Thermo-mechanical modelling of P91 steel weld microstructure and residual stresses in power plant pipework

A circumferentially multi-pass butt-welded P91 steel pipe, typically used for high-temperature applications in power plants, has been numerically analysed to determine residual stresses, induced by the process of welding, as well as microstructural regions in the weld, caused by thermal cycles. The finite element (FE) method has been applied to simulate residual stresses generated in the weld region and heat affected zone (HAZ). The axisymmetric FE simulation incorporates solid state phase transformation (SSPT) by allowing for volumetric changes in steel and associated changes in yield stress due to austenitic and martensitic transformations. The thermal cycles during welding cause different microstructural regions to emerge in the vicinity of the weld metal. Columnar and equiaxed microstructural zones have been numerically modelled in the weld region of the pipe. The predicted FE microstructural regions have been corroborated by columnar and equiaxed zones that have been mapped out on a cross-sectional macro-image of the weld.