MANUFACTURING HEAT TREAT TECH

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.

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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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Answers in the Atmosphere: The Tremendous Value of Industrial Gas Smartphone Apps

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

In this installment of Answers in the Atmosphere, David (Dave) Wolff, an independent expert focusing on industrial atmospheres for heat treat applications, highlights the practical value of smartphone apps designed for industrial gas calculations and conversions.

This informative piece on mobile tools that simplify gas property calculations, unit conversions, and storage or flow-rate estimations — drawing attention to apps developed by major gas suppliers and equipment providers that help heat treaters access critical data in the field — was first released in Heat Treat Today’s January 2026 Annual Technologies to Watch print edition.

The field of industrial gases is complicated by the fact that the physical characteristics of gases depend on the temperature and pressure at the time of measurement. Industrial gases may be delivered and stored as cryogenic liquids and highly pressurized gases, though they are generally used in relatively low-pressure gaseous form. Additionally, gases may be used for different purposes; for example, hydrogen may be used as a metallurgical atmosphere or as a burner fuel. As such, users need a ready source of data on various industrial gases to make necessary calculations.

Image Credit: Open Library/Internet Archive

Years ago, industrial gas users had to rely on data tables in publications like the CRC Handbook of Chemistry and Physics — the nearly 8 lb, $195 hardbound handbook that has been published continuously since 1914 and is currently on its 106th edition.

Today, there are many more mobile solutions in the form of smartphone applications. Several of the major gas providers have developed handy apps available for both Apple and Android operating systems to simplify gas conversions and calculations. Equipment providers have also developed apps to help understand the specifics of their equipment. All of these can be helpful to metals thermal processors, including heat treaters at in-house processing operations.

Some examples follow:

  • Air Products and Linde both provide powerful conversion engines that enable users to convert from imperial to metric units, from mass to volume measurements, and from liquid to gaseous volumes for common industrial gases. For example, users can calculate how many hours of atmosphere coverage 6,000 gallons of liquid hydrogen stored in a tank will provide.
  • Cyl-Tec, Inc. has developed an app that focuses on calculations primarily specific to cryogenic and pressurized gas storage. In addition to unit of measure conversions for each common industrial gas, the app provides detailed information on each of the storage vessels that the company makes.
  • WITT-Gasetechnik of Germany has developed an app to support their gas safety and controls business. Their products include gas mixers, gas analyzers, regulators, and other controls. The app provides a variety of gas blending and measurement information, including welding gas blend suggestions, unit conversion, and flow rate calculators.
  • Gasmet of Finland has developed an app that simplifies calculation of dewpoint and combustion products depending on the fuel being combusted.

While these suppliers hope that you will buy their products, be assured that the measurements and conversions performed with their tools, and the recommendations generated, will be equally applicable to products and systems supplied by others.

I suggest you create a folder called “calculations and conversions” on your smartphone and load it up with several of these apps while you are connected to your home or office internet, so that you will have the apps handy when you are away from your standard technical resources.

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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How To Tame Your Dragon

/ ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, QUENCHING TECHNICAL CONTENT

When a load hangs up during quenching, seconds matter and improvised decisions can escalate risk. In this Technical Tuesday installment, Bruno Scomazzon, general manager of Precision Heat Treat Ltd., outlines a step-by-step emergency response procedure for exactly this scenario, which is one of the most dangerous in atmosphere heat treating. Drawing on real-world experience, this guide is intended to help companies develop their own effective procedures for maintaining safety, controlling furnace conditions, and coordinating with emergency responders in high-risk situations.

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

Scenario Overview

A load has been transferred to the quench and the elevator is lowering into the oil, but the load becomes hung up and fails to fully submerge. The inner door successfully closes, and the outer (front) door remains closed.

This is an extremely high-risk situation requiring strict adherence to emergency procedures. The goal is to protect: first the personnel (minimize the chance of injury or escalation of the situation), then the facility, and finally the equipment.

1. Immediate Actions

DO NOT Open Outer Door

There may be a natural urge to assess the situation but resist temptation. DO NOT stand in front of or directly beside the outer door and never open it during an active hang-up. Opening this door can introduce oxygen to a hot chamber, causing:

  • Explosions or flash fires.
  • Loss of containment due to door warping or mechanical failure.

In extreme cases, the outer door may be compromised (blown off, stuck open, or partially open) with visible flames. This warrants immediate escalation to the fire department.

If Outer Door Cannot Be Closed

In this scenario, immediately notify the fire department and advise them to prepare for a foam response. DO NOT allow the use of water. This may trigger violent reactions with oil or atmosphere and spread the fire!

Internal trained responders should:

  • Don PPE.
  • Retrieve fire suppression gear.
  • Be ready to protect critical systems until responders arrive.

DO NOT shut down the furnace.

Figure 1. Atmosphere furnace during normal
operation | Image Credit: Precision Heat Treat
Ltd.
Figure 2. Vestibule door partially opened during a
controlled simulation to illustrate gas release
behavior — not an actual incident | Image Credit:
Precision Heat Treat Ltd.

2. Maintain Electrical Power

To ensure essential systems stay active, you must maintain electrical power. Ensure these systems stay active:

  • Set the furnace cycle to manual mode from auto mode. This will bypass any PLC sequencing from auto cycling doors, elevators, and handlers.
  • Keep the pilots lit.
  • Keep the oil cooler running to prevent tank overheating.
  • Shut off oil heaters to prevent additional heat loading in the quench tank.
  • Keep quench agitation on low during the entire period to assist in lowering the temperature at the interface surface area between the hot load and the oil. This prevents stratification and dissipates radiant heat into the oil.
  • Keep the recirculating fan running.
  • Keep the instrumentation functioning for monitoring.

NOTE: Loss of these systems eliminates visibility, atmosphere control, and safe response options.

3. Atmosphere Management

Maintain a protective atmosphere and positive furnace pressure to prevent oxygen ingress and uncontrolled combustion:

  • Set the carbon control to “0”.
  • Shut off the enriching gas.
  • Shut off the ammonia.
  • Shut off the dilution air.

Nitrogen Purge

These steps depend on whether a nitrogen purge is available; it is highly advised that nitrogen purge be available for all IQ or straight through units. Be sure you understand how long it takes for your specific furnace to fully purge endothermic gas. While NFPA 86 recommends five volume turnovers, some experts advise planning for up to ten per hour in an emergency. Each furnace should have established purge data under normal conditions so operators can act with confidence when time is critical.

Figure 3. Bulk nitrogen supply used for emergency purging and atmosphere control | Image Credit: Precision Heat Treat Ltd.
  • Begin a nitrogen purge immediately (if available) and maintain it throughout the event.
  • Use at least the minimum flow rate specified in your documentation. If safe, higher flow may be used to help displace flammable gases from the heating and quench chambers.
  • Maintain furnace temperature at 1500°F during the purge.

Residual pockets of Endo gas may remain trapped in less ventilated areas. If the chamber temperature drops below the ignition point before all flammable gas has been displaced, the introduction of oxygen could trigger an explosion. In some cases, trapped Endo and pressure imbalances can lead to sudden releases (“furnace burp”), where oil or gas is expelled due to internal pressure buildup.

After the Purge

The goal of the nitrogen purge is to displace Endothermic gas with an inert atmosphere while maintaining elevated temperature to assist in burning off residual flammable gases and preventing dangerous mixtures. This process must ensure positive pressure throughout the furnace.

  • A purge followed by plunge cooling in nitrogen is a valid approach if the purge is verifiably complete.
  • Depending on furnace size and cooling rate:
  • Larger furnaces may cool slowly enough for a complete purge.
  • Smaller or faster-cooling units may require a brief temperature hold before controlled cooling or plunge cooling.

NOTE: Once the hung-up load cools to a safe temperature (~150°F), perform a standard shutdown.

