MANUFACTURING HEAT TREAT TECH

Ask The Heat Treat Doctor®: What Oil Quenching “Tricks” Help Manage Distortion?

/ MANUFACTURING HEAT TREAT TECH, QUENCHING TECHNICAL CONTENT

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’s April 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).

Figure 3. Distortion (Ishikawa) diagram | Image Credit: The Heat Treat Doctor®

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.

For more information: Contact Dan at dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

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Improve Vacuum Quench, Maximize Tool Life

/ HARDENING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, QUENCHING TECHNICAL CONTENT, VACUUM FURNACES TECHNICAL CONTENT

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’s March 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/WARWICK
Figure 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 Solutions
Figure 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 Solutions
Figure 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 Solutions
Figure 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.

For more information: Contact Paulo Duarte at pauloduarte@treatnote.pt.

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Answers in the Atmosphere: Argon Part 2 — Market Perspectives

/ BULK GASES & SYSTEMS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, OP-ED

In this installment of Answers in the Atmosphere, David (Dave) Wolff, an independent expert focusing on industrial atmospheres for heat treat applications, examines the market realities shaping argon supply and demand.

This informative piece on argon’s sourcing and distribution landscape, safety considerations, and emerging growth drivers — from U.S. titanium refining and powder metallurgy to the reshoring of domestic steel production — was first released in Heat Treat Today’s March 2026 Annual Aerospace Heat Treating print edition.

Akin Malas
Business Development Manager / Metallurgist
Linde

If you are just beginning to read this column, welcome. I encourage you to read the February 2026 installment to have a better understanding of the attributes of argon as an industrial gas for the thermal processing industry. Akin Malas, business development manager and metallurgist at Linde, joins me in this foray into argon, and we’re exploring market realities in this installment.

Though many companies compete for market share in the supply of gases such as nitrogen, oxygen, and hydrogen because they are relatively less expensive to source and process, the number of companies that have sufficient scale and expertise to produce and market argon is generally considered to be limited to the top tier of industrial gas companies like Linde, Air Liquide/Airgas, Air Products, Matheson, and Messer. Many other companies operate ASUs; however, very few of the plants are large enough to separate and purify argon. In the case of some ASUs owned and operated by very large steel mills, they generally sell their crude argon to industrial gas suppliers for purification and subsequent marketing and sales.

Linde gas delivery truck | Image Credit: Linde

Argon is delivered as a liquid cryogenic product (LAR), like liquid nitrogen (LIN), or liquid oxygen (LOX), but there may be differences in the storage and dispensing equipment installed at a client’s site due to the variety of uses for argon. Certain high-volume applications, such as ladle stirring and metals atomization, may require substantially higher pressure than normal cryogenic tanks are set up to store, making the use of boosters or cryogenic pumps necessary. If your application requires argon pressure to be above 100 psig, make sure that you are talking to a supplier that is experienced in providing and maintaining the equipment needed for your process.

From an NFPA codes standpoint, argon storage is comparable to nitrogen storage, and the clearances in NFPA 55 are the same for argon and nitrogen. But there are some key points to consider as far as safety in use:

  • While all cryogenic gases will create a vapor cloud that hugs the ground if there is a release, the clouds from nitrogen and oxygen will disperse and rise relatively rapidly as the gas warms. But because argon has much higher density than oxygen and nitrogen, a release will tend to hug the ground and can create a serious oxygen deficient atmosphere issue in low spots. Users of pit furnaces with argon need to be particularly aware of the unique characteristics of argon.
  • Because argon is generally shipped much longer distances than oxygen or nitrogen, tanks tend to be larger sized so that more can be delivered in a single visit. Since the transportation element of the price is considerable, keep in mind that your tank size (and NFPA clearance calculations) may be different than is typical for nitrogen.

While argon cannot be cost-effectively produced from air by non-cryogenic generation techniques (like membrane and PSA techniques used for nitrogen and oxygen), argon recovery/recycling is possible from argon-based industrial streams. A few of the larger industrial gas providers can provide recycling equipment that uses cryogenic separation technology to re-create very high purity argon from argon-rich offgas streams. Note that these installations are relatively expensive and specialized and are generally only of interest to the largest argon consumers.

After many years of relatively modest growth in the consumption of argon, several market segments are driving potentially important growth in argon use:

  • Several companies are developing U.S.-based titanium refining capabilities. Argon is required for processing and refining titanium metal, and the U.S. titanium production may become an important consumption driver.
  • Virtually all atomization of the special metals for powder metallurgical applications currently is done with argon used as the atomization gas. Though cheaper water-based atomization is being developed, it is not yet suitable for high quality powders. The metals powder industry has experienced strong growth and continues to grow.
  • The U.S. is reshoring iron and steel production, meaning that argon use for ladle stirring will rise. This means that overall argon use will rise, and that some new argon-capable ASU capacity will be built to serve the needs of new and refurbished steel plants.

Akin and I hope that these last two column installments have helped readers understand the factors in the market for argon that make it different in several ways from the more familiar nitrogen/oxygen marketplace.

About The Author:

David (Dave) Wolff
Industrial Gas Professional
Wolff Engineering

Dave Wolff has over 40 years of project engineering, industrial gas generation and application engineering, marketing, and sales experience. Dave holds a degree in engineering science from Dartmouth College. Currently, he consults in the areas of industrial gas and chemical new product development and commercial introduction, as well as market development and selling practices.

For more information: Contact Dave Wolff at Wolff-eng@icloud.com.

