Effects of austenitizing temperature and austenite grain size on the formation of athermal martensite in an iron-nickel
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A M O N G S T the many e a r l y studies of the effect of austenitizing t e m p e r a t u r e and time on the Ms t e m p e r a t u r e of v a r i o u s s t e e l s , t h e r e was a noticeable lack of a g r e e m e n t about the d i r e c t i o n of the change in Ms t e m p e r a t u r e with i n c r e a s i n g austenittztng t e m p e r a t u r e . 1 This confusion was due to the uncertainty introduced by the r e s o l u t i o n of c a r b i d e s and o t h e r changes in the c h e m i s t r y of the austenite during the austenitizing t r e a t m e n t . Using a l l o y s with an Ms t e m p e r a t u r e below 0~ and thus eliminating p r o b l e m s due to c a r b i d e s e p a ration during quenching, Raghavan and Entwlsle 2 showed that the r a t e of i s o t h e r m a l formation of m a r t e n s i t e inc r e a s e s as the grain s i z e of the p a r e n t austenite is i n c r e a s e d . However, Putt and Cohen s showed, by taking Into account the v a r i a t i o n of plate s i z e with austenite grain s i z e , that in these c i r c u m s t a n c e s the Initial r a t e of nucleation is independent of the grain s i z e and the austenttizing t e m p e r a t u r e . There is no s i m i l a r l y c l e a r understanding of the situation in s t e e l s In which m a r t e n s l t e forms athervaally in a b u r s t . Only two studies a r e known 4,s in which the t r a n s f o r m a t i o n was c a r r i e d out at t e m p e r a t u r e s below 273 K and the r e s u l t s of these a r e conflicting. Machlin and Cohen 4 concluded from studies of an Iron-29.5 pct nickel alloy that "the amount of t r a n s f o r m e d phase is independent of the austenitizing t e m p e r a t u r e , g r a i n size being maintained c o n s t a n t . " In s h a r p c o n t r a s t , Entwisle and Feeney 5 who studied 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 in a s e r i e s of 0.5 pct c a r b o n a l l o y s containing between 19 and 27 pct nickel, 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 University, is now Professor o f Materials Science at Massachusetts Institute of Technology, Cambridge, Massachusetts. Manuscript submitted September 2 I, 1973. METALLURGICAL TRANSACTIONS
e x p r e s s e d the opinion that "the austentttzlng t r e a t m e n t is the dominating factor controlling the s t a r t of t r a n s f o r m a t i o n . " The study d e s c r i b e d h e r e was designed to r e s o l v e some of the p r o b l e m s p r e s e n t e d by these d i s c r e p a n t conclusions. The a l l o y s and t r e a t m e n t s were s e l e c t e d to e n s u r e that all the t r a n s f o r m a t i o n s o c c u r r e d below 0~ One a l l o y was an I r o n - n i c k e l a l l o y s i m i l a r to the alloy studied by Maehlln and Cohen and the other alloy had the s a m e nickel content wtth 0.28 pet c a r b o n added. In s p e c i m e n s whteh t r a n s f o r m by bursting, the
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