Effects of A ustenitizing Temperature and Austenite Grain Size on the Formation ofAthermal Martensite in an Iron-Nickel and an Iron-Nickel-Carbon Alloy

M. UMEMOTO AND W. S. OWEN

The effects of austeni t iztng conditions on the kinet ics at the s t a r t of mar tens i te formation in Fe-31 Ni and Fe-31 Ni-0.28 C al loys have been studied using e l e c t r i c a l - r e s i s t a n c e mea- surements during cooling of the specimens to follow the course of the t ransformat ion. The p r i m a r y object of the study was to decide whether or not a change in austeni t izing t e m p e r a - ture , in the absence of a change in austenite grain size, has any effect on the M s t e m p e r a - ture or the burs t cha r ac t e r i s t i c s of a thermal mar tens i te . It is concluded that it does not, suggesting that the potential nuclei (embryos) of mar tens i te a re mechanical ly s table c rys t a l defects . Another in teres t ing observat ion is that when the austeni te grain s ize is smal l , the Mb tempera tu re inc reases with increas ing grain s ize and the burs t is always smal l . When the austentte gra ins a r e coarse , the M b t empera tu re is independent of the grain s ize and the burs t ts la rge . It is suggested that this phenomenon is a resu l t of the e las t ic shea r s t r e s s concentrat ion being re la ted to the s ize of the f i r s t mar tens i te plate and, in turn, to the s ize of the austenite grain.

AMONGST the many ea r ly studies of the effect of au- s teni t izing t empera tu re and t ime on the M s t e m p e r a - ture of var ious s tee ls , there was a noticeable lack of agreement about the di rect ion of the change in M s t em- pera tu re with increas ing austenit tztng t empera tu re . 1 This confusion was due to the uncertainty introduced by the resolut ion of ca rb ides and other changes in the chemis t ry of the austenite during the austeni t iz ing t rea tment . Using al loys with an M s t empera tu re below 0~ and thus el iminat ing problems due to carb ide sepa- rat ion during quenching, Raghavan and Entwlsle 2 showed that the ra te of i so thermal formation of mar tens i te in- c r e a s e s as the grain s ize of the parent austeni te is in- c reased . However, Putt and Cohen s showed, by taking Into account the var ia t ion of plate s ize with austeni te grain s ize , that in these c i rcumstances the Initial ra te of nucleation is independent of the grain s ize and the austent t iz ing tempera ture . There is no s i m i l a r l y c l ea r understanding of the situation in s tee l s In which m a r - tensl te forms athervaally in a burs t . Only two studies a re known 4,s in which the t ransformat ion was ca r r i ed out at t empera tu re s below 273 K and the r e su l t s of these a r e conflicting.

Machlin and Cohen 4 concluded from studies of an Iron-29.5 pct nickel a l loy that " the amount of t r a n s - formed phase is independent of the austeni t iz ing t em- pera ture , gra in s ize being maintained cons tant . " In sharp contras t , Entwisle and Feeney 5 who studied the formation of a thermal mar tens i t e in a s e r i e s of 0.5 pct carbon al loys containing between 19 and 27 pct nickel,

expressed the opinion that " the austentt tzlng t rea tment is the dominating factor control l ing the s t a r t of t r a n s - fo rma t ion . " The study descr ibed here was designed to reso lve some of the problems presented by these d i s - crepant conclusions. The al loys and t rea tments were se lec ted to ensure that al l the t ransformat ions occur red below 0~ One a l loy was an I ron-nickel a l loy s i m i l a r to the al loy studied by Maehlln and Cohen and the other a l loy had the same nickel content wtth 0.28 pet carbon added.

In specimens whteh t r ans fo rm by burst ing, the a t h e r - mal mar tens t te burs t t empera tu re (Mb) coincides with the M s t empera tu re . The magnitude of the effect of au- s teni t iz lng t empera tu re on the M s (Mb) t empera tu re in specimens of the same grain s ize is an Important con- s idera t ion in the selection of a model to descr ibe the potential nuclei of mar tens t te (embryos) in the parent austentte. In the or iginal a thermal nucleation concept of F i sher , Hollomon and Turnbull, e the s ize d i s t r ibu- tion and densi ty of embryos was considered to be a function of the t empera tu re of the austenl te . In general , ff the lat t ice defects which consti tute the embryos a re in a s ta te of the rmal equi l ibr ium, or even if the densi ty and dis t r ibut ion of such defects is changed by a change in t empera tu re , then the Ms(Mb) t empera tu re would be expected to va ry with austentt izing t empera tu re as c la imed by Sas t r l and West ~ and by Entwtsle and Feeney. 5 If, however, the embryos a r e unaffected by

M. UMEMOTO, formerly a Graduate Student in the Department of Materials Science at Northwestern University, is now at the University of Illinois at Urbana. W. S. OWEN, formerly at Northwestern Uni- versity, is now Professor o f Materials Science at Massachusetts Institute of Technology, Cambridge, Massachusetts.

