In this research, a gradient nanostructured surface (GNS) layer was produced on 17-4PH stainless steel by ultrasonic shot peening (USP), which significantly enhanced surface performance. Surface microhardness increased by 24.1% as a result of the severe plastic deformation (SPD) caused by USP, which changed the coarse lath martensite into nanoscale martensitic grains (~60 nm) and produced high-density dislocation structures. Systematic microstructural analyses revealed that the grain refinement proceeded through dislocation multiplication, subgrain segmentation, and continuous dynamic recrystallization. The resulting GNS layer exhibited significantly enhanced wear resistance, with an ~80% reduction in wear rate, attributed to grain boundary strengthening and dislocation-mediated hardening. Post-USP annealing at moderate temperatures (≤500 °C) yielded an additional 22.1% hardness gain over the USPed state. This annealing-induced hardening effect is likely associated with the stabilization of dislocation walls and cell structures during annealing, together with the strengthening contribution of nanoscale Cu-rich precipitates that more effectively hinder dislocation motion. These findings provide a thermomechanical strategy for designing thermally stable GNS layers in precipitation-hardened stainless steels, thereby improving surface performance via the combined contributions of dislocation structures and precipitation hardening.
Liu et al. (Wed,) studied this question.