Structure and properties of the rheocast ferrous alloy 440C
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AND
W.H.
RAND
The i n f l u e n c e of R h e o c a s t i n g upon the m i c r o s t r u c t u r e and, in a m o r e l i m i t e d s e n s e , the m e c h a n i c a l p r o p e r t i e s of a 440~ s t e e l has b e e n i n v e s t i g a t e d . T h e n a t u r e of the R h e o c a s t m i c r o s t r u c t u r e can be e x p l a i n e d in t e r m s of the s o l i d i f i c a t i o n r e a c t i o n s and phase r e l a t i o n s h i p s in the i r o n - c h r o m i u m - c a r b o n t e r n a r y s y s t e m . The R h e o c a s t 440C does not r e s p o n d a d e q u a t e l y to s t a n d a r d heat t r e a t m e n t , but a l t e r n a t e heat t r e a t m e n t s can be dev i s e d to r e s t o r e s t r e n g t h .
IN
the R h e o c a s t p r o c e s s , 1,2 a liquid a l l o y c h a r g e is cooled to a t e m p e r a t u r e within the s o l i d - l i q u i d r a n g e and v i g o r o u s l y agitated by s t i r r i n g . Such s t i r r i n g b r e a k s up the ' f i r - t r e e ' - s h a p e d s o l i d p h a s e d e n d r i t e s , r e n d e r i n g t h e s e into solid, s p h e r o i d a l p a r t i c l e s s u s pended in the liquid r e m a i n i n g . The f r a c t i o n of solid v s liquid is a f u n c t i o n of t e m p e r a t u r e , a s i n d i c a t e d by the alloy p h a s e d i a g r a m . Such a s u s p e n s i o n of s o l i d in liquid has t h i x o t r o p i c p r o p e r t i e s which c a n be r e gained e v e n a f t e r s o l i d i f i c a t i o n s i m p l y by r e h e a t i n g . 2,s T h e u s e of t h i x o t r o p i c alloy s l u r r i e s i n the die c a s t ing of p r e c i s i o n c o m p o n e n t s offers s e v e r a l a d v a n t a g e s o v e r c u r r e n t p r a c t i c e . 2,s T h e l a t t e r i n c l u d e i) i n c r e a s e d die life as a r e s u l t of lower m e t a l t e m p e r a t u r e s and the fact that a s i g n i f i c a n t p o r t i o n of the heat of f u s i o n has a l r e a d y b e e n d i s s i p a t e d , ii) r e d u c e d s o l i d i f i c a t i o n s h r i n k a g e p o r o s i t y , and iii) r e d u c e d gas e n t r a p m e n t due to the c o n t r o l l a b l e v i s c o u s flow p r o p e r t i e s of a t h i x o t r o p i c m e t a l c h a r g e . One p o t e n t i a l a p p l i c a t i o n for m a c h i n e c a s t i n g of R h e o c a s t a l l o y s i s i n the a r e a of f e r r o u s die c a s t i n g . When the c o m p o n e n t p r o d u c e d m u s t m e e t high p e r f o r m a n c e s t a n d a r d s , the i n t e r r e l a t i o n s h i p of p r o c e s s ing, s t r u c t u r e and p r o p e r t i e s b e c o m e s i m p o r t a n t i n a d d i t i o n to the r e q u i r e m e n t f o r s t r u c t u r a l i n t e g r i t y . T h i s is e s p e c i a l l y t r u e if a m a c h i n e c a s t c o m p o n e n t is c o n t e m p l a t e d as a s u b s t i t u t e for a w r o u g h t c o m p o n e n t . F o r e x a m p l e , e v e n a f t e r heat t r e a t m e n t a R h e o c a s t alloy will l i k e l y r e t a i n to s o m e extent the con
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