Effect of Local Strain Distribution on Stress Corrosion Cracking of Cold-Rolled Alloy 690 With Inhomogeneous Microstructure

Abstract

This article undertakes an investigation of the stress corrosion crack growth behavior of a cold-rolled Alloy 690 (UNS N06690) with microstructural inhomogeneity in the primary water of a pressurized water reactor, and ascertains the relationship between this behavior and the local distribution of the residual strain. Stress corrosion cracking (SCC) was found to propagate in a transgranular mode through a banded region composed of intragranular carbides and small-sized grains, and in an inter-granular mode through a normal matrix region of normal-sized grains, with an abnormally high growth rate when aligning the cold-rolling and crack growth directions with the intragranular carbide bands. The Vickers hardness and the degree of misorientation increased as plastic deformation as a result of cold-rolling proceeded. Moreover, these parameters were consistently higher in the banded region than in the matrix region. From an analysis of the residual strain, it was suggested that higher residual strain near intragranular carbide bands in the interior of the grains leads to transgranular SCC growth when the crack tip encounters the banded region, resulting in an abnormally high crack growth rate of the heavily cold-rolled Alloy 690 with microstructural inhomogeneity in the primary water.