Hydrogen-induced cracking in iron: Morphology and crack path dependence
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T H E p r e s e n c e of s m a l l a m o u n t s of h y d r o g e n , e i t h e r i n t e r n a l l y ~'2 or a s an e x t e r n a l e n v i r o n m e n t , 3'4 can have a profound effect on the f r a c t u r e of f e r r o u s m a t e r i a l s . F o r e n g i n e e r i n g d e s i g n , t h i s b e h a v i o r is m o s t c r i t i c a l in s t e e l s having r o o m t e m p e r a t u r e y i e l d s t r e n g t h s in e x c e s s of about 150,000 p s i . B e c a u s e of t h i s , t h e s e h i g h - s t r e n g t h m a t e r i a l s have r e c e i v e d the most attention. However, their complex microstruct u r e h a s p r e v e n t e d a d e t a i l e d u n d e r s t a n d i n g of the m e c h a n i s m of c r a c k n u c l e a t i o n and growth. T h i s d i f f i c u l t y could be p a r t i a l l y a v o i d e d if the p h e n o m e n a c o u l d be s t u d i e d in a s t r u c t u r a l l y s i m p l e r s y s t e m . T h e o b s e r v a t i o n that w e l l - d e f i n e d c r a c k s , a s o p p o s e d to s p h e r i c a l v o i d s , w e r e p r o d u c e d in i r o n in which l a r g e q u a n t i t i e s of h y d r o g e n w e r e i n t r o d u c e d by c a thodic c h a r g i n g , m a d e p u r i f i e d i r o n an o b v i o u s c h o i c e a s a m o d e l s y s t e m in which to study h y d r o g e n e m b r i t t l e m e n t . The m a i n o b j e c t i v e of the study w a s to d e t e r m i n e the r o l e of h y d r o g e n in the p r o d u c t i o n of c r a c k n u c l e i and t h e i r g r o w t h into p o t e n t i a l c r i t i c a l cracks. E X P E R I M E N T A L PROCEDURE S e v e r a l kinds of l o w - i n t e r s t i t i a l - c o n t e n t i r o n w e r e e x a m i n e d ; the c o n c e n t r a t i o n s of i n t e r s t i t i a l s o l u t e s a r e given in T a b l e I. Sheet s p e c i m e n s w e r e p r e p a r e d by c o l d - r o l l i n g to t h i c k n e s s r a n g i n g f r o m 5 to 125 m i l , with m o s t b e i n g ~ 3 0 m i l . P r i o r to c a t h o d i c c h a r g i n g , t h e s e w e r e h e a t e d to t e m p e r a t u r e s up to l l 0 0 ~ in a s t a t i c o r d y n a m i c v a c u u m , A r or H e - p l u s 2 pct H. T h e s u b s e q u e n t c r a c k b e h a v i o r w a s i n d e p e n d e n t of the a t m o s p h e r e e m p l o y e d . H o w e v e r , c r a c k i n g was s e n s i t i v e l y d e p e n d e n t upon both the a n n e a l i n g t e m p e r a t u r e and the c o o l i n g r a t e f o l l o w i n g h e a t t r e a t m e n t . A wide v a r i e t y of cooling r a t e s w e r e e m p l o y e d , r a n g i n g f r o m a w a t e r quench to a slow f u r n a c e cool, a s shown in F i g . 1 f o r a 30 m i l t h i c k s p e c i m e n and an 800~ h e a t treatment. T h e s p e c i m e n s w e r e c a t h o d i c a l l y c h a r g e d at r o o m
t e m p e r a t u r e in a w e l l - s t i r r e d N-H2SO4 s o l u t i o n , with a p l a t i n u m w i r e anode. To f a c i l i t a t e a d s o r p t i o n of h y d r o g e n by i n h i b i t i n g h y d r o g e n g a
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