Without Nitrogen (in Endo)

If there is no nitrogen purge, or it is insufficient, the only option is to let the hung-up load cool in the vestibule while continuing to burn Endo and maintain the furnace temperature at 1500°F. Once the vestibule/oil tank cools below 150°F and the danger has passed, initiate a standard furnace shutdown.

4. Safety Management

  • Alert the local fire department immediately. If the situation becomes unmanageable, or if there is any doubt about the ability to maintain control, evacuate the facility and wait for trained professionals. The safety of plant personnel is paramount.
  • Notify plant safety and site management.
  • Evacuate all non-essential personnel from the heat treat area.
  • Inform all departments that a high-risk incident is in progress.

Fire departments are most effective when they are familiar with your facility before an emergency occurs. Make sure they know the layout of your operation, including:

  • Oil tank locations and sizes
  • Electrical panels
  • Gas shutoffs
  • Hot zones

5. Controlled Cooling Period

  • Maintain atmosphere protection throughout the event.
  • DO NOT open doors until the vestibule’s temperature is low and stable.
  • Cooling time will depend on load mass and heat retention. Expect five or more hours.
  • Use furnace pressure stability, effluent observations, and gas behavior as indirect temperature indicators.

6. Load Recovery Procedure

  • Once cooled and stabilized, perform a standard shutdown, starting with the removal of endothermic gas if applicable.
  • DO NOT attempt manual load removal until the system is verified safe.
  • Only maintenance personnel may retrieve the load, using PPE and appropriate tools.

7. Fire Department Familiarization

Every facility should build rapport with the local fire department before an emergency ever happens. Schedule annual walkthroughs and identify the following:

  • Number of furnaces
  • Quench oil tank volumes
  • Hot zone and live panel locations
  • Emergency shutoff points

Stuck doors are commonly caused by failed pneumatic valves. Shutting off and bleeding compressed air may allow the mechanism to reset. Always consult your equipment manual or the manufacturer before attempting corrective action.

The fire inspector conducting walkthroughs is not the one coming to fight your fires — train the ones who are.

8. Post-Incident Protocol

Before returning the furnace to service:

  • Conduct a formal investigation.
  • Identify and correct root cause(s).
  • Document all key parameters and actions taken.
  • Re-train operators as needed.

Furnace Signage

An operator is likely to read your safety plan but may forget a vital protocol during an emergency. Having bold, brightly colored warnings printed and posted at the panel that the operator can remove and use in an emergency can be invaluable.

Final Reflections

We cannot predict every consequence. No procedure can account for every possible variable in a live emergency. Once an event is in motion, all we can do is respond with the best judgment, training, and intentions — always with the safety of people as the highest priority.

This document is intended as a working reference: a structured reference developed with care, real-world experience, and best practices. It is not a one-size-fits-all solution, but a tool to help teams create or enhance their own effective procedures and respond adaptively in high-risk situations.

Fire preparedness is essential in every heat treating facility. Fires happen, and they are not always small. It is critical to know when to act, when to evacuate, and when to call for help. Equipment manuals provide a foundation, but preparedness through training and planning is the best defense.

Acknowledgments: The author would like to thank Daniel H. Herring, “The Heat Treat Doctor,” The HERRING GROUP, Inc., and Avery Bell with Service Heat Treat in Milwaukee for their valuable input.

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.

For more information: Contact Bruno at bruno@precisionheattreat.com.

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Normalizing and Isothermal Annealing: Which Furnace Is Best?

/ ANNEALING TECHNICAL CONTENT, ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH

Selecting the right furnace is critical to achieving consistent results in normalizing and isothermal annealing of forged steel components. In this Technical Tuesday installment, Arturo Archavaleta of NUTEC Bickley, examines the thermal principles behind each process and evaluates common continuous furnace types to help heat treaters select the best solution for their specific applications and production goals.

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

Introduction

Industrial furnace manufacturers support a wide range of thermal processes across the ferrous and non-ferrous metals industries, including forging, heat treatment, and low-temperature curing and drying applications. Within these areas, furnace design and process selection play a critical role in achieving consistent metallurgical results and efficient production.

This article focuses on continuous furnace systems used for the normalizing and isothermal annealing of forged steel parts, examining how different furnace configurations support the thermal and metallurgical requirements of these heat treatment processes.

Normalizing

From a thermal point of view, normalizing is an austenitizing process followed by slow air cooling. Normalizing steel is carried out by heating it to approximately 30°C–50°C (54°F–70°F) above the critical Ac3 temperature — the temperature at which the transformation to a homogeneous austenitic structure is complete — and then cooling with air to room temperature.

Figure 1. Partial iron-iron carbide
phase diagram showing the typical
normalizing temperature range for
plain carbon steel. (ASM Handbook
1991, p. 35)
Figure 2. Normalizing temperature curve | Image Credit: NUTEC Bickley
Figure 3. Example of a continuous furnace for normalizing forged parts | Image Credit: NUTEC Bickley

Why Normalize?

  • Reduces internal stresses after forging
  • Improves dimensional stability
  • Produces a homogeneous microstructure
  • Ensures a consistent structure across batches of forged parts
  • Helps better control potential problems in subsequent hardening or surface heat treatment processes

Isothermal Annealing

Isothermal annealing is a heat treatment applied to steels to soften their structure, improve machinability, and standardize their mechanical properties. It consists of heating the steel to the austenitizing zone — above Ac3 for hypoeutectoid steels (<0.8% carbon) and above Ac1 for eutectoid steels (≥0.8% carbon) — holding it until the desired austenite is achieved. The parts are then rapidly cooled to an isothermal temperature (usually 550°C–650°C/1020°F–1200°F) and held there until the transformation of the austenite to a fine pearlite is complete. Finally, parts are cooled in air.

Figure 4. Typical isothermal annealing curve | Image Credit: NUTEC Bickley

It is essential to understand the isothermal transformation (IT) diagrams of the steels treated by these processes, as the ITs predict the desired microstructure after transformation, the transformation temperature, and the time required for this to occur.

Figure 5. Example of an isothermal annealing furnace for forged parts | Image Credit: NUTEC Bickley

Main Objectives of Isothermal Annealing

The principal aim is to achieve a more homogeneous and softer structure than that obtained with conventional annealing. This helps:

  • To reduce internal stresses
  • To improve machinability and ductility
  • To achieve reproducible properties (by eliminating variability in the cooling rate during furnace annealing)
Table A. Comparative Summary — Normalizing v. Isothermal Annealing

Types of Furnace

The most typical continuous furnaces used for normalizing and isothermal annealing are as follows:

  • Pusher tray system
  • Roller hearth conveyor
  • Cast-link belt conveyor
  • Rotary hearth system

Let’s look at each one in turn and consider the advantages and disadvantages.

Pusher Tray Furnace

Figure 6. Pusher tray furnace | Image Credit: NUTEC Bickley

Pusher tray furnaces (Figure 6) offer many advantages, including a lower initial investment cost than other options. They have fewer mechanical components exposed to high temperatures requiring extensive maintenance, and the main equipment (tray pusher and puller) requires less maintenance. Short trays can be used in the direction of movement with good stability, and parts can also be loaded hung on the trays. Because the trays are closer together, the length of the furnace is shorter.

There are, however, some drawbacks. Most pusher tray furnaces only have burners firing above the load, which can affect temperature uniformity. Because of this, heating times can increase and there is less space for burners in areas of high heat demand. While main equipment maintenance is low, the trays tend to warp, resulting in additional costs. Finally, loading can be difficult and is not easily automated.

Roller Hearth Furnaces

Figure 7. Roller hearth furnace | Image Credit: NUTEC Bickley

Unlike pusher tray furnaces, roller hearth furnaces (Figure 7) have burners that fire both above and below the load, making it easier to achieve uniform temperature. There is also more space for burners in areas of high heat demand. As with pusher tray furnaces, parts can also be loaded hung on trays.

