Integrated modeling for residual stress and failure behavior in welded Grade 91 steel cladded with nickel alloy

Grade 91 (G91) steel cladded with nickel alloy C22 presents a promising material solution for the Generation 3 concentrating solar power systems. Failure behavior for weld joints of such bimetallic material is complex and affected by many factors. Currently, numerical models considering both welding and loading of bimetallic materials are lacking in the literature. This study introduces an integrated model, encompassing welding, post-weld heat treatment (PWHT), and tensile testing to investigate the fracture behavior of a welded bimetallic plate with G91 steel substrate and alloy C22 clad. The welding and PWHT analyses consider incomplete austenite-martensite transformations and creep, respectively, in the calculation of residual stress and distortion. The tensile testing analysis assesses the fracture behavior of the weld joint, accounting for the effect of residual stress and material inhomogeneity. The simulation results of fusion zone shapes, welding-induced distortion, soft regions in heat-affected zone, and local strains during tensile testing are compared to the respective experimental data. It has been found that PWHT effectively mitigates the residual stress in the weld metal but introduces tensile stress in the clad and a high stress gradient at the bonding interface. The residual stress after PWHT has an insignificant effect on the G91/C22 weld failure during tensile testing. Instead, the fracture location is primarily dictated by the relative strength between the base and weld metals. However, the high stress gradient at the clad-substrate interface results in a large triaxiality factor, potentially contributing to the debonding in cases of weak bonding strength.