If you're having unexpected heat-treating results, quenchant contamination might be the culprit. Find out where contaminants come from and how they affect the quench process.

Quenching in action

A high-performance quench oil must have oxidation resistance and low sludge formation, be nonstaining and have an elevated flash point and acceptable heat-transfer characteristics[1]. Except for some synthetics and vegetable oils, most modern quench oils are based on refined petroleum-base stocks. The use of higher fractions of naphthenic compounds result in lower flash points and greater sludge formation[2]. The presence of sludge reduces the heat transfer efficiency, which could produce inadequately hardened parts. Sludge is the result of oxidation and polymerization of quenching oils in use[3]. In general, the higher the sludge content, the older the quench oil.

A quench oil should not stain parts. Paraffinic oils impart a light gray color to quenched parts. If the oils contain sulfur, unacceptable black stains could appear. While sulfur usually is removed in the refining process, it is possible that sulfur-containing compounds (such as lubricants) could contaminate the oil and create problems with staining.

Quench oils should not catch fire during use. The flash point of oil is a measure of the tendency of the oil to catch fire. An elevated flash point minimizes the tendency of the oil to catch fire during use. As indicated earlier, the use of larger fractions of naphthenic compounds result in lower flash points. This increases the tendency of quench oil to catch fire during use.

Quench oils should be selected on the basis of their heat transfer characteristics. Typically quench oils are classified into three groups: fast, accelerated oils; medium speed, or conventional oils; and marquenching, or hot oils[1, 4-8]. Fast, accelerated oils usually are formulated from mineral oils and contain additives to increase the quenching speed. Medium speed, or conventional oils, also usually are mineral oils. These quench oils contain antioxidation agents to reduce the rate of sludge formation and thermal degradation.

Marquenching oils are used at elevated temperatures ranging from 90 to 200C (195 to 390F). Typically they are formulated from solvent-refined mineral oils. These oils have a large paraffinic fraction to substantially reduce oxidation and sludge formation. Additional antioxidation agents also are added to enhance stability. Different speeds of martempering oils are available. Typical properties of quenching oils are shown in Table 1.

Contamination of quench oils is a problem in most heat-treating shops. Analysis of data from Houghton International's database of customer cooling curves shows that the most common contaminants are water, soot, salt, sludge (oxidation) and hydraulic oils.

Water contamination is the most common contaminant. Its presence can contribute to spotty hardness, distortion, cracking, staining and potential harm to personnel if the concentration is too high (generally greater than 0.1%) [9]. Sources of water include water-cooled bearings, fans, water-oil heat exchangers, and condensation in humid environments.

Soot generally is from the furnace atmosphere, especially if high-carbon potentials are used, or if the atmosphere is not under control. Soot is a very fine particulate, and is difficult to filter, except when using very small filter meshes (2-3 Km) [10]. Initial accumulation of fine particulate reportedly causes an initial increase in the quenching speed of oil, but eventually the accumulation reduces the speed of quenching oil[11, 12]. This occurs because increasing concentration of soot changes the heat-transfer characteristics and causes more rapid oxidation of the oil.

Salt contamination generally occurs from drag-out from salt-bath furnaces, while sludge, or oxidation, is due to extended operation at elevated temperatures or breakdown of the oil during quenching operations.

Hydraulic oils also are a common contaminant. These contaminants are from leaking doors or from contamination from other operations.

Because any or all of these contaminates can be present in a quenching operation, a study was conducted to evaluate the effect of these contaminates on the cooling curve of a medium speed oil, specifically Houghton Houghto-QuenchR G.

Procedure

A test matrix was developed to examine individual and combined effects of these contaminates on the maximum cooling rate and the temperature of maximum cooling. Temperature also was included in the matrix to represent a range of typical operating practices common in the heat-treating industry. Table 2 shows the test matrix used, and the degree and type contaminants used in the study are shown in Table 3.

Fig 1 Representative cooling curve from which quenching behavior was evaluated with respect to quenchant contamination. The Ledenfrost temperature governs the initiation of nucleate boiling and transition from the vapor phase. Heat transfer occurs by radiation through the vapor phase; heat transfer is relatively slow. Convection cooling is the primary region responsible for part distortion and cracking.

