An investigation of fractures in temper brittle steel

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AN INVESTIGATION OF FRACTURES IN TEMPER BRITTLE STEEL

A Thesis Presented to the Faculty of the School of Engineering The University of Southern California

In Partial Fulfillment of the Requirements for the Degree Master of Science in Mechanical Engineering

by Harold Robert Grant August 1950

UMI Number: EP60504

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This thesis, written by

...... Har o 1d__R o be r t __Gr ant........... under the guidance of hX§.... Faculty Committee, and approved by a ll its members, has been presented to and accepted by the Council on Graduate Study and Research in p a rtia l f u lf ill­ ment of the requirements f o r the degree of

Master of Science

AlL.M®chani_ca^ Date.

Faculty Committee

Chairman

TABLE OP CONTENTS

PAGE INTRODUCTION..................................

1

REVIEW OP L I T E R A T U R E .........................

2

EXPERIMENTAL PROCEDURE AND R E S U L T S ............ 10 Determination of Temper Brittle Characteristics. * • • • • • • • • • • • • •

11

Comparison of Fractures ......... • • • • • • •

11

Effect of Grain Size and Speed of F r a c t u r e .............................. ......

15

SUMMARY AND C O N C L U S I O N S ....................... ...... 16 Suggestions for Further S t u d y ............ BIBLIOGRAPHY ...........................................

19 38

LIST OP CHARTS CHART A.

PAGE Impact Strength vs# Tempering Temperature

...

21

LIST OP TABLES TABLE I.

PAGE Impact

Strength and H a r d n e s s ................

22

LIST OP FIGURES FIGURE

PAGE

1.

Specimens Tempered at 800°P ( X 2 ) .................. 23

2.

Specimens Tempered at 900°P (X2) . . . . . . . .

24

3.

Specimens Tempered at 1000°P (X2)

25

4.

Specimens Tempered at I100°P (X2)

26

5.

Specimens Tempered at I200°P ( X 2 ) ................ 27

6.

Specimens Tempered at 1200°F,Tested at Temperature

7.

Impact Specimen (X100)

.........

29

.........

30

Fracture Profile, Embrittled Room Temperature

9.

28

Fracture Profile, Nonembrittled Room Temperature

8.

of Liquid Nitrogen (X2) . . . . .

Impact Specimen (X100)

Fracture Profile, Nonembrittled Low Temperature

Impact Specimen (X100)

.

. • • • 31

iv 10*

Fracture Profile, Embrittled Low Temperature Impact Specimen (X100)

• • • • • •

32

11*

Bend Test Specimens • • • • • • • • • • • • • • •

33

12.

Fracture Profile, Nonembrittled Bend Test Specimen (X100).

13.

. . . . . . . . . . .

35

Fracture Profile, Embrittled Room Temperature Impact Specimen (X500)

15.

34

Fracture Profile, Embrittled Bend Test Specimen (X100)

14.

• • • • • • • • • • • • •

• • • • • •

36

. . . . . .

37

Fracture Profile, Embrittled Low Temperature Impact Specimen (X500)

INTRODUCTION For some time the problem of temper brittleness in steels has been a familiar one to the metallurgist and de­ sign engineer#

Many types of mechanical tests have been

utilized in attempts to differentiate effectively between embrittled and nonembrittled specimens.

Investigations

have been conducted to determine the cause of the brittle­ ness.

Extensive studies have been made to evaluate the

effect of the many variables on the degree of brittleness. Little or no attention has been paid to the fracture itself with the possible exception of a cursory examination of the surface• An understanding of the problem of temper brittleness is vitally important if optimum combinations of mechanical properties are to be developed In steels used In heavy sections.

If a fully quenched and tempered structure is to

be obtained In large sections, it is necessary that the hardenability of the steel be high.

The addition of alloying ele­

ments used to increase the hardenability will also increase the susceptibility of the steel to temper brittleness. With very high alloy content, the impact strength of the steel will be low even though the tempering is followed by a water quench, since the cooling rate at the center of the piece will not be sufficient to prevent embrittle-

2 merit*

Thus, the alloy content for this type of service may

well be decided upon on the basis of temper brittleness rather than upon the hardenability desired* If temper brittleness is to be thoroughly understood, its true cause must be known and the effect of all of the variables upon embrittlement evaluated.

