.WF ,_

“THE EFFECT OF TIME ON
ISOTHERMAL DECGMPOSITION
OF RETAINED AUSTEENITE IN CASE
CARBURIZED LOW ALLOY STEELS

IN THE RANGE 450““) 800°
FAHRENHEIT"

Thai: for fine Dogma of M. S.
MICHIGAN STATE COLLEGE

Donald Adrian Boa-9h
I948

’g H r-Z'S-I 5

This is to certify that the

thesis entitled

“mmror mmoumEIsomERmL
DECOMPOSITION or RETAINED AUSTENITE
IN CASE CARBURIZED LO! mar 3mm
IN THE RANGE 450° 10 W! I

presente I]

Donald Adrian Bergh

has been accepted towards fulfillment
of the requirements for

Mg__degree mmc‘l mn-
coring

Ffléd-  

Major professor

 

 

Datem 1: 19418 n

”-795

"THEEFFECTOFTIIEOETHEISOTHERIAL
DECOIIPOSITION or RETAINED AUSTENITE
IN CASE CARBURIZED Low ALLOI STEELS
II THE RANGE 450° 1'0 800° FARRENI-IEIT "

By
Donald Adrian Bergh
a

Submitted to the School of Graduate Studies of Michigan
State College of Agriculture and Applied Science
in partial fulfillment of the requirements
for the degree of
EASTER OF SCIENCE

'Departnent of Chemical and lbtallurgical Engineering
1948

W
Introduction

kperilental Procedure
' Part I
Part II
Diagram
Carburising bomb apparatus
Carbon train apparatus
Tampering apparatus
Experimental Results
Outline
Carbon Gradient Data
Graphs of Carbon Gradient
Photonicrographs for Part II
Isotherlal Decomposition Data
Graphs of Isothermal Decomposition
Discussion
Conclusion
Possible future nor]:
Selected References

9.11
11-13

15
16

17

23-27
23-51
52-60
61-69
70-74
75

76

77-79

INTRODUCTION

Austenite, so named by Osmond in honor of the late Sir Killian
Roberts—Austen, is a solid solution in which face-centered cubic gamma
iron is the solvent and carbon is the solute. (16, l) The solid solution
austenite is formed by heating a steel containing between 0.05% and 1.7%
carbon to the austenitizing temperature. This austenitizing temperature
is influenced by the alloys added to the steel and varies from 155003“.
for eutectoid steels to 20759F. for hyper-eutectoid steels. The austen—
itic range is important because practically all heat treating starts
from this solid solution range.

Retained austenite, the austenite remaining at room temperature
after rapid cooling, is very detrimental due to its soft and plastic
nature when found with hard martensite, which is the rapid transforma-
tion product. The subject of retained austenite reached new importance
during the war and in recent years has been very carefully studied. (21,
14) The decomposition products have been classified in three general
groupings-—pearlite, bainite, and martensite. Of the three decomposition
products only bainite has received scant attention.

Some of the factors affecting the retention of austenite are as
follows:

(1) Carbon content

(2) Quenching rate (quenching media)
(5) Austenitizing temperature

(4) Sub-atmoSpheric cooling

(5) Cold working

(6) Tampering temperature and time at temperature
(7) Alloy Content
(8) Stress conditions

This retention was first noted in the higher alloy steels-—
especially those containing high manganese, nickel and chromium. This
untransformed product or retained austenite problem has been attacked
from many angles. Investigation with a Tukon micro-hardness tester and
many types of quantitative analysis have been tried. Some of the study
methods used are dilatometric, magnetic (20-25), electrical resistance
(24), X—ray (6), Specific volume changes, hardness, and a combination
photographic point counting and 15.3%; analySis (25 .

The first inves igators thought that more austenite was retained
when the more drastic quenches were used. However Saveur noted nearly
twenty years ago that oil quenches seem to give more of the untransformed
product (17). many investigators noted hat quenches near the critical
cooling rate or those that were just fast enough to yield all martensite-
gave the largest amounts of the retained austenite.

The suggested explanation for this phenomenon was based primarily
on the effect of stresses and stress distribution during quenching. This
bei 3 a very controversal problem, the ideas presented have been numerous.
Since austenite has a greater density than martensite, compressive stress
should promote the retention of austenite while tensional stress will
promote its decomposition. However all evidence does not support this
View; it has been shown that oil quenching leaves more austenite at the
center while water quenching retains more on the outside surface. (30)
Still Scott reasons that oil quenching allows the martensite to temper

while cooling, which allows it to contract and thus sets up compressive

and tension stresses. This holds up the austenite transformation.
Epstein feels that water quenching sets up sharper temperature gradients
giving rise to higher thermal stresses and greater deformations which
tend to promote the transformation of austenite. (50)

Early investigators noted that austenite was usually retained with
high carbon and alloy content. However X-ray studies by Wever and Engel
showed cubic faced—centered austenite in 0.6% carbon. Davenport and Bain
reported austenite present at 0.54 percent carbon, while Tamaru and Sekito
also using X-rays found evidence of retained austenite at 0.4% carbon.
However nearly all investigators reported very minute amounts at these
lower percentages and indicated that as carbon percent increases the per—
cent of retained austenite is greater. (50)

Many previous and all the present investigators also found that
retained austenite was found in greater amounts when cooled just.above
the critical cooling rate. Therefore oil quenches always gave more re—
tained austenite than water quenches.) When quenches were rapid enough to
cause cracking no austenite was retained. This cracking is due to stresses
set up upon quenchi-c and when the cracking starts the structures seem to
be relieved of their internal stress and all austenite changes to marten~
site. However this cracking also depends to some extent uponhS or the
starting temperature for the martensite formation. MS is in turn a function
of the alloy content, and does not vary with tie austenizing temperature.
Higher temperatures for quenching indicate smaller amounts of retained
austenite (so-51) .

