INFLUENCE OF CHROMIUM DEPLETION ON INTERGRANULAR STRESS-CORROSION CRACKING OF 304 STAINLESS-STEEL

Direct correlations are made between the extent of chromium depletion at grain boundaries and intergranular stress corrosion cracking (IGSCC) of 304 stainless steel during slow-strain-rate (SSR) tensile tests in 288-degrees-C air-saturated water. IGSCC was found to depend on the minimum chromium concentration, the spacing of carbides, and the width of the chromium-depleted zone at grain boundaries. Modifying any one of these interfacial characteristics impacted intergranular cracking and ductility. Under the conditions of the SSR test, IGSCC sharply increased as the grain boundary chromium concentration dropped below approximately 13.5 wt%. The presence of a semi-continuous distribution of carbides at grain boundaries prompted some intergranular cracking and ductility loss without localized chromium depletion. This cracking was eliminated by heat treating to form isolated carbides at boundaries. Very narrow chromium depletion widths ( < 4 nm) enhanced IGSCC over those observed for stainless steel with semicontinuous carbides. The extent of intergranular cracking and the degree of embrittlement increased with the width of the chromium depletion zone up to a width of approximately 15 nm.