Weld-Overlay Analyses: An Investigation of the Effect of Weld Sequencing

Abstract

The weld overlay process has been developed and applied to repair of nuclear reactor pipe girth welds for many years in BWR plants. The objectives of such repairs were to induce compressive axial residual stresses on the pipe inside surface, as well as increase the pipe thickness with a weld material that is not susceptible to stress-corrosion cracking. Hence, understanding the residual stress distribution is important to evaluate the reliability of pipe joints with weld overlay repairs. In this paper, a six-inch diameter Schedule 120 stainless steel pipe with an overlay thickness of 7.87 mm (0.31 inch) was picked as a validation case. Weld sequencing effects were thoroughly studied. The residual stresses were calculated by using thermal elasto-plastic finite-element analysis (FEA). After comparing results using different weld sequences, it was found that the calculated weld residual stresses on ID surface were very sensitive to weld sequencing in FE analyses as well as internal cooling rate. The influence of the weld sequencing was relatively secondary to the pipe distortion. An optimum (producing compressive residual stress on the ID surface) weld sequencing was obtained and applied to a 711.2 mm (28-inch) diameter pipe-to-elbow girth weld with an overlay thickness of 24.9 mm (0.98 inch) and a pipe thickness of 29.5 mm (1.16 inch).