tempering of chromium steels s.b. (met), …

S.B. (Met), Massachusetts Institute of Technology

1947

Submitted in Partial Fulfillment of the

Requirements for the Degree of

DOCTOR OF SCIENCE

at the

Massachusetts Institute of Technology

1950

Signature ofDepartment

Authorof Metallurgy

January 7, 1950

Signature of Professor

in Charge of Research

Signature of ChairmanDepartment Committeeon Graduate Research

Signature Redacted

Signature Redacted

Signature Redacted-- !---

TEMPERING OF CHROMIUM STEELS

By

ROBERT WEIERTER BALLUFFI

TABLE OF CONTENTS

Chapter PageNumber Number

List of Figures . . . . . . .. ........... iii

List of Tables . . . ................ viii

Acknowledgments . . ................ ix

I Introduction . . . . . . . . . . . . . . . . . . . . 1

II Quenching Dilatometer Measurements . . . . . . . . . 6

III Kinetics of the Formation of Cementite from the Trans-

ition Structure in the Third Stage of Tempering . . . 19

A. Contraction Curves..... *....... ... 19

B. Aualysis of the Kinetics of the Third stage ... . 27

IV Kinetics of the Decomposition of Retained Austenite in

the Second Stage of Tempering . . * 0 * * . .* * . . 35

A. Expansion Curves. . . . . . . . . . . . . . . . . 35

1. Results for K and T Steels . * . . . . . . . 35

2. Results for Z Steel... . . . . . . . . . . 39

B. Analysis of Kinetics of Austenite Decomposition . 41

V Chemical and Structural Changes during the Decomposition

of Martensite . . . . . . . . . . . . . . . . . . . . 48

A. Introduction . .. . . . . . . .. . . .. . . 48

B. Hardness Results . . . . . .. . . . . . .. 0. . 48

C. Previous X-ray Work on the Transition Phase . . . 49

D. Magnetic Changes During the Third Stage . . . . . 52

E. Electrochemical and X-ray Results . . . . . . . . 56

M~r,. _:W -aw

- ii -

Chapter PageNumber Number

VI The Fourth Stage of Tempering...... . . ..... 60

A. Introduction . . . . . . . . .. . . . . . . . . . 60

B. Quantitative Measurements during Alloy Carbide

Formation . . . . . . * * * . . . * . . . . . * . . 61

1. Metallographic Results . .. .. . . . . . .. 61

2. Specific Volume Measurements . . . . . . . . . 66

3. Hardness Measurements . . . . .. . . . . . .. 66

4. Electrolytic Isolation of Carbides . . . . . . 69

C. Mechanism and Kinetics of the Fourth Stage Alloy

Carbide Formation.... ....... . . .. .. 90

1. Introduction . . . . ........... 90

2. Metallographic Results .9.99.. 99.. 9 91

3. Kinetics of the Cementite to Alloy Carbide

Reaction. . . . . . . . . . . .. . . . . . . . 96

VII Conclusions . . . .. . . . . . . .. . . . . . . . . . 97

Bibliography . . . . . . . . . . . . . . . . . . . . . 100

Appendix A . . . . . . .. 104

Abstract . . . . . . . . . . . . . . . . . . . . . . 107

Biographical Note . . ............. .. . . 109

- iii -

LIST OF FIGURES

Figure PageNumber Number

1 Length Changes of K Steel (1.07% C) During Tempering

Austenitized 14500 F, quenched to Room Temperature 10

2 Length Changes of K Steel (1.07% C) During Tempering

Austenitized 14500 F, Refrigerated -3120 F. . . . . . 11

3 Length Changes of T Steel (1.00% C, 1.56% Cr) During

Tempering. Austenitized 15500 F, Quenched to Room

Temperature . . . . . . . . . . . . . . . . . . . . . , 12

4 Length Changes of T Steel (1.00% C, 1.56% Cr) During

Tempering. Austenitized 15500 F, Refrigerated -3120 F 13

5 Length Changes of Z Steel (1.11% C, 4.11% Cr) During

Tempering. Austenitized 20000 F, Refrigerated to -3120 F 15

6 Length Changes of Z Steel (1.11% C, 4.11% Cr) During

Tempering. Austenitized 20000 F, Quenched to Room

Temperature . . . . . . . . .. . . . . . . . . . . . 16

7 Thermal Expansion of Austenite and Martensite in the

Quenching Dilatometer.... .. . . ........ 17

8 Contraction of Martensite in K Steel (1.07% C) Dur-ing

Tempering. Austenitized 14500 F . . . .. . . . . . 22

9 Contraction of Martensite in T Steel (1.00% C, 4.11% Cr)

During Tempering. Austenitized 20000 F . . . . . . . 23

10 Contraction of Martensite in Z Steel (1.11% C, 4.11% Cr)

During Tempering. Austenitized 20000 F.. .. . . . 24

11 Log Log (-a-) vs. Log Time for Martensite Decompositiona-y

in T Steel (1.00% C, 1.56% Cr).During Third Stage of

Tempering . . . . . . . .. ......... . .... .... . 2

1

29

- iv -


12 Contraction of Martensite in K and T Steels During

Tempering at 450* F ................ 30

13 Log K vs. for Martensite Decomposition in T Steel

(1.00% C - 1.56% Cr) During Third Stage Tempering . . 33

14 Length Changes of Austenite in K Steel (1.07% C)

During Tempering. Austenitized 14500 F . . . . . . . 36

15 Length Changes of Austenite in T Steel (1.00% C -

1.56% Cr) During Tempering. Austenitized 15500 C . . 37

16 Length Changes of Austenite in Z Steel (1.11% C, 4.11%

Cr) During Tempering. Austenitized 20000 F . . . . . 38

17 Structure of Z Steel (1.11% C, 4.11% Cr) after Tempering

28 Hours at 5000 F (2600 C). Tempered Martensite,

Bainite and Austenite Present. Etched with 2 Percent

Nital (150Ox) .................... 40

18 Structure of Z Steel (1.11% C, 4.11% Cr) After Tempering

28 Hours at 7000 F (3700 C). Tempered Martensite, Bainite,

Cementite and Austenite Present. Etched with 2 Percent

Nital (150Ox) . . . . . . . . . . . . . . . . . . . . 40

19 Structure of Z Steel (1.11% C, 4.11* Cr) After Tempering

28 Hours at 8000 F (4250 C). Tempered Martensite, Cementite,

and Austenite Present. Etched with 2 Percent Nital

(150Ox) * . . . . . . . . . . . . . . . . * . . . . . 42


28 Hours at 9000 F (4800 C). Tempered Martensite, Inter-

mediate Structures and Cementite Present. Etched With 2

Percent Nital (1500x) -. . . . .. .0 . . . . - - - . 42

- v -



40 Seconds at 12000 F (6500 C). Tempered Martensite,

Pearlite and Austenite Present. Etched with 2 Percent

Nital (150Ox) . . * . . . . . * & . . . . . . . . . . 4+3

22 TTT Diagram for Retained Austenite in Z Steel (1.11% C,

4.11% Cr) Compared to Primary AuStenite Decomposition in

a Similar One Percent Carbon, Four Percent Chromium

(49)

23 Log Log (a) vs. Log Time for Retained Austenite Decom-

position to Bainite in K and T Steels . . . . . . . . 45

24 Log K vs.- for Retained Austenite Decomposition toT

Bainite in K and T Steels. .. *....... . . 47

25 Rockwell C Hardness of K, T and Y Steels After Ten

Hours at Various Tempering Temperatures . . . . . . . . 50

26 Magnetometer Deflection vs. Temperature for K Steel

(1.07% C) Specimens after Tempering . . . . . . . . . 55

27 X-ray Powder Patterns for Electrochemically Extracted

Carbide Phase in K Steel (1.07% C) Tempered in the Third

Stage. Chromium Radiation Filtered with Vanadium Carbide 58

28 Chrome Carbide in the Y Steel (0.67% C, 4.00% Cr) after

Tempering in the Fourth Stage at the Temperatures and

Times Indicated. Etched with Boiling KMn04-NaOH Solution

(150Ox) * . . * . . . . . . . . . . . . . . * . . . . . 62

- vi -


29 Volume Percent by Lineal Analysis of Chrome Carbide

in Y Steel (0.67% C, 4.00% Cr) after Tempering in the

Fourth Stage . . . . . . . . . . . . . . . . . . . . . 63

30 Structure of Y Steel (0.67% C, 4.00% Cr) after Tempering

28 Hours at 11000 F (5930 C), Etched 4 Minutes with

Boiling KMnO4 - NaOH Etch (1500X) . . .. .. .. . 65

31 Specific Volume of Y Steel (0.67% C, 4.00% Cr) after

Tempering in the Fourth Stage . . .. . . . . . . . 67

32 Brinell Hardness of Y Steel (0.67% C, 4.00% Cr) after

Tempering in the Fourth Stage .. . . . .. . . . . 68

33 Weight Percent Carbide Residue Obtained From Tempered Y

Steel (0.67% C, 4.00% Cr) with HCl and Citrate Cells . 72

34 Schematic Diagram of Citrate Cell . . . . . . . . . . 75

35 Calculated Weight Percent of Chrome Carbide in Y Steel

(0.67% C, 4.00% Cr) after Tempering in Fourth Stage . 80

36 X-ray Powder Patterns of Residues from Y Steel (0.67% C,

4.00% Cr) Tempered in the Fourth Stage . . . . . . . . 83

37 Percent Chromium in Carbide Residues from Y Steel

(0.67% C, 4.00% Cr) Tempered in the Fourth Stage . . . 84

38 Percent Iron in Carbide Residues from Y Steel (0.67% C,

4.00% Cr) Tempered in the Fourth Stage . . . . . . . . 85

39 Percent Manganese in Carbide Residues from Y Steel

(0.67% C, 4.00% Cr) Tempered in the Fourth Stage . . . 86

40 Depression of Cementite Curie Temperature by Alloying 87

41 Magnetometer Deflection vs. Temperature for Y Steel

(0.67% C, 4.00% Cr). Specimens Tempered in the Fourth

Stage . t , . . . , . . . . .* , . . . .. . . . .88

- vii -


42 Temperature of Beginning of Magnetization of Cementite

in Y Steel (0.67% C, 4.00% Cr) after Tempering . . . . 89


28 Hours at 11000 F (5930C). .. .......... 93


28 Hours at 1200* F (6500 C) . . .. . . . . . . . . . 94


at 12000 F (6500 C). Electropolished and Etched in Acetic

Acid-Perchloric Acid Solution. . . . . . . . . . . . . 95

U

- viii -

LIST OF TABLES

Table PageNumber Number

I Composition of Steels Used in Quenching Dilatometer

Experiments . . . . . . . . . . . . . . . . . . . . . 6

II Volume Percent of Retained Austenite and Undissolved

Carbides in Hardened K, T and Z Steels . . . . . . . 20

III Rockwell C Hardness of K and T Steels after Tempering

in the Third Stage. . ....... .. . . . . . . . 51

IV D-Values and Relative Intensities of X-ray Lines

Obtained from Iron Carbides and the Transition Phase

in Tempered Martensite. ..... . .. . . . . . . . 53

V Composition of Y Steel. . .. . . . .. . . . . . . . 60

VI Weight Percent and Chemical Analysis of Carbide Resi-

dues in Tempered Y Steel (0.67% C, 4.00% Cr) Obtained

with the Citrate Cell . . .. . . . . .* . . .. . . . 77

IA Contraction at the Dilation Plateau for the K and T

Steels . * . . . . . . . . . . . * . . . . . . . . . 104

IIA Third Stage Kinetic Data for K and T Steels . . . . . 105

- ix -

ACKNOWLEDGMENTS

The author would like to express his sincere appreciation to

Professor Morris Cohen for his unfailing help and encouragement in

solving the many problems of this investigation. The help of

Professor B. L. Averbach with many problems is also gratefully

acknowledged. Thanks are extended to Mr. Louis Castleman for

numerous retained austenite determinations, to Miss Miriam Yoffa for

metallographic work, to Mr. E. LaRocca for the preparation of

specimens, and to the members of the M. I. T. research staff and

faculty who contributed in many ways.

The financial aid of the Republic Steel Corporation Fund and

the Union Carbide and Carbon Fellowship is also appreciated.

mm

I. INTRODUCTION

Much previous work has been done on the decomposition of martensite

and retained austenite during the tempering of hardened high carbon

steels(1,2,3,4,5,6,7). As a result of dilatometric, specific volume,

metallographic, magnetic, x-ray and hardness investigations the temper-

ing process can be divided into several well defined but overlapping

stages as follows:

1) Immediately after quenching, the hardened steel consists of

untaspered martensite, retained austenite, and undissolved carbides.

If the quenched tetragonal martensite is heated at temperatures less

than approximately 4000 F (2050 C), a carbon-rich transition product

(not cementite) is precipitated, and this process has been called the

first stage of tempering. During this process of carbon rejection, the

martensite lattice contracts in volume and becomes less tetragonal.

2) At temperatures between 400 - 6000 F (205 - 3150 C) the retained

austenite is decomposed into bainite, and this transformation,which is

the second stage of tempering, tends to increase the volume.

3) As the temperature is raised beyond 4000 F (2050 C) the

transition precipitate is converted to cementite and this is called

the third stage of tempering. It is marked by a considerable decrease

in volume, and overlaps the austenite decomposition reaction.

4) At still higher temperatures, above about 10000 F (5400 C),

complex alloy carbides form at the expense of the cementite in steels

containing sufficiently high alloy content. This process is the fourth

stage of tempering.

2

The kinetics of the first stage of tempering in a plain 1 percent

carbon tool steel and in a 1 percent carbon - 1.5 percent chromium ball

(6)bearing steel have been analyzed in detail by Averbach and Cohen

They used precision length measurements to follow the extent of reaction

because of the small length changes produced in this stage. This de.

tailed investigation of the first stage yielded quantitative systematic

information which had been lacking in the more qualitative previous

work. Q(uantitative analysis of these data made possible a postulated

mechanism for the process. One of the aims of the present investigation

was, therefore, to obtain similar information about the conversion of

the transition precipitate into cementite (the third stage of tempering).

For purposes of comparison the same two steels were principally

used in this investigation, but a third higher chromium ball-bearing

steel of 4 percent chromium - 1 percent carbon composition was later

introduced. Since the third stage causes relatively large length

changes and the reactions occur rapidly at the higher temperatures, a

quenching dilatometer was employed instead of precision length measure-

ments to follow the length variations.

