Interlayer ultrasonic impact treatment–induced recrystallization and grain transformation in multi‑pass welded structural steel

Abstract This study evaluates two ultrasonic impact treatment (UIT) strategies for flux-cored arc welded high-strength structural steel: top-surface peening after welding and interlayer peening between successive weld passes. Top-surface UIT increased the surface hardness by more than 130% and introduced compressive residual stresses of up to 582 MPa, consistent with severe plastic deformation and work hardening. In contrast, interlayer UIT did not produce a comparable increase in average microhardness or compressive residual stresses in the interlayers. Instead, it modified the weld metal microstructure by promoting the formation of equiaxed grains between columnar regions. This change indicates that the strain introduced by UIT disrupted columnar grain growth from subsequent weld passes. Interlayer UIT also substantially reduced geometric distortion, decreasing the distortion angle from 5.5° in the as-welded joint to below 1.2°. Top-surface UIT reduced the distortion angle to ~ 3.5°. These results indicate that top-surface UIT may be preferred where enhancement of surface integrity and fatigue resistance is required, while interlayer UIT offers advantages for distortion control and microstructural engineering during weld build-up.