Effect of Residual Stress of Dissimilar Metal Welding on Stress Corrosion Cracking of Bottom-Mounted Instrumentation Penetration Mock-Up

The present article is concerned with the effect of a residual stress induced by a dissimilar metal welding on stress corrosion cracking (SCC) of an Alloy 600 (UNS N06600) bottom-mounted instrumentation nozzle as a penetration mock-up in a pressurized water reactor. From a finite element analysis (FEA), the changes in the inner diameter (ID) of the nozzle and hoop and axial stresses on the inner surface of the nozzle were calculated with various conditions of materials, mock-up design, and welding constraints. The ID change by welding and its dependence on the wall thickness of the nozzle calculated from the FEA were consistent with those measured from the mock-up, verifying the validity of the FEA model. In a highly acidic solution of sodium tetrathionate (Na₂S₄O₆) at room temperature, the SCC behavior of the sensitized Alloy 600 nozzle in the mock-up was closely related with the characteristics of the residual stress in the inner surface of the nozzle. Axial cracks were observed in the inner surface of the nozzle in the J-weld region where the tensile hoop stress was expected, while circumferential cracks were found beyond both the J-weld root and face where the tensile axial stress was predicted.