Mitigation of Stress Corrosion Cracking in Nuclear Weldments Using Low Plasticity Burnishing

Abstract

Stress corrosion cracking (SCC) has been observed for decades in austenitic alloy weldments such as type 304 stainless steel as well as in Ni based alloy weldments including Alloy 600 and 690. SCC continues to be a primary maintenance concern for many components in both pressurized water reactors (PWR) and boiling water reactors (BWR). SCC is understood to be the result of a combination of susceptible material, exposure to a corrosive environment, and tensile stress above a threshold. Tensile residual stresses developed by prior machining and welding can accelerate SCC. A surface treatment is needed that can reliably produce deep compressive residual stresses in austenitic and Ni based alloy weldments in order to prevent SCC. Post-weld surface enhancement processing via low plasticity burnishing (LPB) can be used to introduce deep compression into tensile fusion welds thereby mitigating SCC. LPB has been developed as a rapid and inexpensive surface enhancement method adaptable to existing CNC machine tools or robots. Deep compressive residual stresses produced by LPB are designed to reduce the surface, and near surface stress state to well below the SCC threshold. Residual stress results are shown for 304 stainless steel, Alloy 22 and Alloy 718. SCC test results comparing LPB treated and un-treated 304 stainless steel weldments are presented. Results show that the deep compression produced by LPB eliminates SCC in austenitic weldments.