Grain boundary composition in austenitic stainless steels established by various equilibrium and nonequilibrium processes is assessed and its possible influence on intergranular stress corrosion cracking (IGSCC) in light-water reactor environments examined. Interfacial Cr concentration is shown to be the dominant material variable promoting IGSCC of austenitic stainless steels in oxidizing high-temperature water. Cracking susceptibility is a direct function of the boundary Cr content regardless of depletion width. Small levels of depletion, 1 to 2 wt% below bulk Cr concentrations (below -17 wt% for 304SS), are sufficient to promote IGSCC. As a result, many observations of irradiation-assisted (IA) SCC in boiling-water reactor environments are consistent with radiation-induced Cr depletion. However, grain boundary Cr depletion does not explain IGSCC in (unsensitized) cold-worked stainless steels or IASCC of stainless steels at low electrochemical potentials (hydrogen-water chemistry). Significant interfacial enrichment of Cr, MO and perhaps B are likely in solution- and mill-annealed stainless steels which may play a role in the IGSCC of cold-worked materials and delay IASCC to higher radiation doses. Existing data indicate that impurities and minor elements which segregate to stainless steel grain boundaries do not have a direct effect on cracking susceptibility. Critical research is needed to isolate causes for IGSCC in high-strength (cold-worked and irradiated) stainless steels.
Intergranular stress corrosion cracking (IGSCC) of austenitic stainless steels has been a continuing problem in commercial boiling-water-reactor (BWR) nuclear power plants. The vast majority of failures have been in high-carbon, 300-series stainless steels which were thermally sensitized during fabrication. Extensive basic and applied research activities were initiated about 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.? IGSCC of sensitized stainless steel piping is probably the best understood environmental cracking process and serves as a basis for much of our general understanding of IGSCC phenomena. However, the current understanding of IGSCC that has been observed in unsensitized cold-worked24 or in irradiated stainless steels5.6 is much less clear.
This paper examines stainless steel grain boundary composition and its possible influence on IGSCC. Emphasis is placed on identifying equilibrium and nonequilibrium segregants which may promote susceptibility, or improve resistance, to cracking. In each case, current understanding of grain boundary segregation phenomena in stainless steels is reviewed and assessed relative to IG fracture in reactor water environments.
MEASURING GRAIN BOUNDARY COMPOSITION
Although many authors have attempted to link grain boundary composition and environmental cracking susceptibility, few have made direct measurements. In most cases, bulk composition and/or heat treatment is varied and it is assumed that interfacial segregation is systematically changed. Indirect measurements are often made (e.g., IG corrosion tests) indicating grain boundary composition of an isolated element. Within selected well-understood cases, such approaches can give reproducible results. However, a complete and accurate determination of interfacial composition is never obtained. Direct quantitative measurements of grain boundary composition is essential to enable any reasonable assessment of variables controlling cracking susceptibility.
A wide range of approaches have been employed to a