It has been sug­

gested that the brittleness is due to a precipitate at the grain (prior austenitic) boundaries*^ A metallographic 0 etchant has been developed to detect the presence of this grain boundary precipitate*

An explanation of why the crack

would follow a grain boundary, if there is a grain boundary 3

precipitate, has been presented*

The present investigation

wa§ conducted to determine if the fracture surface actually follows the grain boundaries as would be predicted if there were a precipitate present. REVIEW OF LITERATURE The difficulties encountered due to temper brittle1 John H. Hollomon, "Temper Brittleness,1' Transactions of American Society for Metals, Vol. 36 (Cleveland: American Society for Metals, T946T* p p .473-542• 2 J. B. Cohen, A* Hurlich, and M* Jacobson, (A Metal­ lographic Etchant to Reveal Temper Brittleness in Steel, Transatlons of American Society for Metals, Vol* 39 (Cleveland: American Society for Metals, 1947), pp. 109-38* 3 Clarence Zener, "The Micro-Mechanism of Fracture," Fracturing of Metals, (Cleveland: American Society for Metals, 193^7, pp. 9-10•

3 ness became increasingly apparent during World War I while developing gun and armor steels.

At that time a great

number of investigations were conducted.

During the period

between the two World Wars there was very little work done on the problem in this country.

In the European countries

there was just slightly more attention paid to it.

Since

the end of World War II there have appeared many reports that indicate that interest in this phenomenon has revived. In most details the theories developed by the early inves­ tigators are not commonly accepted today.

In one important

detail, the cause of the embrittlement, early theories are substantiated by recent investigations. There is general agreement among the investigators that the notched bar impact test is the most valuable tool in differentiating between the embrittled and nonem­ brittled condition.

Some reports indicate that in severely

embrittled steels the tensile test may show a longitudinal fracture or a change from the cup—cone to a star-type fracture.4

Other Investigators have found that the notch

bend test^ is valuable in detecting the embrittled condition 4 Hollomon,

0£.

clt.

5 Georges Vidal, "Generalixation de la Detection de Certaines Pragilites des Aciers," (Generalization of the Determination of Certain Types of Steel Brittleness), Comptes Rendus, 224:394-5, February 6, 1947 as abstracted in A.S."M. Rev few of Metals Literature, Vol. 4 (Cleveland: American Society Tor Metals^ 1947,) p • 198.

4 in that th© fracture appears to be all granular for the sev­ erely embrittled condition and all fibrous for the nonem6 brittled condition* The susceptibility ratio (ratio of the impact strength of a nonembrittled specimen to the impact strength of an em­ brittled specimen) has been used extensively as a measure of the degree of embrittlement*' Tests have been run on many steels.

Typical results have been published for several

aircraft steels in the form of correlated curves for Charpy, Izod, and tension impact tests, indicating the variation in the degree of embrittlement of susceptible steels when tempered in the critical embrittling temperature ranges*

7

Hollomon states: The numerical value of the susceptibility ratio of any given steel depends on the conditions of hardening and tempering, and probably also upon the amount of work put on the steel in forging, rolling, etc*, and on the direction in which the tests are made. The conditions should be the same (or should be taken into account) in comparing the susceptibility of different steels." The interpretation of the variation of this ratio with metal­ lurgical variables is extremely difficult.

The ratio is,

therefore, useful principally in determining whether a steel 6 W. S. Pellini and B. R. Queneau, "Development of Temper Brittleness in Alloy Steels,11 Transactions of American Society for Metals, Vol. 39 (Cleveland: American Society for Metals, 1947), pp. 139-53. 7 P. A. Haythorne, "Temper Embrittlement of Aircraft Steels," Iron Age, 158:51-5, August 29, 1946. 8 Hollomon,

0£.

clt.

will become embrittled to a degree sufficient to cause dif­ ficulties in service# The difficulties present in evaluating the meaning of the susceptibility ratio, as determined by room temperature impact tests, led to further study in an attempt to deter­ mine a more reliable means of measuring the extent to which the material became embrittled by a particular treatment# These investigations involved conducting a series of impact tests on the embrittled and nonembrittled material at various temperatures*

From these results the brittle transition

temperature and the amount of displacement of the impact curve were determined#

The embrittling treatment was found

to raise the brittle transition temperature and to shift the entire impact curve to lower values of impact strength# Other attempts at evaluating the degree of embrittle­ ment have been made.

