Linking Grain Boundary Structure and Composition to Intergranular Stress Corrosion Cracking of Austenitic Stainless Stee
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Linking Grain Boundary Structure and Composition to Intergranular Stress Corrosion Cracking of Austenitic Stainless Steels S. M. Bruemmer Pacific Northwest National Laboratory P.O. Box 999, Richland, WA 99532 ABSTRACT Grain boundary structure and composition is assessed in austenitic stainless steels along with its influence on intergranular stress corrosion cracking (IGSCC) in high-temperature water. Brief examples are presented illustrating effects of grain boundary character and segregation on behavior in specific light-water-reactor environments. Although grain boundary engineering can produce an increased fraction of “special” boundaries in austenitic stainless alloys, practical benefits depend on the boundary orientation distribution. It is critical to recognize that only Σ3s appear to be more resistant to SCC and the behavior of other low Σ boundaries is uncertain. Grain boundary composition can have a dominant effect on IGSCC under certain conditions, but altered interfacial chemistry is not required for cracking. In high-potential oxidizing environments, IGSCC susceptibility is a direct function of the boundary Cr concentration. Nonequilibrium thermal segregation of Cr and Mo is often present in mill-annealed stainless steels and may influence cracking susceptibility. This initial grain boundary composition alters subsequent radiation-induced segregation and delays irradiation-assisted SCC susceptibility to higher doses. Other alloying elements and impurities in 300-series stainless steels have been seen to enrich grain boundaries, but few have any significant impact on IGSCC susceptibility. One exception is Si that strongly segregates during irradiation. Recent results suggest that Si may accelerate crack propagation in both low- and high-potential water environments. Critical research is still needed to isolate individual grain boundary characteristics and quantitatively link them to IGSCC. INTRODUCTION Intergranular stress corrosion cracking (IGSCC) of austenitic stainless steels has been a continuing problem in commercial boiling-water-reactor (BWR) nuclear power plants. The majority of failures have been in high-carbon, 300-series stainless steels that were thermally sensitized during fabrication. Extensive basic and applied research activities were initiated more than 25 years ago to develop a mechanistic understanding of the IGSCC process and, more importantly, identify remedial actions and corrective measures. For the most part, those research activities were highly successful. [1] Unfortunately, further observations of environmentinduced cracking in BWR and pressurized-water-reactor (PWR) components have been identified where sensitization is not an important issue. However once again, cracking is nearly always intergranular suggesting that grain boundary structure and composition control susceptibility.
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This paper evaluates austenitic stainless steel grain boundary structure and composition in relation to IGSCC. Emphasis is placed on identifying specific aspects that may promote
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