Behavior of sulfide inclusions during thermomechanical processing of AISI 4340 steel
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the ~ ~ ot t r a n s f o r m a t i o n of the s t e e l under c o n s i d e r a tion. T h e r e is a d e q u a t e e x p e r i m e n t a l e v i d e n c e that the high t e m p e r a t u r e heat t r e a t m e n t of wrought f e r r o u s a l l o y s m o d i f i e s the m o r p h o l o g y of sulfide i n c l u s i o n s and l e a d s to s i g n i f i c a n t l y b e t t e r m e c h a n i c a l p r o p e r t i e s ) %18 R e c e n t l y it was r e p o r t e d 19 that s i g n i f i c a n t c o a r s e n i n g of s u l f i d e s t r i n g e r s o c c u r r e d d u r i n g e a r l y s t a g e s of h o m o g e n i z a t i o n s u b s e q u e n t to h o t - r o i l i n g . T h e p r e s e n t i n v e s t i g a t i o n is a i m e d at g a i n i n g insight into the g e o m e t r i c a l and c o m p o s i t i o n a l m o d i f i c a t i o n s of sulfide i n c l u s i o n s in AISI 4340 low a l l o y s t e e l which o c c u r d u r i n g h o t - r o l l i n g and s u b s e q u e n t h o m o g e n i z a t i o n . The effect of p r o c e s s v a r i a b l e s s u c h a s c o o l i n g r a t e d u r i n g s o l i d i f i c a t i o n , r o l l i n g t e m p e r a t u r e and h o m o g e n i z a t i o n t i m e on s u l f i d e s was s y s t e m a t i c a l l y s t u d i e d .
EXPERIMENTAL
PROCEDURE
The AISI 4340 low alloy steel ingots were prepared and c a s t u s i n g a p p r o p r i a t e q u a n t i t i e s of p u r e i r o n ( F e r r o v a c E), m o l y b d e n u m and n i c k e l , and m a s t e r a l l o y s of c h r o m i u m , s i l i c o n and c a r b o n . The m e l t was p r e p a r e d in an a l u m i n a c r u c i b l e u n d e r an a r g o n b l a n ket. U n i d i r e c t i o n a l l y s o l i d i f i e d ingots w e r e c a s t in 0.16 x 0.09 x 0.05 m e x o t h e r m i c m o l d s , a s p r e v i o u s l y d e s c r i b e d , s~ The l o c a l a v e r a g e cooling r a t e s in the s o l i d i f y i n g ingots w e r e m e a s u r e d at f o u r d i f f e r e n t d i s t a n c e s f r o m the b o t t o m c o p p e r chill, u s i n g P t / P t 10 pct Rh t h e r m o c o u p l e s . T h e c h e m i c a l l y a n a l y z e d c o m p o s i t i o n s of the ingots a r e given in T a b l e I. T h e i n g o t s w e r e s l i c e d t r a n s v e r s e l y (0.09 • 0.05 • 0.03 m) a t v a r i o u s d i s t a n c e s f r o m the c h i l l and h o t r o l l e d at 1223 K up to 85 pct following the p r o c e d u r e d e s c r i b e d in a p r e v i o u s p u b l i c a t i o n . 19 H o m o g e n i z a t i o n t r e a t m e n t s w e r e c a r r i e d out in v a c u u m (10 Pa) at 1583 K for v a r i o u s lengths of t i m e . At the end of the h o m o g e n i z a t i o n t r e a t m e n t the s a m p l e s w e r e f u r n a c e cooled. T h e m i c r o s t r u c t u r e of s u l f i d e s p r i o r to and d u r i n g VOLUME 8A, AUGUST 1977-1275
Table 1. Typical Chemical Analysis of AISI 4340 Steel Ingots Elemental Composition in Weight Percent A1
C
Cr
Fe
Mn
Mo
0.08 to 0.I
0.39 to 0.4t
0.7 to 0.75
Balance
0.55 to 0.65
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