Manufacturing Heat Treat Technical Content Archives

5 Heat Treating Pitfalls — And How To Avoid Them

Have you faced complications from inadequate quenching, tempering, or documentation? You’re not alone. Small oversights can compromise part quality and performance. In this Technical Tuesday installment Ryan Van Dyke, metallurgical engineering manager at Paulo, addresses the top five pitfalls that in-house heat treating operations encounter and when to find another solution.

This informative piece was first released in Heat Treat Today’s July 2025, Heat Treat Super Brands print edition.

When dealing with high-volume production, running an in-house heat treating operation may seem like it makes financial and logistical sense. The ability to immediately process large batches of the same parts, minimize handling time, and tightly integrate heat treatment into the manufacturing workflow can provide critical advantages over outsourcing.

Industries involving high-volume machining of parts (e.g., automotive fasteners and bearings) rely on heat treating in-house to maintain efficiency and cost control. When parts are produced in the millions, outsourcing heat treating risks working with an inadequate supplier, introducing unacceptable lead time delays, transportation risks, and logistical complexities that do not align with high-throughput manufacturing.

Conversely, in-house heat treat operations often lack the flexibility, specialized equipment, and process control systems that commercial heat treaters develop over years of refining best practices. I have worked with countless manufacturers with in-house heat treat who have faced challenges they were unable to solve internally — from unpredictable distortion to process inconsistency, failed audits, and more. When they turn to a commercial heat treater for help, we often find the same core issues at play.

While commercial heat treating is not always the best fit for high-volume operations, there are real risks if you choose to run heat treating in-house. Here are the five most common pitfalls I’ve seen.

Pitfall #1: Inconsistent Mechanical Properties

Understanding the Problem

Gas flow gauges for heat treating furnace

Heat treating sets the foundation for a part’s hardness, toughness, and overall performance. This is done by the controlled heating and cooling of materials in a special atmosphere and then locking in the desired microstructure.

One major challenge that impacts consistency in parts is furnace temperature uniformity. Older or improperly calibrated furnaces can create hot and cold spots, leading to localized variations in hardness and mechanical properties within the same batch. This is a common challenge in-house heat treaters face. To avoid hot spots, heat treaters must go beyond just considering equipment age — they should implement robust preventative maintenance programs and routinely calibrate furnaces to ensure consistent thermal performance across all zones.

Real-World Consequences

Distortion issues from non-uniform heating: Variations in temperature cause inconsistent thermal profiles, leading to unpredictable warping and dimensional instability. For example, a die used for stamping operations requires excessive rework after heat treatment because some areas of the part distorted unevenly due to poor furnace temperature uniformity.

Inconsistent hardness in a load: Hot and cold spots in austenitizing and tempering furnaces can cause parts in some areas to have a different final hardness than others. For example, a load of larger diameter structural bolts was tempered in a furnace with poor uniformity. Bolts located in a hot spot in one corner of the furnace showed below specification mid-radius hardness due to over-tempering.

Pitfall #2: Surface Contamination from Incorrect Gas Atmosphere Control

Understanding the Problem

Many manufacturers with in-house heat treating operations use gas atmospheres to control oxidation and facilitate processes like carburizing and nitriding. However, if the gas atmosphere is not properly monitored, it can lead to oxidation, decarburization, or uncontrolled case hardening.

Heat treaters often rely on Endothermic gas generators that produce a carbon-rich atmosphere. Without precise control of carbon potential, parts may develop non-uniform case depths, excessive soot buildup, or — the opposite extreme — decarburization, in which the surface loses carbon and thus its strength and hardness. Therefore, it’s imperative to monitor and adjust atmosphere parameters in real time using carbon probes to maintain precise control of carbon potential.

Real-World Consequences

Decarburization leading to soft surfaces: If the furnace atmosphere lacks sufficient carbon potential, the steel loses carbon at the surface, reducing hardness and durability. For example, aerospace landing gear components could be rejected if surface hardness tests show excessive decarburization, making them unsuitable for service.

Scaling and oxidation issues: Excess oxygen in the furnace leads to surface oxidation, requiring costly post-processing like machining or pickling. For example, stainless steel medical implants can develop scale during heat treatment, requiring extensive rework to restore a clean finish.

Uneven carburizing creating case depth variations: Fluctuations in furnace gas composition lead to inconsistent carbon diffusion, making case depth unpredictable. For example, a batch of industrial gears can fail inspection because some parts have insu cient case depth while others are over-cased, leading to production delays.

