Hydrogen embrittlement of Ni-Cr-Fe alloys
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I.
INTRODUCTION
THERE appear to be two environmentally enhanced cracking phenomena associated with nickel-base alloys in a hydrogenated water environment. The high-temperature stress corrosion cracking (SCC) behavior of Ni-XCr-9Fe alloys, where Cr is between 15 and 30 pct Cr, in highpurity water has been well documented.[1–4] Low-temperature SCC of the solid solution alloys with the composition of Ni-XCr-9Fe in hydrogenated water has recently been reported.[5] The first phenomenon, high-temperature SCC, has been observed between 250 7C and 360 7C. This phenomenon is characterized by a time to incubate the crack followed by a relatively low crack growth rate, on the order of 0.05 mm/day in 360 7C deaerated hydrogenated water depending on the load. The high-Cr alloys are significantly more resistant to high-temperature pure-water SCC than the lower-Cr alloys. The second phenomenon, low-temperature cracking, occurs at temperatures below 150 7C. This phenomenon is characterized by a decrease in the fracture toughness of the material or rapid stable crack growth. It has been shown that solid solution strengthened Alloy 690 (;30 pct Cr) is very susceptible to this low-temperature embrittlement phenomenon in deaerated hydrogenated pure water, while Alloy 600 (;15 pct Cr) is only slightly embrittled.[5] Alloy 690 showed a fivefold decrease in its fracture toughness along with a very low tearing modulus.[5] Alloy 600 showed a twofold degradation in fracture toughness with almost no change in the tearing modulus.[5] Most tests for low-temperature SCC have been performed on Alloy X750 (15 pct Cr), a high-strength gamma-prime-strengthD.M. SYMONS, Senior Engineer, is with the Bettis Atomic Power Laboratory, Westinghouse Electric Corporation, West Mifflin, PA 15122. Manuscript submitted May 16, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
ened material that shows very localized deformation.[6] It has been shown that the low-temperature SCC is hydrogen embrittlement.[7,8] It was demonstrated 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 higher than can be modeled by anodic dissolution.[7] The primary difference between Alloy 600 and Alloy 690 is the amount of chromium in the alloys, 15 and 30 wt pct, respectively. This difference in chromium content may alter the deformation behavior,[9,10] which, in turn, may change the material susceptibility to hydrogen embrittlement. The slip mode in these alloys is partially controlled by the stacking-fault energy, which is dependent on the chromium concentration.[9,10,11] Also, Cr may produce shortrange order that will increase the degree of slip planarity.[12,13,14] In stainless steels, increased slip planarity results in increased susceptibility to hydrogen embrittlement.[15] It has also been proposed that the slip planarity may control the hydrogen embrittlement phenomenon in the nickel-base alloys.[15,16] In the nickel-chromium system, there appears to be a minimum in stacking
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