Sub—atmOSpheric cooling tends to decompose some of the retained
emistenite. The amount decomposed depends upon the time the Specimen was

zit room temperature, the quenching temperature, and temperature of the sub-

cooling (7-50). Tammann and Scheil found that transformation starts at
—°0°C. (40°F.) for a 1.7 plain carbon steel and that nothing happens again
until the temperature is lowered. Gordon and Cohen indicate hat stopping
the quench at room temperature seems to stabilize the austenite and that
the longer it is held at room temperature the smaller is the amount that
transforms on cooling to ~2109F. They also have dilatometer, magnitic
and specific volume data which indicates this stabilization for approxi—
mately three minutes at room temperature. Here again it was indicated
by Krivobok and Gensamer that the cha "e f austenite to martensite takes
place through the strains which arise. All indicators are that cold work-
ing the steel tends to cause decomposition of the retained austenite.
Here again the internal stresses are great after deformation (50—7).

In transforming retained austenite, Dowel, Harder, and Van Vleet
and Upthegrove (55) state that in most cases the retained austenite de-
composes to a martensite which is very similar to martensite before going
forward with the transformation. Chevanard and Portevin (34) believed that
the austenite decomposed to cementite and an austenite with less carbon;
however, more recent data by Antia and Cohen (52) states that austenite
decomposes to an acicular transition precipitate (55—32—50). Honda and
Nishiyama (52) also:howed that with change from tetragonal martensite to
a cubic form of martensite there was a definite switch of the position of
the carbon atom. However Hagg indicates that a cubic form of martensite
is hard to discern with X—ray and Cohen points out fiB -martensite is harder
than the original tetragonal a; -martensite. There has been a great deal
of discussion aboutfiB-muudensite and a number of recent investigators

wish to refer to this product as cubic ferrite plus cenentite. This for-

mation is also accompanied by a transition precipitate which accounts for

the darkening of the original martensite (52-55).

The second step of tempering, presumed to be the step where austenite
decomposes, occurs at about 450 to 5509F. The decrease in the intensity
of austenite diffraction lines in this range seems to verify this point.
This decomposition can take place either isothermally or upon heating
through this range. The product appears to be somewhat similar to a bain-
itic structure and shows no curie point until nearly all austenite is de—
composed. This would indicate that this product is not a ferrite-cementite
mixture. It is more of a bainitic structure. However to call it a secon-
dary martensite product may be off, as 450°-550°F. is well above the
formation point of martensite and by now the structure has softened up
quite a little. However there are indications of secondary hardening in
high alloy steels due to the decomposition of austenite. (52—55)

The third stage in tempering which occupies the range of 550°F to
7500F. is fundamentally the cementite formation range. The microstructure
indicates that ementite particles develop and grow out of the decomposi-
tion products of austenite and martensite. This would indicate that
ferrite and cementite develOp over both a period of time and temperature
range.

Much o the softening going on around the first and second stages is
thought to be due to stress relief, as both are hardening reactions. These
facts are borne out by magnetic change in cyclic heating and by checking
the magnetic field reading of steels having varying amounts of retained
austenite and martensite. Those having the most martensite showed the
bigger changes in reading, indicating more stress relief since there is no
structual change.

Properties of steels containing retained austenite have not had too

much attention; the results usually looked on as unpredictable. However
French says that small amounts up to 5% retained austenite did improve
the endurance limit and resistance to overstress. He considered the aus-
tenite to having a cushioning effect. He also indicated that high carbon
manganese and nickel tend to give a structure of nearly all retained aus-
tenite (so).

In this paper the primary purpose is to discover how long it takes
retained austenite formed in carburized low alloy steels to decompose
isothermally in certain temperature ranges and to record these changes in

both structure and percent by means of photomicrographs.

EXPERTISE-HAL PROCZSDUPE
Part 1

Determination of Carbon in Case

Five steels were chosen for the eXperimental work, one plain carbon
S.A.E. 1010 and four low alloy steels: S.A.E. 2015, 2540, 5145, and 4640.
The analysis of the five steels are as follows:

Table I

lo:
6:
'1

.0. rm .2 m. in
S.A.E. 1010 0.15 0.55 0.016 0.045

S.A.E. 2015 0.55 0.54 0.017 0.024 0.55 0.75

S.A.E. 2540 0.297 0.71 0.011 0.017 0.22 5.42

S.A.E. 5145 0.40 0.72 0.016 0.020 0.69 1.59

S.A.E. 4640 0.40 0.65 1.82 0.25

The bars were cut to six inches, center drilled and then turned to
the largest possible diameter which would give a taper of no more than
0.001 of an inch from one end to the other. This was checked with taper
gages and the lathe was recentered before the Operation.

The carburizing was done in a 5.00 inch steel pipe carburizing bomb,
using a commercial solid carburizing mixture. The bomb was develOped with
the idea of checking time and temperature lags. A one-half inch pipe
(welded together on the internal side) was in turn welded through one of
the ends of the bomb so that a thermocouple could be placed into the center
of the bomb and readings taken to check (1) the temperature of the bomb vs
the furnace and (2) the time lag that is taken for the bomb to come to
temperature. Later a hole was drilled next to the pipe and the thermocouple

fastened to the bars to see if the bar temperature and lag were the same

as the bomb temperature and lag (see Figure l).

The analysis of the commercial carburizer may be found in Table 2.

Table 2
Ba 005 10-12%
Naacos 2- 5%
Caz CO:5 2__ 5%
Coke 25--50%

Charcoals (Type F.S.R.) Balance

The bars were carburized in groups of two each except for the
4640 bar which was much larger. The bars were carefully placed in the
bomb in such a manner as to give an evenly distributed case to all areas.
The bomb was sealed and placed in a muffle type furnace which was controlled
at 1725-17550F. The lag in the 5.00 inch bomb was about one hour and the
difference of temperature between bar and furnace was about 25°F. for several
hours. The cycle of carburizing was about 16 hours with three hours being
allowed for heating the furnace to 17259F. After 15 hours at 17250F. the
furnace was turned off and the bomb allowed to cool to room temperature
in the furnace.

