Cryogenic fracture toughness of 9Ni steel enhanced through grain refinement
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IN r e c e n t
r e s e a r c h in this l a b o r a t o r y 1 a t h e r m a l c y c l i n g t e c h n i q u e was used to r e f i n e the g r a i n s i z e of an F e - 1 2 N i - 0 . 2 5 T i alloy to ~1 p m . The g r a i n - r e fined alloy showed a n e x c e l l e n t c o m b i n a t i o n of s t r e n g t h and t o u g h n e s s at t e m p e r a t u r e s as low as that of liquid h e l i u m . 2 The work r e p o r t e d h e r e was u n d e r t a k e n to s e e if s i m i l a r r e s u l t s could be o b t a i n e d with 9Ni steel, a c o m m e r c i a l l y a v a i l a b l e alloy which is widely used in c r y o g e n i c s y s t e m s . The 9Ni c r y o g e n i c s t e e l has a n o m i n a l c o m p o s i tion F e - 9 N i - 0 . 1 C - 0 . 9 M n . It is t y p i c a l l y p r o c e s s e d to a r e l a t i v e l y fine g r a i n size of ~10 ~ m with a s m a l l a d m i x t u r e (~5 pct) of a u s t e n i t e p h a s e (added by t e m p e r i n g ) . A r e p r e s e n t a t i v e o p t i c a l m i c r o g r a p h in F i g . 1 shows s u c h a t y p i c a l m i c r o s t r u c t u r e . In its c o m m e r c i a l t r e a t m e n t the alloy c o m b i n e s high s t r e n g t h with good t o u g h n e s s 3 at liquid n i t r o g e n t e m p e r a t u r e (77 K) but r a p i d l y l o s e s t o u g h n e s s as the t e s t i n g t e m p e r a t u r e i s l o w e r e d f u r t h e r . At liquid h e l i u m t e m p e r a t u r e the a l l o y is r e l a t i v e l y b r i t t l e 2 and f r a c t u r e s i n a p r e d o m i n a n t l y t r a n s g r a n u l a r q u a s i c l e a v a g e mode. To o v e r c o m e this e m b r i t t l e m e n t the 9Ni s t e e l was t h e r m a l l y c y c l e d to an u l t r a f i n e g r a i n s i z e , t h e n t e m p e r e d to i n t r o d u c e a s m a l l a m o u n t of a u s t e n i t e p h a s e . As shown below, this t r e a t m e n t was s u c c e s s f u l i n s u p p r e s s i n g the d u c t i l e - b r i t t l e t r a n s i t i o n to below liquid h e l i u m t e m p e r a t u r e .
Fig. l--Typical optical microstructure ventional doable normalize and temper
obtained with the contreatment (NNT).
I. E X P E R I M E N T A L PROCEDURE A) Heat T r e a t m e n t The 9Ni s t e e l u s e d i n t h i s study was o b t a i n e d a s a 24 m m t h i c k plate with the c h e m i c a l c o m p o s i t i o n l i s t e d i n T a b l e I. S a m p l e s cut f r o m the p l a t e w e r e given e i t h e r a c o n v e n t i o n a l (NNT) o r g r a i n - r e f i n i n g treatment. The NNT t r e a t m e n t i s the d o u b l e - n o r m a l i z e and t e m p e r t r e a t m e n t s t a n d a r d for 9Ni s t e e l (ASTM A353-72). T h e s a m p l e is n o r m a l i z e d at 900°C i n a r g o n f o r 1 h, w a t e r quenched, r e n o r m a l i z e d at 790°C in a t CHOL K. SYN and J. W. MORRIS, Jr., are Assistant Research Engineer and Associate Professor, respectively, Department of Materials Science and Engineering, University of California, Berkeley, CA. SUNGHO JIN, formerly an Assistant Research Engin
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