Residual Stress Analysis of Tube Attachment Weld in Pressure Vessel Forging: Baseline FE Analysis and Sensitivity Studies

Full two and three-dimensional single or multi-pass weld simulations are now feasible and practical given the development of improved analysis tools (e.g. ABAQUS), and significantly greater computer power. This paper describes a finite element analysis undertaken to predict the as-welded residual stress field following the welding of a tube attachment weld inside a thick pressure vessel (PV) forging. The coupled thermal-mechanical analysis was performed using the finite element (FE) code ABAQUS, A heat source modelling tool was employed to calculate welding fluxes, which were read into ABAQUS via a user subroutine. The ‘block’ dumped approach was utilised in the 2D thermal analysis such that complete weld rings are deposited instantaneously. Heat inputs were based on the actual weld parameters and bead sizes. The predicted fusion depths matched well with those found in sectioned weld test pieces. 2D FE sensitivity studies were performed examining the effect of variations in a number of parameters (bead sequence, hardening law, inter-pass temperature and annealing temperature). The hardening law was changed from isotropic to kinematic to investigate the effect of material behaviour. Large weld residual tensile stresses were calculated with significant compressive stresses in the adjacent vessel wall. Stress results were generally insensitive in the tube and forging, indicating that the vessel constraint dominates over local welding conditions. Weld hoop stresses were overestimated partly due to the ‘tourniquet’ effect of depositing rings of weld metal and the isotropic hardening law assumed.