Effects of surface integrity on wear resistance of 42CrMo steel subjected to ultrasonic shot peening

To enhance the wear resistance of 42CrMo steel, ultrasonic shot peening (USP) was strategically employed as a surface modification technique to improve the material surface integrity. A systematic investigation was conducted, beginning with the comprehensive surface characterization of the USP-treated specimens, including the quantitative analysis of surface roughness, surface residual stress field, microhardness gradient, grain refinement characteristics, and microstructural transformations. The tribological behavior was then studied through the controlled dry sliding friction and wear tests, with particular emphasis on the coefficient of friction (COF) and specific wear rate under varying operational conditions (normal loads of 10 N and 20 N, sliding speeds of $\pi m/\min$ and $2\pi m/\min$). The experimental results demonstrate that USP treatment using 6 mm-diameter shots converts the surface tensile residual stresses into surface compressive residual stresses, improves the maximum microhardness by 56 %, while significantly increases surface roughness. These surface modifications lead to the reduction of COF and specific wear rate by 15 % and 34 % under the dry sliding condition of 10 N normal load and $\pi m/\min$ sliding speed. SEM was utilized to characterize wear mechanisms through the detailed analysis on the worn surface morphologies, and found that the enhancement of wear resistance by USP is attributed to the transition in wear mechanism from predominant adhesive wear in the as-received specimen to primarily abrasive wear in the USP-treated specimen. The study establishes the correlation between USP-induced surface integrity enhancement and the improved wear resistance, supported by mechanistic analysis of surface strengthening mechanisms. These findings provide fundamental insights into the wear-resistant mechanisms of USP-treated 42CrMo steel, offering a valuable guidance for surface engineering applications in industrial components subjected to severe wear conditions.