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 s tresses exceeds a critical threshold. Residual stresses developed by p rior machining and welding may either accelerate or retard SCC, depend ing on their magnitude and sign. A combined x-ray diffraction and mech anical procedure was used to determine the axial and hoop residual str ess 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 in crease caused by prior machining and weld shrinkage were calculated fr om the line-broadening distributions. Tensile residual stresses on the order of +700 MPa were observed in both the axial and the hoop direct ions at the inside-diameter surface in a narrow region adjacent to the weld heat-affected zone. Stresses exceeding the bulk yield strength w ere found to develop due to the combined effects of cold working of th e surface layers during initial machining and subsequent weld shrinkag e. The residual stress and cold work distributions produced by prior m achining were found to influence strongly the final residual stress st ate developed after welding.