Pitfall #3: Suboptimal Quenching Causing Distortion & Residual Stresses

Understanding the Problem

Quenching is one of the most stress-inducing steps in heat treatment. Rapid cooling causes phase transformations and volume changes within the steel, leading to internal stresses and distortion.

Manufacturers with in-house heat treaters often struggle with choosing the right quench medium, optimizing agitation rates, and positioning parts correctly during quenching. Additionally, many only have access to one quench medium, such as oil, and will attempt to apply it to all materials and geometries — even when a slower or faster quench rate is required. This mismatch can cause excessive distortion, high residual stresses, and even quench cracking.

Another issue is poor part orientation during quenching. If a part is improperly positioned, different areas will cool at different rates, creating non-uniform hardness and residual stress buildup, which can later cause warping or failure in service.

Real-World Consequences

Incorrect quenchant selection: If the wrong quench medium is used, such as oil when polymer or water would be more suitable, the parts could end up having inconsistent hardness in various sections due to insufficient cooling. Conversely, selecting a fast oil as a quenchant when hot oil would be more suitable could cause excessive distortion due to the faster cooling rate. For example, lifting shackles quenched in oil will not have sufficient hardening response throughout the cross-section, causing them to be rejected for service due to low strength values in the center of the part.

Insufficient quenchant agitation: If the quenchant in the quench tank is not sufficiently agitated when the parts are submerged, then cooling rates throughout the load of parts could vary, causing different amounts of hardening. For example, parts near the edges of a batch load show hardness testing within specification, while parts in the center of the load show hardness below specification.

Incorrect positioning of parts: How a part is oriented during quenching can have a large impact on the amount of distortion after heat treatment. If a part is laid horizontally rather than vertically, the amount of distortion can dramatically increase. For example, if a hollow cylinder was laid horizontally for processing, rather than vertically, the cylinder would likely be at risk of material creep during austenization, as well as deformation from the bottom of the part quenching before the top. The result would be distortion in the inner diameter and along the length in excess of the amount of additional material le for machining, causing the part to become scrap.

Pitfall #4: Brittle Failures from Inadequate Tempering

Understanding the Problem

Tempering is a critical post-quench process that reduces residual stresses and brittleness while fine-tuning hardness and toughness. After quenching, steel is in a highly stressed martensitic state, which, if left untreated, can lead to catastrophic failures in service.

If heat treaters are working under tight production schedules or have an incomplete understanding of tempering curves for different steels, then they may fall into the trap of rushing or even omitting tempering cycles. For some in-house heat treat operations, a single tempering cycle may be employed when a double temper is required, particularly for high-alloy steels like D2, H13, or certain aerospace-grade alloys.

Real-World Consequences

Brittle fracture under load: If a part is left untempered or under-tempered, the high internal stresses from quenching remain, making it prone to sudden brittle fracture when subjected to impact or fatigue loading. For example, an induction-hardened gear used in heavy machinery can snap under torque loading due to excessive quench-induced stresses. It is very common to skip tempering on induction-hardened parts, especially in in-house heat treat operations where cycle times are minimized as much as possible.

Reduced wear resistance due to over-tempering: If a steel is over-tempered (held at too high a temperature or for too long), excessive softening can occur, reducing wear resistance and surface hardness. For example, a die used in stamping operations can wear prematurely because it was tempered above its recommended range, leading to a loss of edge retention.

Excessive retained austenite leading to dimensional instability: Some steels, particularly high-carbon and high-alloy grades, require a secondary tempering cycle to stabilize the microstructure. Skipping this can leave excessive retained austenite, which converts to untempered martensite over time, causing unexpected distortion or possibly cracks forming in the material in service. For example, a precision-ground shaft can warp and develop cracks weeks after heat treatment because retained austenite transforms to untempered martensite in service, altering the part’s geometry and encouraging fractures to form.

Pitfall #5: Lack of Process Documentation & Repeatability Issues

Understanding the Problem

Heat treating is a process-sensitive operation where small variations can lead to major differences in final part properties. If a heat treat operation does not have detailed documentation and tracking systems, this will lead to inconsistencies in cycle parameters, atmosphere control, and quenching conditions.

One of the most common issues is manual adjustments without proper record-keeping, which can lead to process drift. Operators may tweak furnace temperatures, quench delays, or gas flow rates without logging the changes, creating batch-to-batch variability.

