Weld residual stress simulations have become an essential tool in structural integrity assessments. In piping, two dimensional axisymmetric simulations generally give good estimations of residual stresses but clearly cannot capture the three dimensional nature of the welding process: the start/stop effects and the constant change in mechanical restraint during a weld pass. In this study, three dimensional welding simulations have been carried out for piping butt welds, first on a dissimilar metal weld in a thin-walled pipe and second on a narrow gap weld in a thicker stainless steel pipe. The effects of mechanical boundary conditions and start/stop positions have been investigated and stress fields are shown to markedly deviate from axisymmetry. As an illustration, a fracture mechanical analysis of a partial internal surface crack reveals noticeable changes in critical crack sizes.

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