Ultrasonic shot peening-induced gradient nanostructures in tempered AISI 9310 gear steel: Microstructural evolution and enhanced wear performance

Ultrasonic shot peening (USP), a surface severe plastic deformation (SPD) technique, provides an effective route to improve the mechanical and tribological performance of high-strength gear steels. In this work, gradient nanostructured (GNS) layers exceeding 200 μm in thickness were fabricated on tempered AISI 9310 steel via USP. The original ferritic–martensitic lath structure (average width ∼6.71 μm) was progressively refined under high strain rates, forming a hierarchical gradient from coarse laths to subgrains and ultimately to equiaxed nanograins near the surface. This transformation was driven by dislocation accumulation, subgrain formation, and the evolution of high-angle grain boundaries (HAGBs), with HAGBs comprising up to 71.8% near the top surface. As a result, surface microhardness increased to 579.8 HV, with an effective hardened depth of ∼960 μm. Pin-on-disk tests revealed a 26.9% reduction in wear width and a 54.1% decrease in wear rate relative to untreated steel. The improved wear resistance is attributed to the stable gradient nanostructure, which enhances strain accommodation and mechanical stability under sliding contact. These findings provide mechanistic insight into ferrite-dominated nanocrystallization induced by USP and offer guidance for surface gradient design in high-performance aerospace gear steels.