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Considerable attention has been focused recently on understanding the detailed mechanism of intergranular stress corrosion cracking (IGSCC) in weld-sensitized austenitic stainless steels used as structural material components in light water nuclear reactors. It is well recognized that three major factors (namely, a sensitized microstructure, a stress and consequent strain-time behavior, and a corrosive environment) need to coexist in order that IGSCC may occur.This paper is only concerned with the characterization of the sensitized microstructure. In order to fully understand weld-sensitized microstructure, it is necessary to carry out detailed microstructural studies on the nature of grain boundary chromium-rich M23C6 carbides in the sensitized region adjacent to the weld bead. In addition, techniques have now been developed for performing detailed quantitative micro-structural measurements that allow the determination of carbide size, number density per unit area of grain boundary, grain boundary areal fraction coverage, and micrograms of chromium depleted from the neighboring grains. Qualitative and quantitative microstructural observations from several pipe heats, pipe diameters, and grain sizes are presented. Since the shape and depth of the chromium depletion profile in the vicinity of carbide particles is expected to govern the extent of susceptibility of the weld heat affected zone to the corrosive environment, a more realistic picture would be obtained if it were possible to map such depletion profiles. Only recently has it become possible with major developments in microanalytical instrumentation to use a combination of scanning-transmission electron microscopy (STEM) and high spatial resolution thin foil X-ray microanalysis for this purpose. Chromium depletion profiles are presented as a function of distance from austenite-austenite and austenite-carbide interfaces to demonstrate the feasibility of performing systematic sensitization studies on austenitic stainless steels used in reactor piping.
steels, microstructure, microstructural studies, microchemical studies, weld sensitization, austenitic stainless steels, transmission electron microscopy, quantitative microstructure, scanning transmission electron microscopy, thin foil X-ray microanalysis
Acting manager, Chemical and Structural Analysis Branch, General Electric Company, Corporate Research and Development, Schenectady, N. Y.