<p dir="ltr">Using oils as quenchants for austempering steels, instead of molten salts, offers advantages such as reduced corrosion, minimized environmental hazards, and easier handling. This study used MarTemp 2565, a mineral-based quenching oil, for austempering high-carbon (SAE 52100) and low-carbon (AISI 4120) steels at temperatures up to 320°C—above its 288°C flashpoint—using a unique low pO₂ vertical tube furnace system to prevent oil combustion. For SAE 52100, the microstructural analysis showed that secondary carbide particles increased significantly in length, aspect ratio, and spacing with rising austempering temperatures (220°C, 270°C, and 320°C). These changes were associated with a reduction in hardness from 59.4 HRC to 51.3 HRC and a decrease in the apparent ductile-to-brittle transition temperature (DBTT) from 250°C (for samples austempered at 220°C and 270°C) to 100°C (for samples at 320°C). For AISI 4120, Charpy tests conducted on both carburized, and non-carburized bars austempered at 300°C showed that i) carburized samples had a lower bainitic case (57 ± 2 HRC) and a tempered martensitic core (44 ± 1 HRC) with compressive residual stress (~-400 MPa), while ii) non-carburized samples exhibited tensile stress (~300 MPa). Charpy impact testing indicated a DBTT of 150°C for both samples, though carburized samples exhibited lower impact energies. High oleic soybean oil (HOSO) was also evaluated as an alternative quenchant under inert conditions. Austempering with HOSO yielded comparable mechanical properties, hardness, and microstructure as for MarTemp 2565 and Quench Salt 330. Stirring in the oil bath proved essential to achieving high cooling rates, by avoiding the vapor blanket phase, to approach or exceed cooling rates typical of salt baths. This study demonstrates how controlled bainitic microstructures and mechanical properties can be achieved in low-carbon and high-carbon steels using oil as a quenchant in an inert atmosphere, which enables a wider range of accessible austempering conditions.</p>