In contrast, the initial investment for roller hearth furnaces is higher. There is additional maintenance due to the roller conveyor, including lubrication of bearings, chains, and roller replacement costs based on lifespan. Longer trays are also needed for good stability, increasing the furnace length.

Figure 8. Cast-link belt furnace | Image Credit: NUTEC Bickley

Cast-link belt roller hearth furnaces (Figure 8) offer a simplified loading system using automation to place parts directly on the conveyor belt (with parts lying flat only) or even in bulk. The configuration also allows for shorter furnaces, distributing more load width-wise.

Conversely, there are several disadvantages, including a very high initial investment cost due to the alloy belt, along with costs associated with belt replacement. These furnaces require more energy because the belt must be reheated as it cools down on its return. They also require maintenance for the roller conveyor, bearings, chains, and the belt traction system. Like pusher tray furnaces, they only have burners firing above the load, making temperature uniformity more difficult to obtain.

Rotary Hearth Furnaces

Figure 9. Rotary hearth furnace | Image Credit: NUTEC Bickley

Rotary hearth furnaces (Figure 9) have a moderate initial investment and carry many advantages. They allow for manual or automatic loading since parts are placed directly on the hearth (flat or in bulk), or can be loaded hung on trays using automatic loaders or robots. They occupy less floor space and have better thermal efficiency, since all the heat is directed to the product.

As with pusher tray and cast-link belt furnaces, most rotary hearth furnaces only have burners firing above the load, which can affect temperature uniformity. They typically require robots or loaders for high-volume, continuous production. While they occupy less floor space, the layout is unconventional because loading and unloading occur from the same side.

In Summary

Selecting the appropriate furnace for normalizing or isothermal annealing ultimately depends on the desired material properties, production volume, parts, and operational priorities. Each furnace type offers distinct advantages and trade-offs in terms of temperature uniformity, flexibility, maintenance, and cost, making it essential to evaluate both metallurgical requirements and practical plant constraints (Table B).

Table B. Comparative Summary

By understanding how heat treatment objectives align with furnace design — and partnering with a supplier who understands as well — you can make informed decisions to select and customize the most suitable furnace for your specific applications.

About The Author:

Arturo Arechavaleta
Vice President, Metal Furnaces
NUTEC Bickley

Arturo Arechavaleta, VP of Metal Furnaces at NUTEC Bickley, is a mechanical and electrical engineer (AA) and holds an MBA. He has 35 years of experience in the furnace industry, including the field of engineering, working on challenging projects, leading multidisciplinary teams, and managing business units.

For more information: Contact Arturo Arechavaleta at arturoarechavaleta@nutec.com.

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CapEx vs. OpEx: Value Beyond the Dollar in Leak Testing

/ GUEST COLUMN, MANUFACTURING HEAT TREAT TECH

What’s the real price of a leak test system? According to Norbert Palenstijn of Nolek, it’s not the number on the invoice. In this guest column, he walks through why total cost of ownership — spanning calibration, consumables, throughput, and quality impact — should drive purchasing decisions more than CapEx alone.

When a factory considers new capital equipment, the first question almost always sounds the same: “What’s the CapEx?”

It is an understandable starting point. Capital expenditure is big, visible, and easy to compare. Numbers sit neatly in a column, budgets are allocated, and decisions get made. But if we stop there, especially when it comes to leak testing equipment, we risk seeing only half the picture.

Leak testing has one main role in production: it is a sorting function. Its job is to distinguish between good and bad parts based on a leak specification. That means it is not just a machine — it is the gatekeeper of quality. And for a gatekeeper, what matters most is not just the cost of admission, but how reliably the gate opens and closes.

The Hidden Cost of “Lower Purchase Price”

Imagine two leak test systems on the factory floor. One has a lower CapEx and looks attractive on paper. But in practice, it requires more frequent calibrations, eats through consumables, and delivers an uncomfortable number of false rejects. Every false reject creates rework and lost time. Every misclassified “pass” creates a risk that defective parts slip through. Suddenly, the lower cost option does not feel so appealing anymore.

Now compare it to a system with a higher upfront price but stable measurement performance, longer service intervals, and better correlation to the specification. Over years of production, this system quietly saves money and protects reputation, even if the original CapEx line was higher.

Beyond the Purchase Price

Image Credit: @TarikVision/AdobeStock

Focusing only on CapEx is like buying a sailboat and budgeting for the hull, but forgetting sails, navigation equipment, and upkeep. The hull might look affordable, but the true cost of ownership is what keeps the ship sailing safely across oceans.

In leak testing, the total cost of ownership (TCO) includes:

  • Purchase and installation (CapEx)
  • Calibration, service, and downtime (OpEx)
  • Consumables and spare parts
  • Impact on throughput (cycle times, operator time)
  • Impact on quality (false rejects and escapes)

These factors flow directly into cash flow, customer satisfaction, and brand reputation.

The Real Measure of Value

Learn the fundamentals of helium leak detection firsthand at Heat Treat Today’s Leak Detection Seminars. Click the image above to register for a session near you.

Leak test systems do not just live on balance sheets, they live in production lines. Their value is measured not just in cost, but in confidence:

  • Confidence that every part has been tested against specification
  • Confidence that defects are caught before they leave the factory
  • Confidence that customers can trust what you ship

That’s why a decision made only on CapEx is incomplete. A leak test system is a long-term partner in your production process. It is not just a one-time payment, it is what you pay and gain every day it runs.

Closing Thoughts

When considering new leak testing equipment, do not just ask, “What is the CapEx?” Ask instead:

  • What will it cost me to run?
  • What will it cost me if it fails to sort correctly?
  • What confidence does it provide in every product leaving my site?

Because in the end, the true price of a leak test system is not the invoice you pay at purchase. It is the trust it secures, or fails to secure, for years to come.

About The Author:

Norbert Palenstijn
U.S. Brand Manager
Nolek|VES|ALPHR|Natgraph

Norbert Palenstijn has built a career as a recognized specialist in helium and hydrogen leak detection, with over 26 years of dedicated experience in industrial vacuum systems, industrial leak testing and detection, and advanced engineering solutions.

For more information: Contact Norbert Palenstijn at norbert.palenstijn@nolek.com and norbert.palenstijn@vac-eng.com.

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Ask The Heat Treat Doctor®: Hot Topic for a Cold Day — Why Is Hot Gaseous Corrosion So Devastating?

/ ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, CARBURIZING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, 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 devastating effects of hot gaseous corrosion on furnace alloys: exploring the mechanisms behind metal dusting, the gas-solid reactions that drive catastrophic carburization, and the mitigation strategies to extend the life of heat treaters’ most valuable furnace components.

This informative piece was first released in Heat Treat Today’s January 2026 Annual Technologies To Watch print edition.

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

Corrosion is a concern experienced by everyone involved in manufacturing industrial products. While there is a plethora of data and information on the effects of corrosion on engineered materials available (sources provided in the references section of this column), most corrosion engineers are focused on aqueous corrosion. By contrast, heat treaters must understand the effects of hot gaseous corrosion, especially on our furnace alloys. Let’s learn more.

Corrosion Basics

It is important to understand that all materials are chemically unstable in some environments and corrosive attack will always occur. In the scientific world, it can often be modeled and its effects predicted by studying thermodynamic data and knowing which of the many corrosion-related chemical states are active. In our world, however, it is equally important to understand the various forms of corrosion, namely:

  • Dezincification (aka selective leaching)
  • Electrolytic
  • Erosion
  • Galvanic (or two metal) action
  • General (aka uniform) attack
  • Intergranular attack
  • Pitting
  • Stress corrosion

The greater the metal’s solubility, the greater the degree and severity of the corrosive attack. There are many important variations of these forms of corrosion; two of the most important are 1) localized corrosive attack (e.g. pits, intergranular attack, crevices) and 2) interaction with mechanical influences (e.g., stress, fatigue, fretting). These actions are frequently rapid and have catastrophic effects.

