Influence of niobium microalloying on rotating bending fatigue properties of case carburized steels

Fatigue properties of three case hardening steels have been investigated by means of rotating bending fatigue tests on smooth bar specimens after carburization. Results show that the 0.04% Nb steel had the highest fatigue limit of 1170 MPa, while those of the Nb-free and 0.08% Nb steels were 995 MPa and 1125 MPa, respectively. It is also shown that the 0.04% Nb steel had the finest prior austenite grain sizes in the carburized case as a result of the pinning effect of fine Nb(C,N) particles on austenite grain coarsening during carburization. Scanning electron micrographic observations on the fracture surface of the fatigue specimens show that fatigue crack usually initiated from oxide inclusions and propagated along prior austenite grain boundaries, indicating that the fine prior austenite grain size in the carburized case of the 0.04% Nb steel can contribute to its high fatigue properties.