Stellite failure on a P91 HP valve - failure investigation and modelling of residual stresses

The sequence of the primary welding process and the following secondary processes of machining, and heat treatment has been modelled to predict the residual conditions in a Stellite 6 overlay weld on a P91 steam turbine valve. The different process steps are coupled in order to transfer the residual conditions from preceding processes to the ones following. The integrated model has been used to analyse a premature rupture in the weld zone between P91 and Stellite. An accurate prediction of the stress level in the weld zone is imperative for the assessment of the quality of the component, and this provides a tool for process modification in order to minimize the risk for future breakdown. Both classical time independent and time dependent plasticity models have been used to describe the different material behaviours during the different process steps. The description of the materials is highly temperature dependent. The welding process is modelled by adding material to the calculation domain, while the machining process is modelled by removing material from the calculation domain to redistribute the stress fields. An in-house iso-parametric FEM-code is used for the implementation of the material models and the discretization of the appropriate addition and removal of material. The return mapping algorithm is used for the time discretization of the time independent plasticity model, and a Norton’s power law model is used for the time dependent model. Results from the numerical calculations are presented.