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Understanding 2 Recent NFPA 86 Updates for Metal Processors

/ BURNERS & COMBUSTION SYSTEMS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH

Is your combustion equipment truly compliant? In this guest column, Mesa Wentling, Marketing/Field Service at PSNERGY, explores two mandatory annual requirements introduced in the latest edition of NFPA 86: Standard for Ovens and Furnaces. The updates — Safety Train Verification and Radiant Tube Integrity Inspections — directly affects combustion-based heat treating equipment. Wentling breaks down what each requirement entails, how to achieve compliance, and the risks operators face if they don’t.

NFPA 86: Standard for Ovens and Furnaces establishes the minimum safety requirements for equipment that uses heat to process materials. The standard is designed to prevent fires, explosions, and hazardous operating conditions in industrial heating systems.

Although there are many updates in the most recent edition, these two mandatory annual requirements directly affect combustion equipment in use by most heat treating operations: Safety Train Verifications and Radiant Tube Integrity Inspections.

Safety Train Verification

The annual Safety Train Verification requirement focuses on confirming that each component of the fuel safety train is present and functioning correctly. Often in older furnace installations, components like gas line drip legs or wye strainers were omitted. Combustion systems rely on a sequence of valves, switches, regulators, and interlocks that must function in a precise way for safe furnace operation. These devices can drift out of adjustment, wear mechanically, or fail electrically over time.

Verifying the gas safety train annually ensures that all protective devices respond when necessary. This procedure confirms valve functionality, switch setpoints, regulator performance, and the integrity of wiring and interlocks. The goal is to identify any signs of degradation of the gas safety train before it becomes a safety hazard.

Radiant Tube Integrity Inspections

RTI Inspection | Image Credit: PSNERGY

Radiant Tube Integrity Inspections are now another required annual check. Radiant tubes operate in severe thermal environments that can lead to cracking, oxidation, warping, or weld deterioration. A tube that loses integrity can leak products of combustion into the furnace chamber, which can contaminate products, affect temperature uniformity, and create unsafe operating conditions. Loss of integrity can occur through thermal cycling, corrosion, or mechanical stress. The annual inspection ensures that any failing tubes are identified before they compromise safety or performance.

Three common ways to perform Radiant Tube inspections are with (a) digital combustion technology, (b) pressure testing, and (c) visual inspection. Digital combustion technology uses furnace atmosphere and O₂ data to identify failing tubes. This method significantly reduces downtime and manpower, improves safety, and increases accuracy. Pressure testing includes furnace shutdown, tube sealing and pressurization, pressure verification, and final seal removal and reassembly. Visual inspection requires furnace shutdown and multi-day cooling, confined space entry with elevated risk, and offers limited accuracy due to restricted access, typically identifying only major cracks in cold tubes.

Noncompliance Is a Liability

It has been observed through industry interactions that many heat treaters have not yet come into compliance with these updated NFPA 86 requirements because of the long-standing belief that their equipment was effectively grandfathered in. Historically, older furnaces and ovens were not always required to meet new verification or inspection criteria. That is no longer true. Due to the grandfather clause being eliminated, every furnace or oven, regardless of installation date, must comply with the current standard.

Failure to comply with the annual requirements can have significant consequences. Noncompliance increases exposure to safety incidents, unplanned outages, and equipment damage. Insurance carriers and auditors are placing greater emphasis on documented conformance to NFPA 86, and missing these verifications can affect coverage or lead to corrective actions.

In the event of an incident, lack of compliance presents substantial liability. Connect with industry experts in combustion like PSNERGY who can provide resources that help heat treaters and metal processors meet these requirements efficiently. You should expect detailed guidance, inspection procedures, and combustion technology for Safety Train Verifications and Radiant Tube Integrity Inspections, along with service options for facilities that need support. These resources assist operators in building compliant, safe, and reliable operations. For more information on the recent edition of NFPA 86, be sure to visit www.nfpa.org.

About The Author:

Mesa Wentling
Marketing/Field Service
PSNERGY

Mesa Wentling specializes in industrial marketing, with hands-on experience supporting furnace efficiency, combustion, and manufacturing-focused initiatives. She works with engineers and furnace specialists to help communicate complex combustion and performance data in a clear, accessible way.

For more information: Contact Mesa Wentling at mwentling@psnergy.com or LinkedIn.

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“Just Balance the Pressure,” They Said

/ BURNERS & COMBUSTION SYSTEMS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, OP-ED

Jim Roberts of U.S. Ignition engages readers in a Combustion Corner editorial about the hidden complexity of balancing furnace pressures — explaining how thermal expansion, gas velocity, and pressure fluctuation interact in modern burner systems, and how flue gas recirculation can push firing efficiency from 30% to 75% while cutting NOx emissions by more than half.

This editorial was first released in Heat Treat Today’s March 2026 Annual Aerospace Heat Treating print edition.

When I made the comment about the negative attitude in Part 1 of this series (Air & Atmosphere Heat Treating, February 2026), I was referring to the fact that most of these burner designs require a suction component (in this case, the eductor) to help pull the exhaust gases out over the heat exchanger portion of the burner. Also, if we just tried to pressurize the burners and force the exhaust gases out through the exchanger section, there would be a pressure buildup in the furnace. With that comes the destruction of door seals. Burner plates begin to leak, and when the doors open, the operators and furnace guys get greeted with a blast of 2000°F flue gas. I can honestly say, I have not, in all my years in this industry, met a furnace guy who likes a thermal haircut.

So, by balancing the pressures, we can save gas, reduce emissions, and probably even heat treat some products along the way.

