In various studies, heat transfer coefficients (HTCs) have been used to characterize the relative ability of a quenching medium to harden steel. In this current work, HTCs are determined for a series of vegetable oils using a stochastic (particle swarm) optimization technique and cooling curves produced via Tensi probe measurements. The vegetable oils investigated include canola, coconut, corn, cottonseed, palm, peanut, soybean, and sunflower oil, and their quenching performance is compared with that of a typical petroleum oil quenchant.

In this investigation, the authors use a Tensi probe to obtain cooling curves for canola and palm oils and determine their heat transfer coefficient profiles. For comparison, the cooling curve of an accelerated petroleum oil quenchant is also presented. Canola oil exhibited minimal evidence of film boiling, while palm oil showed a pronounced film boiling behavior. This behavior suggests the presence of unrefined volatile by-products or subsequent degradation. The petroleum quenchant exhibited wetting front movement along the Tensi probe not observed with the vegetable oils.

The most common probe used for cooling curve analysis of quenchants is a 12.5 mm diameter x 60 mm Inconel 600 cylindrical probe with a Type K thermocouple inserted into the geometric center. The time-temperature cooling curve is obtained at this position and is the basis for national and international standards including ASTM D6200, D6482, D6549, ISO 9950 and others. However, greater insight into the quenching process would be possible if a better profile were available for the uniformity and wetting kinematics of the quenching process. An alternative probe design, proposed by Prof. H.M. Tensi and his colleagues, utilizes a cylindrical 15 mm diameter x 45 mm flat-bottom shape with four thermocouples. One thermocouple is inserted to the geometric center of the probe at 22.5 mm from the bottom. The remaining three thermocouples are located 2 mm below the surface of the probe at 2 mm, at 15 mm, and at 30 mm from the bottom. This alternative probe design was used to characterize the usual centerline cooling curve properties as well as rewetting properties of two vegetable oils, palm oil and canola oil, a commercial fast petroleum oil quenchant, and a conventional petroleum oil quenchant. The probe construction, use, and quenching characterization results are reviewed in this paper.