Steel casting by diffusion solidification
- PDF / 2,538,986 Bytes
- 15 Pages / 612 x 792 pts (letter) Page_size
- 96 Downloads / 248 Views
I. INTRODUCTION D I F F U S I O N s o l i d i f i c a t i o n c a s t i n g is a new p r o c e s s for m a k i n g s t e e l objects. C o m p a r e d with c o n v e n t i o n a l casting, the p r o c e s s t e m p e r a t u r e s a r e about 200~ lower, t h e r e is a one t h i r d as much s o l i d i f i c a t i o n s h r i n k a g e , and s o l i d i f i c a t i o n is f a s t e r . As shown in Fig. 1, s o l i d i f i c a t i o n o c c u r s by the r e d i s t r i b u t i o n of c a r b o n within a m i x t u r e of solid low c a r b o n s t e e l and high c a r b o n liquid i r o n . In c o n t r a s t to c o n v e n t i o n a l s t e e l casting, little o r no heat need be r e j e c t e d to the s u r r o u n d i n g s . Solidification is i n s t e a d c o n t r o l l e d by the diffusion of c a r b o n , so that the s i z e and shape of the object a r e not as i m p o r t a n t as in t h e r m a l s o l i d i f i cation. In p r a c t i c e , the high c a r b o n liquid is f o r c e d to i n f i l t r a t e a bed of p r e h e a t e d , low c a r b o n s p h e r i c a l shot which fills about 5//8 of the space in a r e f r a c t o r y mold. As the liquid l o s e s its c a r b o n , i r o n p r e c i p i t a t e s as a smooth coating on the solid. The s o l i d i f i c a t i o n t i m e can be p r e d e t e r m i n e d by s e l e c t i n g the solid p a r t i c l e s i z e so that it is as s h o r t as a few s e c o n d s o r m i n u t e s ; however, the p a r t i c l e s i z e m u s t be l a r g e enough to a l low i n f i l t r a t i o n to o c c u r r a p i d l y enough to p e r m i t c o m plete filling of the mold. Since the p r a c t i c a l o b j e c t i v e s of diffusion s o l i d i f i c a tion ( r e f e r r e d to h e r e i n a f t e r as SD to avoid confusion with d i r e c t i o n a l s o l i d i f i c a t i o n ) a r e to obtain r a p i d and complete s o l i d i f i c a t i o n without d e f e c t s , and b e c a u s e of the i m p o r t a n c e of s t e e l s , this r e p o r t e m p h a s i z e s the m e c h a n i c s and k i n e t i c s of mixing, the t h e r m o d y n a m i c s
and k i n e t i c s of casting, the f o r m a t i o n and r e d u c t i o n of oxides and t h e i r r o l e in c a s t i n g ductility, and m a c r o and m i c r o s e g r e g a t i o n of C and alloying e l e m e n t s in SD c a r b o n s t e e l s . Diffusion s o l i d i f i c a t i o n in the F e - M n and F e - h e a t s y s t e m a r e also d i s c u s s e d .
a ) PHASE DIAGRAM Dr = COMPOSITION OF PREEXISTING SOLID C2 = AVERAGE COMPOSITION OF THE MIXTURE OF SOLID AND LIQUID
Tj
Cs = SOLUTE CONCENTRATION AT SOLID/LIQUID INTERFACE CL = COMPOSITION OF LIQUID T2-TI = TEMPERATURE RISE DUE TO THE HEAT OF SOLIDIFICATION
METALLURGICAL TRANSACTIONS B
%B--~
b) MECHANISM: SOLID AND LIQUID MIXED INSTANTANEOUSLY IN A REFRACTORY VESSEL
NO DIFFUSION HAS TAKEN PLACE
CI
= CL q/
GEORGE LANGFORD, formerly with the Monsanto Triangle Park Development Center, Inc. is now Associate Professor in the Department of Materials Engine
Data Loading...
