The effects of MO and W on solidification of high speed steels
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Davis-Monthan A. F. B., Arizona. R. W. KRAFT is Professor, Department of Metallurgy and Materials Science, Lehigh University, Bethlehem, Pc. 18015. Manuscript submitted July 30, 1973. METALLURGICAL TRANSACTIONS
the s l o p e change with the c r y s t a l l i z a t i o n of MC c a r b i d e s . B ~ c k e r u d and P f e i f e r 14 contend t h a t t h i s d e f l e c tion s i g n a l s the f o r m a t i o n of MeC, but r e c e n t w o r k b y Snyder is v e r i f i e s B a r k a l o w ' s i n t e r p r e t a t i o n . The r e a c t i o n which o c c u r s at the 1240~ (2264~ s o l i d u s t e m p e r a t u r e would then c o r r e s p o n d to the e u t e c t i c d e c o m p o s i t i o n of the l a s t r e m a i n i n g l i q u i d into a u s t e n ite and M6C. The t y p i c a l a s - c a s t s t r u c t u r e of a high s p e e d s t e e l c o n s i s t s of a r e a s of d e l t a e u t e c t o i d r e a c t i o n b y - p r o d u c t s s e p a r a t e d f r o m i n t e r d e n d r i t i c e u t e c t i c b y a band of a u s t e n i t i c o r m a r t e n s i t i c m a t r i x . 8'9'~2 While the p r e s e n c e of e u t e c t i c i s c h a r a c t e r i s t i c of t h e s e m a t e r i a l s , i t s m o r p h o l o g y is s t r o n g l y i n f l u e n c e d b y the a l l o y c o m p o s i t i o n . ~7 A f a n - s h a p e d o r f e a t h e r y e u t e c tic is c o m m o n to the t u n g s t e n - m o l y b d e n u m and m o l y b d e n u m g r a d e s such a s M2. ~8 High m a g n i f i c a t i o n e x a m i n a t i o n of t h e s e c a r b i d e s , a l o n g with t h e i r s e l e c t i v e e t c h i n g r e s p o n s e and m i c r o a n a l y s i s , i n d i c a t e s that t h e y a r e a l a y e r e d s t r u c t u r e of MC and M6C. ~~ An M C / a u s t e n i t e e u t e c t i c o r i s o l a t e d p a r t i c l e s of MC a l s o o c c u r to s o m e e x t e n t in m o s t high s p e e d s t e e l s . In Ni and Co b a s e s u p e r a l l o y s , such a s M a r M302, MC t y p e c a r b i d e s can f o r m b y a L ~ MC t y p e r e a c t i o n a s w e l l a s b y a e u t e c t i c t y p e r e a c t i o n , L ~ A + MC. The f o r m e r p r o d u c e s b l o c k y MC c a r b i d e s which a r e r a n d o m l y d i s t r i b u t e d w h e r e a s the l a t t e r y i e l d s a fine d i s p e r s i o n s i m i l a r to t h a t o b s e r v e d in o u r M2 s p e c i m e n s . 16 T h e s e o b s e r v a t i o n s s u g g e s t that the fine MC c a r b i d e s in M2 m i g h t b e f o r m e d s o l e l y b y a e u t e c t i c r e a c t i o n . While such a p o s s i b i l i t y cannot b e r u l e d out and m a y o c c u r to s o m e e x t e n t it is not c o n s i d e r e d the p r e d o m i n a n t m e c h a n i s m in M2. E v e n though the i n v e s t i g a t o r s who have s t u d i e d M2 in d e t a i l x~ have s o m e d i f f e r e n c e s of i n t e r p r e t a t i o n on the f i n a l s o l i d i f i c a t i o n r e a c t i o n in M2 none of t h e m have s u g g e s t e d that MC c a r b i d e s in M2 a r e f o r m e d on
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