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Quenching, the most common method of hardening metals, involves continuous cooling from a given austenitization temperature to achieve full transformation into martensite. Thus, optimal mechanical properties are obtained. The quenching media contains a base oil and different types of additives depending on application requirements. The cooling rate mainly depends on the thermal characteristics of metals, section thickness of workpieces, and heat-removal properties of the quenching medium. For the base oil, it is possible to use petroleum oils, synthetic oils, and natural oils separately or in combination. Petroleum base oils are widely used because of their thermal-oxidative stability advantages in comparison to natural oils, or lower prices in comparison to synthetic oils. There are numerous compounds available for potential use based on their functional properties. Environmental and safety requirements also exhibit a strong influence on additive selection. For this reason, the current use ofbarium-containing additives must be replaced with less harmful compounds. Petroleum basestocks should be replaced by renewable basestocks that are biodegradable, as is the current general trend in lubricant development. By changing the composition of quenching oils, base oils, and/or additives, the heat transfer characteristics are also changed. In this study, quenching oils with different types of additives are tested and the results regarding their physical and chemical properties as well as cooling characteristics are presented. Cooling curves of the tested oils have been evaluated according to ISO 9950:1995.