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 -
Figure PageNumber Number
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
20 Structure of Z Steel (1.11% C, 4.11% Cr) After Tempering
28 Hours at 9000 F (4800 C). Tempered Martensite, Inter-
mediate Structures and Cementite Present. Etched With 2
Percent Nital (1500x) -. . . . .. .0 . . . . - - - . 42
- v -
Figure PageNumber Number
21 Structure of Z Steel (1.11% C, 4.11% Cr) After Tempering
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 -
Figure PageNumber Number
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 -
Figure PageNumber Number
42 Temperature of Beginning of Magnetization of Cementite
in Y Steel (0.67% C, 4.00% Cr) after Tempering . . . . 89
43 Structure of Y Steel (0.67% C, 4.00% Cr) after Tempering
28 Hours at 11000 F (5930C). .. .......... 93
44 Structure of Y Steel (0.67% C, 4.00% Cr) after Tempering
28 Hours at 1200* F (6500 C) . . .. . . . . . . . . . 94
45 Structure of Y Steel (0.67% C, 4.00% Cr) after Tempering
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
100 1000TEMPERING TIME - SEC.
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
100 1000TEMPERING TIME - SEC.
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
1000TEMPERING TIME - SEC.
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
10000TEMPERING TIME - SEC.
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
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