The effect of hydrogen on the fracture toughness of alloy X-750
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I.
INTRODUCTION
ALLOY X-750, an age-hardenable high-strength austenitic Ni-Cr-Fe alloy, is widely used in both pressurized water and boiling water nuclear reactors.[1–5] Applications of this alloy, such as bolts, springs, and pins, require high strength, relaxation resistance, and good corrosion properties. Stress corrosion cracking (SCC) has been observed in Alloy X-750 in low temperature (LT) (,100 7C) hydrogenated water.[4] The LTSCC phenomenon appears to be hydrogen-enhanced cracking.[4,6] It has been shown that fracture behavior was sensitive to phosphorus segregation at the grain boundaries and that the crack growth rate was as high as 0.03 mm/s, a rate much higher than can be modeled by anodic dissolution.[4] It was also shown that applying a cathodic potential to the tensile specimens in an aqueous environment decreased the ductility from over 20 pct to below 2 pct.[6] To understand the LTSCC phenomenon better, hydrogen embrittlement experiments have been performed. In an article on the fracture behavior of tensile specimens by Symons and Thompson,[7] it was suggested that the role of hydrogen was to decrease the strain to nucleate voids at the grain boundary carbides. The rate-limiting step was proposed to be hydrogen diffusion. To confirm these conclusions and to develop a model for the fracture behavior, precracked compact tension specimens were tested. DOUGLAS M. SYMONS, Senior Engineer, is with Bettis Atomic Power Laboratory, Westinghouse Electric Corporation, West Mifflin, PA 15122. ANTHONY W. THOMPSON, Staff Scientist, is with the Materials Science Division. Lawrence Berkeley National laboratory, University of California, Berkeley, CA 94720. Manuscript submitted June 13, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fracture toughness values and crack growth rate measurements of hydrogen-charged specimens were examined as a function of hydrogen concentration and loading rates. The fracture morphology in all cases was predominantly intergranular facets covered with ductile dimples. The fracture toughness data were then used to develop a model. The model is based on achieving a critical strain at or over some characteristic distance ahead of the crack tip. The model can correlate the fracture toughness data in the noncharged and hydrogen-charged specimens. A similar model was used by Ritchie et al.[8] to describe the upper shelf fracture toughness of two alloy steels and also by Ritchie and Thompson[9] on comparing micromechanisms of fracture with the macroscopic failure criterion. II.
MATERIAL AND EXPERIMENTAL PROCEDURE
The material used for this program was Alloy X-750 in the HTH condition. The HTH condition consists of a solution anneal at 1094 7C for 1 to 2 hours with a rapid air cool followed by aging at 704 7C for 20 hours. The material was received in the form of a 6.35-cm bar with the composition given in Table I. The mean intercept grain size was 125 mm. Small discrete M23C6 carbide precipitates were decorating the grain boundaries.[7] The alloy was strengthened by 20-nm g ' precipitat
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