Microstructures of deformation and fracture of cementite in pearlitic carbon steels strained at various temperatures
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has b e e n a s s u m e d f r o m e m p i r i c a l o b s e r v a t i o n s that c e m e n t i t e in s t e e l s can d e f o r m p l a s t i c a l l y at e l e v a t e d t e m p e r a t u r e s . F o r e x a m p l e , a c c o r d i n g to s t u d i e s 1'2 on the hot d r a w i n g of p e a r l i t i c s t e e l s , c e m e n t i t e p l a t e s take a r e m a r k a b l y e l o n g a t e d shape a l o n g the d i r e c t i o n of the d r a w i n g . T h e p l a s t i c d e f o r m a t i o n of c e m e n t i t e h a s r e c e n t l y b e e n d e m o n s t r a t e d d i r e c t l y by a t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y study of s t r a i n e d c e m e n t i t e in s t e e l s u b j e c t e d to c o l d - r o l l i n g ? -6 A l a r g e n u m b e r of d i s l o c a t i o n s having a s c r e w n a t u r e w e r e found in t h o s e c e m e n t i t e p a r t i c l e s . S e v e r a l s e t s of c r y s t a l l o g r a p h i c p l a n e s and B u r g e r s v e c t o r s w e r e d e t e r m i n e d a s p o s s i b l e s l i p s y s t e m s of c e m e n t i t e c r y s t a l s . T h e y a r e (100) [010], (010) [001], and (001) [010]. Some o t h e r p l a n e s w e r e found to be a c t i v a t e d in an o p t i c a l m i c r o s c o p y s t u d y of s l i p l i n e s f o r m e d a r o u n d V i c k e r s m i c r o h a r d n e s s i n d e n t a t i o n s on e l e c t r o p o l i s h e d s u r f a c e s of c e m e n t i t e c r y s t a l s . ~ In o r d e r to c l a r i f y the p l a s t i c b e h a v i o r of c e m e n t i t e at e l e v a t e d t e m p e r a t u r e s and the m e c h a n i s m s of d e f o r m a t i o n and f r a c t u r e of s t e e l s c o n t a i n i n g c e m e n t i t e , much i n f o r m a t i o n about the p l a s t i c i t y and f r a c t u r e b e h a v i o r of c e m e n t i t e is s t i l l r e q u i r e d . T h i s p a p e r d e a l s with a t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y study of the d e f o r m a t i o n s t r u c t u r e of c e m e n t i t e in p l a i n c a r b o n s t e e l s s t r a i n e d at v a r i o u s t e m p e r a t u r e s b e t w e e n - 196~ and 700~ T h e i m p o r t a n c e of knowledge of t h i s kind f o r u n d e r s t a n d i n g t h e d e f o r m a t i o n of s t e e l s at e l e v a t e d t e m p e r a t u r e s h a s been pointed out e l s e w h e r e , s
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A p l a i n c a r b o n s t e e l with 1.23 wt p c t C w a s u s e d in the p r e s e n t e x p e r i m e n t . I t s c h e m i c a l c o m p o s i t i o n is shown in T a b l e I. The s t e e l was p r e p a r e d f r o m e l e c t r o l y t i c i r o n and white p i g i r o n by induction m e l t i n g in an a r g o n a t m o s p h e r e . The ingot was h o t - r o l l e d and then c o l d - r o l l e d to p r o d u c e 0.45 m m t h i c k s t r i p s , f r o m which t e n s i l e s p e c i m e n s with gage d i m e n s i o n s of 10 m m width and 30 m m length w e r e m a c h i n e d . T h e y w e r e a u s t e n i t i z e d at 1000~ f o r 30
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