The Morphology and Mechanical Properties of Bainite Formed from Deformed Austenite
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IT is well e s t a b l i s h e d that the
t e n s i l e s t r e n g t h of m a n y quenched f e r r o u s alloys can be i m p r o v e d s i g n i f icantly, without c o n c o m i t a n t loss of ductility, by p l a s t i c d e f o r m a t i o n of the a u s t e n i t e before the t r a n s f o r m a t i o n to m a r t e n s i t e . T h i s p r o c e s s of a n s f o r m i n g was probably p i o n e e r e d by H a r v e y 1 who r e p o r t e d i m proved s u r f a c e p r o p e r t i e s a f t e r shot p e e n i n g at t e m p e r a t u r e s j u s t above Ms, and by Lipps and van Z u i l e n 2 who d e m o n s t r a t e d that the s t r e n g t h of s t e e l wire could be i n c r e a s e d by d e f o r m a t i o n p r i o r to t r a n s f o r m a t i o n . Subsequently, the p r o c e s s has b e e n s u b j e c t to c o n s i d e r a b l e attention, and b e c a u s e of the s t r o n g dependence of m e c h a n i c a l b e h a v i o r on m i c r o s t r u c t u r e , it is g e n e r a l l y b e l i e v e d that the i m p r o v e d t e n s i l e s t r e n g t h and a s s o c i a t e d p r o p e r t i e s d e r i v e dir e c t l y f r o m changes in morphology of the m a r t e n s i t e . Since m a r t e n s i t e and b a i n i t e a r e c r y s t a l l o g r a p h i c a l l y s i m i l a r 3"~ it is expected that analogous changes in morphology, and t h e r e f o r e p r o p e r t i e s , would o c c u r in the b a i n i t e p r o d u c e d f r o m p l a s t i c a l l y d e f o r m e d a u s t e n i t e . T h e r e is some e x p e r i m e n t a l s u p p o r t for this expectation. I r a n i 9 made an e l e c t r o n m i c r o s c o p i c a l study of a c o m m e r c i a l Mo-B s t e e l and found the d i s l o c a t i o n d e n s i t y in the f e r r i t e component of the b a i n i t e to be g r e a t l y i n c r e a s e d by d e f o r m a t i o n of the a u s t e n i t e p r i o r to t r a n s f o r m a t i o n . MetallogTaphic s t u d i e s of a eutectoid s t e e l by J e p s o n and T h o m p s o n 1~ indicated that p r e f e r e n t i a l n u c l e a t i o n of b a i n i t e o c c u r r e d at e i t h e r s l i p band h e t e r o g e n e i t i e s o r twin b o u n d a r i e s in the d e f o r m e d a u s t e n i t e and t h e r e a r e a d d i t i o n a l r e p o r t s n'~z of r e d u c e d plate size of such lower b a i n ires. T h e s e o b s e r v a t i o n s a r e compatible with the r e s u l t s of Kalish, Kulin and Cohen 13 who obtained inc r e a s e s in both yield s t r e n g t h and t e n s i l e s t r e n g t h of a C r - M o - V s t e e l by d e f o r m i n g the a u s t e n i t e 50 pct p r i o r to t r a n s f o r m a t i o n . The effect of d e f o r m a t i o n of a u s t e n i t e on b a i n i t e morphology and a s s o c i a t e d p r o p e r t i e s is f u r t h e r supp o r t e d by the o b s e r v a t i o n of E d w a r d s and Kennon~2 that the d e f o r m a t i o n r e d u c e s the i n c u b a t i o n p e r i o d and i n f l u e n c e s the k i n e t i c s of t r a n s f o r m a t i o
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