The friction behavior and wear mechanism of RV reducer gear steel (20CrMo) subjected to three different heat treatment processes from −20 °C to 100 °C

The gear friction pairs in RV reducer always withstand a broad range of temperature, sliding velocity and heavy load. In the research, three types of heat treatment methods (carburizing + quenching, nitrocarburizing and high concentration nitrocarburizing) are applied on the 20CrMo (cycloidal gear material) disc to study the tribological behavior and wear mechanism. The friction coefficient, wear depth and wear rate of the three types of friction pairs under various temperature (−20 °C, 25 °C, 50 °C, 100 °C) are measured and the microstructure characteristics are analyzed by SEM and EDS. The results reveal that high temperature can dramatically decrease the wear scar dimensions and wear rate due to the more active lubricant. Although the lubricant lost its activity at −20 °C, the friction coefficient and maximum wear depth for GCr15/20CrMo (carburizing + quenching) and GCr15/20CrMo (high concentration nitrocarburizing) are relatively less than 0.1 and 4 μm. The wear rate of GCr15/20CrMo (carburizing + quenching) and GCr15/20CrMo (high concentration nitrocarburizing) are 14.49 μm3/N·m and 17.20 μm3/N·m, which are only 28.7% and 34.1% of GCr15/20CrMo (nitrocarburizing). With the temperature increases to 25 °C, tribofilm is more likely to form on the 20CrMo specimen after nitrocarburizing, leading to better wear resistance. At high temperature (50 °C and 100 °C), GCr15/20CrMo (high concentration nitrocarburizing) system presents excellent tribological behavior by the combination of good lubricant activity, formation of tribofilm and refined nitrides. The wear rate even reaches 0.45 μm3/N·m and 0.43 μm3/N·m at the sliding velocity of 300 mm/s and 562 mm/s under 100 °C. Overall, specimen with high concentration nitrocarburizing process exhibits exceptional wear performance under full operating conditions of RV reducer.