Tempered martensite embrittlement in phosphorus doped steels
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T H E d e v e l o p m e n t of u l t r a - h i g h s t r e n g t h s t e e l s with yield s t r e s s e s in e x c e s s of 200 ksi (1380 MN//m 2) has b e e n r e s t r i c t e d owing to the p r o b l e 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 (TMS), a l s o r e f e r r e d to as 350~ e m b r i t t l e m e n t , 500~ e m b r i t t l e m e n t , and one step t e m p e r e m b r i t t l e m e n t . 1-~6 The e m b r i t t l e m e n t o c c u r s when the a s - q u e n c h e d m a r t e n s i t e is lightly t e m p e r e d (200 to 400~ 1 to 2 h) to produce some d u c t i l i t y and toughness and yet r e t a i n a high yield s t r e n g t h . The h a r d n e s s d r o p s c o n t i n u o u s l y with i n c r e a s i n g t e m p e r Lug t e m p e r a t u r e , and one would expect an a s s o c i a t e d r i s e in t o u g h n e s s . However, a f t e r an i n i t i a l i n c r e a s e in toughness with i n c r e a s i n g t e m p e r i n g t e m p e r a t u r e t h e r e is an a n o m a l o u s loss in notch t o u g h n e s s when the t e m p e r i n g t e m p e r a t u r e is n e a r 350~ T h i s l o s s in toughn e s s is a l m o s t always c o n c u r r e n t with a change in f r a c t u r e mode f r o m t r a n s g r a n u l a r f r a c t u r e to i n t e r g r a n u l a r s e p a r a t i o n along p r i o r a u s t e n i t e g r a i n b o u n d a r i e s . R e c e n t l y i n v e s t i g a t o r s have r e p o r t e d f r a c t u r e e n e r g y r e s u l t s which show the loss in t o u g h n e s s t y p i c a l 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 without the o c c u r r e n c e of i n t e r g r a n u l a r f r a c t u r e . In some c a s e s this has been a t t r i b u t e d to the t r a n s f o r m a t i o n of r e t a i n e d a u s t e n i t e in the m a r t e n s i t e lath b o u n d a r i e s to f e r r i t e and c e m e n t i t e e i t h e r d u r i n g the t e m p e r at 350~ and above ~ or d u r i n g the m e c h a n i c a l t e s t . ~ King, Smith, and Knott ~3 have also o b s e r v e d 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 s t e e l which failed by cleavage a f t e r a l l t e m p e r i n g t e m p e r a t u r e s . They suggested that in this case the d e c r e a s e in f r a c t u r e e n e r g y o c c u r r e d b e c a u s e of the specific c a r b i d e m o r p h o l o g i e s and d i s t r i b u t i o n s c r e a t e d by the d i f f e r e n t t e m p e r i n g t r e a t m e n t s . The fact, however, s t i l l r e m a i n s that 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 c o m m e r c i a l s t e e l s is often a s s o c i a t e d with i n t e r g r a n u l a r f r a c t u r e and it is this i n t e r g r a n u l a r e m b r i t t l e m e n t that is the p r i m a r y c o n c e r n of this p a p e r . T h i s e m b r i t t l e m e n t is u s u a l l y m e a s u r e d by r o o m t e m p e r a t u r e f r a c t u r e t e s t s , such as the notched Charpy i m p a c t t e s t . F i g u r e 1 show
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