Low pressure carbonitriding (LPCN) has the potential to improve impact and fatigue strength, with gears being an example application, through the enrichment of nitrogen in addition to carburizing at higher heat treatment temperatures. In this study, the LPCN response of four different steel alloys is investigated. The influence of unprotected boron is evaluated by comparing the LPCN response of 20MnCr5 with and without boron additions. The influence of Nb microalloying is assessed by comparing the LPCN response of 8620 with and without Nb additions. Low pressure carbonitriding heat treatments were developed to achieve case depths of 0.65 to 0.75 mm in each alloy. The hardness and case microstructure are correlated to bending fatigue response measured with Brugger fatigue specimens, which are designed to simulate the root of a gear tooth.