Jim Roberts of U.S. Ignition engages readers in a Combustion Corner column about the modes of heat transfer at play in the heat treating world — breaking down advection and thermal conduction, from how forced air movement drives heating and cooling in furnaces to how Fourier’s law governs heat flow through direct contact, setting the stage for next month’s look at convection and radiation.
This editorial was first released in Heat Treat Today’sAugust 2026 Annual Automotive Heat Treating print edition.
A furnace guy walks into a heat treat facility and asks, “tropos metadosis thermotitas?” The furnace operators stare and say, “What are you saying? It’s all Greek to me!” But many of us know what it means: What mode of transfer? And that was the question of the last article where we looked at an intro to thermodynamics. This set us up for discussions on the four modes of heat transfer, a non-stop set of interactive processes that we then refine for our use in the heat treating world.
In that earlier discussion, we explained that there are several modes involved in heat transfer. Some are so simple to describe, but there may be several modes working at the same time to target the heat transference we seek. Modes of heat transfer and the Laws of Thermodynamics differ. Think of modes as the actual physical properties of heat transfer and thermodynamics that we can see and feel. The laws are the calculations and theory that allow us to check and produce the heat that is needed or to prove how much we need.
Our first mode of heat transfer is advection. Advection is the mechanism of thermal transfer that occurs when the thermal energy is transferred or transported from one space or object via the movement and the motion of a fluid. Pure convection, in the scientific sense, is strictly the heat transfer between heated bodies by gravitational lift or natural fluid movements.
In other words, heat rises… right? We’ve all heard it, witnessed it in a campfire or elsewhere. Advection is when the transfer media, mostly hot gases, is forced mechanically to swirl around and the item we desire to heat or cool. Speaking of heat transfer, we cannot forget that the door swings both ways. We can affect heat transfer from the standpoint of cooling things down too, and at that point, we discover that the hot gases or heat exchangers with air of liquid are using advection to pull the heat out. The plunge cool function of many furnaces relies on advection to pull the heat out of the parts at a controlled rate to cause a metallurgical change. So, the same laws apply, just in reverse!
One of the easiest ways to remember this difference is that advection actually adds energy to the process. It may be in the form of a blower providing combustion air for a burner, which creates the swirl of gases that are providing the heat transfer to the parts. Or, think of a fan pulling heat from a circuit board of electrical components. Advection is actually forced convection. If you have a modern convection oven or an air fryer, it is not really a convection oven. It’s an advection oven in the scientific world, since the fans involved are adding energy to move the fluid (air/gas) around those chicken wings and fries. The concern over this kind of added energy is in part what makes Data Centers such a volatile topic.
This leads us to our second heat transfer mode, thermal conduction. Thermal conduction is the transfer of heat by direct contact of two objects where there is a differential in temperature and energy. In other words, the temperature difference between two objects is the mechanism for heat transfer. Energy wants equilibrium, so it will always try to bring an item to its temperature by maintaining contact. It can be identified as thermal diffusion in Fourier’s law for heat conduction.
Fourier’s law states that eventually, through contact, the handle on the cooking pot will match the temperature of the pot surface because it is conducting the heat into that handle via thermal conduction. These types of considerations take place during calculations for furnace design, engine components, or household items; anything with a heat differential must be accounted for in our everyday lives.
As they say in the old country: choris thermotita, den yparchei ergo. Without heat, there is no work. That’s the loose translation, but it points out that we need to have all the components for heat transfer to be working for us to do this wonderful work called heat treating. And that shouldn’t be Greek to us.
Next month we will tackle convection and radiation.
About The Author:
Jim Roberts President US Ignition
Jim Roberts president at U.S. Ignition, began his 45-year career in the burner and heat recovery industry focused on heat treating specifically in 1979. He worked for and helped start up WB Combustion in Hales Corners, Wisconsin. In 1985 he joined Eclipse Engineering in Rockford, IL, specializing in heat treating-related combustion equipment/burners. Inducted into the American Gas Association’s Hall of Flame for service in training gas company field managers, Jim is a former president of MTI and has contributed to countless seminars on fuel reduction and combustion-related practices.
Ask The Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues. In this Technical Tuesday installment, Dan Herring explores the fundamentals of vacuum oil quenching — the three stages of oil quenching, the properties that make oil an effective quenching medium, and the ways pressure control, oil circulation, and temperature can be used to minimize part distortion — and offers step-by-step guidance on properly conditioning and degassing vacuum quench oil to ensure consistent, repeatable results.
This informative piece was first released in Heat Treat Today’sAugust 2026 Annual Automotive Heat Treating print edition.
Oil quenching in vacuum furnaces is a technology that has been around for over 65 years. The popularity of oil as a quench medium is due to its excellent performance and stability over a broad range of operating conditions. Proper application, testing, and conditioning of the oil are necessary to ensure its effectiveness. Let’s learn more.
For many, the choice of oil is not only due to metallurgical transformation and resultant properties it can achieve, but also to a number of other factors, including:
The first criterion that any quenchant must meet is its ability to approach an ideal quenching medium.
Figure 1a-b. Three stages of oil quenching
Regardless of the type of liquid (brine, water, polymer, oil, molten salt, or air mist), the ideal quenching medium (Figure 1) is one that would exhibit high initial quenching speed in the critical hardening range (Stages 1 and 2) and a slow final quenching speed through the lower temperature range (Stage 3). Thus, the ideal quenchant is one that exhibits little or no vapor blanket stage, a rapid but controlled nucleated boiling stage, and a slow cooling rate during convective cooling.
The high initial cooling rates allow for the development of full hardness by ensuring the steel misses the “nose” of the time-temperature-transformation diagram (i.e., quenching faster than the so-called critical transformation rate). This is followed by cooling at a slower rate beginning at the time the steel is forming martensite, which allows for better stress equalization so that the potential for distortion and cracking is reduced.
When conventional quenching oils are used, the duration of Stage 1 is longer, the cooling rate in Stage 2 is considerably slower, and the duration of Stage 3 is shorter. As such, the “quenching power” of oil is far less drastic than other quenchants. Water and water-based quenchants 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 low hardenability materials and relatively simple geometries.
Oil has a major advantage over water or polymer 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, however, 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 (α) values are in the 1,000 to 2,500 W/m²K range depending on oil characteristics and degree of agitation. Peak α values are in the order of 4000 to 6000 W/m²K, or a cooling rate greater than 100°C/sec (180°F/sec).
Figure 2. Typical commercial heat treat load | Image Credit: The HERRING GROUP, Inc.Figure 3. Die cutting punches benefiting from controlling the pressure over the oil | Image Credit: The HERRING GROUP, Inc.
The use of vacuum oil quenching has been found to reduce distortion in many components including gears, pinions, and shafts (Figure 2). Oil quench vacuum systems also offer an attractive alternative to conventional atmosphere oil quenching, given their ability to vary a quench related variable not otherwise possible, namely controlling the pressure over the oil (Figure 3). This technique can be used to extend the range of part cross sections and materials that can be successfully hardened (Herring 1987, Sugiyama and Uchigaito 1987).
Pressure control along with oil temperature and oil circulation improves the predictability of distortion. 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 desired hardness if the material’s transformation ranges are accommodating.
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. Also, oils formulated for vacuum service typically have a low vapor pressure, allowing them to be easily degassed.
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. Circulating the oil carries any moisture to the oil surface, where it vaporizes and is removed by the vacuum pumping system.
Maintaining Your Vacuum Quench Oil
One of the aspects of vacuum oil quenching that is seldom documented is how to condition the quench oil. See “Step-By-Step Instructions for Conditioning and Degassing Vacuum Quench Oil” below.
Final Thoughts
As with all quenching, the key is to understand and control the key process variables. Proper selection of the type and use of oil under ideal conditions in a well-designed and well-maintained quench tank will ensure consistent and repeatable results.
Oil quenching should be applied in 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 end use.
References
Brian Barlow. 2025. Private correspondence. Gasbarre Thermal Processing Solutions.
Herring, Daniel. H. 2002. “A Review of Factors Affecting Distortion in Quenching.” Heat Treating Progress Magazine. December. 2012. Vacuum Heat Treatment, Volumes I. BNP Media Group.
Herring, Daniel H. 2016. Vacuum Heat Treatment, Volume II. BNP Media Group.
Herring, Daniel H., Steven D. Balme. 2007. “Oil Quenching Technologies for Gears.” Gear Solutions. July.
Herring, D. H., Sugiyama, M., Uchigaito, M. 1986. “Vacuum Furnace Oil Quenching – Influence of Oil Surface Pressure on Steel Hardness and Distortion.” Industrial Heating Magazine. June.
Sugiyama, M., Uchigaito, M. 1987. “Controlling Oil Surface Pressure in Vacuum Oil Quenching.” Heat Treating Magazine. July.
About the Author
Dan Herring “The Heat Treat Doctor®” The HERRING GROUP, Inc.
Dan Herring has been in the industry for over 50 years and has gained vast experience in fields that include materials science, engineering, metallurgy, new product research, and many other areas. He is the author of six books and over 700 technical articles.
A small amount of hidden water at the bottom of a quench tank can rapidly escalate into a violent fire hazard. In this Technical Tuesday installment, Bruno Scomazzon, general manager of Precision Heat Treat Ltd., discusses how free water develops, why conventional testing can miss it, and the practical steps heat treaters can take to detect and eliminate the risk.
This informative piece was first released inHeat Treat Today’sJuly 2026 Annual Super Brands Issue print edition.
Most heat treat shops never see the danger building beneath the surface of their quench oil. Drop by drop, water stratifies, tipping the balance and awakening the dragon.
In an integral quench furnace, quenching is a controlled process. When a hot load is submerged, vapor is generated below the surface, rising and mixing with the furnace atmosphere. Oil temperature, atmosphere, and vapor generation are managed so that any combustion remains contained, with gases and vapor generated and relieved in a controlled manner. Under normal conditions, the process is stable and predictable.
AI-generated illustration based on a potential quench fire scenario. No actual integral quench furnaces were harmed in the making of this article. | Image Credit: Precision Heat Treat Ltd.
Add water, and you introduce a completely different hazard. It can turn violent before you understand or can react to what you are seeing. In this scenario, when the hot load is submerged, water at the bottom of the tank flashes instantly to steam, expanding roughly 1,600 times in volume. That expansion happens almost instantly, and the resulting increase in volume overwhelms the system.