Additionally, compliance and traceability may present a challenge for manufacturers facing ISO, Nadcap, or AS9100 audits. When an auditor asks for process records, lacking verifiable data is a red flag for non-compliance.

Real-World Consequences

Batch-to-batch variability: When process parameters are not documented or followed precisely, parts in one batch may have different hardness, case depth, or dimensional stability than parts in the next batch — leading to field failures or quality escapes. For example, a manufacturer of automotive control arms may and that some components fail impact testing while others pass, leading to a full production hold to investigate process inconsistencies.

Failed audits and compliance issues: Without traceable process documentation, heat treat operations can fail compliance audits, especially for industries with strict quality requirements. For example, an aerospace supplier could lose Nadcap certification because they cannot provide accurate records of furnace temperature control, atmosphere composition, and quench parameters for critical landing gear components.

Difficulty troubleshooting heat treat issues: When a batch of parts fails post-heat treatment inspection, the root cause can be nearly impossible to determine if there are no detailed process records. For example, a fastener manufacturer might experience high rejection rates due to inconsistent case depths, but if the atmosphere carbon potential wasn’t recorded, they will not be able to pinpoint whether it was a gas mix issue, furnace drift, or soak time variance.

Expensive scrap and rework costs: A lack of process repeatability leads to high scrap rates and expensive rework to bring parts back into spec. For example, a tooling manufacturer might have to scrap an entire run of die components after discovering that an unrecorded furnace temperature deviation softened the steel below acceptable hardness levels.

Lack of lot traceability: When a heat treatment problem does occur, being able to trace it back to exactly which piece of equipment it ran in and when is critical for determining root cause. For example, many automotive seating brackets exhibit low hardness after heat treatment. However, if lot traceability to the furnace cycle was not maintained, root cause of factors like incorrect furnace temperature, inadequate carbon control, or insufficient quench agitation are much more difficult to identify.

When To Call a Commercial Heat Treater

If limited resources and/or lack of specialized expertise are in question, these five pitfalls can easily occur. Even the most well-run in-house heat treat operations must balance production efficiency, heat treat quality, and high-volume demands; additionally, it can be challenging to regularly invest in the most advanced equipment, process monitoring, or specialized personnel.

There are commercial heat treaters that have built their entire business around controlling these variables with precision. These heat treaters have invested decades into refining their heat treating processes, equipment, and metallurgical expertise to eliminate these issues before they ever become problems.

If these five pitfalls are ones your operations cannot easily avoid, consider a partnership with the right commercial heat treater to maintain parts with extreme precision, low distortion, and strict compliance specifications.

About The Author:

Ryan Van Dyke
Manager of Metallurgical Engineering
Paulo

Ryan Van Dyke is the manager of metallurgical engineering at Paulo, where he works closely with customers to solve challenging thermal processing issues. He’s dedicated to pushing the limits of heat treating performance, continuously innovating more efficient, reliable ways to process critical parts. Ryan was an honoree in Heat Treat Today’s 40 Under 40 Class of 2023.

For more information: Contact Ryan Van Dyke at RVanDyke@paulo.com.

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A Microalloyed Solution for High-Temp Applications

July 15, 2025 / ALLOY FABRICATIONS TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content

Alloy R&D has resulted in a material that combines the affordability of 310 stainless steel with the high temperature properties of more expensive higher nickel alloys, like alloy 600. Be it for your muffle belt conveyor or heat treating trays, this Technical Tuesday installment by Hugh Thompson, applications engineer of Rolled Alloys, will explore the strengths of this alloy variety to determine its best application.

This informative piece was first released in Heat Treat Today’s July 2025 Super Brands print edition.

Increasing nickel prices initiated the development of RA 253 MA®, a versatile alloy used in various thermal applications for equipment construction. With low chromium (Cr) and nickel (Ni) levels, this alloy provides a cost-effective alternative to other pricier nickel-based materials. With microalloying control, it is priced alongside 310 stainless steel while offering high strength properties similar to the more costly 600-series alloys.

Chemically similar to 309 stainless steel, the alloy offers significantly higher creep resistance and rupture strength than 310. Its benefits include:

Oxidation resistance up to 2000°F
(1090°C)
Significant hot tensile strength
comparable to that of the 600-series alloys
Noteworthy creep and rupture properties

This lean austenitic stainless steel uses cerium and silicon to create a very adhesive oxide, resulting in excellent oxidation resistance. The combination of nitrogen and carbon provides creep-rupture strength double that of 310 and 309 stainless steel at 1600°F (870°C).

