Permeation measurements of hydrogen trapping in 1045 steel
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THE
p h e n o m e n o n of hydrogen e m b r i t t l e m e n t has b e come r e c o g n i z e d in r e c e n t y e a r s a s one that is s e n s i tive to m e t a l l u r g i c a l v a r i a b l e s such a s c o m p o s i t i o n , m i c r o s t r u c t u r e , and p r o c e s s i n g h i s t o r y , as well a s to s e c o n d a r y v a r i a b l e s , such a s s t r e n g t h level. 1 Howe v e r , much of the i n f o r m a t i o n on this topic has been i n f e r r e d f r o m s t u d i e s in which m e t a l l u r g i c a l v a r i a b l e s were not of p r i m a r y c o n c e r n and t h e r e f o r e were not d i r e c t l y a d d r e s s e d . The p r e s e n t study is p a r t of a p r o g r a m to e x a m i n e d i r e c t l y the role of m i c r o s t r u c t u r e in c r a c k i n i t i a t i o n and p r o p a g a t i o n a s s i s t e d by hydrogen. The focus of the p r o g r a m is to characterize the experimental material, both as to microstructure and as to hydrogen permeation behavior, and to use these data in interpreting the hydrogen fracture observations. This paper reports the permeation results; the fracture results will be reported separately. The experiments were conducted on a single material. In this way, alloy composition could be held constant, while the very important parameter, microstructure, was varied. There is a temptation to restrict experiments to pure or single-phase metals, but the interactions of microstructure with hydrogen effects which are being increasingly recognized in commercial materials~-s cannot be duplicated in model alloys. In order to meet the need for a microstructurally versatile, yet reasonably simple material, SAE 1045 steel was chosen. This material could be manipulated through heat treatment to give several microstructures of interest. The three distinct microstructures to be studied were as follows: i) spheroidized, ii) quenched and tempered martensite, and iii) isothermally transformed to contain pearlite. The need for permeation data on these microstructures is based on the following considerations. There is evidence that cracks initiate below the surface of the material in the presence of internal hydrogen,4 and slow crack growth seems to occur by repeated r e i n i t i a t i o n ahead of c r a c k tips and joining up with WAYNE M. ROBERTSON is Member of Technical Staff, Rockwell InternationalScience Center, Thousand Oaks, CA 91360 and ANTHONY W. THOMPSONis Professor, Department of Metallurgy and Materials Science, Carnegie-MellonUniversity, Pittsburgh, PA 15213. Manuscript submatted August 17, 1979. ME~fALLURG1CALTRANSACTIONSA
the m a i n c r a c k . 5'6 T h u s , c r a c k growth in d e l a y e d f a i l u r e may be c o n t r o l l e d by i n i t i a t i o n e v e n t s . T h e r e has b e e n r e l a t i v e l y little work on the c r a c k i n i t i a t i o n p r o c e s s p e r s e , but t h e r e is c l e a r e v i d e n c e %5 that c r a c k s i n i t i a t e at l o c a t i o n s of m a x i m u m t r i a x i a l s t r e s s . If h y d r o g e n is
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