Annealing Models in Welding Simulation: Conservative and Non-Conservative Residual Stress Distributions

The influence of residual stress on degradation mechanisms must be taken into account when performing structural integrity assessments. Conservative hypothetical distributions are often assumed, in which the maximum stress is equal to yield. Finite element simulation is now being used in an attempt to derive more realistic residual stress profiles for incorporation into assessment procedures. It is now possible to include self-annealing effects in finite element simulations of the welding process, whereby stresses in existing material in the immediate vicinity of a weld pass are relaxed locally on a very short time scale. Annealing models used in practice vary in sophistication from a simple erasure of strain hardening history at a particular temperature, through certain non-physical assumptions about temperature dependence, to phenomenological models based on the assumed kinetics of underlying microstructural processes. Even more sophisticated models based on thermodynamical principles have been proposed. The original incentive for developing such models was the hope that peak residual stress predictions within the weld and heat affected zone would be reduced, removing the need for over-conservative assumptions when performing structural integrity assessments. Preliminary 2-D results using a model developed by one of the authors, however, suggest that peak stresses predicted with annealing may actually be higher, the hypothesis being that it is compressive stresses that are relaxed during the welding cycle, and that the final tensile residual stresses are increased via the Bauschinger effect. This paper considers results from a further 3-D welding simulation and looks a little more closely at different annealing models. A number of different modelling approaches to annealing are described, together with the basic modelling parameters used to simulate a weld that has been the subject of various European round-robins, both theoretical and experimental. A brief overview of how a kinetics based model can be implemented in a finite element code is also presented. Predicted residual stress distributions are compared with each other and with neutron diffraction measurements and conclusions drawn.