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.

It is well known that petroleum oil base stocks possess a number of limitations, such as being non-renewable, but even more importantly, they are considered relatively toxic with limited biodegradability. One class of base stock that is renewable with excellent biodegradability characteristics and that is generally, but not always, non-toxic is animal and seed oils. The quenching performance of many different animal and vegetable oil compositions has been reported in the literature. However, as a class, they suffer from generally poor thermal oxidative stability, even when containing oxidation inhibitors, when compared to quenchants derived from petroleum oil. This factor limits their potential commercial utility. One method of addressing this problem is to chemically modify the vegetable oil to produce increased resistance to thermal-oxidative degradation. This work discusses the physical properties and quenching performance of epoxidized soybean oil-based formulations and the resulting metallurgical properties, hardness, and microstructures obtained. These results have not been reported previously.

Although petroleum oils continue to be the dominant type of base stock for the formulation of vaporizable quenchants, there is increasing pressure to identify an alternative. Petroleum oils are not a renewable base stock, and they possess generally poor toxicity and biodegradability properties. Currently, the most often cited alternative base stocks are those based on seed oils since they are renewable and are readily biodegradable, and usually non-toxic. However, they suffer a critically important deficiency in that they are also typically much less stable to thermal-oxidative degradation than petroleum oils. Various studies have addressed the effect of vegetable oil structure on oxidation and on the use of oxidation inhibitors to provide the necessary stabilization. However, most of these reports do not address the relative effects of specific antioxidant structures on inhibiting oxidation and on quenching performance. This paper describes the use of certain antioxidant structures on inhibition of thermal-oxidation and on the effect of the presence of antioxidants on quenching performance.

The potential use of vegetable oil derived industrial oils continues to be of great interest because vegetable oils are: relatively non-toxic, biodegradable and they are a renewable basestock alternative to petroleum oil. However, the fatty ester components containing conjugated double bonds of the triglyceride structure of vegetable oils typically produces considerably poorer thermal oxidative stability than that achievable with petroleum basestocks under typical use conditions where furnace loads of hot steel (850 °C) are typically rapidly immersed and cooled to approximately 50-60 °C bath temperatures. This is especially true when a vegetable oil is held in an open tank with agitation and exposure to air at elevated temperatures for extended periods of time (months or years). Unfortunately, as vegetable oils degrade, their fluid viscosity is expected to increase resulting in decreased quench severity. Clearly, thermal-oxidative stability is essential. This paper will review thermal-oxidative stability and quenching performance of soybean oil and palm oil and the resulting impact on the heat transfer coefficient.

Vegetable and animal oils as a class of fluids have been used for hundreds of years, if not longer, as quenchants for hardening steel. However, when petroleum oils became available in the late 1800s and early 1900s, the use of these fluids as quenchants, in addition to their use in other industrial oil applications quickly diminished. This was primarily, but not exclusively, due to their generally very poor thermal-oxidative instability and the difficulty for formulating fluid analogs with varying viscosity properties. Interest in the use of renewable fluids, such as vegetable oils, has increased dramatically in recent years as alternatives to the use of relatively nonbiodegradable and toxic petroleum oils. However, the relatively poor thermal-oxidative stability has continued to be a significant reason for their general non-acceptance in the marketplace. Soybean oil is one of the most highly produced vegetable oils in Brazil. Currently, there are commercially produced epoxidized versions of soybean oil which are available. The objective of this paper is to discuss recently obtained results showing the dramatic improvement in thermal-oxidative stability of epoxidized soybean oils and to discuss their potential use and heat transfer properties as viable alternatives to petroleum oils for hardening steel.