In addition to analyzing dilatometrically the kinetics of the

third stage of tempering, the nature of the chemical and structural

changes involved during the conversion of the transition structure to

cementite was also studied. This problem is difficult because of the

extremely fine dispersion of the transition structure and because of the

fine particle size of the first-.formed cementite. Information has

appeared in the literature indicating that the transition structure might

be hexagonal close packed (10,11,12)but no reliable information exists

-3-.

concerning its chemical composition. The phase is more magnetic than

cementite, although only slightly less magnetic than martensite, and

exhibits a higher Curie point than cementite(4). These properties would

be expected to show a progressive change during the conversion process

to cementiteand so magnetic measurements were planned on this reaction

in the plain carbon steel where alloy content would not cause compli-

cations. In order to more definitely settle the question of the

chemical changes involved, electro-chemical work isolating the inter-

mediate structures from the matrix was carried out. X-ray and chemical

analyses were then applied to the isolated carbon-rich phases.

Overlapping the decomposition of the transition precipitate is the

transformation of retained austenite to bainite occurring in the second

stage of tempering. The kinetics of this transformation were of interest,

since there was previous evidence that the reaction was considerably

speeded up by the presence of tempered martensite (6,8)

The highest tempering temperature investigated by Averbach and

Cohen was 5000 F (2600 C), and so only the early part of the second

stage was observed. In order to analyze the austenite decomposition

kinetics more completely, quantitative data were required at higher

tempering temperatures. These data could be obtained concurrently with

the martensite decomposition measurements of the third stage by dilato-

metric methods, since accurate retained austenite determinations(9)

made possible the separation of the simultaneous third stage contraction

and the overlapping austenite expansion in the same specimen (6)

Therefore, second stage data were obtained on the same three steels - 1

percent plain carbon, 1.5 percent chromium - 1 percent carbon and

4 percent chromium - 1 percent carbon - used to investigate the third

stage.

The kinetics and mechanism of the fourth stage of tempering has

previously received little attention, although considerable work has

been done establishing the identity and temperature ranges of stability

of various alloy carbides in tempered steels(13tl4 l5pl6 ). However,

the mechanism of the conversion of cementite to alloy carbide has not

been completely settled,and the kinetics of formation have not been

measured. For the purpose of studying this process a 4 percent chromium -

0.67 percent carbon steel was selected, since at this composition the

cementite existing after completion of the third stage converts entirely

to a chromium carbide at high tempering temperatures. A combined attack

of magnetic, metallographic, x-ray, specific volume, and electro-chemical

separation techniques was planned to investigate these questions.

In summary, then, the following program was outlined:

1) Measure the kinetics of the second and third stages of

tempering by dilatometric methods in three steels - 1 percent carbon,

1.5 percent chromium - 1 percent carbon, and 4 percent chromium -

1 percent carbon.

2) Investigate the nature of the structural and chamical changes

involved in the third stage conversion of the transition precipitate to

cementite, using magnetic, x-ray and electro-chemical separation

techniques on the 1 percent plain carbon steel.

3) Investigate the kinetics of the fourth stage alloy carbide

formation in a 4 percent chromium - 0.67 percent carbon steel, applying

combined magnetic, x-ray, metallographic, specific volume, and electro-

chemical methods to trace the conversion of cementite into chromium

carbide.

- 6-

II. QUENCHING DILATOMETER MEASUREMENTS

Three high carbon steels were subjected to quenching dilatometer

experiments in the temperature range (4500 F - 12000 F) (230* C - 6500 0)

in order to establish the kinetics of the second and third stages. The

compositions are given in Table I.

TABLE I

Composition of Steels Used in Quenching Dilatometer Experiments

Steel C Si Mn S P Cr V

K 1.07 0.23 0.25 0.014 0.011 - -

T 1.00 0.35 0.37 - - 1.56 0.21

Z 1.11 0.25 0.25 - - 4.11 -

Steels K and T are the identical steels used by Averbach and Cohen(6 )

and the Z steel was included in order to further observe the effect of

chromium.

Previous dimensional work on the K and T steels had included quench-

ing dilatometer runs and precision length measurements performed at

room temperature. Because of the comparative slowness of the reactions

at the low tempering temperatures and the small length changes involved,

precision length measurements were used. The quenching dilatometer was

employed to deteot the dimensional changes due to the isothermal decom-

position of retained austenite to martensite directly after the

hardening quench. Measurements could be made at the holding temperature

very soon after the hardening quench with this technique. The more

accurate precision length measurements required considerable time for

thermal equilibrium to be rea;ched after the hardening quench, and the

- 7-

measurements could only be made at room temperature.

Since larger dimensional changes take place in the second and third

stages of tempering than in the first stage,and since the reactior occur

rapidly at the higher tempering temperatures, the quenching dilatometer

was adopted for the present investigation.

The preceding dilatometric work relating to the isothermal retained

austenite-martensite reaction near room temperature had consisted of

austenitizing the specimen in the dilatometer fixture and then quenching

the assembly from the austenitizing temperature into liquid holding baths

at various lower temperatures. The features of the dilatometer and the

quenching techniques are carefully described in that paper. However,

several modifications in technique and apparatus were required in order

to make suitable isothermal tempering runs at elevated temperatures.

Instead of austenitizing and quenching the specimens to the

tempering temperature while in the dilatometer, the hardening operation

was done separately outside the dilatometer. The hardened specimen con-

taining freshly quenched martensite, retained austenite, and undissolved

carbides was then held at room temperature for one day so that the

initially rapid isothermal transformation of retained austenite into

martensite after quenching would essentially cease. The specimen was

then mounted in the dilatometer and quenched upward into a holding bath

at the desired tempering temperature.

The reactions being studied speeded up enormously at the relatively

high tempering temperatures used. In order to observe the initial

stages of the reactions, a specimen which would rapidly come to thermal

equilibrium with the tempering bath was required. Drastic upward

quenching was necessary, since only isothermal measurements were useful

- 8-

in this tempering study. A thin-walled tubular specimen was accordingly

designed, 3.000 + .001 inches long, 0.277 inch outside diameter and

0.217 inch inside diameter. The lower end of the specimen was securely

centered by the small quartz hemisphere which fitted into the tube, and

the top end butted tightly against the end of the dilatometer quartz

extension.rod.

The same general operating procedure described by Averbach and

(6)Cohen was used, but instead of quenching down from the austenitizing

temperature to the desired tempering temperature, the already hardened

specimen was quenched upward from room temperature into the tempering

bath.

Since it was important to establish when the specimen reached

the quenching-bath temperature, thermocouple wires were spot-welded

to the inside of the tubular specimens and the temperature was recorded

by means of a string galvanometer. Simultaneously the dilation was

recorded by a movie camera running freely during the early rapid part

of the run. Later when the dial slowed down sufficiently, an automatic

timer was introduced to take single-shot exposures during the remainder

of the run. By these methods it was established that the specimen re-

quired less than 20 seconds to come to within 50 F of any of the tempering

temperatures investigated. Since the thermocouple wires were located in

the coolest part of the specimen, (the inner walls of the tube which was

heated from the outside) the entire specimen clearly approached the

tempering temperature within this period.

The machined specimens were austenitized in a vertical resistance-

heated tube furnace under a prepurified nitrogen atmosphere. The

temperature was manually controlled to within + 50 F ( 30 C). After

-9-

austenitizing, the specimens exhibited negligible decarburization.

Tempering during the dilatometer run was done in a small resistance-heated

pot equipped with a Wood's Metal bath for temperatures below 7000 F

(3700 C) and a lead bath for higher temperatures. The temperature was

automatically controlled to within + 30 F ( 20 C).

One series of K steel specimens (1.07 percent carbon) was austeni-

tized at 14500 F (7900 C) for 30 minutes and quenched to room temperature

in a 10 percent brine solution and then run in the dilatometer at tem-

peratures of 450, 500, 600, 700, 800 and 9000 F (230, 260, 315, 370, 425

and 4800 C) after aging one day at room temperature. The dilatometric

results are shown in Figure 1. Another series was austenitized and

quenched under similar conditions but was then immediately refrigerated

in liquid nitrogen (-3210 F) (-1950 C) in order to reduce the retained

austenite content. These specimens were subsequently tempered in the

same way as the first series, and the results are shown in Figure 2.

Similarly two series of T steel specimens were austenitized at

15500 F (8400 C) for one-half hour and oil quenched to room temperature.

One set was then refrigerated in liquid nitrogen, and each series was

tempered at the same temperatures used for the K steel. The dilation

curves are shown in Figures 3 and 4.

The Z steel (1.11 percent carbon - 4.11 percent chromium) was

austenitized at 20000 F (11000 C) in order to dissolve all of the

carbides and oil quenched to room temperature. One series was re-

frigerated, and then dilatometer runs were made on both series at

temperatures ranging from 2000 F to 13000 F (950 C - 7050 C).

The low temperature runs were required to establish the end of the

first stage which had not been determined for this steel. The high

temperature runs were necessary, since the more highly alloyed

- 2000

-3000

0

-4000

10

800 F

LENGTH CHANGES OF K STEEL(1.07 C) DURING TEMPERINGAUST 1450'F QUENCHED TO R.T.

5000 F

70 0 F

WUOO F

600F

I~IIIII~ I 11111111 111111111 IlIllill100 1000

TEMPERING TIME - SEC.10 000

FIG. I

H0

LENGTH CHANGES OF K STEEL(1.07 C) DURING TEMPERINGAUST, 1450 F REFRIG. TO -312 F

450 F

0C. 0 _ _ _ _ _ _ _ _ _ _

600 F

700 F

0

100 1000TEMPERING TIME - SEC.

-2000

-3000

0

-4000

10 000FIG. 2

d

10

H

H

i Mftl 74-

yu r I

LENGTH CHANGES OF T STEELDURING TEMPERING (1,00C-1.56CR)AUST 1550*F QUENCHED TO R.T.

450 F

F 600 F800'F

1000TEMPERING TIME - SEC.

10000FIG. 3

-1000

0x

-2000

10 100

1'

LENGTH CHANGES OF T STEEL(1.00 C - 156 CR) DURING TEMPERINGAUST. 1550F, REFRIG. TO -312 F

__________________________________________________________________________________________ _________________________________________-I _____________________________________________

SooF

450 F

600 F

700 F800F

ii __________________________________________ ___________________________________________ __________________________________

-1000

- 2000

0

-3000

~lII~llIIII|I 1 1111 11111II100 1000

TEMPERING TIME10000

- SEC.

H

10

04

-

FIG. 4

-14-

retained austenite persisted to higher temperatures than did the

austenite in the T and K steels. The dilation results are plotted

in Figures 5 and 6.

In order to obtain these curves, where the only dilation is due to

phase changes, the large thermal expansion of the specimens caused by

quenching upward from room temperature to the tempering temperatures

had to be subtracted from the original dial gage readings which

recorded the combined dilation due to heating to temperature plus phase

transformations.

The thermal expansion of the specimens during the quench upward

was determined by indirect methods, since anisothermal transformation

during heating made direct measurements impossible. The thermal ex-

pansion of martensite was assumed to be very closely the same as that

of ferrite, and an Armco iron specimen was run in the dilatometer

establishing the expansion curve for martensite. Similarly, an 18-8

austenitic specimen was used to determine the expansion curve for

austenite, Figure 7. With a known retained austenite content, the

thermal expansion of any specimen in the dilatometer could then be

obtained by interpolation between these two curves.

After the specimen came to thermal equilibrium at the tempering

temperature, the difference between the actually recorded dilation and

the dilation predicted by the expansion curves was attributed to phase

changes occurring at the tempering temperature. This procedure un-

doubtedly caused a slight error which was greatest at the beginning of

the isothermal portion of the run, since the specimens were not heated

instantaneously, and since anisothermal phase transformations occurred

during the heating operation. However, the major portion of the heating

----- 4

LENGTH CHANGES OF Z STEELDURING TEMPERING (1.11 C- 4.11 CR)AUST 20000 F REFRIG. TO -312OF

300 F 2__

400LLF

500 F

700OF

So0 F

1000 F

1100 F

10 0 1000TEMPERING TIME - SEC.

10 000FIG.5

-1000

-2000

0-300c

-400C

- 500C

10

H

Ii

LENGTH CHANGES OF Z STEEL 5(I.IIC-4.IICR) DURING TEMPERINGAUST. 2000 F QUENCHED TO R. T.

600 F

1000 F

2000

1000

0

1.000

-2000

I

I ~I . _________I_ _ _ _ _ __ _ _ _ _ __ _ _ _____ -1 -. i I I 1 1 1


t -

-3000

10

I IJWJ I

900F-0

0u0L r

10 000

kH

FIG. 6

El

-- -

400

300

00

Z100

H A

AU TENI TE_ _

_ ARTEN'ITE

1000 I200FIG. 7

00 600 80TEMPERATURE F

- 17 -

L

A

THERMAL EXPANSION OFAUSTENITE AND MARTENSITEIN THE QUENCHING DILATOMETER

200 4

- 18 -

was accomplished within a few seconds, and the error was therefore quite

small.

All the specimens were obtained from bar stock in the spheroidized

condition. The effect of fiber direction had been previously con-

sidered and was found to be negligible. Therefore, the length

changes measured here (after correcting for the thermal expansion) are

directly proportional to volume changes in the specimens due to phase

transformations . Consequently, the following approximate equation

was assumed valid in all later calculations:

3 AL = AV (1)L V

- 19 -

III. KINETICS OF THE FORMATION OF CEMENTITE FROM THE TRANSITION

STRUCTURE IN THE THIRD STAGE OF TEMPERING

A. Contraction Curves

In order to study the decomposition of martensite and the simul-

taneous transformation of austenite to bainite individually, the effects

of these two reactions had to be quantitatively separated in the curves

of Figures 1 - 6. A method for accomplishing this has already been

(6)described (

The total changes in length of the specimen may be considered as

the sum of the contractions caused by the martensite decomposition and

the expansion caused by the austenite transformation. For two mixtures

containing different percentages of each constituent the total change

in length may be expressed algebraically:

4: = m1M + ajA

6L2 = m2M + a2A

where m, = volume % of martensite in mixture 1 at time = 0

m2 = volume % of martensite in mixture 2 at time = 0

a, = volume % of austenite in mixture 1 at time = 0

a2 = volume % of austenite in mixture 2 at time = 0

AL, = unit change in length of mixturelup to time = t

AL2 = unit change in length of mixtum:2up to time = t

M = unit change in length caused by partial decomposition in

1 percent of martensite up to time = t

A = unit change in length caused by partial decomposition in

1 percent of austenite up to time = t

When the value of Mi, M 2 , a1 and a2 are accurately known, these two

simultaneous equations can be solved for M and A. The retained austenite

- 20 -

contents were, therefore, obtained by lineal analysis and the

x-ray method of integrated line intensities(9). The volume percent

of carbides in all the steels was obtained by the lineal analysis

method and the retained austenite contents of the hardened K and T

steels by the x-ray method. The retained austenite content of the Z

steel was large enough to be measured by lineal analysis. The results

are shown in Table II.

