The effect of an intercritical heat treatment on temper embrittlement of a Ni-Cr-Mo-V rotor steel
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The effect of an i n t e r c r i t i c a l h e a t t r e a t m e n t on t e m p e r e m b r i t t l e m e n t h a s b e e n i n v e s t i g a t e d f o r a r o t o r s t e e l c o n t a i n i n g 0.25 p c t C, 3.5 p c t Ni, 1.7 p c t C r , 0.5 pct Mo, 0.1 p c t V, and d e l i b e r a t e a d d i t i o n s of p h o s p h o r u s , tin, o r antimony. Both m a r t e n s i t i c and b a i n i t i c s t e e l s w e r e h e l d at the i n t e r c r i t i c a l t e m p e r a t u r e of 1380~ (750~ f o r t i m e s up to 40 h and w e r e then quenched o r c o o l e d to obtain m a r t e n s i t i c o r b a i n i t i c t r a n s f o r m a t i o n . The s t e e l s w e r e then t e m p e r e d , followed b y w a t e r quenching o r s t e p cooling f r o m the t e m p e r i n g t e m p e r a t u r e . The r e s i d u a l f e r r i t e m a i n t a i n e d a f i n e p l a t e - l i k e s h a p e even a f t e r 40 h at the i n t e r c r i t i c a l t e m p e r a t u r e . E m b r i t t l e m e n t induced b y s t e p cooling f r o m the final t e m p e r i n g was m a r k e d l y r e d u c e d by the i n t e r c r i t i c a l t r e a t m e n t a s c o m p a r e d to the e m b r i t t l e m e n t o b s e r v e d a f t e r c o n v e n t i o n a l h e a t t r e a t m e n t ; f o r e x a m p l e , A F A T T , the i n c r e a s e in the C h a r p y V - n o t c h 50 p c t s h e a r f r a c t u r e t r a n s i t i o n t e m p e r a t u r e c a u s e d b y s t e p cooling, was r e d u c e d b y at l e a s t 80~ (45~ a s a r e s u l t of the i n t e r c r i t i c a l t r e a t m e n t of s t e e l s containing 0.02 p c t P. M o l y b denum e f f e c t i v e l y r e d u c e d A F A T T in i n t e r c r i t i c a l l y h e a t - t r e a t e d s t e e l s a s w e l l a s in c o n v e n t i o n a l l y t r e a t e d s t e e l s . P o s s i b l e m e c h a n i s m s f o r r e d u c i n g t e m p e r e m b r i t t l e m e n t with the i n t e r c r t t i c a l t r e a t m e n t a r e d i s c u s s e d .
T E M P E R e m b r i t t l e m e n t of s t e e l s h a s b e e n a s u b j e c t of r e s e a r c h f o r s e v e r a l d e c a d e s , ~'z and it has b e e n e s t a b l i s h e d that such i m p u r i t i e s a s p h o s p h o r u s , antimony, and tin c o n t a i n e d in a l l o y s t e e l s a r e p r i m a r i l y r e s p o n s i b l e f o r the e m b r i t t l e m e n t . R e c e n t i n v e s t i g a t i o n s with A u g e r s p e c t r o s c o p y 3-5 have r e v e a l e d that t h e s e i m p u r i t i e s a r e p r e s e n t at a s u b s t a n t i a l l y h i g h e r c o n c e n t r a t i o n in 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 than within the g r a i n s of e m b r i t t l e d s t e e l s . It is a l s o known that contents of n o r m a l a l l o y i n g e l e m e n t s , such a s m a n g a n e s e , c h r o m i u m , o r nickel, influence the extent of e m b r i t t l e m e n t b y i n d i v i d u a l i m p u r i t i e s . 8 Some e l e m e n t s , for i n s t a n c e m o l y b d e n u m , a r e known to r
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