The tempering of a low-carbon internally twinned martensite

THE TEMPERING

OF A LOAM-CAR3ON

INTERNALLY

TWINNED

MARTENSITE* C. J. BARTON? The first stage of tempering of high-carbon martensites results in the precipitation of the metastable epsilon carbide; in low-carbon steels, it is generally considered that epsilon carbide does not form during tempering and only cementito is observed. Since high-carbon steels have an internally twinned martensite morphology, whereas low-carbon martensite (with the exception of high-nickel steels) consists of laths, it is not clear whether the first stage of tempering is controlled by carbon content or by martensite substructure. Therefore, a study was made of the tempering chara~~risti~s of a low-carbon steel in The steel chosen for this investigation contained 28 per which the martensite was internally twinned. cent nickel and 0.1 per cent carbon. Dark-field transmission electron microscopy revealed the presence of carbide precipitates in specimens that had been tempered for 48 hr at 100°C. Selected-area diffraction established that these carbides were the h.c.p. epsilon carbide. This indicates that the first stage of tempering of martensites containing carbon may be more directly related to the martensite morphology than to the carbon content alone. RJEVESU

D’USE

MARTENSITE

A

_MACLES

INTERNES

CONTENANT

PEU

DE

CARBONE LA premier stade de revenu des martensit,es b forte concentration en carbone produit Ia precipitation du carbure epsilon metast.able; dans le aciers 1 faible concentration en carbone on considere gen&alement que Ie carbure epsilon ne se forme pas pendant le revenu, et on observe seulement la cementite. Eta& don& que les aciers it forte concentration en carbone ont une morphologie martensit.ique It maoles imernes, alors que la martensite a faibb concentration en carbone (a l’exception des aoiers $. forte concentration en nickel) se pr&ente SOW forme de rubans, on ne sait pas si le premier stade du revenu est contr61B par la concentration en carbone ou par la sous-structure mart,ensitique. Aussi, une etude des caracteristiques du revenu d’un acier a faible concentration en carbone dam lequel la martensite etait m&cl&e interieurement, a Bte effect&e. L’acier choisi pour cette Ptude contenait 28 pour cent de nickel et 0,l pour cent de carbone. La microscopic Blectronique par transmission a fond noir rtivele la presence de pri?oipit,es de oarbure dans les echantillons revenus a lODoG pendant 48 heures. La diffraction sur des surfaces convonablement choisies montre que ces carbures sont ies carbones epsilon h.c. Ceci indique que le premier stade de revenu des martensites eont,enant, du carbone peut etre relic plus directement, a la morphologic de la marbrtensite qu’a la concentration en carbone uniquement. TEMPERS

VON

KOHLE~STOFFARME~~

MARTENSIT

MIT

INNER,N

ZWILLINGEN

Die erste Temperstufe von Martensiten mit hohem Kohlenstoffgehalt fiihrt zur Ausscheidung metastabiler Epsilonkarbide; es wird allgemein angenommen, dalj sich in kohlenstoffarmen Stahlen wiihrend des Temperns kein Epsilonkarbid bildet und es wird nur Zementit beobachtet. Da kohlenstoffreiche Stiihle eine Martensit-Morphologie mit inneren Zwillingen haben, kohlenstoffarme Martensite (mit Ausnahme von niokelreichrm Stahl) jedoch aus Latten bestehen, ist nicht klar, ob die erste Temperstufe vom Kohlenstoffgehalt oder von der Martensitsubst,ruktur bestimmt wird. Deshalb wurde das Temperverhalten eines kohlenstoffarmen Stahls untersucht, in dem der Martensit innere Zwillinge enthielt. Der fiir diese Untersuchung beniitztc Stahl enthielt 28 % Nickel und 0,l % Kohlenstoff. Dunkelfeld-Elektronenmikroskopie zeigte Karbidausscheidungen in Proben, die 48h bei 100°C getempert worden waren. Die Feinbereichsbeugung ergab, da8 diese Karbide hexagonale Epsilonkarbide waren. Das deutet darauf hin, da9 die erste Temperstufe von kohlenstoff~ltigen Marten&en direkter mit der Morphologic des Martensits als mit, dem Kohlenstoffgehalt e&in zu~menh~n~ en konnte.

