Effect of Friction Stir Processing Parameters on the Surface Wear Resistance of 65Mn Steel

Friction stir processing (FSP) was applied to rolled 65Mn steel to improve its surface wear resistance, with untreated and conventionally water-quenched specimens used as reference conditions. A 3 × 3 full-factorial combination of rotational speeds of 600, 700, and 800 rpm and traverse speeds of 200, 250, and 300 mm/min was investigated. Continuous modified layers without obvious macroscopic defects were obtained under all processing conditions. FSP substantially fragmented and reconstructed the original ferrite–pearlite microstructure, producing a fine and relatively homogeneous acicular/lath-like transformed microstructure. The average grain size decreased from 27.1 μm to 11.6–17.2 μm, corresponding to reductions of 36.5–57.2%, while the high-angle grain-boundary fraction increased from 76.7% to a maximum of 86.2%. Although the hardness of the FSP-treated regions was comparable to that of the quenched specimen, FSP produced a more continuous and spatially uniform gradient-hardened layer. The minimum wear rate was obtained at 800 rpm and 200 mm/min, representing reductions of 97.34% and 96.14% relative to the base material and quenched specimen, respectively. The condition of 800 rpm and 300 mm/min was identified as the optimum overall condition because it provided a favorable balance among wear rate, mass loss, hardened-layer continuity, and wear-track morphology, with wear-rate reductions of 96.57% and 95.02% and mass-loss reductions of 66.67% and 57.14% relative to the two reference conditions. The improved wear resistance is primarily associated with grain refinement, grain-boundary reconstruction, microstructural homogenization, and the formation of a continuous gradient-hardened layer.