Effect of preliminary heat treatment on the microstructure evolution and fatigue strength of ion nitriding 42CrMo steel

Preliminary heat treatment is a key process during gear nitriding, directly affecting microstructure, strength and toughness. In order to further improve fatigue strength of nitriding gears, the paper investigates the microstructure evolution of ion nitriding 42CrMo steel under different preliminary heat treatments, i.e., polyalkylene glycol (PAG) quenching+tempering (PT), oil quenching+tempering (OT), 280°C salt bath quenching+tempering (ST) and normalizing+tempering (NT). The fatigue testing is conducted via the self-developed rolling contact fatigue testing machine. The microstructure characteristics of nitriding layer and core material were systematically investigated by electron backscatter diffraction (EBSD), scanning electron microscope (SEM), XRD, impact testing machine and etc. The experimental results indicate that the average rolling contact fatigue life of the sample PT is the longest, followed by sample OT, sample ST, and sample NT, which are 3.30 × 10 6 cycles, 2.88 × 10 6 cycles, 2.02 × 10 6 cycles and 1.77 × 10 6 cycles, respectively. The heat treatment methods of 42CrMo steel produce different microstructures, grain sizes, and dislocation degrees, affecting the fatigue strength. The XRD test results indicate that PT sample has the best effect on fine grain strengthening and dislocation strengthening. The EBSD test results indicate that the proportion of HAGBs in the PT sample is the highest, reaching 57.6%, and the least in the NT sample, which is 47.7%. The impact energy at -20 °C in the PT sample is the highest, reaching 55.3 J, and the least in the NT sample, which is only 2.7 J. This study reveals the mechanism of preliminary heat treatment on the fatigue strength of ion nitriding 42CrMo steel, providing a theoretical basis for further improving fatigue strength.