Manuscript submit ted September 2 I, 1973.

Table I. Chemi,'Jl Compositions of Alloys (wlt pet)

Ni C Mn Si S P Fe

Fe.3 INi 31.31 <0.01 0.023 0.02 0.002 0.002 bal Fe-31Ni-0,28C 30.70 0.28 <0.005 <0.01 0.016 0.002 bal

METALLURGICAL TRANSACTIONS VOLUME 5, SEPTEMBER 1974-2041

changes in t he rma l energy, as would b e the case if t h e embryos cons is ted of mechan ica l ly s table d is locat ion conf igura t ions , no effect of aus ten i t i z ing t e m p e r a t u r e on Ms(Mb) would be expected. This is the s i tua t ion in - dicated by the r e s u l t s of Machlin and Cohen. 4

EXPERIMENTAL PROCEDURES

The alloys used in this investigation were vacuum melted, cast into ingots and hot forged to sheets 10 -2 m thick. The compositions are given in Table I. All of the alloys were homogenized at 1473 K for 20 h before cold- rolling to sheets 5 x 10 "4 m thick. They were then cut into specimens 4 • 10 -sm by 4 x 10 -2 m by 5 • I0 -4 m,

mechanically and chemically polished and enclosed in evacuated silica capsules. In the first set of experi- ments each specimen was given a single austenitizing treatment at a selected temperature. In this set, both the grain size and the thermal history were different for each specimen. In the second and third sets double austenitizlng treatments were used. In one of these, set two, all specimens were first austenitized at 1473 K for one hour to establish the same grain size in each specimen. This was followed by a second austenitizing treatment at a selected temperature between 873 K and 1373 K for one hour. The grain size of the austenite is not affected by this treatment but, if the martensite em- bryos are responsive to change in the thermal condi- tions they will be affected differently in each specimen. Set three was the inverse of set two. A different grain size was produced in each specimen by first austenitiz- ing for one hour at a selected temperature between 973 K and 1473 K as in set one. The second treatment, designed to establish the same embryo distribution in each specimen, was carried out at 873 K for one hour. There was, of course, no change in the grain size dur- ing this final treatment. The object of this third set of expe r imen t s was to de t e rmine the M s (M b) t e m p e r a t u r e and re la ted kinet ic f ea tu res c h a r a c t e r i s t i c of the g ra in s ize alone by e l imina t ing any effects which might be due to t he rma l condi t ioning of the embryos .

The M s (Mb) t e m p e r a t u r e was m e a s u r e d by r e c o r d - ing the change in e l e c t r i c a l r e s i s t a n c e dur ing cont inu- ous cooling of the spec imen between room t e m p e r a t u r e and the t e m p e r a t u r e of a l iquid n i t rogen or l iquid he - l ium bath.

T h e var ia t ion of Ms(M b) t e m p e r a t u r e with the d i f fer - ent aus ten i t i z ing t r e a t m e n t s applied to the Fe-31 Ni a l - loy is shown in Fig. 3 and pa ra l l e l data for the Fe-31 Ni -0 .28C al loy is given in Fig. 4. The M s ( M b ) t e m p e r - a tu re i nc rea sed with i nc r ea s ing aus ten i t i z ing t e m p e r a - tu re applied in a s ingle step (set one), the r a t e of in - c r e a s e be ing g r e a t e r for the al loy containing carbon. For both a l loys , the Ms(Mb) t e m p e r a t u r e was indepen- dent of the aus ten i t i z ing t e m p e r a t u r e in the spec imens with a constant g ra in s ize (set two). The absence of an effect due to t he r ma l condit ioning of embryos was con- f i rmed by the r e s u l t s of the th i rd set of expe r imen t s which were found to be ident ica l with those of the f i r s t set (Fig. 4). It might be argued that the one hour hold- ing t ime used in these expe r imen t s was inadequate to es tab l i sh the t h e r m a l effects. Holding t ime was shown to be impor tan t in the expe r imen t s c a r r i e d out by Sast r i and West ~ on c o m m e r c i a l s tee l s with M s t e m p e r a t u r e s above room t e m p e r a t u r e . The t ime in t e rva l s involved were much l e s s than an hour . Never the less , to check the adequacy of the holding t ime used in the p re sen t study an addi t ional expe r imen t was done. A spec imen was aus ten i t i zed at 1473 K for one hour to set the au -

0.9

0 .8

"•0.7 0 0 .6

0 .5 ~n

n-- 0 .4

0 .3

0.2 130

I [ I I

Smoll burst, Ms = 210 ~

Groln-s~ze ~ ' ~

/ [ i I 150 170 190 210

TemperGture ( ~

Fig. 1--Typical transformation curve for the Fe-31 Ni alloy showing small bursts. Specimen austenitized at 1473K fol lowed by annealing at 973K. M s temperature 210K. Grain size 12/am.