The carburized bars were then placed on the lathe centers again and
checked for possible warping by use of the dial indicator. If the bars had
warped slightly they were aligned again by pr0per hammering on the lathe
centers. The larger bars showed very little warping. After thorough
checking by the dial indicator the bars were ready to be machined. The
tool was ground with a negative rake to give finer shorter ships, and the
bar was thoroughly washed with alcohol and also parts of the lathe, tool
rest, and tool. The entire length of the bar was turned except the stud

which was used for heat treating and metallographic Specimens.

The chips were collected free of oil and dirt in a Specially
prepared cardboard box. The first cut was 0.002 of an inch and the rest
were 0.005 of an inch until the core was reached. Each cutting was placed
in a prOperly labled envelOpe which was designated both by tool rest travel
and by the change of diameter of the bar. Bar diameters were determined by
micrometer readings.

The steel chips were analyzed for carbon content in a carbon train
(see Figure 2). The carbon train was checked by using Bureau of Standard
samples and the results were very good being well within the allowable
error of t 0.05 percent (slide rule error). The steel chips were now
analrzed for carbon content in a carbon train. The graph of carbon content
vs. distance from edge gives a good indication of the carbon gradient and
depth of case.

Part 2

The studs which were all between 0.8 and 1.0 inch long were cut
into 0.4—0.5 inch pieces and then quartered across the case. These Spec-
imens were now divided into two groups each having an equal number of
samples of the five steels. After carefully marking they were heated to
17250F. throughout,and four samples of each steel quenched into both
agitated heated water (lZOQF.) and agitated heated oil (lZOOF.). All of
the Specimens were tempered for 1.5 hours at 400°F., except the S.A.E.
4640 which showed some decomposition at 400019“. However 1.5 hours at 550%“.
gave the desired transformation of the tetragonal martensite to a dark etching
constitutent in the S.A.E. 4640 steel. The samples were mounted in steel
clamps, very carefully rough ground on the emery wheel, the three stages
of the belt grinder, and the #400 set emery paper. Polishing was completed
roughly in four steps depending upon the scratches which were present.

First after careful cleansing of the cleup and sample in water and alcohdl,

'I l

the lead lap was resurfaced and impregnated with 302% Buehler lapping
compound. The sample was carefully polished until the entire edge near
the case was flush with the clamp. Best results were obtained with

lead lap when polishing was completed simultaneously with the drying up of
the wheel. The lowest possible number of revolutions per minute of the
Slow Speed wheel was used.

The Second step involves polishing with #1576 microcloth and £1549
micropolish. The micrOpolish gives variable results depenling upon the
quantity of polish on cloth and amount.of knap on cloth. Best results
were obtained on these edge polishes by using worn knaps and pasty micro-
polish before the first etch with 5% nital. However just before the second
etch usually about five seconds on the wheel with very little micropolish
present gave the best results. To test the validity of the etch, each
Specimen was polished and etched at least twice with 5% nital before tak—
ing the final photomicrograph at 1500 diameters. In order to keep photo—
graphing the same area continuously it was necessary to mark the surface
with a hardened tool steel Scribe, or use tukon hardness tester points
with a 20 gram load. At 600°F. and eooQF. tempering treatments it was
necessary to start on the lead lap after each tempering interval, thus
it was necessary to notch the Specimen on the edge of the cases. Photo-
micrographs of the 5 steels were taken at intervals developed through
eXperience for each tempering range. The first picture was taken in the
as-received condition, the second after blackening the tetragonal marten—
site at 550 deg. F. for 1.5 hours in the lead bath, and the rest at l,

5, 10, 15, and 20 minute intervals for the 450 deg. F. treatment (see
Figure 5)- At 600°F. and 800°F. the intervals were much shorter using

50 seconds and of this interval 20—25 seconds were involved in coming to

12

the constant temperature of the lead bath. hese times varied depending
upon the Specimen Size and were checked by drilling holes into the sample
and placing the thermocouple inside. Alundum cement was placed around the
bottom insulation piece and allowed to dry for txo days over a Steam rati-
ator. Then it was immersed in the lead and the time recorded to reach
nine tenths of the lead—bath temperature and the final temperature. It
took about 15-17 seconds to reach this first temperature and 5-10 seconds
more to reach the lead-bath temperature. After finding the row of marks
all pictures were taken in the area of heaviest austenite concentration
and at a standard interval from the edge (0.20 mm). The metallograph
was set with a constant field and aperture Opening to give better time
adjustments and similar contrasts.

The method of "Point Counting" was used to determine the quantity
of retained austenite present after the individual heat treatments. The
method consists of placing a grid on tOp of the plate and then over ex-
posing the plate to give a good black and white contrast. The next step
involves counting the number of intersections of the white and then check-
ing these against counting the black intersections. The value obtained
is the number of points per square inch which in turn can be transferred
to percent of retained austenite for that particular square inch by a
simple formula.

(Ho of points shonins austenite per sqJ inch) (100?!
Total points per sq. inch

 

= Percent austenite in area
Graphs of temperature vs. log of time were plotted to indicate
graphically the positions of cifferent percents of retained austenite at

the various temperatures.

13

FIGUREI

The apparatus used for the carburizing of the steels

 

E!

