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For evaluation of quenchants under real industrial conditions, the new Liščić/Petrofer probe is designed. Liščić/Petrofer is special upon design, because it is instrumented with 3 thermo-couples on the same radius of the cross- section at the middle of the probe length. The outer thermocouple measures the temperature at 1 mm below surface, the second one at 4.5 mm below surface, and the third one at the center of the probe. The working principle of the probe is the measurement of the dynamics of heat extraction which is best represented by the change of temperature gradients. Second type of probe developed to evaluate the cooling properties of relatively small samples of a quenchant used in this paper is standard ASTM D6200 probe, which consists from cylindrical test specimen having one thermocouple at its geometrical center. Comparative testing was recently performed with the Liščić/Petrofer probe and standard probe with one vegetable oil (Palm Oil) and a commercially available conventional mineral oil (Kalenol 32). The work was performed at the Quenching Research Centre (QRC) of the Faculty for Mechanical Engineering, University of Zagreb, Croatia. The results of this paper showed distinctive differences in the quenching behavior of these two quenchant media. The results of this comparative study are reported herein.

Quenching is the most common method of hardening metals, which involves continuous cooling from a given austenite temperature to achieve full transformation into martensite structure. The cooling rate mainly depends on thermal characteristics of metals, section thickness of workpieces, and the quenching medium heat-removal properties. Oils as quenching media contain a base oil and different types of additives according to application requirements. As the base oil, it is possible to use mineral oils, synthetic and natural oils, either separately or in combination. Mineral base oils are widely used because of their advantages in stability in comparison to natural oils, or lower prices in comparison to synthetic oils. There are numerous additives available for use but, besides functional properties, the environmental and safety requirements have to be considered in the selection of additives, e.g. barium additives must be replaced with less harmful compounds. Petroleum derivatives should be replaced with renewable base stocks that are biodegradable, which is a general trend in lubricant development. By changing the composition of quenching oils, base oils and additives, heat transfer characteristics are also changed. This study presents results of the analysis of physical and chemical properties, as well as of cooling characteristics of new quenching oils with different compositions. Cooling curves for different compositions have been evaluated according to ISO standard 9950.