Metallography of bainitic transformation in silicon containing steels
- PDF / 7,217,213 Bytes
- 14 Pages / 612 x 792 pts (letter) Page_size
- 62 Downloads / 226 Views
T
H
E
t e r m b a i n i t e , so n a m e d in honor of E. C. Bain, r e f e r s to t h e m i c r o s t r u c t u r a l c o n s t i t u e n t f o r m e d in s t e e l s when a u s t e n i t e d e c o m p o s e s at t e m p e r a t u r e s a b o v e that for m a r t e n s i t i c f o r m a t i o n but b e l o w that for pearlite through either isothermal transformation o r continuous cooling. The m o r p h o l o g y of b a i n i t e c h a n g e s g r a d u a l l y with r e a c t i o n t e m p e r a t u r e , s o no p r o n o u n c e d s t r u c t u r a l c h a n g e s a r e o b s e r v e d o v e r any s m a l l t e m p e r a t u r e r a n g e . Although the m i c r o s t r u c t u r e m a y a s s u m e a v a r i e t y of f o r m s , two m a j o r v a r i a n t s of b a i n i t e m o r p h o l o g y , n a m e l y u p p e r and l o w e r b a i n i t e , a r e o b s e r v e d in s t e e l s t r a n s f o r m e d in d i f f e r ent r a n g e s of t e m p e r a t u r e . 2"~ T h e t r a n s f o r m a t i o n m e c h a n i s m of b a i n i t e has b e e n a topic of d i s c u s s i o n e v e r s i n c e the s t r u c t u r e was f i r s t r e c o g n i z e d . T h e p r o b l e m i s c o m p l i c a t e d in view of the f a c t that the b a i n i t e r e a c t i o n often e x h i b i t s f e a t u r e s c h a r a c t e r i s t i c of two b a s i c c l a s s e s of t r a n s f o r m a t i o n in i r o n a l l o y s , the diffusion c o n t r o l l e d and the m a r t e n s i t i c type. A s p o i n t e d out in a r e c e n t d e b a t e on b a i n i t e , 1 both the n a t u r e of c a r b i d e s and w h e t h e r the c a r b i d e s in b a i n i t e p r e c i p i t a t e d f r o m s u p e r s a t u r a t e d f e r r i t e o r f r o m a u s t e n i t e is of fundam e n t a l s i g n i f i c a n c e to the b a i n i t e r e a c t i o n m e c h a n i s m . It is g e n e r a l l y a c c e p t e d that c e m e n t i t e c o n s t i t u t e s the c a r b i d e p h a s e in u p p e r b a i n i t e . Although c e m e n t i t e is l i k e w i s e f r e q u e n t l y o b s e r v e d in l o w e r b a i n i t e , the exact carbide precipitation process is still uncert a i n . It h a s b e e n r e p o r t e d that e p s i l o n c a r b i d e i s the f i r s t c a r b i d e f o r m e d in low t e m p e r a t u r e b a i n i t e which i s s u b s e q u e n t l y r e p l a c e d b y c e m e n t i t e on f u r t h e r t r a n s f o r m a t i o n . 5-7 T h e s e r e p o r t s w e r e b a s e d e i t h e r on conclusions drawn indirectly from physical measureDER-HUNG HUANG, formerly with the Department of Materials Science and Mineral Engineering, University of California, is now with Rohm and Haas Company Research Laboratories, Bristol, PA 19007 and GARETH THOMAS is Professor of Metallurgy, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. Manuscript submitted December 23, 1975. METALLURGICAL TRANSACTIONS A
ments or from direct electron microscopy observat i o n s . 7 U n f o r t u n a
Data Loading...
