GRAIN BOUNDARY SEGREGATION IN AUSTENITIC STAINLESS STEELS AND EFFECTS ON INTERGRANULAR STRESS CORROSION CRACKING IN LIGHT-WATER REACTOR ENVIRONMENTS

摘要

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.