In applying equation (2), the basic assumption is made that A and

M are independent of the austenite-martensite ratios in the same steels.

TABLE II

Volume Percent of Retained Austenite and Undissolved Carbides

in Hardened K, T, and Z Steels

Steel

K (1.07% C) Austenitized 14500 F,brine quenched to RT

Austenitized 1450 F,refrigerated liquidnitrogen

Volume Percent Volume Percent MatrixUndissolved Retained CgpositionCarbides Austenite _r

3.0 7.8 0.80 None

3.0 3.8 0.80 None

T (1.00% C - Austenitized 15500 F, 8.0 10.0 0.53 0.901.56% Cr) oil quenched to RT

Austenitized 15500 F, 8.0 2.0 0.53 0.90refrigerated liquidnitrogen

(1.11% C -4.11% Cr)

Austenitized 20000 F,oil quenched to RT

Austenitized 20000 F,refrigerated liquidnitrogen

0.0

0.0

75.0

25.0

1.11 4.11

1.11 4.11

-.4

-21-

The effect of stresses caused by refrigeration has already been

investigated(6) and found to have negligible effect upon the martensite

contraction. In the case of retained austenite, the decomposition kinetics

are probably not exactly the same in quenched and refrigerated specimens

of the same steel. Different stress conditions are present, and in-

creasing amounts of tempered martensite have been shown to speed up the

austenite-bainite reaction (8 . Since the austenite contents in the

quenched and refrigerated K and T steels differ by only a few percent,

the austenite decomposition behavior is probably very similar.

The austenite contents in the Z steel specimens differ widely,

however. The decomposition kinetics do not seem greatly different in

the quenched and refrigerated Z steel specimens after reference to

Figures 5 and 6. The expansion peaks due to the austenite decomposition

show about the same time dependence (see especially curves at 5000 and

6000 F (2600 and 3150 C). Also after calculating A and M values for

-this steel by equation (2) the M values appeared consistent. Any

widely varying austenite kinetics would cause non-consistent M calcu-

lations. For these reasons it was concluded that the austenite and

martensite kinetics were sufficiently similar in the refrigerated and

quenched steels to warrant applying equation (2). The resulting A

values may be considered 'average values" for a 45 - 55 martensite-

austenite mixture in steel Z.

Equation (2) was then applied to the quenching dilatometer results

for the three steelsand the calculated values for 100M are shown in

Figures 8, 9 and 10.

The third stage behavior of martensite shown by the K and T steels

fits in well as a continuation of the first stage data of Averbach and

I I V

CONTRACTION (IN K STEEL (I.

TEMPERING A(

500 F

600 "F

90040"

FTT~mi il 7l 111000

TEMPERING TIME-SEC.

4

)F MARTENSITE)7 C) DURINGJST. 145 0*F

450 F

10 000FIG. 8

ii

&

I-2000

-3000

0

-4000

100

II

"004

-1000

- 2000

0

x_ji

-3000

t0

CONTRACTION OF MARTENSITE INT STEEL (1.00 C- 1.56 CR) 0

DURING TEMPERING AUST 550 F

450 F

600 *F

800 'F 70 0 F

---------- .*F

- I i l 110 u00

TEMPERING TIME-SEC.

A

I.

1000100

FIG, 9

-1000

-200C

-300C

-400C

-500C

-600

I 2,00F

)

)

*1

10

300 F

00-F

~ 5&0F-~ + ____________________

700 F

,800 F

900 F

t It 4

III' I 11111 I I I liii Ii

CONTRACTION OF MARTENSITE IN ZSTEEL (IIIC-4.11 CR) DUWZINGT I N AUIT. 2IJIIF

_________TEMPERING,_AUST. 2000 F


10000FIG. 10

-i

N)

-=-

1 ro 0 6*F

-25-

(6)Cohen . The dilation plateau caused by the transition structure is

present at short times at 450 and 5000 F (230 and 2600 C) and disappears

as overaging takes place. The conversion to cementite occurs rapidly

at higher temperatures and is essentially complete at 8000 F (4250 C)

after twenty-four hours.

Dilatometric runs were made at higher temperaturesand the calcu-

lAted values of 100 M gave flat curves which were asymtotic to the

final shrinkage values of the 9000 F (404 C) curves. At temperatures

above 11000 F (5950 C) the thin walled dilatometric specimens flowed

plastically due to the extension rod pressure making further transfor-

mation measurements impossible.

In order to verify the final martensite contractions obtained by

the above procedure, the total contraction which could be expected was

checked by an independent method. The amount of carbon in solution was

calculated by subtracting the carbon combined in the undissolved car-

(3)bides from the total carbon. Previous work had established that a

total contraction of 6000 microinches per inch could be expected for

complete decomposition of a 1 percent carbon martensite at room tempera-

tures. It was therefore easy to calculate the total possible contraction

of the martensite in the three steels. Since the coefficients of

thermal expansion are essentially the same for martensite and its

decomposition products, these contraction values are applicable for any

temperature. The total contractions obtained this way agreed closely

with the dilatometric results and affirmed the accuracy of the dilatometric

technique.

- 26 -

The initial slopes of the 450 and 500* F (230 and 2600 C) curves

indicate that the 5000 F plateau would rise above the 4500 F level if

extrapolated to shorter times. The shrinkage compared to the as-hardened

state iszof course,a direct measure of the amount of carbon rejected by

the tempered martensite. This behavior indicates that the transition

precipitate may be in pseudo-equilibrium with slightly increasing

percentages of carbon in the martensite lattice as the temperature rises,

up to the temperature at which the cementite conversion occurs before

(6)the time of the.earliest observations . The effect of the chromium

in the T steel is to about halve the shrinkage involved in the first

stage. This agrees with the previous work and indicates that chromium

in solid solution in the martensite increases the solubility of carbon

in the presence of the transition precipitate.

The increase in solubility of carbon due to the chromium is more

strikingly shown in the martensite-decomposition curves for the Z steel

(1.11 percent carbon - 4.11 percent chromium) in Figure 10. In this

case all the 4.11 percent chromium is initially in solution in the

martensite and is more than four times the amount present in the T

steel martensite. Evidently, very early in the tempering process and

at a low temperature, the martensite is able to reject the small amount

of carbon in excess of the amount in *.seudo-equilibrium with the

transition precipitate. At 2000 F ('0O C) the transition precipitate

is already overagedand further contraction takes place at this

temperature. This behavior is expected, since less than 1 percent of

chromium in the martensite had previously decreased the contraction

involved in the plain carbon steel K first stage by about one half.

The length changes involved in approaching the plateau at the end of

- 27 -

the first stage are too small to be measured with the quenching dila-

tometer,and, therefore, no attempt was made to investigate temperatures

below 2000 F.

B. Analysis of the Kinetics of the Third Stage

It was first assumed that the third stage reaction proceeded by a

process of nucleation and growth. The decomposition curves should then

follow the general equation (3) developed by Mehl and Johnson (28) and

Avrami(21).

v= k (a -y) ta-(3)dt()

where a = total extent of reaction possible

y = extent of reaction at time = t

k = a temperature dependent constant

when k j f (t) equation (3) may be integrated tom+l1

y a(l - e -kt (4

k includes the rates of nucleation and growth which are often time

dependent themselves. However, it is possible for the two terms to

compensate partially for each other in the expression producing an over-

all k value which is relatively insensitive to time.

Various powers of t have been used to fit special cases (22) The

factors affecting the experimental power of t in equation (4) are the

time dependence of the rate of nucleation and growth and assumptions

regarding the shape and distribution of the particles being formed.

Mehl and Johnson(28) obtained a value of m = 3,when they assumed that

the rates of nucleation and growth were constant, and that spherical

new-born particles were formed at random in the matrix. In these

equations k is temperature-dependent according to equation (5).

. Q

k = Ae RT (5)

where Q = a constant which in certain cases may be interpreted

as an activation energy for a simple mechanism.

T.= temperature (OK)

Since the martensite shrinkage is proportional to the volume

change and therefore to the extent of reaction, the shrinkage was used

in these equations as a measure of y. The total extent of reaction

possible in the third stage (a) was obtained by extrapolating the

Averbach and Cohen(6) values of the shrinkage at the end of the first

stage to higher temperatures and then taking the difference between

these values and the final shrinkage values of the third stage.

Equation (4) may be put into logarithmic form:

log log (a )I~ l ka - y (m + 1) logt+log 2 3 (6)

The data for the T steel were then plotted on coordinates of log

log (a y) vs. log t and a series of straight lines was obtained at

different temperatures with slopes = (m + 1) and intercepts of log 2

at log t = 0 (Figure 11). The average value of m was -0.7$.

Similarly, for the K steel a value of m =-0.74 was obtained. The

negative values of m when put into equation (3) indicate that the reaction

rate is extremely rapid near t = 0. By inspection of equation (3) it

may be seen that the reaction rate ( d) is 0 at the beginning if m 0.

However, if U-< 0 the reaction rate is extremely rapid near t = 0.

The initially rapid reaction is more evident when the dilation curves

for the K and T steels are replotted on linear coordinates (Figure 12).

Since the data follow the general form of equation (3) but do not

show the incubation period characteristic of ordinary nucleation and

- 29 -

-I- ___________ ______________ ______________________

0

>2-.p

9.,

500 F/

A50 F

LOG LOG( .- ) VS LOG TIME FOR

MARTENSITE DECOMPOSITIONIN T STEEL (1.00 C - 1,56 CR)

2.0 3.0 4.01.0

LOG TIME - SEC.

0.21

0.4

-0.6

-0.8

0-%

0

0-J

-1.0

-1.2

-1.4

-1.6

-1.8

FIG. I I

- 30 -

ASY MPTo~: CONTRACTION OF MART -To ZERO -- -- _- ENSITE IN K AND T STEELS

DURING TEMPERING AT 450 *F

--STE E-.(I.CO0C -- 1.56> C R

AS MP'roT ICTO ZERO

- - - --) - - -

'~~~~1 _______ _______ _______ - t - -.-.t-=-- - -I-I- -_______ 1. _______ _______

100

K S

200 300TEMPERING

TEE ( 1.97 Cl)

400TIME - SEC

500

F I G. 12

-900

-100C

- 110

-200C

-21 C

09

-2200

-2300

0

--- -- -t1-

600

- 31 -

growth reactions, there are two alternatives for regarding the reaction

mechanism. Either the reaction proceeds by a nucleation and growth

mechanism which is greatly speeded at the beginning by some additional

factor, or an entirely different mechanism having a reaction rate

equation similar to (3) is present. The dditional factor causing the

high rate of nucleation and growth at short reaction times would have

to follow the general form of equation (5).

(20)Stress relief data are empirically found to follow equation (3)

quite well. The formation of the transition precipitate causes an early

(6)increase in hardness , and the literature indicates that the structure

is probably in somewhat localized registry with the matrix. According

to a recent investigation(35), the first stage results in the formation

of precipitated volumes continuous with the parent lattice rather than

of discrete precipitate particles. These volumes later transform to

discrete particles which have non-equilibrium structure and composition,

and gradually convert to the more stable cementite. Direct precipi-

tation of ortiorhombic cementite is apparently relatively difficult

in the tempered marten-ite lattice. The transition structure forms

first and then concerts to the final cementite at a later time. The

lattice registry characteristic of the transition precipitate produces

localized stresses which may be responsible for the high initial reaction

rate of the third stage.

Zener (23) and West(24) have given equations similar in form to

(3) for thk rate of growth of variously shaped particles in a solid

solution when the growth is only limited by diffusion. They assume that

nuclei are already present, and so nucleation is not required for

reaction. However, the values of m predicted for this case range from

.5 - 1.5 depending upon the shape of particle precipitated. These

- 32 -

positive values of m indicate an initially slow reaction rate which

does not agree with the kinetics observed here.

Averbach and Cohen(6) found this same behavior during the first

stage decomposition of martensite and attributed the high initial

reaction rate to the effect of the high stresses present in quenched

martensite. By suitably modifying a first order reaction equation with

a stress factor empirically derived from stress relief behavior(20)

they obtained an equation similar to equation (3) and were able to account

for the observed kinetics. Their empirical equation closely fits the

third stage reaction data presented here.

However, the mechanism and chemistry of the transition process

are not precisely known,and so further conjecture is not warranted here.

The temperature-dependent reaction constant k was evaluated from

Figure 11 from the intercept at log t = 0. By plotting log k versus

according to equation (5) a Qk value of i4,j00 cal/mol was

calculated (Figure 13). and for the K steel a value of 11,000 cal/mol

was similarly obtained. Since the third stage depends upon the formation

of cementite by withdrawal of C from the matrix, the reaction rate

constant k might depend upon the rate of diffusion of carbon in tempered

martensite. Consequently the "activation energy" of the constant k was

compared to the activation energy for the diffusion of carbon in low-

carbon ferrite as follows.

Since only the heats of activation of quantities having the same

dimensions in time may be directly compared, a correction factor had

(25) -e(m + 1)to be applied to Qk . k has the dimensions of time and

the diffusion coefficient of carbon in iron has the dimensions time

If a new constant T having the dimensions of time is introduced by

mo

- 33 -6

LOG K VS + FOR T STEEL (1.00 C-

1.56 CR) MARTENSI TE DECOMP-OSITION DURING TEMPERING

2

4

6

8

0

2

4

0\

1.6 1.8x K

T ( K)

2.0

FIG. 13

-0.

-0.

-0.

-0.

'50-J

- 1.

- 1.

-I.

-1.

- 34 -

the relation

k= 1 (7)1(m +

- will have the dimension of , and the activation energy of

(Q ) should be comparable to the activation energy for the diffusionQk

of carbon in iron. Dimensional analysis shows that = ncer M + 1 Si

(m + 1) is of the order of 0.25, Q- is much larger than the value of

18,100 cal/mol for the activation energy of carbon in iron given by

(47)Stanley . Therefore, it appears that the diffusion rate of carbon

is not the controlling factor in the kinetics of the third stage of

tempering.

More detailed and additional calculations concerning the third stage

kinetics are given in Appendix A.

- 35 -

IV. KINETICS OF THE DECOMPOSITION OF RETAINED AUSTENITE

IN THE SECOND STAGE OF TEMPERING

A. Epansion Curves

The expansion curves for retained austenite decomposition were

obtained by applying equation (2) to the quenching dilatometer results

and solving for values of 100 A. The results are plotted in Figures

14, 15 and 16.

