Effects of Mo in Combination with Nb on Austenite Grain Size Control in Vacuum Carburizing Steels

Vacuum carburizing is increasingly employed to reduce near-surface intergranular oxidation, and reduce quenching distortion. Vacuum carburizing can be conducted at higher operating temperatures, as high as 1100°C, to reduce the processing times and increase furnace productivity. Processing at elevated temperatures may, however, result in excessive austenite grain coarsening, leading to the degradation of fatigue performance of case carburized steels. Micro-alloying with grain growth inhibitors is one effective method to limit austenite grain size of carburized steels. Core properties might also be enhanced; in this study, the effects of micro-alloying a carburizing steel with Mo and Nb on microstructural refinement in the core have been investigated. Additions of Nb alone provide some control of the abnormal austenite grain growth. Further additions of Mo in combination with Nb provide enhanced resistance to austenite grain growth, especially at high carburizing temperature conditions (≥ 1050°C). The enhanced control is attributed to the formation of complex Mo, Nb carbides.