These consist of investigating the

time--temperature--transformation relationship for the exnq

brittlement#

These results indicate that the transforma­

tion occurs at a low rate at tempering temperatures just below the critical where both the nucleation rate and the rate of growth are small.

As the tempering temperature is

9 iiollomon, 0£# clt *; Pellini and Queneau, op. cit.; P. B. Michailow-Michejew, ^Temper Brittleness and Heat Em­ brittlement of Alloy Steels,11 Archlv fur das Elsenhuttenwesen, 17:177-80, January— February, 1944 as abstracted in A.S.M# Review of Metals Literature, Vol. 2 (Clevelandi American Society for Metals^ 1945), p# 39#

6 decreased the rate of transformation increases to a maximum and upon further decrease of the tempering temperature the transformation rate decreases again.

These curves were

plotted for isothermal transformation and no method has been devised to correlate this information with the con­ dition of continuous cooling*

In this respect the use of

these curves is similar to that of the S-curves for the quenching of steel, indicating that if the cooling rate is sufficiently rapid the "nose" of the curve can be missed and the transformation avoided* It has been found that the transformation is a rever­ sible one so that an embrittled specimen can be retempered and if quenched from the tempering temperature the result is a nonembrittled specimen.^

Jones found that on repeated

reheating followed by slow cooling a slow recovery takes place so that the impact strength after slow cooling ap­ proaches that of the water quenched m a t e r i a l S o m e

steels

required as few as ten reheatlngs and the more susceptible steels as many as thirty reheatings* Many investigators have come to the conclusion that the transformation which causes the embrittlement is simply 10 Pellini and Queneau, o p . olt * 11 J* A* Jones, “Temper Brittleness in Alloy Steels,“ Metal Treatment, 4:97-101, January 1938*

7 a grain boundary precipitate*

12

most students of the phenomenon.

This cause is accepted by There is no definite in­

dication of the composition of the precipitate.

It has been

suggested that the precipitate may be a carbide, a phosphide, a nitride, or an oxide, but the most likely are nitrides or carbides Investigation of the effect of various alloying ele­ ments indicate that manganese, chromium, and nickel appear to Increase the susceptibility.

Their effect probably is an

increase in the amount of transformation that occurs at a given temperature, with only an Indirect effect on the rate of transformation.^

Their relative effect appears to de­

crease in the order manganese, chromium, and nickel.

Van­

adium appears to increase the susceptibility slightly, al­ though this result has not received general acceptance. Phosphorous raises the brittle transition temperature which would increase the apparent temper brittleness.

Molybdenum

seems to decrease the amount of transformation in mild em-^2 Hollomon, op. oit ♦;Pellini and Queneau, op. cit*; J. H. Andrew and H. A. Dickie,“Physical Investigation into the cause of Temper Brittleness," Engineering, 122:553-5 582-4, 1926; K. Yokoyama, "The Cause of Temper Brittleness in Steels and Analogous Phenomena in some Age-Hardening Alloys," Nippon Klnzoku Gakukal-Sl, 1:43-58, 92—103, May-June, 1937 as abstracted in Chemical Abstracts, Vol. 32 (Columbus: American Chemical Society, 1938), column 2486.

13 Hollomon,

op. oit.

14 Hollomon,

op. cit.

8 brittling treatments*

This is accomplished by reducing the

rate of transformation rather than by limiting the total 15 amount that may occur. Some data indicate that tungsten and possibly titanium and columbium effect the transformation in a manner similar to molybdenum but this had not been de­ finitely established# Until recently it has been believed that plain carbon steels were free from temper brittleness.

Jaffee and Buffum'LO

reexamined the means of de-termining susceptibility to temper brittleness by comparing the brittle transition temperature of the embrittled and nonembrittled specimens.

On this basis

plain carbon steels are not considered to be susceptible to temper brittleness.

If it were assumed that plain carbon

steels were embrittled very rapidly, so rapidly in fact that they were embrittled even when water quenched from the tem­ pering temperature, then this means of determining suscepti­ bility would not be valid for them#

The brittle transition

temperature of the plain carbon steel, without intentional embrittlement, was compared with the transition temperature for a nonembrittled alloy steel and an embrittled alloy steel.

It was found that the transition temperature for 15"Hollomon, op. cit ♦

16 L. D. Jaffee and D. C. Buffum, "Temper Brittleness of Plain Carbon Steels," Metals Technology, Vol. 15, No. 8, December 1948, Tech. Pub. 2432#

9 the plain carbon steel was higher than that for the nonembrittled alloy steel and was approximately the same as that for the embrittled alloy steel#

On this basis it was con­

cluded that plain carbon steels are temper brittle and that the transformation is so rapid that it occurs even upon water quenching from the tempering temperature.