A comment like, “just balancing the pressures,” seems like such an easy thing to accomplish. And, for all the experienced furnace guys out there, that is probably regarded as pretty simple stuff. But we have to give proper respect to the myriad of moving parts in today’s modern burners and heating systems. When I say moving parts, perhaps the better description is designing around the fluctuations in pressures, temperatures, and flows that these modern systems all perform to operate at these efficiencies.

When Combustion Corner covered pressures and velocities in August and September 2025, you will recall that under these temperatures, everything starts moving around under the temperature growth and pressure increases. Velocity increases like crazy, and at heat treating temperatures, the very components expand significantly enough to affect the pressure and delivery of flue gases.

High temperatures cause flue gases to expand significantly because increased thermal energy boosts gas molecules’ kinetic energy, making them move faster and spread out. This principle, described by gas laws like Charles’s Law, leads to volume increases that necessitate expansion joints in equipment to prevent system damage and maintain integrity. This expansion can create immense stress on combustion systems, requiring specialized components like expansion joints to absorb thermal growth and maintain seals, while the high heat can also induce chemical changes and dissociation, influencing performance in other ways.

For example, can you begin to envision how furnace designers and burner design engineers have to pay attention to component growth while maintaining the critical pressures of the furnace and the burners and heat exchangers? It’s a dance, let me tell you! I believe I pointed out a while back that a 6-inch diameter radiant tube or burner combustor will grow almost an inch in length when running at 1400°F and above. If it’s growing in length, it is also trying to grow in diameter. It’s like trying to produce a constant flow of water at a constant spray rate on your garden hose, all the while the hose is changing dimensions. Not so easy is it?

To sum up, with heat recovery, and then with the addition of flue gas recirculation and high velocity burners, it is really quite remarkable how well many of these systems perform. The firing efficiency of a flue gas recirculation system over a conventional cold air burner can be the difference of 30% fuel efficiency and 75% fuel efficiency! We are talking about some serious fuel dollar savings when that all happens. And now, with recirculation, you are also cutting NOx by better than half as well.

Next time we will talk about how these systems do all of this.

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.

For more information: Contact Jim Roberts at jim@usignition.com.

“Just Balance the Pressure,” They Said Read More »

Answers in the Atmosphere: Argon Part 1 — An Inert Alternative

/ BULK GASES & SYSTEMS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, OP-ED

In this installment of Answers in the Atmosphere, David (Dave) Wolff, an independent expert focusing on industrial atmospheres for heat treat applications, explores the practical role of argon as a truly inert alternative to nitrogen in thermal processing.

This informative piece on argon’s unique properties, production challenges, and applications — from vacuum heat treating of titanium to powder metallurgy and additive manufacturing — was first released in Heat Treat Today’s February 2026 Annual Air & Atmosphere Heat Treating print edition.

Akin Malas
Business Development Manager / Metallurgist
Linde

In this column, I’ve invited Akin Malas, business development manager and metallurgist at Linde, to bring his deep expertise in the subject of argon gas. What follows is the fruit of our discussion and continued conversations about this specialized yet indispensable industrial gas in thermal processing applications.

Compared to nitrogen (the industrial gas this column last covered), argon exhibits actual inertness, enabling its use in high-temperature environments and for processing metals that cannot tolerate nitrogen atmospheres, such as titanium and certain high-performance stainless steels. While argon is significantly higher cost than nitrogen, it remains far more economical than helium, another highly inert alternative.

Argon plays a vital role across multiple stages of metal processing, including:

  • Primary metallurgy: ladle stirring
  • Powder metallurgy: atomization of metal powders
  • Additive manufacturing: laser and electron-beam processes requiring inert chamber atmospheres
  • Vacuum heat treating: backfill gas for titanium and specialty alloys

Argon is used differently than nitrogen in most cases. Inexpensive nitrogen is often used as a utility pressurization gas, for scavenging, and blended with other gases (such as hydrogen); however, argon is most often used in pure form. Nitrogen is considered inert for heat treatment applications except in extraordinarily high temperatures or heat treatment of reactive metals, such as titanium and stainless steels. In this case, using an actual inert gas like argon or helium is necessary. Also, while nitrogen is virtually the same density as air and thus will diffuse throughout a vessel, argon is much denser than air and can be used to form a stratified inert layer.

Linde gas storage tanks | Image Credit: Linde

Both argon and nitrogen are separated from air in a cryogenic air separation unit (ASU), but there are three main factors that make argon much harder to make than nitrogen and thus much more expensive:

  • Argon is only 1% of air while nitrogen is 78% of air. Argon boils at nearly the same temperature as oxygen, making a separate purification process necessary. Those two factors mean that only the largest ASUs make enough argon to make it worth purifying.
  • Argon cannot economically be separated from air non-cryogenically (primarily because the percentage in air is so low), so there is no low-cost competition to cryogenic argon. Also, because argon is prized for its inertness, there is much less interest in argon that might be lower purity.
  • Because argon is made in only the largest ASUs (typically those serving very large steel mills) and because those plants tend to be geographically grouped, shipping distances for argon tend to be much farther than for nitrogen and oxygen, further driving up the costs.

Processors of titanium parts and parts made of some stainless steels, such as the 300 series stainless alloys (SS), cannot be processed in nitrogen-containing atmospheres, because the metals will nitride at heat treating temperatures. Hence these metals may be processed in a pure argon (for Ti) or hydrogen (for SS) atmosphere blends.

We’ll pick up this discussion next month to see what market options are available, particularly in the U.S.

