Properties of carbon and alloy steels vary with their composition and microstructures, which are dependent on the heat treatment and quenching process used.

Typically, steel is heated to its austenitization temperature and then cooled sufficiently fast to avoid pearlite transformation and obtain maximum hardness and strength. The critical cooling rate is dependent on both the specific heat capacity and thermal conductivity of both the steel and the quenchant in addition to quench-bath temperature and agitation.    

Among the most common quenchants used is water. However, petroleum-oil-derived quenchants are used when lower cooling rates and more uniform cooling is desired for better distortion control and crack prevention. In addition to being susceptible to global supply issues and price volatility, petroleum oil possesses a number of other significant disadvantages, including: poor biodegradability,[1] toxicity[2] and flammability. Therefore, it is of continuing interest to identify a viable alternative to petroleum oil as a basestock for quenchant formulation.[3]    

Currently, vegetable oils are one of the most commonly identified renewable, biodegradable and non-toxic basestocks. In addition to their relatively narrow viscosity range, vegetable oils exhibit considerably lower thermal-oxidative stability relative to petroleum oil, which has been a significant factor inhibiting its use in the industry.    

This paper will provide an overview of the recent history of the use of vegetable oils as quenchants for steel. A brief comparison of the quenching performance exhibited by vegetable oils relative to petroleum oil will be illustrated. Also discussed is the effect of antioxidants on thermal-oxidative stabilization of soybean oil.

Historical Overview

There have been many comparative investigations on the use of vegetable oils and animal oils as quenchants. One study involving cooling-curve and heat-transfer analysis of quenching properties of rapeseed oil was conducted by Rose in 1940.[4] Higher cooling rates were observed for rapeseed oil compared to petroleum oil, which was attributed to the relatively poor stability of the vapor blanket formed by the rapeseed oil.    

Tagaya and Tamura subsequently compared the quench severity of different vegetable oils (soybean, rapeseed and castor oils) with mineral oils and fish/animal oils with respect to fluid source, viscosity and oxidative stability for various naturally derived fluids.[5] These data showed that the Grossmann quench-severity factors were comparable for both castor oil (H=0.199) and soybean oil (H=0.200).    

Fujimura and Sato also studied the quenching performance of vegetable oils reported earlier by Tagaya and Tamura. They added castor oil to their study in addition to ethyl esters of oleic, palmitic and stearic acid versus different petroleum-oil quenchants and concluded that the performance of soybean and rapeseed oil were essentially equivalent.[6] Quenching performance of the ethyl esters were also equivalent to each other and exhibited greater quench severity than the vegetable oils. Castor oil, however, was found to be thermally unstable, and this was subsequently confirmed.[7, 8]    

Currently, the most commonly cited vegetable-oil basestocks used for quenchant formulation in the U.S. are based on canola oil[9,10] and soybean oil.[11] Recently, a crambe-oil-based fluid has been reported as a potential quenchant.[12] Przylecka and Gestwa have reported the use of an unidentified vegetable-oil-based quenchant for hardening carburized steels.[13]    

Prabhu studied the heat-flux properties of palm oil[14] and extensive heat-transfer and wetting studies of coconut, sunflower, groundnut, palm and castor oils.[14-17] With the exception of palm oil, these vegetable oils are not commonly considered as basestocks for quenchants in North and South America. Interestingly, with the exception of castor oil, relatively little difference in surface wetting properties were observed in Prabhu’s work, and quench severities comparable to conventional, non-accelerated mineral oil were observed.    

Totten, et. al. reported the results of cooling curve, hardening performance, heat-transfer and rewettability characterization studies conducted with crude and partially hydrogenated and winterized soybean oils[18] provided by Honary.[19] Because of the strong influence of the test specimen surface on the wetting behavior (and therefore on the quenching behavior), unalloyed (Ck 45) steel probes were used. This work provided a simultaneous examination of the martensitic transformation with local differences and dependencies on the rewetting time (and on the steel’s metallurgical condition).    

Cooling-curve analyses showed that immersion quenching was primarily influenced by the sequence of the cooling phases and different heat-transfer rates. Heat transfer on the specimen surface was primarily determined by the rewetting conditions and depended on the cooling characteristics of the quenching medium used, bath temperatures and agitation rates.    