As the steam rises, each bubble becomes coated with oil. Rapid expansion displaces oil and generates a large volume of vapor in a very short period of time. In a confined quench chamber, that surge carries oil and vapor together toward the burn-off vent and the doors.
There are typically two doors in the system, and they behave very differently. The inner door separates the hot zone from the quench tank. During quenching, the hot zone is typically operating around 1550°F. If oil is forced into the hot zone, it will vaporize and burn, generating products of combustion that can lead to a more severe internal event. However, this is not where the external fire develops. The outer door separates the quench chamber from the outside. During a surge, oil and vapor push upon this door and can be forced out. When the flammable oil vapor and furnace atmosphere reach an ignition source such as the flame curtain pilot, it will ignite violently.
At that point, the fire is no longer confined or controlled. Oil that reaches the exterior can spread along the floor and around the base of the furnace. Once outside the chamber, any oil present becomes fuel, and the fire can spread quickly.
How Water and Oil Interact
Two different conditions exist: dissolved water and free water. Dissolved water is moisture within the oil, typically when the oil is hot. It is part of normal operation and must be monitored. Elevated dissolved water levels are mainly a performance and control problem, not the immediate hazard. But if it continues to rise, it leads to free water, which is the hazard. Free water is water that has separated from the oil and settled to the bottom of the tank.
As a general guideline for dissolved water:
Below 100 ppm (0.01%): very good
100–200 ppm (0.01–0.02%): acceptable
200–500 ppm (0.02–0.05%): caution range
Above 500 ppm (0.05%): corrective action required
These values apply to dissolved water in the oil — not the free water condition at the bottom of the tank, which is the condition of greatest concern. As the oil cools, its ability to hold moisture decreases. Excess water comes out of solution, forms small droplets, and over time settles to the bottom. This creates stratification — the formation of distinct layers. Oil floats above, leaving a layer of free water at the bottom. That bottom layer is the dangerous condition, the same condition that drives the surge event described earlier.
Water Does Not Just Appear — It Gets Introduced
Common water sources include condensation during shutdowns and startups, along with operator or maintenance oversights that allow water or contaminated oil to enter the system. On older units, water-cooled components can develop leaks over time, allowing water to enter the oil slowly and go unnoticed. Leaking roof components or failed hood caps can also allow water to reach the furnace and make its way into the tank.
Water does not always find its way into a quench tank through a leaking cooler, heat exchanger, or outside source. During extended shutdowns, particularly during cooler months or periods of high humidity, moisture can condense on the ceiling and sidewalls of the quench vestibule and tank area. As the furnace heats up and the colder oil tank lags behind, water droplets can form and eventually fall into the oil. | Image Credit: Precision Heat Treat Ltd.
Because this develops gradually and often out of sight, everyone in the shop needs to stay alert. If something does not look right, or behave abnormally, report it immediately.
Detecting Water: What the Operator Sees First
Detecting free water in a quench tank is not as straightforward as it sounds. In many cases, the first indication is not a test result but a change in furnace behavior.
Changes in burn-off flame height, flame recovery time, unusual oil discharge (“burping”), or abnormal sounds during quenching are often early warning signs that something in the system has changed. All furnace operators should be trained to recognize these changes and treat them as indicators that the condition of the quench may no longer be normal.
Confirming with Sampling and Testing
Most monitoring systems measure dissolved water in circulating oil — not free water at the bottom. That distinction matters. Water that is mixed in the oil can be measured and trended. Water that has separated and settled to the bottom may not be detected by standard sampling methods. A sample taken from a circulating line or mid-depth in the tank can show acceptable results while free water remains undetected. These values are typically determined through lab analysis or in-shop test kits that measure dissolved water in quench oil. Standard tank sampling does not distinguish between dissolved and free water. That is why where you take the sample matters as much as how you test it.
Bottom sampling is critical. Pulling oil from the lowest point in the tank after the furnace has been idle, such as over a weekend, is often the best way to identify free water. If you do not already have a way to sample from the bottom of your tank, you should plan to install a dedicated drain or sample port. A bottom sample should be part of your weekly oil monitoring.
In the absence of a dedicated bottom drain, a simple method can still be used. Tubing can be inserted down through the fill or access point until it reaches the bottom. By sealing the top of the tube, the oil column inside is held in place as it is withdrawn, allowing a sample from the lowest point in the tank. The sample must be taken before any agitation or circulation begins. If the oil has been disturbed, the water can be temporarily mixed and the true condition at the bottom may not be seen.
The sample is placed in a clear glass beaker and allowed to stand. If water is present in any significant amount, it will usually be obvious, especially when compared to a sample taken from mid-depth or circulating oil. If it is not obvious, a crackle test can be used as a quick field check. A small sample of oil is placed on a hot surface, typically around 300–350°F. If free water is present, it flashes to steam and produces visible bubbling or crackling.
In practice, the severity of the reaction gives a clear visual indication of the condition:
No reaction: acceptable, continue to run
Light fizz or fine bubbles: trace water present, monitor
Moderate crackle or popping: plan corrective action
Strong crackle or aggressive bubbling: correct the condition before continuing
A consistent reaction across samples is the key indicator. This is not a precise measurement, but it is a reliable field guide. If it is reacting hard, you are already past where you want to be.
Sending samples for quarterly lab analysis provides a more complete picture of oil condition, including water content, oxidation, viscosity, contamination, and quench performance. This helps track dissolved water levels and overall oil health. In-shop test kits and commercial monitoring systems are also available, but none replace the need to understand what is happening at the bottom of the tank.
Removing Free Water from the Quench Tank
When free water is present at the bottom of the tank, it must be removed. This is an immediate hazard. In practice, removing oil from the top-down is often the most controlled approach. Oil is siphoned from the surface, working downward and stopping about a foot above the bottom to avoid disturbing the settled water layer.
This allows clean oil to be removed first while leaving the water undisturbed. Attempting to remove water directly through a bottom drain is not always effective, particularly in larger tanks, as it can pull both water and usable oil and disturb the interface between the two.
Left: Accumulated oil sludge, soot, and scale deposits inside a quench tank. Right: The same area after cleaning. These deposits can build up over time and provide fuel for a fire. Regular tank cleaning is an often-overlooked part of quench oil stewardship and fire prevention. | Image Credit: Precision Heat Treat Ltd.
The removed oil can be placed into totes and allowed to sit undisturbed for several days so any remaining water can separate and settle. Clean oil can then be recovered from the top. The remaining oil and water mixture in both the tank and tote should be recycled. Other methods, such as vacuum dehydration or oil reclamation systems, can be used where available and are often more effective at removing both free and dissolved water, but are not always practical in every shop.
With the oil removed, this is an ideal time to carry out thorough tank cleaning. Over time, quench oils form sludge, a combination of oxidation byproducts, degraded oil, carbon, scale, and fines from processed parts. This material settles to the bottom, can trap and hold water, and hide it from normal sampling. As it builds up, it interferes with oil flow and agitation, affecting quench performance.
Tanks require periodic cleaning, typically every 12 to 18 months, depending on usage and condition. This is the time to remove sludge and clean deposits from the walls and ceiling.
It is also an opportunity to inspect agitation systems, elevators, rollers, and other components, and carry out preventative maintenance. Proper lockout procedures and confined space protocols must be followed.
The Bottom Line
Free water is an immediate hazard. If allowed to accumulate, it can trigger a rapid pressure event and an uncontrolled fire. Once it starts, it escalates quickly and is difficult to contain, putting personnel and the entire operation at risk.
Regular bottom sampling for free water must be part of your quality control.
Acknowledgements
The author would like to thank Daniel H. Herring, “The Heat Treat Doctor®” at The HERRING GROUP, Inc.
About The Author:
Bruno Scomazzon General Manager Precision Heat Treat Ltd.
Bruno Scomazzon is the general manager of Precision Heat Treat Ltd. in Surrey, British Columbia, Canada, with over 40 years of experience in metallurgical processes and heat treating operations.
In the second of the two-part series, the Advanced Rapid Quench Jominy Tester application for automotive underbody parts is examined. In this article, Dr. Gopal Nadkarni, an associate professor of mechanical engineering at the University of Akron, explains how automotive manufacturers can integrate this little-used test methodology following existing industry practices. It also makes the case for updating the ASTM A255 Jominy test method to measure enhanced hardenability under modern industrial quenching conditions.
This informative piece was first released in Heat Treat Today’sAugust 2026 Annual Automotive Heat Treating print edition.
Introduction
In “The Jominy and Rapid Quench, Part 1: An American Story” (see Heat Treat Today: Induction Heat Treating, April 2026), the Jominy test adopted in the 1940s was shown to revolutionize how complex metallurgical phenomena in most steels during cooling could be understood using a simplified method of cooling a 4-inch bar of steel from one end. Since then, the ASTM A255: Standard Test Method for Determining Hardenability of Steel (Jominy test) has become the dominant form of characterizing steel behavior under multiple cooling conditions (ASTM International 2020).
Figure 1. Grossman H values for quenching in water shows the gap between ASTM A255 and current industrial quench practices (modified from Liscic 2010) | Image Credit: Gopal Nadkarni
When the Jominy test was developed, quenching technology relied primarily on relatively mild water agitation (Figure 1). Industrial heat treatment has evolved dramatically since then. These advances raise an important question: Does the traditional Jominy test still represent the cooling conditions experienced by modern industrial components?
The answer is both yes and no. The conventional Jominy test remains an outstanding comparative tool because of its simplicity, reproducibility, and enormous historical database (Chandler 1994). However, improved understanding of quench heat transfer — specifically, the laminar umbrella flow underneath rapidly quenched Jominy test ends hides a “film boiling layer” that impedes heat transfer in the first critical seconds of quenching — suggests that the standard unintentionally limits the maximum cooling rate during the first critical seconds of quenching. Consequently, the hardenability curves engineers have relied upon for decades may not fully represent the hardening potential of steels subjected to today’s rapid or intensive quenching processes.
This article proposes extending the Jominy test’s usefulness through a modified methodology that better reproduces modern industrial quenching conditions while preserving the standardization that has made ASTM A255 indispensable.
