Investigation of the evolution of welding residual stress in A350-LF2 steel multi-layer and multi-pass joint

For multi-layer and multi-pass welding (MMW), two different structural layers, including the upper fine-grained layer caused by the reheating effect of the subsequent weld passes and the lower coarse-grained layer determined by the peak temperatures of the current thermal cycle, are likely to generate in most weld passes (except for the final one). In this study, to explore the formation and evolution of the welding residual stress (WRS) in MMW joint, a finite element model considering the thermo-metallurgical-mechanical interaction effects was developed to calculate WRS. Then a novel heat conduction model was proposed to quantitatively estimate the reheating effect. The effectiveness of the above models was verified by the experimental measurements. Then Satoh test was further performed to explore the effects of the peak temperature, phase transformation, and reheating on WRS evolution. The results demonstrate that the formation of WRS in MMW joint is similar to a multilayer residual stress stacking, which is mainly influenced by not only the peak temperature of the welding thermal cycle but also the reheating effect of the weld passes and consequent phase transformation. In the event of‌ the peak temperature of the welding thermal cycle being in the range of Ac1 to Ac3, the mixture phase with relatively higher yield strength tends to be generated in the reheated layer in weld passes, as a result, the welding residual tensile stress in MMW joint can be decreased.