High strength low carbon sheet steel by thermomechanical treatment:II. Microstructure
- PDF / 2,450,526 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 82 Downloads / 210 Views
A t h e r m a l m e c h a n i c a l t r e a t m e n t f o r d e v e l o p i n g high s t r e n g t h in a low s t r e n g t h low c a r b o n s t e e l was d e s c r i b e d in a p r e v i o u s p a p e r x which e x a m i n e d the r e l a t i o n s h i p s b e t w e e n p r o c e s s i n g and m e c h a n i c a l p r o p e r t i e s . T h a t p a p e r r e f e r r e d to the m i c r o s t r u c t u r a l changes o c c u r r i n g d u r i n g p r o c e s s i n g and t h e i r eff e c t on the p r o p e r t i e s o b s e r v e d , without p r e s e n t i n g a n y d i r e c t e v i d e n c e to s u b s t a n t i a t e that a n a l y s i s . TMT p r o c e s s i n g h a s b e e n a p p l i e d to SAE 1010 s t e e l , SAE 1010 n i t r o g e n i z e d s t e e l , and SAE 945X 2 s t e e l . T h e p u r p o s e of t h i s w o r k was to d e t e r m i n e and c o m p a r e the effect of T M T p r o c e s s i n g on the m i c r o s t r u c t u r e of t h e s e t h r e e s t e e l s . PROCEDURE T h e T M T p r o c e s s i s i n d i c a t e d s c h e m a t i c a l l y in F i g . 1. S a m p l e s of hot r o l l e d SAE 1010, SAE 1010 n i t r o gnnized, and SAE 945X s t e e l s w e r e s u b j e c t e d to v a r i ous s t e p s in t h e p r o c e s s and the m i c r o s t r u c t u r e s obs e r v e d u s i n g both o p t i c a l and 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 . T h e c h e m i c a l c o m p o s i t i o n of e a c h s t e e l i s given in T a b l e I. Samples for optical microscopy were sectioned, m e c h a n i c a l l y p o l i s h e d , e t c h e d , and e x a m i n e d at 1100 t i m e s m a g n i f i c a t i o n u s i n g a R e i c h e r t MeF1 m e t a l l o graph. Samples for transmission electron microscopy w e r e c h e m i c a l l y thinned to 0.15 m m and s u b s e q u e n t l y e l e c t r o p o l i s h e d to e l e c t r o n t r a n s p a r e n c y . A l l thin f o i l s w e r e e x a m i n e d in a S i e m e n s 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 e o p e r a t i n g at a n a c c e l e r a t i n g p o t e n t i a l of 100 kV, u t i l i z i n g s e l e c t e d a r e a d i f f r a c t i o n and d a r k f i e l d t e c h n i q u e s f o r a n a l y z i n g the m i c r o s t r u c t u r e s .
Conditioning T r e a t m e n t T h e e f f e c t of h e a t i n g to 760~ and r a p i d l y q u e n c h ing is to p r o d u c e a s e c o n d p h a s e with d i f f e r e n t e t c h ing c h a r a c t e r i s t i c s f r o m the f e r r i t e . T h i s s e c o n d p h a s e is l o c a t e d p r e f e r e n t i a l l y at g r a i n b o u n d a r i e s and grain boundary triple points. Tempering markedly i n c r e a s e s the c o n t r a s t b e t w e e n the f e r r i t e and the s e c o n d p h a s e . T h e s e c o n d p h a s e can r e a d i l y be noted in F i g . 3. T h e s e c o n d p h a s e o b s e r v a b l e on the o p t i c a l m i c r o graphs is revealed by transmission electron micros c o p y to be m a r t e n s i t e . E x a m p l e s of both d i s l o c a t e d
Data Loading...
