Stress history effect on incubation time for stress corrosion crack growth in AISI 4340 steel
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total t i m e for a s t r u c t u r a l f a i l u r e due to s t r e s s c o r r o s i o n c r a c k i n g (SCC) i n c l u d e s the t i m e for c r a c k n u c l e a t i o n and the t i m e for a c r a c k to grow to a c r i t i c a l u n s t a b l e length w h e r e c a t a s t r o p h i c f a i l u r e r e s u l t s . F o r h i g h - s t r e n g t h s t e e l s , c r a c k n u c l e a t i o n a p p e a r s to occur in two s t e p s . The f i r s t step is dependent only on the s u r f a c e p h e n o m e n a and fails u n d e r the c a t e g o r y of g e n e r a l c o r r o s i o n s t u d i e s on pit and c r e v i c e f o r m a t i o n . F o r e x a m p l e , T i n e r and Gilpin ~ found that pits on AISI 4340 in 3 pet NaC1 solution f o r m e d at i n c l u s i o n s i t e s . T h i s was a r e s u l t of p r e f e r e n t i a l i r o n d i s s o l u tion about i n c l u s i o n p a r t i c l e s due to v a r i a t i o n s in c h e m i c a l c o m p o s i t i o n and s t r e s s . The pits grew by slow c h e m i c a l attack of the p r i o r a u s t e n i t i c g r a i n b o u n d a r i e s . T h e s e pits then b e c a m e p o s s i b l e n u c l e a tion s i t e s for c r a c k s . The second step i s a s t r e s s - d e p e n d e n t step that r e q u i r e s t i m e for diffusion of the i n t e r s t i t i a l hydrogen e l e m e n t to the point of m a x i m u m t r i a x i a l s t r e s s t y p i c a l l y behind the pit. When the hydrogen c o n c e n t r a t i o n which has been g e n e r a t e d by c o r r o s i o n p r o c e s s e s r e a c h e s a c r i t i c a l value, a c r a c k is n u c l e a t e d . T r o i ano, 2 u s i n g a notched round b a r , Steigerwald, 3 u s i n g a p r e c r a c k e d notched t e n s i o n s p e c i m e n , and Slaughter et al., 4 u s i n g a smooth s p e c i m e n , have d e m o n s t r a t e d the i m p o r t a n c e of s t r e s s in the n u c l e a t i o n and p r o p a gation of a c r a c k by showing that a m i n i m a l applied s t r e s s is r e q u i r e d for SCC. The v a r i o u s s t r e s s c o r r o sion m e c h a n i s m s for hydrogen e m b r i t t l e m e n t in s t e e l s have been d i s c u s s e d by T e t e l m a n , 5 E l s e a and Fletcher,~ and B e a c h e m . 7 I n c u b a t i o n t i m e p e r i o d s have been r e p o r t e d by T r o i ano and c o w o r k e r s . 2, a-lo By u s i n g e l e c t r i c a l r e s i s t a n c e t e c h n i q u e s on h y d r o g e n - c h a r g e d notched r o u n d b a r s , they found that a p e r i o d of t i m e e x i s t e d w h e r e no a p p a r e n t c r a c k growth was o b s e r v e d . T h i s incubation p e r i o d was r e q u i r e d for hydrogen to diffuse to the point of m a x i m u m t r i a x i a l s t r e s s . They a l s o found that if the hydrogen was baked out before this incubation t i m e p e r i o d elapsed, then hydrogen e m b r i t t l e m e n t would not o c c u r . In other w o r d s , the m e c h a n i s m was r e v e r s i b l e . Loomba 11 has defined the i n c u b a t i o
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