INTRODUCTION

The tempering received subject,

behavior

considerable of

many

tempering,

was

attention

research

A theory of tempering,

time.

of ferrous martensites and

in particular

developed

has

and review by

been

uous process resulting in the simultaneous

eo-

workers(1-4), in the 1950’s; however, no theoretical approach has been applied to the problem since that * Received October 4, 1968; revised December 16, 1968. t United States Steel Corporation, Applied Research Laboratory, Monroeville, Pennsylvania 15146. Now at: Research and Development Laboratory, Chase Brass and Copper Company, University Circle Research Center, Cleveland, Ohio 44106. ACTA 11

METALLURGICA,

VOL.

17, AUGUST

1969

is that the

the

articles.(l-s) and

aspect of this theory

first stage of tempering of epsilon carbide

the first stage of Cohen

A significant

has

carbon),

of martensite

and low-carbon

is a discontinformation

martensite

and that these two products

(0.25%

cannot

form

independently of each other. A corollary of this statement is t,hat epsilon carbide cannot form during the tempering of a martensite containing 0.25 per cent carbon or less. Since that time, the presence of epsilon carbide has been reported in tempered steels containing less than 0.2 per cent carbon.@~rO) Also, some work has been published on the structural aspects of the first stage 1085

ACTA

1086

METALLURGICA,

of tempering in high-carbon martensites.(11-15) The work of Tekin and Kelly(ll) has shown that the lowtemperature tempering (below about 200°C) of an iron-20% nickel-0.8°/0 carbon martensite results in the precipitation of epsilon carbide across the transformation twins in individual martensite plates. Epsilon carbide was also observed in areas that appeared to be nntwinned. In the present investigation, low-~mperature tempering was performed on a 28% nickel, 0.1% carbon steel for various times at 100°C and 15O’C. This steel was chosen because, although it has an internally twinned morphology, the carbon content is far below 0.25 per cent. Tra~mission electron microscopy was used to follow the tempering reaction. MATERIALS

AND

EXPERIMENTAL

WORK

The composition of the steel used in this investigation is shown in Table 1. Specimens from &-in.thick sheet were austenitized in molten salt for 10 min at 850°C. Quenching consisted of rapidly transferring the specimens into an iced brine bath, where they were held for 24 set, and then immersing them in liquid nitrogen. The specimens were then stored in liquid nitrogen, and were removed only for tempering heat treatments or for metallographi~ preparation and examination. Transmission electron microscopy was performed on as-quenched specimens as well as on specimens tempered for various times at 100°C and 150°C. Because of the possibility of unintentional tempering at comparatively low ~mperature~, care was taken to avoid heating the specimens during metallographic preparation, especially those in the as-quenched condition. Surface grinding was done on a Sanford SGl surface grinder with a wheel (Robertson WA543FlOVC) selected for its ability to minimize specimen heating. Flood cooling was also employed, and sheets approximately 0.006 in. thick were produced by grinding from both sides. Finish cuts were made at a depth of less than 0.0005 in. and the width of cut was less than 0.005 in. Preparation time on each specimen (time out of liquid nitrogen) was kept to a minimum. Foils were prepared from disk specimens that were electrolytically thinned in a jet polishing aparatus(l*) with a chromic-acetic electrolyte.05) Polishing was stopped as soon as the disk perforated, and the foil TABLE

VOL.

