Influence of Solid-State Phase Transformation and Softening Effect on Welding Residual Stress of Ultra-High Strength Steel

In recent years, ultra-high strength steel (UHSS) has been widely utilized in engineering structures, mining machinery, and military equipment. However, UHSS is prone to brittle fracture and fatigue failure due to high strength and relatively low plasticity. Moreover, residual stress induced by welding process affects both brittle fracture and fatigue failure. In this work, a single-pass butt-welded joint was fabricated by metal inert-gas welding. The base metal was 1600 MPa grade UHSS with a 5 mm thickness, and the filler metal was ER307Si. The distributions of welding residual stress and hardness of the butt-welded joint were measured using the hole drilling method and a microhardness tester, respectively. Based on measured values of hardness in the heat-affected zone (HAZ) and softening zone (SZ), SYSWELD software was used to develop an advanced computational approach with consideration of "thermal-metallurgical-mechanical"coupling behaviors. In addition to the strain hardening and annealing effects of weld metal, the established computational model accounted for both the solid-state phase transformation (SSPT) of HAZ and softening effect of SZ. The temperature field and residual stress distribution of the UHSS single-pass butt-welded joint were simulated. Furthermore, the simulated results were compared with the corresponding measured data. The simulation results revealed the effect of SSPT and softening on welding residual stress. The numerical results indicated that SSPT has a strong influence on both the magnitude and distribution of the longitudinal residual stress; however, it has a limited effect on transverse residual stress. Meanwhile, the softening effect drastically affects the peak values of the longitudinal residual stress, while it hardly influences transverse residual stress. When both SSPT and softening effects are simultaneously considered in the numerical model, the computed results of welding residual stress are in good agreement with the experimental measurements.