Their ex­

planation of the difference in the behavior of plain carbon steels and alloy steels was that the alloy content merely retarded the rate of transformation rather than limiting the amount of transformation# Though the exact nature of the precipitate that forms is not known it has been found useful to be able to detect its presence by microscopic examination#

Many metallographic

etchants have been investigated to determine a suitable re­ agent for detecting the presence of the precipitate#

The

one found to give the most satisfactory results consists of fifty grams of picric acid dissolved in two hundred fifty milliliters of ethyl ether with two hundred fifty milli17

liters of zephiran-water solution added#A '

The zephiran-

water solution is made by adding ten milliliters of 12#8 per cent zephiran chloride solution to two hundred forty milliliters of water.

When this reagent is mixed thoroughly

and allowed to stand over night it will separate into two layers •

It is the upper layer that is used as an etchant

~11~Cohen, Hurlich, and Jacobson,

0£#

cit♦

10 when diluted with ether.

The etchant will attack and cause

pitting of the brittle constituent.

The etched specimen can

be polished to remove the microstructural etch and the pits in the brittle constituent will still be in evidence. This etching procedure locates the grain boundary, but it doesn*t establish that there is actually a brittle con­ stituent present.

The etching action could be merely a

preferential grain boundary attack. Although efforts have been made (1) to evaluate the degree of embrittlement and the effect of time and tempera­ ture on the embrittlement;

(2) to detect and identify the

brittle constituent; and (3) to classify the fracture accord­ ing to its visual appearance (fibrous or granular); no evi­ dence of examination of the type of fracture (intergranular or transgranular) has been found.

If it could be established

that in the embrittled condition the fracture follows the grain boundaries and in the nonembrittled condition that it does not, the presence of a brittle constituent at the grain boundary would be further confirmed*

It was the purpose of

this investigation to establish the course of the fracture* EXPERIMENTAL PROCEDURE AND RESULTS The material selected for this Investigation was a steel known to be susceptible to temper brittleness*

This

material is used in the petroleum industry and in the manu-

11 facture of gun barrels*

In each Instance temper brittleness

has been a problem in the manufacturing of a suitable product* Chemical analysis supplied by the manufacture was as follows: 0-0.34#, Mn— 0.52$, P— 0.012$, S— 0.010$, SI— 0.28$, Cr— 1.49$, and Ni— 4.2$. Determination of Temper Brittle Characteristics *

To

determine the effect of tempering temperature and rate of cooling on the impact strength of this material, 10 standard Charpy V-notch impact specimens were hardened and then tem­ pered at various temperatures.

Two specimens were tempered

at 800°P, 900°F, 1000°P, 1100°P, and 1200°P, one was water quenched from the tempering temperature and the other furnace cooled* For hardening the specimens were packed in a mixture of approximately 90% coarse alumina abrasive and 10# graphite, heated for 2 hours at 1500°F and oil quenched*

The specimens

were packed In the same material for tempering. The results of the room temperature impact tests are presented on Chart "A" •

The curves show that as the temper­

ing temperature is increased the effect of cooling rate becomes more pronounced* Comparison of Fractures *

The difference in appear­

ance between the fractures of the furnace cooled (embrittled) specimens and the water quenched (nonembrittled) specimens at the various tempering temperatures is shown in Figs. 1,

12 2, 3, 4, and 5.

The difference in appearance of the fractures

becomes more pronounced as the tempering temperature increases. The tempering temperature of 1200°F was selected for further study and an additional 6 specimens were hardened and tempered at this temperature for 2 hours*

Half of these

were water quenched from the tempering temperature and the other half were furnace cooled*

One each of the embrittled

and nonembrittled specimens were tested in impact at room temperature*

The remaining 4 specimens were fractured in

impact at the temperature of liquid nitrogen (boiling point is -320.4°F).

The time between removal from the liquid

nitrogen and the impact was from 5 to 7 seconds. The typical difference in appearance between the fractures of an embrittled and a nonembrittled specimen (for the specimens tempered at 1200°F and fractured at room temperature) is shown in Fig. 5.