The soybean oils investigated showed no significant differences in cooling behavior and rewetting conditions on the test-specimen surface, and cooling rates were similar. Comparison of the cooling time-temperature and cooling-rate curves showed that vegetable oils exhibited faster cooling rates than the mineral oil used as a reference. More recent results confirmed the faster cooling rates and showed nearly no full-film boiling behavior.[20]  

Quenching Mechanism Comparison
It is well known that conventional (“slow”) petroleum-oil quenchants exhibit pronounced film boiling (vapor-phase cooling) and nucleate-boiling behavior as shown in Fig. 1. This is a disadvantage when faster, high-temperature cooling is necessary, particularly for crack-sensitive, low-hardenability steels. In such cases, additives are used to formulate accelerated (“fast”) quenching oils that exhibit decreased film-boiling behavior by accelerating the wetting of the hot steel upon initial immersion by the quench oil.     Figure 1 shows that soybean oil (typical of other commonly available vegetable oils) does not exhibit the same film-boiling or nucleate-boiling behavior as exhibited by petroleum-oil quenchants. This is because of the very high boiling points (low vapor pressure) exhibited by vegetable oils under atmospheric-pressure conditions. It should be noted that contamination and fluid degradation can produce more volatile components during use, which may lead to a slightly prolonged film-boiling region. However, the surface temperature of the steel is usually below the boiling point of the vegetable oil throughout most of the quenching process, and cooling is predominantly by convection.

Quenching Behavior of Vegetable Oil
The quenching performance of a series of commonly available vegetable oils was compared. The oils, which were obtained from a local supermarket, included canola, soybean, corn, cottonseed and sunflower oils. These vegetable oils were evaluated by cooling-curve analysis according to ASTM D6200-01 – Standard Test Method for Determination of Cooling Characteristics of Quench Oils by Cooling Curve Analysis, which was conducted at 60°C under unagitated conditions. For comparison, two different petroleum-oil quenchants were used: Micro Temp 157 (a “slow” oil) and Micro Temp 153B (a “fast” oil). The cooling time-temperature and cooling-rate curves obtained are shown in Fig. 2.    

Overall, Figure 2 shows that the cooling properties of the series of vegetable oils appeared to be comparable to each other. It is notable that only convective cooling was obtained with no extended vapor blanket cooling as expected from the curves shown in Fig. 1. This cooling profile would suggest that vegetable oils generally require no cooling-rate accelerator and would be acceptable even for difficult-to-harden, crack-sensitive carbon steels. The cooling rates, however, were somewhat faster, especially in the critical martensitic temperature transition range (300°C), than the petroleum-oil quenchants evaluated, suggesting that they may not be as desirable for higher-alloy, crack-sensitive steels.    

The petroleum-oil-based quenchants, Micro Temp 157 and Micro Temp 153B, did behave as expected. The fast oil, Micro Temp 153B, exhibited shorter film-boiling times, and both oils exhibited similar lower-temperature cooling rates. The Micro Temp 153B was somewhat slower, which was probably due to the higher bulk fluid viscosity of the basestock used to formulate the quenchant.    

Another method of comparing the quenching performance of different quenchants is to compare their heat-transfer coefficients.[21] A plot of the heat-transfer coefficient as a function of the centerline (core) temperature of the cooling curves obtained under unagitated conditions according to ASTM D6200 are shown in Fig. 3. These data were obtained using the commercial code HTMod.[22,23] The maximum heat-transfer coefficients exhibited by the vegetable oils in this study decreased as follows:  

sunflower > corn > soybean > canola  > cottonseed      

Comparison of the heat-transfer coefficient as a function of temperature afforded a better differentiation of the quenching performance exhibited by these vegetable oils than afforded by the more conventional cooling-curve analysis.    

While both of the petroleum quench oils Microtemp 157 (T 157) and Microtemp 153B (T 153B) exhibited full-film boiling, this was typically not observed for any of the vegetable oils evaluated in this study. As expected, the Microtemp 157 slow oil exhibited a longer full-film boiling duration than did the Microtemp 153B fast oil. Furthermore, the Microtemp 153B fast oil exhibited a significantly higher maximum heat-transfer coefficient at a higher temperature than the Microtemp 157 slow oil and was comparable to the vegetable oils evaluated.

