Extended crack propagation in nitrided and carburised steels subject to rolling contact fatigue

To predict the Rolling Contact Fatigue life of components such as rolling element bearings, it is important to understand the failure mechanisms. A rather unusual failure was observed on integrated races into a planetary gear train. The failure of the planet of this gearbox was attributed to the very long propagation, at an unusual depth, of a crack, which probably started at the surface. The present preliminary study aims to gain a better understanding of the factors that can lead to this type of failure. Firstly, a twin-disc machine is used to reproduce the contact conditions identified in the application. A second set of more severe conditions is also tested. A comparison is made between a carburised steel (16NiCrMo13) and a nitrided one (32CrMoV13). Two distinct crack patterns and spalling characteristics between the two materials are observed. Some unusual crack characteristics are observed in the carburised sample for a specific set of contact conditions.