1. Results for K and T Steels. The relative height of these

curves is somewhat in doubt, since the dilatometric errors during

heating are significant compared to the effect produced by the small

amounts of retained austenite in the specimens. A possible error of

+ 2000 microinches/in is estimated in the level of these curves. How-

ever, the dilatometric errors were negligible during the isothermal part

of the run, and hence the shape of the curves is considered to be quite

accurate. As the temperature increases, the expansion measured at tem-

perature produced by the reaction decreases due to the difference in the

thermal coefficient of the austenite and its reaction products (Figure 7).

The sets of curves for the K and T steels are quite similar. At

7000 F (3700 C) the retained austenite has already completely transformed

before the time of earliest measurement. Comparison of these data with

the rate of bainite formation from primary austenite after hot quenching

shows that the ferrite-nucleated austenite-to-bainite transformation is

speeded by the presence of the tempered martensite.

The slight contraction exhibited by the bainite after complete

transformation of the austenite indicates that the initially formed

LENGTH CHANGES OF K STEEL AU TENITE(1.07C) DURING TEMPERINGAUST. 1450'F

700 F

800F

/500 F /450 F

0450


10000

8000

2 600Cx

400C

200

10 0UUUUFIG. 14

Aj" : JI H 1 -6. hil"

ONI

100

LENGTH CHANGES OF AUSTENITEIN T STEEL (1.00 C- 1.56 CR)DURING TEMPERINGAUST. 1550OF

700 F800OF

0600F __ __ _ _I 7-I%-'

0 0

I-i

100 1000TEMPERING TIME -

10000SEC.

10000

8000

>0000

~j~i40001-

2000

10

-~'1

FIG. 15

II

I19

6100 a F-

L 001

Al

LENGTH CHANGES OF AUSTENITEIN Z STEEL(.IIC-4.1ICR) DURINGTEMPERING AUST, 2000F j

1 .5.

LU I I

U I

600 F7

a -

-

-

- U I -

0500 F

01 100 F

120 %FI200'F1300 F

** 7000F

y0900 F

800OF

1111111II~I~II1111111I~II!I100

TEMPERING1000

TIME - SEC.

o~x

FIG. 16

II

3000

2000

I00

0 0

x

-1000

10

-4 -000 wmmwmmm

'4 ftft

- 39 -

bainite is not stable and transforms to a more stable form slowly at

temperature with an accompanying shrinkage. This phenomenon is most

noticeable at the lower tempering temperatures.

. Most of the previous theories of bainite formation have depended

upon the transformation of austenite to supersaturated ferrite followed

(18, 26,27)by the precipitation of cementite ( 2 The precipitation of

cementite occurs as a function of temperature and time. Magnetic

(4)data show no evidence of cementite even after heating well into the

second stage, indicating that no cementite is formed when retained

austenite first decomposes. The contractions observed here could,

therefore, be a result of the progressive precipitation of the cementite

from a transition structure in the initially formed bainite.

2. Results for Z Steel (L4.11 percent chromium - 1.11 percent carbon)

The transformation of retained austenite in the Z steel is more complex,

and metallographic work was required to fully explain the dimensional

behavior of the retained austenite in Figure 16. Metallographic speci-

mens were tempered for suitable temperatures and times to reveal the

structural changes responsible for the expansion and contractions.

The expansions at 5000 F (2600 C) and 6000 F (315* C) are due to

the expected austenite-bginite reaction (Figure 17). The first exparsLon

at 7000 F (3700 C) is due to partial bainite formation, and the follow-

ing contraction results from the precipitation of cementite directly from

the austenite in WidmanstAtten form (Figure 18). The structure at the

end of the 7000 F temper (Figure 18) shows clearly the nucleating effect

of the tempered martensite upon the bainite. Carbide precipitation

precedes bainite formation at 8000 F (4250 C) (Figure 19). At 9000 F

(4800 C) cementite appears first causing the initial contraction, but a

- 40 -

FIGURE 17. Structure of Z steel (1.11% CJ411% Cr) after

tempering 28 hrs. at 5000 F (2 C). Tempered

martensite, bainite, and austenite present.

Etched with 2% nital (1500x).

FIGURE 18. Structure of Z steel (1.11% C, 4.11% Cr) aftertempering 28 hrs. at 7000 F (3700 C). Temperedmartensite, bainite, cementite and austenitepresent. Etched with 2% nital (1500x).

-41-

cementite-nucleated product forms at longer times resulting in an

expansion (Figure 20). At higher temperatures carbide precipitation

followed by pearlite formation occurs until at 1200 and 13000 F (650

and 7050 C) the carbide precipitation is suppressed and pearlite forms

directly. Figure 21 shows the pearlite nodules after 40 seconds at

12000 F.

These transformation data were conveniently summarized in a TTT

diagram (Figure 22) and compared to the transformation characteristics

of primary austenite in a /4 percent chromium - 1 percent carbon steel49)

during hot quenching (dotted lines). The transformation data for re-

tained austenite and primary austenite exhibit the same characteristics.

The transformation products appear in similar order, and the tempera-

ture dependence is the same. However, the beginning of the retained

austenite transformation to bainite(26 ,49) is considerably speeded up

by the presence of martensite.

B. Analysis of Kinetics of Austenite Decomposition

The retained austenite to bainite reaction seems to occur by a

process of nucleation and growth, and so the data might be expected to

follow the form of the general equation (6). The data for the K and T

steels were plotted on log log a vs. log t coordinatesand a seriesa - y

of straight lines were obtained at the different temperatures (Figure 23).

In this case yis the expansion observed at time t,and a is the total

expansion possible for the complete reaction. The kinetic behavior of

the austenite in both steels is essentially the same as shown by the

coincidence of the curves at the three temperatures. The slope of the

curves (m + 1) is equal to 1.6.

- 42 -

FIGURE 1%.

FIGURE 20.

preset. ttchsd iih9I and cementite

nital (1500x).

- 43 -

p

r

- 44 -

I% Carbon - 4 % Chromium Stee

Retained AusteniteTransformationury MU-

Primary AustTransformati

1100 [- Start of EPearlite Pearli

Reaction

Start ofCarbidePrecipitation

S t art o f \BBoirnite --Reac tion

N-4%

N

ofReaction

100

/II

II

N

10Time4Hour

FIGURE 22. TTT diagrom for retained, aVtAite inl Z, $teal (1.11% C, 4.11% Cr)cpaAO# t* primary qptenite decomposition siaiilar onep reent carbon - four percent chromium steel

//I

r/

/

1200 I

1300

eniTeion

nd ofte Reaction

'4-

Endiinite

1000

900LL0

0OC

E800H

700[-

600t

500 -

4000.01 0.1

I

0

-1.0

2.0 3.0LOG TIME - SEC.

4.0

FIG. 23

0

@

600* F

500* F

450*F o T STEEL

@ K STEEL

LOG LOG(e) VS. LOG TIME FOR RETAINEDAUSTENITE DECOMPOSITION TO BAINITEIN K AND T STEELS

11

-2.0(~ I(VU-0-j

U0-j

-3.0

-4.01.0

r- 46 -

The temperature dependent constant k (equation 5) was obtained

from the curves in Figure 23, and log k was plotted vs. T (OK)(Figure 24) to obtain the"activation energy" Qk. A value of

Qk= 58,000 cal/mol was found for the austenite reaction in both

steels from the slope of this curve. This Q value depends on a quantity

having the dimensions time-l 6. If Q is the "activation energy" for a

quantity having the dimensions time 1, then dimensional analysis shows

that Q 580 = 36,000 cal/mol.

The"activation energy" for the quantity ( ) having the dimensions-1

t was next obtained from the experimental data. For temperature

dependent reactions such as the austenite decomposition equation (8)

usually appl&fe,

= Ae RT (8)t

where t is the time required for any given fraction of austenite to

transform at the various reaction temperatures. By plotting values of

log t vs. a value of Q = 38,000 cal/mol was obtained for the reaction1PT 6 3800ca/o

in both steels. A good check is thus obtained for both methods. The

value of 38,000 cal/mol as a heat of activation for the austenite-

bainite reaction checks well with the identical value obtained

previously by Averbach and Cohen (6)

I -

LOG K VS. 4FOR RETAINED

AUSTENITE DECOMPOSITIONIN K AND T STEELS

1 4 t

1.9

N

2.0FIG. 24

il

-2.0

-3.01

-4.0

U0-J

-5.0

-6.0

1.7 1.8- X10-3T(K)

I I

44,4,

- 48-

V. CHEMICAL AND STRUCTURAL CHANGES DURING THE

DECOMPOSITION OF MARTENSITE

A. Introduction

The extremely fine dispersion of the transition structure and

the first formed cementite make direct observation of the third stage

difficult. A unification of evidence in the literature and the kinetic

behavior (discussed in preceding section) indicate the following probable

decomposition mechanisms

1. The first stage ends with this precipitation of small volumes

in some sort of registry with the matrix. This process corresponds to

reaching the plateau on the dilation curves.

2. These volumes are the so-called transition precipitate and

exist with the matrix in pseudo-equilibrium,until at high temperatures

and long times they form discrete platelets. The dilation plateau

then shows signs of overaging by dropping off.

3. The discrete platelets gradually convert to cementite by an

undetermined prouess of structural and compositional changes. Carbon

is simultaneously withdrawn from the matrix. These discrete platelets

will be called "early cementite" for convenience, because their

structure and composition differ from the final stoichiometric cementite.

B. Hardness Results

Previous hardness data(6) have shown an early increase before

reaching the pl:teau and a gradual decrease during the period of pseudo-

equilibrium with the transition precipitate. Further hardness readings

were taken during the third stage and are given in Table III. The

-49 -

hardness continuously decreases as tempering proceeds. Softening is

due to stress relief, loss of carbon from the matrix and spheroidization

of the cementite. In order to compare directly the hardness of the K

and -T steels after identical tempering treatments, the hardnesses after

ten hours at the different tempering temperatures are plotted in

Figure 25. Chromium is found to retard softening at the higher tempering

temperatures. The effect of chromium on hardness will be discussed

further in Section VI-B-3.

C. Previous X-ray Work on the Transition Phase

Previous x-ray work indicates that the transition phase is probably

hexagonal close packed. The structures investigated were tempered at

temperatures and times near the end of the dilation plateau. Arbusov

and Kurdjumov(10) by using a single crystal technique obtained five

diffuse diffraction lines attributed to the transition structure.

Heidenreich, Sturkey and Woods by electron diffraction work on

martensitic steel tempered at 3920 F (2000 C) obtained seven lines which

they erroneously identified with a standard hexagonal close packed

Fe3N pattern. Attempts to isolate the transition structure by electro-

chemical methods in a 10 percent HCl cell were made by Crafts and

Lamont(29). They obtained residues which gave weak diffuse lines, and

three typical d-values (interplanar spacing) are given.

While the x-ray patterns obtained by the above means are too

incomplete to be individually analyzed, the d-values may be compared to

other possible standard patterns. The two other carbides existing in

the iron-carbon system are the hexagonal close packed FepC carbide and

the more complex Fe2 0C9 carbide which is probably hexagonal or

orthorhombic(32,3). Hofer, Cohn, and Peebles have recently checked

70

____________ ~ - ~ F ~ - , -~

601-

50

40

30

1 __ __ 11 __ __ __ __ __ __ __

_ _ _ _ _ NtiN2 I I IN

I--- -4-.- -

o YSTEEL(0.6 7C- .00 R)T STEEL

K ST E.EL\6

ROCKWELL C HARDNESS OFK, T, AND Y STEELS AFTER10 HOURS AT VARIOUS TEMP-ERING TEMPERATURES

200 400 600 8000TEMPERATURE - F

(I.)Lf)Liz0a:

U-I

Id

U0a:

0-

1400FIG. 25

-1

1000 1200

- 51 -

TABLE III

Rockwell C Hardness of K and T Steels After Tempering

in the Third Stage

K Steel (407% C)liquid nitrogen.

TemperingTemperature OF

400

500

600

700

800

900

1000

1100

austenitized

10 min.

63.0

61.5

57.0

54.5

51.0

45.0

39.0

36.0

14500 F for 1/2 hour - refrigerated in

Tempering Time1/2 hour 1 hour

62.5

60.5

56.0

53.5

50.0

43.0

37.0

32.0

62.0

60.0

55.0

53.0

49.0

41.0

35.5

30.0

T Steel (1.00% C - 1.52% Cr) austenitized 15500 F for 1/2 hour -refrigerated in liquid nitrogen.

400

500

600

700

800

900

1000

1100

63.5

61.5

58.5

56.5

54.0

50.0

44.5

63.0

61.0

58.0

56.0

53.0

49.0

43.5

62.5

60.5

57.5

55.5

52.0

42.5

42.5

61.5

59.5

56.0

54.0

50.0

44.5

38.0

140.0 31.5

10 hours

62.0

59.0

53.0

50.0

46.0

36.5

30.5

23.0

43.0 41.5

- 52 -

the identity of these carbides by chemical and x-ray methods and have

determined their Curie temperatures and magnetic moments. They measured

Curie temperatures of 4770 F (2470 C) and 7160 F (380P C) for the

Fe20 9 and Fe2C carbide respectively and found the saturation magnetic

moments of these carbides to be about the same as that of cementite.

All these iron carbide investigations were made using synthetically

produced powders.

For purposes of direct comparison, the d-values obtained by

various investigators for the Fe2009 and Fe2C carbides are given in

Table IV along with the d-values reported for the unknown transition

structure in martensite. The correlation between the d-values in

column II for the transition structure in tempered martensite and

known carbides is not perfect, but seems quite real. Three of the

Arbusov and Kurdjumov lines check well with the hexagonal close

packed carbide pattern, while all of the Heidenreich, Sturkey and

Woods values agree. The transition structure is, therefore, probably

hexagonal close-packed. The composition is not necessarily Fe2C,

however, since similar sets of d-values have been obtained for hexagonal

Fe 3N and the carbonitrides of Jack(32). Large variations in N and

C in these structures are possible without radically changing the

pattern of d-values.

D. Magnetic Changes during the Third Stage

Since the compositional changes involved in the third stage could

not be followed by means of x-ray data alone, a combined program con-

sisting of magnetic, electro-chemical and x-ray work was followed.

