Engineered Residual Stress to Mitigate Stress Corrosion Cracking of Stainless

Stress corrosion cracking (SCC) is the result of the combined influence of tensile stress and a corrosive environment on a susceptible material. Austenitic stainless steels including types 304L and 316L are susceptible alloys commonly used in nuclear weldments. SCC typically has a long incubation period followed by sudden and often catastrophic failure. Detection of SCC is difficult as there is often no gross corrosion or detectable cracks. Component failure often occurs at stress levels well within the engineering design limits of materials further complicating the problem and increasing risk. SCC is a major primary maintenance concern for many components in both pressurized water reactors (PWR) and boiling water reactors (BWR). Tensile residual stresses are developed in components by cold working, forming, prior machining, grinding and welding. Tensile residual stresses in the heat-affected zone of weldments can reach the yield strength of the material. A surface treatment is needed that can reliably produce stable, deep compressive residual stresses in austenitic stainless steel weldments in order to prevent SCC at these critical locations.

Post-weld surface enhancement processing via low plasticity burnishing (LPB) can be used to introduce a deep layer of residual compression into weldments thereby mitigating SCC through removal of the tensile stress. 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 engineered to reduce the surface, and near surface stress state to well below the SCC threshold. X-ray diffraction residual stress analyses and SCC testing in boiling MgCl₂ were performed to compare LPB treated and un-treated 304L and 316L stainless steel weldments. The results show conclusively that the deep, stable compression produced by LPB completely mitigated SCC in the tested weldments while the un-treated weldments suffered catastrophic SCC damage. LPB can be used to dramatically extend the life of welded components, increasing safety, and decreasing operational cost and inspection intervals.