Finite Element Modelling of a Tube to Vessel Attachment Weld and Local Post-Weld Heat Treatment

Abstract

This paper describes Finite Element (FE) modelling of a weld between a tube and a machined feature on a curved pressure vessel surface. The components were manufactured from a ferritic steel with a matched weld metal deposited by a mechanised TIG process. The weld region then underwent a local Post-Weld Heat Treatment (PWHT) which used heating bands and cooling air flows to control the temperature distribution. The PWHT’s aim was to provide stress relief and HAZ tempering, while minimising the stresses due to thermal gradients in the component. Trial welds on representative test pieces had predicted significant welding-induced distortions. Therefore, during the weld and PWHT, restraints were applied to the tube to prevent excessive deformation. The material behaviour was represented using Abaqus’ built-in material options, with the same properties for both the base metal and the filler. Isotropic hardening was assumed and the stress relaxation during the PWHT was modelled by applying a Norton creep law only during the hold time. Phase transformation effects in the ferritic material were not included. Initial modelling used a 2D axisymmetric model to allow sensitivity studies to inform the development of the PWHT process. These showed that the degree of stress relief was much more sensitive to the soak temperature than the hold time. Subsequent runs analysed a 3D model using a segmented block-dumping technique, with the deposition modelled by introducing the weld elements in 90° segments. The 3D modelling was undertaken in order to more accurately model potentially asymmetric welding distortions and residual stresses. The torch was represented by a body flux into each segment after its introduction. This model was also run without restraint to provide validation by comparing the predicted distortion with measurements from the welding trials; a good match was demonstrated. Further comparisons were made between the predicted stresses and results of Incremental Centre Hole-Drilling (ICHD) stress measurements made on the trial specimens both in the as-welded condition and after PWHT. The measured stresses were close to those predicted by the FE analysis and the key features of the predicted stress field were apparent in the measurement data. Due to the location of the tube’s attachment to the pressure vessel, thermal expansion of the vessel during the PWHT caused the tube to bend. The induced bending stresses were then relaxed during the soak and re-introduced in the opposite sense as the system cooled. This effect was captured by running the analysis as a submodel of a global FE model with displacements read across at nodes in the pressure vessel shell immediately below the weld.