Mechanistic Study of Surface Nanocrystallization for Surface Modification in High-Strength Low-Alloy Steel

This study systematically investigates the surface nanocrystallization mechanism of 35CrMo high-strength low-alloy (HSLA) steel induced by ultrasonic surface rolling processing (USRP), with particular emphasis on elucidating its optimization effects on surface integrity, mechanical properties, and wear resistance. Through USRP treatment with varying static pressure parameters combined with multi-scale characterization techniques, we demonstrate that high-frequency impact and rolling effects promote martensite lath fragmentation and dislocation multiplication, thereby forming a gradient nanostructured layer composed of equiaxed nanocrystals and high-density dislocations in the surface region. After USRP treatment, the surface roughness was minimized to 0.029 μm, attributed to the synergistic "peak-cutting and valley-filling" effect and plastic flow-induced surface smoothening. Concurrently, compressive deformation during rolling induces lattice distortion effects, successfully transforming the residual stress state from tensile to high compressive stress. A remarkable 32.3% enhancement in surface microhardness was observed, primarily originating from multiple strengthening mechanisms including grain refinement, dislocation strengthening, and carbide dispersion strengthening, with an effective hardened layer depth reaching 300 μm. In terms of wear performance, the USRP-0.35 MPa sample exhibited optimal wear resistance with a 28.9% reduction in mass loss, owing to the significantly improved load-bearing capacity of the gradient nanostructured layer and effective suppression of crack initiation and propagation by compressive residual stresses. High-resolution electron microscopy and diffraction analyses captured the dynamic evolution process of dislocations, from their initiation at martensite boundaries to subsequent propagation and eventual formation of dislocation pile-ups and subgrain boundaries, providing direct experimental evidence for the Hall-Petch and Taylor strengthening theories. This work not only clarifies the microscopic mechanisms of USRP-induced surface strengthening in 35CrMo steel but also offers important theoretical guidance and practical references for optimizing surface treatment processes of high-strength alloy components.