A two-liter volume of each solution was made according to the test matrix. Solutions were split into one-liter volumes and heated to 28 and 82C (80 and 180F). Cooling curves were generated in moderately agitated solutions using the IVF Quenchotest[13]. A representative cooling curve is shown in Fig. 1.

Recorded temperature at maximum cooling rate, maximum cooling rate and comments are shown in Table 4.

Fig 2 Effects of contamination on the maximum cooling rate (CR). Contamination by oxidation is based on the number of days at a temperature of 125?C. Fig 3 Effects of contamination on the temperature at which maximum cooling occurs. Contamination by oxidation is based on the number of days at a temperature of 125?C.

Contamination effects

A statistical analysis of the effects of each contaminate was performed and results are shown in Fig. 2 and Fig. 3. Typically, when the hot probe was immersed into the test oil, there was a small flash and the fire subsequently self-extinguished. However, in some quench tests (3, 4, 11 and 12), a fire occurred that did not extinguish on its own, a phenomenon that was not observed in any other test sequence. This suggests that the interaction of salt and hydraulic oil substantially reduces the flash point of the oil. This effect will be verified in further work.

Of all the main variables that can affect the maximum cooling rate during nucleate boiling, temperature has the most significant effect on the maximum cooling rate. Increasing the temperature increases the maximum cooling rate. While this appears to be counter intuitive, it can be explained by the change in viscosity of the quench oil. At room temperature (28?C, or 80?C), the oil is viscous and does not wet the surface of the part well. A viscous oil does not carry the heat away from the part. However, the viscosity of the oil decreases as the temperature of the oil increases, which results in increased wetting of the part, and, consequently, better heat transfer. Increasing the temperature also causes the temperature at which the maximum cooling rate occurred to increase. This is believed to be associated with the decreased viscosity of the oil, which improves wettability.

Soot has the second largest impact on the maximum cooling rate. The maximum cooling rate increases as the amount of soot increases in the quench oil. This probably is due to the very fine soot particles functioning as nucleation sites for bubble formation during nucleate boiling. However, additions of soot also caused the temperature at maximum cooling to decrease. If soot served as a nucleation site for bubble formation, it is likely that the temperature of maximum cooling would also increase. The reason for this contradiction is not fully understood.

Salt additions have an effect similar to that of soot, and it is likely that the mechanism is similar. Salt crystals will not dissolve in oil because of the polar nature of the salt. Therefore, it would serve as a discrete nucleation site for bubble formation during nucleate boiling.

Water increases the maximum cooling rate and substantially decreases the temperature of maximum cooling. This increases the distortion of a part by increasing the thermal gradients within the part. It also can create spotty work by means of an insulating effect of water-bubble formation.

Contamination by hydraulic fluid increased the maximum cooling rate and the temperature at which maximum cooling occurs. Because organic hydraulic fluids are miscible in mineral oil, the properties of the quench oil change. The boiling point of the oil-hydraulic fluid mixture is likely to increase causing an increase, in the maximum cooling rate and the temperature where maximum cooling occurs.

Oxidation causes the maximum cooling rate and the temperature of maximum cooling to decrease, which is likely caused by increases in the viscosity of the quenching oil. This increase causes a decrease in the wettability of the quench oil. Because the oil is more viscous, bubble formation is more difficult, and the maximum cooling rate and the temperature of maximum cooling is reduced.

Precautionary measures

Water is the most common and dangerous contaminant and should be avoided. Water in quench oil causes foaming and even explosions in certain systems. The foam greatly enhances the tendency for fires to initiate. Water also increases the tendency toward distortion and cracking of the quenched material.

Entire heat-treating facilities have been destroyed by fires resulting from a quench cycle into an oil containing small amounts of water. If water is detected, settling, centrifuging, filtration and heating methods can be used to remove it. Care should be taken that additives are replaced if removed with the water. If heat separation is used, the temperature and time must be controlled to minimize possible oxidation of the oil.

Solvents and other low-flash materials should be kept away from quenching oils. Quench oils reclaimed from washers, centrifuges, etc., can be returned to the quench tank. However, care must be taken to segregate the quench oil from other products and check its condition prior to adding it to the system.