17,

1969

was rinsed in glacial acetic acid followed by methyl alcohol. The specimens were examined in either a Siemens la or a JE&l7 electron microscope at 100 kV. Selected-area diffraction patterns were analyzed with the use of an EFFA measuring device, and the carbide diffraction patterns were indexed through the use of LabIes of d-spacings and angles between crystallographic planes calculated for noncubic crystal systems.(16j RESULTS

As-quenched

AND

martensite

The composition iron-28% nickel-O.1 % carbon has an H, ~m~rature, as calculated by the equation developed by Andrews,07) of approximately - 10°C. Since the M, temperature was only slightly above the iced brine temperature, the quenching procedure included a quench in liquid nitrogen. In order to minimize any differences in auto-tempering or austenite thermal stabilization between specimens, it was established that the procedure should be to quench in iced brine, and then transfer the specimens as quiokly as possible into liquid nitrogen. When the &-in.thick specimens were held in the iced brine for 2-4 set (a procedure that, was found to be easily repr~u~ible), the amount of retained austenite, determined by X-ray techniques, was consistently between 11 and 14 volume per cent. Very high retained austenite values resulted, however, if the specimen transfer from iced brine to liquid nitrogen was delayed. For example, a delay time of several minutes resulted in a high degree of austenite thermal stabilization and in a concomitant retained austenite value of 36 volume per cent. Such thermal stabilization has been reported in high-carbon, plain carbon and in high-carbon, iron-nickel-carbon steels.(ls*is) The present observations, however, indicate that this steel is much more sensitive to austenite stabilization than would be predicted by the earlier work. Transmission electron microscopy revealed that the martensite produced in this steel was entirely that of the internally twinned mo~hology,(~-~’ as opposed to the lath martensite observed in plaincarbon, low-carbon steels,(21*N) and in high alloy steels.(Z1~25’Figure 1A shows a representative area of a martensite plate in an as-quenched specimen; fine twins, about 50 A in thickness, are seen consistently

1. Chemical composition, wt. %

C

Mn

P

s

Si

xi

Cr

0.11

<0.005

0.001

0.004

0.21

28.00


MO <0.01

Al*

N

Co

0.034

0.001

0.02

._ * Soluble

DISCUSSION

O(PPm) 11

BARTON:

TEMPERING

OF

IXTERNALLY

in the plates. The selected-area diffraction pattern from the martensite plate shown in Fig. IA is presented together with its solution, in Fig. lB, and a matrix dark-field micrograph showing the twins in strong contrast is shown in Fig. 1C. The character of the transformation twins is clearly revealed in Fig. 2. The specimen has been oriented so as to present the twin-matrix interface to the incident beam, rather than viewing the twins on-edge as was illustrated in Fig. 1. Note that some transformation twins are continuous across the martensite plate boundary (labeled A-A). It has been reported that the martensite plates are frequently not twinned across their entire width,(s”) a behavior that wa8 often observed in the present investigation. Sometimes, however, the twins did extend across the entire plate and end in the plate boundary. The continuity of twins at the boundary of two plates as shown in Fig. 2, however, implies that a cooperative intera&ion of the

TWINTU’ED

MARTENSITE

1087

inhomogeneous shear systems is possible between adjacent martensite plates. The dark-field micrographs shown in Fig. 3, taken from the same area as Fig. 2, show irregular fringes at the twin-matrix interfaces that are attributed to a semicoherent boundary condition. These boundaries, therefore, are slightly higher in energy than a perfect twin-matrix interface, and contain a network of dislocations that were probably int~rodueedduring the transformation. Finally, in a.11the observations of the as-quenched martensite, no indication of the presence of a carbide phase was detected either in the transmission images or in selected-area diffraction patterns, Tempered structures Specimens tempered at 100°C for short times, for about ten min, had structures that were unchanged from those of the as-quenched martensite. No

Frc. 1. (A) Bright field of martensite plate showing fine scale internal twins. (B) Selected area diffraction pattern of twinned regions and analysis. (C) Matrix dark field from area above showing twins in dark contrast.

Frc.

Pm.

2. Bright field of internally twinned martensite with specimen tilted to present twin-matrix interface to the incident beam. Insert shows continuity of tramformation twins wxoss martensite p&xl boundary.

3. Dark field of the area shown in Fig. 2. Note fringes at twin-matrix coherent boundary. 1088

interface

indicating

s semi.

