Residual stress and deformation in UHS quenched steel butt-welded joint

For the low-alloy ultra-high strength (UHS) quenched steels with a yield strength of 1500 MPa, the welded joints have two obvious characteristics, namely solid-state phase transformation (SSPT) in the heat-affected zone (HAZ) and softening effect in the subcritical heat-affected zone (SCHAZ). In the current study, a UHS single-pass butt -welded joint with filler wire of ER307Si was fabricated, and thermal cycles at specific points were measured by thermocouples. After welding, relevant measurements of residual stress, deformation, microstructure and hardness were carried out. In this paper, based on the experimental hardness on the cross-section of the joint, a novel softening model was proposed. Meanwhile, based on coding subroutines of ABAQUS software, an advanced computational approach considering the thermal-metallurgical-mechanical coupling behaviour including the strain hardening as well as the annealing effect in fusion zone, SSPT in HAZ, and softening in SCHAZ was developed to implement thermal, metallurgical and mechanical analyses of the UHS quenched steel butt-welded joint. Comparing numerical and experimental results of temperature field, residual stress distribution and welding deformation of the joint, the calculated results of welding residual stress, angular distortion and transverse shrinkage are in good agreement with the measured data when both SSPT and softening effect were considered in the finite element model. The computed results indicate that SSPT has a significant influence on both the magnitude and the distribution of longitudinal residual stress, but it has a small effect on transverse residual stress. Moreover, the softening effect dramatically decreases the peak value of longitudinal residual stress in the SCHAZ, while it has an insignificant influence on transverse residual stress. The simulation results suggest that SSPT has a small effect on both transverse shrinkage and angular distortion, while the softening effect has very limited influence on welding deformation.

Automated summary

The welded joints of low-alloy ultra-high strength (UHS) quenched steels with a yield strength of 1500 MPa exhibit solid-state phase transformation (SSPT) and softening effect. In this study, a UHS single-pass butt-welded joint was fabricated and various measurements were carried out. A novel softening model was proposed and an advanced computational approach considering thermal-metallurgical-mechanical coupling behavior was developed. The simulated results showed good agreement with the experimental data and indicated that SSPT significantly influenced the longitudinal residual stress, while the softening effect had a dramatic influence on the peak value of longitudinal residual stress in the SCHAZ.