The number of ways to combat corrosion have been well-documented, including alloying to produce better corrosion resistance materials; cathodic protection (via sacrificial anodes); coatings (metallic or inorganic); organic coatings (e.g. paints); metal purification; alteration of the environment; and nonmetallic or design (i.e., physical) changes.

Heat Resistant Alloys

Furnace interiors contain numerous examples of heat-resistant nickel-chromium-iron (Ni-Cr-Fe) alloys, including radiant tubes, fans, heating elements, roller rails and rollers, thermocouple protection tubes, chain guides, and atmosphere inlet tubes, to name a few. Baskets, grids, and fixtures are other examples. These alloys are normally selected based on their strength (at temperature) rather than resistance to corrosive attack.

Since these heat-resistant alloy parts are often the most expensive furnace components, heat treaters must understand how they can be attacked and what can be done to extend their life by minimizing or preventing corrosion.

Gas-Solid Reactions

A chemical reaction involving a (non-equilibrium) gas or gas mixture and a solid is classified as a gas-solid reaction. Examples of intermediate and high temperature reactions of this type include oxidation, sulfidation, carburization, and nitriding. Effects of gases containing vapors of chlorine, fluorine, and effluents from deposits of various alkaline chemicals (from cleaning compounds) and even phosphates are also problematic. The principles are the same for all types — only the details differ. As heat treaters, our interest is in controlling, retarding, or suppressing these reactions to prevent unwanted corrosion, gasification, or embrittlement of the furnace alloy or materials being processed.

Examples of Catastrophic Carburization (a.k.a. Metal Dusting)

Figure 1. Pusher furnace alloy fan and shaft assembly | Image Credit: The Heat Treat Doctor®

Metal dusting (Figure 1) is a hot gaseous corrosion phenomenon in which a metallic component disintegrates into a dust of fine metal and metal oxide particles mixed with carbon.

Generally, metal dusting occurs in a localized area, and how rapidly the disintegration progresses is a function of temperature, the composition of the atmosphere and its carbon potential, and the material. Other significant factors include the geometry of the system, reaction kinetics, diffusivities of alloy components, the specific-volume ratio of new and old phases, and the ultimate plastic strain.

Metal dusting usually manifests itself as pits or grooves on the surface, or as an overall surface attack in which the metal can literally be eaten away in a matter of days, weeks, or months. As an example, this writer has seen a 330-alloy plate mounted underneath a refractory-lined inner door of an integral quench furnace (where atmosphere passes underneath the door and into the quench vestibule) reduced in thickness from 12.5 mm (0.50 in) to less than 0.75 mm (0.03 in) in a little over two months.

Figure 2. 330 alloy radiant tube removed after six months of use (rotary retort furnace) | Image Credit: The Heat Treat Doctor®
Figure 3. Microstructural view: catastrophic carburization | Image Credit: The Heat Treat Doctor®

In another example, a metallographic investigation performed by this writer on a failed wrought 330 alloy radiant tube (Figure 2) was conducted. Optical microscopy of the inside (Figure 3) and outside diameter surfaces in the attacked area revealed evidence of massive carbides. These carbides are formed by the reaction of carbon with chromium, depleting the matrix of chromium in regions adjacent to the carbides. Grain detachment and subsequent failure by erosion then occurred.

How Does It Occur?

In general, catastrophic carburization of ferrous alloys proceeds via the formation and subsequent disintegration of metastable carbide. The first step in the process is absorption of the gaseous phase on the surface of the metal; the more reactive this phase, the easier it decomposes or is catalytically decomposed (in the case of iron) on the surface. This step is followed by diffusion of carbon atoms from the surface into the bulk metal.

As a result, there is a continuous buildup of carbon within the surface layer. As this layer becomes saturated with carbon, a stable carbide, metastable carbide, or an active carbide complex forms, which then grows until it reaches a state of thermodynamic instability, at which point it rapidly breaks down into the metal plus free carbon.

It’s at this stage that the metal disintegrates to a powder as the result of plastic deformation and subsequent fracture in the near-surface layer. The process is controlled by internal stresses due to phase transformation; in other words, competition between stress generation and relaxation exceeds the ultimate strength in this near-surface layer and causes fracture to occur.

In Ni-Cr-Fe alloys, the phenomenon occurs slower (but does not stop) since the disintegration leads to larger metal particles, which are less active catalysts for carbon deposition than the fine iron particles that form with ferrous metals. Therefore, the mass gain from carbon depositing onto high-nickel alloys is much lower. Also, the decomposition of high-nickel alloys occurs by graphitization and not via unstable carbides.

Pourbaix-Ellingham Diagrams

Thermodynamics can be applied to solid-gas reactions to obtain equilibrium dissociation pressures below which no reactions occur. Data and diagrams are available for the free energies of formation versus temperature for most metallic compounds. An interesting use of Pourbaix diagrams (generally reserved for mapping out possible stable equilibrium phases of an aqueous electrochemical system) as a predictor of stable alloy systems is found by superimposing the various elemental constituents. These diagrams are read much like a standard phase diagram (with a different set of axes).

In Summary

Hot gaseous corrosion should be an area of focus for every heat treater to extend the life of alloy components, reduce downtime, and save money. Mitigation in the form of alloy selection, equipment design, type of atmosphere, process/cycle selection, and idling temperatures will play a huge role in extending the life of our furnace alloys, baskets, and fixtures.

References

ASM International. 1971. Oxidation of Metals and Alloys.

ASM International. 2003. ASM Handbook. Vols. 13A–C.

Fontana, Mars G., and Norbert D. Greene. 2008. Corrosion Engineering. New York: McGraw-Hill.

Herring, D. H. 2003. “What to Do About Metal Dusting.” Heat Treating Progress, August.

Herring, Daniel H. 2015. Atmosphere Heat Treatment. Vol. 2. Troy, MI: BNP Media Group.

Javaheradashti, Raza. 2008. Microbiologically Induced Corrosion. Berlin: Springer-Verlag.

NACE International. www.nace.org.

Nateson, K. 1980. Corrosion–Erosion Behavior in Metals. Warrendale, PA: Metallurgical Society of AIME.

National Bureau of Standards. 1978. Gas Corrosion of Metals.

Pourbaix, Marcel. 1974. Atlas of Chemical and Electrochemical Equilibria in Aqueous Solutions. Houston, TX: NACE International.

Pourbaix, Marcel. 1998. Atlas of Chemical and Electrochemical Equilibria in the Presence of a Gaseous Phase. Houston, TX: NACE International.

Schweitzer, Philip A. 1996. Corrosion Engineering Handbook. New York: Marcel Dekker.

Staehle, R. W. 1995. “Engineering with Advanced and New Materials.” Materials Science and Engineering A 198 (1–2): 245–56.

Stempco, Michael J. 2011. “The Ellingham Diagram: How to Use It in Heat-Treat-Process Atmosphere Troubleshooting.” Industrial Heating, April.

Uhlig, Hubert H. 2008. Corrosion and Corrosion Control. Hoboken, NJ: Wiley-Interscience.

Fabian, R., ed. 1993. Vacuum Technology: Practical Heat Treating and Brazing. Materials Park, OH: ASM International.

The Boeing Company. n.d. “Practical Vacuum Systems Design Course.”

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®: Hot Topic for a Cold Day — Why Is Hot Gaseous Corrosion So Devastating? Read More »

IN 718 Part 1: History, Applications, and Production

/ ENERGY HEAT TREAT TECHNICAL CONTENT, FOUNDRY CASTING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, VACUUM FURNACES TECHNICAL CONTENT

Today’s Technical Tuesday highlights this first installment in a multi-part series by Nikolai Alexander and The Heat Treat Doctor® Daniel H. Herring, which introduces Inconel® Alloy 718, one of the most widely used nickel-based superalloys, tracing its history, applications, and production fundamentals. Understanding why this alloy performs so well in extreme environments is critical as manufacturers consider material choices available for demanding components, especially alloys more typically sourced outside of one’s own industry. As demanding performance capabilities are being required of new engineered solutions, selecting the right alloy becomes a strategic decision to meet the need for higher temperatures, pressures, and corrosive environments.

