Assessment of Tensile Residual Stress Mitigation in Alloy 22 Welds Due to Laser Peening

Abstract

This paper examines the effects of laser peening on Alloy 22 (UNS N06022), which is the proposed material for use as the outer layer on the spent-fuel nuclear waste canisters to be stored at Yucca Mountain. Stress corrosion cracking (SCC) is a primary concern in the design of these canisters because tensile residual stresses will be left behind by the closure weld. Alloy 22 is a nickel-based material that is particularly resistant to corrosion; however, there is a chance that stress corrosion cracking could develop given the right environmental conditions. Laser peening is an emerging surface treatment technology that has been identified as an effective tool for mitigating tensile redisual stresses in the storage canisters. The results of laser-peening experiments on Alloy 22 base material and a sample 33 mm thick double-V groove butt-weld made with gas tungsten arc welding (GTAW) are presented. Residual stress profiles were measured in Alloy 22 base material using the slitting method (also known as the crack-compliance method), and a full 2D map of longitudinal residual stress was measured in the sample welds using the contour method. Laser peening was found to produce compressive residual stress to a depth of 3.8 mm in 20 mm thick base material coupons. The depth of compressive residual stress was found to have a significant dependence on the number of peening layers and a slight dependence on the level of irradiance. Additionally, laser peening produced compressive residual stresses to a depth of 4.3 mm in the 33 mm thick weld at the center of the weld bead where high levels of tensile stress were initially present.