Role of Hardness Distribution on Pitting Strength in Rolling-sliding Contact Fatigue for Nitrided-SCM440H

This study aims to clarify the influence of hardness distribution on the pitting fatigue strength of nitrided steels. Gas nitriding with controlled nitriding potential was applied to SCM440H and 31CrMoV9 steels, and roller pitting tests were conducted. The nitrided specimens exhibited superior pitting fatigue strength compared with carburized specimens at 107 cycles, corresponding to the fatigue limit. In contrast, under high contact pressure, early pitting occurred within 105 cycles, indicating reduced fatigue performance in the finite-life. The pitting fatigue strength of nitrided steels strongly depended on the case-hardened depth, and specimens with deeper hardened layers demonstrated higher durability. Under high contact pressure, most of the plastic deformation occurred in the early stage of testing, whereas work hardening developed progressively with increasing number of cycles. In nitrided steels, such plastic deformation induces tensile stress at the contact surface, and in combination with frictional stress, promotes the formation of vertical cracks originating from the compound layer. Therefore, increasing the case-hardened depth is effective for suppressing early pitting by reducing plastic deformation. For nitride steels, pre-running to induce work hardening also mitigated early pitting without reliance on alloying elements or prolonged nitriding durations. These findings indicate that, even when flaking initiates from the surface, the hardened layer depth plays a critical role in the rolling–sliding contact fatigue performance of nitrided steels. Consequently, for components such as gears that operate under rolling–sliding contact, it is essential to consider the influence of internal shear stress in strength design to achieve superior pitting fatigue strength.