Transformation Plasticity and Residual Stresses in Single-Pass Repair Welds

Prediction of the residual stresses caused by welding is important when post-weld stress relief is not feasible. Phase changes and transformation plasticity have a significant effect on the residual stresses generated by welding and heat-treatment of some alloys. Transformation plasticity occurs when the stresses generated by the transformation of individual grains interact with the macroscopic stress state to produce plastic strains. Heuristic methods requiring empirical constants have been used in the past. A method based on the fundamental laws of plasticity and basic material properties is proposed to incorporate transformation plasticity in a finite element program. The transformation plasticity which occurs depends on the stress state. During any increment the stress state can change substantially. If the step size is too large, the analysis may become unstable. A method which allows larger steps while eliminating the instability and improving the convergence is presented. A three-dimensional (3D) analysis of a short longitudinal pipe weld in a typical pressure vessel steel is shown. The significance of this phenomenon in welds is demonstrated by comparing the residual stress states predicted with and without transformation plasticity.