THE Ms(Mb) TEMPERATURE

Typical experimental results showing the variation of the resistance of the Fe-31 Ni and the Fe-31 Ni- 2.38 C alloys with temperature are shown in Figs. i and 2. These transformation curves for the Fe-31 Ni alloys were reproducible and were similar to those reported earlier by Krauss and Cohen. 8 The start of the transformation to martensite was easily identified even when it was the occasion of only a small burst. The curves for the Fe-31 Ni-0.28C alloy were also re- producible but when, in the very fine-grained speci- mens, the initial burst was difficult to identify (Fig. 2) even by the sensitive resistance technique, the M s tem- perature was revealed by a relatively small change in slope of the curve. Fortunately, this difficulty did not introduce any serious ambiguity and the recorded Ms(bib) t e m p e r a t u r e s were reproduc ib le to within + 2 K .

2 0 4 2 - V O L U M E 5, SEPTEMBER 1974

09 / I I l

Ms = t53~

0.8 Gram-size ~ - -

~ 0.7

o ~ 0 . 6

"~ o5

re" 0.4

I I I 0.3 120 130 140 J50 160

Temperature ( ~ ) Fig. 2--Typical transformation curve for the Fe-31 Ni-0.28 C alloy showing detectable, but extremely small, bursts. Speci- men austenitized at 1473K followed by annealing at 1073 K. M s temperature 1473 K. Grain size 88/am.

METAI.LURGICAL TRANSACTIONS

2:30

o 220

~ 210

E 200

190

I i I t

Set 2 o ~] g--O--8--g--g.-~--O~

0 [3 O / O

i I I I 900 HOO ~300 1500

hustenitiz mg temperature (~ F i g . 3 - - F e - 3 1 Ni. V a r i a t i o n of Ms(Mb) w i t h a u s t e n i t i z i n g t e m -

p e r a t u r e . Set 1 (D) single-step treatment. Set 2 (�9 specimens austenitized at 1473 K for one hour followed by an annealing treatment for one hour at the temperature indicated.

J70

o 130 a)

9o

E

~ 5o

JO

t I f I

Set 2 ^ o o O

o o X 5

Set :3 El Set 1 /El I

I ! !

.& L ~ j h

gOO ,o0 1300 150~ Austenitizmg temperature (~

F i g . 4 - - F e - 3 1 N i - 0 . 2 8 C. V a r i a t i o n of M s ( M b ) w i t h a u s t e n i t i z - ing temperature. Set 1 (D) single-step treatment. Set 2 (O) specimens austenitized at 1473K for one hour followed by an annealing treatment for one hour at the temperature indicated. Set 3 ((3) specimens austenitized at temperature indicated fol- lowed by an annealing treatment at 873 K for one hour.

240

2:30 o v

220 o

E 210

200

I I I ]

Single c r ys ta l . - 7 - ~

I I I ~

e �9

I 190 1 i I I I l I,~

0 I 0 2 0 3 0 4 0 5,0 6 0 7 0 Austenite, groin-s~ze (p.m)

F i g . 5 - - F e - 3 1 Ni. V a r i a t i o n of M s ( M b ) t e m p e r a t u r e w i t h g r a i n

size. �9 This investigation. �9 Machlin and Cohen, Ref. 2. QBokros and Parker, Ref. 9.

190

170 L

150 o

o J 130

~- I IQ

70 i

5C 0

1 I 1 I I I I I

o

% , , ~ o ~

/. f

Fe-31 N i -0 .28 C

�9 Detectible burst o Small burst �9 Large burst

I I I J I l 1 I 50 I00 150 200 250 30(3 :350 4 0 0 4 5 0

Austenite, grain-size (/~m )

Fig. 6--Fe-31 Ni-0.28 C. Variation of Ms(Mb) temperature with austenite grain size.

s t en t t e g ra in s i ze and then held at 1173 K for 24 h. Com- p a r e d with the s p e c i m e n in se t t h r e e he ld at 1173 K only one hour , no change in Ms(Mb) could be de tec t ed .

