Thermal stabilization of austenite in a high-nickel steel
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T. K. SANYAL,
AND
G. WHITE
The k i n e t i c s of the t h e r m a l s t a b i l i z a t i o n of a u s t e n i t e have b e e n studied in a 0.5 pct C 19 pct Ni s t e e l within the r a n g e +300~ to - 7 8 ~ The K i n s m a n - S h y n e model for s t a b i l i z a tion is s e e n to be inadequate outside the t e m p e r a t u r e r a n g e +150~ t o - 20~ for this alloy. S t a b i l i z a t i o n at s u b z e r o t e m p e r a t u r e s can exhibit t h e r m a l r e v e r s i b i l i t y without, however, r e q u i r i n g the f u n d a m e n t a l m e c h a n i s m i t s e l f to be s i m i l a r l y r e v e r s i b l e .
IN
c e r t a i n s t e e l s the s t a b i l i t y of the a u s t e n i t e within the t e m p e r a t u r e r a n g e of m a r t e n s i t e t r a n s f o r m a t i o n can be i n c r e a s e d e i t h e r by cooling slowly o r by a r r e s t i n g the cooling (aging). C e r t a i n f e a t u r e s of s t a b i l i z a t i o n a r e well e s t a b l i s h e d . When s t a b i l i z a t i o n is due to aging below the m a r t e n s i t e - s t a r t t e m p e r a t u r e (M s) the extent of s t a b i l i z a t i o n can be quantified 1,2 in t e r m s of the d e g r e e of u n d e r c o o l i n g (0) below the a r r e s t t e m p e r a t u r e that is r e q u i r e d to r e s t a r t t r a n s f o r m a t i o n (at M s , ) . F o r a given aging t r e a t m e n t , the g r e a t e r the a m o u n t of m a r t e n s i t e the l a r g e r will be 0. 2,a F o r a given m a r t e n s i t e content and aging t e m p e r a t u r e , 0 i n c r e a s e s with t i m e and m a y p a s s through a m a x i m u m (0 max). An i m p o r t a n t c r i t e r i o n on s t a b i l i z a t i o n is its r e v e r s i b i l i t y ; 1-3 when the s a m e s p e c i m e n is s u b j e c t e d to a s e r i e s of a g i n g t r e a t m e n t s without c a u s i n g o v e r a g i n g the r e s u l t a n t effect is c h a r a c t e r i s t i c of the f i n a l t r e a t m e n t only, The effect b e c o m e s p a r t i c u l a r l y p r o n o u n c e d in s t e e l s which t r a n s f o r m below 100~ and a n i n c r e a s ing n u m b e r of c o m m e r c i a l h i g h - s t r e n g t h s t e e l s a r e in this category. S t a b i l i z a t i o n has b e e n a t t r i b u t e d to s t r e s s - r e Iaxation 3'~ or to s t r a i n - a g i n gs,6 of the a u s t e n i t e . The l a t t e r t h e o r y a g r e e s with o b s e r v a t i o n s that c a r b o n is a p p a r e n t l y e s s e n t i a l for s t a b i l i z a t i o n and a m o d e l of i n t e r f a c e diffusion has, in fact, b e e n p r o p o s e d which r e l a t e s s t a b i l i z a t i o n to the e x c e s s c o n c e n t r a t i o n of c a r b o n at the a u s t e n i t e - m a r t e n s i t e b o u n d a r y . 7 On the other hand, s t a b i l i z a t i o n at s u b z e r o t e m p e r a t u r e s is i n c o m p a t i b l e with the k i n e t i c s of c a r b o n diffusion, as they a r e c u r r e n t l y u n d e r s t o o d , s The work to be d e s c r i b
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