Design and study of a new QPQ combined with ultrasonic roller pressing method for gear surface strengthening

This study investigates a hybrid surface enhancement technique that combines Quench Polish Quench (QPQ) with ultrasonic rolling to augment the durability of 42CrMo gears against wear and fatigue. Analysis reveals that post-treatment, the composite nitride layer on the gear surface maintains its original phase composition, evidenced by the absence of new diffraction peaks in X-ray diffraction (XRD) patterns. Nevertheless, a noticeable broadening in these peaks suggests grain refinement and the presence of microstrain due to the rolling process. Incremental static pressures improve the mechanical characteristics of the surface, with the layer treated at 0.6 MPa exhibiting the lowest wear rate (3.15 × 10−8 mm3N−1 m−1). Oxidative wear increases at elevated temperatures (100 °C), yet under lubricated conditions, Coefficient Of Friction (COF) of the treated layer significantly reduces. The predominant wear mechanisms include mild abrasive wear, oxidative wear, and adhesion. Excessive impact force, particularly at 0.6 MPa, risks surface cracking and subsequent degradation in fatigue resistance. Optimal fatigue performance is achieved with a static pressure of 0.4 MPa, corresponding to a surface strength of 2300 MPa and a fatigue life of 3.23 × 106 cycles.