter 6 on ‘Quenching and Quenchability’ • However, water quenching is required for hardening heavier section medium carbon or low-alloy steels. For water-quenching, hardening temperature is about 10 to 20 degree lower than the oil-quenching. Care is necessary in water-quenching for avoiding cracking or distortion due to higher quench severity. • Tempering temperature will depend on the strength and toughness combination wanted in the steel. For alloy steel, especially containing Mo, tempering temperature is relatively higher; above 530°C for taking advantage of secondary hardening. However, for all tempering operation, care is necessary to avoid temper embrittlement while tempering in the brittle range (vide Chapter 7 for details of embrittlement temperatures of steel). Therefore, from metallurgical point of view, there are two important factors for shop floor heat treatment:

(1) Steel composition and quality has to be procured with closer range of composition for consistency in the response to hardening or heat treating temperature. Table 11-2 illustrates that heat treating temperature and time for these grades of steels are rationalised to an extent, requiring strict conformance to grade specification. (2) With rationalised temperature and time for heat-treating these grades, focus for shop floor control on heat treatment goes to (a) control of distortion of the component, and (b) atmosphere control for protecting the surface quality – in addition to getting required structure and properties. For the forging grade hardening quality steels, fixturing for long jobs and even layout of jobs on the furnace floor / trays are the common practices, in addition to control over the cooling condition, for controlling distortion. Since the scope of atmosphere control is rather limited in direct hardening of medium carbon steels, care is taken to control the air flow in the furnace for avoiding excessive scaling and pitting of the surface. However, shot-blast cleaning of the jobs after heat treatment is a must for further processing and uses. Case-carburising grade steels are generally forged, normalised (if required) and machined first, and then case carburised under controlled conditions (vide Chapter 8) for the development of final structure and properties. Few such grades of case carburising steels along with their shop floor heat treatment parameters are shown in Table 11-3:

Important points for hardening of case carburised parts are: (1) Hardening is generally carried out at below 850°C (preferably around 830°C) in order to avoid any retained austenite. Higher temperature of hardening tends to produce higher retained austenite due to more carbon taken into solution along with alloys – which pushes the MS temperature down. (2) Salt-bath hardening temperature is considerably lower than the oil-quenching; because (a) heat transfer is faster in salt-bath than in oil-bath due to better convective heat transfer, and (b) higher salt bath temperature might lead to additional carbon pick up on the surface, giving rise to ‘angular carbide’ precipitated at the triple corner of grain boundaries. (Salt bath composition contains high cyanide salt from which carbon gets picked up) (3) Carburising temperature is limited by the possibility of grain growth at that temperature and also the possibility of higher oxidation of the surface – interfering with uniform carbon pick up during the process. (4) Level of case depth and core strength is so maintained as not to allow development of harmful tensile residual stress in the component

after the hardening. Generally, high core strength and high case depth tend to produce tensile residual stress in the surface area. (Also, vide Section 11.1.1 on Residual Stress) (5) Control of distortion due to heating and quenching is done by appropriate fixturing and care during quenching. At times, plug quenching is resorted to for circular parts prone to distortion. (6) After hardening the case-carburised parts, they are stress-relieved (rather than tempered) at lower temperature (between 180 and 220°C) in order to keep the case hardness as high as possible for wear resistance and, at the same time, some toughness is introduced in the steel. Modern fine-grained Al-killed steels are capable of developing tough and strong core in a single heat treatment involving hardening in oil or salt bath or by direct quenching from carburising, as in seal quench furnace. (7) Steel containing higher Cr, Ni and Mo, which are prone to formation of retained austenite after quenching, are generally given a second stress-relieving or tempering in order to transform and temper as much retained austenite as possible. Generally, a temperature difference of 20°C is maintained between first and second treatment; first treatment being at higher temperature. Retained austenite content should generally be kept below 5% (a ball path figure) for optimum results in engineering applications. However, processes mentioned in Table 11-3, mostly relate to small components like gears, cams, small shafts, pins, etc. For larger components, like the differential gears, pinion, crown-wheels of vehicle drive system, etc, oil quenching from an appropriate furnace with suitable atmosphere control and proper fixture / plug quenching is used. Atmosphere control and hardening precautions have been discussed in Chapters 5 and 8. Based on the foregoing information and logic of heat treatment, few cases of heat treatment practices for critical automobile components have been illustrated in Section 11.5.

11.5

CASES OF APPLICATION SPECIFIC HEAT TREATMENT OF STEELS

Application specific heat treatment refers to selection and execution of heat treatment process for developing exact end-use specific properties and quality attributes for the application. Heat treatment process is commonly guided by the steel grade used, and that is, by and large, also true for application-specific heat treatment. Distinction between them

is in special control and care to be taken for heat-treating these components that are used for more rigorous application. Component-specific heat treatment often calls for greater control on the processes in order to ensure that no un-favourable factor gets introduced during heat treatment that might jeopardise the functionality of the part. Common examples of such extra control are steps for minimising bend / distortion, covering up of key-hole notches during hardening for avoiding cracks, quenching precautions for uneven cooling due to variation of sections or degradation of oil quality, etc. Moreover, at times, a component may call for more than one type of heat treatment for developing the multi-faceted functionality of the part. For example, crankshaft of an automobile may require (a) through hardening and tempering of the steel for developing general load bearing capacity in uses, and (b) additional selective area surface hardening (e.g. in the crank pin and bearing pin areas) for higher wear and fatigue resistance in those locations. In such cases of application specific heat treatment of crankshaft, it is not enough to only focus for developing the specified strength, hardness and other mechanical property parameters; the processes must also ensure that: • The microstructure after through hardening is uniform across the section (as far as possible) for enabling the part to withstand high torsional force in service • The depth and profile of hardening the pins are within the specified limits as per design, which should be enough to withstand the hoop stress (a peak applied stress which generally occurs at the sub-surface area), and • The surface hardened profile of the pin does not encroach onto the collar portion of the pin – which can then make the collar area stiff and cause a notch effect at the inter-junction of the hard surface and the softer collar area. Thus, the task of application-specific heat treatment is more explicit and comprehensive than the grade-based heat treatment, practised in general. It is in these contexts, some cases of application-specific heat treatment of steel have been discussed in this section.

