Effects of grain boundary chemistry on the intergranular cracking behavior of Ni-16Cr-9Fe in high-temperature water
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INTRODUCTION
CORROSION-resistant Ni-Cr-Fe alloy 600, which is used extensively as pressurized water reactor (PWR) steam generator tubing in nuclear power plants, has experienced intergranular stress corrosion cracking (IGSCC) on both the primary and secondary sides. [1-51 Although considerable research has been performed on identifying the basic mechanism of this phenomenon, our understanding is still very poor. This lack of understanding of the IGSCC mechanism is due to a complex interaction of numerous variables, such as microstructure, thermomechanical processing, strain rate, water chemistry, and electrochemical potential. Among those variables, microstructure is most difficult to control because most studies have been conducted with commercial alloys which not only had heat-to-heat differences in composition but also experienced different thermomechanical histories. For this reason, the relationship between microstructure (not heat treatment), such as impurity segregation, carbide precipitation, chromium depletion, etc., and IGSCC susceptibility in alloy 600 is not clearly understood. For example, while high mill-anneal temperatures generally produce less susceptible microstructures compared to low mill-anneal temperatures, t6[ the microstructural or microchemical feature responsible for this effect has not been identified. Low mill-anneal temperatures generally yield a microstructure consisting of small grains and heavy intragranular carbides with a high ultimate tensile strength (UTS). High mill-anneal temperatures produce a microstructure consisting of large grains, G.S. WAS, Professor, Departments of Nuclear Engineering and Materials Science and Engineering, and J.K. SUNG and T.M. ANGELIU, Graduate Research Assistants, Department of Materials Science and Engineering, are with The University of Michigan, Ann Arbor, MI 48109. Manuscript submitted November 4, 1991. METALLURGICAL TRANSACTIONS A
abundant intergranular carbides, some intragranular carbides, and more carbon in solution. The beneficial effect of high mill-anneal temperatures seems to be a result of slow crack initiation. Norring e t al. tTI showed that an increase in the mill-anneal temperature increased the crack initiation time in pure water with hydrogen at 365 ~ Although this may seem to suggest that small grains and intragranular carbides influence cracking, subsequent thermal treatments at 700 ~ for 15 hours have been shown to improve IGSCC without significantly changing grain size or removing intragranular carbides, t8'91 Of course, the thermal treatment may be responsible for redistributing chromium at the grain boundary, coarsening carbides, relieving stress (and dislocation density), as well as inducing grain boundary segregation of impurities-all of which may affect IGSCC. In fact, Garriga-Majo e t al. 1~~ have recently proposed a relationship between carbide distribution and carbon content, grain size, and cold work for mill-annealed and heat-treated tubes, which attempts to explain the observed cracking behavior. This behavior i
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