Hydrogen embrittlement studies of a trip steel
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HIGH-STRENGTH ferritic and martensitic steels are notoriously susceptible to hydrogen-induced failure, commonly called hydrogen embrittlement (H.E.). Hydrogen can be acquired by these steels relatively easily during steelmaking or processing, or from their environment while in service.~-3 The mechanism by which hydrogen embrittles ferrite and martensite is still in dispute. Also, it is not well understood why austenite is relatively immune to H.E. Because of the widespread commercial use of high-strength steels and because insufficient knowledge of the influence of composition, processing, design, and environment continues to contribute to service failures, further study in the area of H.E. is of special technological importance. A new class of potentially useful high-strength steels, TRIP steels (TRansformation Induced Plasticity), obtain their high elongation4 and fracture toughness5 from the strain-induced austenite to martensite transformation. In 1969 Dulls and Chadhok6 reported that cathodically charged TRIP steel specimens were immune to hydrogen cracking. However during that same year, Gold and Koppenaal7 reported that TRIP steels could be embrittled under certain conditions of tempera t u r e and s t r a i n r a t e . At the outset of this study it was hypothesized that in the a u s t e n i t i c s t a t e , TRIP s t e e l s should be r e l a t i v e l y i m m u n e to H . E . ; w h e r e a s , after the s t r a i n - i n d u c e d t r a n s f o r m a t i o n b e g i n s , a d v e r s e e f f e c t s m i g h t be expected. A c c o r d i n g l y in this i n v e s t i g a t i o n , the following obj e c t i v e was sought: The d e t e r m i n a t i o n and u n d e r s t a n d ing of the conditions of cathodic c h a r g i n g , g a s e o u s h y d r o g e n e n v i r o n m e n t , and loading for which T R I P s t e e l s m a y or may not be s u s c e p t i b l e to H. E. P u r s u a n t to this o b j e c t i v e , a p p r o p r i a t e e x p e r i m e n t s were designed and p e r f o r m e d u s i n g t e n s i l e and s i n g l e - e d g e - n o t c h (SEN) s p e c i m e n s which had been c a t h o d i c a l l y charged with hydrogen or m e c h a n i c a l l y tested in p u r e H2.
I) E X P E R I M E N T A L A) M a t e r i a l Selection and P r e p a r a t i o n The c o m p o s i t i o n s and p r o c e s s i n g of the s t e e l s t e s t e d in this i n v e s t i g a t i o n a r e l i s t e d in T a b l e I. The t h e r m o m e c h a n i c a l t r e a t m e n t chosen for alloy C was c o n s i d e r e d to be o p t i m a l for this p a r t i c u l a r T R I P s t e e l . 8 As a check on the H. E. t e s t p r o c e d u r e s and for a c o m p a r i s o n of H. E. p r o p e r t i e s with T R I P s t e e l , alloy 4340, which is a t e m p e r e d m a r t e n s i t i c s t e e l with yield and u l t i m a t e t e n s i l e s t r e n g t h s a p p r o x i m a t e l y equal to alloy C, was a l s o e v a l u a t e d . T e n s i l e and SEN s p e c i m e n s w e r e m a c h i n e d f r o m r o l l e d s h e e
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