The water quenched specimen

is fibrous in appearance while the furnace cooled specimen is not* In Fig. 6 is shown typical fractures of an embrittled and a nonembrittled specimen (tempered at 1200°F and fractured at the temperature of liquid nitrogen) but here the difference in the fracture is not as pronounced.

The embrittled speci­

men in Fig. 6 does not look much more granular or brittle than the embrittled specimen in Fig. 5, while there is a great deal of difference in their impact strengths.

The

13 nonembrittled specimen in Pig. 6, which had an impact strength of only 23 ft.-lbs., does not appear to be as granular as the embrittled specimen in PLg. 5 that had an impact strength of 31 ft.-lbs. Some metallurgists use the appearance of the fracture of a piece of steel to estimate the austenitic grain size. This method should be used only when the steel is in the hardened (i. e., as quenched) condition.

Otherwise, as is

apparent from these fractures the grain size is not revealed. It is evident from these photographs that the macroappearance of the fracture is not a completely reliable indication of temper brittleness.

In order to determine

whether or not the micro-appearance would be a better cri­ terion, the specimens

(all tempered at 1200°P) were copper

plated and prepared for microscopic examination of their edges• The purpose of plating the specimens was to preserve the edge of the specimen when the profile of the fracture was exposed and polished.

The profile exposed is in a plane

that is perpendicular to the notch. A great deal of difficulty was experienced In obtain­ ing a plating on the fracture that would adhere to the sur­ face and not peel off during the preparation of the specimen for examination.

Difficulty was also experienced in getting

the plating into the crevices as there was a definite ten-

14 dency to form bridges and leave the lower portions not plated* The technique that proved to be successful is briefly described in the following paragraph.

The fracture surface

was cleaned thoroughly by use of carbon tetrachloride fol­ lowed by ethyl ether.

A flash plating was applied using a

cyanide plating bath composed of 22.5 g. cuprous cyanide, 34.0 g. sodium cyanide, 15.0 g. sodium carbonate, and 1000 ml. of water.

A current density of about 0.15 ampere per square

decimeter was used with a mean distance between the fracture surface of the specimens and the copper anode of about 2.5 inches. anode.

The fracture surface was turned directly toward the This plating bath was operated for 1 hour.

The main

portion of the plating was carried out in a copper sulphate plating bath.

This bath was made up of 250 g. copper sul­

phate, 75 g. concentrated sulphuric acid, and 1000 ml. of water.

A current density of 3 amperes per square decimeter

was used*

The spacing between the specimens and the copper

anode was about 2.5 inches.

This portion of the plating

process was continued for 12 hours giving a copper plate of approximately 1/32 inch thick. To expose the fracture profile without causing the plate and the specimen to separate also gave some trouble* The successful method used consisted of mounting each speci­ men separately in bakelite using a metallographic specimen mounting press and dies.

After mounting, approximately l/8

15 to 3/16 inch was ground from the side of the mounted specimen to expose the fracture profile.

During the grinding, the

specimen was held so that the movement of the abrasive wheel was always from the plate toward the specimen.

The specimens

were polished using the usual metallographic technique. Final polishing was accomplished with very fine levigated alumina• Typical results of the microscopic examination are shown in Figs. 7, 8, 9, and 10.

Since the specimens were

unetched it was impossible to obtain the desired contrast in the photomicrographs by use of standard metallographic plates and the usual filters.

The combination that yielded

most satisfactory results was a red filter and Kodak M plates (very high contrast) developed in Kodak D-19 developer for maximum contrast.

The similarity of the two embrittled

specimens, Figs. 8 and 10, and of the two nonembrittled ones, Figs. 7 and 9, is immediately apparent.

It would

appear that the specimens that are temper brittle fail around the austenitic grain boundaries on impact, while those that are not temper brittle fail by trans-crystalline fracture, regardless of the temperature of test. Effect of Grain Size and Speed of Fracture.

To check

the reliability of this microscopic method of detecting temper brittleness, 6 pieces of the steel, about 1/4” x 1/4” x 1”, were grain coarsened by heating for 2 hours at 2200°F,

16 on© half were water quenched and the other half were furnace cooled.

The average hardness values along with the values

for the specimens previously discussed are given in Table I. The specimens were fractured by first notching with a hack­ saw and broken by bending in a vise.

Typical resulting

fractures are shown, macroscopically, in Fig. 11.

Here the

embrittled specimen shows an extremely coarse granular type of fracture while the nonembrittled one is fibrous• fracture profiles are shown in Figs. 12 and 13.

