Design of strong tough Fe/Mo/C martensitic steels and the effects of cobalt
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tion of a c o h e r e n t w-16 alloy c a r b i d e , but is a c c o m p a n i e d by a r e d u c t i o n in t o u g h n e s s . 9'~7 The p r e c i p i t a t e r e s p o n s i b l e for s e c o n d a r y h a r d e n i n g in m o l y b d e n u m c o n t a i n i n g s t e e l s has b e e n i d e n t i f i e d to be Mo2C.8- ~o,13- ~6, is, 19 Detailed i n v e s t i g a t i o n s of the n a t u r e and i d e n t i f i c a t i o n of this p r e c i p i t a t e have b e e n conducted by R a y n o r , e t a l . 2~ A c c o r d i n g to I r v i n e , 9 Mo is p r e f e r r e d as a s e c o n d a r y h a r d e n i n g e l e m e n t o v e r other s e c o n d a r y h a r d e n e r s , n o t a b l y W and V, b e c a u s e Mo is c h e a p e r t h a n ' W and is m o r e e a s i l y taken into s o l u t i o n than V. This i n v e s t i g a t i o n e n t a i l s m e a s u r e m e n t s of y i e l d and u l t i m a t e t e n s i l e s t r e n g t h , p l a n e s t r a i n f r a c t u r e t o u g h n e s s , C h a r p y v a l u e s , h a r d n e s s , and d e t a i l e d a n a l y s e s of the m i c r o s t r u c t u r e s u s i n g o p t i c a l and e l e c t r o n m e t a l l o g r a p h y . The s t e e l s w e r e e x a m i n e d in t h e i r a s - q u e n c h e d f u l l y m a r t e n s i t i c c o n d i t i o n and a f t e r t e m p e r i n g for one h o u r at 673 K (400~ and 873 K (600~ L E X P E R I M E N T A L PROCEDURE Table I l i s t s the c o m p o s i t i o n s and m e a s u r e d M s t e m p e r a t u r e s of the a l l o y s e x a m i n e d in this i n v e s t i g a tion. The a l l o y s and t h e i r c h e m i c a l a n a l y s i s w e r e kindly s u p p l i e d by the Republic Steel C o r p o r a t i o n .
Table I. Alloy Compositions and M s Temperatures
Ms Alloy
C
Mn
Si
Mo
Co
C198 C214 C215 C216 C217
0.t8 0.41 0.43 0.43 0.42
0.30 0.35 0.34 0.33 0.33
0.15 0.15 0.15 0.15 0.15
3.85 4.2 3.85 2.2 2.1
9.0 5.2
Measured ~ 435 341 465 332 397
VOLUME 6A, MAY 1975
969
Each m a t e r i a l was v a c u u m m e l t e d and i n i t i a l l y c a s t in 22.7 kg (50 lb) ingots. The ingots w e r e s u b s e q u e n t l y hot r o l l e d to n o m i n a l l y 1.3 c m (0.5 in.) plate and a f t e r s a n d b l a s t i n g any s c a l e off all s u r f a c e s , the p l a t e s w e r e v a c u u m h o m o g e n i z e d at 1473 K (1200~ for 48 h then f u r n a c e cooled. At this point, m e c h a n i c a l t e s t s p e c i m e n s for Charpy V - n o t c h , t e n s i l e , and p l a n e s t r a i n f r a c t u r e t o u g h n e s s (KIc) w e r e rough m a c h i n e d . Rough m a c h i n i n g for t e n s i l e s p e c i m e n s c o n s i s t e d of s q u a r i n g b l o c k s o f m a t e r i a l , with s i m i l a r d i m e n s i o n s to the KIc s p e c i m e n s , and p l a c i n g four c e n t e r holes in each block to d e l i n e a t e f u t u r e m a c h i n i n g . This p r o c e d u r e e n s u r e d that d u r i n g s u b s e q u e n t heat t r e a t m e n t , h e a t i n g and cooling r a t e s in
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