This informative piece is from Heat Treat Today’s February 2026 Annual Air & Atmosphere Heat Treating print edition.

History

Inconel® Alloy 718 (IN 718) is a nickel-iron base superalloy known for its exceptional strength, resistance to high temperatures and ability to withstand harsh environments, where oxidation, creep, and corrosion resistance are paramount. The alloy was created by Dr. Herbert L. Eiselstein, who began his research in 1958, culminating in a patent assigned to The International Nickel Company in 1962 (U.S. Patent No. 3,046,108). In the many years since its creation, IN 718 remains the most widely used of all superalloys due to its availability in both wrought and cast products with high strength and stress-rupture life up to 650°C (1200°F), good hot working characteristics, castability, weldability, and cost effectiveness — all in an alloy with nominally 18% iron! The alloy’s superior performance is due in large part to its unique strengthening mechanisms.

There are different classifications of a superalloy, all based around the predominant metal present in the alloy. These categories include (Akca and Gursel 2015):

  • Nickel-based
  • Iron-based
  • Cobalt-based

The microstructural design makes IN 718 one of the best alloys for service applications below 650°C (1200°F) (Loria 1988, Herring 2011). It is widely used in extreme environments where components are subjected to high temperature, pressure, and/or mechanical loads. When heated, IN 718 forms a thick, stable, passivating oxide layer that protects the surface from further attack.

The alloy retains strength over a wide temperature range, making it attractive for high-temperature applications where materials like aluminum and steel would fail due to creep caused by thermally induced crystal vacancies. Inconel’s high-temperature strength is developed through heat treatment by solutionizing and precipitation hardening.

IN 718 is an alloy used around the world, but you might know it better by one of a variety of trade names (see sidebar).

The alloy has been modified numerous times to extend its operating temperature and service life. The alloy is readily available in all of these modified variations, each having slight differences in chemistry, cast and wrought processing methods, and heat treatments.

Applications

There is a wide variety of IN 718 applications across many industries, including aerospace, nuclear, oil and gas, automotive, motorsport, chemical processing, non-nuclear power generation, medical, tooling and molds, and fire protection systems.

In the automotive and motorsport industry, IN 718 is used for turbocharger rotors, exhaust manifolds, and valve springs in high-performance engines, such as those found in Formula 1 or the 24 Hours of Le Mans race cars. Naval warships are also purported to use IN 718 for components in their nuclear reactors (Table A).

Table A. Possible Uses of IN 718 in Naval Warship Nuclear Reactors
Table B. Oil & Gas Industry Use Examples for IN 718
Figure 1. A “Christmas tree”: the complex assembly of valves, gauges, and controls installed at the surface of a completed oil or gas well which has the primary function of regulating and controlling the flow of oil from the well. | Image Credit: Croft Systems

Perhaps surprisingly, IN 718 is also widely used in the oil and gas industry, which in addition to its many other benefits has remarkable resistance to sulfide and chlorine stress corrosion cracking at both high and low temperatures (Table B). Stress corrosion cracking is a failure mechanism that is caused by a combination of environment, a susceptible material, and the presence of tensile stress. Oil and gas applications like downhole tools, wellhead components, and subsea equipment benefit from IN 718’s other valuable properties as well, some of which include:

  • High strength and toughness at temperatures up to 700°C (1290°F)
  • Excellent resistance to pitting, crevice corrosion, and stress corrosion cracking
  • Sustained strength in hydrogen sulfide (H2S) and CO2-rich environments
  • Good weldability and fabrication

Continuous innovations in processing and material chemistry have enhanced superalloy properties resulting in the extension of its use into other industries, such as the energy and more conventional transportation sectors (Loria 1988).

Production Methods

IN 718 is available in cast and wrought alloy form and follows a stringent production process (Figure 2). Basic melt practices are used, such as vacuum induction melting (VIM), vacuum arc remelting (VAR), and electro-slag remelting (ESR).

Figure 2. Flow diagram of processes widely used to produce superalloys (Data reference: Akca and Gursel 2015)

VIM

The VIM process produces liquid metal under vacuum in an induction-heated crucible. It is used as a primary melting step in the route to producing wrought and cast products. Before being melted, the raw material can be refined and purified, and its composition can be controlled. VIM has been widely used in the manufacture of all types of superalloys, which must be melted under vacuum or in an inert gas atmosphere because of their reactivity with atmospheric oxygen and nitrogen.

VAR

The VAR process, a secondary melting technique, converts VIM-processed electrodes into ingots whose chemical and physical homogeneity have been significantly improved. In this process, a stub is welded to one end of an electrode, which is then suspended over a water-cooled copper crucible. Next, an arc is struck between the end of the electrode and the crucible bottom. Maintaining the arc generates the heat required to melt the electrode, which drips into the crucible and can subsequently be poured into molds. Many inclusions can be removed by flotation or chemical and physical processes before the molten material solidifies.

ESR

The ESR process, another secondary melting technique, is similar to the VAR process, but with notable differences. Remelting does not occur by striking an arc under vacuum. Instead, an ingot is built up in a water-cooled mold by melting a consumable electrode that is immersed in a slag, which is superheated by means of resistance heating. Rather than operating in a vacuum, the process is conducted in air under the molten slag. During melting, metal droplets fall through the molten slag, and chemical reactions reduce sulfur and nonmetallic inclusions. Both ESR and VAR processes allow directional solidification of an ingot from bottom to top, yielding high density and homogeneity in its macrostructure, as well as an absence of segregation and shrinkage cavities.

Casting Methods

IN 718 can also be produced by several casting methods. The most common of these are investment casting and (vacuum) die casting:

  • Investment casting: This process involves creating a wax pattern, coating it with a ceramic shell, melting out the wax, and then pouring molten IN 718 into the ceramic mold.
  • Vacuum die casting: This method uses a vacuum to fill the mold, resulting in a refined grain structure, minimal porosity, and good dimensional reproducibility, making it suitable for components like airfoils.
  • Sand casting: This method is far less common due to its inherent limitations in precision and surface finish, but the technology has been used for large castings.

A Metallurgical Perspective: The Role of Gamma Prime and Double Prime

IN 718 is a precipitation hardening superalloy. Its principle strengthening phases are gamma prime (γ′) or Ni3Al and gamma double prime (γ″) or Ni3Nb. The relationship between these precipitates (and others) and the gamma (γ) nickel matrix is critically important. For example, the coherency strain (i.e., the elastic deformation that occurs between two phases when their lattice structures do not perfectly match) is due to the fact that γ′ is face-centered cubic and γ″ is body centered tetragonal. In the case of IN 718, these strengthening effects are influenced more by γ″ than γ′ (ASM International 2016, Lee et al. 2023).

In addition, IN 718 has a natural tendency to precipitate rapidly by homogeneous nucleation in the noncompressible γ matrix. Depending on chemistry, γ′ volume percentage can vary over a wide range (3%–65%). Practically speaking, creep strength is proportional to volume percent over this range at temperatures between 700–980°C (1290–1800°F). As a result, the ratio of titanium to niobium/aluminum is key to hardening. High ratios imparted by niobium assure high strength at intermediate service temperatures around 600°C (1110°F). For higher service temperatures, higher aluminum content and molybdenum additions minimize the γ and γ′ mismatch, thus contributing to more stable alloys (Decker 2006, Guan et al. 2023).