Intensive Quenching for Industrial/Automotive Parts
Figure 2. Schematic of heat transfer experienced by quenched parts during cooling with and without the vapor barrier (i.e., Leidenfrost layer) (Modified from Kobasko, et al. 2010)
If methods existed to consistently remove the vapor barrier during the first few seconds of quenching, engineers could increase heat transfer substantially (Figure 2) to maximize hardenability. Dr. Nikolai Kobasko and Dr. Michael Aronov have worked at this theory for the last three decades, demonstrating significant material processing benefits with intensive (or rapid) quench practices. Other scientists have also teamed up with heat treaters in the U.S. to demonstrate commercial adoption of industrial parts (Totten, et al. 2024).
Today, intensive quench practices are being seriously assessed by several automotive manufacturers as a way to not only maximize hardenability, but also to control distortion after quenching, impart beneficial compressive stresses below the surface, and reduce spotty hardness, as well as lower cost and improve sustainability practices by employing lower alloy steels.
Automotive Designs Require Updated Tests
Figure 3. The part production cycle requires a deep understanding of the supply chain variables.
The automotive part production cycle is complex and requires close coordination across multiple stakeholders to achieve desired property outcomes (Figure 3). Designers often rely on “real test” data as input to finite element models to predict critical mechanical properties that correlate with structural life for fatigue, wear, or fracture resistance, with most of these parts being heat treated. They also lean on the correlating structure properties for required parts using the universal ASTM A255 Jominy test as a reference. While this serves as a unique fingerprint for the alloy used, it is by no means an ideal representation of hardenability because chemistry variations that result in a large H-band (Figure 4a).
This approach also falls short because Jominy cooling rates do not reflect real-world quenching, which can be seen as enhanced hardenability under industrial conditions (Figure 4b). Heat treaters use high-velocity forced convective flow to create violent agitation, improve quench consistency, increase hardness, and break up the film boiling layer. However, complex parts like gears pose an issue: it is difficult to guarantee uniformity of instantaneous cooling rates and may only break up parts of the persistent vapor layer (Figure 4b). The result is uneven heat transfer and spotty hardness across the part.
Figure 4. Hardenability is dependent on steel chemistry and microstructure management for the same heat of alloy (a) and uniform quench processing management between batch to batch (b) | Image Credit: Gopal Nadkarni
Automotive part designers need consistent hardness to ensure reliable service and accurate life predictions, but industrial quench conditions do not ensure 100% removal of the vapor barrier. To ensure performance, automotive designers specify higher alloy steels than necessary. In addition, engineering drawings often specify suboptimal surface hardness (suboptimal hardenability) to stay within hardenability realities of accepted heat treatments. Increased costs are often necessary if parts do not hit design targets and require secondary processing (e.g., tempering, carburizing, carbonitriding).
Overcoming Slow Automotive Adoption
Despite excellent data over the past two decades (Kobasko, et al. 2010; Totten, et al. 2024) showing the usefulness of rapid quench technology in maximizing hardenability for bar/plate steels, the paradox is that the technology has been slow to be adopted. We can learn lessons on adopting new steel/processing technologies from the flat steel side used for body-in-white/chassis, where the entire supply chain came together (Auto/Steel Partnership) in 1987 and produced significant lightweighting programs like the 1994 ULSAB (UltraLight Steel Autobody).
If automotive OEMs desire to use the benefits of enhanced hardenability, they must work with the supply chain and standardization bodies (ASTM) to promote widespread adoption. This would allow performance and productivity gains to be adopted at scale quickly.
Intensive quenching has seen limited adoption in mass-produced automotive parts because engineers lack standardized material data that comes after industrial adoption. Therefore, their confidence in process repeatability at scale is not in place. Past experience with promising technologies like austempered ductile iron (ADI) reinforced caution around safety-critical components, especially when processing risks remain difficult to validate. As a result, rapid quenching faces less of a technology adoption challenge than a technology translation challenge: the industry must reduce perceived risk through shared standards, comparable data, and coordinated supply chain validation.
Figure 5. The standard Jominy test is universally used across the supplychain because of its usefulness in specification, standardization, andease of use. The proposed rapid Jominy test could be easily adapted toextend the overall usefulness to measure enhanced hardenability anddevelop new alloy chemistries. | Image Credit: Gopal Nadkarni
The good news is that we have standard methods characterizing hardenability that are well-established and reliable for thousands of grades of steel. For rapid quenching to be acceptable, we need to reduce risk and increase accountability across the supply chain. One way is to extend the current ASTM A225 standard Jominy test used by all (Figure 5) and use proven strategies that flat steel producers used to implement ultrahigh strength designs in steel.
From Theory to Practice: Removing the Vapor Blanket in the ASTM Jominy Test
In short, rapid quenching uses high-pressure, high-velocity water impingement to disrupt the vapor film almost immediately. This allows the surface to transition quickly into nucleate boiling and forced convection, dramatically increasing heat transfer.
The Jominy test can track the improved hardenability results, if we can make a slight modification to the ASTM 255 test bar design. The modified Jominy test’s critical feature is a flat, tapered configuration versus the traditional flat end face (Figure 6a), and it ensures that the standard uniaxial quench condition is imposed when the bar is sealed in an enclosed chamber. This refinement improves flow control for consistent and repeatable results, therefore providing better alignment with standardized quench conditions used in industry. Another test bar version with a bar end was developed through a university-industry collaborative venture that is useful to measure enhanced compressive stresses (Funk 2021).
The modified bar has been proven to be a close twin of the standard 1-inch diameter bar. The current setup uses a high-pressure nozzle to break up the vapor layer similar to industrial quench conditions and is noticeably different from the laminar flow (H < 1.4) used in the ASTM A225 test. The flow conditions (velocities > 40 m/s, pressures > 120 kPa) have been optimized to ensure that the persistent vapor barrier never forms, achieving high heat transfer rates (H > 5) from the start of quenching till the end of the cooling period.
Figure 6. (a) Schematic of modified Jominy bar to achieve rapid quenching; (b) example AISI 1045 data developed with new tester; and (c) enhanced hardenability due to rapid quenching is estimated to produce ~50% martensite (45 HRC) deeper in a 3-inch diameter bar | Image Credit: Gopal Nadkarni
As a result, current low alloy automotive grades can be quickly analyzed and compared for properties as well as depth of hardening to published hardenability curves, allowing a useful screening tool for adoption.
If heat treaters and steelmakers want to cater to automotive clients who want to reap the benefits of rapid quenching, they need to understand whether their grades are suitable. The simplest approach is to use the rapid quench methodology that has been described here for low alloy steels of interest. This will ensure that true and maximized hardenability data can be used by design engineers to optimize automotive steel parts.
In a recent research study (Nadkarni, et al. 2025), both standard hardenability and true (enhanced) hardenability curves were developed for a benchmark AISI 1045 grade (Figure 6b). The 50% martensite line (45 HRC) is clearly deeper, indicating a significant increase in hardenability, with corresponding Jominy (J in 1/16″) distances moving from J2 to J3.5 (from 3.2 mm to 5.5 mm). An example of how the data can be used practically for a sample 3-inch bar shows that depth of hardening to HRC45 is likely to be significantly higher in rapidly quenched steels (Figure 6c).
The increased hardenability results shown for the benchmark AISI 1045 have been replicated for a wide variety of steels including AISI 1020, AISI 8620, AISI 6150, M50, and tool steels like H13. However, we need to be cautious about extrapolating the results to steels where significant retained austenite can form under enhanced quench conditions.
The significance of adopting such a practice into a new or modified ASTM standard will have global implications. Standardized adoption of advancing technology will support automotive manufacturers in optimizing the performance of their current steel compositions through more controlled distortion and reduced variability in material properties.
Acknowledgements
Figure 7. The integrated Jominy Rapid Quench Tester developed at Induction Tooling Inc. for student training and research (Funk 2021). Students at the University of Akron standing with research leadership: Dr. Gopal Nadkarni, University of Akron (second from the left); Bill Stuehr, Induction Tooling, Inc. (third from left); and Joe Powell, Integrated Heat Treating Solutions, Inc. (fourth from left).
ASTM International. 2020. Standard Test Methods for Determining Hardenability of Steel. ASTM A255-20a. West Conshohocken, PA: ASTM International. https://doi.org/10.1520/A0255-20A.
Chandler, Harry, ed. 1994. Heat Treater’s Guide: Practices and Procedures for Irons and Steels. Materials Park, OH: ASM International.
Kobasko, N. I., M. A. Aronov, J. A. Powell, and G. E. Totten. 2010. Intensive Quenching Systems: Engineering and Design. West Conshohocken, PA: ASTM International.
Nadkarni, Gopal. 2026. “The Jominy and Rapid Quench Part 1: An American Story.” Heat Treat Today 9, no. 4 (April): 25–27.
Nadkarni, Gopal, Nafi Bhuiyan, Saikishan Suryanarayanan, and Ryan Souders. 2025. “The Relevance of the Standard Jominy Test for Rapid/Intensive Quenching.” Paper presented at the Heat Treat 2025 Conference & Exposition, Huntington Place, Detroit, MI, October 21–23.
Totten, George E., Rosa Simencio Otero, Xinmin Luo, and Lauralice C. F. Canale, eds. 2024. ASM Handbook, Volume 4F: Quenchants and Quenching Technology. Materials Park, OH: ASM International. https://doi.org/10.31399/asm.hb.v04F.9781627084505.
About The Author:
Dr. Gopal Nadkarni Associate Professor of Mechanical Engineering University of Akron
Dr. Gopal Nadkarni is an Associate Professor of Mechanical Engineering at the University of Akron and manages its Manufacturing Graduate Certificate Program. He brings extensive industry and innovation experience, having held previous leadership roles at industry and in technology ventures, with research and teaching focused on manufacturing, materials, and product design.
To contribute to ongoing industry-academia research regarding this topic, please contact Professor Gopal Nadkarni.
Ask The Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues. In this installment, Dan Herring discusses the science behind pH — what it really measures, and why it matters — and offers practical guidance on monitoring water quality in open and closed systems found throughout the heat treat shop.
This informative piece was first released in Heat Treat Today’sMay 2026 Sustainable Heat Treat Technologies print edition.
Introduction to pH
The term “pH” is used to describe a unit of measure that indicates the degree of acidity or alkalinity of a solution. It is measured on a scale of 0 to 14 (Table A). pH is an abbreviation that stands for the “potential of hydrogen”; the “p” being the symbol for potential (or power) and “H” the symbol for hydrogen.