The r e s u l t s of the e x p e r i m e n t s with s ing le and double aus t en i t i z ing t r e a t m e n t s c l e a r l y point to the conc lus ion that of the v a r i a b l e s a f fec ted by aus t en i t i z i ng t e m p e r a - t u r e , only g ra in s i ze has a s ign i f i can t effect on the Ms(M b) t e m p e r a t u r e . This is d e m o n s t r a t e d a l so by the plots of M s (M b) t e m p e r a t u r e v e r s u s g r a i n s i ze , F i g s . 5 and 6. Al l the r e s u l t s f r om a l l t h r e e s e t s of e x p e r i m e n t s on the Fe-31 Ni a l l oy can be r e a s o n a b l y r e p r e s e n t e d by a s ing le cu rve (Fig . 5) and the s a m e is t r ue of the r e s u l t s for the Fe-31 N i - 0 . 2 8 C a l loy (Fig. 6). The r a n g e of g ra in s i ze (4 to 50 ~m) of the c a r b o n - f r e e a l l oy was, unfor tuna te ly , c o n s i d e r a b l y s m a l l e r than the r ange (8 to 450 ~m) of the a l l oy conta in ing 0.28 C. At - t emp t s to extend the r a n g e for the Fe -31 Ni we re un- s u c c e s s f u l . The Ms(Mb) t e m p e r a t u r e of th is a l l oy was found to i n c r e a s e r a p i d l y with i n c r e a s e in g ra in s i ze f rom the s m a l l e s t g r a i n s i z e up to about 40 pm. Be - tween 40 and 50 ~ m the Ms(Mb) t e m p e r a t u r e a p p e a r s to be cons tan t but in the a b s e n c e of s p e c i m e n s with c o a r s e r g r a i n s , th is cannot be dec ided . However , t h e r e a r e some r e l i a b l e r e s u l t s for the M s ( M b ) t e m p e r a t u r e of s ingle c r y s t a l s of th is a l loy 4,~ which sugges t s t r o n g l y that t h e r e is no change in the Ms(Mb) t e m p e r a t u r e on changing the g r a i n s i ze f rom 40 to 50 ~m to a c e n t i - m e t e r o r m o r e . The r e s u l t s for the Fe-31 Ni-0 .28 C

a l loy a r e s i m i l a r to those for the c a r b o n - f r e e a l l oy but the r a n g e of g r a i n s i ze ove r which the Ms(Mb) t e m p e r - a t u r e i n c r e a s e s with i n c r e a s i n g g ra in s i ze is ex tended. At the s m a l l e s t g r a i n s i z e s ( l e s s than about 40 g m ) the M s (M b) t e m p e r a t u r e increases e x t r e m e l y r a p i d l y with i n c r e a s i n g g r a i n s i z e . The i n c r e a s e cont inues in an in - t e r m e d i a t e r ange (40 to 150 vm) but the r a t e of i n c r e a s e is l e s s r ap id , be ing about the s a m e a s in the s m a l l e s t g r a i n - s i z e range of the F e - 3 1 Ni a l loy . The Ms(M b) t e m p e r a t u r e of s p e c i m e n s of F e - 3 1 N i - 0 . 2 8 C with a g ra in s i ze of 150 p m and l a r g e r does not change s i g - n i f i can t ly with a change in g r a i n s i ze .

The p r e s e n t r e s u l t s a r e c o m p a r e d with those ob - t a ined by Entwis le and F e e n e y 5 in F ig . 7. The gene ra l t r e n d of i n c r e a s i n g M s (Mb) t e m p e r a t u r e with i n c r e a s - ing g ra in s i ze is the s a m e for a l l the a l l oys . However , the s lope of the c u r v e s at the f i n e - g r a i n e d end of the r ange is shown a s be ing a p p r e c i a b l y s m a l l e r in the e a r l i e r s tudy. En twis le and F e e n e y used a d i l a t o m e t - r i c technique for de t ec t ing Ms(M_b). Pos s ib ly , the b e t - t e r s e n s i t i v i t y of the e l e c t r i c a l r e s i s t a n c e method used in the p r e s e n t s tudy can account for th is d i f f e rence . Data c o v e r i n g the c o a r s e g r a i n - s i z e r ange a r e r e p o r t e d for F e - 1 9 N i - 0 . 5 C and F e - 2 4 Ni-0 .5 C which show that in t hese a l l oys a l so the M s (Mb) t e m p e r a t u r e is indepen- dent of g ra in s i ze when the g r a i n s i ze is g r e a t e r than about 150 pm. However , En twis le and F e e n e y chose to

M E T A L L U R G I C A L TRANSACTIONS VOLUME 5, SEPTEMBER 1 9 7 4 - 2 0 4 3

310

270

o ~ 22s

~ 190'