Thermal-Oxidative Stability
Although soybean oil is of particular interest as a potential basestock for quenchant formulations, it has been shown previously to be one of the oxidatively unstable vegetable oils that might be selected for this purpose.[24,25] Therefore, if soybean oil could be effectively stabilized by the addition of an antioxidant, other vegetable oils with greater oxidative stability could potentially be even more stable with the addition of the same antioxidant. Therefore, soybean oil was selected for an antioxidant study. In addition, the oxidative stability of various inhibited soybean oils was compared to fully formulated, commercial petroleum quench oils.    

The test and test conditions selected[8] were modeled after the system reported earlier by Bashford and Mills.[26] In this test, the fluid viscosity increase with time is monitored, and increasing viscosity indicates increasing instability. Ideally, no viscosity change with time would be observed.    

The results obtained show that the uninhibited soybean oil (SO) and two inhibited fluids SO1 and SO4 exhibit comparatively poor stability. Two inhibited fluids SO2 and SO3 exhibited significantly better stability than the other soybean-oil-based fluids. However, even SO2 and SO3 did not exhibit the excellent oxidative stability shown by both petroleum-oil-based quenchants Microtemp 157 and Microtemp 153B. Because this is an accelerated test, it is difficult to translate the relative oxidative-stability performance indicated to actual commercial quench-tank performance. However, these data do indicate that the soybean-oil-based fluids are notably poorer than petroleum oil and considerably more work must be done to achieve comparable performance.    

Oxidative-stability performance improvements of vegetable oils can be achieved by chemical[27,28] or genetic modifications[29,30] or by process improvements such as winterization and partial hydrogenation.[31,32] Winterization (fractionation) is performed to remove crystallized fats and improve the pour point of the base oil. The performance objective is to reduce the linolenic and linoleic ester content of the vegetable oil to increase the oxidative stability, making the resulting vegetable oil more suitable for use in industrial oil applications.[31] Genetic engineering has produced soybean oil with >85% oleic acid ester contents.[27,28] There are, however, a number of substantial problems with genetic engineering to reduce polyunsaturation.    

Another approach that could produce vegetable-oil-derived basestocks for quenchant applications is epoxidation. Epoxidation of vegetable oils has been shown to significantly improve oxidative stability, although data comparing the resulting oxidative stability of the epoxidized vegetable oils with their functionally equivalent petroleum oil basestocks is not yet available. However, Wu reported that epoxidization of rapeseed oil exhibited greater oxidative stability than rapeseed oil without chemical modification, and the biodegradability was not affected.[33]   

It has also been reported that epoxidized soybean oil demonstrated improved performance of thermal-oxidative stability relative to soybean oil and genetically modified high-oleic soybean oil in certain high-temperature lubricant applications.[34,35] Thus, these and other methods are excellent candidates for producing oxidatively stable, vegetable-oil-derived basestocks for quenchant formulation.

Conclusions

A brief historical overview of the use of vegetable oils as quenchants for steel has been provided. It was also shown that the quenching mechanism exhibited by vegetable oils is fundamentally different than that traditionally observed for petroleum-oil quenchants. While vegetable oils mediate cooling predominantly by convective heat transfer, petroleum oils exhibit an inherently non-uniform mechanism involving full-film boiling, nucleate boiling and convective cooling with often very different heat-transfer coefficients.    

The cooling-curve performance of a series of commonly available vegetable oils (canola oil, corn oil, soybean oil, sunflower oil and cottonseed oil) was examined. Although the cooling curves of each of these vegetable oils seemed comparable, comparison of the maximum heat-transfer coefficients showed the following decreasing order of magnitude:  

sunflower > corn > soybean > canola > cottonseed      

Oxidative stability testing showed that although the stability of soybean oil could be improved, all of the soybean-oil candidates exhibited poor stability relative to fully formulated petroleum-oil quenchants. Potential methods of structurally modifying vegetable oils to improve their stability to rival that of petroleum oil were identified. IH  

For more information: Contact George E. Totten, Portland State University, G.E. Totten & Associates, LLC, PO Box 30108, Seattle, 98113: tel: 206-788-0188; fax: 815-461-7344; e-mail: getotten@getottenassociates.com. Other authors are Ester Carvalho de Souza (esterfisica@yahoo.com.br) and Lauralice C. F. Canale (lfcanale@sc.usp.br) of the University of São Paulo, Department of Materials, Aeronautical and Automobile Engineering, School of Engineering, São Carlos, SP, Brazil