The plain carbon K steel was used, since the simplest possible

system was desired. The specimens were austenitized in the vertical

TABLE IV

D-Values and Relative Intensities of X-ray Lines Obtained fromIron Carbides and the Transition Phase in Tempered Martensite

Transition Phase in

Hexagonal or Orthorhombic Fe ,C Carbide

JackPercarbide

Hofer,C hn &Peebles c34)Carbide

H.C.P. Fe2C Carbide Tempered Martensite

Hofer,C Oan & Jack k' Tutiya( HeidenreichPeebles 34) Carbo- Carbide Sturk y &Hex. Carbide nitride Woods RA)

Arbusow &Kur d-mowMT

Extracted ExtractedResidue of Residue ofCrafts and this Inves-Lamont(29 )tigation

vw

vvwmmmmmsmwmwmwvsvsmw

msmwmmsvwm

2.642.492.412.272.202.18

2.102.072.042.012.00

2.622.482.392.262.18

vwwmwmmw

mws

ww

3.312.922.862.642.492.412.282.202.182.112.102.072.042.022.001.991.981.921.901.821.811.781.771.731.681.661.641.621.581.531.511.501.441.431.401.381.341.341.321.281.271.25

ma 1.81vwmmwmwwvwmw

8vw

wvwvwvwwmmw

1,761.721.68

1.631.57

1.511.47

1.411.381.33

mw 1.32wwms

1.271.251.211.201.191.161.151.121.101.08

2.062.03

3.13 V

2.63vwwwmm

s

2.38

2.16

2.08

w 2.35

m 2.18

vs 2.07

2.32

2.20

2.07

2.18

2.09

8

vs 2.06 m

2.37 w 2.392.282.21

2.07 m 2.082.03

1.97 V

1.981.91

wm

1.80 aDI

V

V

V

1.761.721.67

1.621.57

1,50

vwm

ww

www

w

wwswwwm

aV

V

1.371.34

IvIvIV

vw

s

Iv

1.88 m

1.82

1.71

1.60 DI 1.60 1.61 a

1.49

1.37mvv

vw

1.36 1.38 1.38 a 1.39 vw

1.32 vw

1.271.251.21

wwm

1.24m

1.17 w1.161.141.131.111.09

wwmVrn

1.16 m

1.24

1.18

1.151.14

1.09

1.251,21

1.17

1.23 s

1.16 a

1.22 vw 1.22 s

1.14 vw

HfggCarbide

1.97 w1.91 mw

mmms

ss

-54 -

tube furnace at 20000 F (10900 C) in order to dissolve the carbides and

put all the carbon in solution. Refrigeration in liquid nitrogen was

employed directly after quenching to room temperature to eliminate most

of the retained austenite. A series of four-inch long by 1/4-inch

diameter specimens was then tempered at 4500 F (230* C) for three hours

and eleven hours, and at 700, 900 and 11000 F (370, 480 and 5900 C)

for 15 minutes, 100 minutes, and 11 hours in metal baths controlled

to within + 5* F ( 30 C). Each tempered specimen was run in a field

of /400 gauss in a ballistic-type magnetometer equipped with a furnace,

in order to obtain curves of comparative magnetization versus temperature.

The curves are shown in Figure 26. Absolute values of the

intensity of magnetization were not calculated, since all the specimens

were run under identical conditions, and the galvanometer deflection

serves as an accurate measure of the comparative magnetization. A

heating rate of 50 F/min. was used. The specimens tempered at 700,

900, and 11000 F were run in the temperature range 200 - 5000 F (93 -

2600 C).. The specimens tempered at 4500 F were only heated up to

4500 F in the magnetometer, since simultaneous phase changes were

not desired. In all cases the magnetization curve on cooling was

obtained to make sure that no phase changje had occurred during the

run and that the magnetic changes were reversible. Pure Fe3 C loses

its magnetism in this temperature ranc:e and passes through its Curie

temperature at about 4000 F (2040 C). The magnetic inversion of the

carbides is shown by the sudden drop in magnetization of the specimen

as the temperature increases. The specimens tempered at 11000 F (5900 C)

show the typical rapid decrease in magnetization near 4000 F (204* C),.

due to the presence of well formed stoichiometric cementite in the

matrix. The 9000 F (4800 C) specimens exhibit a higher initial

40TEMPERATURE -OF

MAGNETOMETER DEFLECTIONVS. TEMPERATURE FOR KSTEEL (1.07 C) SPECIMENSAFTER TEMPERING

IIHR.T 450 F

0

5tIN. AT 700 F

700 F

5 dN.AT 900 F0

I I HR.AT 900 F,II HR.AT I I 0F

500FIG. 26

I I V

z

H

z0IrULi

LiaLi

0Liz

200 300

3 HR. ATje.50 F

%il

- 56 -

magnetic moment and a higher Curie temperature. This trend is continued

at 700 and 450* F (370 and 2300 C). Increased tempering time has the

same effect as raising the temperature. For the specimen tempered at

4500 F for three hours, the decrease in magnetization with temperature

is only slightly greater than the decrease due to the normal tempera-

ture coefficient of ferromagnetic materials.

This behavior is explained by the chemical changes going on within

the carbide phase as tempering proceeds. The early cementite formed

from the transition precipitate is very rich in iron, and recent x-ray

work has indicated that the early cementite is also not perfectly

orthorhombic ( This iron-rich phase, therefore, has a higher

magnetic moment and Curie temperature than stoichiometric cementite.

As tempering proceeds, the magnetization progressively decreases,and the

Curie point is lowered as the carbon content approaches that of

cementite.

E. Electrochemical and X-ray Results

In order to follow the compositional and structural changes more

exactly, attempts were made to isolate the carbide phases for purposes

of chemical and x-ray analysis.

Four specimens tempered at 450, 700, 900 and 11000 F (230, 370,

480 and 5900 C) for eleven hours were used. The carbide phases were

extracted by means of a sodium citrate - copper sulfate electrolytic

cell according to a technique described later. However, it was found

impossible to obtain accurate iron and carbon percentages in the carbide

residue by chemical analysis, since the dissolved carbon from the

matrix was unavoidably present in uncombined form in the residues.

Since these residues were of chief interest, the compositional changes

in the early cementite could not be studied by this technique.

- 57 -

Powder patterns were next made of the extracted iesidues. A

standard 57.3 mm. diameter Debye-Scherrer camera was employed. Ex-

posures were made using chromium radiation filtered with vanadium

carbide.

The carbide residue extracted from the specimen tempered at 4500 F

(2300 C) for eleven hours gave the first pattern (a) shown in Figure 27.

The lines are weak and diffuse, and the calculated d-values are given in

Table IV. The d-values show some correlation with the values for the

Fe2C and Fe 20C carbides. However, the correlation is not uniform

enough to warrant identifying the carbide residue with the transition

structure. The inherent features of the extraction process may

account for this lack of correlation. The residue may not be truly

representative of the transition phase, since lattice registry is

lost during the extraction. Also, oxidation and surface conditions

on the very fine particles could result in an un-representative x-ray

pattern. The d-values of Crafts and Lamont, obtained from an electro-

lytically extracted residue, show a similar lack of correlation. As

the carbide phase becomes discrete and attains a larger particle size

the above disadvantages are largely eliminated.

The residue from the specimen tempered at 7000 F (3700 C) for

eleven hours gave an extremely faint diffuse pattern which could not

be analyzed. Electron microscope work(29,35) has indicated that many

of the discrete platelets found even after the decomposition of the

transition precipitate are approximately the same size as the cementite

particles found much later at high temperatures. This indicates that

the conversion of the early cementite to the final state does not occur

by a simple growth mechanism, and that complex structural changes occur.I

- 58 -

(a) Tempered 11

J

(b)

(C)

FIGURE 27. ,ed

- 59 -

The small particle size and inhomogeneity of the particles account for

the poor powder pattern obtained. The residues tempered at 900 and

11000 F (480 and 5900 C) for eleven hours gave the cementite patterns

shown in Figure 27 (b and c). Even though the magnetic analysis indi-

cated that chemical changes occur in the cementite between 700 and 11000 F

370 and 5900 C), corresponding structural changes were too subtle to be

picked up by the x-ray powder technique.

Since iron rich-transition cementite forms from the transition

precipitate of the first stage, the iron content of the hexagonal close-

packed transition precipitate is undoubtedly higher than the iron con-

tent of cementite. This observation is also supported by the high

magnetization values of the specimens tempered at 4500 F (2300 C)

(Figure 26).

The tempering process in martensite may then be considered as the

formation of a series of carbon-rich phases which become increasingly

rich in carbon at the expense of the matrix. The direct precipitation

of orthorhombic Fe3C is relatively difficult in the tempered martensite

lattice, and so the process is preceded by the initial formation of the

hexagonal close-packed transition precipitate. An analogous hexagonal

transition phase has been observed to form in a-iron containing dis-

solved nitrogen (36)While the transition precipitate seems to give

an x-ray pattern similar to hexagonal close-packed Fe2C, the former

phase contains much more iron than indicated by the formula. This is

possible ?since the hexagonal close-packed structures in the iron-carbon

and iron-nitrogen systems can undergo wide variations in carbon and

nitrogen content without much change in their x-ray patterns.

A. Introducti

A 4.00 per

purposes of stud

bide formation.

C

0.67

VI. THE FOURTH STAGE OF TEMPERING

- 60 -

o)n

cent chromium - 0.67 percent carbon steel was chosen for

lying the fourth stage of tempering, involving alloy car-

The composition is given in Table V.

TABLE V

Composition of Y Steel

Cr

4.00

Si

0.28

Mn

0.31

S,

0.009

0P

0.014

Previous work(13,14,15) indicated that at this composition of chromium

and carbon the carbide (CrFe) 7C3 is the only carbide phase present

after tempering at elevated temperatures near the critical.

In order to observe both the temperature and time dependence of

the chromium carbide formation, a series of specimens was hardened and

then tempered at 1000, 1100, 1200, 1300 and 14000 F (540, 590, 650, 705

and 7600 C) for periods of time ranging from 15 minutes to 28 hours.

These tempering treatments completely transformed the retained austenite

in all the specimens except the one tempered at 14000 F for 15 minutes.

This specimen was, therefore, tempered at 12000 F near the nose of the

pearlite c-curve for 15 minutes prior to the 15 minutes temper at 14000 F.

The specimens were austenitized in the vertical tube furnace at

20000 F (10900 C) for 1/2 hour and then oil quenched to room temperature

and refrigerated in liquid nitrogen. This treatment dissolved all the

carbides and retained 6.0 percent austenite as determined by the inte-

grated intensity x-ray method.

Tempering

B. Quantitati

- 61 -

was carried out in lead baths controlled to + 50 F ( 30 C).

ve Measurements During Alloy Carbide Formation

1. Metallographic Results. Metallographic specimens were prepared

from the tempered specimensand efforts were made to find a differential

etch which would distinguish only the (Cr,Fe)7C3 carbide and leave any

coexisting cementite and the matrix unaffected. After considerable

experimentation, the boiling KMnO4 - NaOH etch recommended by Groesbeck038 )

was found most satisfactory. By boiling one minute in a solution of

4 grams potassium permanganate, 1 gram sodium hydroxide and 100

millilters of water, the chromium carbide was darkened leaving cementite

unattacked. After 5 minutes in the boiling solution, the cementite

and matrix were also darkened. Photomicrographs of the tempered series

etched only for the chromium carbide were prepared (Figure 28). The

increasing amount of chromium carbide with increasing temperature and

time is evident. A more quanitative measure of the amount of chromium

carbide was obtained by the method of lineal analysis (19). However,

instead of using the Hurlbut counter as in the Howard and Cohen(l9)

method the photomicrographs were enlarged 3X and marked with a series of

parallel lines. The length of these lines intersected by the carbide

was then equal to the length of these lines intersected by the carbides

divided by the total line length. The results are shown in Figure 29.

Since the distance covered on the specimen by this method was

only 0.7 mm, high accuracy was not expected. Longer counts would be

necessary to obtain precise statistical averages and eliminate any segre-

gation effects. However, the scatter of points is not excessive, and the

values are considered quite indicative of the metallographic structure.

.w 62.

* -0

its

VOLUME PERCENT BY LINEALANALYSIS OF CHROME CARBIDE IN YSTEEL (0.67 C- 4.00 CR) AFTERFOURTH STAGE TEMPERING

0

1000 F

O ||00

x 120 O OF

030

1400wF

0

1000 10000 100 000TEMPERING TIME - SEC. FIG. 29

0

2

-

z8i

Ii

1

i

- 64 -

The metallographic results show that very little (Cr,Fe)7C3 forms

at 10000 F (5400 C) within 28 hours. At the higher temperatures tnis

process occurs with increasing rapidity.

At 13000 F (7050 C) after 28 hours the cementite is completely

converted to the chromium carbide. At 14000 F (7600 C) the process is

completed much earlier, and considerable spheroidization occurs within

100 hours. The complete conversion of carbide was checked by etching the

same areas with picral and photographing the structures. These photo-

micrographs were then compared to the earlier ones etched only for the

chromium carbide. If the reaction were complete, no extra cementite

particles appeared due to the picral etch, and the structures appeared

the same.

The structures tempered at 1000 and 11000 F (540 and 5930 C)

(Figure 28) show clearly the original structure of the martensite plates.

The interstices around each plate are full of extremely fine dark-

etching chromium carbide particles, while the martensite plates them-

selves appear light and unattacked. The microstructure indicates that

the original retained austenite between the plates has transformed into

chromium rich carbide-ferrite aggregates which convert rapidly to chromium

carbide and ferrite at these elevated tempering temperatures. The behavior

agrees with the retained austenite decomposition in the Z steel (4.11 per-

cent chqromium - 1.11 percent carbon) and with previous observations that

pearlite carbides have relatively high alloy content in alloy steels .

The chromium carbide in the originally austenitic regions is even more

clearly shown in Figure 30 where the structure has been etched long

enough to partially attack the tempered martensite.

- 65 -

1 .

FIGURE 30. #tertnr. of T Steel (6.6*% C 4.00% Cr) after*a hows at 1100 F (593' 0). Etched

t With boiling V04 -409 etch. (1500x)fAu flrtamsite plate in center surrounded by

(P*100 U-lb *tde a ferrite aggregate in originallyatftettle regions.

-66-

2. Specific Volume Measurements. A change in specific volume of

the specimens was expected due to the formation of alloy carbide. An

effort was made to calculate these changes from lattice parameter data

for the carbide and matrix given by Westgren et al . The calculations

showed that the changes would be very small, but they were not suf-

ficiently accurate to predict an expansion or a contraction.

A series of specimens for specific volume measurements was

hardened and tempered at the same temperatures and times used for the

metallographic specimens. Cylindrical samples of 1/4-inch diameter and

weighing approximately 20 grams were used. Specific volume measurements

(Figure 31) were obtained using the method described in detail by

Fletcher and Cohen(AU) with an estimated accuracy of + .00002 cc. per

gram. The curves show a very small general contraction with increasing

tempering and exhibit the same trends as the family of volume percentage

curves in Figure 29. The specific volume of the carbide matrix aggregate,

therefore, depends directly upon the amount of conversion of cementite

to chromium carbide and is a measure of the extent of reaction. In a

Crafs ad Laont(29)previous investigation Crafts and Lamont found a small expansion

during the tempering of chromium steels which they erroneously

attributed to the formation of chromium carbide. In view of the con-

traction caused by the chromium carbide formation found here plus other

dilatometric evidence presented in the discussion of the Crafts and

Lamont paper, the observed expansion must have been due to retained

austenite decomposition.