About The Author:

David (Dave) Wolff
Industrial Gas Professional
Wolff Engineering

Dave Wolff has over 40 years of project engineering, industrial gas generation and application engineering, marketing, and sales experience. Dave holds a degree in engineering science from Dartmouth College. Currently, he consults in the areas of industrial gas and chemical new product development and commercial introduction, as well as market development and selling practices.

For more information: Contact Dave Wolff at Wolff-eng@icloud.com.

Answers in the Atmosphere: Argon Part 1 — An Inert Alternative Read More »

Perspectivas de Sostenibilidad: Calculadors de Cuantificación de Emisiones de Carbono

/ CARBURIZING TECHNICAL CONTENT, CONTROLS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH

El reporte de emisiones de carbono ya no es opcional para los especialistas en tratamiento térmico — se está convirtiendo en una necesidad competitiva y regulatoria. En esta entrega de Perspectivas de Sostenibilidad, Heat Treat Today examina la investigación del Profesor Fu Zhao y la candidata a Doctorado Lakshmi Srinivasan del Heat Treating Consortium de Purdue University, detallando una nueva calculadora de carbono basada en Python, desarrollada específicamente para operaciones de tratamiento térmico, cómo modela las emisiones del Alcance 1, 2 y 3 a partir de la geometría del horno y los parámetros del proceso, y cómo los especialistas en tratamiento térmico con operaciones internas pueden utilizarla para cumplir con las crecientes exigencias de transparencia con un mínimo de intervención manual.

Este artículo informativo se publicó por primera vez en Heat Treat Today’s February 2026 Annual Air & Atmosphere Heat Treating print edition.

Si tiene comentarios o preguntas sobre este artículo, háganoslo saber en: editor@heattreattoday.com.

To read this article in English, click here.

El reporte de emisiones se ha convertido en un paso esencial. Navegar los requisitos en un entorno político cambiante solo añade complejidad al desafío. ¿Cómo pueden los especialistas en Tratamiento Térmico mantenerse en el cumplimiento normativo? Una herramienta diseñada a través de Purdue University’s Heat Treating Consortium (PHTC, por sus siglas en inglés) podría ser la respuesta.

El consorcio ha financiado investigaciones en proyectos de tratamiento térmico que abarcan desde la eficacia de nuevos medios de temple hasta la mejora de dureza de los materiales. Hace aproximadamente dos años, las empresas miembros del PHTC solicitaron una investigación para el desarrollo de una herramienta que hiciera posible la estimación de carbono.

Lakshmi Srinivasan, Candidata a Doctorado en School
of Mechanical Engineering at Purdue University
Professor Fu Zhao, Miembro del Profesorado de School
of Mechanical Engineering and the School of
Sustainability Engineering and Environmental
Engineering at Purdue University

El Profesor Fu Zhao, miembro del profesorado de School of Mechanical Engineering and the School of Sustainability Engineering and Environmental Engineering at Purdue decidió asumir esta solicitud de investigación. Incorporando a la candidata a Doctorado Lakshmi Srinivasan, una destacada investigadora en el modelado de sistemas energéticos y evaluación del ciclo de vida en School of Mechanical Engineering y la School of Sustainability Engineering and Environmental, para la investigación y desarrollo de esta herramienta. “Este proyecto tiene como objetivo modelar los requerimientos energéticos del horno en función de su geometría y los parámetros de entrada de tratamiento térmico”, explicó Srinivasan. “A partir de estos flujos energéticos modelados y de los insumos asociados a la construcción del horno, calculamos las emisiones de carbono del Alcance 1, Alcance 2 y Alcance 3 asociados a la operación del horno”.

  • Alcance 1: Emisiones directas de carbono derivadas del consumo de energía dentro de la planta (por ejemplo, combustión de gas natural u otros combustibles)
  • Alcance 2: Emisiones indirectas provenientes de electricidad, vapor, calor o enfriamiento adquiridos
  • Alcance 3: Todas las demás emisiones indirectas a lo largo de la cadena de suministro (por ejemplo, proveedores, transporte, uso del producto)

La herramienta es una aplicación de escritorio basada en Python, diseñada pensando en la escalabilidad. Dado que el desarrollo está orientado al proceso de carburizado tanto por razones de mercado como regulatorias, se encuentra altamente enfocada en las necesidades de la industria. Adicionalmente, Zhao y Srinivasan diseñaron la herramienta para que los usuarios puedan integrar características adicionales y conjuntos de datos que se alineen con nuevos requerimientos o tecnologías emergentes. También subrayaron que la arquitectura de la herramienta está pensada para su crecimiento como una aplicación basada en la web.

Imagen de la herramienta digital de seguimiento de carburizado | Image Credit: Srinivasan and Zhao

La facilidad de uso es un aspecto esencial. Zhao y Srinivasan han refinado la herramienta para limitar la cantidad de entradas únicas requeridas por el usuario para generar un resultado preciso. El equipo explicó que este aspecto fue particularmente desafiante, ya que se examinaron alternativas para simplificar la interfaz sin simplificar en exceso la “física subyacente”. Describieron como funcionará la versión final de la herramienta, explicando que una vez que se introduzcan los parámetros clave (tipo de horno, temperaturas de proceso, tiempo, pieza) la herramienta automáticamente calculará la energía usada y las emisiones con una intervención manual mínima.