11.5.1

Heat Treatment of Front Axle Beam for Commercial Vehicles

A front axle beam is a part of front suspension system in which front wheels are connected laterally by a single shaft (stub-axle) and the assembly with the help of steering wheel and its connected system should ensure smooth driving of the vehicle in right, left or straight direction.

Figure 11-2 illustrates a popular front axle beam assembly for commercial vehicle. The front axle beam is assembled with stub-axle for mounting of wheels, king pin to lock the stub-axle in position, connecting arms for regulating control of direction and tie rod for balancing and co-ordinating between the right and left wheels. Shape wise front axle beam is basically an ‘I’-section with strong rib at the middle of the cross-section.

Fig. 11-2 An illustration of a typical Front Axle Beam assembly for commercial vehicle applications

Main functions of front axle beam are: • To support the weight of the front part of the vehicle, including the engine weight under the motion of the vehicle – which cause application of sizeable resolved stress on the part • To facilitate smooth steering on the road, including turning and negotiation of curves on the road • To absorb shock and varying degree of load transmitted due to reactions of dynamic vehicle load with the road surface irregularities, and • To absorb torque applied or experienced due to application of brake, including sudden brake, while on high motion. Therefore, the part (Fig. 11-2) is a complex mechanical engineering design where metallurgical properties and correctness of heat treatment play a critical role for good service life. Metallurgically, the part is designed with: • Very high strength with good toughness; in the range of 100 to 120 kgf/mm2 with an elongation of at least 10% and room temperature impact toughness of above 43 joules.

• No residual stress that can add to the working stress, and reduce service life • No point of sharp bend or dent that can act as stress raiser in service Principal heat treatment for this part is quenching and tempering to develop the strength and toughness. The other requirement is freedom from distortion so that the camber of the beam does not get distorted. Camber of the beam is a critical dimension for assembly and alignment of the beam assembly – influencing the load carrying capacity of the beam. Shop floor practices for manufacturing this safety critical part are as follows: (1) Selection of Steel; by considering various alternatives and suitability as per application conditions. Generally, a medium carbon low-alloy steel capable of producing high strength and toughness is selected; e.g. 42CrMo4 (SAE 4140 type). (2) Forging in stages (pre-forming and finishing) with aim for (a) shape optimisation, and (b) maximisation of material yield, i.e. generation of less material in flash and fins (3) Heat treatments – including normalising/softening for machining and hardening for final properties (4) Quenching in fixture for controlling bend and distortion (5) Finishing and correction, which involves checking the dimension, mass balance, section profile of critical areas, and straightness over a surface table (6) Surface checking, rectification and crack testing. Front axle beam is an extremely critical component for forging – requiring pre-forming by roller bending, mass gathering, and forging in stages under die in a heavy press for material balance as per the design, correct dimension, and for better material yield. Manufacturing stages and parameters for forging and heat-treating this component are shown in Table 11-4:

Critical points in the heat treatment of front axle beam are: • Charging into the furnace for normalising or hardening, which should be done preferably by using suitable fixture for avoiding excessive distortion. The furnace should have neutral atmosphere for control of oxidation and scaling. • Quenching in oil bath after soaking, which should be done under fixture in a conveyor system by ensuring entry of Front Axle (FA) Beam into the quench tank individually. • Ensuring sufficient agitation of quench bath so that temperature of oil near the quenched job is maintained at constant level of around 50–60°C for efficient quenching / heat removal • Allow sufficient time in oil bath after quenching for temperature of the job to come down to about 150°C or below in order to ensure sufficient transformation to be completed before the job is taken out of the bath. (This calls for sufficiently large oil bath for quenching) • Temper early after quenching using air circulating furnace for better heat transfer.

• Shot blast cleaning of the surface for removal of scale • Any mechanical working or correction of bend, etc., should be followed by another tempering but at lower temperature for relieving stresses. Thus, in addition to care for developing specified strength and toughness in the part, control of bend and camber during heat treatment of this long part are the crucial control points in the shop floor heat treatment practice.

11.5.2

Heat Treatment of Crown Wheel / Pinion for Commercial Vehicle Power Transmission

Crown wheel is a part of gearing system that – combined with appropriate assembly of a pinion shaft gear – permits rotation of two shafts at different speeds – which, in turn, allows the wheels mounted on the shafts (called axle shafts) to rotate at different speeds on curves and curvature of the road. Figure 11-3 depicts a set of crown wheel – pinion for commercial vehicle application, generally manufactured by using alloy-carburising grade steels – like 20MnCr5, 14CrNi6, SAE 4320, etc.

