Effect of ultrasonic surface rolling process on the surface integrity of A100 ultra-high-strength steel

This study investigated the influence of multi-pass ultrasonic surface rolling processing (USRP) on the surface integrity of a hard-to-deform metal—A100 steel. Compared with the as-received A100 substrate, fifteen USRP passes markedly reduced the surface roughness. The sharp decrease mainly stemmed from the cemented carbide sphere’s rapid collision, triggering a swift shear instability in a notably slender zone of contact. The peaks of the machining grooves underwent localized melting and plastic side-flow, while the valleys were leveled by high-energy dislocation glide and grain rotation; simultaneously, a micro-forging effect forced secondary micro-cutting debris into the groove bottoms, producing a mirror-like finish. This intense deformation accumulated a 94 µm-thick severe plastic flow layer at the surface, where grains were heavily fragmented and the dislocation density increased significantly. The resulting volumetric expansion, constrained by the underlying material, generated a residual stress of − 1.3 GPa. With increasing depth, the stress wave amplitude decayed exponentially, the plastic-strain gradient diminished, and the dislocation multiplication–annihilation process approached equilibrium. Consequently, the microhardness rose continuously from 653 HK in the bulk to 812 HK at the surface, establishing a 200 µm gradient-hardened layer and a 350 µm compressive residual stress layer. This synergistic “plastic flow–gradient hardening–residual stress” barrier effectively suppressed fatigue-crack initiation and hindered abrasive wear, leading to a significant improvement in the material’s wear and fatigue performance.