Oil quenching is a logical choice for gears. Atmosphere and vacuum carburizing carbonitriding of gears followed by oil quenching is a popular choice among manufacturers to achieve consistent mechanical and metallurgical properties and predictable distortion patterns. The reason oil quenching is so popular is due to its excellent performance results and stability over a broad range of operating conditions. For many the choice of oil as the best quenchant is due to an evaluation of a number of factors, including :
• Economics/cost (initial investment, maintenance, upkeep, life)
• Performance (cooling rate/quench severity)
• Minimization of distortion (quench system)
• Variability (controllable cooling rates)
• Environmental concerns (recycling, waste disposal, etc.)
Cooling Rate Characterization
Measuring the efficiency, or "speed," of an oil quench can be done one of two ways: by measuring the oil’s "hardening power"—that is, its ability to harden a steel—or by measuring the cooling ability of the liquid. Since cooling ability is independent of steel selection (composition and grain size) this method is popular since it provides information about the oil itself, independent of its end use application.
The preferred test method today is cooling curve analysis (ASTM D6200/ISO 9950), which involves a laboratory test using a nickel–alloy probe for the determination of the cooling characteristics of industrial quenching oils. The test is conducted in non–agitated oils, and thus is able to rank the cooling characteristics of the different oils under standard conditions, providing information on the cooling pathway, which must be known if the ability of quench oil to harden steel is to be determined. Older methods such as the GM Quenchometer (ASTM D3520) or the hot wire test are still in common use. The GM Quenchometer method, for example, measures the overall time to cool a 7/8" (22 mm) nickel ball from 1625°F (885°C) to 670°F (355°C), while the hot wire test is influenced by the heat extraction rate of the oil at temperatures close to the melting point of Nichrome, about 2750°F (1510°C).
Oils are generally classified by their ability to transfer heat as fast–, medium–, or slow–speed oils (%%Herring0707Tab1.jpg%%). Fast (8–10 second) oils are used for low hardenability alloys, carburized and carbonitrided parts, and large cross sections that require high cooling rates to produce maximum properties. Medium (11–14 second) oils are typically used to quench medium to high hardenability steels, and slow (15–20 second) or marquench (18–25 second) oils are used where hardenability of a steel is high enough to compensate for the slow cooling aspects of this medium .
Mechanisms of Heat Removal During Quenching
The mechanism of cooling a gear in liquid is largely dependent on geometry for a given material which dictates the requirements of the quench system.
There are three distinct stages of cooling (Figure 1). Stage A is called the "vapor blanket" (or "film boiling") stage. It is characterized by the Leidenfrost phenomenon, which is the formation of an unbroken vapor blanket that surrounds and insulates the work piece. It forms when the supply of heat from the surface of the part exceeds the amount of heat that can be carried away by the cooling medium. In this stage the cooling rate is relatively slow, in that the vapor envelope acts as an insulator and cooling is a function of conduction through the vapor envelope.
Stage B is the second stage of cooling, known as the "vapor transport" (or "nucleate boiling" or "bubble boiling") stage. It is during this portion of the cooling cycle that the highest heat transfer rates are produced. It begins when the surface temperature of the part has cooled enough so that the vapor envelope formed in Stage A collapses. Violent boiling of the quenching liquid results, and heat is removed from the metal at a very rapid rate, largely due to the heat of vaporization. The boiling point of the quenchant determines the conclusion of this stage. Size and shape of the vapor bubbles are important in controlling the duration of this stage. The majority of gear distortion occurs during this stage.
Stage C is the third stage of cooling, called the "liquid" (or "convection") cooling stage. The cooling rate during this stage is slower than that developed in the second stage. This final stage begins when the temperature of the metal surface is reduced to the boiling point (or boiling range) of the quenching liquid. Below this temperature boiling stops and slow cooling takes place by conduction and convection. The difference in temperature between the boiling point of the liquid and the bath temperature is a major factor influencing the rate of heat transfer in liquid quenching. Viscosity of the quenchant plays a major role in the cooling rate in this stage.
These stages of cooling may not occur at all points on a part at the same time (Figure 2). As the internal heat moves to the surface differences in heat rejection may vary based on the surface configuration. Consequently, the need for a uniform and controllable agitation of liquid over the part surface is imperative. Controlled movement of the quenching liquid is vital as it causes an earlier mechanical disruption of the vapor blanket in the first stage and produces smaller, more–frequently detached vapor bubbles during the vapor transport cooling stage. Agitation constantly provides a cooler liquid to the part surface providing a greater temperature difference that allows for improved heat rejection.