A— 3" pipe

3- Furnace

C— Potentioloter

n- Internally welded 1/2 - pipe
3- Bench

14

FIGUREZ

The apparatus use for the carbon determinations

.—

  

A- Onset! supply tank

B- Gas pressure regulator

(13- Combustion furnace

D- Oxygen washing bottle (cone. 32804)

E- Ascarite tube ((302 removal)

!'- Conlmstion tube

(3- Zinc pellets

H- 002 and 02 washing bottle (cone. 112804 and Cr 03)
I- 002 and 02 washing bottle (cone. 32804 )
J- Ascerite weighing bottle

I— Balancing Bottle

1'..— Bench

I- Alundul end escarite bottle

1* liekle boats

15

FIGURE 3
The apparatus used for tempering steels

 

E!
A- Furnace (lead bath)

3- Innuslly controlled potentioneter
c. Bron automatic potentioneter

D- Bench

E- Cover

mm RESULTS
The experinentel results were divided into two IaJ or divisions:
Part 1
(s) Cerbon gradient date. for surmised an 1010, 2015,
2340, 3145, end 4640
(1)) Graphs of the date. for the five steels listed
Part 2
(s) Photonicrographs of listed steels-st 1500 dis-store
(b) Isotherlel decomposition date. for the- various steels
(c) Graphs of the isotherlsl deco-position date.

l7

Table‘z

Data of barbon Gradient in Carburized
SAE 1010 Steel

Feed
Readings In Sample It. Grams

MEL-MW

1111112. ML *
1 .130 - .135 0.6 0.0289 1.345
2 .110 0.6 0.0257 ‘ 1.165
3 .145 0.7 0.0253 1.03
4 .150 0.7 ‘ 0.0280 1.08
5 .155 0.7 0.0242 0.940
6 .60 0.7 0.0218 0.845
7 .165 0.7 0.0185 0.722
8 .170 1.5 0.0199 0.362
9 .175 1.5 0.0265 0.482
10 .180 1.5 0.0178 0.323
11 .185 1.5 0.0105 0.296

* All readings were checked with a micrometer.

18

Ipb1044

Data of Carbon Gradient in Carburized

SA! 2015 Steel

Reading of
Iicrameter

E

.577 - .566
.557
.549
.539
.530
.523
.511
.502

@QQOWbWNH

~.486
11 .480

H
O

* Diameter Change Of Bar

h‘ F4 r4 54 ta ta k‘ +4 rd

H

H

Sample Wt.
£3.12202241. ..Jh:g§L__.

l9

'te 602

. .QIana_.

0.0444
0.0362
0.0360
0.0329
0.0305
0.0288
0.0262
0.0232
0.0229
0.0205
0.0166

Garbo
1.195
0.986
0.978
0.895
0.829
0.783
0.713
0.632
0.621
0.558
0.452

 

E

Iab1e.2
Data of Carbon Gradient in Carburized

SAE 3145 Steel

Reading of

Micrometer Sample fit. Wt. 002

In nghgg * Gramg _§§§!§__ Garbo
1 .688 - .681 1 0.0548 1.49
2 .661. i 1 0.0401 1.09
3 .653 1 0.0391 1.063
4 .644 1 0.0360 _ 0.978
5 .640 1 0.0324 0.882
6 .629 1 0.0318 0.865
7 .621 1 0.0287 0.782
8 .612 1 0.0259 0.704
9 .604 1 0.0246 0.669
10 .597 1 0.0222 0.604
11 .589 1 0.0205 0.557
12 .579 1 0.184 0.501

* Diameter Change In Bar

20

333.3122
Data of Carbon Gradient In Carburized

SAE 2340 Steel

Reading of
lierometer Sample Wt. Wt. 002
t o. In Incheg * 0mg m M
1 .580 - .570 0.7 0.0298 1.160
2 .560 0.7 0.0340 1.320
3 .550 1 0.0426 1.060
4 .540 1 0.0357 0.961
5 .530 1 0.0255 0.694
6 .521 1 0.0253 0.688
7 .511 1 0.0244 0.664
8 .502 1 0.0218 0.594
9 .491 1 0.0172 0.467
10 .480 1 0.0170 0.462
11 .470 1 0.0160 0.435
12 .460 1 0.0147 0.40
13 .450 1 0.0110 0.30

* Diameter Change in Bar

21

kble 1
Data of Uarbon Gradient In Carburized

SAE 4640 Steel

 

Reading of
Hicrometer Sample Wt. It. 002
t No a Incgeg * Gramg Grams &
1 1.011 - .969** 1 0.0339 0.924
2 .966 1 0.0327 0.891
3 .956 1 0.0246 0.670
4 .946 1 0.0196 0.534
5 .937 1 0.0193 0.525
6 .927 1 0.0156 0.425

'* Dianeter Change In Bar
** Bumped Head of T001 in Traverse

22

 

 

 

 

Pm II

he photonicrogaphs in this section represent steels that

were all subjected to the following treatment and specification

1.

2.

3.

4.

5.

6.

7.

8.

9.

carburized at 1725°lI for 13 hours.

Carefully sectioned and four samples of each steel were
quenched free 1700”!" into agitated water at 120°! and
sauna 011 at 130°! .

SAE 1010, 2015, 2340, and 3145 were tempered for 1.5
hours at 400°! .

811: 4640 was tempered at 350°1r for 1.5 hours.

Etchant- 3 percent nital (used with and without swabs).
Alchol was used for the rinse.

Transverse sections were used.

lagnification used was 1500 diameters.

the tempering tine and temperature will be listed on the
page Just preceding each page of pictures.

All pictures were taken at a standard distance fro- the
edge (0.20 II) .

The isothermal decomposition data follows the photonicrographs.

10. Graphs of the isotherlal deco-position data follow last.

28

 

 

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PHOTOMICROGBAPHS 0F RETAINED AUSTENITE IN
WATER QUENCHED SAE 2340 CARBURIZED STEEL

As Received 1.5 hr. 20 min.
4000?. 4500?!
3.0 . ‘ ' ‘ 50 mine 60 111111.
4503f.l 450°F. 450°r.
120 min. 150 min. 180 min.

450°F. 450°F. 450°F.

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1.5 hr.
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400°F. 4500?.
30 min. 50 min. 60 min.
1.50%“. 450°F. 450%.
:20 min. 150 min. 180 min.
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Experimental Data For Isothermal Decomposition 0f

Iable g

30
2O

10

30
20

Tampering
Temperature Corrected
Wm n19. _....__.._'rime
400 1.5 hr. --
450 20 min. --
450 40 min. ...
450 60 min. —-
450 120 min. -—-
450 200 min. --
450 230 min. --
450 ' 320 min. —-
450 410 min. --
450 470 min. --
450 530 min. --
600 1 min. 15 sec. 50 sec.
600 2 min. 20 sec. 1 min.
600 3 min. 35 sec. 2 min.
600 4 min. 50 sec. 3 min.
600 6 min. 5 sec. 4 min.
- 600 11 min. 8 min.“
600 16 min. 25 sec. 13 min.
600 21 min 40 sec. 18 mio.
800 30 sec. 5 sec.
800 60 sec. 10 sec.
800 90 sec 15 sec.