I I I I

~'~ . . . . . . . I 9 N ( - 0 . S C

31Ni j , . . . . . 24Ni -0 .5C

~26Ni-0.5C E ~' "" ~ 51N i -0 .28C

~'~- 150 ~s ' ,, ~ N i - 0 . 5 C

IlO - - Present study ---- Entwisle and Feeney (5)

70 I I I I I I o 50 Ioo 150 200 250 5o0

Austenite, grain-size (Fro)

Fig. 7--Variation of Ms(Mb) temperature with austenite grain size of some Fe-Ni and Fe-Ni-C alloys.

i n t e r p r e t th is p a r t of the cu rve as be ing " a l m o s t " h o r - i zonta l . M s(Mb) t e m p e r a t u r e s for c o a r s e - g r a i n e d s p e c - imens of F e - 2 6 Ni-0 .5 C and F e - 2 7 Ni-0 .5 C a r e not given.

Although the e x p e r i m e n t a l r e s u l t s r e p o r t e d by En t - w i s l e and F e e n e y B a r e compa t ib l e with those found in the p r e s e n t s tudy, the conclus ion which the:~ di:aw f rom t h e i r r e s u l t s is d i r e c t l y opposed to that d e r i v e d h e r e f r o m a c o n s i d e r a t i o n of the s ing le and double s t ep a u - s t en i t i z l ng t r e a t m e n t s . The i r s t a t e m e n t that " . . . a u - s t en i t i z ing t r e a t m e n t is the domina t ing f ac to r c o n t r o l - l ing the s t a r t of t r a n s f o r m a t i o n , g r a i n s i ze m e r e l y b e - ing a dependent v a r i a b l e with e s s e n t i a l l y no inf luence on the s t a r t of t r a n s f o r m a t i o n " r e l i e s heav i ly on the work of S a s t r i and Wes t 7 who c a r r i e d out e x p e r i m e n t s with double aus t en i t i z ing t r e a t m e n t s . En twis le and F e e n e y s did not. Unfor tunate ly , the s t e e l s used by S a s t r i and West 7 had an M s t e m p e r a t u r e above r o o m t e m p e r a t u r e and t h e i r r e s u l t s we re p r o b a b l y inf luenced by e f fec t s d e r i v e d f r o m the t e m p e r i n g which must , in- ev i tab ly , have o c c u r r e d dur ing quenching. The r e s u l t s of e x p e r i m e n t s involving double aus t en i t i z ing t r e a t - men t s of a l l o y s with M s be low 273 K r e p o r t e d above show that g r a i n s i ze a lone is the f ac to r d e t e r m i n i n g Ms(M b) in a given a l loy . The aus t en i t i z i ng t e m p e r a - t u r e p e r se has no effect . This is in c o m p l e t e a c c o r d with the conclus ion r e a c h e d by Machl in and Cohen 4 f r o m t h e i r s tudy of s ing le c r y s t a l s of Fe -31 Ni.

T h e r e a r e s e v e r a l p o s s i b l e r e a s o n s why t h e r e is no d i r e c t effect of au s t en i t i z i ng t e m p e r a t u r e on the M s t e m p e r a t u r e . One p o s s i b i l i t y is that the equ i l i b r i um e m b r y o d i s t r i bu t i on is r e a c h e d so r a p i d l y that even dur ing quenching to r o o m t e m p e r a t u r e i t ad jus t s con- t inuous ly to the changing t e m p e r a t u r e . Another is that the magni tude of the change in e m b r y o d i s t r i bu t ion p r o - duced b y changing the aus t en i t i z i ng t e m p e r a t u r e over the r ange used in t h e s e e x p e r i m e n t s is so s m a l l that i t is i m p o s s i b l e to de tec t the r e s u l t i n g d i f f e rence in the M s t e m p e r a t u r e . T h e r e is no way of t e s t i n g e i t he r of t he se p r o p o s i t i o n s unti l a subs t an t i a l i m p r o v e m e n t in the s e n s i t i v i t y of the e x p e r i m e n t a l t echniques is ach ieved . Meanwhile , i t i s thought that the mos t p r o b - ab le explana t ion is that the aus t en i t l z i ng t r e a t m e n t does not a f fec t the d i s t r i bu t i on o r po tency of the e m b r y o s r e - spons ib l e for the f o r m a t i o n of a t h e r m a l m a r t e n s i t e by bu r s t i ng . A s s u m i n g th is is so, i t sugges t s that the m a r - t e n s i t e e m b r y o i s some f o r m of nonequi l tb r tum m e c h a n - i c a l l y - s t a b l e d i s loca t ion conf igura t ion .

