Investigation of Shot Peening Residual Stress on Alloy 718 Leaf Springs for Primary Water Stress Corrosion Cracking Analysis

摘要

Primary Water Stress Corrosion Cracking (PWSCC) is a form of material degradation characterized by crack formation and growth related to particular combinations of material, environmental and tensile stress conditions in the primary loop water of pressurized water reactors (PWRs). PWSCC of stainless steel, Nickel Alloy 600, and Nickel Alloy 718 components in PWRs has occurred, thus failure of these components by PWSCC is a concern. Nickel Alloy 718 is used in various components of the fuel assembly. Specifically, some fuel assembly hold down springs are made from Nickel Alloy 718 and shot peened after fabrication. Nickel Alloy 718 has shown very good resistance to PWSCC, especially when surface improvement by shot peening is employed to create a residual compressive stress layer at the material surface. This compressive stress layer reduces the tensile stress at the surface of the operating springs, thereby reducing the potential for PWSCC. While peened Nickel Alloy 718 has shown good resistance to PWSCC, fuel assembly hold down springs can operate at high levels of stress, sometimes in the plastic range for the material. Loading the hold down springs into the plastic range at the beginning of a cycle of operation and then unloading the hold down springs at the end of the cycle of operation may reduce the magnitude and depth of the compressive stress layer from shot peening, reducing the beneficial effects for subsequent cycles of operation as the springs are loaded again. Depending on their size, geometry and relation to the residual compressive stress layer, small surface defects from manufacturing processes or other sources may locally increase tensile stress concentrations thereby decreasing margins to crack initiation by PWSCC. The investigation of the residual compressive stress layer from shot peening over the operational life of Nickel Alloy 718 hold down springs, is therefore of interest. This paper investigates the residual stress distributions from shot peening of Nickel Alloy 718 fuel assembly hold down spring material Precision X-Ray Diffraction (XRD) testing has been performed to quantify residual stress versus depth on shot peened Nickel Alloy 718 samples that have been plastically deformed to different levels of stress. The results from these XRD tests have been used to benchmark a finite element model (FEM) that includes the effects of the residual compressive stress layer from shot peening. The benchmarked method of applying shot peening residual stresses can be applied to FEMs of Nickel Alloy 718 fuel assembly hold down springs to investigate the stress distributions in the springs at different loading conditions. XRD test results with a comparison to the benchmarked FEM are presented Future work planned is to perform slow strain rate testing (SSRT) and to use the results of this analysis and the XRD work to interpret and understand the results from the SSRT system, now in commissioning. The SSRT will provide comparatively rapid evaluation of various parameters in component fabrication, including heat treatment cycle, surface treatment, surface condition (including small surface defects), and alloy selection, and in application, including chemical environment and applied stress over the service life.