Insights into the strengthening mechanisms and improved surface integrity of 18CrNiMo7-6 carburized steel in radial high-speed ultrasonic vibration cutting

18CrNiMo7-6 carburized steel is extensively used in high-load transmission components owing to its superior strength and toughness. Nevertheless, balancing machining efficiency with surface quality when processing high-hardness materials remains challenging. To overcome this limitation, Radial high-speed ultrasonic vibration cutting (R–HUVC) has been proposed as a hybrid machining approach to improve both the cutting performance and surface characteristics. A finite element model incorporating the gradient properties of the carburized layer was developed to analyze the multiphysics evolution during the R–HUVC process. The results demonstrate that high-frequency radial vibration dynamically modifies the tool–workpiece contact condition, reducing the cutting force and temperature by 13.8% and 9.03%, respectively, compared with conventional cutting (CC). Simultaneously, the vibration-induced impact and ironing effects enhanced the plastic deformation of the material, leading to a significant increase in the equivalent stress and plastic strain. Furthermore, the simulation results were validated through cutting experiments on 18CrNiMo7-6 carburized steel. The results demonstrated that under R–HUVC conditions, the thrust force decreased by as much as 34.6%, accompanied by a 13.34% reduction in the cutting temperature. Analysis of surface integrity indicates that R–HUVC enhanced the subsurface deformation zone and increased compressive residual stress (up to −1460.5 MPa, representing a 39.1% improvement over CC). In addition, a more desirable microhardness gradient was obtained. These strengthening effects primarily stem from the ironing-impact coupling mechanism introduced by the ultrasonic vibration. This study elucidates the surface modification mechanism of the material under R–HUVC conditions from a multi-scale perspective, providing a new approach for achieving “cutting-strengthening integration” in the machining of carburized steel and offering a theoretical basis for the active regulation of surface integrity tailored for service performance.