2 0 4 4 - V O L U M E 5, SEPTEMBER 1974

THE BURST

Both the Fe-31 Ni and the Fe-31 Ni -0 .28 C a l loys t r a n s f o r m e d by bu r s t i ng . In the Fe -31 Ni s e r i e s those s p e c i m e n s with a g r a i n s i ze s m a l l e r than 10 ~m t r a n s - f o r m e d with a b u r s t that was so s m a l l that i t was d i f f i - cul t to de tec t it . S l ight ly c o a r s e r g r a ine d s p e c i m e n s (about 12 ~m) t r a n s f o r m e d with a s m a l l but e a s i l y d e - t e c t ed b u r s t (F ig . 1) and the r e l a t i v e l y c o a r s e - g r a i n e d s p e c i m e n s (40 to 50 ~m) exhib i ted a l a r g e b u r s t (Fig . 8). These l a r g e b u r s t s we re a c c o m p a n i e d by a m a r k e d r i s e (about 10 K) in the t e m p e r a t u r e of the spec imen . In the Fe-31 Nt-0 .28 C a l loys , those s p e c i m e n s with an a u s t e n - t t ic g r a i n s i ze of l e s s than 150 ~m a l l showed only s m a l l b u r s t s , the magni tude of the b u r s t i n c r e a s i n g f rom jus t de t ec t ab l e (Fig . 2) to e a s i l y de t ec t ab l e (Fig. 1) a s the g ra in s i ze was i n c r e a s e d within th is r ange . When the g r a i n s i ze was 150 ~m or l a r g e r the t r a n s f o r m a t i o n was in i t i a t ed by a l a r g e b u r s t ( s i m i l a r to Fig . 8).

En twis le and F e e n e y s m e a s u r e d the magni tude of the b u r s t s f r o m d i l a t o m e t e r r e c o r d s by noting the p e r c e n - tage of the aus t en i t e t r a n s f o r m e d to m a r t e n s i t e in the f i r s t b u r s t . This a p p e a r s to be an unce r t a in p r o c e d u r e b e c a u s e of the s low r e s p o n s e of the d i l a t o m e t e r and the subs t an t i a l r i s e in t e m p e r a t u r e , a function of the s i ze and shape of the s p e c i m e n and unknown a mb ie n t cond i - t ions , which a c c o m p a n i e s l a r g e b u r s t s . Cer t a in ly , no a t t e mp t to a s s i g n a quant i ta t ive m e a s u r e to the b u r s t s r e c o r d e d by the r e s i s t a n c e changes in the p r e s e n t s tudy could be jus t i f i ed . The b u r s t s could, however , e a s i l y be c l a s s i f i e d into b u r s t s which could be de t ec t ed only with d i f f icu l ty (Fig . 3), s m a l l b u r s t s (Fig . 1), and l a r g e b u r s t s (F ig . 8). T h e r e is no s h a r p d e m a r c a t i o n be tween the ju s t de t ec t ab l e and the s m a l l b u r s t s . However , the d iv i s ion be tween s m a l l and l a r g e b u r s t s c l e a r l y c o i n , c tdes with the change f rom the range of fine g ra in s i z e s over which the M s (Mb) t e m p e r a t u r e i n c r e a s e s cont inu- ous ly with i n c r e a s i n g g ra in s i ze to the r ange of c o a r s e r g ra in s i z e s ove r which the M s (blb) t e m p e r a t u r e is con- s tant . Th is r e s u l t conf l i c t s with the f indings of Entwis le and F e e n e y s who r e p o r t e d that a s the g r a i n s i ze and the M b t e m p e r a t u r e we re i n c r e a s e d the s i ze of the b u r s t i n c r e a s e d , r e a c h e d a ma x imum and then, a t the l a r g e s t g r a in s i z e s , d e c r e a s e d again . The magni tude of the b u r s t is a function of both the g r a i n s i ze and the t h e r - modynamic d r iv ing f o r c e ( A G T ~ a ) . As the g r a i n s i ze is d e c r e a s e d the Ms(M b) t e m p e r a t u r e d e c r e a s e s and

1.0

0.9

E 0.8

o 0.7

.~ 0.6 $ IX:

0 .5

I I

Large burst, Ms = 232 ~

Grain - size ~ I \

2; I I

230 2:40 Temperature ( ~ )

0 .4 210 250

Fig. 8--Fe-31Ni. Typical transformation curve showing large burst. Specimen austenitized at 1573K. Ms(Mb) temperature 232 K. Grain size 50 pm.