BARTOK:

TEMPERING

OF

(A)

I

r

INTERNALLY

TWINNED

IP

7

MARTESSITE

1089

I

FIG. 4. Bright field (A) area showing twins almost out of contrast and faint indications of precipitate particles. Dark field (B) of precipitate which was idmtified as epsilon carbide; tempered for 48 hr at 100°C.

0

0

X E - CARBIDE, 0 FIG. 5. Selected area diffraction

MATRIX,

pattern of area shown in Fig. 4 and analysis. surface oxide, and T is a twin reflection.

[I Is]

[OIO]

ZONE

ZONE

Reflections

labelled 0 are from

ACTA

1090

METALLURGICA,

VOL.

17,

1969

ii0

Zl .

I

/

/

OEI

.

023 a -._

l

To2

265 103

iI4_T_

Z!4

223

521

oil

\

__. .

' ill

-

012

I ;b

-

't4 Jo5

0025 _ _ 0.013

0014

E25i!2 .

l 216

706

Tl4.-• . II3 'ii5 $6 $4

O?l

230

r?l

i.23 $2

igl

i20

015 '0.6 .I05 *lo4

.

*IO3 a205

.

2pl

,014 0'3 0025 0012 .ll6 0023 II4 mnll .-. .

i3l .

l

l

.

021 .

.I02 24

r

II2 .

293

ii0

i30

IfI

3pl

201 .

3il .

3EO 2io

\

FIG. 6. Standard projection of epsilon carbide, a = 2.763, c = 4.355.

evidence

of carbide was obtained

graphs or in the selected-area After

the specimens

lOO”C, selected-area very

low

reflections

intensity

were tempered

diffraction from

were in poorly

and a satisfactory

either in the micro-

diffraction

patterns. for 24 hr at

revealed reflections

a second

phase.

defined patterns,

analysis

of

These

however,

could not be made.

across

the

directions.

The

twins

the reflections these

and

perpendicular

Selected-area from

reflections,

diffraction

a second dark-field

to

their

long

patterns revealed

phase.

By

micrographs

imaging of

the

second phase were obtained

as shown in Figure 4B.

The selected-area

patterns

diffraction

were analyzed

with the use of the computer program prepared for the calculation

of d-spacings and angles between crystallo-

second phase could not be clearly imaged in transmission by either bright-field or dark-field techniques,

graphic

although

second phase detected in specimens tempered for 48 hr

some faint indications

could occasionally that

tempering

occurrence

be resolved.

of very fine particles This evidence indicates

for 24 hr at 100°C resulted

of the initial

stages

which the particles formed resolution of the microscope.

in the

of precipitation

were

at the

edge

in of

After tempering for 48 hr at lOO”C, a general precipitation occurred throughout the steel. Figure 4A is a bright-field micrograph of an area where the twins are nearly out of contrast. Very faint indications of rod-like precipitate particles can be seen running

planes

for

non-cubic

at 100°C was determined diffraction

pattern

taken

systems,o@

and

to be epsilon carbide. from

the

area shown

the The in

Fig. 4A and its analysis are shown in Fig. 5. The matrix zone in this pattern is a [ 112],, and is tilted off a [lli], orientation. The epsilon carbide zone was [OIO],, and the dark-field micrograph in Fig. 4B was from the (103), reflection. The diffraction patterns thus obtained were checked against, and found to be consistent with, the orientation relationship

first specified by Jack,@‘) then later

BARTON:

TEMPERING

OF

INTERNALLY

TWINNED

MARTENSITE

i!o

iA1

010 .

011 . I!0 O?l

III .

