Effect of hydrogen on fracture of U-notched bend specimens of spheroidized AISI 1095 steel
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studies have been performed of the hydrogen embrittlement of steels in recent y e a r s . While hydrogen is known t o d e g r a d e mechanical behavior in various ways,1 the detailed mechanisms of degradation are not clearly understood. More than five mechan i s m s for hydrogen embrittlement, some conflicting even qualitatively, have been proposed2-6 and hydrogen has been reported to produce the various experimental effects of a change in fracture mode, a reduction in ductility without change in fracture mode, a change in hardness, an increase or d e c r e a s e in flow s t r e s s and a change in elastic moduli,7 as reviewed elsewhere. ~ The present work was undertaken following a suggestion by Rice8 that the role of hydrogen would be clarified by carrying out tests on notched specimens of varying r a d i i of curvature mad hence s t r e s s and s t r a i n state. If the role of hydrogen is internal, influencing c r a c k nucleation at the site of m a x i m u m n o r m a l s t r e s s , for example, t h e r e would be a strong d i r e c t effect of notch curvature. On the other hand, if the hydrogen effect is largely a surface one, t h e r e would be little effect of notch curvature. Spheroidized plain carbon s t e e l was selected as the initial m a t e r i a l for study because extensive studies of ductile fracture, both by metallography and by mechanical testing, have been completed on notched and unnotched tensile bars of such m a t e r i a l by A r g o n and Im. 9 They studied the course of ductile fracture proceeding by void nucleation, void growth and void coalescence. U-notch bend testing was selected because it facilitates the study of the role of plastic instability in ductile fracture as shown by Spretnak and coworkers. ~°'11 All of the above effects can conceivably be influenced by hydrogeta. In o r d e r to provide a b a s i s for comparison, ductile T. D. LEE, formerly withOhio State University, is now Metallurgist with KoreaInstituteof Scienceand Technology, Seoul, Korea, and T. GOLDENBERG, formerly with Ohio State University, is now Metallurgist with Bell Telephone Laboratories, Allentown, PA 18103, and J. P. HIRTH is Professor, Metallurgical Engineering Department, Ohio State University, Columbus, OH 43210. Manuscript submitted March 9, 1978. METALLURGICAL TRANSACTIONS A
fracture in the absence of hydrogen was studied12 on the same m a t e r i a l and specimen type used for the present work. In the following development the influence of hydrogen on both precharged and dynamically c h a r g ed specimens is reported. Hydrogen was found t o r e d u c e ductility by enhancing void nucleation and growth and by promoting plastic instability in prec h a r g e d specimens and by changing fracture m o d e and c r a c k morphology in dynamically charged specim e n s . The invariance of the degradation of properties t o notch curvature showed that the hydrogen effect was a near surface effect. 2. EXPERIMENTAL PROCEDURE U-notched bend specimens of spheroidized AISI 1095 s t e e l were prepared wit
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