The influence of Mo on P-lnduced temper embrittlement in Ni-Cr steel
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h as b e e n known f o r d e c a d e s that P is one of the principal impurities responsible for temper embrittlemerit in al l o y s t e e l s I and that the addition of Me a c t s to inhibit this e m b r i t t l e m e n t . ~ M o r e r e c e n t l y , it h as b e e n shown 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 (AES) s,4 that P - i n d u c e d e m b r i t t l e m e n t is a s s o c i a t e d with a b u i l d - u p of P at g r a i n b o u n d a r i e s , which t h e r e b y b e c o m e s i t e s f o r low e n e r g y f r a c t u r e . M u l f o r d et al s h a v e d e m o n s t r a t e d that t e m p e r e m b r i t t l e m e n t by P in a N i - C r s t e e l i n v o l v e s e q u i l i b r i u m s e g r e g a t i o n of both P and Ni to g r a i n b o u n d a r i e s . T h e e q u i l i b r i u m l e v e l s of t h e s e c o s e g r e g a t e d e l e m e n t s i n c r e a s e d i r e c t l y with the bulk c o n c e n t r a t i o n of P and i n v e r s e l y with the t e m p e r a t u r e of e q u i l i b r a t i o n ; the r a t e of
a s d e s c r i b e d e l s e w h e r e . 5,s S o m e s p e c i m e n s of 3.7 m m d i a m , which w e r e g i v en i d e n t i c a l h e a t t r e a t m e n t s , w e r e f r a c t u r e d at "~-90~ in a v a c u u m of ~10 -1~ T o r r and a n a l y z e d by A E S a s d e s c r i b e d e l s e w h e r e . ~ T h e s e f r a c t u r e s u r f a c e s w e r e s u b s e q u e n t l y e x a m i n e d by ~canning e l e c t r o n m i c r o s c o p y (SEM) and the p e r c e n t i n t e r g r a n u l a r f r a c t u r e d e t e r m i n e d . T h e l a t t e r data w e r e u s e d to weight the A E S d a t a to r e f l e c t the c o m p o s i t i o n of the g r a i n b o u n d a r i e s ; 5 peak h e i g h t s w e r e c o n v e r t e d to at. pct by m e a n s of c a l i b r a t i o n c u r v e s . %9 Th e g r a i n s i z e s a r e shown in T a b l e I.
segregation can be described by a McLean-types analysis, and it appears to be controlled by diffusion of P in the alloyed ferrite matrix. Schulz7 examined the effect of the addition of 0.6 pct Me to a Ni-Cr steel with 0.4 pct C and 0.06 pct P with coarse grain sizes (0.2 to 0.5 ram) and found that the Me addition did not appear to eliminate the P-induced temper embrittlement at long aging times (up to 19,000 h). The present study was undertaken to clarify the Me effect in Pdoped Ni-Cr steels using lower P and C contents and a finer grain size than those employed by Schulz.
The response to the 475~ embrittlement treatment of the 0.02 pct P steel with and without the addition of 0.6 pct Me is shown in Fig. 1. The P-doped steel without Me underwent fairly substantial embrittlement, the transition temperature rising about 100~ after I00 h aging before decreasing slightly at longer aging times. As shown by Mulford e t a [ , s the rise corresponds to equilibrium segregation of P to grain boundaries (steady state at 480~ being reached within I00 h) and the decrease is due to the softening which occurs during prolonged aging, as indicat
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