Influence of filler metal on residual stress in multi-pass repair welding of thick P91 steel pipe

Based on the thermal-elastic-plastic theory, a three-dimensional finite element (FE) model for residual stress in multi-layer and multi-pass repair welding of P91 steel pipe with wall thickness of 60 mm is established, which considers the sequential coupling among thermal field and microstructure and mechanical properties. The arc heat input is described as a double-ellipsoid heat source, and the solid-state phase transformation (SSPT) concerning austenite and martensite during welding of P91 steel is modeled through allowing for the changes of material volume and yield stress as well as phase transformation plasticity in the finite element analysis. Using the established model, the residual stresses in repair welding of P91 steel pipe with similar and dissimilar filler metals are calculated through SYSWELD software, which are also compared with experimental results to validate the accuracy of the numerical model. The distribution features of residual stresses under the two conditions are comparatively investigated, and the influence of different filler metals on them is also analyzed. When using similar filler metal, the stress components and equivalent stress have strong fluctuations on the weld surface due to martensitic transformation, and there exists relatively wide compressive stress region on the weld surface. At the fusion zone (FZ) near the weld surface, a serious stress concentration appears. The tensile stress peak can reach 813 MPa, close to the yield stress of untempered martensite. While using Ni-based alloy as filler metal, the peak values of tensile stress components and equivalent stresses in the HAZ of weldment surface are higher than those for the former owing to lack of relaxation by phase transformation. In the interior of weldment, the region with high stress in the FZ is much smaller, and the welding stress in the HAZ is also lower compared with those for the former.