On the formation of macrosegregations in ingots
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S. O. N I L S S O N
A new quenching technique f o r s t u d y i n g the f o r m a t i o n of m a c r o s e g r e g a t i o n s in ingots h a s been d e v e l o p e d and u s e d in a study of the f o r m a t i o n of A - s e g r e g a t i o n s in ingots. The f o r m a t i o n of a n e g a t i v e s e g r e g a t i o n in the b o t t o m p a r t of an ingot and a p o s i t i v e s e g r e g a t i o n in the top p a r t have a l s o b e e n s t u d i e d . It was found that the t h r e e t y p e s of m a c r o s e g r e g a tion p h e n o m e n a could be e x p l a i n e d by the o c c u r r e n c e of i n t e r d e n d r i t i c convection d u r i n g the s o l i d i f i c a t i o n p r o c e s s .
B Y definition, m a c r o s e g r e g a t i o n gives r i s e to a d e v i a t i o n in the a v e r a g e c o m p o s i t i o n of d i f f e r e n t p a r t s of an ingot o r a c a s t i n g . It d e p e n d s p r i m a r i l y upon m i c r o s e g r e g a t i o n by which the a l l o y i n g e l e m e n t s u s u a l l y c o n c e n t r a t e in the m e l t r e m a i n i n g b e t w e e n the dendrite arms. Macrosegregation occurs as a result of the t r a n s p o r t of s u c h e n r i c h e d liquid o v e r long d i s t a n c e s . T h i s t r a n s p o r t may, f o r e x a m p l e , be c a u s e d by gas f o r m a t i o n d u r i n g s o l i d i f i c a t i o n , b y s o l i d i f i c a t i o n s h r i n k a g e o r by convection due to d e n s i t y d i f f e r e n c e s in the m e l t e i t h e r d e p e n d i n g on d i f f e r e n c e s in c o m p o s i tion o r d i f f e r e n c e s in t e m p e r a t u r e . The t r a n s p o r t of liquid due to s o l i d i f i c a t i o n s h r i n k age c a u s e s a type of m a c r o s e g r e g a t i o n c a l l e d i n v e r s e s e g r e g a t i o n , w h e r e the s u r f a c e l a y e r of a c a s t i n g has an i n c r e a s e d a l l o y content. T h i s type of m a c r o s e g r e gation was f i r s t d e a l t with by M a s i n g ~ and h a s b e e n t r e a t e d t h e o r e t i c a l l y b y Scheil, 2 A d a m s , a and Y o u d e l i s and K i r k a l d y . 4 The o c c u r r e n c e of m a c r o s e g r e g a t i o n a s a r e s u l t of s o l i d i f i c a t i o n s h r i n k a g e has s u b s e q u e n t l y b e e n d e a l t with in a m o r e g e n e r a l m a n n e r by F l e m i n g s and his c o l l e a g u e s . 5-7 T h e i r s t u d i e s have i n c l u d e d the t h e o r e t i c a l and e x p e r i m e n t a l study of the influence of the s h a p e of the c a s t i n g . T h e m e c h a n i s m of the f o r m a t i o n of the V and A s e g r e g a t i o n in ingots has b e e n s t u d i e d d u r i n g a v e r y long t i m e . H u l t g r e n , a and L a r s e n , 9 have s u g g e s t e d that the s o l i d i f i c a t i o n s h r i n k a g e is a l s o r e s p o n s i b l e f o r the o c c u r r e n c e of those p h e n o m e n a in ingots. H o w e v e r , a l r e a d y 1933 C a r l s s o n *~ in a study of the s o l i d i f i c a t i o n of s t e
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