Dissolution of hydrogen from the gas in steel at room temperature
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i s well k n o w n t h a t d i s s o l v e d h y d r o g e n e m b r i t t l e s c e r t a i n m e t a l s . (In m i l d s t e e l t h i s i s o b s e r v e d a t h y d r o gen c o n t e n t s a s l o w as a f e w p p m . ) But e v e n h y d r o g e n free m e t a l s m a y lose part of t h e i r ductility if t h e y are d e f o r m e d u n d e r c o m p r e s s e d h y d r o g e n g a s . ~-7 F i g s . 1 and 2 show tensile test s a m p l e s w h i c h have been t e s t e d u n d e r c o m p r e s s e d h y d r o g e n . B o t h h a v e d e v e l o p e d fiss u r e s o n t h e m e t a l s u r f a c e . A s o n e w o u l d e x p e c t , such a s a m p l e y i e l d s less plastic e l o n g a t i o n until f r a c t u r e t h a n a s a m p l e w h i c h is t e s t e d in a i r w h e r e such c r a c k s do n o t f o r m . T h e i n t e r p r e t a t i o n o f t h i s p h e n o m e n i s controversial. Essentially, t h r e e explanations have been proposed: 1) It i s s u p p o s e d 1'5'a that t h e h y d r o g e n d i f f u s e s i n t o t h e m e t a l d u r i n g t h e tensile t e s t . T h e d i s s o l v e dh y d r o g e n e m b r i t t l e s t h e m e t a l just a s it w o u l d if it h a d b e e n i n s o l u t i o n f r o m t h e b e g i n n i n g of t h e t e s t . N o a t o m i s t i c m e c h a n i s m f o r t h i s type of e m b r i t t l e m e n t i s g e n e r a l l y
of h y d r o g e n ( a s e x t r a p o l a t e d f r o m h i g h t e m p e r a t u r e s ) a p p e a r s too low to have much mechanical importance. T h e p u r p o s e of o u r e x p e r i m e n t s w a s t o f i n d o u t w h i c h o f t h e s e hypotheses i s c o r r e c t . It t u r n e d out that neither the s e c o n d nor the t h i r d assumption i s compatible with our e x p e r i m e n t s . F u r t h e r m o r e , a q u a n t i t a t i v e t r e a t m e n t of t h e d i f f u s i o n p r o b l e m u s i n g a c c e p t e d
accepted so far. Possibly the model given by Troiano9 for the delayed fracture of high strength steel could be modified* for this purpose. This, however, is not the *A modification would be necessary since delayed fracture depends on stress and time, whereas the embrittlement of soft steel depends on strain and strain rate.I° a i m of t h e p r e s e n t p a p e r . 2) It h a s b e e n a s s u m e d n-13 t h a t t h e s i m u l t a n e o u s plastic s t r a i n h e l p s the hydrogen t o e n t e r the m e t a l by m e a n s of m o v i n g d i s l o c a t i o n s w h i c h m a y c a r r y c l o u d s of h y d r o g e n a l o n g w i t h t h e m . T h i s a s s u m p t i o n s e e m e d t o be n e c e s s a r y b e c a u s e p u b l i s h e d v a l u e s o f t h e d i f f u s i v i t y o f h y d r o g e n in s t e e l at r o o m t e m p e r a t u r e a r e e x t r e m e l y l o w , so t h a t i t a p p e a r e d d o u b t f u l w h e t h e r a n a p p r e c i a b l e a m o u n t of h y d r o g e n c o u l d a c t u a l l y e n t e r the m e t a l by a c o n v e n t i o n a l d i f f u s i o n p r o c e s s . 3
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