Tempered martensite embrittlement in SAE 4340 steel
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M A R T E N S I T I C m e d i u m C s t e e l s show a d e c r e a s e in r o o m t e m p e r a t u r e i m p a c t t o u g h n e s s and an i n c r e a s e in t r a n s i t i o n t e m p e r a t u r e when t e m p e r e d in the t e m p e r a t u r e r a n g e 523 K (250~ to 673 K (400~ 1-4 T h i s e m b r i t t l e m e n t has b e e n v a r i o u s l y r e f e r r e d to a s 500~ e m b r i t t l e m e n t , 3-5 350~ e m b r i t t l e m e n t , 6 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 , 7-1~ and o n e - s t e p t e m p e r e m brittlement. 1 Tempered martensite embrittlement (TME) will be the t e r m u s e d in this p a p e r ; t h e r e i s the l i t e r a t u r e p r e c e d e n t for i t s u s a g e , and it d o e s not tie the p h e n o m e n o n to a s p e c i f i c t e m p e r a t u r e , c o m p o s i t i o n o r s e q u e n c e of p r o c e s s i n g o p e r a t i o n s . B o t h i n t e r g r a n u i a r 1 and t r a n s g r a n u l a r 9 f r a c t u r e have been o b s e r v e d to a c c o m p a n y T M E . I n t e r g r a n u l a r f r a c t u r e in s p e c i m e n s t e m p e r e d in the c r i t i c a l r a n g e was e x p l a i n e d in the 1940's and 1950's by the d e c o m p o s i t i o n of r e t a i n e d a n s t e n i t e to f e r r i t e , 1 the f o r m a t i o n of c e m e n t i t e with an a d j a c e n t thin f e r r i t e zone at 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 , ~ o r the f o r m a t i o n of c e m e n t i t e along m a r t e n s i t e p l a t e b o u n d a r i e s c o i n c i d ing with p r i o r a u s t e n i t e g r a i n b o a n d a r i e s P C a p u s , 4 h o w e v e r , in the e a r l y 1960's p r o p o s e d that c a r b i d e m o r p h o l o g y alone was not r e s p o n s i b l e f o r T M E , and showed that i m p u r i t y e l e m e n t s s u c h a s P , A s , Sb, and Sn m u s t c o n t r i b u t e to TME t o g e t h e r with the f o r m a t i o n of c a r b i d e f i l m s . Kuia and A n c t i l 8, in 1969, d e m o n s t r a t e d the e x i s t e n c e of TME in 4340 s t e e l b y Kic m e a s u r e m e n t s , and e x t e n d e d the i m p u r i t y - c a r b i d e i n t e r a c t i o n e x p l a n a t i o n of TME by p r o p o s i n g that i m p u r i t y a t o m s at p r i o r a u s t e n i t i c g r a i n b o u n d a r y a r e f i r s t i n c o r p o r a t e d into the c e m e n t i t e and then r e j e c t e d f r o m p l a t e l e t c e m e n t i t e to p r o v i d e an e a s y p a t h f o r c r a c k propagation. Recently, Auger electron spectroscopy (AES) and s c a n n i n g e l e c t r o n m i c r o s c o p y have made p o s s i b l e b e t t e r c h a r a c t e r i z a t i o n of i m p u r i t y e f f e c t s and JAMES P. MATERKOWSKIis Metallurgist, Republic Steel, Corporation Research Center, Cleveland, OH 44131. GEORGE KRAUSS is AMAX Foundation Professor, Colorado School of Mines, Golden, CO 80401. Manuscript submitted December 19, 1978.
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