The Fatigue Performance of High Temperature Vacuum Carburized Nb Modified 8620 Steel

The bending fatigue performance of high temperature (1050 °C) vacuum carburized Nb modified 8620 steel, with niobium additions of 0.02, 0.06 and 0.1 wt pct, was evaluated utilizing a modified Brugger specimen geometry. Samples were heated at two different rates (20 and 114 °C min^-1) to the carburizing temperature resulting in different prior austenite grain structures that depended on the specific Nb addition and heating rate employed. At the lower heating rate, uniform fine grained prior austenite grain structures developed in the 0.06 and 0.1 Nb steels while a duplex grain structure with the presence of large (>200 μm grains) developed in the 0.02 Nb steel. At the higher heating rate the propensity for abnormal grain growth was highest in the 0.02 Nb steel and complete suppression of abnormal grain growth was achieved only with the 0.1 Nb steel. The fatigue properties were correlated with measurements of austenite grain size and grain size distributions, microhardness profiles, carbon profiles, and fracture behavior as evaluated with scanning electron microscopy resulting from the different modes of grain growth. The higher Nb alloys, processed to produce fine, uniform prior austenite grain structures exhibited the best fatigue performance as evidenced by higher endurance limits and higher low cycle fatigue life. The presence of large prior austenite grains in samples that exhibited abnormal grain growth enhanced fatigue crack nucleation leading to lower fatigue performance. The results indicate that controlling austenite grain coarsening characteristics during carburizing, through additions of niobium effectively limits grain growth and removes the susceptibility of austenite grain growth to heating rate, and subsequently increases fatigue performance.