Vacuum carburizing with high pressure gas quenching is increasingly employed to reduce near-surface intergranular oxidation and quenching distortion. It has also been shown to reduce processing times because it can be conducted at higher temperatures, up to 1100 °C. These temperatures, however, may cause austenite grain coarsening, making steel more susceptible to fatigue failure. This paper presents a study showing how microalloying carburizing steels with Mo and Nb improves resistance to austenite grain growth. The control of grain size is attributed to solute and precipitation effects.