Finally, the size and shape of these precipitates is important; larger precipitates enhance the strengthening effect. Key to the formation of these two precipitates is the aging treatment temperature, time, and alloy composition. According to existing research, higher aging treatment temperatures and longer times can lead to an increased amount of γ″ while extended aging coarsens the γ′ and γ″ particles, potentially leading to a reduction in strength and creep resistance. Furthermore, the composition ratios of Al, Ti, and Nb in the alloy influence the shapes of γ′ and γ″ precipitates, forming so-called co-precipitates that also affect the properties (Table C).

*SS = solid solution; + = enhancement; — = negative effect
Table C. Effect of Various Alloying Elements (Data Reference: Decker 2006)

The highest strength and hardness, coupled with reduced impact toughness, have been observed after heat treatment at 718°C (1325°F), due to an increase in the size and quantity of γ′ and γ″ precipitates.

In addition, as a result of surface analysis of Charpy bars, intergranular fracture occurs due to abundant small-sized precipitates formed within the boundary. In the case of the Charpy impact test, the absorbed energy decreases as the aging temperature increases. The formation of carbide, γ′ and γ″ precipitates can reduce the impact toughness of materials because precipitates may cause more obstacles to dislocation movement and promote crack initiation and propagation (Lee et al. 2023).

This article’s discussion continues in Heat Treat Today’s Annual Aerospace Heat Treat (March 2026) print edition to address heat treatment methods for this superalloy.

References

Akca, Enes, and Gursel, Ali. 2015. “A Review on Superalloys and IN718 Nickel-Based INCONEL Superalloy.” Periodicals of Engineering and Natural Sciences 3 (1): 15–27.

ASM International. 2016. ASM Handbook, Volume 4E: Heat Treating of Nonferrous Alloys. ASM International.

Babu, S. S., N. Raghavan, J. Raplee, S. J. Foster, C. Frederick, M. Haines, R. Dinwiddie, M. K. Kirka, A. Plotkowski, Y. Lee, and R. R. Dehoff. 2018. “Additive Manufacturing of Nickel Superalloys: Opportunities for Innovation and Challenges Related to Qualification.” The Minerals, Metals & Materials Society and ASM International: 3764–3780.

Bradley, Elihu F., ed. 1988. Superalloys: A Technical Guide. ASM International.

Chandler, Harry, ed. 1996. Heat Treater’s Guide: Practices and Procedures for Nonferrous Alloys. ASM International.

Croft Systems. n.d. “The Difference between a Wellhead & Christmas Tree.” https://www.croftsystems.net/oil-gas-blog/the-difference-between-a-wellhead-christmas-tree/

Decker, R. F. 2006. “The Evolution of Wrought Age-Hardenable Superalloy.” Journal of The Minerals, Metals & Materials Society, September: 32–36.

del Bosque, Antonio, Fernández-Arias, Pablo, and Vergara, Diego. 2025. “Advances in the Additive Manufacturing of Superalloys.” Journal of Manufacturing and Materials Processing 9 (215): 1–31.

Eliasen, K. M., T. L. Christiansen, and M. A. J. Somers. 2010. “Low-Temperature Gaseous Nitriding of Ni-Based Superalloys.” Surface Engineering 26 (4): 248–255.

Guan, Hao, Wenxiang Jiang, Junxia Lu, Yuefie Zhang, and Ze Zhang. 2023. “Precipitation of δ Phase in Inconel 718 Superalloy: The Role of Grain Boundary and Plastic Deformation.” Materials Today Communications 36 (August).

Herring, Daniel H. 2011. “Stress Corrosion Cracking.” Industrial Heating, October: 22–24.

Herring, Daniel H. 2012. Vacuum Heat Treating: Principles, Practices, Applications. BNP Media II, LLC.

Herring, Daniel H. 2019. “The Heat Treatment of Inconel 718.” Industrial Heating, June: 12–14.

Lee, Gang Ho, Ang Ho, Minha Park, Byoungkoo Kim, Jong Bae Jeon, Sanghoon Noh, and Byung Jun Kim. 2023. “Evaluation of Precipitation Phase and Mechanical Properties According to Aging Heat Treatment Temperature of Inconel 718.” Journal of Materials Research and Technology 27 (Nov–Dec): 4157–4168. https://doi.org/10.1016/j.jmrt.2023.10.196

Lee, Shin-Chin, Shih-Hsien Chang, Tzu-Piao Tang, Hsin-Hung Ho, and Jhewn-Kuang Chen. 2006. “Improvements in the Microstructure and Tensile Properties of Inconel 718 Superalloy by HIP Treatment.” Materials Transactions 47 (11): 2877–2881.

Loria, Edward A. 1988. “The Status and Prospects of Alloy 718.” Journal of Materials, July: 36–41.

Polasani, Ajay, and Vikram V. Dabhade. 2024. “Heat Treatments of Inconel 718 Nickel-Based Superalloy: A Review.” Metals and Materials International: 1204–1231.

Sharghi-Moshtaghin, Reza, Harold Kahn, Yindong Ge, Xiaoting Gu, Farrel J. Martin, Paul M. Natishan, Arrell J. Martin, Roy J. Rayne, Gary M. Michal, Frank Ernst, and Arthur H. Heuer. 2010. “Low-Temperature Carburization of the Ni-Base Superalloy IN718: Improvements in Surface Hardness and Crevice Corrosion Resistance.” Metallurgical and Materials Transactions A 41A (August): 2022–2032. https://doi.org/10.1007/s11661-010-0299-y

Shipley, Jim. 2023. “Hot Isostatic Pressing and AM: How to Improve Product Quality and Productivity for Critical Applications.” Metal AM 9 (3).

U.S. Patent No. 3,046,108.

Acknowledgments: This paper would not have been possible without discussions, guidance and contributions from a number of individuals in both the heat treat industry and academia.

Special Note: Inconel® is a registered trademark of Special Metals Corporation group of companies.

About the Authors:

Dan Herring
“The Heat Treat Doctor®”
The HERRING GROUP

Dan Herring, who is most well known as The Heat Treat Doctor®, has been in the industry for over 50 years. He spent the first 25 years in heat treating prior to launching his consulting business, The HERRING GROUP, in 1995. His vast experience in the field includes materials science, engineering, metallurgy, equipment design, process and application specialist, and new product research. He is the author of six books and over 700 technical articles.

Nikolai Alexander Hurley
Intern
The Heat Treat Doctor®

Nikolai Alexander Hurley is a young academic, interning with The Heat Treat Doctor®.

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

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Q&A: AI, MCP, and Heat Treat

/ CONTROLS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH

AI is moving from concept to practice in heat treating — driving furnace optimization, smarter scheduling, and predictive compliance. In this Q&A, Peter Sherwin, strategic marketing at Watlow, highlights how Model Context Protocol (MCP) will connect data, tools, and operators to reshape the industry’s digital future.

This informative piece was first released in Heat Treat Today’s October 2025 Ferrous & Nonferrous Heat Treatments/Mill Processing print edition.

Q1. What do we mean by “AI” in industrial heat treat?

It is probably best to start with a contrast. We have fixed code in heat treat applications, such as a setpoint programmer that is pre-programmed with ramps and soaks at specific temperatures for specific times. I like to think of AI (artificial intelligence) as introducing the concept of flexible code that learns from data over time.

AI has been used for a surprisingly long time in heat treatment. The original autotune algorithms used a form of AI and machine learning to adapt the PID parameters to a specific furnace, learning from real equipment process signals (such as temperature sensors) to provide optimum control.

Q2. Where is AI already working in heat treat?

AI is most obviously used in equipment optimization, and there are a growing number of cases expanding from process control to energy optimization. Less obvious uses are within the heat treating plants. For example, AI in contract review can highlight key customer requirements, pull together relevant specifications, and help craft recipe design or selection.

A common issue across plants is the need to continually optimize and re-optimize production planning and scheduling. Because heat treating occurs near the end of the manufacturing chain, last-minute changes are common. The ability to quickly re-plan based on specific requirements is a typical use of AI.