Fig. 11-3 An illustration of a typical Crown wheel–Pinion set that are assembled in a vehicle after proper heat treatment and matching of the tooth contact figures

Crown wheel is a special type of bevel / hypoid gear, capable of transmitting high torque required for driving the wheels on the road. Important functional parameters of this part are: • Robust and strong tooth for carrying and transmitting the load / torque • Good tooth contact between wheel and the pinion for even and gradual distribution of load, without causing any localised high point loading or impact loading

• Tooth roots must be smoothly blended radius, free from sharp notch or cut for avoiding any chance of stress concentration at the tooth route. (Tooth route generally experiences the highest bending moment while load gets transmitted) • Surface hardness of the wheel – as well as pinion – must be high for resisting the sliding contact wear. (Wear of the face will disturb the contact picture between the matching gear tooth and shift the load unevenly) • Since the applied load on a tooth varies as the wheel rotates (i.e. the loading is dynamic in nature), the component must have high fatigue strength. As such, the part after heat treatment must be free from any harmful tensile residual stress; if possible, the part should be produced with beneficial compressive residual stress. • The crown wheel must be produced with least distortion after heat treatment – both back face and tooth profile. This is the most critical shop floor issue for heat treating crown wheels. Principal heat treatment route for meeting the foregoing parameters and points are: (1) Forging and Normalising of the Crown wheel blanks (2) Turning / machining the blanks (to pre-gear cutting dimensions) (3) Gear teeth cutting / generation as per designed profile in a special purpose machine (e.g. Gleeson, Orlikon, etc), including generation and drilling of mounting holes (4) Heat treatment of finish machined crown wheels by: (i) Case carburising - with care to avoid carburising of threaded mounting bolt holes (ii) Hardening in special furnace for control of distortion and decarburisation (iii) Oil-quenching under press for hardening in order to avoid distortion of tooth flanks and the back face run-out of the wheel (5) Tempering in air circulating furnace for stress-relieving (6) Shot blast cleaning, distortion checking and contact picture matching Most critical control points for crown wheel heat treatment are (a) control of hardness and microstructure of forged crown wheel blanks for machining, (b) control of case depth in carburising, (c) control of core strength in hardening, and (d) control of distortion during quenching and hardening. Table 11-5 outlines the parameters of heat treatment of crown wheels, starting from forging till the end of production cycle.

Carburised case depth for such critical component generally ranges between 0.80 mm and 1.20 mm, depending on the vehicle type and expected load in the service. For consistency in surface properties, variation of case depth is kept limited within +/– 0.10 mm, i.e. range is fixed within the level of 0.80 to 1.0 mm, 0.90 to 1.10 mm, or 1.0 to 1.20 mm. However, it is to be noted that the carbon content or hardness within a given case depth is not constant; it varies from surface towards the core – decreasing inwardly with distance. Purpose of diffusion cycle in carburising operation is to flatten the variation of carbon profile from surface to the core, so that there is sufficient case with uniform or near uniform hardness. This control is necessary for increased resistance to contact fatigue (i.e. resistant to pitting under load). Figure 11-4 depicts how the hardness varies with depth across the case, which is caused by

the change in carbon profile from surface to core. A gradual transition of case to core is preferred over a sharp case-core interface. Generally, about 50% of case is aimed with near uniform hardness for heavy duty gears, like crown wheels.

Fig. 11-4 Graphical depiction of the hardness profile of case carburised pinion caused by change in carbon profile of the case. (Line with arrow indicates the case depth with respect to 580 VPN hardness criteria)

The figure illustrates how the hardness profile can change from surface towards core and the importance of maintaining the right core strength and hardness profile for ensuring good strength for fatigue resistance. The hardness profile shows that the hardness is not uniform across the case depth; it drops with distance from the surface. The profile can be made more uniform by controlling the ‘diffusion cycle’ of the carburising process (vide discussions in Chapter 8). However, gradient in carbon and hardness profile can neither be totally eliminated nor a sharp transition of case to core can be tolerated. The latter is known to give rise to notch effect due to sharp transition of case. Hence, adjustment and control of diffusion cycle during carburising is very important. Figure 11-5 illustrates the case depth profile of a crown-wheel gear tooth after carburising and hardening, as obtained in practice. The figure shows the variation of hardness from the surface at different location of the tooth profile. This type of hardness profile checking – necessity of which arises due to variation of carbon profile – is important for understanding how the case depth can change from one point of the tooth to other, especially between tooth pitch line and tooth route or fillet. While pitch line hardness controls the wear in service, tooth fillet hardness contour controls the fatigue strength – but both are important

Fig. 11-5 A graphical presentation of carburised case depth and hardness profile at different locations of the tooth

for life cycle of the gear and must be achieved. Hence, attempts should be made to even out this difference in case depth and hardness profile by improving the carbon profile by appropriate measures during carburising – such as adoption of low-pressure carburising or other improved means

for even carbon pick up in geometrically constrained areas, such as the tooth root / fillet, bends and radius. Depending on the shop practice, case depth is determined by a minimum hardness level in this curve. For example, if minimum hardness is fixed as 580 VPN (equivalent to 58HRC), the case depth in this case is about 0.90 mm at the tooth fillet and 1.3 mm at the tooth pitch line. This situation arises mostly due to accessibility of carbon atoms to areas of geometric constraint of the gear profile. Hence, attempts should be made to minimise the difference of case depth between different points in the gear by selecting appropriate carburising technique, e.g. gas carburising with additional control parameters or even vacuum carburising, if cost justifies this process route. Another important control for carburising of fatigue sensitive load bearing parts is the control of surface carbon – which should not exceed 0.80% carbon. Higher carbon on the surface generally leads to higher percentage of retained austenite in alloy carburised steels, especially those containing Ni and Mo. High volume of retained austenite not only lowers fatigue strength of the case but also increases the possibility of pitting under load from those soft spots. Figure 11-6 shows area of a case containing high volume of retained austenite, shown under high magnification.

Fig. 11-6 A case microstructure of an alloy carburised steel with 20% retained austenite in a matrix of needle type martensite, shown at x1000

While some amount of retained austenite is desirable for increased fracture toughness, high percentage of retained austenite may weaken the gear in strength. Hence, the gear is subjected to second tempering or a sub-zero treatment at about –50°C for conversion of retained austenite to martensite. Sub-zero treatment has to be followed by another short tempering cycle for stress relieving of newly formed martensite from retained austenite.

