Mechanisms of tempered martensite embrittlement in low alloy steels
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AND
ROBERT
O. RITCHIE
An i n v e s t i g a t i o n into the m e c h a n i s m s of 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 (TME), a l s o know as "500~ ' ' or "350~ ' ' or o n e - s t e p t e m p e r e m b r i t t l e m e n t , has b e e n made in c o m m e r c i a l , u l t r a - h i g h s t r e n g t h 4340 and S i - m o d i f i e d 4340 (300-M) alloy s t e e l s , with p a r t i c u l a r focus given to the r o l e of i n t e r l a t h f i l m s of r e t a i n e d a u s t e n i t e . Studies w e r e p e r f o r m e d on the v a r i a t i o n of i) s t r e n g t h and t o u g h n e s s , and ii) the morphology, volume f r a c t i o n and t h e r m a l and m e c h a n i c a l s t a b i l i t y of r e t a i n e d a u s t e n i t e , as a function of t e m p e r i n g t e m p e r a t u r e , following o i l - q u e n c h i n g , i s o t h e r m a l holding, and continuous a i r cooling f r o m the a u s t e n i t i z i n g t e m p e r a t u r e . TME was o b s e r v e d as a d e c r e a s e in both Klc and Charpy V - n o t c h i m p a c t e n e r g y after t e m p e r i n g a r o u n d 300~ in 4340 and 425~ in 300-M, w h e r e the m e c h a n i s m s of f r a c t u r e w e r e e i t h e r i n t e r l a f h cleavage o r l a r g e l y t r a n s g r a n u l a r cleavage. The e m b r i t t l e m e n t was found to be c o n c u r r e n t with the i n t e r lath p r e c i p i t a t i o n of c e m e n t i t e d u r i n g t e m p e r i n g and the c o n s e q u e n t m e c h a n i c a l i n s t a bility of i n t e r l a t h f i l m s of r e t a i n e d a u s t e n i t e d u r i n g s u b s e q u e n t loading. The r o l e of s i l i c o n in 300-M was s e e n to r e t a r d these p r o c e s s e s and hence r e t a r d TME to higher t e m p e r i n g t e m p e r a t u r e s than for 4340. The m a g n i t u d e of the e m b r i t t l e m e n t was found to be s i g n i f i c a n t l y g r e a t e r in m i c r o s t r u c t u r e s c o n t a i n i n g i n c r e a s i n g v o l u m e f r a c t i o n s of r e t a i n e d a u s t e n i t e . Specifically, in 300-M the d e c r e a s e in KIc, due to TME, was a 5 MPavCm i n oil quenched s t r u c t u r e s with l e s s than 4 pct a u s t e n i t e , c o m p a r e d to a m a s sive d e c r e a s e of 70 M P a v ~ in slowly (air) cooled s t r u c t u r e s c o n t a i n i n g 25 pct a u s t e n i t e . A c o m p l e t e m e c h a n i s m of 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 is p r o p o s e d involving i) p r e c i p i t a t i o n of i n t e r l a t h c e m e n t i t e due to p a r t i a l t h e r m a l d e c o m p o s i t i o n of i n t e r l a t h f i l m s of r e t a i n e d a u s t e n i t e , and ii) s u b s e q u e n t d e f o r m a t i o n - i n d u c e d t r a n s f o r m a t i o n on loading of r e m a i n i n g i n t e r l a t h a u s t e n i t e , d e s t a b i l i z e d by c a r b o n depletion f r o m c a r b i d e p r e c i p i t a t i o n . The d e t e r i o r a t i o n in t o u g h n e s s , a s s o c i a t e d wit
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