A fracture mechanics study of stress corrosion cracking of type-316 austenitic steel
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I N r e c e n t y e a r s , the a p p l i c a t i o n of the t e c h n i q u e s of l i n e a r e l a s t i c f r a c t u r e m e c h a n i c s to the s t u d y of s t r e s s c o r r o s i o n c r a c k i n g (SCC) has p r o v i d e d u s e f u l q u a n t i t a t i v e d a t a upon which to b u i l d a m o r e t h o r o u g h u n d e r s t a n d i n g of m e c h a n i s m s . T h e s e s t u d i e s have involved d e t e r m i n i n g the r e l a t i o n s h i p b e t w e e n c r a c k v e l o c i t y , v, and opening mode s t r e s s i n t e n s i t y f a c t o r , KI, t o g e t h e r with the e f f e c t s of t e m p e r a t u r e , T, a n d / o r e l e c t r o c h e m i c a l p o t e n t i a l , E . S e v e r a l a l l o y s y s t e m s have b e e n i n v e s t i g a t e d in this m a n n e r , including A l - a l l o y s , ' T i a l l o y s , z b r a s s e s , 3 and high s t r e n g t h s t e e l s . 4,5 H o w e v e r , with the e x c e p t i o n of R o b i n s o n and Scully, ~ L e f a k i s and R o s t o k e r , 7 S p e i d e l , s'9 and B u r s l e , 1~ few w o r k e r s have adopted the f r a c t u r e m e c h a n i c s a p p r o a c h to the study of SCC in a u s t e n i t i c s t a i n l e s s s t e e l s . The p r e s e n t i n v e s t i g a t i o n was an i n t e n s i v e study of SCC in T y p e - 3 1 6 a u s t e n i t i c s t a i n l e s s e x p o s e d to a hot aqueous solution of 44.7 wt pct MgCI2. The effect of KI, E, and T upon both v, and c r a c k f r a c t o g r a p h y w e r e s t u d i e d . A new t e s t s p e c i m e n c o n f i g u r a t i o n was u s e d and c a l i b r a t e d with r e s p e c t to K I. T h i s s p e c i m e n , when e m p l o y e d with w o r k h a r d e n e d 316 s t e e l , gave well b e haved c r a c k s which m i n i m i z e d the t r o u b l e s o m e m a c r o s c o p i c c r a c k b r a n c h i n g phenomenon o b s e r v e d in a n n e a l e d a u s t e n i t i e s t e e l s by p r e v i o u s workers.6-z~
The s t e e l was r e c e i v e d as 6.4 m m t h i c k hot r o l l e d and p i c k l e d p l a t e s t o c k . It was r o l l e d at 25~ with i n t e r m e d i a t e a n n e a l s , to p r o d u c e a f i n a l t h i c k n e s s (B) of 3.2 m m with e i t h e r 25 o r 50 pct cold w o r k (CW). A n n e a l i n g t r e a t m e n t s w e r e c a r r i e d out in s e a l e d SenP a k s t a i n l e s s s t e e l c o n t a i n e r s at 1050~ for one h, followed by quenching in w a t e r . The 0.2 pct offset y i e l d s t r e s s ((~y) n o r m a l to the r o i l i n g d i r e c t i o n at 150~ was 534 M N / m 2 at 25 pct CW, and 903 M N / m ~ at 50 pct CW. Q u a n t i t a t i v e s t u d i e s of SCC w e r e conducted only on the cold w o r k e d m a t e r i a l . R e s i d u a l s t r e s s e s r e m a i n i n g f r o m cold w o r k i n g w e r e r e l i e v e d by pulling in t e n s i o n to p r o d u c e a u n i f o r m 2 pct p l a s t i c s t r a i n p a r a l l e l to the r o l l i n g d i r e c t i o n . X - r a y d i f f r a c tion s t u d i e s d i d not r e
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