Effect of Strain Hardened Inner Surface Layers on Stress Corrosion Cracking of Type 316 Stainless Steel in Simulated PWR Primary Water

In this study, the relationship between SCC growth rate and Vickers hardness for Type 316 stainless steel at various temperatures in simulated PWR primary water was investigated. In addition, IGSCC growth measurements were conducted using so-called functionally graded cold-rolled specimens made of Type 316 stainless steel, in simulated PWR primary water. Residual stress measurements were performed using the contour and slitting methods on the functionally graded cold-rolled specimens. The contour method measurements found highly tensile stress (350 MPa) toward the surface of the side with a larger thickness reduction and low-magnitude stress elsewhere. The purpose of these latter specimens was to evaluate the effect of heavily cold-worked layers on the inner surface of components made of Type 316 stainless steel on the possibility that IGSCC could penetrate through the wall thickness. The test data showed that the work-hardened inner surface does not always have a detrimental effect on SCC susceptibility. However, if any surface cracks exist within the work-hardened inner surface layer of any components, the stress at the crack tip becomes very large and SCC easily initiates. Nevertheless, even if IGSCC initiates from a heavily cold-worked layer at the inner surface, IGSCC growth rate in Type 316 stainless steel was found to eventually become negligibly small or arrests in the soft base material.