The enhancement of strengthening dislocated martensite
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P R O C E S S E S for o b t a i n i n g high s t r e n g t h s t e e l s involve v a r i o u s t h e r m a l and t h e r m a l - m e c h a n i c a l t r e a t m e n t s , e . g . a u s f o r m i n g , which e v e n t u a l l y p r o d u c e m a r t e n s i t i c s t r u c t u r e s . However, a p a r t from a u s f o r m i n g , little is known about the o r i g i n and c h a r a c t e r of the s u b s t r u c t u r e of the m a r t e n s i t e p r o d u c e d by o t h e r c o m p l e x p r o c e s s e s and its r e l a t i o n to m e c h a n i c a l p r o p e r t i e s . This p a p e r e x p l o r e s this p r o b l e m and ways of duplicating the b e n e f i t s of a u s f o r m i n g without r e q u i r i n g p l a s t i c d e f o r m a t i o n as a stage in the p r o c e s s . It is useful f i r s ~ to r e v i e w what is known about ausforming. Ausforn-ring is a t h e r m o m e c h a n i c a l t r e a t m e n t , whereby the m e t a s t a b l e a u s t e n i t e is d e f o r m e d at e l e vated t e m p e r a t u r e s b e f o r e i s o t h e r m a l d e c o m p o s i t i o n t a k e s place and then cooled to allow the m a r t e n s i t e t r a n s f o r m a t i o n to occur. The m e c h a n i c a l p r o p e r t i e s of a u s f o r m e d m a r t e n s i t e in s t e e l s c o n t a i n i n g c a r b o n and c a r b i d e f o r m i n g e l e m e n t s , a r e in g e n e r a l s u p e r ior to those obtained by c o n v e n t i o n a l l y h e a t - t r e a t e d m a r t e n s i t e ; for e x a m p l e , the t e n s i l e s t r e n g t h m a y be i n c r e a s e d by as m u c h as 30 pct with g e n e r a l l y an i m p r o v e m e n t in ductility. ~-'~ It has been shown that c a r b i d e s a r e f o r m e d d u r i n g d e f o r m a t i o n of m e t a s t a b l e a u s t e n i t e , e . g . by d i r e c t o b s e r v a t i o n s u s i n g e l e c t r o n m i c r o s c o p y ~'2 and i n d i r e c t l y f r o m the o b s e r v e d i n c r e a s e in M s t e m p e r a t u r e . 8 T h e s e c a r b i d e s m a y pin the d i s l o c a t i o n s in a u s t e n i t e so that they a r e not likely to be swept away by the advancing m a r t e n s i t e b o u n d a r i e s on s u b s e q u e n t 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 . Consequently, the d i s l o c a t i o n s may be r e t a i n e d in the r e s u l t i n g m a r t e n s i t i c s t r u c t u r e . The m a r t e n s i t e t r a n s f o r m a t i o n itself g e n e r a t e s d i s l o c a t i o n s and by i n t e r a c t i o n and m u l t i p l i c a t i o n at the c a r b i d e s the total d i s l o c a t i o n d e n I-LIN CHENG, formerly Graduate Student, Department of Materials Science and Engineering,University of California, Berkeley, Calif., is now with Borg-WarnerCorp., Des Haines, Ill. G. THOMASis Professor of Metallurgy,Department of Materials Science and Engineering,University of California. Manuscript submitted April 8, 1971. METALLURGICALTRANSACTIONS
sity can be v e r y high c o m p a r e d to n o r m a l m a r t e n s i t e s .
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