Properties of an Ideal Quenching Medium
The ideal quenching medium is one that would exhibit high initial quenching speed in the critical hardening range (through Stage A and B) and a slow final quenching speed through the lower temperature range (Stage C). Thus the ideal quenchant is one which exhibits little or no vapor stage, a rapid nucleated boiling stage, and a slow rate during convective cooling. The high initial cooling rates allow for the development of full hardness by getting the steel past the "nose" of the isothermal transformation diagram (quenching faster than the so–called critical transformation rate) and then cooling at a slower rate beginning at the time the steel is forming martensite. This allows stress equalization; thus distortion and cracking are reduced. The first criterion that any quenchant must meet is its ability to approach this ideal quenching mechanism.
When conventional quenching oils are used the duration of Stage A is longer, the cooling rate in Stage B is considerably slower, and the duration of Stage C is shorter. As such, the "quenching power" of oil is far less drastic than that of water. Water and water solutions exhibit high initial cooling rates. Unfortunately, because of water’s low boiling point, this fast cooling persists until the steel is cooled to below 300°F (150°C). As most steels have formed or are forming martensite by this point, stresses are given little time to equalize. Thus water is typically limited to simple shapes or low hardenability materials.
Oil has a major advantage over water due to its higher boiling range. A typical oil has a boiling range between 450°F (230°C) and 900°F (480°C). This causes the slower convective cooling stage to start sooner, enabling the release of transformation stresses. Oil, therefore, is able to quench intricate shapes and high hardenability alloys successfully.
As it is heated, oil has a proportional drop in viscosity. This allows the quenchant to move more freely, increasing—in general—the tendency to break the vapor blanket layer. The nucleate boiling stage is not drastically altered by changes in bath temperature. The cooling rate in the convection stage of an oil quench will slow as the bath temperature increases. This is advantageous for obtaining a slower rate of cooling through the austenite–to–martensite transformation range. In general, as the temperature of a quenching oil increases, the overall quenching rate increases.
Practical heat transfer coefficient (‘±) values in the 1000 to 2500 W/m2¬…K range can be achieved depending on oil characteristics and degree of agitation. Peak values of ‘± in the cooling range of oil are 4000 to 6000 W/m2¬…K, or a cooling rate greater than 100°C/s (180°F/sec). shows the same portion of the tooth at a still higher resolution. The spherical shape of the coating micro–structure is evident, and a portion of the surface has been smoothed. By using backscatter electron micrographs and energy dispersive x–ray spectrums, it was found that in localized areas the coating had been removed from the surface.
Effect of Increasing Bath Temperature
The temperature of the quenchant has dramatic influence on the rate of part cooling. Increasing the bath temperature from, say, 70°F (21°C) to 250°F (120°C) produces a slightly faster cooling in Stage A because the viscosity of the oil decreases. In Stage B cooling is only slightly increased, and in Stage C cooling decreases near the end of the quench because the temperature differential between the bath and the steel is decreased.
Quenching oils have various characteristics that allow for varying cooling rates as a function of not only their boiling point, but also their temperature. This has a direct bearing on properties such as viscosity, conductivity, and heat rejection (based on the log mean temperature differential, or LMTD) of the heat exchange system. Bubble size as well as the conductivity of the vapor barrier in all three stages of cooling are, to a degree, selectable by oil choice.
For most quench oils other than marquench oils, the optimum rates of cooling are normally obtained when the bath temperature is between 120°F (50°C) and 150°F (65°C). In this temperature range properly refined mineral oils are indefinitely stable and the effect of viscosity is drastically reduced. Various manufacturers usually have an optimum temperature range for their product. The instantaneous rate of rise of the entire quench bath is also important. This is normally design dependent, but usually averages between 20°F (–7°C) to 40°F (4°C).
Effect of Increasing Degree of Agitation
Even with a properly selected oil and correct quench environment design, the stages of cooling described above may not occur at all points on a part configuration at the same time. Part cooling is a function of the quality of the oil as well as the part’s geometry, fixturing, and loading techniques. As the part’s internal heat moves to the surface, and based on the surface configuration, differences in heat rejection will vary. Consequently, the need for a uniform and controllable agitation of clean properly controlled liquid flow over the part surface is imperative. Controlled movement of the quenching liquid is vital as it causes an earlier mechanical disruption of the vapor blanket in the first stage and produces smaller, more–frequently detached vapor bubbles during the "e;vapor transport" cooling stage. Agitation constantly provides a cooler liquid to the part surface, providing a greater temperature difference that allows for improved heat rejection.
