Oil is one of the most common quenchants used in the heat treating industry.Since the cooling properties of a quench oil varies with the degree of aging, it is important that quenching cooling rates be monitored with respect to use time. In addition, there is insufficient general information about various oil quenchants in general and cooling rates in particular. This problem is compounded by complaints from heat treating shops that quenching oils of nominally the same type, and which are alleged to contain the same additive packages, exhibit very different quenching performance in practice. Therefore, at the request of the heat treating industry in Iran, measuring system was designed and built to determine the cooling rate of quenching oils. This system includes of a tubular furnace, oil tank, probe for measuring the temperature, a system for probe transfer from the furnace to the quenchant, and a data acquisition system. This new system is capable of measuring the cooling rate of quenching oils according to various national and international standards. In the study reported here, two types of oils were comparatively evaluated: Behran 145 Oil (New Oil) and Used Behran 145 Oil. The cooling rate of these quenching oils was measured, and the effect of their cooling rate differences on the hardness and the microstructure of a low carbon steel was determined.

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