Choice of steel composition, and control over the case depth and core strength are very important parameters for crown-wheel or any gear manufacturing and hardening (after case carburising). If these parameters can be appropriately controlled and quenching is suitably adjusted, case carburised surface of such components may additionally develop some residual compressive stress – which works favourably for improved fatigue resistance capability. Conditions for development of compressive residual stress have been pointed out in Section 11.1.1 earlier.

11.5.3

Heat Treatment of Ball Bearing Races

Ball or taper roller bearings are used in automobile (or any other engineering component assembly) for smooth rotation and simultaneous transmission of radial / axial force from one part to the other. Figure 11-7 depicts one such set of races of ball bearing, which is generally manufactured by using special bearing steels, containing chromium which is a hard carbide forming element (e.g. 100Cr6 – a popular chromium containing bearing grade steel).

Fig. 11-7 An illustration showing the construction of a ball bearing – assembled with outer bearing race, inner race, balls and ball bearing cage for linearly holding the balls

Balls are used in the assembly for maintaining a separation between the bearing races – one of which is fixed and the other rotates. The purpose of ball bearing is to reduce friction between two mating parts in dynamic rotation and to support radial and axial loads. Rotating balls transmit the load from upper race to the lower race, where one race is stationary and the other is attached to the rotating assembly, such as hub or shaft. An example of bearing application in automobile is the mounting of wheels using ball/taper roller bearings. As such, bearings experience high specific

contact load and can give rise to (a) wear and pitting, if the hardness is not high enough, and (b) can lead to pre-mature fracture and failure, if the steel is brittle, i.e. lacks toughness. Properties required in a ball bearing steel are: • High surface hardness for wear resistance • Good strength and toughness for fatigue and fracture resistance • Total freedom from decarburised layer for prevention of pitting and dents of the surface – leading to increased friction and failure of the bearing. Heat treatment of bearing races and balls also calls for freedom from distortion for ensuring smooth movement of balls and rollers within the groove. Dimensions being a very critical parameter for bearing, the finish dimensions are generated after heat treatment by special grinding and lapping. Bearing races can be manufactured by using high carbon special quality ball bearing grade steel (e.g. SAE 52100 / 100Cr6) or by carburising the low-carbon alloy steel with appropriate precautions and control. However, use of high carbon ball bearing steel after through hardening is the common route for bearing manufacturing, which involves the following steps and heat treatment:

Primary concerns in heat treating ball bearing steels are: • Control of decarburisation of surface – which leads to soft spots on the finished job and might lead to failure by pitting under contact fatigue load • Control of oxidation of the surface - which might cause brittleness in the structure • Control of retained austenite, which should not exceed 5 to 10% max.; else might cause inferior wear resistance of the surface. This is controlled by controlling austenitising temperature and time. More is the carbon in the solid solution after austenitising more is the chance of retaining untransformed austenite. • Control of uneven / non-uniform hardness on the surface after quenching. This could be due to inadequate soaking, un-even quenching or non-uniform carbide structure in the initial material. • Control of over soaking and overheating, which might give rise to micro-cracks / distortion after quenching, especially if the quenching rate is high and / or austenitising temperature is high. • Control of distortion and damage to the surface at every stages of heat treatment, and • Ensuring fine and even dispersion of carbides in the structure. After finish heat treatment, ball bearing races should have uniform surface hardness of 60HRc and above, uniform and fine dispersion of carbides, and limited retained austenite. If the retained austenite percentage is higher, the races can be given a sub-zero treatment at (– 50°C) before tempering. Figure 11-8 illustrates a typical microstructure of SAE52100 steel after hardening and tempering. The fineness and distribution of carbide particles – which are generally chromium carbide in case of SAE52100 steel – are controlled by initial spheriodised structure of the steel and the austenitising temperature and time, which control the amount of carbide dissolution into the austenite. Too high a carbon in the austenite will lead to higher retained austenite,

Fig. 11-8 A typical micrograph of SAE 52100 ball bearing steel after hardening from 840°C, showing fine and uniform dispersion of carbide particles (x500)

and too low a carbon level in the austenite will create problem in developing uniform hardness and leave behind some coarse and non-uniform carbides in the matrix. Thus, austenitisation of bearing steel requires careful balancing of carbon in the austenite before hardening. This calls for careful adjustment of austenitising temperature and time with respect to heat transfer efficiency of the furnace used. Salt bath furnace generally takes lower temperature and time for austenitisation than oil fired or electrically heated furnace due to higher heat transfer efficiency. Hence, salt bath hardening is very popular method of shop floor practice for ball bearing steel heat treatment.

11.5.4

Heat Treatment of High Speed Steel Tools

Heat treatment of high speed steels has been discussed in Section 9.6, and this section further elucidates the process of heat treatment of HSS-tools, which calls for many extra precautions that normal tool steel hardening may not demand. High-speed steels are widely used for metal cutting and drilling operations in industries; requiring special properties for efficient and reliable performance justifying costs. High-speed steels are used for manufacturing various tools, cutters, broaches, and tool bits for machine shops. These tools are critical items for determining the shop efficiency and productivity. Therefore, quality of their heat treatment is very important for economy in tool life and cutting efficiency. High-speed steels are hard, high-alloy steels – containing Mo, W, V and Co as alloying elements – designed to cut other materials at high speeds, despite extreme heat generation at the tool’s cutting edge. The heat can reach to the level of ‘red heat’ condition – indicating the heat level of 600°C or above. Figure 11-9 depicts few high speed steel (HSS) tools that are used for cutting/drilling on the shop floor.

