Isolation of carbon and grain boundary carbide effects on the creep and intergranular stress corrosion cracking behavior
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INTRODUCTION
IT is well established that the intergranular stress corrosion cracking (IGSCC) susceptibility of alloy 600 (Ni16Cr-9Fe-xC) in high-temperature, primary water is a sensitive function of carbon distribution.[1,2] Previous work shows that the cracking resistance of alloy 600 generally increases with an increasing level of carbon in solution or with an increasing grain boundary carbide density. Norring et al.[1] demonstrated that crack initiation times for alloy 600 in 365 7C water increased by a factor of 10 as the amount of carbon in solution increased, when the annealing temperature was raised from 927 7C to 1024 7C. With increasing annealing temperature, the density of intergranular carbides increased as well, making it difficult to determine the true reason for the high cracking resistance. Majo et al.[2] found that crack initiation times for alloy 600 in 360 7C primary water increased from less than 500 hours to greater than 5000 hours when the carbide distribution was changed from mainly transgranular to predominantly intergranular precipitation, although the carbon distribution was not quantified. With regard to creep behavior, it has been demonstrated that carbon in solution reduces the creep rate of these alloys by orders of magnitude,[3] while the role of grain boundary carbides in creep remains unclear. Leclercq and Vaillant[4] found that alloy 600 tubing containing mostly grain boundary carbides exhibited a lower creep rate at 350 7C than J.L. HERTZBERG, formerly Graduate Student Research Assistant, Department of Materials Science and Engineering, University of Michigan, is Engineer, Failure Analysis Associates, Menlo Park, CA 94025. G.S. WAS, Professor and Chair, is with the Nuclear Engineering Department, University of Michigan, Ann Arbor, MI 48109-2104. Manuscript submitted June 23, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
tubing containing mostly matrix carbides. Boursier et al.,[5] however, reported that a post anneal thermal treatment at 700 7C for 16 hours, resulting in extensive grain boundary carbide precipitation, increased the creep rate of alloy 600 compared to the mill-annealed condition. Since the amounts of carbon in solution and as carbides are interdependent and undefined in commercial alloy 600, it has only been possible to determine general trends for the creep and cracking behaviors for different carbon distributions. The influence of microstructure on cracking propensity can be strongly dependent upon the environment itself, although very little work exists which clearly demonstrates this dependency. Preliminary constant extension rate tensile (CERT) test results of alloy 600 for an initial strain rate of 3 3 1027 s21 suggest that the effect of carbon distribution on cracking in primary water conditions is strongly dependent upon hydrogen overpressure.[6] While variations in carbide distribution did not alter IGSCC behavior in 360 7C primary water with 0 bar hydrogen, Rios et al.[6] found that for CERT tests conducted with 4 bar hydrogen, the crack depths of a
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