Table A. pH Chart (Herring 2015b)
A Slightly Deeper Dive
What most people don’t realize is that pH is a complex concept rooted in chemical equilibrium, thermodynamics, and electrochemistry. The formal definition of pH is “the negative logarithm of the hydrogen ion activity” and can be expressed mathematically by the following formula where au+ is the activity of hydrogen ions, a dimensionless quantity (Rumble 2024):
In this form, pH provides a way of expressing the degree of the activity of an acid or base in terms of its hydrogen ion activity. Acids and bases have, respectively, free hydrogen [H+] and free hydroxyl [OH−] ions. Since the relationship between hydrogen ions and hydroxyl ions in a given solution is constant for a given set of conditions, either one can determine the other. In other words, pH is really a measurement of both acidity and alkalinity, even though by definition it is a selective measurement of hydrogen ion activity.
Since pH is a logarithmic function, a change of one pH unit represents a tenfold change in hydrogen ion concentration, that is, of both the hydrogen ion and the hydroxyl ion at different pH values (Table A). Note that each decrease in pH by one pH unit means a tenfold increase in the concentration of hydrogen ions.
A Little Chemistry
In school, we learned that all substances are made up of millions of tiny atoms. These atoms combine to form molecules. In water, for example, each molecule is made up of two hydrogen (H) atoms and one oxygen (O) atom. The formula for a molecule of water is expressed by the familiar symbol H2O. That is, there are two hydrogen atoms needed for each oxygen atom to form a stable compound.
Now, the behavior of pH in aqueous systems is governed by the equilibrium of water to form positive and negative ions (so-called self-ionization), which can be expressed as:
or in the following form we more commonly think of:
Hency, at 25°C (Kw = 1.0 x 10-14), the equilibrium constant for this process is:
Then for pure water, where aH+ = a0H-, we have that aH+ = 10-7 hence pH = -log10 (10-7) = 7.00 which is neutrality at 25°C (77°F).
Finally, it is important to note that Kw is temperature-dependent: it increases with temperature, meaning neutral pH decreases slightly as temperature rises (e.g., ~6.14 at 100°C). Therefore, “neutral pH” is not always 7 — it depends on thermal conditions.
A Practical Application — Water Quality in the Heat Treat Shop
Water is used in most of our heat treat shops for a variety of purposes, perhaps less than before but still vitally important. Examples include parts washers, heat exchangers, water cooled bearings on fans and rolls, seals on pit furnace covers, water cooled jackets on continuous furnaces, water cooled jackets for quench tanks, top or side cooling chambers, inner doors and plate coils, and make up water for water systems, to name a few.
Table B. Typical Water Requirements for Open Systems (Decelles 2002)Table C. Water Requirements for Closed Hydronic Systems (Heatlink Group 2006)
Water quality requirements are often defined differently for open systems (Table B) and closed (recirculated) systems (Table C). Open systems are typically more problematic as the issue of water quality varies. Water is often classified as “soft” or “hard” depending on its mineral content (i.e., the amount of calcium and magnesium dissolved in the water). Soft water has an ideal hardness of approximately 120 ppm (7 grains/gallon). Hard water often results in the formation of mineral deposits, which can lead to blockages in water systems (Figure 1).
Figure 1. Sludge buildup and flow blockage in the top cool of an integral quench furnace | Image Credit: The HERRING GROUP, Inc.
Furthermore, we must ensure that the water being discharged from our heat treatment operations is clean and meets EPA standards. Finally, we must be especially careful to avoid cross-contamination from other sources in the shop (e.g., polymers, quench oils, chemicals).
In Summary
Two little consonants, pH, are deceptively simple yet so profoundly important. They represent the thermodynamic state of solutions, but in reality, link microscopic interactions with real world issues. As heat treaters, our focus is to not take our water supply and water systems for granted since unexpected surprises, unwanted downtime, and expensive repairs can result. When is the last time you tested your water?
References
Herring, Daniel H. 2015a. Atmosphere Heat Treatment. Vol. 2. Southfield, MI: BNP Media.
Herring, Daniel H. 2015b. “The Importance of pH.” Industrial Heating, January.
Heatlink Group. 2006. Water Quality in Hydronic Systems. June 21, 2006. https://www.heatlink.com/sites/default/files/Info%20Sheet/L2329-Water-Quality-in-Hydronic-Systems-2006-06-21.pdf.
Decelles, P. 2002. The pH Scale. Johnson County Community College. Archived webpage. http://staff.jccc.net/pdecell/chemistry/phscale.html.
Rumble, John R., ed. 2024. CRC Handbook of Chemistry and Physics. 105th ed. Boca Raton, FL: CRC Press.
About the Author
Dan Herring “The Heat Treat Doctor” The HERRING GROUP, Inc.
Dan Herring has been in the industry for over 50 years and has gained vast experience in fields that include materials science, engineering, metallurgy, new product research, and many other areas. He is the author of six books and over 700 technical articles.
In this two-part series, Dr. Gopal Nadkarni, an associate professor of mechanical engineering at the University of Akron, revisits the American origins and impacts of the Jominy test while exploring how rapid quenching technologies are exposing its limitations. Discover how a new approach builds on ASTM foundations to better reflect today’s high-performance cooling methods.
This informative piece was first released in Heat Treat Today’sApril 2026 Annual Induction Heating & Melting print edition.
Introduction
For nearly a century, the Jominy End-Quench Test has shaped how North American engineers design alloys, specify steels, and heat treat critical components across automotive, oil and gas, heavy machinery, and aerospace industries. It involves one small piece of steel and a stream of water. Simple, repeatable, and powerful, it was revolutionary for its time. But it derived from a different world of manufacturing. Furnaces were batch loaded. Parts required hand transfer. Quenching utilized tanks with modest agitation — not high-pressure sprays, induction-to-quench lines, or high-performance water systems.
Today’s rapid quench technologies reveal a key limitation: the classic Jominy test can systematically underestimate what many steels are capable of under aggressive convective cooling. Why? Because film boiling — a vapor layer at the steel surface — chokes heat transfer right where it matters most.
In order to take advantage of a wider range of materials and processing methods, a shift in how to leverage this test is necessary. The discussion that follows elaborates on this American origin story of the Jominy’s test and what challenge the film boiling layer posed 75 years ago versus today.
Pursuing Performance: The Origins of the Jominy Test
In 1947, Fred P. Peters wrote in Scientific American that
hardenability
was no longer just a technical trend; it was a revolution changing how the steel industry did business. He was describing a shift in which clients wanted to move away from buying a grade based on chemistry specs alone to buying a product that would guarantee performance.
*Hardenability: the ability of a steel to harden to a certain depth. Hardness, by comparison, is the measurement of how hard a material is at a given location. High hardenability ensures desired properties and microstructure at a given depth for critical components; this leads to more efficient optimized part designs.
But there was a problem: Steelmaking was, and still is, a complex science with unavoidable variability arising from chemical, thermal, and metallurgical processes involved. From batch-to-batch, a single supplier could achieve different hardenability results. That variability caused headaches for manufacturers.
Two metallurgists working with General Motors, Walter Jominy and A.L. Boegehold, proposed an elegant solution: standardize the cooling conditions instead of chasing exact chemistry. Pinpointing the “maximum hardness at center” had been an estimate derived from submerging several steel bars of various diameters in a quench tank. Speaking to the Detroit Chapter of American Society of Metals, the metallurgists pushed for an end-quench test using a standardized one-inch round bar, heated uniformly, and quenched at one end with a water jet. Their practical solution replicated the range of cooling rates from the maximum cooling rate (water) at the quench face to the slowest cooling rate (air) at the other end of the bar. The resulting hardness profile became a “fingerprint” of that alloy’s hardenability.
That fingerprint changed everything.
Figure 1. Diagram of the Jominy End-Quench Test | Reference: ASTM A255-02
Metallurgists could now rapidly compare alloys from different suppliers or with minor chemistry differences. Additionally, design engineers could specify hardenability bands rather than tight chemistry limits. Steelmakers could adjust compositions and quickly verify performance. Scrap heats dropped. Costs fell. Customers gained predictable, repeatable results. ASTM A255 formalized the method, and the Jominy test became the global language of hardenability (Figure 1).
Known Limitations and Rapid Quench
Heat treaters know the Jominy test is a simplification. The specimen is a straight bar, not a gear or forging. Only one end is quenched. The cooling method represents one type of quench: water, not oil, polymer, or gas.
There’s also a compositional limitation; highly alloyed steels often show little variation along the four-inch test length.
But one limitation has taken on new importance in modern heat treatment: the vapor blanket.
The Vapor Blanket Problem
When red-hot steel hits water, a vapor layer instantly forms on the surface. This “Leidenfrost layer” acts like insulation. Heat transfer drops until the vapor film collapses and nucleate boiling begins.
That means the cooling severity at the Jominy face is not just “water quench.” It is water quench through a steam barrier.
So, the hardenability curve we measure reflects steel transformation behavior plus a boiling-limited surface condition. If that vapor layer is removed or shortened, the cooling rate at the surface rises, and the steel may harden deeper than the standard Jominy curve suggests.
Over the past two decades, a new approach has gained traction: do not accept film boiling — remove it.
Researchers, such as Dr. Kobasko and Dr. Aronov, showed through modeling and experiments that high-velocity, high-pressure water flow can consistently suppress the vapor layer, a method known as Intensive Quenching™. This approach pushes the hot surface quickly into high heat transfer by removing the vapor film and has been referred to as High Convective Quenching, High-Pressure Convective Quenching, and Rapid Quenching.
The result is more than faster cooling.
Figure 2. Quench rate and surface reactions
Early formation of a martensitic surface shell creates compressive stresses that enhance fatigue and wear resistance. Parts can show deeper effective hardening and improved surface performance without increasing alloy content (Figure 2). Some studies even suggest differences in martensite morphology (twinned morphology) compared to conventional quenching (lath morphology). This is not just “harder steel.” It is a different thermal-mechanical response at the surface.
Rethinking the Jominy Test
If quenching technology has changed, should the hardenability test evolve too?
Research at the University of Akron has shown that the standard Jominy setup itself forms a vapor layer. Raising the jet height does not eliminate it. That means the test measures hardenability under a boiling-limited condition, not under maximum achievable heat transfer.
