E-axle潤滑油下における歯車鋼の耐疲労摩耗特性に及ぼすガス軟窒化処理の影響

抄録

To achieve further improvement in EV energy efficiency, the size and weight of E-axle is being promoted to decrease. For the trial of decreasing the size and weight of E-axle while maintaining the conventional power of transmission, higher rotation speeds of gear are necessary. However, the number of contacts between gear tooth flanks increases; therefore, in the E-axle, fatigue wear, such as pitting, will increase. As a conventional method to improve fatigue wear resistance, heat treatment has been widely used for a long time. Especially, carburizing is the mainstream heat treatment for gear surfaces. Carburizing is the main heat treatment for tooth surfaces, but it is known that as the sliding speed increases and the frictional heat increases, the amount of wear increases because the base material is softened by tempering. Therefore, gas nitrocarburizing treatment, which shows excellent wear resistance even in the high sliding speed range, is attracting attention. In this study, rolling fatigue friction tests were performed under different conditions of slide-roll-ratio (SRR) load and specimen heat treatment to evaluate fatigue wear resistance and investigate its mechanism. The damaged area percentage (DAP) of carburizing and nitrocarburizing specimens increased and decreased as SRR increased, respectively. Cross-sectional observation showed that in the case of gas nitrocarburizing, crack propagation stopped in the middle of the compound layer. The mechanism improving fatigue wear resistance is considered to be the effects of the compound layer produced by the nitrocarburizing. We considered that the soft surface layer decreased the contact pressure and the compound layer protected against the crack propagation due to the high hardness on the improvement of fatigue wear by nitrocarburizing.