Cleanliness of the oil is an important issue, and it must be free of particulate materials such as carbon, sludge, and water. Carbon is formed after evaporation and fractionation under conditions of insufficient oxygen or is introduced by processes such as carburization. Oil breakdown on the part surface may occur if sufficient quenchant agitation is not provided.
Important considerations with respect to agitation are as follows: type and design of agitators (mixers) or pumps, and draft tube design. For example, we often give little consideration to an internal component such as a draft tube, but we should. Draft tubes are important in the overall performance of the system and should have the following characteristics :
- A down pumping flow path (to take advantage of the tank bottom)
- An angle of 30° on the entrance flare (to minimize head loss and establish a uniform velocity profile at the inlet)
- Liquid coverage over the top of the draft tube of at least one–half the tube diameter (to avoid flow restriction and disruption of the inlet velocity profile)
- Anti–cavitation or internal flow straightening vanes (used to prevent fluid swirl)
- Proper impellor positioning (both insertion depth into the draft tube—a distance equal to at least one half of the tube diameter as dictated by the required inlet velocity profile—and diameter, fitting tight enough to prevent fluid flow along the sides of the draft tube)
- Anti–deflection capability (to compensate for occasional high deflection)
Effect of Quench Tank Design
Draft tubes are just one component that highlights an often–overlooked aspect of quenching. The limitations imposed by the design of the quench tank can have a significant (negative) effect on the ability of the quench oil, or any quench medium, to perform properly. The volume of oil, the localized instantaneous temperature rise of the bath, the ability to circulate the quench medium through the load (measured in ft/sec or m/sec), the capacity of the heat exchanger system, and the overall maintenance of the tank all influence quenching.
The volume of oil contained in a quench tank is important for controlling the overall rate of temperature rise after quenching. The common rule of thumb for oil quench tanks is one gallon of oil per pound of steel. Quench tanks that utilize less than this ratio must be designed with highly effective agitation systems. Of course, having a large volume of oil is no guarantee of success if the quench tank design is inadequate for the job. For example, a mesh belt conveyor furnace utilized a 3000 gallon (11,350 liter) tank but failed to properly circulate oil and dissipate heat from the area of the quench chute—where the actual product was being transformed. The result was improper hardness and excessive distortion even at production rates of only several hundred pounds per hour.
Distortion and Cracking
Another important advantage of oil quenching is that it minimizes the tendency to cause distortion and cracking. While better than some mediums such as water or brine, other mediums such as salt or high pressure gas quenching tend to produce less overall distortion. A key consideration, however, is the uniformity/repeatability of the distortion profile, and oil quenching has the ability to produce a very consistent profile.
Fast oils that are highly agitated tend to produce the highest rates of distortion while slow or marquench oils tend to minimize distortion. Quenching in still (non–agitated) oil is often used as a means of distortion control on critical parts.
Water in Quench Oil
One of the major concerns regarding oil quenching is the presence of water in the quench oil. It is dangerous since on quenching it will form steam, resulting in an enormous volume expansion. As the steam bubble rises out of the quench tank its surface is coated with oil, and as it exits from the furnace—usually under extremely high pressure—it is readily ignited. Water detectors with a sensitivity in the range of 0.2 –0.3 percent should be provided on all quench tanks. They should be properly maintained and tested daily. Some manufacturers believe that as little as 0.1 percent may cause dramatic changes in quenching and part surface contamination.
Relationship of Physical Properties of Quenching Oils to their Performance
Oil is often analyzed to determine its performance characteristics, and the testing laboratory issues a report that contains information about the physical property characteristics of the oil. The following describes various test procedures and provides insights into the meaning of the results obtained. 
Viscosity. As discussed earlier, quenching performance is dependent on the viscosity of the oil. Due to degradation (the formation of sludge and varnish), oil viscosity changes with time. Samples should be taken and analyzed for contaminants, and a historical record of viscosity variation should be kept and plotted against a process control parameter such as part hardness.
