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 fa-tigue failure due to high strength and relatively low plasticity.Moreover,residual stress induced by weld-ing 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,respec-tively.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 trans-formation(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 com-pared with the corresponding measured data.The simulation results revealed the effect of SSPT and soft-ening 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 longi-tudinal residual stress,while it hardly influences transverse residual stress.When both SSPT and soften-ing effects are simultaneously considered in the numerical model,the computed results of welding residu-al stress are in good agreement with the experimental measurements.
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