The relation between the fracture behavior of 4340 bend specimens and the observation of tempered martensite embrittleme
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V - n o t c h e d and fatigue p r e c r a c k e d Charpy s p e c i m e n s of v a r i o u s s i z e s , t e s t e d in i m p a c t and slow bend, w e r e used to study t e m p e r e d m a r t e n s i t e e m b r i t t l e m e n t in a 4340 steel. When plotted a s a function of t e m p e r i n g t e m p e r a t u r e , the r e s u l t s showed that the m a g n i t u d e of the t o u g h n e s s d e c r e a s e caused by e m b r i t t l e m e n t v a r i e d with the type and size of the s p e c i m e n s . E m b r i t t l e merit was always d e t e c t e d u s i n g thin s a m p l e s but i t s detection in t h i c k s p e c i m e n s depended on w h e t h e r o r not they c o n t a i n e d a p r e c r a c k . In p a r t i c u l a r , no e m b r i t t l e m e n t - a s s o c i a t e d fall in t o u g h n e s s was o b s e r v e d u s i n g s t a n d a r d s i z e p r e c r a c k e d s a m p l e s t e s t e d in slow bend. S e p a r a tion of the s h e a r and flat f r a c t u r e c o m p o n e n t s of the a b s o r b e d e n e r g i e s showed that the v a r i a tion of s h e a r e n e r g y i s a m a j o r f a c t o r c o n t r i b u t i n g to e m b r i t t l e m e n t . The r e s u l t s a r e i n t e r p r e t e d as i n d i c a t i n g that i n t e r g r a n u l a r f r a c t u r e o c c u r s m o r e as the r e s u l t of i n h i b i t i o n of p l a s tic flow within the g r a i n s r a t h e r than d i r e c t l y a s the r e s u l t of the a p p e a r a n c e of a low r e s i s tance c r a c k path at the g r a i n b o u n d a r i e s . T H E e n e r g y requixed to b r e a k a s t a n d a r d V-notch Charpy s p e c i m e n in an i m p a c t t e s t is a widely u s e d m e a s u r e of the toughness of a m a t e r i a l . U n f o r t u n a t e l y , the e n e r g y value is not a m a t e r i a l c o n s t a n t , for the e n e r g y p e r unit a r e a of c r o s s s e c t i o n a b s o r b e d by a C h a r p y s p e c i m e n i s dependent in some complex f a s h i o n upon i t s size. ~-s An i m p o r t a n t f a c t o r g o v e r n i n g this size effect i s the state of s t r e s s i n the s p e c i m e n . If the s i z e of the s p e c i m e n i s such that the p l a s t i c d e f o r m a t i o n p r e c e d i n g f r a c t u r e o c c u r s u n d e r plane s t r e s s c o n d i t i o n s , a h i g h e r e n e r g y p e r unit a r e a of c r o s s s e c tion will u s u a l l y be r e c o r d e d than if the f r a c t u r e o c c u r s u n d e r plane s t r a i n c o n d i t i o n s . T h i s s i z e dependence of the e n e r g y is i l l u s t r a t e d s c h e m a t i c a l l y in Fig. 1, w h e r e specific e n e r g y , W/A, i s plotted as a function of s p e c i m e n t h i c k n e s s , t. H e r e , as in the r e m a i n d e r of this p a p e r , W r e p r e s e n t s the e n e r g y a b s o r b e d in b r e a k i n g the s p e c i m e n , and A i s the a r e a of c r o s s section u n d e r the notch; it i s a s s u m e d that all d i m e n s i o n s except t h i c k n e s s r e m a i n unchanged.
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