• PDF / 1,814,954 Bytes
• 13 Pages / 612 x 792 pts (letter) Page_size
• 17 Downloads / 306 Views

DOWNLOAD

REPORT

---

IN

the wide v a r i e t y o f r e a c t i o n k i n e t i c s o b s e r v e d i n t h e m a r t e n s i t i c t r a n s f o r m a t i o n o f i r o n a l l o y s it i s p o s s i b l e to d i s c e r n four distinct t y p e s o f b e h a v i o r : i) the " a t h e r m a l " t r a n s f o r m a t i o n o f c a r b o n a n d low alloy s t e e l s , ii) " b u r s t " t r a n s f o r m a t i o n , typical o f F e - N i , F e - N i C alloys b e l o w room temperature, iii) completely isothermal t r a n s f o r m a t i o n a s e x h i b i t e d in s o m e F e - N i - M n , F e - N i - C r a l l o y s b e l o w room temperature, iv) s u r f a c e t r a n s f o r m a t i o n , a n i s o t h e r m a l t y p e o f t r a n s f o r m a t i o n s h o w n by m a n y i r o n a l l o y s b e l o w r o o m temperature, and w h i c h s e e m s particularly active w h e n the c o n c e n t r a t i o n of i n t e r s t i t i a l a l l o y i n g e l e m e n t s is v e r y low. ATHERMAL

TRANSFORMATION

In carbon and low alloy steels, transformation from austenite to martensite begins at a fairly well-defined temperature, Ms, which is insensitive to cooling rate up to 50,000°C per sec. '-3 The results from such experiments at high cooling rates indicate that for a given alloy Ms is constant to within +20°C. Below Ms transformation proceeds to an extent characteristic of the temperature interval AT, below Ms . The reaction rate is very great and within the few seconds required for a sample to reach thermal equilibrium with its surroundings in a liquid quenching bath, the reaction has stopped, no further transformation to martensite occurring during isothermal holding.4'~ The characteristics have been taken to imply that the nucleation processes take place without thermal activation i . e . they a r e a l h e r r n a l . T h e Ms i s a f u n c t i o n o f c o m p o s i t i o n ( s e e b e l o w ) , but the p r o g r e s s o f t r a n s f o r m a t i o n b e l o w Ms i s r e m a r k a b l y s i m i l a r i n a l l o y s w i t h Ms > 1 0 0 ° C . H a r r i s a n d C o h e n6 h a v e d e r i v e d a n e m p i r i c a l e q u a t i o n e x p r e s s i n g the r e lationship between volume fraction of transformation f a n d u n d e r c o o l i n g b e l o w M s , A T :-

s t a n t w i t h i n t h e b r a c k e t [ 4 5 5 , w h i c h i s t h e Ms (°K) f o r a 1.1 p c t c a r b o n s t e e l ] , a n d a l s o t o o b t a i n a v a l u e f o r Ms by l i n e a r e x t r a p o l a t i o n o f t h e o r i g i n a l d a t a f r o m t h e f i r s t 50 p c t o f t r a n s f o r m a t i o n . A l o g - l o g plot i s t h e n used to o b t a i n t h e r e m a i n i n g p a r a m e t e r s . T h i s p r o c e dure g i v e s m o s t w e i g h t to t h e d a t a f r o m t h e l a t t e r h a l f of t h e t r a n s f o r m a t i o n a n d r e s u l t s i n d i s c r e p a n c i e s o f a few percent near Ms . The e x p r e s s i o n o f K o i s t i n e n a n d M a r b u r g e r7 f = 1 - e x p - (1.10 x 10-2AT) is d e r i v e d f r o m d a t a r e l a t i n g