Figure 3. Standard vs. Rapid Quench Hardenability | Image Credit: Gopal Nadkarni
Working with industry partners, the university researchers developed a modified end-quench configuration that uses high-convective water impingement to strip the vapor barrier. The measurement philosophy remains Jominy-based i.e. measure hardness along the length of the bar. What changes is that the end of the flat bar is given a slight taper to allow it tightly seal into a chamber where a water jet is sprayed on the surface, much like a jet pressure washer. Modeling this scenario allows us to predict that the new pressure and flow conditions are sufficient to strip the vapor and keep it from reforming, thus creating conditions of maximum heat transfer without the continuous formation of the film. The old “umbrella” method of cooling does not ensure the removal of the vapor or film on surface. The result is a new method that reveals how steels behave under rapid quench — conditions increasingly used in advanced heat treat operations (Figure 3).
Why This Matters to Industry Now
For 75 years, engineers have relied on handbooks filled with Jominy curve diagrams. Those curves remain valuable, but they reflect a quenching severity rooted in mid-20th-century practice.
Today, heat treaters, steelmakers, and designers have a chance to expand that framework. A rapid-quench Jominy approach could help:
Optimize alloy design for modern quench systems
Improve simulation accuracy in digital twins
Reduce over-alloying and cost
Increase part performance and consistency
This is not about redefining hardenability. It is about recognizing that hardenability is expressed under a defined cooling boundary. As quenching technology advances, our standardized ways of describing steel response should advance with it.
In Part 2, we’ll look at how this modified Jominy approach aligns with ASTM philosophy, what simulation reveals, and how rapid quenching translates into real improvements for gears, heavy components, and other critical parts.
About The Author:
Dr. Gopal Nadkarni Associate Professor of Mechanical Engineering University of Akron
Dr. Gopal Nadkarni is an Associate Professor of Mechanical Engineering at the University of Akron and manages its Manufacturing Graduate Certificate Program. He brings extensive industry and innovation experience, having held previous leadership roles at industry and in technology ventures, with research and teaching focused on manufacturing, materials, and product design.
To contribute to ongoing industry-academia research regarding this topic, please contact Professor Gopal Nadkarni.
Cuando una carga se atasca durante el temple, cada segundo cuenta y las decisiones improvisadas pueden aumentar el riesgo. En esta entrega de “Martes Técnico“, Bruno Scomazzon, gerente general de Precision Heat Treat Ltd., describe un procedimiento de respuesta de emergencia paso a paso para este escenario, uno de los más peligrosos en el tratamiento térmico con atmósfera. Basándose en la experiencia real, esta guía tiene como objetivo ayudar a las empresas a desarrollar sus propios procedimientos eficaces para mantener la seguridad, controlar las condiciones del horno y coordinarse con los servicios de emergencia en situaciones de alto riesgo.
Este artículo informativo se publicó por primera vez en Ingles enHeat Treat Today’sFebruary 2026 Annual Air & Atmosphere Heat Treating print edition.
Si tiene comentarios o preguntas sobre este artículo, háganoslo saber en: editor@heattreattoday.com.
Traducido por Víctor Zacarías. To read this article in English, click here.
Descripción del escenario
Se ha transferido una carga a la cámara de temple y el elevador está descendiendo al aceite, pero la carga se atasca y no se sumerge por completo. La puerta interior se cierra correctamente, y la puerta exterior (frontal) permanece cerrada.
Esta es una situación de altísimo riesgo que requiere el estricto cumplimiento de los procedimientos de emergencia. El objetivo es proteger: primero al personal (minimizar la posibilidad de lesiones o que la situación empeore), luego las instalaciones y, finalmente, el equipo.
1. Medidas inmediatas
NO abra la puerta exterior
Es posible que sienta la tentación de evaluar la situación, pero resista la tentación. NO se coloque frente a la puerta exterior ni justo al lado de ella, y nunca la abra mientras la carga esté “colgada”. Abrir esta puerta puede introducir oxígeno en una cámara caliente, lo que provocaría:
Explosiones o incendios repentinos (flash fire).
Pérdida de contención debido a deformación de la puerta o falla mecánica.
En casos extremos, la puerta exterior puede resultar dañada (arrancada, atascada o parcialmente abierta) y presentar llamas visibles. Esto requiere alertar inmediatamente a los bomberos.
Si la puerta exterior no se puede cerrar
En este caso, notifique inmediatamente a los bomberos e indíqueles que se preparen para una respuesta con espuma. NO permita el uso de agua. ¡Esto podría provocar reacciones violentas con el aceite o la atmósfera y propagar el fuego!
El personal de respuesta capacitado debe:
Colocarse el equipo de protección personal (EPP).
Preparar el equipo de extinción de incendios.
Estar listos para proteger los sistemas críticos hasta que lleguen los bomberos.
NO apague el horno.
Figura 1. Horno de atmósfera durante su operación normal. | Image Credit: Precision Heat Treat Ltd.Figura 2. Puerta del vestíbulo parcialmente abierta durante una simulación controlada para | Image Credit: Precision Heat Treat Ltd.
2. Mantener el suministro eléctrico
Para garantizar que los sistemas esenciales permanezcan activos, debe mantener el suministro eléctrico. Asegúrese de que los siguientes sistemas permanezcan activos:
Cambie el modo del horno de automático a manual. Esto evitará cualquier secuenciación del PLC que active automáticamente las puertas, los elevadores y los manipuladores.
Mantenga las llamas piloto encendidas.
Mantenga el enfriamiento del aceite en funcionamiento para evitar el sobrecalentamiento del tanque.
Apague los calentadores de aceite para evitar una carga térmica adicional en el tanque de temple.
Mantenga la agitación del temple a baja velocidad durante todo el proceso para ayudar a disminuir la temperatura en la superficie de interfaz entre la carga y el aceite. Esto evita la estratificación y disipa el calor por radiación en el aceite.
Mantenga el recirculador en funcionamiento.
Mantenga la instrumentación en funcionamiento para el monitoreo.
NOTA: La pérdida de estos sistemas elimina la visibilidad, el control de la atmósfera y las opciones de respuesta seguras.
3. Gestión de la atmósfera
Mantenga una atmósfera protectora y una presión positiva en el horno para evitar la entrada de oxígeno y la combustión incontrolada:
Ajuste el control del potencial de carbono a “0”.
Cierre el suministro de gas de enriquecimiento.
Cierre el suministro de amoníaco.
Cierre el suministro de aire de dilución.
Purga de nitrógeno
Estos pasos dependen de si se dispone de una purga de nitrógeno; se recomienda encarecidamente que esté disponible para todas las unidades con temple integral o de paso directo. Asegúrese de comprender cuánto tiempo tarda su horno en purgar completamente el gas endotérmico. Si bien la norma NFPA 86 recomienda cinco ciclos de purga, algunos expertos aconsejan prever hasta diez por hora en caso de emergencia. Cada horno debe contar con datos de purga establecidos en condiciones normales para que los operadores puedan actuar con confianza cuando el tiempo es crucial.
Figura 3. Suministro de nitrógeno utilizado para purga de emergencia y control de la atmósfera. | Image Credit: Precision Heat Treat Ltd.
Inicie inmediatamente una purga de nitrógeno (si está disponible) y manténgala durante todo el evento.
Utilice al menos el flujo mínimo especificado en su documentación. Si es seguro, se puede utilizar un flujo mayor para ayudar a desplazar los gases inflamables de las cámaras de calentamiento y temple.
Mantenga la temperatura del horno a 815°C (1500°F) durante la purga.
Pueden quedar espacios residuales de gas Endo atrapados en zonas con poca ventilación. Si la temperatura de la cámara desciende por debajo del punto de ignición antes de que se haya desplazado todo el gas inflamable, la entrada de oxígeno podría provocar una explosión. En algunos casos, el Endo atrapado y los desequilibrios de presión pueden causar fugas repentinas, en las que se expulsa aceite o gas debido a la acumulación de presión interna.
Después de la purga
El objetivo de la purga con nitrógeno es desplazar el gas endotérmico con una atmósfera inerte, manteniendo una temperatura elevada para facilitar la combustión de los gases inflamables residuales y prevenir la formación de mezclas peligrosas. Este proceso debe garantizar una presión positiva en todo el horno.
Una purga seguida de enfriamiento por inmersión de nitrógeno es un método válido si la purga se ha completado de forma verificable.
Según el tamaño del horno y la velocidad de enfriamiento:
Los hornos más grandes pueden enfriarse lo suficientemente lento como para completar la purga.
Las unidades más pequeñas o de enfriamiento más rápido pueden requerir un breve mantenimiento de la temperatura antes del enfriamiento controlado o el enfriamiento por inmersión.
NOTA: Una vez que la carga suspendida se enfríe a una temperatura segura (aproximadamente 65°C), realice el apagado estándar del equipo.
Sin nitrógeno (en Endo)
Si no hay purga de nitrógeno, o esta es insuficiente, la única opción es dejar que la carga acumulada se enfríe en el vestíbulo mientras se continúa quemando Endo y se mantiene la temperatura del horno a 1500°F. Una vez que el vestíbulo/tanque de aceite se enfríe por debajo de 150°F y haya pasado el peligro, inicie el apagado estándar del horno.
4. Gestión de la seguridad
Alerte inmediatamente al cuerpo de bomberos local. Si la situación se vuelve incontrolable o si existe alguna duda sobre la capacidad de mantener el control, evacúe las instalaciones y espere a que lleguen profesionales capacitados. La seguridad del personal de la planta es primordial.
Notifique al departamento de seguridad de la planta y a la administración del sitio.
Evacúe a todo el personal no esencial del área de tratamiento térmico.
Informe a todos los departamentos que se está produciendo un incidente de alto riesgo.
Los bomberos son más eficaces cuando conocen sus instalaciones antes de que ocurra una emergencia. Asegúrese de que conozcan la distribución de sus operaciones, incluyendo:
Ubicación y tamaño de los tanques de aceite
Paneles eléctricos
Válvulas de cierre de gas
Zonas calientes
5. Periodo de enfriamiento controlado
Mantenga la protección con atmósfera durante todo el evento.
NO abra las puertas hasta que la temperatura del vestíbulo sea baja y estable.
El tiempo de enfriamiento dependerá de la masa de la carga y la retención de calor. Prevea cinco horas o más.
Utilice la estabilidad de la presión del horno, las observaciones de los efluentes y el comportamiento de los gases como indicadores indirectos de la temperatura.
6. Procedimiento de recuperación de la carga
Una vez enfriado y estabilizado, realice el apagado estándar, comenzando con la eliminación del gas endotérmico, si corresponde.
