The effect of hydrogen on the fracture of alloy x-750
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INTRODUCTION
A L L O Y X-750, an age-hardenable high-strength austenitic Ni-Cr-Fe alloy, is widely used in both pressurized water and boiling water nuclear reactors, t~,2] Applications of this alloy, such as bolts, springs, and pins, require high strength, relaxation resistance, and good corrosion properties. Over the past 2 decades, stress corrosion cracking of alloy X-750 has been observed in nuclear reactor environments.[~,2] Due to these failures, work has focused on determining the effect of heat treatment and metallurgical structure on stress corrosion cracking (SCC) resistance, t3,41 The early alloy X-750 components were heat treated to maximize high-temperature creep resistance for aerospace applications.t3] This heat treatment, condition AH, consisted of an anneal at 890 ~ for 24 hours, air cooling, and aging at 704 ~ for 20 hours. The subsequent microstructure was susceptible to both low-temperature (250 ~ SCC in primary reactor water.t3] A microstructure that is much more resistant to SCC was later developed, as demonstrated by extensive testing.t41 This treatment, referred to as HTH, included a solution anneal at 1093 ~ for 1 to 2 hours followed by rapid cooling and aging at 704 ~ for 20 hours. The low-temperature cracking phenomenon appears to be hydrogen-enhanced cracking.t3,51 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 ram/s, a rate much higher than can be rood-
DOUGLAS M. SYMONS, Senior Engineer, is with the Bettis Atomic Power Laboratory, Westinghouse Electric Corporation, West Mifflin, PA 15122. ANTHONY W. THOMPSON, Staff Scientist, is with the Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720. Manuscript submitted October 17, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
eled by anodic dissolution.t3] It was further 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.ts] While many theories that have been developed to phenomenologically describe the cracking process in nuclear reactor primary water, a basic micromechanistic understanding of the fracture phenomenon is lacking. Three mechanisms have been proposed to explain the hydrogen embrittlement in nickel-base alloys. The first mechanism is a decohesion mechanism in which hydrogen decreases the cohesive energy between atoms and promotes cleavage, as originally proposed for steels by Steigerwald et al. t6j and furthered by Oriani and Josephic, t71 who suggested that the decohesion mechanism was appropriate for interfacial fracture as well. For the microvoid coalescence process observed in this system, the concept of decohesion could be used to characterize the reduction in the cohesive strength of the carbide-matrix interface. The second mechanism, in which hydrogen alters the deformation behavior, was first proposed for steels by Beachem.tS] One process of altering the deformation behavior by hydrogen
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