Following the process, quality analysis is now supported by AI with optical microscopy that leverages microstructural datasets. AI can also be used for financial analysis, recruitment, and customer support.

Q3. What is MCP?

Model Context Protocol (MCP) is a structured method for AI applications and agents to securely discover data, call tools, and share context. Developed by the engineering team at Anthropic in 2024, it has now received widespread adoption across major technology providers, such as Microsoft and OpenAI.

In simple terms, it enables large language models (LLMs) to communicate reliably with other data sources.

Q4. What MCP adoption is happening today?

It is still early, but MCP adoption is accelerating rapidly. Most software companies are developing MCP servers. Many B2C applications already exist, and there are now a growing number of industrial applications, such as those from Highbyte, Flow Software, and Siemens.

Q5. What will “MCP-compliant” mean for AI developers?

From a developer’s perspective, this should be easier than crafting individual application programming interfaces (APIs) that require strict mapping between software products. Any changes on the other end of the system would normally require the API to be restructured. MCP is expected to support inheriting updates without code changes and provide a more uniform setup.

Figure 1a. MCP Standard screen capture of how to use the tool. (Screen capture from the “Architecture overview” page of modelcontextprotocol.io.)
Figure 1b. Toggle to the “Tool Call Response” to view the response for that example input request. (Screen capture from the “Architecture overview” page of modelcontextprotocol.io.)

Q6. How would MCP specifically benefit heat treat?

In the last 30 years, I have seen three waves of technology. The first wave was automation that leveraged PLCs, setpoint programmers, and carbon probes to reduce manual errors and improve utilization.

The second wave focused on regulations in aerospace (AMS2750) and automotive (CQI-9) to harmonize auditing processes, improve quality, and reduce in-use failures (reducing recalls). These regulations focused on ensuring ongoing equipment capability (such as TUS for furnaces and ovens), instrumentation and quality thermocouples via SATs, independent calibration, and operator procedures and training.

The last wave focused on Industry 4.0 and IIoT to further automate and optimize previous improvements. However, apart from some isolated cases, many Industry 4.0 solutions have not delivered the expected value. There are many potential reasons, but one standout is the focus on continued machine automation at the expense of human intervention.

The benefit of MCP is that it acts as a bridge between data and the people who need to use that data to improve processes.

Q7. What are the biggest adoption barriers (and how to reduce them)?

I am typically an early adopter of technology. I was asked to automate a manual sealed quench furnace (batch integral quench) to automatic setpoint and carbon control in the early 1990s, which was one of my first projects. I began exploring technology solutions for Industry 4.0 and IIoT back in 2013. There will always be both early adopters and laggards.

Sometimes it makes sense to wait until technology matures and becomes more reliable, but this feels different. For the first time, data will build upon data, and learning early from that data will put companies ahead.

Cybersecurity and IT policies will scrutinize any new technology. One opportunity for AI is to also strengthen cybersecurity robustness. I recently heard that if you do not respond to a technology breach within 30 minutes, you will lose significant data. Human intervention alone will not be fast enough. AI is truly a double-edged sword.

There is also a growing fear that AI will take jobs. This has been demonstrated in the software industry, where it is estimated that 30 percent of code is now written by AI. I do not believe a heat treater can reduce staff further, since most are already operating with skeleton crews. The real opportunity is to enable all individuals to accomplish more, supported by AI.

The final point is when to adopt this technology. The pace of improvement over the past two years has been tremendous, and we are only now reaching the point where new models are robust enough for industrial application.

Q8. Pace of change: start now or wait?

The base LLMs needed time to improve and become more reliable while reducing hallucinations. Each version of ChatGPT has made significant leaps in knowledge and robustness. The latest model, GPT-5, is beginning to provide the level of reliability needed for industrial applications; this progress will continue.

Q9. What AI-powered products or services will emerge with MCP?

We can do a bit of future gazing. I compiled several ideas as part of my preparation for my presentation at ASM Heat Treat in October. In each example below, you will notice that a human remains in the loop. Instead of manually fetching specific data and information, the agent provides timely information.

EnergyOptimizerAgent — Subscribes to “Power/Furnace*/kW” tags and day-ahead tariff feeds. Models alternate start times and sends a proposal called “propose_shift” to a PlanningAgent. If planners accept, the new schedule is written back to the UNS so control logic and enterprise resource planning (ERP) software stay aligned.

ComplianceAgent — Monitors SAT and TUS counters published by the Edge Process Management (EPM) platform. When drift approaches a set threshold, it issues “propose_sat” with a suggested window and part list. After the test, AuditPackAgent gathers .uhh files and publishes a cryptographic hash so auditors can verify authenticity without manual file transfers.

UniformityMonitorAgent — Streams zone temperatures and compares each batch with stored “golden” fingerprints. If deviation grows, it assembles options, such as rerouting the load or adding a soak. Operators approve or reject through a dashboard.

MaintenanceSchedulerAgent — Reads valve-cycle counts, fan-vibration spectra, and motor current signatures. Calls a computerized maintenance management system (CMMS) tool to open a work order, reserve a slot, and order spare parts when limits are reached.

OperatorCopilotAgent — Listens to every proposal on the MCP bus and presents it in chat form. For example: “Shift Load B932 to 13:30 to avoid the peak tariff. Accept or ask why.” One tap reveals historian trends, specification clauses, and the agent’s reasoning trail, giving junior staff instant context while keeping humans in charge.

Q10. Any drawbacks or cautions with MCP?

AI and MCP will continue to be targets for cybercrime. It is important to architect any solution so that the base control and operation of equipment remain safe, even if the AI layer is breached.

At ASM Heat Treat, I will touch on some architectural solutions that can support safer AI implementations. As with anything internet-related, precautions must be taken. With AI, you also introduce the possibility of human-like imposters.

There is risk in everything we do, and everyone needs to continually assess risk versus reward. In many cases, MCP may tip the balance by providing more value than past technology solutions.

The responses in this article represent Peter Sherwin’s personal views and not necessarily those of his organization.

About The Author:

Peter Sherwin
Strategic Marketing
Watlow

Peter Sherwin is passionate about offering best-in-class solutions to the heat treatment industry. He is a chartered engineer and a recognized expert in heat treatment control and data solutions.

For more information: Contact Peter Sherwin at peter.sherwin@watlow.com.

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CO2-Neutral Heat Generation Technology Progress

/ ANNEALING TECHNICAL CONTENT, ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, FOUNDRY CASTING TECHNICAL CONTENT, HARDENING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH

A new study from the Umweltbundesamt (the Federal Environment Agency in Germany) outlines a clear, technically grounded pathway for achieving CO2-neutral process heat across energy-intensive industries. This Technical Tuesday installment highlights the study’s key findings, offering North American heat treaters a concise look at the technical feasibility, economic pressures, and strategic choice involved in moving beyond fossil-fuel-based thermal processing.

This informative piece was first released in Heat Treat Today’s January 2026 Annual Technologies To Watch print edition.

Introduction

Figure 1. Metal Industry – distribution of total annual energy consumption by energy source | Image Credit: Schwotzer
Figure 2. Mineral industry – distribution of total annual energy consumption by energy source | Image Credit: Schwotzer
Table A. Overview Examined Dectors, Associated Reference Technologies, and Thermal Processing Systems | Image Credit: Schwotzer

Efforts to mitigate climate change are crucial, particularly in Germany where there is a significant amount of energy-intensive industry, to achieve ambitious climate targets while preserving jobs and international competitiveness. Currently, process heat generation is heavily dependent on the use of fossil fuels, especially natural gas, with a low utilization of renewable energies. Fossil energy sources dominate the metal industry, accounting for 87.3%, while electricity represents 10.8%, and hybrid heating systems make up 2.0%. The mineral industry shows an even stronger dependence, with fossil fuels accounting for 99.7%. These figures illustrate the challenges and potential for technological innovations to provide CO2-free process heat in these sectors.

