The influence of intercritical heat treatment on the temper embrittlement susceptibility of aP-Doped Ni-Cr steel
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T H E s u s c e p t i b i l i t y of alloy s t e e l s to t e m p e r e m b r i t t l e m e n t can be r e d u c e d s i g n i f i c a n t l y by what is known as a n ' i n t e r c r i t i c a l ' heat t r e a t m e n t . 1'2 I n s t e a d of the c o n v e n t i o n a l a u s t e n i t i z a t i o n above the Ac3 t e m p e r a t u r e , the s t e e l is quenched f r o m below this t e m p e r a t u r e , so that its m i c r o s t r u c t u r e p r i o r to quenching c o m p r i s e s a m i x t u r e of the y and ot p h a s e s . A l o w e r - t h a n - u s u a l t e m p e r i n g t e m p e r a t u r e is n e c e s s a r y to obtain a h a r d n e s s e q u i v a l e n t to that of the c o n v e n t i o n a l l y t r e a t e d steel. The r e a s o n for the r e d u c e d e m b r i t t l e m e n t s u s c e p t i b i l i t y is s t i l l a m a t t e r for c o n j e c t u r e . In addition to the m i c r o s t r u c t u r a l r e f i n e m e n t , which is known to r e duce e m b r i t t l e m e n t s u s c e p t i b i l i t y , 3 it is p o s s i b l e that b e n e f i t s a c c r u e f r o m solute p a r t i t i o n i n g b e t w e e n the a and y p h a s e s , and p e r h a p s f r o m the f o r m a t i o n of c a r b i d e s d u r i n g the i n t e r c r i t i c a l t r e a t m e n t . 1 Also, t h e r e is some q u e s t i o n about the s t a b i l i t y of the r e s i s t a n c e to e m b r i t t l e m e n t in c o m p o n e n t s which m u s t o p e r a t e for long t i m e s at elevated t e m p e r a t u r e s . In an a t t e m p t to elucidate the m e c h a n i s m of the effect of IHT, a study was made of t e m p e r e m b r i t t l e m e n t induced by P and Sb in a s i m p l e N i - C r s t e e l which has b e e n e x t e n s i v e l y studied and c h a r a c t e r ized.4"~ The a p p r o a c h was s i m p l y to c o m p a r e the e m b r i t t l e m e n t of i n t e r c r i t i c a l l y heat t r e a t e d (IHT) s t e e l and c o n v e n t i o n a l l y heat t r e a t e d (CHT) steel, both with the s a m e h a r d n e s s , taking account of the effect of g r a i n size v a r i a t i o n in the CHT steel. T h i s p a p e r r e p o r t s the work on the P - d o p e d steel; the Sb-doped s t e e l will be r e p o r t e d upon s u b s e q u e n t l y .
Table I. Alloy Composition C
Ni
Cr
P
0.4
3.5
1.7
0.061
a t m o s p h e r e , and then forged and hot swaged to 6.35 m m d i a m rod. The heat t r e a t m e n t s e m p l o y e d a r e d e s c r i b e d s c h e m a t i c a l l y in Fig. 1. A u s t e n i t i z a t i o n was c a r r i e d out in a d y n a m i c v a c u u m of
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