3D FE simulation of the welding process to optimise residual stress profiles in complex geometries

Abstract

Welded, thick-walled, tee branch junctions are complex piping components commonly used in the nuclear power and petrochemical industries. Owing to the relatively large wall thickness, weldments are often constructed in several passes. Each successive pass alters the stresses caused by previous passes. Consequently, complex residual stresses of significant levels can develop at the welding stage. Tensile welding residual stresses can, in combination with operating stresses, lead structures to be prone to catastrophic premature failure. It is most desirable that residual stresses be predicted and optimized well in advance of welding execution. This dissertation documents the development of a full 3D FE model for multipass welding simulation. A generalized plane strain model was first developed. Modelling techniques, including standard versus contact boundary conditions, sequentially versus fully coupled models, were investigated. A 3D sequentially coupled model with standard constraint was then proposed and applied to multipass butt-welded plates and pipes for validation. Good agreements between the predictions and independent experimental measurements have been obtained. To extend the work to thick and intricate welded structures, a newly developed all-hexahedral meshing technique was employed to mesh the complex intersection area in a tee branch junction. The moving heat source and filler material deposition were simulated by assigning reactivated elements with a volumetric heat flux progressing along the weld path. Temperature dependent material properties, latent heat and large deformation were considered. Detailed temperature and residual stress distributions have been reported. The correlations between welding parameters and residual stresses have been established and issues concerning residual stress profile optimization have been addressed via extensive parametric studies. The parameters investigated included the number of passes, welding sequence, heat input, preheat and interpass temperature and cooling rate. Cooling rate and interpass temperature were found to be the most important parameters affecting residual stresses. The model can be applicable to other multipass welded complex geometries for residual stress prediction and optimization.