METALLURGICAL TRANSACTIONS

so the driving force increases. Thus, the direction of the two effects is opposed. It was claimed ~ that in the case of the Fe-XNi-0.5 C series there is in fact a crossover, and that when the effect of the change in driving force is subtracted the size of the burst in- creases continuously with increase in grain size. s In the case of the Fe-31 Ni and Fe-31 Ni-0.28 C alloys the effect of changing the dr iving force appears to be domi- nated by the effect of gra in s ize over the whole range and the magnitude of the burs t does not pass through a maximum.

GENERAL DISCUSSION

Adding carbon to an Fe-31 Ni al loy changes the t e m - pe ra tu re range over which mar tens i t e forms and causes the t rans i t ion f rom smal l to la rge bu r s t s to occur at a l a rge r gra in s ize. This change in the t rans i t ion grain s ize may be due to carbon dec reas ing the driving force needed to produce mar tens i te at the Ms(Mb) t e m p e r a - ture . In the Fe-31 Ni a l loy the t rans i t ion f rom smal l to la rge burs t s occurs at 50/zm and an Ms(bib) t e m p e r a - ture of 233 K. Following Kaufman and Cohen, '~ who combined der ivat ions by F i s h e r " and Jones and Pum- phrey, la the driving force is es t imated to be - 780 J/mol. The corresponding transition in the Fe-31 Ni- 0.28C alloy occurs at a grain size of 150 ~tm and an Ms(M b) temperature of 123 K. The driving force is es- timated to be only - 635 J/tool. Thus, if, as suggested by Entwisle and Feeney, s l a rge burs t s a r e a ssoc ia ted with a l a rge driving force a l a rge burs t would be ex- pected at a sma l l e r grain s ize in the Fe-31 Ni alloy.

There is another in teres t ing effect introduced by the addition of carbon. In the range of ve ry smal l g ra in - s izes ( less than 40 ~m) the Ms(Mb) t empera tu re of the al loy containing carbon inc reases ve ry rap id ly with in- c r e a s e in gra in s ize . Between grain s i zes of 40 and 150 ~m the ra te of continuous inc rease of Ms(M b) t e m - pera tu re with increas ing g ra in - s i ze is reduced and is about the same as the ra te of inc rease in the smal l grain s ize , smal l burs t , range of the ca rbon- f r ee al loy. It was found that this t rans i t ion in the Fe-31 Ni-0.28 C al loy from a s teep to a more gradual slope coincided with a t rans i t ion in the morphology of the mar tens i te . In the range of 40 to 150 ~m the mar tens i t e is lent icu- far , as it is a lso in the Fe-31 Ni al loy. In the f inest g r a i n - s i z e specimens of the Fe-31 Ni-0.28C al loy the mar tens i te formed by the t rans format ion is th in-pla te mar tens i te as desc r ibed by Maki, Shimooka, Umemoto and Tamura . Is However, in both this Japanese work and in e a r l i e r observat ions of the th in-pla te m i c r o - s t ruc ture by Entwisle and Feeney s the t rans i t ion f rom lent icular to plate mar tens i t e was brought about by changing the carbon concentrat ion. In the presen t work it was brought about by changing the grain s ize .

R is cus tomary s to explain the inc rease of M s with increas ing grain s ize in t e r m s of the austeni te grain par t i t ioning model f i r s t d i scussed by F i she r et el, 14 according to which the volume f rac t ion of mar tens i te formed in the ea r ly s tages is propor t ional to the cube of the mean grain d iamete r . Then, assuming that a smal l but definite volume-f rac t ion (say 1 pct) of m a r - tens i te must form before it can be detected, the de- p ress ion of the M s with a dec rea se in grain s ize can

be a t t r ibuted s imply to the inc reased diff iculty of de - tect ing the reduced amount of mar tens i t e formed ini - t ia l ly . In an a l loy in which the mar tens i te forms in a burs t this explanation is inadequate. In such al loys ve ry thin isola ted plates may well form before the f i r s t successful burs t occurs but, ff they do, they p ro - duce only a ve ry smal l , undetectable, volume fract ion of mar tens i te . It is c l ea r f rom the appearance of the se r r a t ions in the e l ec t r i ca l r e s i s t ance ve r sus t e m p e r - a ture curves that the Ms(Mb) t empera tu re measured exper imenta l ly is de termined not by the f i r s t isolated plate nucleated but by the f i r s t burs t . Since the ten- dency to burs t is increased by increas ing the driving force and the austenite gra in s ize , it follows that to produce a burs t in a smal l grained specimen requ i res a l a rge driving force and hence, a low M s t empera tu re .

