The strength and fracture toughness of 18 Ni (350) maraging steel
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RACK AND DAVID KALISH
T h e i n f l u e n c e of m i c r o s t r u c t u r e o n t h e s t r e n g t h a n d f r a c t u r e t o u g h n e s s of 18 N i ( 3 5 0 ) m a r a g i n g s t e e l w a s e x a m i n e d . C h a n g e s i n m i c r o s t r u c t u r e w e r e f o l l o w e d by X - r a y a n d n e u t r o n d i f f r a c t i o n a n d by o p t i c a l a n d e l e c t r o n m i c r o s c o p y . T h e s e o b s e r v a t i o n s h a v e b e e n c o r r e l a t e d w i t h the f r a c t u r e m o r p h o l o g y e s t a b l i s h e d by 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 . A i r c o o l i n g t h i s a l l o y f r o m t h e a u s t e n i t i z i n g t e m p e r a t u r e r e s u l t s in a d i s l o c a t e d m a r t e n s i t e . D u r i n g t h e i n i t i a l s t a g e o f age h a r d e n i n g , m o l y b d e n u m a t o m s t e n d t o c l u s t e r ( f o r m i n g p r e p r e c i p i t a t e s ) a n d t h e cobalt a s s u m e s s h o r t r a n g e o r d e r e d p o s i t i o n s . Subsequent a g i n g r e s u l t s in N i a M o a n d a F e T i with overaging b e i n g a s s o c i a t e d with the formation of e q u i l i b r i u m r e v e r t e d austenite a n d F e 2 M o . T h e f r a c t u r e b e h a v i o r is e x a m i n e d in t e r m s o f e l e m e n t a r y d i s l o c a t i o n p r e c i p i t a t e i n t e r a c t i o n s . It i s s u g g e s t e d t h a t the d e v e l o p m e n t of c o p l a n a r s l i p i n t h e u n d e r a g e d c o n ditions l e a d s t o its i n c r e a s e d s t r e s s c o r r o s i o n susceptibility and d e c r e a s e d f r a c t u r e toughn e s s . T h e o p t i m u m a g e d c o n d i t i o n is t h e n a s s o c i a t e d w i t h c r o s s - s l i p d e f o r m a t i o n . T h e f r a c t u r e b e h a v i o r o f t h e o v e r a g e d c o n d i t i o n is a d y n a m i c b a l a n c e b e t w e e n a b r i t t l e m a t r i x a n d t h e d u c t i l e ( c r a c k blunting) r e v e r t e d austenite.
ALTHOUGH
m a r a g i n g s t e e l s have been commercially available for a l m o s t t e n y e a r s , t h e factors controlling t h e i r f r a c t u r e toughness are s t i l l unknown.1 P r e v i o u s i n v e s t i g a t i o n s of t h e s t r e n g t h a n d f r a c t u r e b e h a v i o r o f 18 p c t n i c k e l s t e e l s h a v e b e e n d e v o t e d t o t h e 8 p c t c o b a l t - t y p e w i t h s t r e n g t h s up t o 300 k s i . 2'3 R e c e n t l y , a 12 p c t c o b a l t g r a d e h a s b e e n d e v e l o p e d w h i c h c a n be h e a t t r e a t e d t o t h e 350 k s i s t r e n g t h l e v e l . 4'5 H o w e v e r , a i r c r a f t structural applications of this h i g h e r strength g r a d e have been limited b e c a u s e of its low f r a c t u r e t o u g h n e s s ; t h e c r i t i c a l c r a c k d e p t h is a p p r o x i m a t e l y f i v e t i m e s s m a l l e r t h a n that f o r t h e m o s t c o m m o n l y u s e d l a n d i n g g e a r m a t e r i a l ( a s i l i c o n m o d i f i e d 4 3 4 0 ) .5 T h i s p a
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