Residual Stress Analysis of Tube Attachment Weld in Pressure Vessel Forging: Comparison of FE Predictions and Measurements

Residual stresses were analysed in a partial penetration weld attaching a tube inside a thick pressure vessel forging, both made of SA508 steel. 2D finite element (FE) analyses methods were used to simulate this multi-pass manual TIG weld. The weld preps are buttered and the forging subsequently heat-treated prior to making the closure weld. Buttering of the forging J-prep and subsequent PWHT creep stress relaxation were modelled. Generally the buttering was found to have minimal influence on the final stress state, although some difference in local peak stress and stress gradients were calculated. Representative test blocks were manufactured, with and without buttered weld preps. Each test block contained two tube penetrations and attachment welds, in order to examine interaction effects. Welding details were captured and peak temperatures recorded by thermocouples were reasonably consistent with the FE model predictions. Surface stresses were measured both in the as-welded condition and after machining, using the hole drilling strain gauge method. Good agreement with FE results was achieved in surface stress levels in the vessel forging, buttering and tube wall. However the 2D model overestimates weld hoop stresses. Large yield magnitude tensile stresses in the vicinity of the joint are balanced by lower compressive stresses in the surrounding PV forging. Interaction effects between the stress fields produced by adjacent tube welds are negligible.