Thermal embrittlement of 18 Ni(350) maraging steel
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T h e e m b r i t t l e m e n t of a s - s o t u t i o n i z e d 18 Ni(350) M a r a g i n g s t e e l w a s m o n i t o r e d a s a f u n c t i o n of h e a t t r e a t m e n t v a r i a b l e s by m e a n s of C h a r p y i m p a c t t e s t s . T h e p r o c e s s i n g p a r a m e t e r s of i n t e r e s t w e r e a n n e a l i n g t e m p e r a t u r e s i n the r a n g e of 1900 ° t o 2 4 0 0 ° F , i n t e r m e d i a t e h o l d i n g t e m p e r a t u r e s i n the r a n g e o f 1300 ° t o 1 8 0 0 ° F , a n d t h e q u e n c h i n g r a t e . T h e c h a n g e s i n f r a c t u r e m o d e w i t h h e a t t r e a t m e n t w e r e c h a r a c t e r i z e d by r e p l i c a a n d s c a n n i n g e l e c t r o n m i c r o s c o p y . T h e s e v e r i t y of t h e r m a l e m b r i t t l e m e n t i n c r e a s e s w i t h d e c r e a s i n g c o o l i n g r a t e f r o m the a n n e a l i n g treatment upon d i r e c t quenching t o r o o m temperature. Intermediate isothermal holding, part i c u l a r l y at 1500 ° t o 1 6 0 0 ° F , f u r t h e r a c c e n t u a t e s t h e e m b r i t t l e m e n t . A l a r g e g r a i n s i z e i s b e n e f i c i a l t o t h e t o u g h n e s s w h e n r a p i d d i r e c t q u e n c h e s f r o m t h e a n n e a l i n g r a n g e a r e i m p o s e d but i s d e t r i m e n t a l u p o n a i r c o o l i n g o r i n t e r m e d i a t e h o l d i n g . T h e m a j o r l o s s i n t o u g h n e s s m a y be a s s o c i a t e d w i t h t h e d i f f u s i o n of i n t e r s t i t i a l i m p u r i t y a t o m s (C + N) t o t h e a n s t e n i t e g r a i n b o u n d a r i e s d u r i n g c o o l i n g o r i n t e r m e d i a t e i s o t h e r m a l h o l d i n g b e l o w 2 0 0 0 ° F . A n a d v a n c e d s t a g e of the embrittlement i s c h a r a c t e r i z e d by the d i s c r e t e p r e c i p i t a t i o n o f T i ( C , N ) p l a t e l e t s o n t h e s e b o u n d a r i e s . T h e r m a l e m b r i t t l e m e n t i s a c c o m p a n i e d by c h a n g e i n f r a c t u r e m o d e f r o m t r a n s g r a n u l a r d i m p l e d r u p t u r e t o intergranular quasi-cleavage.
THE 18-pct Ni m a r a g i n g steels are recognized for their high fracture toughness as c o m p a r e d to q u e n c h and t e m p e r steels at tensile strengths of 200,000 to 300,000 psi. ~ H o w e v e r , m a r a g i n g steels show a considerable loss in fracture resistance, as m e a s u r e d in t e r m s of the C h a r p y i m p a c t e n e r g y or the plane-strain fracture toughness (Kic), w h e n subjected to certain heat treatment schedules, z'3 This degradation of toughness, t e r m e d " t h e r m a l embrittlement", w a s f o u n d to o c c u r in 18 Ni (250) M a r a g i n g steel upon heating a b o v e 2 0 0 0 ° F followed by slow cooling or by interrupted cooling and holding in the temperature r a n g e of 1500° to 1800OF.Z T h e s o u r c e of the embrittlement has been attributed to the precipitation of the intermetallie c o m p o u n d Ti (C, N) on austenite grain bou
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