Fig. 11-9 An illustration of few high speed drills made of high speed steel (HSS)

For providing efficient cutting / drilling performance, the high speed steel must have three basic characteristics: • Capacity to attain high room temperature hardness after heat treatment (usually between 62 – 66 HRc) for resistance to wear during cutting • Capacity to maintain high hardness at elevated temperatures i.e. high red hardness – due to generation of heat during cutting, arising from frictions between tool and the work piece, and • Enough impact toughness to withstand the interrupted cutting forces that tools may experience during the cutting operations. Table 11-7 provides the list of few important high speed steel composition and their relative properties, including toughness. These are functional properties. In addition to these properties, steel condition should be appropriate for fabrication by machining – which requires soft machinable structure. Thus, high speed steels for tools have two distinct set of property requirements – both of which are developed by appropriate heat treatment. The set of properties are: (1) For fabrication / manufacturing: The steel should be soft for machining and fabrication – achieved by spheriodising annealing to produce soft structure with spheriodised carbide matrix. The size and distribution of carbides are determined by the degree of forging or hot working given to the steel during shaping – but this is an important parameter for response to correct hardening of HSS tools.

(2) For functional performance: The tool should be very hard, strong and tough – with hardness level of 64 to 66 HRc at room temperature along with good hot-hardness and high toughness property. For developing these functional properties, HSS tools are subjected to a series of hardening and tempering treatment for producing very finely dispersed alloy carbides in the matrix of tempered martensite.

HSS contains special alloying like Mo, Co and W for providing a stable structure at the higher temperatures i.e. for high hot hardness properties. Because of fabrication difficulty, tungsten (W) containing HSS tools are often produced by compact powder metallurgy route. But, tools through this route also require heat treatment for producing the final set of properties. Though the cost of heat treatment of tool steel is less than 10% of the tool, a tool is as good as its heat treatment quality. Skill and care in heat treating tool steel is, therefore, critical for its performance. Tool steels, in general and including HSS, are high alloy steels and they are prone to distortion and cracking due to thermal shock, if heating is not uniform. Thus, the very first step in heat treating tool steels is the selection of heating process – which should be able to heat uniformly

and evenly without causing any thermal shock during heating. In this regard, use of salt bath furnace for hardening of tool steels is very helpful. Salt bath provides heating by conduction, which is superior to heating by radiation and convection for uniform heat transfer. Further, buoyancy from the liquid salt where the tools are suspended during heating also supports lowering of distortion while being heated. Though there has been considerable improvement in the technique of heat treating tool steels in controlled atmosphere electrically heated furnace, salt-bath hardening of tool steels is preferred by most heat treaters due to: (a) Uniform heat transfer by conduction in a molten salt bath (b) Facility to hang the long and slender tools vertically down in straight line in a salt bath, which is necessary for reduced distortion during heating cycle (c) Buoyancy support of the molten bath to the hanging tools (d) Faster rate of heating at higher temperature, and (e) Flexibility of the process for small lot heat treatment. However, high-quality tool hardening can also be carried out by using vacuum furnace where the risk of decarburisation or surface oxidation is nil. Main steps in the hardening of tool steels are: (1) Preheating the annealed tools, (2) Austenitising, involving soaking at high temperature, (3) Quenching, and (4) Tempering. Annealing is the starting point for using a tool steel – such as high speed steel, which is a popular example in this group of steels. Forged or hot worked HSS steel blanks are always supplied in the annealed condition in the hardness range of 200 to 220 BHN for ease of machining and fabrication. However, if a tool is re-hardened or welded during fabrication, the tool is required to be annealed again; this is to take the initial structure back to original. Annealing is generally carried out in the shop floor in atmosphere controlled conventional furnace in the temperature range of 850 to 900°C with a soaking time of around 2 hrs – which can vary with the diameter of the tool or blank. After soaking, the steel is cooled slowly till the temperature is below 650°C – followed by normal air-cooling. Alternatively, the steel is cooled inside the shut furnace overnight for softening. Annealing should produce a hardness level between 200 and 220 BHN max with uniformly fine spheriodised carbide structure in the matrix – which is required as starting structure for hardening. Preheating is an essential step for taking the HSS tools to hardening temperature. It minimises thermal shock, reducing the danger of distortion and cracking. For HSS tools, preheating is done in two steps; first, below the transformation temperature i.e. between 650 and 750°C, and

the second one just at the transformation temperature, i.e. around 810 to 830°C. Preheating does not require any soaking; its purpose is to reduce thermal shock when taken up to higher austenitising temperature. Austenitising of HSS tools is generally carried out in salt bath. In fact, in a salt bath hardening set-up, there are at least three salt bath pots: one for preheating – maintained at around 830°C, the other for austenitising – maintained at higher heat of 1100 to 1280°C (depending on the grade of HSS), and the last one for quenching – maintained at about 540 to 620°C. Generally, the process of austenitising of tool steels involves heating to higher temperature and soaking – for producing full austenite with balancing of carbides in the solid solution for getting the optimum microstructure that is hard, tough and wear resistant. But, for HSS tools, the soaking time is very short – because of the fact that austenitising temperature is too high and very close the melting point of the steel, vide Table 10-8. Hence, purpose of soaking of HSS tools is to equalise the temperature and then quench – again in a separate salt bath where heat is removed in more uniform manner by conduction and convection. That is the reason for very short soaking time of HSS in the bath, which is only few minute. Table 11-8 illustrates various heat treatment parameters of HSS tools of different grades.

