Internal hydrogen supersaturation produced by dislocation transport
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T HE study of h y d r o g e n e m b r i t t l e m e n t in s t e e l i s h i s t o r i c a l l y i n t e r t w i n e d with the concept that b r i t t l e n e s s is c a u s e d b y h y d r o g e n gas u n d e r p r e s s u r e in v o i d s . In t h i s view h y d r o g e n d o e s not influence m a terial properties; rather brittleness occurs because t h e e l e v a t e d l o c a l s t r e s s f i e l d s a s s o c i a t e d with the high p r e s s u r e h y d r o g e n p o c k e t s allow a c r i t i c a l f r a c t u r e event to o c c u r at a much l o w e r a p p l i e d s t r e s s than in the a b s e n c e of h y d r o g e n . P r e s s u r e m o d e l s m a y be c l a s s i f i e d a s e i t h e r t h e r m o d y n a m i c o r k i n e t i c , d e p e n d i n g on the p r o c e s s w h e r e b y the p r e s s u r e i s p r e s u m e d to d e v e l o p . In t h e r m o d y n a m i c m o d e l s , which have a m o s t e x t e n s i v e h i s t o r y , h y d r o g e n g a s p r e c i p i t a t i o n and p r e s s u r e b u i l d - u p a r e j u s t t h e r m o d y n a m i c c o n s e q u e n c e s of a l a t t i c e highly s u p e r - s a t u r a t e d with h y d r o g e n . In the e m b r i t t l e m e n t r a n g e of t e m p e r a t u r e s , s u b s t a n t i a l p r e s s u r e s a r e r e q u i r e d to e q u i l i b r a t e v e r y s m a l l l a t t i c e c o n c e n t r a t i o n s , s i n c e the S i e v e r t ' s law c o e f f i c i e n t f o r h y d r o g e n in i r o n is quite s m a l l . T h e s e conc e n t r a t i o n s m a y be e a s i l y p r o d u c e d b y e l e c t r o c h e m i c a l o r c h e m i c a l c h a r g i n g at t e m p e r a t u r e o r , s i n c e h y d r o g e n i s an e n d o t h e r m i c s o l u t e in i r o n , b y e x p o s u r e to h y d r o g e n at e l e v a t e d t e m p e r a t u r e s and p e r h a p s p r e s s u r e , followed by r a p i d cooling. Although the t h e r m o d y n a m i c s a r e s t r a i g h t f o r w a r d , t h e concept that an a s s o c i a t e d p r e s s u r e c a u s e s b r i t t l e n e s s has b e e n c o n t r o v e r s i a l . U n d e r s e v e r e c h a r g i n g c o n d i t i o n s i r r e v e r s i b l e d e t e r i o r a t i o n of the s t r u c t u r e m a y o c c u r , a s is shown in d i f f e r e n t a l l o y s by t h e a p p e a r a n c e of b l i s t e r s and i n t e r n a l f i s s u r e s , v o i d s , and c r a c k s . Although o c c a s i o n a l e x p e r i m e n t s have i n d i c a t e d the p r e s e n c e of h y d r o g e n gas u n d e r p r e s s u r e in t h e s e m a c r o s c o p i c d e f e c t s , e s p e c i a l l y b l i s t e r s , t h e r e i s l i t t l e d e t a i l e d u n d e r s t a n d i n g of the r o l e of p r e s s u r e in d e f e c t n u c l e a t i o n and growth. Reversible embrittlement is observed under less s e v e r e c h a r g i n g c o n d i t i o n s . The d e t a i l e d i n v e s t i g a t i o n s of T r o i a n o I e s t a b l i s h e d that the b e h a v i o r p a t H. H. JOHNSON
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