NO intente retirar la carga manualmente hasta que el sistema esté verificado como seguro.
Solo el personal de mantenimiento puede recuperar la carga, utilizando equipo de protección personal (EPP) y las herramientas adecuadas.
7. Familiarización con el cuerpo de bomberos
Toda instalación debe establecer una buena relación con el cuerpo de bomberos local antes de que ocurra una emergencia. Procure revisiones anuales e identifique lo siguiente:
Número de hornos
Volumen de los tanques de aceite de temple para extinción de incendios
Ubicación de las zonas calientes y los paneles de control
Puntos de parada de emergencia
Las puertas atascadas suelen deberse a fallos en las válvulas neumáticas. Cerrar el suministro de aire comprimido y purgarlo puede permitir que el mecanismo se reinicie. Consulte siempre el manual del equipo o al fabricante antes de intentar cualquier solución.
Considere que el inspector de bomberos que realice las revisiones no es necesariamente quien acudirá a combatir los incendios; capacite a quienes sí lo harán.
Protocolo posterior al incidente
Antes de volver a poner en funcionamiento el horno, asegúrese de:
Realizar una investigación formal.
Identificar y corregir la(s) causa(s) raíz.
Documentar todos los parámetros clave y las acciones tomadas.
Capacitar nuevamente a los operadores según sea necesario.
Señalización del horno
Es probable que un operador lea el plan de seguridad, pero podría olvidar un protocolo vital durante una emergencia. Contar con advertencias claras y llamativas, impresas y colocadas en el panel, que el operador pueda retirar y utilizar en caso de emergencia, puede ser de gran utilidad.
Reflexiones finales
No podemos predecir todas las consecuencias. Ningún procedimiento puede contemplar todas las variables posibles en una emergencia real. Una vez que un evento se pone en marcha, lo único que podemos hacer es responder con el mejor criterio, capacitación e intenciones, priorizando siempre la seguridad de las personas.
Este documento pretende ser una referencia práctica: una guía estructurada elaborada con esmero, experiencia real y buenas prácticas. No es una solución universal, sino una herramienta para ayudar a los equipos a crear o mejorar sus propios procedimientos eficaces y a responder de forma adaptativa en situaciones de alto riesgo.
La preparación contra incendios es esencial en toda planta de tratamiento térmico. Los incendios ocurren, y no siempre son pequeños. Es fundamental saber cuándo actuar, cuándo evacuar y cuándo pedir ayuda. Los manuales de equipos proporcionan una base, pero la preparación mediante capacitación y planificación es la mejor defensa.
Agradecimientos: El autor agradecer a Daniel H. Herring, “The Heat Treat Doctor,” a The HERRING GROUP, Inc., y a Avery Bell de Service Heat Treat en Milwaukee por sus valiosas contribuciones.
Acerca del autor:
Bruno Scomazzon Gerente General Precision Heat Treat Ltd.
Bruno Scomazzon es el gerente general de Precision Heat Treat Ltd. en Surrey, Columbia Británica, Canadá, y cuenta con más de 40 años de experiencia en procesos metalúrgicos y operaciones de tratamiento térmico.
Ask The Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues. In this installment, Dan Herring discusses practical strategies for managing distortion through oil quenching, focusing on how subtle adjustments — such as delaying agitation to extend the vapor blanket phase — can influence heat transfer behavior and improve dimensional stability in challenging geometries like thin-walled, large-diameter gears.
This informative piece was first released in Heat Treat Today’sApril 2026 Annual Induction Heating & Melting print edition.
The Question
A reader’s question caught the Doctor’s eye and will provide some valuable information we all can benefit from. Let’s learn more:
“I have a question about a technique we used sometimes in my factory for distortion reduction. As you know, in the oil quench cooling there are 3 steps: 1. Vapor Blanket Phase (≈ 840–700°C) 2. Boiling Phase (≈ 700–400°C) 3. Convection (≈ 400–40°C) In addition to [running] a martempering oil (Houghton M240) and a high oil temperature of 80–100°C, a technique we used successfully to reduce the distortion in thin wall large (> 1m) gears was to wait 1 minute without agitation just after placing the parts in the oil tank. Once the minute has passed, we start with the agitator speed at 1,700 rpm. The technical reason for this improvement is to extend the vapor blanket step and hence reduce the distortion created by the boiling step. My questions are: What effect does the vapor blanket step have on thermal uniformity, and is it possible to get a similar result in the agitator speed, for instance, start with a low rotating speed and finishing with a high speed?”
The Three Phases of Quenching
As a brief reminder, let’s revisit the three distinct stages of cooling (Figure 1). The first stage, the “vapor blanket” (or “film boiling”) stage, 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.
The stability of the vapor layer, and thus the ability of the oil to harden steel, is dependent on: the metal’s surface irregularities; oxides present; surface-wetting additives, which accelerate the breakdown and destabilize the vapor blanket; and the quench oil’s molecular composition, including the presence of more volatile oil degradation by-products (Herring 2015). 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.
The second stage, the “vapor transport” (or “nucleate boiling” or “bubble boiling”) stage, is where the highest heat transfer rates are produced — and where the greatest amount of distortion occurs. The point at which this transition occurs and the rate of heat transfer in this region depend on the oil’s overall composition (base oil, speed accelerators, and antioxidant package). It begins when the surface temperature of the part has cooled enough so that the vapor envelope formed in the first stage collapses. Violent boiling of the quenching liquid results, and heat is removed from the metal at a very rapid rate, largely due to 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.
Figure 1. The three stages of liquid quenching | Image Credit: The Heat Treat Doctor®
The third stage of cooling is called the “convection” (or “liquid”) cooling stage. The cooling rate during this stage is slower than that developed in the second stage and is exponentially dependent on the oil’s viscosity, which will vary with the degree of oil decomposition. Heat transfer rates increase with lower viscosities and decrease with increasing viscosity. This final stage begins when the temperature of the metal surface is reduced to the boiling point (or boiling range) of the quenching liquid.
The Answer
A sage veteran once reminded the Doctor that we cannot control distortion, only manage it.
As we know, if we were able to control the heat transfer during the nucleate boiling phase, the result would be less gear distortion, especially when the geometry (in this case thin wall, large diameter gears) makes it even more challenging.
What many people do not realize is that in addition to the correct choice of oil, oil temperature, the proper size and design of the quench system (which is fixed for all part or load configurations), and the uniform removal of the vapor blanket in the first stage of quenching influences the development and type of heat transfer that will occur in the nucleate boiling phase — yes, it is uncontrolled, but it can be influenced.
A delay in the start of agitation ensures the vapor blanket phase is extended and (in a sense) more uniformly conforms to the part geometry than it would otherwise. The result is that it is easier to be uniformly swept away once the agitation begins. Interestingly, the vapor blanket begins to form within the first few seconds of quenching and begins to collapse (often in a nonuniform way) as the surface temperature drops. Agitation delay times ranging from 1 to 2 minutes have been used in industry, which are primarily a function of material, (gear) geometry, and tooth profile/thickness.
As to the other question, some manufacturers recommend quenching into slowly agitated oil (100–125 rpm) — the slower agitation only intended to push any moisture molecules around, then increasing the speed to normal agitation rates once the load is fully submerged. Appropriate safety precautions must be followed with either method. A great deal of success has been reported using this method for many of the same reasons as above.
On another note, there is some merit in vacuum oil quenching to vary the pressure over the oil. Interestingly, the characteristics (i.e., size and distribution) of the “bubbles” formed in the nucleate boiling phase changes and the end result is that they can be more easily and more uniformly swept away.
In Summary
A word or two is in order about measuring and maintaining the quench oil. Measuring the efficiency (i.e., speed) of an oil can be done in one of two ways. The first method is by measuring the oil’s cooling ability (i.e., hardening power). 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 (Figure 2).
Figure 2. Typical cooling curves and cooling-rate curves for new oils | Image Credit: The Heat Treat Doctor®Table A. Classification of Quench Oils
The older GM Quench-O-Meter method (Table A) can be used as well.
Variables Affecting Dimensional Change
A number of factors influence post-heat treat distortion, including those related to material, manufacturing, and heat treating (Figure 3).
When selecting an oil quench process, some of the many factors to consider include:
Material — form, chemistry, hardenability, grain size, homogeneity, cleanliness, microstructure
Heat treatments performed at the mill
Starting microstructure — mill or third-party heat treating prior to manufacturing
Manufacturing process — sequence of operations, tooling, speeds & feeds
Part orientation during manufacturing, as opposed to grain orientation
Grids, baskets, and fixtures — both material & design
Load configuration — part spacing, orientation, arrangement (load density)
Load weight (gross or net)
Maximum quench fixture size, weight, shape
Part geometry and mass — maximum/minimum part section thickness, consideration for whether the component part is uniform in thickness or has thin and thick sections next to one another
Residual stress state before heat treatment
Targeted hardness range (initial or final)
Type of process being run (e.g., hardening, case hardening)
Free quenching or restricted (press or roll) quenching
Oil type — quenching characteristics, cooling curve data
Oil speed, condition, viscosity (fast, 7–9 second oil; medium, 10–14 second oil; slow, 15–18 second oil; or marquench, >20 second oil)
Oil temperature (initial, instantaneous rate of rise, recovery time to initial temperature)
(Effective) quench tank volume
Height of oil above the load
Agitation — agitators or pumps
Quench tank design factors
Agitation method and number of agitators or pumps
Type of quench tank baffling
Location/size of agitators or pumps
Type of agitators (e.g., fixed, two speed, variable)
Propeller size (e.g., diameter, clearance in draft tube)
Internal tank baffling (e.g., draft tubes, directional flow vanes)
Flow direction
Flow restrictions (quench elevator and baffling design)
Volume of oil
Maximum (design) temperature rise
Heat exchanger-type, size, heat removal rate (instantaneous and total demand)
Quench elevator design (e.g., hearth type, sidewalls, flow restrictions)
Flow velocity (with and without a load present)
Number of furnaces to be served by the quench system
Duty cycle (i.e., the frequency of quenching or time between quenches)
Post heat treatment operations, if applicable
Furnace temperature uniformity
Furnace repeatability
Type of furnace atmosphere
Post processing (e.g., washing, deep freeze or cryogenic treatment, number of tempers)
Time delay between heat treat operations (especially important for high hardenability materials to avoid cracking)
References
Herring, Daniel H. 2015. Atmosphere Heat Treatment. Volume 2, BNP Media II.