Although some sectors are already either using technologies for CO2-neutral process heat supply or are planning to do so, there is no comprehensive overview of the technical possibilities for generating process heat in energy-intensive industries in the context of future economic framework conditions.

In this study, technologies for the CO2-neutral supply of process heat are considered from a technical, economic, and ecological perspective. The study was conducted for thirteen industries and thirty-four exemplary applications in the metals and minerals industries, as well as for the cross-cutting technology steam generation industry (Table A). For each application, alternative CO2-neutral technologies are examined for their technical feasibility, economic viability, and ecological impact. The focus is on the electrification of plant technology, the use of hydrogen, but also hybrid systems, and, in some cases, the use of biomass. From this comprehensive review of the current situation and the possible alternative technologies, findings and recommendations for implementation will be developed for industry, policymakers, and researchers to support the transformation to CO2-neutral process heat generation.

Study Method

Figure 3. Study approach | Image Credit: Schwotzer

The study is based on an industry and technology assessment of the state of the technology (Figure 3). The results from the metal and mineral industries and the cross-sectional technology of steam generation were analyzed and summarized in consultation with experts. The central process chains were examined for each sector and the most important processes in terms of energy were identified. Each process chain contains several processes in which specific thermal process plants (industrial furnaces) are used, which are grouped into plant types. Based on the selected processes and plant types, applications are defined for further consideration. A reference technology and two to four CO2-neutral alternative technologies (new technologies) are assigned to each application. Key figures such as specific energy requirements, process-related emissions, or investment costs are used for comparison.

Table B. Theses Summary of Study Results | Image Credit: Schwotzer

The central findings of the study are summarized in eleven theses on the transformation of process heat generation (Table B). In this article, Theses 1, 2, 6, and 9 are presented in detail, providing a broad overview of the essential findings. For a more in-depth examination of the theses, see the link to the original study.

The Plant Fleet of Industrial Furnaces is Heterogeneous

The metal and mineral industries are characterized by numerous small process plants (throughput of less than 20 tons per hour and plant capacity of less than 20 MW). At the same time, there are large facilities with significantly higher throughput and corresponding higher plant capacities. Figure 4 shows a selection of technical examples from the study. Examples of large plants include heating and annealing furnaces in the steel industry with capacities of up to 170 tons per hour or cathode shaft furnaces in the copper industry with throughputs of up to 80 tons per hour. It is observed that the specific energy requirement of a plant correlates with the process temperature. The higher the required temperature of a process, the higher the specific energy requirement.

Figure 4. Classification of the considered applications and reference technologies in the plant fleet in Germany based on characteristic parameters | Image Credit: Schwotzer

Additionally, the cross-sectional technology of steam generation was examined. The most up to date technology includes natural gas boilers or combined heat and power (CHP) systems. Industry-specific characteristics play a minor role in the selection of technology for achieving CO2 neutrality. The technical requirements for end applications are less different compared to industrial furnaces. This includes performance, throughput, pressure, and temperature.

A transition to CO2-neutral process heat generation encompasses various technical possibilities and obstacles, as well as investment costs and space requirements, depending on the industry and application. Accordingly, the necessary adaptation measures require a differentiated approach to the transition to CO2-neutral process heat generation. An effective strategy to achieve CO2 neutrality should take into account the unique characteristics of each industry’s production processes, as well as the specific challenges and opportunities they present.

Technical Transformation to CO2-Neutral Production is Feasible

Despite the wide variety of plants and specific challenges, the transition to CO2-neutral process heat generation is technically feasible by 2045. The solutions will vary depending on the industry and application, and the effort required to transition from currently used reference technologies to CO2-neutral alternatives varies significantly.

The heterogeneity of industrial furnaces has a significant impact on the feasibility of deploying CO2-neutral technology in the future. While electrification is already highly advanced and most up to date in applications such as the foundry industry, bulk forming, or melting of aluminium with induction furnaces, it shows comparatively low technological maturity in sectors like the lime and cement industry, which are associated with fundamental technical challenges; see Figure 5. This significant heterogeneity in the existing plant stock and terms of technology readiness level (TRL) (European Commission 2014) requires consideration in transformation strategies.

Figure 5. Technology readiness level (TRL) of the alternative technologies (summarized) | Image Credit: Schwotzer

Both hydrogen and electrification can have a significant impact, although further research and development are needed in many areas. Across applications, it is evident that electrification generally requires the construction of new facilities. Transitioning from natural gas-operated reference technology to hydrogen involves less technical effort in terms of plant technology and can be accomplished by retrofitting the burner technology. Additionally, using hydrogen requires local infrastructure (pipes, valves) and its impacts on process and product quality need to be tested. Industrial-scale facilities are not yet available, resulting in a TRL of < 5, according to the study. However, with ongoing research and development in many projects, the TRL for many applications is expected to rise quickly in the coming years.

Scaling all alternative technologies to an industrial level and testing them in operational deployments are crucial. Some technologies face significant technical barriers, such as the continuous heating in steel rolling mills. These processes and their plant technology are characterized by very high process temperatures and production capacities, requiring heating technologies with a high energy density, which are not possible with current most cutting-edge electrical heating technologies. The use of hydrogen also presents a particular technological challenge, especially in areas where solid fuels like coke are currently used, such as in shaft kilns for lime burning or in cupola furnaces of iron foundries. As a result, alternative, bio-based fuels are being considered for these applications.

However, for these fuels to be a viable option, they need to be produced in sufficient quantity and quality. On the other hand, CO2-neutral techniques for steam generation using hydrogen and for electrification are already available for industrial use today.

The continuation of this article will be released in Heat Treat Today’s Sustainable Heat Treating Technologies edition (May 2026) where electrification versus hydrogen and a frank reckoning with the cost of new investments will be examined.

References

European Commission. 2014. Annex G – Technology Readiness Levels (TRL). Extract from Part 19 – Commission Decision C(2014)4995, “Horizon 2020 – Work Programme 2014–2015. General Annexes.” Brussels: European Commission.

Fleiter, Tobias, et al. 2023. CO2-Neutrale Prozesswärmeerzeugung: Umbau des industriellen Anlagenparks im Rahmen der Energiewende. Dessau-Roßlau: German Environment Agency (Umweltbundesamt).

All results in this article derive from the study “CO2-neutral process heat generation” (German: „CO2-neutrale Prozesswärmeerzeugung – Umbau des industriellen Anlagenparks im Rahmen der Energiewende: Ermittlung des aktuellen SdT und des weiteren Handlungsbedarfs zum Einsatz strombasierter Prozesswärmeanlagen”). The authors of this article would like to thank everyone who contributed to the study, listed in the published study. The study and further documents are on the website of the Federal Environment Agency in Germany (Umweltbundesamt).

This editorial is published with permission from Heat Treat Today’s media partner heat processing, which published this article in March 2024.

About The Authors:


Dr. Christian Schwotzer
Department for Industrial Furnaces and Heat Engineering
RWTH Aachen University, Germany
schwotzer@iob.rwth-aachen.de

Katharina Rothhöft, M.Sc.
Department for Industrial Furnaces and Heat Engineering
RWTH Aachen University, Germany
rothhoeft@iob.rwth-aachen.de

Dr. Tobias Fleiter
Fraunhofer Institute for Systems and Innovation Research
Karlsruhe, Germany
tobias.fleiter@isi.fraunhofer.de

Dr. Matthias Rehfeldt
Fraunhofer Institute for Systems and Innovation Research
Karlsruhe, Germany
matthias.rehfeldt@isi.fraunhofer.de

Dr. Fabian Jäger-Gildemeister
Federal Environment Agency of Germany (Umweltbundesamt)
Dessau-Roßlau, Germany
fabian.jaeger-gildemeister@uba.de

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