Environmentally Assisted Cracking of Types 304L/316L/316NG Stainless Steel in 288°C Water

The objective of this study is to identity the causes of intergranular environmentally assisted cracking in non-chromium-depleted stainless steels, a phenomenon that has been observed under irradiated and unirradiated conditions. Special emphasis is placed on quantifying the possible effects of sulfur, phosphorus, and nitrogen relative to the effects of chromium depletion and aqueous impurities in altering both the crack-growth rate and cracking morphology in 288°C water. Eleven custom and three commercial heats have been examined, including several containing exceptionally high sulfur levels. Slow strain rate (SSR) tests on smooth, cylindrical specimens and fracture mechanics, crack-growth rate tests on 1-T compact-type (CT) specimens were performed in 288°C water. The resulting cracking morphology and crack-growth rate data were interpreted using grain-boundary characterization data obtained by Auger electron spectroscopy and analytical electron microscopy. In both SSR and CT tests, the amount of intergranular cracking correlated well only with grain-boundary sulfur segregation and not with phosphorus and nitrogen segregation; however, in no instance was a statistically significant enhancement in crack-growth rate observed in the fracture mechanics specimens. The ability of sulfur to produce intergranular cracking was much greater in the SSR tests in the potentiostatically controlled, 2.5 pH H2SO4 than in SSR or CT tests in pure water. Chromium depletion and/or aqueous impurities can have a much more pronounced effect on crack-growth rates in stainless steels than grain-boundary segregants. However, effects of these and other segregants cannot be precluded since, for example: (1) even small changes in crack-growth rate can be significant in terms of component lifetime; (2) their role may be more important for very short cracks in the early stages of cracking, where the crack-tip corrosion potential in oxygen-containing solutions is much more oxidizing than for long cracks; and (3) despite the use of very high sulfur levels in this study, the impurity profiles near grain boundaries resulting from irradiation-induced segregation may lead to vastly more impurity in the vicinity of the grain boundary.