FIG. 7. Matrix projection which is parallel to the epsilon carbide standard projection in the Jack, Pitsch and Schrader orientation relationshipfP7~**).

by Pitsch and Schrader,cz8) and Arbuzov and Khayenko, tzg) for epsilon carbide precipitation in steels. This orientation relationship was prepared in stereographic projection for use in checking the diffraction-pattern analyses. The standard projection of epsilon carbide, calculated by using the lattice parameters a = 2.763A and c = 4.35511, is shown in Fig. 6; the matrix projection, which superimposed on the epsilon carbide standard projection yields the Jack, Pitsch and Schrader orientation relationship, is shown in Fig. 7. As mentioned in the description of the martensite morphology of this steel, the fine-scale twinning usually does not extend across the full width of the martensite plate. Epsilon carbide was also observed in these untwinned areas after tempering for 48 hr at 100°C. An example of this is shown in Fig. 8 which shows a bright field (A) of an untwinned region of a martensite plate, and a corresponding epsilon dark

field (B) from the same area. The particles appear to lie along (lOO), directions, but closer inspection reveals that they are composed of groups of rods, as shown in the insert in Fig. 8B. Even though epsilon carbide was observed in regions other than the twinned areas, this observation does not contradict the idea that the first stage of tempering depends on martensite morphology; it is the type of martensite which is important, not merely the presence or absence of twins in the substructure. After tempering for 48 hr at 150°C particles were observed that could not be indexed as epsilon carbide; Fig. 9. Analyses of d-spacings and angles between crystallographic planes obtained from selected-area diffraction patterns were consistent with the existence of cementite; however, Jack and Wild(m) demonstrated that diffraction evidence of this type would not distinguish between cementite and Hilgg carbide. Since 15O’C is usually considered to be low for the

ACTA

1092

METALLURGICA,

VOL.

Ii, 1969

‘P

i------------i FIa. 8. ISright field (A) of untwinned region and epsilon carbide dark field (B) of the same area. Tempered 48 hr st 100°C. Insert shows 8 x enlargement of circled area in dark field.

FIG. 9. Bright field showing cementite particles in a specimen tempered 48 hr at 15O*C.

for

BARTON:

precipitation cannot

of the

TEMPERJNG

stable

be dismissed

carbide,

OF

the

INTERNALLY

possibility

that these particles

are Hagg

carbide and not cementite. SUMMARY

In contradiction

to the earlier theories

ing, it has been demonstrated tempering

of temper-

that low-temperature

can lead to the precipitation

of epsilon

carbide in an internally twinned martensite

containing

less than 0.2 per cent carbon. Tempering

a 28%

nickel-O.1 y0 carbon

48 hr at 100°C resulted in a precipitation epsilon

Two separate types of sites were in the twinned areas of the martensite

plates

the epsilon

matrix

interfaces,

precipitation

carbide

(loo),

precipitated

in the twin-

and in the untwinned

occurred

epsilon carbide

wit,h groups

directions.

of

the

with the Jack, Pitsch

relationship.

The results suggest that the tempering istics of a martensite

regions the

of rods forming

The orientation

was consistent

and Schrader orientation

directly

for

carbide.

detected :

along

steel

of rod-like

containing

related to the morphology

than to the carbon content

character-

carbon may be more of the martensite

alone.

ACKNOWLEDGEMENTS

The author Dr.

J.

wishes to express

M. Chilton

W. W. Corbett

for

helpful

and R. F. Zamba

with the experimental

his appreciation discussions,

and

to to

for their assistance

work.

REFERENCES 1. C. S. ROBERTS, B. L. AVERBACH and M. COHEN, The mechanism and kinetics of the first stage of tempering. Trans. Am. See. Metals 45, 576 (1953). 2. B. S. LEMENT, B. L. AVERBACHand M. COHEN,Further study of microstructural changes on tempering ironcarbon alloys. Trans. Am. Sot. Metals 47, 291 (1955). 3. B. S. LEMENT, B. L. AVERBACH and M. COHEN Microstructural changes on tempering iron-carbon alloys. Trans. Am. Sot. Metals 46,851 (1954). 4. B. S. LEMENT and M. COHEN, A dislocation attraction model for the first stage of tempering. Acta Met. 4, 469 (1956). 5. M. P. ARB~JZOVand B. V. KHAYENKO, X-ray diffraction analysis of the crystal structure of the carbide phase at different stages of steel tempering. Physics Metals Metdogr. 13,48 (1962). 6. G. V. KTJRDJUMOV, Phenomena occurring in the quenching and tempering of steels. J. Iron Steel Inst. 195, 26 (1960).