Los miembros del PHTC, de los cuales muchos representan compañías manufactureras que cuentan con tratamiento térmico interno, han mostrado interés, proporcionando retroalimentación y recursos para dar forma al desarrollo de la herramienta. Un entusiasmo adicional se observó durante el IHEA’s annual SUMMIT en agosto de 2025, donde Srinivasan presentó el desarrollo de la herramienta. Cuando se les preguntó qué interrogantes han guiado su investigación, Zhao y Srinivasan compartieron lo siguiente:

  1. Versatilidad y funcionalidad: ¿Qué tan flexible es la herramienta para adaptarse a diferentes geometrías de horno, geometrías de piezas, tipos de hornos y procesos de tratamiento térmico?
  2. Asignación basada en piezas: ¿Cómo asigna la herramienta las emisiones de manera precisa a piezas individuales o lotes de una carga dentro del horno?
  3. Emisiones específicas por ubicación: ¿Cómo considera las variaciones regionales en las emisiones del Alcance 2 y Alcance 3, tales como las diferencias en la generación de electricidad o los impactos de la cadena de suministro?

Otro desafío ha sido garantizar la calibración y verificación cuidadosa de la herramienta. Para ello el equipo ha utilizado datos reales y precisos de consumo de gas natural y electricidad provenientes de operaciones de tratamiento térmico, cortesía de los miembros del PHTC, con el fin de verificar el consumo energético predicho por el modelo a temperaturas de operación definidas del horno.

Eventualmente alguna versión de esta herramienta estará disponible para usuarios fuera del consorcio. Sin embargo, actualmente, los miembros del PHTC se encuentran a la vanguardia tanto del desarrollo como del uso. Los investigadores enfatizaron este punto: “Esta herramienta es particularmente oportuna y esencial para la industria, ya que las empresas enfrentan una creciente expectativa de proporcionar reportes de emisiones transparentes y precisos”.

Si bien el mundo de las normas y regulaciones puede sentirse como un campo minado, las discusiones comparativas sobre esta herramienta revelan aplicaciones prometedoras a corto plazo para los especialistas en tratamiento térmico con operaciones internas.

Perspectivas de Sostenibilidad: Calculadors de Cuantificación de Emisiones de Carbono Read More »

Sustainability Insights: Quantifying Carbon Calculator

/ CARBURIZING TECHNICAL CONTENT, CONTROLS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH

Carbon emissions reporting is no longer optional for heat treaters — it’s becoming a competitive and regulatory necessity. In this Sustainability Insights installment, Heat Treat Today examines research from Professor Fu Zhao and PhD candidate Lakshmi Srinivasan of Purdue University’s Heat Treating Consortium, detailing a new python-based carbon calculator built specifically for heat treat operations, how it models Scope 1, 2, and 3 emissions from furnace geometry and process parameters, and how in-house heat treaters can use it to meet growing transparency demands with minimal manual effort.

This informative piece was first released in Heat Treat Today’s February 2026 Annual Air & Atmosphere Heat Treating print edition.

If you have any comments or queries, on this article, let us know at editor@heattreattoday.com.

Para leer el artículo en español, haga clic aquí.

Emissions reporting has become an essential step. Navigating the requirements in an influx political environment only adds to the challenge. How can heat treaters remain in compliance? A tool designed through Purdue University’s Heat Treating Consortium (PHTC) may be the answer.

The consortium has funded research across heat treat projects ranging from the efficacy of novel quenchants to improving materials hardness. Roughly two years ago, the PHTC member companies requested research to develop a tool that would make carbon estimation possible.

Lakshmi Srinivasan, PhD Candidate in the School
of Mechanical Engineering at Purdue University
Professor Fu Zhao, Faculty Member at the School
of Mechanical Engineering and the School of
Sustainability Engineering and Environmental
Engineering at Purdue University

Professor Fu Zhao, faculty member at the School of Mechanical Engineering and the School of Sustainability Engineering and Environmental Engineering at Purdue, decided to take on this research request. He brought on PhD candidate Lakshmi Srinivasan, an astute researcher of energy systems modeling and life cycle assessment in the School of Mechanical Engineering, to research and develop the tool. “This project aims to model furnace energy requirements based on furnace geometry and heat treating input parameters,” Srinivasan explained. “From these modeling energy flows and furnace build inputs, we calculate Scope 1, Scope 2 and Scope 3 carbon emission associated with operating the furnace.”

  • Scope 1: Direct carbon emissions from energy consumption within the plan (e.g. combustion of natural gas or other fuels)
  • Scope 2: Indirect emissions from purchased electricity, steam, heat, or cooling
  • Scope 3: All other indirect emissions across the supply chain (e.g., suppliers, transportation, product use)

The tool is a python-based desktop application with scalability in mind. Since development targets the carburizing process for both market and regulatory reasons, it is highly focused on industry needs. Additionally, Zhao and Srinivasan built the tool for users to integrate additional features and data sets to align with new requirements or emerging technologies. They also underscored that the tool’s architecture is designed for growth as a web-based application.

Image of the digital carburization tracking tool | Image Credit: Srinivasan and Zhao

Ease of use is central. Zhao and Srinivasan have refined the tool to limit how much unique user input is required to generate an accurate output. The team explained this as particularly challenging, having examined alternatives to simplify the interface without oversimplify the “underlying physics.” They described how the final form of the tool will work, saying that once key parameters are entered (furnace type, processing temperatures, time, part geometry), the tool will automatically calculate energy usage and emissions with minimal manual intervention.

