Welding processes induce residual stresses and distortions which can play a major role in the mechanical strength of a component. 3D modelling of multipass welding of a 316L stainless steel pipe. Numerical simulation of processes is of big help to control these effects as it provides the evolution of physical quantities such as temperature, stresses and strains at any point in the structure. Simulations of single pass welding stresses give today rather satisfying results. Prediction of distortions is far much difficult as it needs 3D simulations to accurately take account of the constraining conditions induced by the weld bead. The difficulty is still increased when considering multipass welding.