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T H E s t a b i l i t y of a u s t e n i t e a g a i n s t the m a r t e n s i t i c t r a n s f o r m a t i o n is u s u a l l y m e a s u r e d by the M s and M d t e m p e r a t u r e s . The M s s i g n i f i e s the t e m p e r a t u r e at which the a u s t e n i t e s t a r t s to t r a n s f o r m s p o n t a n e o u s l y to m a r t e n s i t e on cooling, while M d is the t e m p e r a t u r e above which d e f o r m a t i o n s t r e s s e s cannot i n i t i a t e the t r a n s f o r m a t i o n . In the case of 304 s t a i n l e s s s t e e l s , * *Compositionof alloygivenin Ref. 1. the a u t h o r s 1 have shown that the t r a n s f o r m a t i o n s e quence i s Z - - 9 with M~a) < 4~ and M ~ ) ~ 293~ In fact, a is only o b s e r v e d a f t e r m e c h a n i c a l or t h e r m a l - m e c h a n i c a l p r o c e s s i n g . It is thus p o s s i b l e to i n duce the m a r t e n s i t i c t r a n s f o r m a t i o n in these a l l o y s by s u b z e r o d e f o r m a t i o n ( " Z e r o l l i n g " ) , and this i s known to c o n s i d e r a b l y i n c r e a s e the yield and t e n s i l e s t r e n g t h s . ~-~~ F u r t h e r s i g n i f i c a n t i n c r e a s e s i n these p r o p e r t i e s have been a c h i e v e d by s t r e s s r e l i e f t r e a t m e n t between 225 ~ and 425~ 2'8'9 or by aging. 6'7 An o p t i m u m aging t e m p e r a t u r e of 400~ was e s t a b l i s h e d by Chukhleb and M a r t y n o v 8 where the m a x i m u m i n c r e a s e in p r o p e r t i e s was o b s e r v e d . The i n c r e a s e in p r o p e r t i e s at a c e r t a i n a g i n g t e m p e r a t u r e depended both on the time of aging and the amount of p r i o r subz e r o d e f o r m a t i o n . Most of the i n c r e a s e o c c u r r e d d u r ing the f i r s t hour of a g i n g followed by a v e r y slight i n c r e a s e but no a p p a r e n t s o f t e n i n g effect o c c u r r e d even a f t e r 120 h r , which m a k e s the phenomenon quite diff e r e n t from that of p r e c i p i t a t i o n h a r d e n i n g . With the i n c r e a s e in m e c h a n i c a l p r o p e r t i e s , the m a g n e t i c s u s c e p t i b i l i t y also i n c r e a s e s . The only a t t e m p t made so far to explain the s t r e n g t h e n i n g o b s e r v e d on s t r e s s - r e l i e f aging is that due to Chukhleb and Martynov. 8 They proposed that p r e c i p i tation of c a r b i d e s in the r e t a i n e d a u s t e n i t e o c c u r s followed by n u c l e a t i o n of the a phase a r o u n d the c a r b i d e s . Additional s t r e n g t h e n i n g was also p r e s u m e d to r e s u l t from p r e c i p i t a t i o n of c a r b i d e s in the s t r e s s i n d u c e d ~ phase. Aside f r o m m a g n e t i c s u s c e p t i b i l i t y PAT L. MANGONON,JR., formerly with the Department of Materials Science and Engineering, University of California, Berkeley, Calif., is with Inland Steel Company, East Chicago, Indiana. GARETH THOMAS is Professor of Metallurgy in the Department of Materials Science and E