Percent_Re~

tained Austenite
13 Picture

SOC.
sec.

800‘

BBC.

sec.

sec.

58%
28%
20%
19%
19%
10%
12%
10%
7-5%
11%
4-5%
37%
20%
16%
25%
12%
10%
2-3%
1%
2-3%
Doubtful

Overall
Percent

53%

2—3%
1%
2-3%

Salt and Pepper Effect

gable 2_

Experimental Data For Isothermal Decomposition

Of Retained Austenite In Water Quenched SAE 2015 Carburized Steel

Tampering
Temperature

°£ggreg§ei§

§§§§§§§

. 15 sec.

2 min. 30 sec.

3 min. 45 sec.
5 min.
47 min.
30 sec.

60 sec.

Corrected

Time

50 sec.
1 min.

2 min.

3 min.
44 min.

5 sec.

10 sec.

 

40 sec.
30 sec.
20 sec.

27 sec.

Percent Re-

tained Austenite Overall

12.212M991..___. 1223330;
29% 29%
10% 10%
10% 10%
34% 3-45:
1—?% 1%
19% 19%
5% 5%
1-2% 1-2%
1-2% 1—2%
Little if any

Salt and Pepper Effect

Table :_1_<_)_
hperinental Data For Isothermal Decomposition 0f
Retained Austenite In Oil Quenched 3145 Carburized Steel

 

Percent Re-
Tenperature Corrected tained Austenite Overall
° Pahrgggeit 2.2119. Time ficture 132.133.2311.

mo 1.5 hr. -—-- 1.8% 48%
£50 20 min. -- 37% 37%
450 40 min. --- 36% 36%
450 60 min. --- 20% 20%
£50 120 min. --- 17% 17%
1.50 200 min. -- 16% 16%
[.50 230 min. --- 12% 12%
1.50 320 min. -- 12% 12%
450 410 min. ‘ . -- 13% 8-10%
450 470 min. --- 10% 4-85
1.50 530 min. --- 3-5% 14%
600 1 min. 15 sec. 50 sec. 37% 37%
600 2 n19.20seec. l min. 30 sec. 23% 23%
600 3 min. 35 sec. 2 min. 20 sec. 26% 26%
600 4 min. 50 sec. , 3 min. 10 sec. 38% 38%
600 6 min. 5 sec. 1. min. 12% 12%
600 11 min. 20 sec. 8 min. 50 sec. 17% 17%
600 16 min.» ..‘5 Sec. 13 min. 30 sec. 36% 36%
600 p 21 min. 40 sec. 18 min. 20 sec. 1% 1%
800 3038963 5 sec. 24% 2-3%
800 60' see; 10 sec. Some Some
800 90 sec. 15 sec. 2 ‘2

3
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(Table 1;;
Experimental Data For Isothermal Decomposition 0f

Retained Austenite In Oil Quenched 2340 Carburized Steel

, Percent Re-
3.33123th 2112 0°33?“ $313333; mm 32.3;

400 1;- hr. --- 52% 52%
450 20 min. --- 47% 1.7%
450 30 min. --—- 1.0% 40%
450 50 min. —- 45% 45%
450 60 min. -- 41% 41%
1.50 120 min. --- 45% 45%
450 150 min. -—- 36% 30%
450 180 min. -- 36% 20%
L50 240 min. -- 36% 15%
1.50 270 min. --- 30% 10%
1.50 300 min. ~- 28% 340%
600 1 min. 15 sec. 50 sec. 65% 65%.
600 2 min. 20 sec. 1 min. 30 sec. 45% 45%
600 3 min. 55 sec. 2 min. 40 sec. 55% 55%
600 4 min. 50 sec. 3 min. 10 sec. 33% 38%
600 6 min. 5 sec. 4 min. 42% 42%
600 11 min. 20 sec. 9 min. 20 sec. 25% 25%
600 21 min. 40 sec. 18 min. 45 sec. 22% 10%
600 32 min. 40 sec. 29 min. 20 sec. 3’5%. 3-5%
800 30 sec. 5 sec. 14% 14%
800 60 sec. 10 sec. 3-5% ?

55

Retained Austenite in Oil Quenched SAE 4640 Carburized Steel

Tampering
Temperature

oFagrgggeit

350
450
450
450
450

°8°°8°§§§§§§§

339g;
1.5 hr.
20 min.
40 min.
60 min.
80 min.
110 min.
140 min.
170 min.
200 min.
260 min.
320 min.
380 min.
30 sec.
1 min.

3 min.

6 min.
12 min.
112 min.
30 sec.

60 sec.

' Table 13.

45 sec.

45 Sec.

Corrected
Time

~
--
..-
..-

5 sec.
55 sec.
1 min.

5 min.

9 min.
109 min.
5 sec.

10 sec.

