Effect of carburizing process on high cycle fatigue behavior of 18CrNiMo7-6 steel

Carburizing heat treatment is widely used in the fabrication of gears to improve their fatigue performance. The effects of carburizing process on the material properties surface hardness, residual stresses, retained austenite of case-hardened 18CrNiMo7-6 and the consequential fatigue strength have not been studied in detail. In this research, the high-cycle fatigue performance of uncarburized and carburized 18CrNiMo7-6 gear steel were studied. In addition, the evolution processes of hardness, microstructure and residual stress of carburized specimens during the fatigue test were observed quasi-in situ. The initiation of fatigue cracks was also analyzed by SEM and the transformation of retained austenite during the fatigue test was observed by TEM. The results showed that the fatigue strength of carburized specimens has an increase-decrease trend with the increase of effective case depth. Besides, the fatigue failure mode of steel changes from surface failure to internal failure after being carburized, during which retained austenite plays an important part. In addition, if a large amount of retained austenite exists in the specimen with internal failure, the high-density twin martensite region formed by the retained austenite transformation can act as the crack initiation site, which is harmful to fatigue performance.