The

Here the

tendency for the temper brittle specimens to fracture on the austenltlc grain boundaries is even more apparent.

There are

two very definite grain boundary cracks protruding into the specimen.

However, if the fine grained embrittled impact

specimens be magnified sufficiently, Figs. 14 and 15, it will be seen that they look just like the coarse grain em­ brittled specimen of Fig. 13. SUMMARY AND CONCLUSIONS It was the object of this investigation (1) to criti­ cally examine the fracture of temper brittle steels;

(2) to

compare the brittle fracture with a similar fracture in nonembrittled material; and (3) to classify the brittle fracture as transgranular or intergranular.

This has been

accomplished for the material selected for the several types of fractures Investigated.

17 Embrittled and nonembrittled specimens were fractured by impact at room temperature and at the temperature of liquid nitrogen.

The room temperature test was selected

because of its extensive use in differentiating between the embrittled and nonembrittled condition.

A low temperature

test will induce a brittle fracture in material that is ductile at room temperature.

A comparison of this type of

fracture with a temper brittle fracture was desirable. Therefore, the second type of test was selected.

The same

material was also treated in such a manner as to produce a large grain size and fractured by a notched bend test, to evaluate the effect of grain size and speed of fracture on the temper brittle type of fracture. The steel tested, a chromium— nickel steel with 0.34/6 carbon, was selected because of its known susceptibility to temper brittleness. It was found that while a comparison of the impact strengths obtained at the same temperature will give an indication of the presence of temper brittleness, the com­ parison of impact strengths obtained at two different tem­ peratures may lead to erroneous conclusions about the degree and presence of temper brittleness.

The decrease in tempera­

ture will lead to a loss in impact strength in the nonem­ brittled specimen, while the embrittled specimen will show an even greater percentage loss.

Therefore, the suscepti-

18 bility at a fixed temperature should not be used as an absolute measure of the amount of embrittlement that is present. A visual examination of the fracture surface of the impact specimen is not a reliable means of detecting the pre­ sence of temper brittleness.

This point was brought out by

comparison of the results of the impact tests at different temperatures.

In each instance the temper embrittled speci­

men showed a granular type of fracture regardless of its impact strength while the fracture of the nonembrittled material was more fibrous.

Even though their impact strengths

were very nearly the same, there was no similarity between the appearance of the fractures of the embrittled room tem­ perature specimen and the nonembrittled low temperature test specimen. The nonerabrittled material showed a random type of fracture with no preference for grain boundaries.

The type

of fracture was the same regardless of (1) prior austenitic grain size,

(2) method of fracture (impact or otherwise),

or (3) the temperature of the impact test.

This would in­

dicate that the brittleness or loss of impact strength due to the decrease in testing temperature is not a result of a change in the mode of fracture but due to some other ef­ fect not studied in this investigation. In the embrittled material the fracture surface fol-

19 lowed the prior austenitic grain boundaries.

This type of

fracture occurred regardless of the method of fracture or the temperature of the test.

This phenomenon is indicative

of the presence of a grain boundary precipitate.

The de­

termination of the type and composition of the precipitate was not within the scope of this investigation. The results obtained, though conclusive for the con­ ditions and material tested, should not be interpreted as applying to all materials in all conditions of temper brittle­ ness without further verification. Suggestions for further study.

Additional information

regarding the effect of a grain boundary precipitate on the type of fracture is needed.

Ferrite could be precipitated

on the grain boundaries by quenching to a predetermined temperature.

The temperature and the time required for this

transformation can be obtained by the S-curve for the mater­ ial. gated.

The impact strength and fracture could then be investi­ If there were a definite difference in impact strength

accompanied by an intergranular fracture, this would substan­ tiate the conclusions drawn about the effect of the grain boundary precipitate. It Is suggested that, since the temper brittle mater­ ial fails on the austenitic grain boundaries, examination of the impact fracture might be used for determination of the grain size In susceptible steels.

A visual or macroscopic

20 examination of the surface or a microscopic examination of the profile could he used. The method of examining fracture profiles, while relatively simple, has not been exploited.

It could be

used to verify the presence of temper brittleness in plain carbon steels.

The results of examinations of this sort

might be useful in obtaining a fuller understanding of the embrittling effects such as; reduction of test temperature, increasing the notch severity, and increasing the speed of the test.

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