56>

Experimental Data For Isothermal Decomposition Of

Percent Re—
tained Austenite Overall
in Picture 'Eggggnt
39% 39%
16% 16%
17% 17%
9% 9%
7% 7%
5% 5%
5% 3-5%
5% 3-5%
5% 3-5%
5% 3—5%
5% 3-%
Salt and Pepper Effect
29% 29%
17% 17%
45 sec. 9% 7-9%
5 sec. 17% 7-8%
55 sec. 3-5% 1—2%
30 sec. Salt and Pepper Effect
5-6% 1-2%
Salt and Pepper Effect

Tablellz

Experimental Data For Isothermal Decomposition Of

Retained Austenite in Water Quenched SAE 2340 Carburized Steel

 

Tampering Percent re-
gemperature Corrected tained Austenite Overall
Fahrenheit 115; Time 1g Picgye m
400 1.5hr. ~— 48% 48%
450 20 min. -- 40% 40%
450 30min. —-- 43% 43%
450 50 min. -- 31% 31%
450 60 min. ~— 35% 35%
150 120 min. --- 40% 1.0%
450 150 min. -- 30% 30%
1.50 180 min. --- 33% 33%
450 21.0 min. --- 23% 23%
450 270 min. -—- 22% 16~12%
450 300 min. -- 3-10% 3-10%
600 l min. 15 sec. 50 sec. 30% 30%
600 2 min. 20 sec. 1 min. 30 sec. 30% 30%
600 3 min. 35 sec. 2 min. 20 sec. 24% 24%
600 4 min. 50 sec. 3 min. 10 sec. 25% ~ 25%
600 6 min. 5 sec. 4 min. 13% 13%
600 11 min. 20 sec. 8 min. 50 sec. 22% 3-5%
600 21 min. 40 sec. 18 min. 40 sec. 1~3% 1-3%
600 32 min. 40 sec. 29 min. 20 sec. 1-3% 1-3%
800 30 sec. 5 sec. 10% 10%
800 60 sec. 10 sec. Trace Trace

57

Igble 14,

Experimental Data for Isothermal Decomposition of

Retained Austenite in Oil Quenched SAE 2015 Carburized Steel

Tampering
Tenperature

Eissssagsah

Jase.
1.5 hr.

20 min.
40 min.
120 min.
200 min.
1 min.
2 min.
3 min.
5 min.
47 min.
Boasec.

60 sec.

Corrected
Tine

15 sec. 50 sec.
30 sec. 1 min.
45 sec. 2 min.
3 min.
44 min.
5 sec.

10 sec.

58

Percent Re-

tained Austenite Overall

13 P;gture
28%
15%
10%
3%
1-3%
30%
40 sec. 14%
30 sec. 13%
20 sec. 5%
27 sec. -~

17%

EENBQE

28%
15%
10%
8%
1-3%
30%
14%
13%
1-5%

10-17%

Salt and Pepper effect

Experimental Data For Isothermal Decomposition Of

Retained Austenite in Oil Quenched SAE 1010 Carburized Steel

Tampering
Temperature
oiggggggeig

400

450

450

450

450

50

;~

§§§§§§i§§

.1131
1.5 hr.

15 min.
35 min.
85 min.
95 min.

145 mini.

Table‘li

Corrected

Time

“-

l min. 50 sec. 50 sec.

3 nine 45 Sec.

5 min.
27 min.
67 min.
30 sec.
60 sec.

90lsec.

2 min;
3 nine
24 min.
5 sec.
10 sec.

15 sec.

Percent Re-
tained Austenite
in Picture

30 sec.
20 sec.

55 sec.

19%
17%
15%
3-5%
3-5%
Trace
4~5%
7-8%
2

11%

Trace

Overall
Percent

19%
17%
15%
3—5%
3-5%
Trace
4~5%
3-5%

?

3—5%
Trace

Salt and Pepper Effect

 

Experimental Data For Isothermal Decomposition Of

Table Jhé

Retained Austenite In water Quenched SAE 1010 Carburized Steel

Tempering
geeperature

Lahggggeig

§§§§§§§§§§§§§§

Corrected
13251 Time
1.5 hr. -—
15 min. --
35 min. -—-
85 min. ---
95 min. --
145 min. -—

1 mine 15 ”Ce 50 8630

3 min 45 sec. 2 min.

5 min. 3 min.
27 min. ' 24 min.
67 min. 15 BBC. 63 min.
30 sec. 5 sec.
60 sec. 10 sec.
90 sec. 15 sec.

60

Percent Re-
tained Austenite

in Picture

30 sec.
20 sec.
55 sec.

42 BGCe

15%
16%
11%
10%
10%
1—3%
5-6%
3-5%
4%
1%
Trace
20%
Trace

Overall
Percent

15%
16%
11%
3-5%
3-5%
1%
5-6%
3-5%
1.4%
1%
Trace
10%

Trace

Salt and Pepper Effect

Retained Austenite In water Quenched SAE 1010 Carburized Steel

Tablgllé,

Tempering
gelperature '- Corrected
W 11:19. .1112...
400 1.5 hr. --
450 15 min. --—
450 35 min. ~-
450 85 ltn. ---
450 95 min. --
450 145 min. ...
600 l min. 15 sec. 50 sec.
‘ 600 3 min 45 sec. 2 min.
600 5 min. 3 min.
600 27 min. 24 min.
600' 67 min. 15 sec. 63 min.
800 30 sec. 5 sec.
800 60 sec. 10 sec.
800 90 sec. 15 sec.

0C)

Experimental Data For Isothermal Decomposition Of

Percent Re-
tained Austenite

in Picture

30 BGCe
20 sec.
55 sec.

42 sec.

15%
16%
11%
10%
10%
1-3%
5-6%
3-5%
4%
1%
Trace
20%
Trace

Overall
Percent

15%
16%
11%
3«5%
3-5%
1%
5-6%
3-5%
1.4%
1%
Trace
10%

Trace

Salt and Pepper Effect

qt-

 

FIGURE9
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TABLE 11

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O,» V

$157

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69

Discussion
Sons of the problems which were encountered have already

been discussed lightLy in the procedure. Etching in itself was
one of the big factors and was seriously affected by the temper-
ing temperature and time. Figure 24 shows a 2015 structure etch-
ed short and is contrasted with a 2015 etched long as in figure 25.
It is apparent here that the etch needs to be standardized if possible.
‘usually from 5 to 10 seconds was long enough in lost cases, but at
600°? it was apparent that the same structure could be etched to
7 three different results. Here again the etching time had to be
increased slightly over the tine used at 450°F. the above facts
would seem to show why everything was double etched before pictures
were recorded.