General hardening cycles for HSS tool steels in salt bath hardening process are: • Preheating of the tool – prior to hardening – at the temperature of about 820 to 850°C – with holding time 5 to 10 mins as per the size of the tool, if treated from a neutral salt bath

• Austenitisation of the tool in the temperature range of 1190 to 1220°C by holding for about 3 to 5 minutes as per cross-section. • Quenching in a salt bath maintained at the temperature range of 540 to 620°C for holding and temperature equalisation – followed by air cooling to room temperature. • Tempering in an air circulated furnace at temperature of 540 to 570°C for about 45 minutes. Since austenitisation time is short, initial annealed structure of the tool should be with fine carbides – which can easily go into solid solution at the pre-heating and soaking temperature. Care is necessary that during quenching, the job is uniformly immersed in the quench bath without allowing uneven cooling at any point. Each step in the heat treatment of HSS tool is critical for the end result. HSS tools can be also quenched in oil, but the oil temperature and cooling condition must be closely controlled. For oil quenching, generally an interrupted quenching where the job is first quenched in a bath maintained at about 530°C and then air cooled in even manner. A critical part of HSS heat treatment is tempering the tool quickly after the quenching, as soon as the job reaches the handling temperature. If there is any chance of delay for tempering, the jobs should be placed in a container of boiling water or salt bath pot maintained at 150 – 160°C in order to prevent cracking. HSS tools generally require multiple tempering – about 2–3 cycles – in order to improve hardness and lowering of retained austenite (vide discussion in Section 9.6). Figure 11-10 shows the typical microstructure of HSS tools after hardening through salt bath.

Fig. 11-10 An illustration depicting two micrographs of heat treated HSS tools using salt bath hardening, where A pertains to M4 steel, austenitised at 1190°C for 5 mins and salt quenched at 590°C, and B pertains to T15 steel, austenitised at 1215°C for 3 mins and salt quenched at the same temperature – followed by air cooling to room temperature. (Both micrographs are at x1000)

In these micrographs, while Mo-bearing M4 steel shows fine carbides and some retained austenite, tungsten-cobalt bearing T15 steel shows lot many more hard carbides, which are slightly coarser. The T15 steel, which might have slightly lower hardness at room temperature than M4 steel, has higher hot hardness properties than the M4 steel. High-speed steel tools are also stress relieved after grinding at a slightly lower temperature than the last tempering temperature. This is to ensure that the tool tips and cutting edges are totally free from any internal or residual stresses. Since high speed steel and its heat treatment is an expensive one, many heat treaters try to optimise the life of cutting tools by giving additional surface treatment like nitriding and sulphur coating for reducing friction. Also, trend is developing to give a thin coating of very hard compound layer by CVD/PVD process. The foregoing are few illustrations of the heat treatment processes from the shop floor practices. Illustration of these industrial heat treatment cases sums up most of the heat treating tasks that are faced by the heat treaters on the shop floor. While metallurgical principles of heat treatment are mostly concerned with the development of structure and properties in the component, the utility and life cycle of the component in actual service depends on few more factors, such as: • Freedom from decarburisation or oxidation of the working surface • Freedom from distortion (i.e. variation of dimensions and contour from the design specification) of any kind in the main and matching components • Freedom from micro-cracks and internal stresses • Freedom from tensile residual stresses at any portion of the surface or body of the component, and • Freedom from dents and damages or stress raisers that can be caused during heat treatment The chain of heat treatment processes followed in the shop floor is, therefore, more than just heating and cooling at a predetermined rate for developing some physically measurable properties – like the hardness, strength, ductility, toughness, etc. The process begins with an elaborate preparation of ‘process chart’ that includes: • Types of furnace to be used and the methods / degree of controls in the furnace • Identifying the need for cleaning / degreasing the jobs, which otherwise interfere with the surface chemical reactions as well as furnace atmosphere after vapourisation

• The way the components are to be placed and heated inside the furnace • Fixturing for distortion sensitive components while being heated inside the furnace and, particularly, when cooled (quenching) • Identifying the need for atmosphere control and using an efficient atmosphere generation and control system, wherever necessary • Methods of monitoring or signalling any process deviation during the operation, and • Safety and emergency measures Each and every component that goes for heat treatment in an organised shop floor is covered by a process chart elaborating the method and precautions for heat treatment, including the way of loading inside the furnace and controls for distortion. Furnace / furnace maintenance and methods of atmosphere generation and control play important roles in any heat treatment shop.

Summary 1. The chapter discusses various tasks of heat treatment practices in the shop floor and illustrates the issues with reference to heat treatment practices of some common automobile components. 2. To start with, the chapter reiterates the common rules and principles of heat treatment discussed in the book at one place – in order to facilitate understanding and examination of the industrial practices from a common thread of knowledge and practice. 3. The chapter provides an overview of heat treatment at-work on the shop floor and different considerations that guide the process selection and practice. 4. Industrial heat treatment practices have been grouped into two types: one is the grade specific heat treatment and the other is the application specific heat treatment, e.g. heat treatment of Ball bearing, High speed tools, etc. 5. To illustrate the grade specific heat treatment, heat treatability of different grades of steels have been first given in tabular form, and thereafter applicable heat treatments have been discussed as per the processing route – like through hardening, case hardening, etc. Tables and charts have been used to illustrate different heat treatment parameters and properties. 6. Emphasis of application wise heat treatment of different components has been to illustrate the need for additional control for dimensions and distortions – in addition to the traditional needs for control of structure and property. 7. Cases from industries have so chosen as to bring out the intricacies and tasks of practical heat treating problems for all engineering applications; namely the heat treatment of: Front Axle Beam of a commercial vehicle, Crown-Wheel / Pinion of automobile transmission system, Ball bearing for high speed rotation and load transfer, and finally the heat treatment of High speed steel tools for demonstrating the complexities of heat treating tool steels.

