The hydrogen attack of HSLA steels
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AND
P. G. SHEWMON
T h r e e high s t r e n g t h low alloy s t e e l s with low c a r b o n c o n t e n t s and v a r y i n g a l l o y c o n t e n t s were exposed to 21 M P a (3000 psi) hydrogen p r e s s u r e in the t e m p e r a t u r e r a n g e 350 to 510~ The r e s u l t i n g s a m p l e e x p a n s i o n r a t e was m e a s u r e d u s i n g a highly s e n s i t i v e c a p a c i t a n c e d i l a t o m e t e r . In all t h r e e HSLA s t e e l s , the e a r l y e x p a n s i o n r a t e was found to be i n d e p e n d e n t of e x p o s u r e t i m e , but a f u n c t i o n of e x p o s u r e t e m p e r a t u r e and p r e s s u r e . The t e m p e r a t u r e dependence of the s a m p l e e x p a n s i o n r a t e was s i m i l a r for a l l t h r e e HSLA and the r e f e r e n c e c a r b o n s t e e l s , and c o r r e s p o n d e d r o u g h l y to an a c t i v a t i o n e n e r g y (Q) of 190 K J / m o l e . The l a t e r a c c e l e r a t i n g e x p a n s i o n rate fitted a Q of 160 K J / m o l e . Though the s a m p l e e x p a n s i o n r a t e s exhibited the s a m e t e m p e r a t u r e d e p e n d e n c e , they v a r i e d by as much as two o r d e r s of magnitude a m o n g the t h r e e HSLA and the c a r b o n s t e e l s . T h i s m a r k e d v a r i a t i o n in r a t e was p r o p o r t i o n a l to the e s t i m a t e d c a r b o n a c t i v i t y in each s t e e l . C o m p a r i s o n between the N e l s o n c u r v e s p u b l i s h e d by the A m e r i e a l P e t r o Ieum I n s t i t u t e (API) for 1/2 pet Mo s t e e l and the e x p e r i m e n t a l c u r v e s c o n s t r u c t e d for HSLA s t e e l s i n d i c a t e d that these s t e e l s should p r o v i d e c o m p a r a b l e or b e t t e r r e s i s t a n c e to hydrogen a t t a c k than the 1/2 pet Mo s t e e l .
H Y D R O G E N attack (HA) is a f o r m of m a t e r i a l d e g r a d a t i o n which o c c u r s when a s t e e l is exposed to a high t e m p e r a t u r e , high p r e s s u r e h y d r o g e n e n v i r o n m e n t . It involves the f o r m a t i o n of m e t h a n e b u b b l e s along the g r a i n b o u n d a r i e s , t h e i r s u b s e q u e n t growth at a r a t e dependent on t e m p e r a t u r e and p r e s s u r e , and f i n a l l y t h e i r l i n k - u p to f o r m f i s s u r e s . In the e n g i n e e r i n g s e n s e , a s t e e l is said to have undergone HA a f t e r its m e c h a n i c a l p r o p e r t i e s suffer a d e t e c t a b l e d e t e r i o r a t i o n f r o m t h e s e b u b b l e s . T h u s , u n d e r c o n d i t i o n s where a s t e e l will u l t i m a t e l y u n d e r g o HA t h e r e e x i s t s an i n cubation p e r i o d d u r i n g which no a p p r e c i a b l e change in m e c h a n i c a l p r o p e r t i e s o c c u r s followed by p r o nounced HA. A n u m b e r of a u t h o r s 1-6 have t r e a t e d the effects of low alloy additions on the r e s i s t a n c e of the s t e e l to HA. T h e s e i n v e s t i g a t i o n s c l e a r l y showed that
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