Background
The driving force for crack growth near welds in nuclear application piping may arise from weld residual stresses. Experience from nondestructive examination show that cracks in dissimilar metal welds are often found near weld repairs. The repair welding may affect the residual stresses through e.g. increased tensile stresses and increased plastic straining on the surfaces of the piping in contact with the light water reactor environment. This may in turn reduce the margin towards initiation of stress corrosion cracking.
Objectives
This study investigates how weld residual stresses in dissimilar metal welds are affected by weld repairs for select cases relevant to Swedish nuclear power plants.
Results
The most common configuration of dissimilar metal weld connecting ferritic nozzles to safe-ends in Swedish nuclear power plants were chosen for further investigation. Five significant cases of weld repairs are identified for that particular configuration of dissimilar metal weld, three inside and two outside repairs for pipes of different sizes.
The weld residual stresses for the five cases are investigated by numerical simulation with two dimensional axi-symmetric assumptions. Conclusions are drawn based on comparison of the weld residual stresses before and after weld repair.
The results show that the inside repairs generate high tensile stresses at the inside surface. This is generally unfavourable since stress corrosion cracking would be prevented if stresses at the surface were compressive. Therefore, careful consideration is recommended before performing inside weld repairs to avoid introducing unnecessary plastic strains from welding and high tensile stress after repair as driving forces for stress corrosion cracking.
The effect from an inside repair was similar for both the thin and thick pipe cases studied. Sensitivity studies were performed with respect to heat input and repair thickness, and the results show small effects.
Repair welding from the pipe outside surface results in small changes of the residual stress field compared to the unrepaired weld. However, for deep repairs from the outside, the material at the pipe inside surface can be subjected to a significant increase in plastic hardening. Material highly hardened from plastic deformation at welding is generally more susceptible to initiation of stress corrosion cracking, provided that the material is in a region of tensile stress and exposed to the corrosive environment. The sensitivity to an offset between the outside repair and the original weld was investigated, and the effect was found to be small.