The lethod of point counting has certain limitations. The
person who is taking the pictures is the biggest variable and
he lust be careful to choose representative sections. If at all
possible more accurate results could be obtained if the same area
was photographed continuously and more than one picture recorded
near that area to give a better average. Also small percentages
are hard to count and do not give reliable results. However the
method is far above visual estimation and would be very good in
structures where the constitutents do not vary; With this in mind
it is not hard to see that the variable carbon case present in

this problem conplicated things no end.

70

Even though very good checks were obtained on the carbon
train the results indicate that there might have been some
contamination of some samples. This could be due to a slight
amount of oil or other carbonaceous material in the high results
while incomplete combustion could account for the low results.
however this last factors was held to a minimum by a three hour
preheat period.with oxygen slowly bubbling through the system
and then checked against ASTM standards. One other problem was
that of plotting the results. the percent carbon present was over
an interval of distance and thus was plotted against the mid point
of each cut. This yields a curve which is more representative of
the actual carbon gradient..

The plot of the isothermal curves should not be taken as
absolutely true as all points were obtained from only one set of
results. Even though the same area was apparently photographed
over and over in.the SA! 1010 steel; the pictures which were record-
ed gave no indication of likeness. At 1500 diameter one millimeter
makes quite a difference in the picture obtained and consequently
a change in results.

One of the big factors in doing the experimental work was
control of carbon and alloy content of the various steels. Since
it was impossible to control any of the alloys except carbon and
carbon was only controlled between limits it is difficult to draw

valid conclussions.

In all of the steels the amount of retained austenite varied directly
with the carbon content and showed fairly homogeneous areas of austenite
whenever the percent of carbon present was close to the eutectoid carbon
content or higher. At lower percentages it was Spotty depenCing upon
alloy segregation and varied in persistence to tempering along these same
lines. For instance some of the longer tenpering treatments showed
groupings of the austenite, Where the amount present in the photonicro-
graph taken was nearly equal to the original structure. This was esocc-
ially true in SAE 5145 and 2540. The plain carbon steel decomposed
faster than the other steels, but did however Show areas where the austen—
ite was slow to break up. It s>ens that as the austenite decomposes (a)
the martensite needles develop little warts and grow wider and (b) at

the same time there is a divisionirg of the austenite present with fine
needles develOping and slowly spreading out (figure 19, 21, 22). Figure
18 shows the structure just after darkening of the martensite needles and

.15

note how sharp and clear they are in contrast to libure 20 where the needles
seem to shade and contain a lighter etching area. Figure 19 shows the

start of divisioning and it continues on and at about 500 minutes at 450
deg. F. in both SAE 2540 and 5145 the lines have developed into needles

and the austenite seems to be quartered away. Now in figure 25 you have

the end result of all the different heat treatments thich yields nearly

a perfect salt and pepper effect for 550 min. at 450°F. for the SAE 5145
carburized steel. To reach this last step in 800°F. tempering it is

only a matter of seconds and in GOOOF. it taxes only a few minutes to

yield this similar structure.

7'2

Figure 18
2340,

011 Quenched
1.5 hr. 450%.

Figure 21

2340,

Oil Quenched
300 min. 450%.

Figure 24

2015,

Oil Quenched
100 min. 450 0?.

Figure 19
2340,

Oil Quenched
60 min. 450°1I'.

Figure 22
3145,
Oil Quenched

320 min. 450°F.

75

Figure 20
2340,

Oil Quenched
105 min. 1.50%.

Figure 23
3145,
Oil Quenched

530 min. 450°F.

Figure 25
2015,

Oil Quenched
80 min. 450°F.

 

Time is very in ortant and one second at 8009F. or one minute at

P "a

BOOOF. is as good a retained austenite decomposer as 40 to 60 minutes at
45OOF. It should be noted here that just heating through the higher
temperature ra ges tends to rapidly decompose the retained austenite.
hhether this decomposition is a matter of stress relief or carbon move-
ment is beyond the scope of this paper. However either would be possible
in the range of temperatures used.

Perhaps this paper vould give further evidence to the support of many
old and recent investigators that austenite seems to be sensitive to
decomposition at about 450°F. and increas s in Speed of decomposition
for the next 400°. However on interrupted quenching to determine S—curves
some investigators found austenite to stablize itself at 800°F. and they
developed neuer C-curves showing this. Retained austenite indicated none
of this stability at ggOOF. In fact the SAE 2015 and 4640 carburized
Specimens indicated some decomposition at 400°F. and a loner honing
temperature for the martensite was used.

The 1010 carburized Specimens were checked for the effect of the
400° treatment in darkening the<7L —martensite and indicated no effect
whatsoever as the end decompositions came at exactly the sane time.

Just why the 2015 and 4640 should be more sensitive to 400°F. is hard
to say, unless it be from an alloy standpoint. But then, he 1010 would
also be affected in the same way. The 4640 does yield a loner carbon

case upon examination and carbon seems to be the most powerful influence

present in the alloy field in this problem.

Conclusions

 

The amount of retained austenite varies directly vith the carbon
content, higher carbon yielding more retained austenite, when the
other factors have remained cons ant.

Oil quenched steels give slightly more even distributions of retained
austenite. however, the total percentage difference is not great.
The three tempering emperatures 450°F., 6009F., and SOOOF. all yield
the same end result if tempering times are long enough at the lower
temperatures.

The retained austenite seems to decompose upon going through a range
of temperatures as well as decomposing at temperature.

High all y steels retained the most austenite upon quenching.

'»r alloy steels.

U)
Id.
[1
(+
9
5.)
(‘1'
He
:5
(‘1'
:fi
(D
p.
0
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(3

The retained austenite is more per
The point counting method can be readily applied and is more accurate
than visual estimation.

he isothermal diagrams should not be accepted as absolutely true.

‘7

Future Work

This problem presents many possibilities for future nork:

A.

Develop a method to center Specimens so that the same area can
be photographed continuously.

Use smaller specimens so that more accurate estimates can be
made of time at temperature for the 6OOQF. and BOOOF. temper—
ing temperature.

Use specimens that are carburized all the way through; thus,

technique more valuable.