PHTC members, many of whom represent manufacturers with in-house heat treating, have shown great interest, providing feedback and resources to shape the development of the tool. Additional enthusiasm was found at IHEA’s annual SUMMIT in August 2025, where Srinivasan presented the tool’s development. When asked what inquiries have directed their research, Zhao and Srinivasan shared the following:

  1. Versatility and functionality: How flexible is the tool in accommodating different furnace geometries, part geometries, furnace types, and heat treatment processes?
  2. Part-based allocation: How does the tool allocate emissions accurately to individual parts or batches within a furnace load?
  3. Location-specific emissions: How does it account for location-based variations in scope 2 and scope 3 emissions, such as differences in electricity generation or supply chain impacts?

Another challenge has been ensuring careful tool calibration and verification. To do so, the team has taken accurate, real-world natural gas and electricity consumption from heat treat operations, courtesy of PHTC members, to verify the model’s predicted energy consumption at defined furnace operating temperatures.

Eventually, some form of this tool will be made available to those outside the consortium. Currently, however, PHTC members are at the forefront of development and usage. The researchers underlined this, commenting, “This tool is particularly timely and essential for industry, as companies are increasingly expected to provide transparent and accurate emissions reporting.”

While the world of standards and regulations can feel like a minefield, benchmarked discussions of this tool reveal promising applications for in-house heat treaters in the near future.

Sustainability Insights: Quantifying Carbon Calculator Read More »

Ask The Heat Treat Doctor®: Why and How Do We Heat Treat Gears? Part One

/ ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, CARBURIZING TECHNICAL CONTENT, HARDENING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, NITRIDING TECHNICAL CONTENT, NITROCARBURIZING TECHNICAL CONTENT, OP-ED

Ask The Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers, answer questions about heat treating, brazing, sintering, and other types of thermal treatments, as well as metallurgy, equipment, and process-related issues. In this installment, Dan Herring examines the essential role of heat treatment in gear performance: exploring the key material and design considerations for power transmission gears, the difference between through hardening and case hardening, and the atmosphere heat treatment processes — from carburizing and carbonitriding to nitriding and nitrocarburizing — that determine how well a gear handles load, wear, and fatigue in heavy-duty applications.

This informative piece was first released in Heat Treat Today’s February 2026 Annual Air & Atmosphere Heat Treating print edition.

Have questions or feedback? We’d love to hear from you — reach out to our editorial team at editor@heattreattoday.com.

Gears play an essential role in the performance of many products that we rely on in our everyday lives. When we think about gears, we generally separate them into two categories: motion-carrying and power transmission. Motion-carrying gears are generally nonferrous alloys or plastics, while load bearing power transmission gears (Figure 1) are usually manufactured from ferrous alloys and are intended for heavy-duty service applications.

Figure 1. Typical off-highway truck power transmission gears | Image Credit: The Heat Treat Doctor®

Gear Materials & Engineering

Power transmission gears involve a wide variety of steels and cast irons. In all gears, the choice of material must be made only after careful consideration of the performance demanded by the application end-use and total manufactured cost, taking into consideration such issues as pre- and post-machining economics.

Key design considerations require an analysis of the type of applied load, whether gradual or instantaneous, and the desired mechanical properties, such as bending fatigue strength or wear resistance — all of which will define core strength and heat treating requirements.

Figure 2. Stress profile in a heavy-duty transmission gear | Image Credit: The Heat Treat Doctor®

It is important for the designer to understand that each area in the gear tooth profile sees different service demands (Figure 2). Consideration must be given to the forces that will act on the gear teeth with tooth bending and contact stress, resistance to scoring and wear, and fatigue issues being paramount. For example, in the root area, good surface hardness and high residual compressive stress are desired to improve endurance or bending fatigue life. At the pitch diameter, a combination of high hardness and adequate subsurface strength are necessary to handle contract stress and wear and to prevent spalling.

Some of the factors that influence fatigue strength are:

  • Hardness distribution, a function of:
    • Case hardness
    • Case depth
    • Core hardness
  • Microstructure, a function of:
    • Retained austenite percentage
    • Grain size
    • Carbide size, type, and distribution
    • Non-martensitic phases
  • Defect control, a function of:
    • Residual compressive stress
    • Surface finish and geometry
    • Intergranular toughness

In the total manufacturing scheme, a synergistic relationship must exist between the material selection process, engineering design, and manufacturing (including heat treatment). A balance of the priorities in each discipline must be reached to achieve the optimization necessary for the ultimate performance of the gear design. This is often not an easy task.

Various atmosphere heat treatment methods are used for most types of gears including pre-hardening steps (e.g., annealing, normalizing, stress relief) and hardening processes (e.g., neutral hardening and case hardening).

Hardening

Neutral (aka through hardening) refers to heat treatment methods that do not produce a case. Examples of commonly through-hardened gear steels are AISI/SAE grades 1045, 4130, 4140, 4145, 4340, and 8640. It is important to note that hardness uniformity should not be assumed throughout the gear tooth. Since the outside of a gear is cooled faster than the inside, there will be a hardness gradient developed. The final hardness is dependent on the amount of carbon in the steel. The depth of hardness depends on the hardenability of the steel.

Through hardening can be performed either before or after the gear teeth are cut. When gear teeth will be cut after the part has been hardened, machinability becomes an important factor based on final hardness. The hardness is achieved by heating the material into the austenitic range, typically 815°C–875°C (1500°F–1600°F), followed by quenching and tempering.

Case Hardening

By contrast, case hardening is used to produce a hard, wear resistant case (surface layer) on top of a ductile, shock resistant interior (core). The idea behind case hardening is to keep the core of the gear tooth at a level under 40 HRC to avoid tooth breakage while hardening the outer surface to increase pitting resistance.