References / Suggested Reading ASM Handbook, Vol. 4, Heat Treating, ASM International, USA, 1991 ASM Handbook, Volume 4, Defects and Distortion in Heat Treated Parts, Heat Treating, (p 601 to 619), Metals Park, OH, USA, 1998 ASM International, Heat Treater’s Guide: Practices and Procedures for Iron and Steels, 2nd Edition, Metals Park, Ohio, 1995 Dossett, John L. and Howard E. Boyer, Practical Heat Treating, ASM International, USA, 2006 Higgins, R. A., Engineering Metallurgy (Applied Physical Metallurgy), Viva Books, New Delhi Mandal, S. K. Steel Metallurgy: Properties, Specifications and Applications, McGraw-Hill Education, New Delhi, 2014 Reed-Hill, Robert E., Physical Metallurgy Principles, Van Nostrand Reinhold Co., 1973 Rollason, E. C., Metallurgy for Engineers, Edward Arnold (Publishers) Ltd., 1973 Rose, A. and H. P. Hougardy, Transformation Characteristics and Hardenability in Carburising Steels, Symposium on “Transformation and Hardenability in Steels”, Climax Molybdenum Co., Michigan, USA, 27-28, February, 1967 Totten, George E. (Ed), Steel Heat-Treatment Handbook, 2nd edition, CRC Press, 2006

Review Questions 1. Pinpoint five important features in steel which are considered critical for heat treatment of steels and discuss why they are critical. 2. Why control of residual stress is important in the hardening of steels? Discuss the controls necessary for developing ‘compressive residual stresses’ in a case carburised steel part. 3. Name the factors that need to be controlled in industrial heat treatment operations – other than the structure and strength – for improved service performance. How distortion problem is handled in industrial practices of heat treatment for automobile crown-wheel gear? 4. Critically discuss the hardening operations and controls for hardening of Axle shaft. 5. What type of heat treatment is applied in industry and why: for (a) Low-carbon sheet metal steel (b) Mild plain carbon steel (c) Low-carbon alloy steel for case hardening (d) Medium carbon low-alloy steels, and (e) High carbon low-alloy steels 6. Why steels containing higher carbon are preferably annealed instead of normalising? 7. What is the purpose of application specific heat treatment of steels? Illustrate your answer with reference to the heat treatment of automobile ‘Front Axle Beam’.

8. Draw a process chart for heat treatment of automobile crown-wheel – starting after blank forging. 9. Discuss how retained austenite content can be controlled in the hardening of ‘Ball Bearing Steel’. How presence of retained austenite can influence the performance of an engineering component in service? 10. Critically discuss the heat treatment of Tungsten bearing high speed steel tool (e.g. T-15 grade) for ensuring long service performance.

Appendices

A. Steel Grades & Compositions 1. Compositions of Hardening Grade Steels (H Steels)

2.1 Common Case-carburising Grade steels and their near

B.

Physical Data & Tables 1. Physical Constants: HANDY PHYSICAL CONSTANTS

2.

Conversion Tables CONVERSION FACTORS

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Glossary

Ageing: It is a time-temperature dependent process where changes in properties take place in certain steel types, such as mild, low carbon and some special grade alloy steels. It is generally a result of precipitation from the matrix of a solid solution (e.g. Austenite or ferrite in steel) whose solubility decreases with temperature. Steel with smaller solute atoms containing nitrogen has the marked tendency for ageing. Annealing: It is a thermal treatment consisting of heating uniformly to a temperature (within or above the upper critical range) and cooling (at a slow but controlled rate) to a temperature below the critical range, generally to room temperature. This treatment is used to produce a softer microstructure, usually designed for improved machinability, homogenisation of structure, and to remove stresses. Austenite: It is a high-temperature crystallographic phase of steel having ‘face centred cubic’ (FCC) arrangement of atoms in the lattice structure. Austenite is not stable at temperature below the ‘lower critical temperature’ of steel (around 723°C), unless the steel contains higher percentage of alloying elements like Cr, Ni, and Mn. It is non-magnetic in nature. Austenitising: It is the process of producing 100% austenite in the steel structure by heating above the transformation temperature A3 and holding it for the required time for transformation to 100% austenite. Austempering: It is a process of quenching the steel from austenitising temperature to a temperature just above the martensite transformation temperature (Ms) in a suitable media having cooling rate higher than required critical cooling rate, and then holding at that temperature until transformation is complete. Generally, lower bainitic structure is obtained by this treatment to alloy steels and alloy cast irons. Burning: It is a phenomenon of overheating of steel during rolling or heattreating causing incipient melting at the grain boundaries or inter-granular oxidation. Burning causes permanent damage to the steel. Carbon potential: A term used to define the ability of an environment containing active carbon to contribute, alter and maintain a defined carbon content of the steel surface exposed to it under a given condition. In any particular environment, the carbon level attained by the surface will depend on such factors as temperature, time and steel composition.

Cementite: It is a chemical compound of iron (Fe) and carbon (C), expressed by chemical equation Fe3C, containing carbon per cent of 6.67%. It is a hard compound and can be present in steel in globular, lamellar or dendritic form. Controlled cooling: A term used to describe a process by which a steel object is cooled from an elevated temperature, usually from the final hot-forming or hot rolling operation in a predetermined manner of cooling to avoid hardening, cracking, or internal damage and to produce a desired microstructural combination. Corrosion: It is an electrochemical phenomenon by which metal surface gets attacked by atmospheric or acidic environment causing decay or damage to the surface. Presence of moisture and oxygen is a vital factor for the corrosion process. With salt solution and presence of electric current, corrosion process gets greatly accelerated. Critical cooling rate: The cooling rate necessary for a steel to transform to 100% martensite upon quenching (vide CCT-diagrams in the text). Critical cooling rate is dependent on the composition of the steel; higher the carbon and alloying (i.e. Hardenability of the steel) lower is the critical cooling rate required for 100% martensite formation. Critical temperatures: Steel undergoes different allotropic changes during cooling or heating, which are associated with evolution of heat at the points where allotropic (i.e. Phase change) changes take place. This causes a momentary arrest of fall of temperature at those points – called critical points or critical temperatures, denoted by A. Since steel undergoes number of allotropic changes during cooling or heating, there would be number of ‘critical points’, which are denoted by A. Of these critical points, A3, marking the change of austenite (g) to ferrite (b variety), is referred as ‘upper critical temperature’ (910°C on cooling) and A1, marking the change from austenite to pearlite which occurs at lower temperature, is called the ‘lower critical temperature’ (723°C on cooling). Though critical points occur both on heating and cooling, there is minor difference in their practical values; values on heating are a bit more. Decarburisation: The process of loss of carbon from the steel surface as a result of heating in a medium that reacts with the carbon in the steel. Higher the temperature, higher is the chance of decarburisation. Decarburisation results to loss of strength of the steel in the surface areas. Ductility: It may be defined as the property which enables the steel to be drawn into wire by use of tensile force. Ductility is used as a measure of how much the steel can deform plastically before fracture. Eutectic: It is a composition in an alloy system (e.g. Fe-C system) where the alloy system solidifies at a lower temperature than any other combination of composition in the same system. The composition is called eutectic composition and the temperature is called the eutectic temperature. In Fe-C system, eutectic composition is about 4.3% carbon.

