Temper embrittlement of Ni-Cr Steels by phosphorus
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C. J. McMAHON,
Jr., D. P. POPE,
AND
H. C. FENG
T e m p e r e m b r i t t l e m e n t in 3.5 pct Ni, 1.7 pct Cr s t e e l s doped with P and i s o t h e r m a l l y aged at s e v e r a l t e m p e r a t u r e s was studied by m e a s u r e m e n t s of d u c t i l e - t o - b r i t t l e t r a n s i tion t e m p e r a t u r e and h a r d n e s s , which w e r e c o r r e l a t e d with o b s e r v a t i o n s of the i n t e r g r a n u l a r f r a c t u r e s u r f a c e s by A u g e r e l e c t r o n s p e c t r o s c o p y and s c a n n i n g e l e c t r o n f r a c tography. It is shown that if all other f a c t o r s r e m a i n constant, the effect of a s m a l l change in the m a t r i x h a r d n e s s can be v e r y large; " o v e r a g i n g " (a m a x i m u m in e m b r i t t l e m e n t with r e s p e c t to aging time) was found to r e s u l t f r o m s o f t e n i n g r a t h e r than f r o m a r e v e r s a l of s e g r e g a t i o n of P. Nickel was found to be s e g r e g a t e d at the g r a i n b o u n d a r i e s , and both Ni and Cr a p p e a r to enhance the a m o u n t of s e g r e g a t e d P. The m a j o r r o l e of C r was found to be its effect of i n c r e a s i n g m a t r i x h a r d n e s s (by e n h a n c i n g h a r d e n a b i l i t y and r e s i s t a n c e to s o f t e n i n g d u r i n g t e m p e r i n g ) , r e s u l t i n g in an i n c r e a s e d s u s c e p t i b i l i t y to t e m p e r e m b r i t t l e m e n t . The effect of v a r i a t i o n s in the r o u g h n e s s of g r a i n b o u n d a r y t o p o g r a phy a p p e a r s to be s m a l l . It is shown that the s e g r e g a t i o n of P to g r a i n b o u n d a r i e s can be accounted for by diffusion f r o m the m a t r i x and is c o n s i s t e n t with the hypothesis of e q u i l i b r i u m (Gibbsian) s e g r e g a t i o n . The r e s u l t s a r e in q u a l i t a t i v e a g r e e m e n t with the t h e r m o d y n a m i c theory of G u t t m a n n .
T E M P E R e m b r i t t l e m e n t r e f e r s to the loss in toughn e s s which o c c u r s when a low alloy s t e e l is h e a t e d i n the t e m p e r a t u r e r a n g e of a p p r o x i m a t e l y 350 to 600~ (623 to 873 K) and is now known to be caused by the s e g r e g a t i o n of c e r t a i n i m p u r i t i e s to g r a i n b o u n d a r i e s , p r o d u c i n g a t e n d e n c y for i n t e r g r a n u l a r b r i t t l e f r a c t u r e and thus a shift in the d u c t i l e - b r i t t l e t r a n s i t i o n to h i g h e r t e m p e r a t u r e s 1-3 The technique of A u g e r e l e c t r o n s p e c t r o s c o p y (AES), i n t r o d u c e d by H a r r i s i n 1968, 4 p r o v i d e s a m e a n s to m e a s u r e g r a i n b o u n d a r y s e g r e g a t i o n and has b e c o m e a potent tool for the study of this p h e n o m e n o n . 5-s N e v e r t h e l e s s , a c o m p l e t e u n d e r s t a n d i n g of t e m p e r b r i t t l e n e s s has not yet b e e n achieved. While the m e c h a n i s m of t e m p e r e m b r i t t l e
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