3. Hardness Measurements. In order to investigate the effect of

alloy carbide formation on the hardness during tempering, hardness

readings were taken on each metallographic sample (Figure 32). Rockwell C

SPECIFIC VOLUME OF YSTEEL (0.67C - 4.00 CR)TEMPERED IN THE FOURTH - 1000 FSTA GE 0- 1100 F

G __ - 1200&FA - 1300'F

u X - 14000F

u

:D.12840

1300 FU.128 30A

ANNEALEDSSTEEL VV

.12820_______________________ _____________________ __

1000 10000 100000TEMPERING TIME - SEC. FIG. 31

Ii

BRINELL HARDNESS OFY STEEL (0.67C - 4.00 CR)AFTER FOURTH STAGET EMPE RIN G

400

0 3601000 F

360

z

1100 F

Z280

240

1000 10000 100000TEMPERING TIME - SEC. FIG.32

and B readings were originally taken and then converted to the Brinell

scale. The curves of decreasing hardness have the same general

characteristics shown by the curves of volume percent chromium carbide

in Figure 29. This behavior indicates that the growth and spheroidization

of the alloy carbide and the loss of chromium from solid solution in the

ferrite are mainly responsible for the softening. The hardness values

after 10 hours at each tempering temperature are compared to the cor-

responding hardness values of the K and T steels in Figure 25, in order

to show the effect of chromium on hardness after tempering. Chromium

is seen to retard this softening at higher tempering temperatures. This

retardation is due to the effect of chromium on the dispersion of the

carbides and the solid solution hardening of the matrix. The cementite

formed during the third stage of tempering in the Y steel does not

spheroidize but remains finely dispersed until it converts to the

chromium carbide. Chromium remains in solution in the matrix, and the

fine dispersion of cementite is maintained up to the relatively high

temperatures where chromium can diffuse more readily. At this point

the alloy carbide begins to form. Crafts and Lamont (29) have observed

a positive rehardening due to the newly-formed finely dispersed alloy

carbide in some alloy systems but observed only a retardation of softening in

tempered chromium steels. Evidently rehardening may or may not occur de-

pending upon the dispersion and type of alloy carbide found in different

alloy systems. As the alloy carbide grows, and more and more chromium

is removed from the ferrite, rapid softening takes place.

4. Electrolytic Isolation of Carbides. The chemical changes

- 69 -

-1

- 70 -

occurring in the carbides during tempering were investigated by means

of chemical analysis of electrolytically extracted carbide residues.

The ferrite is anodically dissolved in an electrolytic cell by -this

method leaving behind the carbide as a residue which is then chemically

analyzed.

Much successful quantitative work has been done with this technique,

notably with the massive stable alloy carbides found in high speed tool

steels.

Summaries of the techniques and literature are given in recent papes

by Koch (41) and Blickwede and Cohen (4.

While the extraction of the stable carbides in high speed steel is

oelatively straightforward, the technique becomes more difficult in

tempered plain carbon and low-alloy steels because of the fine particle

size and low stability of the carbides.

Factors causing difficulty are:

(a) After the carbide particles are freed from the matrix and

are no longer in electrical contact, they may react with the electrolyte

or the oxygen in the air. The fine particles present in tempered steels

possess a large surface area and small curvature and so are reactive in

acids and oxygen. The cementite in plain carbon and low-alloy steels is

chemically more unstable than the alloy carbides.

(b) Fine carbides tend to stick tenaciously to the specimen

anode during electrolysis forming a dense impermeable layer which tends

to cover the active anodic ferrite. When the active ferrite area is

decreased, the current density increases,,and concentration gradients are

set up in the electrolyte adjacent to the anode. This polarization may

change the solution potentials of ferrite and cementite and cause cementite

to dissolve electrolytically.

-71-

(c) After electrolysis, difficulty is encountered in separating

the extremely fine carbide particles from the electrolyte by filtering

methods.

Electrolytic experiments were made using the H0 and the citrate

(42)cells described by Blickwede and Cohen with modifications designed

to minimize the dangers described above.

(43)It had been indicated that by using a high current density and

short time of electrolysis cementite could be successfully isolated from

plain carbon steels in an HCl cell. The short electrolysis time evidently

decreased the amount of reaction of cementite with the acid and was not

long enough for a thick adherent carbide layer to build up and polarize

the anode. Accordingly, a cell similar to the one used by Blickwede

and Cohen(42) was constructed. Instead of the copper cathode, however,

a combined cooling coil and cathode of bent tubular 1/4-inch diameter

stainless steel was used. A more dilute 1:20 HU0 electrolyte was used

instead of the 1:10 solution,and the current density was increased to

0.5 amps/cm2. In order to decrease the time of exposure of the carbides

to acid and air, the carbides were separated from the electrolyte by

centrifuging techniques instead of the previous slower filtering

method. The wet centrifuged cake was then dried according to previous

methods(42).

Specimens tempered at 1300 and 14000 F were electrolyzed, and

the weight percent and chemical composition of the carbides were obtained.

The weight percents are shown by the dotted lines in Figure 33. For the

shorter tempering times at 13000 F (7050 C), the weight percent decreased

rapidly, indicating that the carbide was going into solution in the

electrolyte as the particle size decreased. In the specimens tempered

for 15 minutes at 13000 F more than half of the carbides was destroyed.

0

11000 F

8 41200 F

1300 F- 1400 F

Z - ----- ---

- 1400 F (HCL)

H, /

UJ /

1300 F (H C ) WEIGHT PERCENT CARBIDE RESIDUE5 /OBTAINED FROM TEMPERED Y

STEEL (0.67 C - 4.00 CR) WITHHCL AND CITRATE CELLS

1000 10.000TEMPERING

100000TIME - SEC. FIG.33

Considerable gas evolution from the carbide particles in suspension in

the acid electrolyte after electrolysis indicated that H2 and hydrocarbon

gases were being evolved according to reactions (1) and (2) suggested

by Koch .1)

(1) FeC+ HCl= FeCl2 + C+ H2

(2) Fe3C + HCL = FeCl2 + C + H?

The gas evolution became more pronounced as the degree of tempering and

particle size decreased.

These results proved quite conclusively that finely divided cemen-

tite and (CrFe) 7C3 carbides cannot be quantitatively separated from

tempered plain carbon or low-alloy steels in the ACl cell.

The sodium citrate-copper sulfate cell was tried next, since the

almost neutral electrolyte prevents the decomposition of the isolated

carbides. The cell contains two compartments, the anod!e and cathode

chambers being separated by a porous diaphragm. A solution of 15

percent sodium citrate, 2 percent potassium bromide and 1 percent

potassium iodide is used for the anolyte, while thecatholyte consists

of a 10 percent solution of copper sulfate, with copper serving as the

cathode. The cell has the desirable characteristic that:

(1) The anolyte has a pH value close to 7 at the beginning and

does not fall below 6 during the latter part of the run.

(2) The iron dissolving from the anode specimen forms a water

soluble complex iron with the citrate preventing the precipi-

tation of iron hydroxide.

(3) Copper deposition instead of hydrogen evolution occurs at the

cathode, thereby avoiding an increasing hydroxyl ion concen-

tration which might cause the precipitation of insoluble

hydroxides in an otherwise neutral solution.

- 73 -

r Several changes in the techniques described by Blickwede and

Cohen (42) were introduced. Since somewhat less than 1 gram of carbide

residue was required for chemical analysis, much shorter runs were

made eliminating the necessity for the continuous flow apparatus. A

cell very similar in construction to the H01 cell was used except for

the presence of the porous cup acting as a diaphragm between the anolyte

and catholyte chambers (Figure 34). The anolyte and catholyte chambers

were filled at the beginning of the run, and a higher electrolyte level

in the anolyte was maintained manually. The anolyte was completely

changed once at the middle of each run thereby keeping the pH always

above 6.

A relatively high current density of 0.2 amp/cm2 was used. The

runs were, therefore, shortened, and the time of exposure of carbides

to oxygen was decreased. The disadvantage of polarization due to the

higher current density was partially offset by mechanically knocking the

carbides off the specimens every 10 minutes.

Cold working the anode specimens had been found to improve the

extraction of the carbides in pearlitic specimens . Consequently,

two specimens tempered at the lowest temperature (10000 F) (5400 C) for

28 and 56 hours were electrolyzed according to the above technique in

the worked and non-worked condition. Cold working was accomplished by

two 50 percent reductions in thickness at right angles under a drop

hammer. The weight percent and chemical analysis of carbides obtained

for the two sets of samples were almost identical (Table VI) and indi-

cated that little advantage could be gained by cold working. Evidently,

the chief advantage gained by cold working pearlitic structures is due

to breaking up of the lamellae and not in increasing the difference in

solution potential between ferrite and cementite.

- 74 -

- 75 -

+

RUBBER- -TUBING

1000 ML,BEAKER

SPECIMEN

ANOLYTE - -

CATHOLYTE -4

211

4. I-I'I

*1~

Ii ~

SPECIMENSUPPORT

A)i. - _

r-POROUS CUP

-CATHODE

SCHEMATIC DIAGRAM OFCITRATE CELL FIG. 34

.4

-

k

I I

- 76 -

A complete set of tempered specimens 3 inches X 1/4-inch diameter

was run in the citrate cell for 3 ampere-hours. Residues ranging from

0.5 to 1.0 gram in weight were extracted, and weight percent and chemi-

cal composition of tae residues were determined. The results are shown

in Table VI. Three general criteria may be applied to indicate the suc-

cess of the extraction technique:

(1) The total weight percent analysis for all elements in the car-

bide should equal one hundred percent.

(2) Since essentially all the carbon in the steel is combined in

(44)the carbides at these tempering temperatures , the product

of the weight percent carbides and percent carbon in the

residues should equal the carbon content of the steel (equation

9).

67 = (weight % carbides) (carbon % in carbides) (9)

(3) The carbon content of the residue should correspond to the

stoichiometric carbon content of the carbide present.

For the coarse high-chromium residues obtained after long tempering

times at 1300 and 14000 F (705 and 7600 C), these criteria are well

satisfied (Table VI). However, as the particle size and chromium con-

tent of the carbi!i.es in the less tempered specimens decrease, the total

analysis falls towards 90 percent. The decrease in total analysis

percentage may be attributed to oxidation of the residues and the pos-

sible formation of an adsorbed moisture layer after exposure to the air

following vacuum drying. This increased the carbide residue weight but

11

TABLE VI

Weight Percent and Chemical Analysis of Carbide Residues in Tempered Y Steel

(0.67% C - 4.00% Cr) Obtained with the Citrate Cell

TemperingTemp. 0F

1400140014001400

1300130013001300

1200120012001200

110011001100

TemperingTime (sec.)

360,00020,0003,0001,000

100,00020,0003,0001,000

100,00020,0003,0001,000

100,00035,00011,800

Weight %Carbides

7.57.88.28.4

7.88.18.69.2

8.28.79.59.9

9.09.5

10.0

ExerimentalWeight %

Fe

47.5348.1048.8448.84

46.2046.5351.4854.78

43.5649.1755.4459.73

51.1554.1260.72

Weight %Cr

40.3239.5138.1136.33

41.2739.5934.6529161

41.0535.2827.3022.05

34.6520.0923.31

ResultsWeight %

Mn

1.201.231.241.06

1.401.361*201.20

1.521.481.061.04

1.561.401.18

Weight %C

8.868.438.328.95

8.738.738.187.91

8.688.328.187.64

7.918.187.21

TotalAnalysis

97.9197.2796.5194.68

97.6096.2195.5193.50

94.8194.2591.9890.96

95.2792.7992.42

Corrected

Weight %Carbides

7.47.67.98.0

7.67.88.28.6

7.88.28.79.0

8.68.89.2

ResultsWeightCarbon

9.08.88.58.4

8.88.68.27.8

8.68.27.77.4

7.87.67.3

7.34 92.607.41 87.99

Cold Worked Specimens

7.31 92.657.36 88.93

10001000

200,000100,000

10.210.3

62.3961.05

10001000

21.6318.37

200,000100,000

1.241.16

10.110.3

62.3762.37

21.6318.06

9.49.1

1.341.14

7.17.4

- 78 -

was not accounted for in the total analysis. Therefore, the total

percentages of Fe, Cr, Mn and C do not equal one hundred percent.

The experimental carbide weight percentages (Figure 33) were corrected

for these effects by the multiplying factor total1 alysis, and the

corrected values are listed in Table VI.

Since the experimental carbon percentages showed irregularities.

which were partly due to difficulty in analyzing small samples from

the residue, a set of more reliable calculated carbon values was ob-

tained by using the calculated weight percent values in equation (9).

These values (Table VI) show an entirely consistent Irend of decreasing

carbon percent as the amount of tempering decreases and more cementite

is present.

The chromium carbide (Cr,Fe)7C3 is capable of dissolving up to

about 55 percent Fe while (Fe,Cr)3C can absorb about 15 percent chromium(1/

The carbon content of (Cr,Fe)7C3 , therefore, decreases only from 9.0 to

8.7 percent as the amount of dissolved iron increases, and the carbon

content of (Fe,Cr)3 C increases only from 6.7 to 6.8 percent as the

dissolved chromium increases. The corrected carbon contents of the

carbides after long tempering times at 1300 and 14000 F (705 and 760 0C)

(Table VI) correspond closely to the known carbon content of the

chromium carbide. As the amount of tempering decreases and more and

more cementite is present, the carbon contents regularly decrease and

approach that of cementite. The 'eneral consistency of the carbon contents

obtained by this method indicates that the corrected weight percent of

acarbide is/more accurate and reliable quantity than the experimentally

determined carbon content of the extracted residue.,

now

- 79 -

Fortunately, the atomic weights of iron and chromium are nearly

the same, and so large compositional changes of chromium and iron in the

two carbides are permissible without greatly affecting the carbon percent

in each carbide. By knowing the overall carbon content of the residues

and the carbon contents of the two carbides constituting the mixture,

the fraction of each carbide in the mixture may be calculated. The

weight percent of each carbide can then be calculated, since the total

weight percent of carbide is known. The values of weight percentage of

chromium carbide calculated by this method are shown in Figure 35. Of

course, the weight percentage of cementite may be obtained at any

point by subtracting the amount of chromium carbide from the total

amount of carbide phase (Table VI).

At 10000 F (5400 C) slight conversion has taken place, and the

(CrFe) 7C3 present is mainly the result of chromium carbide formation

from the transformation products of the retained austenite in the

original hardened specimen. The calculated values of weight percent

chromium carbide at 10000 F are not precise enough to indicate whether

noticeable conversion to alloy carbide takes place at this temperature.

