Effects of carburizing and nitriding sequential treatment on rolling contact fatigue of M50NiL bearing steel

The study aims to investigate the effects of carburizing (C), carburizing and nitriding (C+N) sequential treatment on microstructure, microhardness distribution, residual stress distribution and rolling contact fatigue (RCF) properties of M50NiL bearing steel. The results showed that the fatigue limit of the C+N samples was 14.4% higher than that of the C samples. According to analyses of fatigue crack initiation and propagation behavior of RCF specimens revealed that the failure mechanism of the C specimens was deep spalling caused by fatigue crack in a large-angle propagation mode; whereas the C+N specimens had higher surface hardness and residual compressive stresses, which suppressed fatigue crack initiation and propagation, and the failure mechanism was changed to fatigue crack in a small-angle propagation mode caused by shallow spalling. Transmission electron microscopy analysis of C+N specimens after RCF experiments revealed that the dislocations proliferate and shear the carbonitride precipitation phases during the RCF experiments, and the microstructure and precipitation phases of the M50NiL steel are refined, which effectively inhibited the rate of fatigue crack initiation and propagation, and substantially enhanced the RCF life of M50NiL steel.