TWINSED

MARTENSITE

1093

7. V. KERLINS and C. ALTSTETTER,Kinetics of the initial stage of decomposition of low-M, iron-nickel-carbon martensites. Trans. Am. Inst. Min. Engrs 227, 94 (1963). 8. M. RON and H. SCHECHTER,Precioitation of iron carbides in tempered martensite. J. a~$. *Phys. 39,265 (1968). 9. S. MODIN, s-Karbidbildning vid anliipning av lagkolhaltig martensit. Jernkont. AnnZr 142, 209 (1958). 10. C. ALTSTETTER, X-my diffraction study of carbides formed during tempering of low alloy steel. Trans. Am. Inst. Min. Engrs 224, 394 (1962). 11. E. TEKIN and P. M. KELLY, Tempering of steel. Precipitation From Iron Base Alloys. Gordon & Breach (1965). 12. M. G. H. WELLS, An electron transmission study of the tempering of martensite in an Fe-Ni-C alloy. Acta Met. 12, 389 (1964). 13. V. N. GRIDNEVand Yu. N. PETROV,Mechanism of carbide formation on the tempering of martensite with a twin structure. Physics Metals MetaZZogr. 22, 55 (1966). 14. R. D. SCHOONEand E. A. FISCHIONE,Automatic unit for thinning transmission electron microscopy specimens of metals. Rev. Gent. Instrum. 37, 1351 (1966). 15. C. E. MORRIS, Electropolishing of steel in chrome-acetic acid electrolyte. Metals Progr. 56, 696 (1949). 16. J. M. CHILTON and C. J. BARTON, Identification of strengthening precipitates in 18Ni (250) aluminum, vanadium and titanium maraging steels. Trans. Am. Sot. Metals 60, 528 (1967). 17. K. W. ANDREWS, Empirical formulae for the calculation of some transformation temperatures. .J. Iron Steel Inst. 201, 721 (1965). 18. S. G. GLOVER,The characteristics of the thermal stabilization of austenite in high carbon steel. J. Iron Steel Inst. 198, 102 (1962). 19. B. EDMONDSON,Thermal stabilization of austenite in a 10% Ni, 1% C steel. Aeta Met. 5, 208 (1957). 20. P. M. KELLY and J. NUTTINQ, The martensite transformation in carbon steels. Proc. R. Sot. A250, 45 (1960). 21. P. M. KELLY and J. NUTTING,The morphology of martensite. J. Iron Steel Inst. 197, 199 (1961). 22. K. SHIMIZU, Direct observation of substructures of the martensite in Fe-S? alloy by means of electron microscopy. J. phys. Sot. Japan 17, 508 (1962). 23. K. SHIMIZU and Z. NISHIYAMA, Direct observation of substructures in martensite. Acta Met. 7, 432 (1959). 24. J. M. CHILTON, C. J. BARTOK and G. H. SPEICH, The martensite transformation in low alloy steels. J. Iron Steel Inst. in press. 25. P. M. KELLY, Martensitic transformation in high alloy steels conference proceedings: Metallurgical Developments in. High Alloy Steels. The Iron and Steel Institute, London (1964). 26. H. WARLI~ONT, On the martensite structure of an iron30.9 ‘A nickel alloy. Fifth International Congress for Electron Microscopy, p. HH6. Academic Press (1962). 27. K. H. JACK, Structural transformations in the tempering of high carbon martensitic steels. J. Iron Steel Inst. 100, 26 (1951). 28. W. PITSCHand A. SCHRADER,Die Ausscheidungsform des s-Karbids in Ferrit und im Martensit bein Anlassen. Arch. Eisen 29, 715 (1958). 29. M. P. ARBUZOVand B. V. KHA~ENKO, Orientation of the low-temperature carbide phase Fe,C. Physics Metals Metallogr. 13,128 (1962). 30. K. H. JACK and S. WILD, Nature of Z-carbide and its possible occurrence in steels. Nature, Lord. 212,248 (1966).