Since the chromium content of the residue increases slightly at this

temperature and magnetic data discussed later indicates that the

(Fe,Cr)3C is not being appreciably enriched in chromium at this tempera-

ture, slight alloy carbide formation must take place. At higher

temperatures the reaction occurs more rapidly and is complete at 13000 F

(7050 C) and 14000 F (7600 C) after tempering for 28 hours.

This family of curves shows the same trends apparent in the volume

percent carbides, specific volume and hardness measurements (Figures 29,

CALCULATED WEIGHT PERCENTOF CHROME CARBIDE IN Y STEEL(0.6o7 C -4.00 CR) AFTER FOURTHSTAGE TEMPERING

2 1000 F -____+

80

1100 F

r6 1300 F

1000 10000 100000TEMPERING TIME -SEC.

0

FIG.35

(

F-LU

LUI0-

-

LJ

I- 81 -

31, and 32). These values all depend directly upon the relative amount

of (Cr,Fe)7C3 formed and so exhibit the same general characteristics.

The weight percent carbides and volume percent carbides should be

related by equation (10).

Volume percent carbides= (Ste) weight % of carbides (10)

where = density

This equation was applied for values at 14000 F (7600 C) where only the

chromium carbide is present. By substituting values of the weight per-

cent and using 0 (steel) = 7.8 and p (Cr,Fe)7C3 = 7.3, values of the

volume percent were calculated which agreed well considering the limi-

tations of the method of lineal analysis used for the direct measurement

of volume percentage (Figure 29). For instance, after tempering

360,000 seconds at 14000 F (7600 C) a value of 7.4 was obtained for the

volume percent of chromium carbide. The calculated value is

(7.4) 78 7.97.3

One more check on the phases present at the different tempering

temperatures was obtained by x-ray means. Debye-Scherrer powder

patterns were made of the residues from specimens tempered for long

times at the five temperatures using chromium radiation filtered with

vanadium carbide. The patterns are shown in Figure 36 compared to

standard patterns of pure Fe3 C and Cr7C3. The line shift, most

apparent at the high angles, is due to the presence of chromium in the

Fe3C and iron in the Cr7C3 . However, direct comparison of the patterns

shows that at 14000 F (7600 C) and 10000 F (7050 C) (Cr,Fe)703 is the

only carbide present. At 1200, 1100, and 10000 F (650, 590 and 5400 C)

the cementite pattern appears and becomes increasingly strong. The

cementite pattern of the 10000 F residue is rather broad due to fine

-A

---. m .. NNW-'--

- 82 -

particle size at the low tempering temperature. The patterns were

measured, and the calculated d-values confirmed the observations made

above.

The changes in composition of the residue are shown graphically in

Figures 37, 38 and 39. The curves for iron and chromium show that the

solubility of iron in the chromium carbide increases as the temperature

is raised. Conversely, the manganese solubility in the chrome carbide,

decreases as the temperature is raised.

The compositional changes in each carbide could not be followed,

since no method was available for separating the two carbides in the

residue mixture and analyzing each individually. However, the approxi-

mate composition of the cementite present before appreciable conversion

to chromium carbide occurred was obtained by magnetic measurements.

Previous work indicated that the Curie temperature of cementite is

considerable lowered by chromium and manganese in solution(45). The

Curie temperature depression is shown in Figure 40. Tempered speci-

mens were, therefore, run in the magnetometer and the magnetization

versus temperature curves were run between room temperatures and

4000 F (2040 C) according to the technique described in section V-D.

For the slightly tempered specimens containing low-alloy cementite,

magnetization curves such as shown in Figure 41 were obtained. The

sudden rise in magnetization near 1000 F (380 C) is due to the

cementite becoming ferromagnetic. The temperatures at which the

cementite begins to become ferromagnetic is shown in Figure 42 for the

specimens tempered for various short times at 1000, 1100 and 12000 F

(540, 590 and 6500 C). At longer tempering times, the cementite

- 83 -

FIGURE 36. X-ray powder patterns of residues from Y steel (0.67% C -4.00% Cr) tempered in fourth stage - chromium radiation.

Cr7C3 - pure Cr7C3 carbide

(C Ii) C2 1 - residue from specimen tempered 14000 F (7600 C) -100 hrs.

% N. 5 - residue from specimen tempered 13000 F (7050 C) -28 hrs.

j4' ) 10 - residue from specimen tempered 12000 F (6500 C) -28 hrs.

C ) 13 - residue from specimen tempered 11000 F (5900 C) -28 hrs.

(FC L C. 16 - residue from specimen tempered 10000 F (5400 C) -56 hrs,

cem.- pure Fe 3C carbide

PERCENT CHROMIUM INCARBIDE RESIDUES FROMY-STEEL (0.67 C -4.CR)TEMPERED IN FOURTHSTAGE

200

F10

000-0 1300*F 12 0 F


100 000FIG. 37

40

~35

0

zLufl 25u

20

1000

li

PERCENT IRON IN CARBIDERESIDUES FROM Y STEEL(0.67 C - 4.00 CR) TEMPEREDIN THE FOURTH STAGE

1000 F

5

1100*F

-1400 "F

12000 F

1000 10 000 100000TEMPERING TIME - SEC

z

F-z

bi

IrAmoloolm - llol I,opow"

4

I04

FIG.38

I,

/0

1400'F

K00J4

z

zuI-

a-

10000TEMPERING

cz

100000TIME-SEC.

I.6

1.4

1.2

1.0

PERCENT MANGANESE INCARBIDE RESIDUE FROMY STEEL ( 0.67 C - 4.00 CR)TEMPERED IN THE FOURTHSTAGE

r0.8

1000

I '

0001-

u

I

FIG. 39

- 87-

2 4 6PERCENT ALLOYING

ELEMENT

8

FIG. 40

DEPRESSION OFCEMENTITE CURIETEMPERATURE BYALLOYING

MN

CR

400

D 3 0 0

LJ200

U 00

0

A

MAGNETOMETER DEFLECTION VS.TEMPERATURE FOR Y STEEL(0.67C-4.OOCR) SPECIMENS TEMP-ERED IN THE FOURTH STAGE

90 C5E .. A T 1200F

D

H - - - -__--_

100 200TEMPERATURE F

JUL) 44L)FIG. 41

z0

HLJi

IL-

Ld

LL2

~In

u 200ui

D

0-:

Li

150

100

TEMPERATURE OF BEGINNINGIOF MAGNETIZATION OF CEMVENT-ITE IN Y STEEL(0.67 C-4.0CR)AFTER TEMPERING ____

1200OF 1100 0F 1000

_ _ - - - - - - - - - - - -I

1000 10000TEMPERING

100000TIME-SEC.

0?

FIG. 42

- 90 -

dissolves sufficient chromium and manganese to depress the whole

carbide Curie point considerably below room temperature.

The Curie temperature, defined as the point of inflection of the

magnetization curve, may be estimated,if the temperature of the be-

ginning of the magnetization curve is known. If chromium and manganese

are assumed to have additive effects on depressing the Curie point,

then reference to Figure 40 indicates that the chromium content of the

cementite in the specimens tempered at 10000 F (5400 C) is nearly the

same as the average chromium content of the steel. At 10000 F (5400 C),

then, the cementite does not become noticeably enriched in chromium.

At 11000 F (5900 C) and 12000 F (6500 C) the cementite dissolves

chromium after longer tempering times, and the Curie temperature is

eventually depressed below room temperature. Simultaneously at these

temperatures the coromium carbide is forming. These observations

indicate, therefore, that cementite has about the chromium content

of the steel until at temperatures where chromium can diffuse readily,

chromium enrichment occurs, and chromium carbide simultaneously forms.

C. Mechanism and Kinetics of the Fourth Stage Alloy Carbide Formation

1. Introduction

Attempts were made to determine the mechanism of the formation of

the alloy carbide from the cementite and chromium-rich ferrite matrix.

Two mechanisms are generally possible.

a. Alloy carbide nuclei form in the chromium-rich ferrite, and the

cementite goes into solution providing the carbon necessary for the new

carbide. The alloy carbide grows by the diffusion of chromium from the

matrix and of carbon from the dissolved cementite(46)

- 91 -

b. The cementite converts to the alloy carbide in place by a

process of chromium enrichment and structural changes(29)

The evidence in the literature given for both these mechanisms is

not conclusive, however, and further work was indicated.

2. Metallographic results

Since a method of differential etching of the two carbides was

available, attempts were made to observe the relative positions of the

particles during the conversion process. The specimens were first etched

with the boiling KMnO4 - NaOH etch and photographed at 1500x. The same

area was then etched with 5 percent picral and again photographed.

Picral was found to give better etching results than any of a wide variety

of acid etchants. The two photomicrographs of the differently etched

area were then compared. Unfortunately, the acid etch removed the

basic oxidized film in the chrome carbide due to the KMnQ4 - NaOH

etch, an'a, therefore, the chrome carbide particles did not appear the

same after both etc-hes. Efforts to find another basic oxidizing etch

which would sharply reveal the cementite after the initial KMnO4 - NaOH

etch were unsuccessful.

Even though the same particles could not be observed in the two

photomicrographs, the general size and pattern of the (Cr,Fe)7 3

particles could be determined from the KMnO4 - NaOH etched structure

and then be compared to the acid etched structure. Electropolishing and

simultaneous etching in a acetic acid-perchloric acid electrolyte was

found to reveal an even more sharply defined duplex microstructure than

shown by the picral etch.

-92-

Two sets of photomicrographs etched with KMn04 - NaOH and

electropolished and etched are shown in Figures 43 and 44. The

photographs were taken at 1500 magnification but were enlarged 2x in

an enlarger. The "empty magnification" does not increase the

resolution but does make the small particles easier to see. The speci-

men tempered at 11000 F (5900 C) is about half way through the con-

version process (see Figure 35). Since the small dark particles in

both structures appear similar, they must be the (CrFe)7C3 carbide.

The conversion process is almost over in the specimen tempered at 12000 F

(6500 C) for 28 hours. The cementite particles are very poorly defined

in both acid etched structures, however, and the relationship between

the cementite and chromium carbide particles is difficult to observe.

In order to show the cementite particles more clearly, specimens con-

taining more cementite after tempering for shorter times at these

temperatures were electropolished and etched. The structures after

tempering at 12000 F (6500 C) for 50 minutes and 5.6 hours are shown

in Figure 45. The series of photomicrographs shows the white cemen-

tite particles becoming smaller, the dark chromium carbide particles

becoming larger, and the general structure becoming more clearly

defined as tempering proceeds. The relationship between the two-

type particles, however, cannot be definitely determined by these

methods. The particles are too small and close together, and the

cementite is not clearly outlined. The undefined appearance of the

cementite may indicate that it is dissolving or is inhomogeneously

converting to the alloy carbide. In many cases it appears that the

dark chrome carbide particles are growing directly on the periphery

of the large cementite particles, but higher resolution would be

J

0'0 I

0

S

0

94 0

'II. *''0~

0~ - ~4~9*0%

~ *0Os09 0 0~ * 0 . S

046% ~t* 0.

4* 9 .. ~9 *~15 ~** 0.: ~* ~. 4~

0 4. 0

* .g~4

.6'~. .0

@10 9'p.

0~ ~e0'

~:** I -9..*

4

A*'~~ *~

'PDS * 0

.4

-V

*0

4 1. *

'1.

00

4 AP

0

0

0.. *0

0 -9.

(a) Etched 1 minute in boiling KnO4 - NaOH.Shows the chrome carbide (3000x)

(b) Electropolished and etched in acetic acid-perchloric acidsolution. Shows chrome carbice and cementite (3000x)

FIGURE 43. Structure of Y steel (0.67% carbon - 4.00% chromium)after tempering 28 hours at 11000 F (5900 ).

- 93 -

***.,*l~ ~ S

U.

of of* &

* 411664 6. . 1 *

*ow -s4*

0

14S, .. j- , 0.

- -.- . .* *

S- - . --, - *

or -- ..1o itw

**O

0*

0 90* ~0

.:e.~. *

(a) Etched 1 minute in boiling KMnO4 - NaOH.Shows only chrome carbide (3000x).

-*

(b) Electropolished and etched in acetic acid-perchloric acid

solution. Shows chrome carbide and cementite (3000X).

FIGURE 44. Structure of Y steel (0.67% carbon - 4.00% chromium)after tempering 28 hours at 12000 F (6500 C).

- 94 -

I

- 95 -

4,

UKS)ishedtion

- 96 -

required to definitely settle this relationship.

3. Kinetics of the Cementite to Alloy Carbide Reaction

The curves of volume percent and weight percent of chromium carbide

versus time at different temperatures (Figures 29 and 35) show directly

the kinetics of the cementite to carbide reaction. The reaction has a

maximum rate at the beginning which slowly decreases with time. The

data do not fit the general form of equation (4).

A complete interpretation of these kinetics is not warranted,

since the mechanism of the carbide formation could not be definitely

determined. The high initial rate indicates that the reaction does

not follow ordinary nucleation and growth kinetics. When the

cementite-ferrite aggregate is heated rapidly into the high tempera-

ture range of alloy carbide formation, chromium is capable of diffusion,

and the reaction occurs relatively rapidly. As the reaction proceeds,

however, the chromium concentration in the matrix decreases, and the

close growing alloy carbide particles interfere with each other as a

result of an overlapping of their diffusion zones.

A Q value of about 60,000 cal/mol (equation (8)) was obtained.

This value is based upon a quantity having the dimensions time-1 and

may be compared directly to diffusion data. The heat of activation

for the self-diffusion of iron in alpha iron has been found to be

59,700 cal/mol , and chromium would be expected to have only a

slightly lower value because of its atomic similarity. Since the Q

values for the fourth stage alloy carbide formation and for the dif-

fusion of chromium in alpha iron are about the same, the formation of

alloy carbide must depend directly upon the diffusion rate of chromium

in ferrite.

- 97 -

VII. CONCLUSIONS

1. The decomposition of martensite during tempering occurs by a

two-step process. The first step ends with the precipitation of a

transition precipitate which remains in pseudo-equilibrium with the

tempered martensite over a considerable range of temperatures and

times.

2. The transition precipitate is probably hexagonal close packed

having a structure similar to Fe2C but possessing a higher iron

content than the iron content of cementite. The solubility of carbon

in the tempered martensite coexisting with the transition precipitate

increases with temperature. Chromium increases the solubility of

carbon in tempered martensite in the presence of the transition precipi-

tate.

3. The transition precipitate which is in registry in the

tempered mrartensite ultimately converts to a discrete plate-like form

of iron-rich cementite during the second step of martensite decom-

position. This "early cementite" gradually converts to stoichiometric

cementite.

