Effect of prior machining deformation on the development of tensile residual stresses in weld-fabricated nuclear components

Austenitic alloy weldments in nuclear systems may be subject to stress- corrosion cracking (SCC) failure if the sum of residual and applied stresses exceeds a critical threshold. Residual stresses developed by prior machining and welding may either accelerate or retard SCC, depending on their magnitude and sign. A combined x- ray diffraction and mechanical procedure was used to determine the axial and hoop residual stress and yield strength distributions into the inside- diameter surface of a simulated Alloy 600 penetration J- welded into a reactor pressure vessel. The degree of cold working and the resulting yield strength increase caused by prior machining and weld shrinkage were calculated from the line- broadening distributions. Tensile residual stresses on the order of +700 MPa were observed in both the axial and the hoop directions at the inside- diameter surface in a narrow region adjacent to the weld heat- affected zone. Stresses exceeding the bulk yield strength were found to develop due to the combined effects of cold working of the surface layers during initial machining and subsequent weld shrinkage. The residual stress and cold work distributions produced by prior machining were found to influence strongly the final residual stress state developed after welding.