About the Author
Dan Herring “The Heat Treat Doctor” The HERRING GROUP, Inc.
Dan Herring has been in the industry for over 50 years and has gained vast experience in fields that include materials science, engineering, metallurgy, new product research, and many other areas. He is the author of six books and over 700 technical articles.
Vacuum furnaces performing hardening have been in use for over 50 years, yet many heat treaters may not be taking full advantage of newer, more advanced analysis tools and methods. Controlling the cooling pressure can dramatically improve toughness and tool life, but only if applied with precision. In this Technical Tuesday installment, Paulo Duarte, technical director at Treatnorte, explores the science behind gas quenching, the role of step cooling, and why measuring and adjusting cooling curves is critical for consistent, high-performance results.
This informative piece was first released in Heat Treat Today’sMarch 2026 Annual Aerospace Heat Treating print edition.
Introduction
It has been a long time since the invention of the vacuum hardening process, yet innovation in this field continues. In recent years, industrial furnaces capable of operating with higher cooling gas pressures — up to 15 bar now commonly offered on the market — have become standard. But do we truly know how to make the best use of such high pressures?
Pressures up to 10 bar were first applied to cool small parts made from cold-work tool steels, such as sheet metal stamping tools. However, such high pressures can lead to cracking in larger hot-work steel dies when cooled directly. Step cooling was introduced as a solution: start with a fast initial cooling at higher temperatures to avoid carbide formation, then gradually lower pressure stages during the final cooling phase to reduce distortion and minimize the risk of crack appearance.
Despite this empirical knowledge, the question remains: do we really understand what we are doing? Are we routinely measuring cooling rates to determine where they stand on the CCT diagram, predicting microstructure and properties, and adjusting quenching parameters accordingly? And are we certain about which pressures to use for producing high-performance, demanding tools?
Cooling in Vacuum Furnaces
Quenching is one of the most critical steps in the hardening cycle. It transforms austenite into the optimal final microstructure, avoiding the formation of coarse carbides and pearlitic constituents during cooling. This ensures the finest possible microstructure.
Figure 1. Gas quenching in a vacuum hardening furnace | Image Credit: SECO/WARWICKFigure 2. Surface cooling rates region on systematic analysis of parts quenching in a 600 mm x 600 mm x 900 mm furnace. Parts comprising weights from 500 up to 1,000 kg. Cooling pressures varies from 4 to 5 bar. Hot work tool steel. | Image Credit: Metaltec Solutions
In vacuum furnaces, this is typically achieved by injecting cooling gas through nozzles directed at the surface of the parts located in the furnace hot zone. During cooling, the gas circulates through the chamber, being drawn through furnace ports into contact with the heat exchanger tubes. A turbine then blows the cooled gas back into the hot zone where the load is located (Figure 1).
The higher the programmed cooling pressure, the greater the volume of gas passing through the nozzles over the same period of time. This increases the heat transfer from the parts to the cooling gas, resulting in a faster cooling rate.
By measuring successive cooling curves for different loads, specifically for single hot-work steel tools weighing over 500 kg, surface cooling rates pass through the bainitic–martensitic domain (the green area of the CCT diagram shown in Figure 2). Thinner parts tend to cool closer to the martensitic end at the Ms-Bs intersection, while larger tools tend to approach the pearlitic nose.
These observations highlight the importance of adjusting cooling pressure to produce the desired microstructure and account for the different cooling behaviors of large, medium, and small parts.
Investigative Approach: Testing Furnace Data Against CCT Diagrams
Measuring part temperatures during cooling began over 20 years ago, using thermocouples and data loggers, and comparing the results to steel continuous cooling transformation (CCT) diagrams. Most vacuum furnaces do not include this capability as standard, and when available as optional software, many companies choose not to invest in it. In 2005, it was discovered what few in the industry knew at the time: hardening hot-work tool steels in industrial vacuum furnaces often results in a bainitic–martensitic microstructure. This phenomenon is now more widely recognized, with published cooling curves overlaid on CCT diagrams for larger tools becoming more available.
Even so, open discussion remains rare, partly because many heat treaters are reluctant to present this evidence to academia, fearing criticism that their results do not match the fully martensitic microstructure taught at universities. This is not a debate about right or wrong, but rather an opportunity for research and improvement in heat treatment practices worldwide.
After initial testing with a 600 mm × 600 mm × 900 mm French-made single-chamber furnace, trials continued with a larger 900 mm × 900 mm × 1,800 mm German-made vacuum furnace. These tests began by measuring both surface and core temperatures for repeated cycles with small and large charges ranging from small cold-work tools to hot-work tool steel parts weighing 500–1,500 kg. Leading vacuum furnace manufacturers in North America and Europe have developed technologies capable of successfully heat treating small, medium, and large tools, resulting in microstructures that often contain both bainite and martensite. This is, in fact, an inherent characteristic of the technology. Such tools have performed well in service for decades. That said, heat treaters using higher cooling pressures have seen improved tool life significantly, while also increasing the risk of treatment failures if the pressure is too high.
In the last 10 years, properties and microstructure analyses have shown that variations in cooling rate can significantly change the microstructure and toughness of the part even within the same bainitic–martensitic domain of the CCT diagram.
With the emergence of Industry 4.0 and 5.0, along with digitalization and AI, systematic research into heat treatment processes combined with quenching deformation simulation can lead to better selection of cooling pressures. This is a critical parameter in controlling the hardening process, and it has a direct impact on part toughness and service performance. Metaltec Solutions introduced one of the first software tools aimed at improving vacuum heat treatment through Industry 4.0 concepts in 2017. This technology represents a step toward greater awareness and precision in tool steel hardening, helping heat treaters program their cycles for optimal performance in demanding applications.
Regulating Pressure in Vacuum Hardening Furnaces
To obtain the best possible microstructures, gas quenching must be programmed in the furnace so that the cooling rate is kept as close as possible to the martensitic end, i.e., at the Ms-Bs intersection, of the CCT diagram, avoiding the formation of coarse and undesirable microconstituents in the steel. This is achieved by selecting the highest permissible cooling pressure that still prevents cracking or excessive deformation. While small parts can withstand direct high-pressure cooling, larger tools require a reduction in cooling pressure.
Preliminary Pressure Comparison
For optimal quenching of large parts, the cooling pressure should not remain constant throughout the entire cooling cycle. Instead, high pressure should be applied during the initial cooling stage to prevent coarse carbides and pearlite formation and then reduced when the surface temperature reaches approximately 550°C (1022°F). This creates a martempering stage at lower pressures, reducing the risk of distortion and cracking.
Figure 3a. Cooling pressure effect on Vidar Superior (an H11 steel grade variation) part surface toughness | Image Credit: Metaltec SolutionsFigure 3b. Cooling pressure effect on 400 mm x 400 mm x 400 mm block surface toughness | Image Credit: Metaltec Solutions
If we measure the toughness of steel pieces quenched at different cooling pressures, then tempered together to achieve a typical 46–48 HRC hardness (in hot work tool steel), we find that higher cooling pressures result in greater toughness. Using older furnace pressures (around 3 bar) yields lower toughness, whereas increasing cooling pressure can improve toughness by approximately 60% (Figure 3a). This translates into longer tool life, since high-pressure-quenched tools better absorb stress, delaying the initiation and propagation of cracks. These benefits result from higher cooling rates (Figure 3b) and the corresponding finer microstructures achieved.
Although quenching at 3, 6, and 9 bar passes through the same transformation domain on the CCT curve, differences in the resulting internal steel structure, whether coarser or finer, are clearly observable.
True Toughness and Speed
Looking in more detail at the above findings, we can observe that when parts are cooled in a 900 mm × 900 mm × 1,800 mm vacuum furnace, the gas temperature drops below the Ms temperature (for typical hot work tool steels) in less than one minute. The gas temperature then remains near room temperature during the subsequent cooling of the parts (Figure 4a).
Figure 4a. Cooling NADCA block in a large vacuum hardening furnace; gas cooling rate according to gas pressure used | Image Credit: Metaltec SolutionsFigure 4b. Cooling NADCA block in a large vacuum hardening furnace; surface cooling curves and its respective toughness after tempering, with the alteration of the cooling curve behavior provided by the martempering (final hardness level 46–48HRC hot work tool steel | Image Credit: Metaltec Solutions
The parts, however, take considerably longer to cool down to the furnace unloading temperature, depending on the cooling pressure applied. When analyzing the cooling of large dies using the NADCA block as the standard size for comparison, the surface cooling curves vary according to the applied pressure, falling into the bainitic–martensitic domain for 3, 6, and 9 bar cooling pressures.
From this data, it can be seen that hardness is not significantly affected by using 3, 6, or 9 bar cooling pressures, even though the higher pressures produce cooling rates up to twice as fast as the slower ones. Toughness, however, is largely influenced by the way the cooling curves pass through the bainitic–martensitic domain, whether crossing the Bs and Ms intersection closer to the martensitic end (9 bar), near the center (6 bar), or closer to the pearlitic nose (3 bar).
Tuning Pressure and Time
These results show that, within the typical cooling rates of vacuum hardening (Figure 2), toughness varies significantly with cooling pressure, corresponding to finely tuned cooling speeds ranging from approximately 9 to 16°C/min (48 to 61°F/min) between 800°C and 500°C (932°F and 1472°F). This highlights the need to use the highest possible cooling pressures to achieve excellent properties while avoiding direct high-pressure cooling of large parts by applying step cooling with an initial fast cooling phase, followed by reduced pressure.
How Microstructure Drives Toughness
The reason for achieving better properties at higher cooling pressures lies in the resulting microstructure, as shown in Figure 5. Fine bainite and martensitic needles, formed through faster cooling rates, are responsible for the higher toughness observed. When lower cooling pressures are used, the cooling rate decreases, leading to coarser needle sizes (Figres 5a–c) and, consequently, lower toughness values.
