Heterogeneous nuclei and graphite chemistry in flake and nodular cast irons
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N o d u l a r and flake g r a p h i t e have b e e n e x t r a c t e d f r o m c o m m e r c i a l c a s t i r o n s and s t u d i e d by a v a r i e t y of t e c h n i q u e s , including high v o l t a g e t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y . F o r n o d u l a r g r a p h i t e the n u c l e i a r e (in t h e s e c a s t i r o n s at l e a s t ) 2 to 5 /~m d i a m s p h e r i c a l p a r t i c l e s of p r i m a r i l y CeO2, MgO, and Fe203. In a d d i t i o n the n u c l e i contain v a r i o u s l y Ca, A1, la, S, and Si. T h e c o m p o s i t i o n of the n u c l e i is highly v a r i a b l e , p r e s u m a b l y due to the highly n o n e q u i l i b r i u m conditions u n d e r which they a r e f o r m e d . F o r flake g r a p h i t e the n u c l e i a r e a p p a r e n t l y s u l f i d e p a r t i c l e s , a g a i n of v a r i a b l e c o m p o s i t i o n . T h e y a r e s p h e r i c a l and a p p r o x i m a t e l y the s a m e d i a m e t e r a s the n o d u l a r n u c l e i . M o r e o v e r , c h e m i c a l a n a l y s i s of flake and n o d u l a r g r a p h i t e s shows that t h e r e is a r e l a t i v e l y high c o n c e n t r a tion of oxygen in s o l u t i o n in f l a k e g r a p h i t e but not in n o d u l a r g r a p h i t e , which s u g g e s t s oxygen i s a growth m o d i f i e r .
G R A P H I T E n u c l e a t i o n and g r o w t h in c a s t i r o n s has been the focus of an e n o r m o u s a m o u n t of r e s e a r c h in the p a s t t h r e e d e c a d e s . 1 But the p h y s i c a l p h e n o m e n a a s s o c i a t e d with the m o r p h o l o g i c a l c o n t r o l of g r a p h i t e in c a s t i r o n s a r e not y e t u n a m b i g u o u s l y d e m o n s t r a t e d . T h i s m a y be a t t r i b u t e d to the e x p e r i m e n t a l p r o b l e m s i n h e r e n t in c a s t i r o n work, w h e r e high t e m p e r a t u r e s , c o m p l e x c h e m i s t r y , and i n t e r v e n i n g s o l i d s t a t e r e a c tions m a k e e x p l i c i t n u c l e a t i o n and growth d a t a difficult to obtain. While d a t a on many a s p e c t s of c a s t i r o n s o l i d i f i cation (for e x a m p l e the e f f e c t s of c o m p o s i t i o n on cooling c u r v e s , the e f f e c t s of i n o c u l a n t s and i n o c u l a t ing p r a c t i c e on nodule counts, o r the e f f e c t s of v a r i ous d e l e t e r i o u s e l e m e n t s , lab, A s , and so f o r t h , on g r a p h i t e s h a p e ) i a r e now abundant s o m e v e r y c r u c i a l a s p e c t s have s c a r c e l y b e e n t o u c h e d . Two of the l a t t e r a r e the c h e m i s t r y of the g r a p h i t e in c a s t i r o n s and the m o r p h o l o g y and c h e m i s t r y of the h e t e r o g e n e o u s nuclei for graphite formation. The heterogeneous nuclei are important because they exert a powerful influence on the s o l i d i c a t i o n of the c a s t i r o n , and u l t i m a t e l y on the m e c h a n i c a l p r o p e r t i e s ; t h i s influence is e x e
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