The effect of structure on the deformation of as-quenched and tempered martensite in an Fe-0.2 pct C alloy
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MANY
l e v e l s of s t r u c t u r e m a y affect the m e c h a n i c a l p r o p e r t i e s and d e f o r m a t i o n b e h a v i o r of low c a r b o n f e r r o u s m a r t e n s i t e s . C a r b o n e x e r t s a p o w e r f u l and v a r i e d i n f l u e n c e on s t r u c t u r e by a c t i n g a s a s o l i d s o l ution s t r e n g t h e n e r , by s e g r e g a t i n g to b o u n d a r i e s and d i s l o c a t i o n s in the m a r t e n s i t e a n d / o r by f o r m i n g c a r bide p a r t i c l e s e i t h e r a s a r e s u l t of a u t o t e m p e r i n g o r c o n v e n t i o n a l t e m p e r i n g . Speich and W a r l i m o n t ~ have shown t h a t the y i e l d s t r e n g t h of tow c a r b o n m a r t e n s i t e i s p r o p o r t i o n a l to t h e s q u a r e r o o t of c a r b o n content up to 0.18 wt p c t C, a r a n g e of c o m p o s i t i o n w h e r e a l l of the c a r b o n i s e f f e c t i v e l y s e g r e g a t e d to d e f e c t s i t e s in the m a r t e n s i t e a f t e r quenching to r o o m t e m p e r a t u r e . ~ Speich and W a r l i m o n t conducted t h e i r e x p e r i m e n t s on m a r t e n s i t e s f o r m e d in a u s t e n i t e with a r o u g h l y c o n s t a n t g r a i n s i z e r a n g i n g between ASTM No. 7 and 9. H o w e v e r , when a u s t e n i t i c g r a i n s i z e i s m a d e to v a r y over a wide range in a given steel, significant changes in strength according to Hall-Perch type relationships with austenite grain size as the structural parameter are observed.3'4 More recently, the strength of lath martensite has been related to the size of the packets of parallel laths that subdivide the parent austenite grains into smaller, roughly equiaxed units. Hall-Perch relationships with packet size as the structural parameter have been found for the martensites formed in an F e - 0 . 2 p e t C a l l o y with c o a r s e s and fine 6 a u s t e n i t e g r a i n s i z e s and in Fe-lVha a l l o y s ] An i m p o r t a n t d i f f e r e n c e b e t w e e n t h e m a r t e n s i t e with c a r b o n and that f o r m e d in the F e - M n a l l o y without c a r b o n was a much s t e e p e r s l o p e , ky, of the F e - 0 . 2 p c t C m a r t e n s i t e H a l l P e r c h p l o t . P a c k e t s i z e h a s a l s o been d i r e c t l y r e t a r e d 8'~ to t h e c l e a v a g e f a c e t s of low c a r b o n t e m p e r e d m a r t e n s i t e f r a c t u r e d below i t s d u c t i l e to b r i t t l e t r a n THOMAS SWARR is Engineer, Knolls Atomic Power Laboratory, Schenectady, N. Y. 12309. GEORGE KRAUSS is AMAX Foundation Professor, Colorado School of Mines, Golden, CO 80401. The authors were formerly Research Assistant and Professor, respectively, at Lehigh University, Bethleilem, PA. Manuscript submitted December 3, 1974.
METALLURGICAL TRANSACTIONS A
s i t i o n t e m p e r a t u r e . The v e r y s t r o n g influence of packet size, a relatively coarse microstructural feat u r e , on the d e f o r m a t i o n and f r a c t u r e b e h a v i o
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