Treat samples from higher temperature to coarsen structure.

This would then allow a smaller magnification, better ability

to stay in the same area, and more representative percentages by
point counting.

DevelOp a method to give data showing the rate of cooling.

Shorten the increments of time in the high temperature treatments.

70

l.

10.

11.

Selected References

Dowdel, R. L. and Harder, O. 3., "The Decomposition of the Austenitic
Structure in Steels", Transaction for Steel Treating, 1927, Vol. II
April, P. 585.

Meh1,R. F. and Roberts, G. A., "Hechanism and Rate of Formation of
Austenite from Ferrite-Cementite Aggregates: Transactions ASH, V01.
51, 1945, P. 615.

Digges, T. G. and Rosenberg, S. J., "metallographic Study of Forma-
tion of Austenite in .5% Carbon Alloys", Transactions ASH, V01. 51, 1945.
Greninger, A. B. and Troiana, A. R., "Kinetics of Austenite to
hartensite Transfer in Steel," Transactions ASH, Vol. 50, 1942.

Mehl, R. F. and Roberts, G. A., "Effect of Inhomogeneity in Austenite
on Rate of.Austenite—Pear1ite Reactions in Plain Carbon Steels”,
Transactions AIME, Vol. 154, 1945.

Gardner, F. S., Cohen, M., and Antia, D.P., "Quantitative Determination
of Retained Austenite by X-rays", Transactions AILE, Vol. 154, 1945.
Gordon, P. and Cohen, M., "Transformation of Retained Austenite in

High Speed Steels at Sub-AtmOSpheric Temperatures", Transactions ASH,
Vol. 50, 1942.

Lyman, T. and Troiano, A.R., "Influence of C Content upon Transforma—
tion in 5% Cr Alloys, Transactions ASH, Vol. 57, 1946.

Klier, E.P. and Troiano, A. R., "Ar in Cr Alloy Steels," Tech. Pub.

No. 1799, AIKE, Ketals Tech., Feb. ‘45.

Saveur, A., "Metallography and Heat Treatment of Iron and Steel,

1955, McGraneRill Book Company.

Austin, J. B. and Ricket, R. L., "Kinetics of Decomposition of Austénite

at Constant Temperature", Transactions AIEE, Vol. 155, 1959.

'7?

12.

15.

15.

16.

17.

18.

19.

20.

21.

24.

25.

Savarine, 1.11., "Phase Char ures in 5.5% 111 Steel in the A01 Region",
Transactions AIEE, Vol. 155, 959.

Wells, C. d Lb ehl, R. F., "Rate of Diffusion of Carbon in Austenite,"
Transactions A “E,Vol.140, 1940.

Greninger, A. B. and Troiano, A. R., "Crystallography of Austenite
Decomposition", Transactions AIEE, Vol. 145, 1940.

Greninger, A. B. and Troiano, A. R., "mechanism of Martensite Formation",
Transactions AIHE, Vol. 145, 1940. 1

Gordon, P. and Cohen, M., "Retained Austenite in High Speed Steels,
Transactions ASH, Vol. 50, 1942.

mirkin, I. L. and Spektor, A. 6., "Transformation of Residual Austen-

ite during Dr atring", met illurgy 12 No. 8, 9-14, 1957 (Russian).

3

lier, E. P. and Troiano, A. R., "Ar in Cr S eels", Metals Tech.,

Feb. 1945.

Lyman, T. and Troiano, A. R., "Isothermal Transformation of Austenite
in One Percent Carbon High Cr Steels", Ietals Tech., Feb. 1945.
Gordon, P., Cohen, R., Rose, R. S., "The Kinetics of Austenite
Decomposition in High Speed Steel, Trans .ASM, Vol. 51, 1945.

Smith, G. V., and Mehl, R. F., "Lattice Relationships in Decomposition
of Austenite to Pearlite, Bainite and Martensite," metals Tech., April
1942.

Bain, E. C., "Alloying Elements in Steel", A'H, Cleveland, Ohio, 1959.
Howard, R. T. and Cohen, h., "Quantitative m etallography by Point-
Counting and Lineal Analysis", Metals Tech., August 1947.

Rote, C. B., Truckenmiller, W. C., and Wood, R. P., "Electrical
Resistance Method for Determining of Isothermal Austenite Transfor-
mation", ASH Preprint #46, Oct. 20, 1941.

Zimeskal, 0. and Cohen, R., "Simultaneous Measurements of Magnetic

and Dilatometrical Change, Rev. Sci. Instruments, Vol. 15, 1942.

'71}

26. Flectcher, S. G. and Cohen, R., "The Effect of Carbon on Tempering
of Steel," Transactions ASH, 1944.

'27. Davenport, E. S. and Bain, E. C., "Transformation of Austenite at
Constant Sub—Critical Temperatures", Transactions AIIL, I and S
Division, 1950.

28. Davenport, E. S., "Isothermal Transformation in Steel", Transactions
ASE, Vol. 27, 1959.

29. Allen, N. P., Pfeil, L. B., and Griffths, W. T., "The Determination
of the Transformation Characteristic of Alloy Steel", British
Iron and Steel Institute, Sp. Report, No. 24, 1959, p. 569-590.

50. Epstein, Samuel, "The Alloys of Iron and Carbon", Vol. I, 1956 Ed.
McGraw—Hill Book Company.

51. Zmeskal, O. and Cohen, M., "The Tempering of Two High—Carbon, High-
Chromium Steels", Transactions ASH, 1945, Vol. 51.

52. Antia, D. P., Fletcher, S. C., and Cohen, R., "Structural Changes
during he Tempering of High—Carbon Steel", ASH, Vol. 52, 1944.

55. Van Vleet, H. S. and Upthegrove, C., "Decomposition of Austenite",
Metal Progress, 1950, Vol. 18, October, p. 68—70.

54. Chevanard, P., Portevin, A., "Mechanism of Tempering Martensite",

Comptes Rendus 191, 1059—82, 1950.

79

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