4. The kinetics of the formation of cementite follow the form

of the general equation (3)

- k (a - y)tm (3)dt

where a = total extent of reaction possible

y = extent of reaction at time = t

k = a temperature dependent constant

The kinetics differ from ordinary nucleation and growth kinetics

by proceeding most rapidly at the beginning of reaction indicating that

- 98 -

no nucleation period is required. Coherency stresses from the transition

precipitate probably speed the reaction initially.

5. Retained austenite decomposes to the same products as does

primary austenite after hot quenching. However, the retained auste-

nite to bainite reaction is initially speeded by the presence of tem-

pered martensite.

6. The retained austenite to bainite reaction follows equation (3)

with nucleation and growth characteristics, and proceeds with a "heat

of activation" of about 38,000 cal/mol.

7. The low temperature bainite formed from the retained austenite

is dimensionally unstable and contracts with increasing tempering time.

8. The cementite and alloy carbides existing in tempered plain

carbon or low alloy steels may be quantitatively separated from the

matrix with a neutral sodium citrate electrolytic cell.

9. The cementite existing in a 0.67 percent carbon - 4.00 per-

cent chromium steel at the end of the third stage of tempering con-

verts progressively to the chromium carbide (Cr,Fe)7 C3 at higher

tempering temperatures during the fourth stage causing a slight

contraction.

10. The formation of the chromium carbide has a maximum rate at

the beginning of reaction and takes place with a "heat of activation',

of about 60,000 cal/mol. This value indicates that the reaction is

controlled by the diffusion rate of chromium in ferrite.

11. Essentially all the carbon in the steel is combined in the

cementite at the end of the third stage and remains in the carbide

phases during the fourth stage conversion process.

- 99 -

12. Chromium retards softening during tempering by maintaining

a fine dispersion of cementite and by solid solution hardening. When

sufficient chromium is present for the chromium carbide to form, the

fine dispersion of the newly formed carbide contributes to the hard-

ness.

13. The cementite formed at the end of the third stage of temper-

ing has approximately the same alloy content as the overall steel.

At higher temperatures during the fourth stage of tempering where

chromium diffuses more readily and the alloy carbide begins to form,

raoid chromium enrichment of the cementite takes place.

- 100 -

BIBLIOGRAPHY

1. P. K. Koh and M. Cohen, "The Tempering of High Speed Steel", Trans.

A.S.M. 2 (1939) p. 1015.

2. 0. Zmeskal and M. Cohen, "The Tempering of Two High-Carbon High Chromium

Steels", Trans. A.S.M. (1943) 1 p. 380.

3. S. G. Fletcher and M. Cohen, "The Effect of Carbon on the Tempering of

Steel", Trans. A.S.M., 2 (1944) p. 333.

4. D. P. Antia, S. G. Fletcher and M. Cohen, "Structural Changes During

the Tempering of high Carbon Steels", Trans. A.S.M. 2 (1944) p. 290.

5. D. P. Antia and M. Cohen, "The Tempering of Nickel and Nickel-

Molybdenum Steels", Trans. A.S.M. .32 (1944) p. 363.

6. B. L. Averbach and M. Cohen, "The Isothermal Decomposition of Marten-

site and Retained Austenite," Trans. A.S.M. 4 (1949), p. 1024.

7. G. Kurdjumov and L. Lyssak, "The Application of Single Crystals to

the Study of Tempered Martensite", J. Iron and Steel Institute 156

Part 1, May 1947, p. 29.

8. H. J. Elmendorf, "The Effect of Varying Amounts of Martensite upon

the Isothermal Transformation of Austenite Remaining after Controlled

Quenching," Trans. A.S.M. U (1944) p. 236.

9. B. L. Averbach and M. Cohen, "X-ray Determinations of Retained Austenite

by Integrated Intensitibs," T.P. 2342, Metals Tech., Feb. 1948.

10. Arbusow and G. Kurdjumov, "The Condition of Carbon in Tempered Steel,"

J. of Physics, USSR 5 (1941) p. 101.

11. F. D.Heidenroich, L. Sturkey and H. L. Woods, "Investigation of

Secondary Phases in Alloys by Electron Diffraction and the Electron

Microscope," J. Applied Physics, _17 (1946) p. 127.

- 101 -

12. K. H. Jack, "Fe-N, Fe-C and Fe-C-N Interstitial Alloys: Their

Occurrence in Tempered Martensite," Nature ; 8 (1946) p. 60.

13. W. Crafts and C. N. Offenhauer, "Carbides in Low Chromium Steel",

Trans. A.I.M.E. 150 (1942) p. 275.

14. A. Westgren, G. Phragnen, T. Negresco, "On the Structure of the

Fe-Cr-C System", J. Iron & Steel Inst. 117 (1928) p. 383.

15. F. Wever and W. Jellinghaus, "The Influence of Chromium upon the

Transformations in Chromium Steels", Mitt. K-W. Inst. Eisenforschung

-& (1932) p. 105.

16. W. Koch, "Electrolytic Isolation of Carbides in Alloyed and

Tfralloyed Steels", Stahl und Eisen, 1 (1949) p. 1.

17. B. L. Averbach, M. Cohen and S. G. Fletcher, "The Dimensional

Stability of Steel. Part III - Decomposition of Martensite and

Austenite at Room Temperature", Trans. A.S.M. 40 (1948) p. 728.

18. Davenport and Bain, "Transformation of Austenite at Constant Sub-

Critical Temperatures", A.I.M.E. 2Q 117, (1930).

19. R. T. Howard and M. Cohen, "Quantitative Metallography by Point

Counting and Linel Analysis", Trans. A.I.M.E. 172 (1947) p. 413.

20. E. A. Rominski and H. F. Taylor, "Stress Relief and the Steel

Casting", Trans. A.F.A. 51 (1943) p. 709.

21. M. Avrami, "Granulation, Phase Change and Microstructure", J. Chem.

Physics, 2 177 (1941).

22. M. Cook and T. L. Richards, "Observations of the Rate and Mechanism

of Recrystallization in Copper", J. Inst. of Metals 7_ 1 (Sept. 1946).

23. C. Zener, "Theory of Growth of Spherical Precipitates from Solid

Solution", J. App. Phys. 20 (1949) p. 950.

24. C. H. Wert, "Precipitation from Solid Solutions of Carbon and

Nitrogen in Alpha-Iron", J. App. Phys. 20 (1949) p. 943.

- 102 -

25. C. Zener, discussion of reference 6.

26. E. P. Klier and T. Lyman, "The Bainite Reaction in Hypoeutectoid Steels",

Metals Tech. June 1944,

27. C. Zener, "Kinetics of the Decomposition of Austenite", Metals Tech.

Jan. 1946.

28. W. A. Johnson and R. F. Mehl, "Reaction Kinetics in Processes of

Nucleation and Growth", Trans. A.I.M.E., 13 (1939) p. 416.

29. W. Crafts and J. L. Lamont, "Secondary Hardening of Tempered

Martensitic Alloy Steel", Metals Tech. Sept. 1948.

30. H. Tutiya, Bull. Inst. Phys. Chem. Research (Tokyo) 10 (1931) p. 556.

31. U. Hofmann and E. Groll, Z. Anorg. Allgem. Chem., 191 (1930) p. 414.

32. K. H. Jack, "Study of Interstitial Alloys of the Fe-C-N System",

British Iron and Steel Research Assn. Project MG/C/26.

33. G. Hlgg, Z. Krist. _2 (1934) p. 92.

34. L. J. E. Hofer, E. M. Cohn and W. C. Peebles, "The Modifications of

the Carbide, Fe2C; Their Properties and Ident.", J. Amer. Chem. Soc.

71 (1949) p. 189.

35. J. Trotter and D. McLean, "Electron Microscope Study of Quenched

and Tempered Steel", J. British Iron & Steel Inst. 163 Sept. (1949)

p. 9.

36. L. J. Dijkstra, "Precipitation Phenomena in the Solid Solutions

of Nitrogen and Carbon in Alpha Iron below the Eutectoid Temperature",

J. of Metals, 1 (March 1949) p. 252.

37. K. H. Jack: private communication.

38. E. C. Groesbeck, Scientific Papers Bureau of Standards, 20 (1925) p. 518.

39. A. Hultgren, "Isothermal Transforiation of Austenite", Trans. A.S.M.

2_2 (1947).

- 103 -

40. S. G. Fletcher and M. Cohen, Authors reply to paper, "The Effect of

Carbon on the Tempering of Steel", Trans. A.S.M. 32 (1944) p. 333-357.

41. W. Koch, "Electrolytic Isolation of Carbides in Alloyed and Unalloyed

Steels", Stahl und Eisen, Jan. 1949 _6 p. 1.

42. D. J. Blickwede and M. Cohen, "The Isolation of Carbides from High

Speed Steel", A.S.M. Metals Transaction 185 (1949) p. 578.

43. A. Hultgren, private communication.

44. G. Kurdjumov and E. Entin, "Temper Brittleness of Steels", Metal-

lurgsdat, 1945.

45. P. Chevenard and A. Portevin, "Dilatometric Analysis of Alloys", Revue

de Metallurgie, 22 (1925) p. 357.

46. Houdremont, Bennek and Schrader, Archiv f. d. Eisenhtittenwesen (1932)

6 p. 24.

47. J. K. Stanley, "The Diffusion and Solubility of Carbon in Alpha

Iron", A.I.M.E. J. of Metals 1 Oct. (1949) p. 752.

48. F. S. Buffington, I. D. Bakalar and M. Cohen, Unpublished research,

M.I.T.

49. T. Lyman and A. R. Troiano, "Transformation of Austenite in One

Percent Carbon High Chromium Steels", Trans. A.I.M.E. 162 (1945)

p. 196.

- 104 -

APPENDIX A

Calculations of the Kinetics of The Decomposition of

Martensite During the Third Stage of Tempering

1. Values of the extent of reaction (y) at time t were obtained

directly from the contractions in Figures 8 and 9 for the K and T

steel martensite. Since the beginning of the third stage involves the

breakdown of the dilation plateau caused by the transition precipitate,

the contraction at the dilation plateau was obtained from the first

stage data of Averbach and Cohen(6) (Table IA). The total possible

contraction at any temperature (a) during the third stage is therefore

the difference between the contraction at the dilation plateau at that.

temperature and the final contraction at the end of the third stage.

The extent of reaction (y) at any time and temperature is the difference

between the contraction at the time and temperature in question and the

contraction of the dilation plateau at the same temperature.

TABLE IA

Contraction at the Dilation Plateau for the K and T Steels

K Steel (1.07% C)

ContractionTemp. (''F) (microinches/inch)

450 1400500 1300600 1100700 900

T Steel (1.00 C - 1.56 Cr)

450 700500 660600 580700 510

- 105 -

Values of a and (a - y, at various times and temperatures for

the K and T steels are given in iable IIA.

TABLE IIA

Third Stage Kinetic Data for K and T Steels

K Steel (1.02 C)

TemperingTeEperature (F)

450

500

600

700

a(Microinches/inch)

33502550

3450

3650

3850

(a - y)(Microinches/inch) (oeconds)

27002550230020001860

250022001350

860760

1800U50

500200100

730350140

9080

20

1000WOO

10000

200o4

10006000

10000

2000

0006000

10000

20100

10006000

10000

T Steel (1.00 C - 1.56 Cr)

2600

2640

2720

2790

2350230021001800

2350225019501550175014001100

800

10507505504UG

450

500

600

700

40100

100010000

;r-0.00

100010000

2000

A00010000

'0100

10000000U

- 106 -

2. Dimensional analysis shows that =m + 1 Therefore:

QT (K steel) = = 42,300 cal/mol.

QT (T steel) - 14s_ = 57,500 cal/mol..25

Since QT depends upon a quantity having the dimensions, time ,

the heat of activation (QT) for the time for a given fraction to

transform was obtained directly from the contraction curves in Figures

8 and 9. Values of

Q (K steel) = 40,100 cal/mol.

Q (T steel) = 51,000 cal/mol.

were obtained. These values check reasonably well with the QT values

given above.

- 10.7 -

ABSTRACT

The decomposition of martensite and austenite and the formation of

the chromium carbide (CrFe)7C3 in chromium bearing steels during temper-

ing have been studied by quenching dilatometer, metallographic, x-ray,

electrochemical, and magnetic methods.

Martensite decomposition takes place by a two-step process. The

first step ends with the formation of an iron-rich coherent transition

precipitate of probable hexagonal close packed structure. Carbon

solubility in the tempered martensite in pseudo-equilibrium with the

transition precipitate increases with increasing temperature and

chromium content of the martensite.

During the second decomposition step the transition precipitate

ultimately decomposes to "early" iron-rich cementite of imperfect

structure. This early cementite then gradually converts to stoichio-

metric cementite at higher temperatures.

The kinetics of cementite fornation do not exhibit ordinary

nucleation and growth characteristics, since the reaction proceeds

most rapidly at the beginning. Coherency stresses from the transition

precipitate probably cause this high initial reaction rate.

Retained austenite transforms to the same products as does

primary austenite during hot quenching. However, the retained auste-

nite to bainite reaction, which proceeds with a "heat of activation" of

about 38,000 cal/mol, is initially speeded by the presence of the

tempered martensite.

" 108 --

The cementite at the end of the third stage contains essentially

all the carbon in the steel and has about the same chromium content

as the overall steel. When the chromium content of the steel is

sufficiently high, this cementite converts to the chromium carbide

(Cr,Fe)7C3 at higher temperatures where chromium can diffuse more

readily. The chromium content of the cementite increases rapidly during

this process. Essentially all the carbon in the steel remains in the

carbide phases during the chromium carbide formation. The formation of

the chromium carbide has a maximum rate at the beginning of reaction

and takes place with a heat of activation of about 60,000 cal/mol.

This value indicates that the reaction is controlled by the diffusion

rate of chromium in ferrite.

Increasing chromium is found to retard softening during tempering.

This effect is due to the influence of chromium on maintaining a fine

dispersion of cementite and also to slight solid solution hardening.

- 109 -

BIOGRAPHICAL NOTE

The author was born in Bayshore, New York on April 18, 1924. He

attended public schools in New York City and Peekskill, New York and

entered M. I. T. as an undergraduate in 1941. After serving three

years in the Army during 1943 - 1946, he returned to M. I. T. and

received the degree of S. B. in Metallurgy in 1947. After working as

a metallurgist at the Naval Research Laboratories, Washington, D. C.

during the summer of 1947, he started graduate work at M. I. T. in

the fall. He was appointed a research assistant to study the tempering

of steel and in 1948 became the Union Carbide and Carbon Fellow.

-4

tempering of chromium steels s.b. (met), …

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