Carburizing

Figure 3. Atmosphere carburizing of large gears | Image Credit: Photograph courtesy of Aichelin Group

Atmosphere carburizing is the most common of the case hardening methods in use today and can handle a diverse range of part sizes and load configurations (Figure 3). In general, a properly carburized gear will be able to handle somewhere between 30–50% more load than a through-hardened gear. Examples of commonly carburized gear steels include AISI/SAE grades 1018, 4320, 5120, 8620, and 9310, as well as international grades, such as 20MnCr5, 17CrNiMo6, 18CrNiMo7-6, and 20MoCr4.

Atmosphere carburizing is typically performed in the temperature range of 870°C–955°C (1600°F–1750°F) although temperatures up to 1010°C (1800°F) are used for deep case work. Carburizing case depths can vary over a broad range, typically 0.13–8.25 mm (0.005–0.325 inches).

Carbonitriding

Carbonitriding is a modification of the carburizing process, not a form of nitriding. This modification consists of introducing ammonia into the carburizing atmosphere to add nitrogen to the carburized case as it is being produced. Examples of gear steels that are commonly carbonitrided include AISI/SAE 1018, 1117, and 12L14.

Carbonitriding is done at a lower temperature than carburizing, typically between 790°C–900°C (1450°F–1650°F), and for a shorter time. Combine this with the fact that nitrogen inhibits the diffusion of carbon, and what generally results is a shallower case than is typical for carburized parts. A carbonitrided case is usually between 0.075–0.75 mm (0.003–0.030 inches) deep.

Nitriding

Nitriding is another surface treatment process that has as its objective increasing surface hardness. One of the appeals of this process is that rapid quenching is not required, hence dimensional changes are kept to a minimum. It is not suitable for all gear applications; one of its limitations is that the extremely high surface hardness case produced has a more brittle nature than say that produced by the carburizing process. Despite this fact, in a number of applications, nitriding has proved to be a viable alternative. Examples of commonly nitrided gear steels include AISI/SAE 4140, 4150, 4340, and Nitralloy® 135M.

Nitriding is typically done in the range of 495°C–565°C (925°F–1050°F). Case depth and case hardness properties vary not only with the duration and type of nitriding being performed but also with steel composition, prior structure, and core hardness. Typically, case depths are between 0.20–0.65 mm (0.008–0.025 inches) and take from 10 to 80 hours to produce.

Nitrocarburizing (Ferritic or Austenitic)

Nitrocarburizing is a modification of nitriding, not a form of carburizing. In the process, nitrogen and carbon are simultaneously introduced into the steel while it is in a ferritic or at times an austenitic condition. A very thin “white” or “compound” layer is formed during the process, as well as an underlying “diffusion” zone. Like nitriding, rapid quenching is not required. Examples of gear steels that are commonly nitrocarburized include AISI/SAE grades 4140, 5160, 8620, and certain tool steels, such as H11 and H13.

Nitrocarburizing is normally performed at 550°C–600°C (1025°F–1110°F) and can be used to produce a 58 HRC minimum hardness, with this value increasing dependent on the base material. White layer depths range from 0.0013–0.056 mm (0.00005–0.0022 inches) with diffusion zones from 0.03–0.80 mm (0.0013–0.032 inches) being typical.

In Summary

There are many ways to heat treat gears. While atmosphere heat treatment (discussed above) is perhaps the most widely used technology today, other types of heat treatments, namely vacuum and induction hardening, are becoming more and more common methods. These will be discussed in Part Two.

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.

For more information: Contact Dan at dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

Ask The Heat Treat Doctor®: Why and How Do We Heat Treat Gears? Part One Read More »

Redundant Flame Safety

/ BURNERS & COMBUSTION SYSTEMS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH

What do aerospace and industrial heating vessels have in common? Backups for essential systems. In this Technical Tuesday installment, Bruce Yates, president of Protection Controls Inc., explores how NFPA 86 Standard for Oven and Furnaces addresses redundant flame safety, compares common sensing approaches, and highlights recent advances in UV scanner technology that improve reliability and reduce maintenance risks.

This informative piece was first released in Heat Treat Today’s February 2026 Air & Atmosphere Heat Treating print edition.

Introduction

Boeing Aircraft lost billions of dollars before realizing that the 737 MAX’s MCAS (Maneuvering Characteristics Augmentation System) needed a redundant angle-of-attack vane to prevent erroneous MCAS-induced drive commands. Lockheed Martin uses dual-redundant MIL-STD-1553 data bus (that is, a shared communication pathway for exchanging data between electronic systems) on its Apache Guardian attack helicopter for target acquisition and cueing for the helicopter’s fire-control radar system. Spacecraft internal Active Thermal Control Systems (ATCSs) can either be a fully redundant thermal-control loop or a single loop system that is equipped with a redundant accumulator to be activated if needed. The accumulator represents a single point of failure that can result in a loss of crew.

Aerospace is not the only industry where redundancy is an important aspect of safety. It is critical in the industrial heating industry. NFPA 86 Standard for Ovens and Furnaces has for many years required redundant pilot gas valves and redundant main gas valves.

Let’s discuss redundant flame safety.

Redundancy in Industrial Heating

There are two types of flame sensors generally used on industrial burners: flame rods and ultraviolet scanners. Flame rods are simply stainless steel rods that intersect the burner flame. A voltage potential from the combustion safeguard is applied to the flame rod. When a flame is present, an electrical current (measured in millionths of an amp) flows from the flame rod through the ionized gases of the flame to the burner, which is grounded. This current is amplified in the combustion safeguard and energizes a relay output to power the fuel valves (see Main Image).