Furnace atmosphere should be controlled to reduce excessive carbon fines from being introduced to the quench oil.

All quenching oils should be checked periodically for their chemical and physical characteristics. If the user does not have in-house capability, the oil supplier or a qualified independent laboratory facility should be used. The results of the laboratory tests should show the quality of the used oil, together with an indication of the type of contamination present. The supplier also should have the expertise to help the user eliminate further degradation. For example, quality checks provided by Houghton International Heat Treating Laboratory on a quench oil include:

  • % Water - This contaminant in amounts as low as 0.1% can cause foaming, fires, and explosions, and adversely affect the quality of the work.
  • Flash point - This physical property of an oil ensures a safety factor, and changes in its value usually indicate contamination or degradation.
  • % Sludge - This is the result of oxidation and polymerization. It can affect the oil's quenching characteristics and reduces the effectiveness of heaters and coolers. Parts hardened in oil containing sludge usually are dirty in appearance. If the oil isn't too far degraded, filtration could extend its useful service life.
  • % Ash - This is the amount of incombustible material present in the oil. Increased readings can indicate contamination.
  • Kinematic viscosity - The lower the viscosity, the easier an oil can transfer heat through the bath. As oil degrades, viscosity usually increases. Some contaminants can lower the viscosity and usually reduce the flash point at the same time.
  • Saponification and precipitation numbers - These tests give the present condition of an oil regarding its original properties. For example, they measure the oxidation rate and the tendency to form sludge. Also, the staining of parts usually accompanies higher numbers.
  • Neutralization number - Increased oxidation usually increases this number. As the number increases, the oil becomes more acidic.
  • Quenching speed - Either a GM Quenchometer test or a cooling rate curve should used to evaluate the cooling/quenching characteristics of an oil.

Quench oil can be reclaimed even when it is severely contaminated. Reclamation of contaminated quench oil usually can be performed by a qualified oil reclaimer. However, restrictions on waste-oil transportation and high trucking costs usually eliminate any cost savings. Today's disposal problems and the eventual cycling of oil economics could again make reclamation and revitalization processes attractive. Many times, centrifuging, ultrafiltration, drying and the addition of specially selected additives can extend the use of a quench oil. To obtain a quality reclaimed and treated oil, the user should segregate his various quench oils and keep contamination to a minimum. If martempering oil and medium-fast oil are mixed prior to a treatment, the resulting oil could be too viscous for use as a standard oil and have too low a flash point for use in the original martempering application. The quench-oil user must be conscious of the changing economic and legal factors associated with reclamation processes, such as disposal versus on-site or outside treatment. For high-volume oil users, purchasing reclamation equipment is becoming economically attractive.

Conclusions

Conclusions based on this investigation show that:

  • The presence of hydraulic fluid, soot, salts or water increases the maximum cooling rate of conventional quench oils.
  • Oxidation decreases the maximum cooling rate and the temperature of maximum cooling by increasing the viscosity of the quench oil.
  • Increasing temperature increases the maximum cooling rate of the oil and the temperature of maximum cooling rate (within the temperature range studied) because of decreases in viscosity and improved heat transfer.
  • The presence of salt and hydraulic fluid increases the temperature of maximum cooling by means of different mechanisms. Salt provides additional sites for bubble formation, causing the temperature of maximum cooling to increase, whereas the presence of hydraulic fluid reduces the viscosity of the quench oil and enhances bubble formation because of the different boiling points of oil and hydraulic fluid.
  • Water decreases the temperature of maximum cooling, which, in turn, can cause increased distortion.

The effects of contamination of quench oil can cause significant changes in the maximum cooling rate and temperature of maximum cooling. This can result in increased part distortion, cracking and nonuniformity of properties. A control program to monitor and track quench oil performance is necessary to ensure quality parts and customer satisfaction.

For more information: D. Scott MacKenzie is Technical Specialist-Heat Treating Products, Houghton International Inc., Madison & Van Buren Aves., Valley Forge, PA 19482; tel: 610-666-4007; fax: 610-666-5689; e-mail; smackenzie@houghtonintl.com; Internet: www.houghtonintl.com<