Eutectoid: It is a composition when similar analogous to eutectic transformation occurs in solid state, rather than liquid, where one solid solution (e.g. Austenite in steel) transforms to other phases (e.g. Ferrite and carbide – forming pearlite) at a temperature lowest in the system. In steel, eutectoid composition in plain carbon steel is about 0.85% carbon. This might change with alloy content in steel. Ideal critical diameter (DI): The diameter of a round steel bar that will harden at the centre to a given percentage of martensite or a definite hardness value when subjected to an ideal quench (i.e. Grossman quench severity H = infinity). Isothermal annealing: A process involving austenitising followed by cooling and holding at a temperature where austenite transforms to a relatively softer ferrite-carbide structure. The isothermal holding temperature is generally below the lower critical temperature of the steel. Isothermal transformation: A change in phase by transformation of austenite at constant temperature. Elongation: In tensile testing, it is the increase in gage length, measured after the fracture of a specimen within the gage length; usually expressed as a percentage of the original gage length. This is taken as measure of ductility in the steel. Fatigue: Fatigue is the progressive and localised structural damage leading to fracture which occurs when a material is subjected to cyclic loading. Fatigue damages occur at stress level less than the tensile stress limit of the steel and can be even below the yield stress level. Fracture toughness: It is a property which describes the ability of the steel containing a pre-existing crack to resist fracture. Since all materials will have some kind of pre-existing flaw, which is not exactly avoidable, some fracture toughness is necessary to avoid catastrophic failure, especially in high strength steels. A parameter called the stress-intensity factor (Kt) is used to determine the fracture of most materials, and the value is expressed in the unit: __ __ toughness MPa÷m (psi÷in ). Free energy: A thermodynamic quantity that defines the change in energy between the internal energy of the system and the product that forms from the system at standard temperature and pressure (STP). If free energy change is negative, i.e. Energy is lowered due to the product formation, the process is thermodynamically favourable. It is also called ‘Gibbs free energy’. Hardness: Resistance of a metal to plastic deformation, usually by indentation. However, this may also refer to stiffness or temper or to resistance to scratching, abrasion, or cutting. Hardenabiity: It is a measure of the property of steel that determines the depth and distribution of hardness obtained by quenching. Hardenability of steel depends on the carbon and alloy content in the steel; higher carbon and

alloy contributes to higher hardenability of the steel. It is not a measure of hardness, but the depth and distribution of a pre-fixed minimum hardness value measured by Jominy test. Hertzian stress: Hertzian stress (also referred as Hertzian contact stress) refers to the localized stresses that develop as two curved surfaces come in contact and deform slightly under the imposed loads. The amount of deformation is dependent on the modulus of elasticity of the material in contact. It gives the rolling contact stress as a function of the normal contact force, the radii of curvature of both bodies in contact and the modulus of elasticity of both bodies. Hertzian contact stress forms the foundation for equations for load bearing capabilities and fatigue life in bearings, gears, and any other bodies where two surfaces are in contact. Impact test: It is a test to determine the behaviour of materials when subjected to high rates of loading, usually in bending, tension or torsion. The test measures the energy absorbed in breaking a specimen by a single blow, as in the Charpy or Izod tests. Killed steel: Steel treated with a strong deoxidizer (like Al, Si, etc.) To reduce oxygen to a level where no reaction occurs between carbon and oxygen during solidification. Machinability: This is a generic term used for describing the ability of a material to be machined. To relate to practice, machinability is qualified in terms of tool wear, tool life, chip control, and/or surface finish, and a machinability index value is assigned. Overall machining performance is affected by a series of variables relating to the machining operation and the work piece microstructure. Martempering: A hardening method in which the austenitised steel piece is quenched into an appropriate medium maintained at around Ms temperature of the steel and held in the medium until the temperature in the piece is uniform throughout the body but not long enough to form bainite, and then cooled in air. Aim of the process is to form 100% martensite in high alloy steels without cracking or distortion due to residual stresses. Martempering should follow by tempering. Normalising: It is a thermal treatment consisting of heating uniformly to temperature at least 30°C above the critical temperature range and cooling in still air at room temperature. The treatment produces a recrystallisation and refinement of the grain structure and gives uniformity in hardness and structure to the product. Strength of the steel also increases due to refinement of grains. Pickling: An operation by which oxide scale of the surface which gets formed during heating or holding to higher temperature is removed by chemical action. Sulphuric acid is typically used for carbon and low-alloy steels; after the acid bath, the steel is rinsed in water. Quenching: A treatment consisting of heating uniformly to a predetermined temperature and cooling rapidly in water or a suitable fluid medium to

produce one or a set of desired microstructure structure. This is a critical step for successful hardening of steels. Recrystallisation: It is a thermal process by which deformed grains are replaced by a new set of un-deformed grains that nucleate and