Figure 5a-c. Microstructures obtained after quenching Orvar Supreme (premium H13 steel): a) 100°C/min; b) 12°C/min; c) 3°C/min (or, a) 180°F/min; b) 22°F/min; c) 5°F/min) | Image Credit: Metaltec SolutionsFigure 6. Toughness model | Image Credit: Metaltec Solutions
This can be explained by Figure 6. In a coarser microstructure, cracks can propagate more easily because there are fewer obstacles to their advance. In finer microstructures, the higher density of needles forces cracks to deviate repeatedly from their path due to the branching effect, altering the directions of crack propagation. This “shock absorber” effect — caused by the frequent detours a crack experiences when traveling through a greater number of fine needles — is the reason for the toughness improvement observed when higher cooling pressures are used to achieve faster cooling rates.
Figure 7. Convection coefficients for a 900 mm × 900 mm × 1,800 mm vacuum hardening furnace according to the pressure being used | Image Credit: Metaltec Solutions
Each furnace behaves differently, from one furnace builder to another and also depending on the level of maintenance of a furnace. So a similar furnace to the one used for obtaining cooling curves and corresponding toughness values (Figure 4b) was used to obtain the convection coefficients (Figure 7). We can see a strong correlation between convection coefficient, pressure, and final toughness obtained, indicating that these features must be carefully adjusted to reach optimal part properties and longer service life.
Conclusion
Properly applying cooling pressures, through direct high-pressure cooling for small loads or step cooling for larger tools, can significantly increase part toughness and extend tool life. The key lies in understanding how cooling curves interact with the bainitic–martensitic microstructure and adjusting pressure according to part size, geometry, and furnace characteristics.
By measuring temperatures, analyzing microstructures, and fine-tuning cooling cycles, heat treat operators can achieve consistent, high-performance results, as demonstrated with the above studies on tool steels. Faster, well-controlled cooling typically produces finer bainitic–martensitic microstructures which results in a part with “shock absorber” qualities.
Ultimately, maximizing cooling pressure, not just for minimal distortion, creates more durable tools, reduces downtime, and strengthens competitiveness through part performance.
About The Author:
Paulo Duarte Technical Director Treatnorte
Paulo Duarte is an independent researcher and consultant on heat treat technologies, also working as technical director at Treatnorte. His education and expertise in metallurgy have culminated in several articles and patents. Previously, he was the project manager at Metalsolvus and also had been the technical manager and heat treatment manager within bohler-uddeholm group for the Portuguese market. Currently, Paulo focuses on helping heat treaters by providing innovative, more efficient, and profitable heat treatment services to companies.
Ask The Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues. In this installment, Dan Herring continues his discussion on gear heat treatment, exploring vacuum and induction hardening methods for gears — from low-pressure carburizing for advanced materials to single shot and tooth-by-tooth induction techniques — and how each can be matched to the specific demands of any gear application.
This informative piece was first released in Heat Treat Today’sMarch 2026 Annual Aerospace Heat Treating print edition.
In Part One of this discussion (Air & Atmospheres Heat Treating, February 2026), we discussed various gear types, materials, and how they can be atmosphere heat treated. This month, we are focusing on vacuum and induction heat treating methods. Let’s learn more.
Vacuum Heat Treatment Processing Methods
Table A. Advanced Materials Processed by LPC
Vacuum processing can be used for most of the atmosphere treatments mentioned in Part One including carburizing (Figure 1). Low pressure carburizing (LPC) is a proven technology and the choice for many advanced applications in aerospace, automotive, off-highway, and motorsports markets, as well as the development of carburizing cycles for high-performance materials (Table A).
Figure 1. Typical commercial heat treat load of gears for vacuum carburizing (Otto and Herring 2007) | Image Credit: Photo courtesy of Midwest Thermal-VacFigure 2. Pyrowear 675 – LPC – anneal – double normalize – harden – anneal – deep freeze – double temper | Image Credit: The HERRING GROUP, Inc.
The range of effective case depths for most of these grades can range up to 2.0–3.0 mm (0.080–0.120 inches) without significant sacrifice of microstructure (Figure 2). Furnace variables, such as temperature uniformity (± 3°C or ± 5°F), control of cycle parameters (boost/diffuse times, gas flow rate, pressure, hydrocarbon type) and surface carbon optimize the microstructure, producing case uniformities of ± 0.05 mm (± 0.002 inches). Where permitted, the range of carburizing temperatures now includes the use of high temperature (> 980°C, or 1800°F) techniques.
All these advanced materials required extensive development testing to produce custom designed recipes to optimize cycle parameters. Also, quenching methods (Otto and Herring 2002) have improved, allowing us to achieve desired core properties with quenching parameter selection (high-pressure gas or oil) for distortion-sensitive and distortion-prone part geometries (Otto and Herring 2005, 2008).
Induction Hardening Methods
Various methods of hardening via applied energy are used in manufacturing gears, including flame hardening, laser surface hardening, and induction hardening.
Of the various types of applied energy processing, induction hardening is the most common. Induction heating is a process that uses alternating electrical current that induces a magnetic field, causing the surface of the gear teeth to heat. The area is then quenched resulting in an increase in hardness within the heated area. This process is typically accomplished in a relatively short time. The final desired gear performance characteristics are determined not only by the hardness profile and stresses but also by the steel composition and prior microstructure. External spur and helical gears, bevel and worm gears, racks, and sprockets are commonly induction hardened. Typical gear steels include AISI/SAE grades 1050, 1060, 1144, 4140, 4150, 4350, 5150, and 8650.
Figure 3. Patterns produced by induction hardening (Rudnev 2000)
The hardness pattern produced by induction heating (Figure 3) is a function of the type and shape of inductor used, as well as the heating method. Quenching or rapidly cooling the workpiece can be accomplished by spray or submerged quench. The media typically used for the quench is a water-based polymer. The severity of this quenchant can be controlled by the polymer’s concentration. Cooling rates are usually somewhere in between what would be obtained from pure water and oil. In some unusual situations compressed air or nitrogen is used to quench the part.
The most common methods for hardening gears and sprockets are by single shot (Figure 4) or the tooth-by-tooth method (Figure 5). Single shot often requires large kW power supplies but results in short heat/quench times and higher production rates. This technique uses a circumferential copper inductor, which will harden the teeth from the tips downward.
Figure 4. Typical single shot induction hardening operation | Image Credit: Photo courtesy of Ajax-Tocco-MagnethermicFigure 5. Tooth-by-tooth induction hardening of a helical gear | Image Credit: Photo courtesy of Ajax-Tocco-Magnethermic
The larger and heavier loaded gears (where pitting, spalling, tooth fatigue, and endurance are issues) need a hardness pattern that is more profiled like those produced by carburizing, which can be obtained by tooth-by-tooth hardening. This method is limited to gear tooth sizes with modulus 4.23–5.08 (6 or 5 DP) using frequencies from 2 to 10 kHz and about 2.54 (10 DP) using a range of 25 to 50 kHz.
The lower the frequency, the deeper the case depth. Tooth-by-tooth hardening is a slow process and usually reserved for gears and sprockets that are too large to single shot due to power constraints. The process involves heating the root area and side flanks simultaneously, while cooling each side of the adjacent tooth to prevent temper-back on the backside of each tooth. The induction system moves the coil at a pre-programmed rate along the length of the gear. The coil progressively heats the entire length of the gear segment while a quench follower immediately cools the previously heated area. The distance from the coil to the tooth is known as coupling or air gap. Any changes in this distance can yield variation in case depth, hardness, and tooth distortion. The gear is indexed after each tooth has been hardened, often skipping a tooth. This requires at least two full revolutions in the process to complete the hardening of all teeth. Straight, spur, and helical gears up to 5.5 m (210 inches) weighing 6,800 kg (15,000 lb) have been processed with this method. The entire process yields a repeatable soft tip of the tooth with hard root and flank. In other applications, the tip and both flanks can be hardened simultaneously and yield a soft root.
In Summary
Today’s design engineer has the good fortune of being able to choose from a number of heat treatment technologies for any given type of gear material and design. When selecting a gear hardening method, it is essential to specify not only the desired mechanical and metallurgical properties, but the critical dimensions that must be held and even the desired stress state of the gears themselves. The secret to success is understanding the advantages and limitations of each technology and taking these into consideration when determining the overall cost of gear manufacturing.
References
Herring, Daniel H. 2004a. “Gear Heat Treatment: The Influence of Materials and Geometry.” Gear Technology, March/April.
Herring, Daniel H. 2004b. “Reducing Distortion in Heat-Treated Gears.” Gear Solutions, June.
Herring, Daniel H. 2007a. “Oil Quenching Technologies for Gears.” With Steven D. Balme. Gear Solutions, July.
Herring, Daniel H. 2007b. “Heat Treating Heavy Duty Gears.” With Gerald D. Lindell. Gear Solutions, October.
Herring, Daniel H. 2012–2016. Vacuum Heat Treatment. Vols. 1–2. BNP Media Group.
Herring, Daniel H. 2014–2015. Atmosphere Heat Treatment. Vols. 1–2. BNP Media Group.
Herring, Daniel H., Gerald D. Lindell, D. J. Breuer, and B. Matlock. 2001. “Atmosphere vs. Vacuum Carburizing.” Heat Treating Progress, November.
Herring, Daniel H., Gerald D. Lindell, D. J. Breuer, and B. Matlock. 2002. “An Evaluation of Atmosphere and Vacuum Carburizing Methods for the Heat Treatment of Gears.” In Off-Highway Conference Proceedings. SAE International.
Otto, Frederick J., and Daniel H. Herring. 2002a. “Gear Heat Treatment: Today and Tomorrow, Part 1.” Heat Treating Progress, June.
Otto, Frederick J., and Daniel H. Herring. 2002b. “Gear Heat Treatment: Today and Tomorrow, Part 2.” Heat Treating Progress, July/August.
Otto, Frederick J., and Daniel H. Herring. 2005. “Vacuum Carburizing of Aerospace and Automotive Materials.” Heat Treating Progress, January/February.
Otto, Frederick J., and Daniel H. Herring. 2007. “Advancements in Precision Carburizing of Aerospace and Motorsports Materials.” Heat Treating Progress, May/June.
Otto, Frederick J., and Daniel H. Herring. 2008. “Improvements in Dimensional Control of Heat Treated Gears.” Gear Solutions, June.
Rudnev, V. 2000. “Gear Heat Treating by Induction.” Gear Technology, March/April.
About the Author
Dan Herring “The Heat Treat Doctor” The HERRING GROUP, Inc.
Dan Herring has been in the industry for over 50 years and has gained vast experience in fields that include materials science, engineering, metallurgy, new product research, and many other areas. He is the author of six books and over 700 technical articles.
