Mechanical stability of retained austenite in tempered 9Ni steel
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P R O P E R L Y t e m p e r e d F e - (5- 9)Ni 1-~ s t e e l s for c r y o g e n i c use contain a s m a l l a d m i x t u r e of r e t a i n e d a u s t e n i t e (5 to 10 pct by volume) which is b e l i e v e d to c o n t r i b u t e to good low t e m p e r a t u r e t o u g h n e s s . The r e t a i n e d a u s t e n i t e is p r e c i p i t a t e d a s s m a l l e l o n g a t e d p a r t i c l e s along m a r t e n s i t e lath b o u n d a r i e s and p r i o r a u s t e n i t e g r a i n b o u n d a r i e s , a s i l l u s t r a t e d in F i g . 1. If i n t r o d u c e d by t e m p e r i n g in the lower p o r t i o n of the t w o - p h a s e (~ + ~,) r e g i o n of the e q u i l i b r i u m p h a s e d i a g r a m , at t e m p e r a t u r e s in the r a n g e 500 to 600~ the a u s t e n i t e is t h e r m a l l y s t a b l e and d o e s not t r a n s f o r m to m a r t e n s i t e when cooled to liquid n i t r o g e n t e m p e r a t u r e (77 K). While it is widely a c c e p t e d that p r e c i p i t a t e d and r e t a i n e d a u s t e n i t e has a b e n e f i c i a l effect on c r y o genic m e c h a n i c a l p r o p e r t i e s the s o u r c e of the effect is not w e l l u n d e r s t o o d . P l a u s i b l e e x p l a n a t i o n s have b e e n b a s e d both on the c h e m i c a l p r o p e r t i e s of the a u s t e n i t e , 3,4 in that it m a y " g e t t e r " p o t e n t i a l l y e m b r i t t l i n g s p e c i e s , and on i t s low t e m p e r a t u r e m e c h a n i c a l p r o p e r t i e s ~-5 in that a u s t e n i t e is a r e l a t i v e l y d u c t i l e p h a s e which m a y e n h a n c e l o c a l d u c t i l i t y o r blunt p r o p a g a t i n g f r a c t u r e s . E i t h e r e x p l a n a t i o n i s , however, c o m p l i c a t e d by the q u e s t i o n of the m e c h a n i c a l s t a b i l i t y of the a u s t e n i t e ; while the a u s t e n i t e is t h e r m a l l y s t a b l e it m a y n o n e t h e l e s s undergo c a t a s t r o p h i c t r a n s f o r m a t i o n to m a r t e n s i t e in the i n t e n s e d e f o r m a tion f i e l d n e a r the tip of a p r o p a g a t i n g c r a c k . R e s e a r c h in r e c e n t y e a r s has p a r t i c u l a r l y e m p h a s i z e d the i m p o r t a n c e of the m e c h a n i c a l s t a b i l i t y of r e t a i n e d a u s t e n i t e to the low t e m p e r a t u r e toughn e s s of F e - N i c r y o g e n i c s t e e l s . It was noted s o m e t i m e ago that the b e n e f i c i a l effect of r e t a i n e d a u s tenite formed during tempering decreases dramatic a l l y a s the t e m p e r i n g t e m p e r a t u r e is r a i s e d above ~575~ a r e s u l t which has b e e n a t t r i b u t e d 6 to a d e c r e a s i n g m e c h a n i c a l s t a b i l i t y of the a u s t e n i t e due to C. K. SYN, B. FULTZ, and J. W. MORRIS, Jr. are Assistant Research Engineer, Research Assistant, and Professor of Metallurgy and Principal Investigator, respectively, all with the Department of Materials Science and Engineering ahd the
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