Water Content. Water from oil contamination or degradation may cause soft spots, uneven hardness, staining and, perhaps worst of all, cause fires. When water–contaminated oil is heated, a crackling sound may be heard. This is the basis of a qualitative field test for the presence of water in quench oil. The most common laboratory tests for water contamination are either Karl Fisher analysis (ASTM D 1744) or by distillation.
Flash Point. The flash point is the temperature where the oil in equilibrium with its vapor produces a gas which is ignitable but does not continue to burn when exposed to a spark or flame source. There are two types of flash point values that may be determined: closed–cup, or open–cup. In the closed–cup measurement, the liquid and vapor are heated in a closed system. Traces of low–boiling contaminants may concentrate in the vapor phase, resulting in a relatively low value. When conducting the open–cup flash point the relatively low boiling byproducts are lost during heating and have less impact on the final value. The most common open–cup flash point procedure is the "Cleveland Open Cup" procedure described in ASTM D 92. The minimum flash point of an oil should be 90°C (160°F) above the oil temperature being used.
Neutralization Number. As an oil degrades it forms acidic byproducts. The amount of these byproducts may be determined by chemical analysis. The most common method is the neutralization number. The neutralization number is determined by establishing the net acidity against a known standard base such as potassium hydroxide (KOH). This is known as the "acid number" and is reported as milligrams of KOH per gram of sample (mg/g).
Oxidation. This variable may also be monitored and is especially important in tanks running marquenching oil or oils being run above their recommended operating range. Oxidation is detected by infrared spectroscopy. Nitrogen blanketing of the oil is one way to reduce both oil oxidation and sludge formation.
Precipitation Number. Sludge is one of the biggest problems encountered with quench oils. Although other analyses may indicate that a quench oil is performing within specification, the presence of sludge may still be sufficient to cause non–uniform heat transfer, increased thermal gradients, and increased cracking and distortion. Sludge may also plug filters and foul heat–exchanger surfaces. The loss of heat–exchanger efficiency may cause overheating, excessive foaming, and possible fires.
Sludge formation is caused by oxidation of the quench oil and by localized overheating, or "frying," of the quench oil. The relative amount of sludge present in a quench oil may be quantified and reported as a "precipitation number." The precipitation number is determined using ASTM D 91. The relative propensity of sludge formation of a new and used oil may be compared providing an estimate of remaining life.
Accelerator Performance. Induction coupled plasma (ICP) spectroscopy is one of the most common methods for the analysis of quench oil additives. When additives such as metal salts are used as quench rate accelerators, their effectiveness can be lost over time by both drag–out and degradation. Their effectiveness can be quantified by performing ICP spectroscopy—a direct analysis for metal ions—and compensating measures can be taken, such as the addition of a specific percentage of new accelerator.
A Look at Oil Quenching in Vacuum
Not all materials and part configurations behave as one would hope within a given oil properties range. Oil quench vacuum systems offer an attractive alternative, given their ability to vary a quench related variable not possible with atmosphere equipment; namely the pressure over the oil. As such, this technique can extend the range of part cross sections and materials that can be processed. In addition, the use of a vacuum oil quenching has been found to reduce distortion in such components as gears, shafts, and ball bearings.
Pressure control offers a method of assisting in developing desired part results, along with oil temperature and oil circulation. The lower pressure allows for longer "vapor blanket" stages and a somewhat long "vapor transfer" stage, due to the reduced boiling point of the oil. This may reduce distortion and provide the sought after hardness, if the materials transformation ranges are accommodating.
Distortion minimization techniques using pressure variation have proven effective [6, 7]. Altering pressure over the quench oil allows for a change in the boiling point of the quenchant. The position of the boiling point—i.e. "characteristic temperature"—determines where and for how long the various stages of oil cooling take place.
Distortion minimization methods have been used in combination with changes to flow characteristics—some manufacturers pull oil down through the workload as opposed to pushing it upward, for example—and oil compositions specially blended (Table 1) for use in vacuum having low vapor pressure oils so that they are easily degassed.
The design of an integral vacuum oil quench system requires considerations beyond those of atmosphere oil quenching. For example, the boiling point and vapor pressure of the base oil, as well as the accelerate additives characteristics, must be taken into consideration along with the quench oil temperature, agitation, cleanliness, PH, and viscosity. Also, the vapor pressure of the quench oil must be compatible with the selected operating vacuum level.