Redundancy can be achieved by using a two-burner control with one flame rod. The flame signal from the flame rod goes to the sensor input of both positions of the two-burner control (Figure 1).

We will devote the rest of this article to UV scanners (Figure 3).

Figure 1. Redundant flame safety with a single burner flame safeguard with a flame rod sensor
Figure 2. Solar radiation begins at approximately 2,800 Å and is therefore not detectable by the flame rod sensor.
Figure 3. Demonstration of two independent UV tubes producing UV rays out of sync with one another | Image Credit: Protection Controls

Redundant Flame Safety with UV Scanners

The tube of a UV scanner responds only to radiation in the spectrum of 1,900 to 2,300 Å (Figure 2). Peak response is at 2,100 Å (210 nm). Solar UV starts at about 2,800 Å, as shown in Figure 2, and is therefore not detectable by the device. Solar radiation, of course, extends into the visible spectrum (4,000 Å) and extends into the infra-red spectrum. A UV tube consists of a fused silica or UV glass envelope, two electrodes, and a gas contained in this envelope. This is called a cold-cathode gas-discharge tube.

This tube conducts or ignites when it is irradiated with ultraviolet light and when sufficient voltage potential exists across the two electrodes. The electrodes can be made of tungsten, molybdenum, or nickel. When a photon of sufficient energy is absorbed into the cathode electrode, electrons are emitted and are drawn to the anode. A larger cathode allows more electrons to avalanche, causing higher current flow and thus higher sensitivity to UV. There are high sensitivity UV scanners designed for special burners that will produce low UV, such as one designed by Protection Controls, Inc.

The gas in the tube is usually a helium-hydrogen ionizable mix. Electrons released by the cathode release electrons in the ionized gas, becoming a self-sustaining discharge much greater than that of the originally generated electrons and producing a very high current gain or avalanche effect. The sensitivity of a tube will very slowly decrease over a period of time. Replacement should be made after 8,000 hours of operation. The current produced by the photoelectrons is measured in millionths of an ampere, so this current is amplified in the combustion safeguard to energize a relay that can then energize the fuel valves.

Critical Maintenance to Avoid Tube Gas Contamination

While UV scanners are very reliable, tube gas contamination may occur with large temperature shock (ΔTEMP/ΔTime) or large physical shock (a 2-inch drop may cause 100G shock), causing the electrode to UV glass envelope seal integrity to be compromised. Because of this, it is possible for a UV tube to conduct current when no UV is incident upon it. This would normally be detected during the flame safeguard safe start check. When an indicated flame on condition exists prior to purge or ignition, the safe start check relay prevents ignition and gas valve energization.

In addition to safe start check before every heating cycle, a monthly preventative maintenance schedule should be in place if the burner is used daily. This consists of closing a manual gas valve. The electrically powered gas valves should close in two to four seconds as the UV scanner and combustion safeguard respond to loss of flame.

If a burner is in continuous service, we recommend that this maintenance schedule be performed weekly. An alternative to this is to use a self-checking ultraviolet scanner and control. In the past, this type of scanner involved an electrically operated shutter, which alternately would block and allow UV to the tube. However, having a mechanical device operating close to the burner heat and vibration is a recipe for frequent and premature failures; it is typically rated for only 140°F to 175°F maximum and is quite expensive.

Going Shutterless

Figure 4. Note how each amplifier has its own flame relay | Image Credit: Protection Controls

Newer designs are available that completely avoid using a mechanical operating device to moderate the UV, increasing reliability and durability. For example, the Dual/Redundant Self Check UltraViolet Flame Sensor and Combustion Safeguard Control from Protection Controls includes two UV tubes in one ultraviolet sensor to monitor one burner flame. UV tubes respond to welding sparks, ignition sparks, lightning, bright incandescent or fluorescent light, solar radiation, gamma rays, and x-rays.

Since UV tubes produce UV rays when they conduct, two UV tubes in one sensor would not normally be suitable for sensing a burner flame, as one UV tube could be responding to the other tube and not the flame. But in the case of this safety control, two voltage supplies to the UV tubes are out of phase with each other. When one UV tube is powered and may respond to UV rays, the other UV tube is off. Additionally, the two UV tubes are powered through two rectifier circuits from two transformers that are out of phase with each other. The two UV tubes are powered and sense UV from the flame on alternating half cycles (Figure 3).

Each UV tube and rectifier circuit provides input to its amplifier. Each amplifier provides input to its own flame relay (Figure 4). Upon burner startup, before burner ignition, if either UV tube is in conduction, the safe start check circuit does not permit powering the fuel valve.

During the burner run cycle, if either UV tube fails in the conduction state, the cycle will safely continue with the other UV tube sensing the burner flame. See Figure 5.

Regardless of which sensor option you choose, accounting for flame redundancy and ensuring your maintenance plan is proactive enough for the method chosen is key to a safe manufacturing environment.

Figure 5. Redundant flame safety for single- and multi-burner flame safeguards: (a) redundant flame safety with a single burner flame safeguard with an ultraviolet sensor and (b) redundant flame safeguard (2-burner shown) with an ultraviolet sensor. | Image Credit: Protection Controls

About The Author:

Bruce Yates
President
Protection Controls, Inc.

Bruce Yates is the president of Protection Controls and is involved with management, sales, and engineering responsibilities. He graduated from the University of Illinois with a Bachelor of Science in Electrical Engineering in 1968. He works with his brother Douglas in the family-owned flame safeguard control manufacturing company, started by his father, James, and uncle, Robert, in 1953.

For more information: Contact Bruce Yates at email@protectioncontrolsinc.com.

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