Fatigue properties of niobium-microalloyed case hardening steels for heavy duty gears

Fatigue properties of three case hardening steels for heavy duty gears after carburization were investigated by means of rotating bending fatigue tests. The results indicate that Nb microalloyed steels possess finer microstructure and higher case hardness, resulting in higher fatigue limits. Intergranular fracture is observed in the fatigue crack propagation region of the carburized case, indicating that the finer prior austenite grain size in the carburized case of the Nb microalloyed steels can more effectively suppress fatigue crack propagating. Scanning electron micrographic observations on fracture surface show that fatigue cracks usually initiate from matrix or inclusions, and thus the smaller the size of inclusions is, the better the fatigue properties are.