Finally, vacuum systems do not permit the buildup of water in the quench tanks. In a vacuum furnace system, where vacuum is used to process the work or purge the quench environment, moisture will be removed as the system is evacuated and the oil circulated. The circulated oil brings any moisture to the surface where it is vaporized and removed from the oil by the pumping system.
Truck transmission shafts (Figure 3) of AISI 8620 material are oil quenched to develop the required properties. A core hardness of 25 HRC at mid–radius is achieved by quenching in 195 °F (90 °C) oil. The effective case depth developed was 0.50" (1.2 mm). Load weight was 1000 lbs (450 kg).
Large marine transmission gears (Figure 4) of 8620 weighing 64 lbs (29 kg) each are oil quenched to develop a surface hardness of 62–64 HRC and a core hardness greater than 25 HRC. Load weight is approximately 1000 lbs (450 kg). The effective case depth is 0.045"–0.065" (1.14–1.65 mm).
Drive flange gears (Figure 5) of 4620 weighing four lbs (1.8 kg) each are oil quenched to achieve a surface hardness of 61–63 HRC and a core hardness greater than 28 HRC. Load weight is approximately 780 lbs (350 kg). The effective case depth is 0.030"–0.035" (0.076–0.89 mm).
Pinion gears (Figure 6) of 8822 weighing 1.7 lbs (0.75 kg) each are oil quenched to develop a surface hardness of 59–62 HRC and a core hardness greater than 35 HRC. Load weight is approximately 800 lbs (360 kg). The effective case depth is 0.025"–0.035" (0.63–0.89 mm).
One example of the advances occurring today in quenching is in the area of sensor technology. Most heat treating shops occasionally sample the oil, check the temperature and level, and look to see that the oil agitators are rotating. These are operator dependent quality methods. Since heat removal from the parts is due in large part to the fluid flow activity, sensors have been designed to measure the heat transfer inside a quench bath . They measure the difference between the temperature of the quenchant and a higher temperature produced by a constant power heat source inside the sensor itself so quench flow is monitored by measuring the heat produced by the sensor and the convective heat transferred to the quenchant.
These types of units are an example of tools that can be used to monitor quench intensity in the quench tank in real time as a quality control tool. This ensures that the quench conditions are known and corrective action or repairs can be implemented before more parts are quenched.
Oil quenching is so routinely used in the gear industry today that it is almost taken for granted—and this is a significant concern in that misapplication and misuse are all too common. As with all quenching, the key is to understand and control the key process variables. Proper selection of the type of oil and use of that oil under ideal conditions in a well designed and well maintained quench tank will assure consistent and repeatable results.
Oil quenching should be applied in those applications where its advantages outweigh its disadvantages, and—as with all technologies—should be as completely understood as possible with respect to the performance requirements of the product so as to meet the application’s end use.
1) Herring, D. H., "A Review of Factors Affecting Distortion in Quenching," Heat Treating Progress Magazine, December 2002.
2) "Typical Cooling Rate Curves for Various Liquid Quenchants," Advanced Materials & Processes Magazine, February 1998.
3) ASM Handbook, Volume 4: Heat Treating, ASM International, 1991.
4) Totten, G.E., and Lally, K.S., "Proper Agitation Dictates Quench Success, Part 1," Heat Treating Magazine, 1992.
5) Wachter, D.A., Totten, G.E., and Webster, G.M., Quenching Fundamentals: Quench Oil Bath Maintenance
6) Herring, D. H., Sugiyama, M., and Uchigaito, M., "Vacuum Furnace Oil Quenching ’Äì Influence of Oil Surface Pressure on Steel Hardness and Distortion," Industrial Heating Magazine, June 1986.
7) Herring, D. H., Sugiyama, M., and Uchigaito, M., "Controlling Oil Surface Pressure in Vacuum Oil Quenching," Heat Treating Magazine, July 1987.
8) Unpublished data, C. I. Hayes Inc.
9) Hebauf, T. and Goldsteinas, A., "Experience in Low Pressure Carburizing," Industrial Heating, September 2003.
10) Otto, Frederick J. and Daniel H. Herring, "Gear Heat Treatment, Part I & II," Heat Treating Progress, May & June, 2002.
11) Lohrmann, M., "The Fluid Quench Sensor¬Æ—a New Development for Ensuring Reproducible Quenching Results in Liquid Quench Systems," 3rd International Conference on Quenching and Control of Distortion, Prague, Czech Republic; 24–26 March 1999.