Microcracks associated with inclusions in as-cast iron and their role during fracture
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n u m b e r of r e c e n t s t u d i e s l - s have been m a d e on the e f f e c t s of n o n m e t a l l i c i n c l u s i o n s on the m e c h a n i c a l p r o p e r t i e s and f r a c t u r e of a l l o y s . It h a s b e e n o b s e r v e d that the m i c r o c r a c k s a s s o c i a t e d with i n c l u s i o n s can c a u s e p r e m a t u r e f r a c t u r e and d e c r e a s e d d u c t i l i t y . R o s e n f i e l d 4 h a s p u b l i s h e d a t h o r o u g h r e v i e w of the m e c h a n i s m s of i n i t i a t i o n , g r o w t h , and c o a l e s c e n c e of microcracks resulting from either inclusion-matrix i n t e r f a c e s e p a r a t i o n or i n c l u s i o n f r a c t u r e d u r i n g m e c h a n i c a l t e s t i n g . M o s t of the s t u d i e s m a d e on the e f f e c t of i n c l u s i o n s on f r a c t u r e b e h a v i o r e i t h e r a s s u m e the c r a c k s a r e a l r e a d y p r e s e n t in the p a r t i c l e s o r a r e f o r m e d due to e x t e r n a l l y a p p l i e d s t r e s s e s . In t h i s w o r k we have e x a m i n e d the f o r m a t i o n of m i c r o c r a c k s and h o l e s in and a r o u n d the i n c l u s i o n s d u r i n g c o o l i n g of liquid i r o n . Of s p e c i a l i n t e r e s t to this w o r k i s the p r e d i c t i o n of Backofen 5 that c r a c k s a s s o c i a t e d with i n c l u s i o n s could be f o r m e d d u r i n g heating o r c o o l i n g (due to unequal t h e r m a l e x p a n s i o n of i n c l u s i o n and m a t r i x ) . G e l l and L e v e r a n t 6 have shown that M C type c a r b i d e s in c a s t MAR-M200 c r a c k e d d u r i n g c o o l ing a f t e r s o l i d i f i c a t i o n . W o r k by B r o o k s b a n d and A n d r e w s 7 has shown that the s t r e s s e s such a s those c a u s e d by quenching could c a u s e the c r a c k i n g of s o m e inclusions.
E X P E R I M E N T A L PROCEDURE T h e f a c t o r s r e l a t e d to i n c l u s i o n s which a f f e c t the m e c h a n i c a l p r o p e r t i e s of i n c l u s i o n - b e a r i n g m a t e r i a l s a r e : a) v o l u m e f r a c t i o n of i n c l u s i o n s , b) s i z e and s h a p e of i n c l u s i o n s , c) d i s t r i b u t i o n of i n c l u s i o n s , and d) c h e m i c a l n a t u r e of i n c l u s i o n s . Out of t h e s e v a r i a b l e s the v o l u m e f r a c t i o n s , the shape and the d i s t r i b u tion of i n c l u s i o n s was k e p t c o n s t a n t d u r i n g the c o u r s e of the p r e s e n t i n v e s t i g a t i o n . Two t y p e s of i n c l u s i o n s w e r e e x a m i n e d ; F e O and SiO2. F o r F e O i n c l u s i o n s an a l l o y F e - - 0 . 0 7 p e t O w a s u s e d and f o r SiO2 i n c l u s i o n s an a l l o y F e - - 0 . 0 5 p c t Si W. E. BROWER, Jr. is Research Associate, Mechanical Engineering Department, Duke University, Durham, N. C. S. N. SINGH is Senior Research Engineer, Process Metallurgy Development Division, U. S. Steel Corporation, Research Laboratory, Monroeville, Pa. 15146.
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