Influence of Local Stress and Strain on Intergranular Cracking of 316L Stainless Steel in Supercritical Water.

Abstract

The objective of this study was to determine how the deformation propensities of individual grains of 316L stainless steel influence intergranular cracking behavior in supercritical water (SCW). The grain-to-grain variations in deformation propensities were estimated from the Schmid and Taylor factors of grains. Resulting stress inhomogeneities and strain incompatibilities which were evaluated to determine the conditions that promoted intergranular cracking in SCW. Proton irradiation of 316L caused hardening and radiation induced depletion of chromium at grain boundaries and was found to increase intergranular cracking severity. The SCW environment increased the crack density on the gage surfaces of the specimen by a factor of 18 compared to a 400˚C argon environment. Intergranular cracks preferentially occurred along grain boundaries oriented perpendicular to the tensile axis and adjacent to grains with low Schmid factors. The Schmid-Modified Grain Boundary Stress (SMGBS) model was developed to analyze local grain boundary stresses. The model was validated by showing that the Schmid factor dependence of cracking in SCW could be predicted from the trace inclination distribution, and confirmed that cracking was driven by the normal stresses acting on grain boundaries. The similar dependencies of slip discontinuity and intergranular cracking on trace inclination, Schmid factor, Taylor factor, and grain boundary character suggest that slip discontinuity contributes to intergranular cracking. Grains with low Taylor factors decreased slip discontinuity propensity at grain boundaries with trace inclinations >50˚ because they provided multiple favorably oriented slip systems on which deformation could occur. Grain boundary engineering reduced the intergranular cracking propensity of 316L stainless steel in SCW by virtue of the fact that special grain boundaries were more resistant to intergranular cracking in SCW than random high angle grain boundaries. The findings of this study indicate that the intergranular cracking resistance of 316L stainless steel in a SCW environment would be greatest for a microstructure with a large population of grains with high Schmid factors (for a specified stress state), a high frequency of grain boundaries oriented nearly parallel to the tensile direction, and a large fraction of special grain boundaries.