The effect of g r a i n - s i z e on the magnitude of the bu r s t is a lmos t ce r ta in ly a r esu l t of the coupling of e las t ic shea r s t r e s s e s with the nucleation p rocess as f i r s t d i s - cussed by Patel and Cohen ~s and e labora ted by Bokros and Pa rke r . a The e las t ic shear s t r e s s concentration in the austeni te is a maximum near the t ip of a m a r - tens i te plate and is propor t ional to the square of the d iamete r of the f i r s t plate which, in turn, can be ex- pected to be r e l a t ed to the d iamete r of the austenite grain, and inve r se ly to the radius of the tip, or the thickness, of the plate. Thus, the concentrat ion of e l a s - t ic s t r e s s e s will be g rea tes t in the specimen with the l a r ge s t grain s ize . It is difficult, at th i s ' t ime , to de- velop a mechanis t ic model using this concept because a sa t i s fac to ry model of the re levant p r i m a r y nuclea- tion p rocess does not exist .

CONCLUSIONS

In Fe-Ni and Fe-Ni-C alloys which athermally trans- form to martensite with a burst:

I. In the absence of a change in composition or ex- ternal conditions such as imposed stress, the M s tem- perature is determined by the grain size of the austen- ite.

2. Thermal treatment prior to transformation has no effect on the density, distribution or potency of the martensite embryos in the austenite.

3. A transition from acicular to thin-plate marten- site can be induced in an Fe-31 Ni-0.28 C alloy by re- fining the grain s ize .

4. The ra te of inc rease of Ms(M b) with inc rease in gra in s ize is l a rge for f ine-gra ined specimens which t r ans fo rm to mar tens i te with a th in-pla te morphology, l e s s for spec imens with a fine austeni te g r a i n - s i z e which produce mar tens i te with an ac icu la r morphology and zero for c o a r s e - g r a i n e d specimens .

5. The M s t empera tu re measured exper imenta l ly is de termined by the f i r s t successful burs t . A la rge d r i v - ing force and, consequently, a low M s t empera tu re is requi red to produce the f i r s t bu r s t in a f ine-gra ined specimen.

6. The t rans i t ion from a smal l to a l a rge burs t co- incides with a change f rom fine to c o a r s e - g r a i n e d au- stenite in spec imens in which the mar tens i te is ac icu- lax. Over the range in which the burs t is l a rge the Ms(Mb) t empera tu re is independent of the austenite g r a in - s i ze .

METALLURGICAL TRANSACTIONS VOLUME 5, SEPTEMBER 1974-2045

ACKNOWLEDGMENTS

The work was carried out in the Department of Ma- terials Science of the Technological Institute of North- western University and was supported by a grant from the National Science Foundation (GK-23985) and by a Fellowship from the University. We are grateful to Professor Tamura of Kyoto University, Japan, for providing us with specimens and related unpublished data.

RE FERENCES

1. M~ R. Meyerson and S. J. Rosenberg: Trans. ASM, 1954, vol. 46, p. 1225. 2. V. Raghavan and A. R. Entwisle: Physical Properties of Martensite and Bainite,

p. 30, Iron and Steel Institute Special Report No. 9, London, 1965.

3. S. R. Pati and M. Cohen: ActaMet., 1966, vol. 14, p. 100I. 4. E. S. Machlin and M. Cohen: Trans. AIME, 1951, vol. 191, p. 746. 5. A. R. Entwisle and J. Feeney: Institute o f Metals Monograph No. 33, 1968,

p. 156. 6. J. C. Fisher, J. H. Holloman, and D. Tumbull: J. Appl. Phys., 1948, vol. 19, p.

775. 7. A. S. Sastd and D. R. F. West: J. Iron and Steel Inst., 1965, vol. 203, p. 138. 8. G. Krauss and M. Cohen: Trans. TMS-AIME, 1963, vol. 227, p. 278. 9. J. C. Bokros and E. R. Parker: Acta Met., 1963, vol. 11, p. 1291.

10. L. Kaufman and M. Cohen: Prog. MetalPhys., 1958, vol. 7, p. 165. 11. J. C. Fisher: Trans. AIME, 1949, vol. 185, p. 688. 12. F. W. Jones and W. I. Pumphrey: J. Iron Steetlnst., 1949, vol. 163, p. 121. 13. T. Maki, S. Shinooka, M. Umemoto, and I. Tamura: J. Japan Inst. Met., 1971,

vol. 35, p. 1073. 14. J. C. Fisher, J. H. Holloman, and D. Turnbull: Trans. AIME, 1949, vol. 185,

p. 691. 15. J. R. Patel and M. Cohen: Acta Met., 1953, vol. 1, p. 531.

2 0 4 6 - V O L U M E 5, SEPTEMBER 1974 M E T A L L U R G I C A L TRANSACTIONS