Cyclic plasticity of welded P91 material for simple and complex power plant connections

•Detailed finite element investigation of cross weld specimens highlights the key role of plastic strain localisation in the heat affected zone of a welded connection.•Identification and calibration of cyclic plasticity material constants for parent, weld and heat affected zone regions using a non-linear kinematic–isotropic hardening material model.•Application of a multi-material cyclic plasticity model to a T-joint welded connection. The predicted critical location corresponds to the location of failure observed in-service.

This paper focuses on with the cyclic plasticity modelling of welded, ex-service P91 material. A multi-material model is developed for high temperature cyclic plasticity, including the effects of the three different material zones in the vicinity of the weld: parent metal, weld metal and heat-affected zone. The cyclic plasticity behaviour of the three zones is identified from previously-published high temperature, low cycle fatigue experimental tests on uniaxial parent metal, weld metal and cross-weld (repair weld) specimens on ex-service P91 material. The heat-affected zone is shown to be significantly softer than the parent and weld metals and to act as a focus for concentration of plastic strain, leading to significantly inhomogeneous distributions of stress and strain in the weldment. The multi-material cyclic plasticity model is applied, via a three-dimensional finite element analysis of a welded T-piece header-branch connection, to predict the effects of cyclic internal pressure and small amplitude thermal fluctuations.