Chemistry

RA 253 MA has a specified chemistry, as indicated in Table A.

High Temperature Properties

Figure 1 shows the hot tensile strengths of different materials. RA 253 MA can be seen to have higher hot tensile properties than alloy 600, 310 stainless, and RA330® but lower than RA 602 CA®. It’s worth noting that while its hot tensile strength is reported up to 2200°F (1200°C), practical use is limited to 2000°F (1090°C) in oxidizing environments due to a loss of oxidation resistance at this temperature.

Figure 2 displays the allowable design stresses for pressure vessel plates according to Section II-D of the ASME 2023 (2024 revision) code. One can see that the allowable stresses for RA 253 MA are higher than those for 310 stainless and RA330 but not as high as alloy 601. ASME allows design stresses for this alloy up to 1650°F (900°C). However, RA 253 MA is utilized at higher temperatures for various applications because this temperature limit is only for pressure vessels.

Figure 3 displays the actual 10,000-hour rupture strengths of different high temperature alloys. The data reveal that RA 253 MA exhibits high creep and rupture stress values comparable to alloy 601 and RA 602 CA, and it surpasses RA330; this would also surpass alloy 600.

In Figure 4, data are presented for the minimum creep rate of 0.0001% per hour. Creep refers to the rate at which metal stretches, and it is usually measured in percentage per hour. There is a phase where the creep rate remains relatively constant, known as the secondary creep rate. This rate is a key factor in designing for high temperatures. It’s important to consider that metal will creep even under light loads, as the effects of creep can be observed in material with no load other than its own weight. Therefore, in practical applications, a creep criterion is utilized for design purposes.

Figure 4. Minimum creep rate of 0.0001% per hour

The furnace industry has traditionally used a design criterion based on the stress required for a minimum creep rate of 1% in 10,000 hours or 0.0001% per hour. The design stress is typically set at a fraction of this value. For one of its criteria, ASME uses 100% of the extrapolated stress for 1% in 100,000 hours (or 0.00001% per hour). It is not recommended to extrapolate stress rupture and creep data to 100,000 hours above 1800°F (980°C). Th is comparison is provided for general guidance only.

Rupture strength is reported as a stress and number of hours. It is the stress required at a specific temperature to break a specimen within a given time. In the furnace industry, a standard criterion for setting design stresses is to use a fraction of the stress that would result in rupture at 10,000 hours. ASME uses the lower of 67% of the extrapolated 100,000 rupture stress or 100% of the extrapolated 1% in 100,000 hours minimum creep rate.

Strengths and Limitations

When compared to alloys like 309 and 310, RA 253 MA has demonstrated equal or superior oxidation resistance. At 2000°F (1090°C), it displays outstanding oxidation resistance, on par with the limit for 310 stainless steel and surpassing 309. It is important to note that although short furnace excursions up to 2100°F (1150°C) can be tolerated, consistent oxidizing temperatures above 2000°F (1090°C) can quickly degrade the material. Therefore, it is best to avoid excursions above the suggested temperature limits for any alloy.

This material has also proven to perform well in mildly carburizing environments, despite its lower alloy content. Even small amounts of oxygen in the gas, like carbon dioxide or steam, can create a thin and tough oxide layer on RA 253 MA, offering excellent protection against carbon and nitrogen pickup. However, it’s not recommended to use it in carburizing environments. Due to its lower nickel content, it is less resistant to carburization compared to higher nickel alloys such as RA330.

Table B. Ductility based on room-temperature tensile tests

In a simulation where coupons were exposed to fifteen weeks of simulated bake cycles between 1700°F–1950°F (930°C–1065°C) in “green mix” used for producing carbon electrodes, room-temperature tensile tests revealed the ductility as shown in Table B.

For RA 253 MA, the sigma phase formation process is much slower compared to 310S and 310, as shown in the TTT diagram in Figure 5 and the micrographs in Figure 6. At temperature, it is very unlikely material containing sigma phase will behave adversely. When the material is cooled to room temperature, it becomes very brittle, making it less resistant to thermal cycling. The material may crack if highly constrained and unable to expand freely during subsequent ramp-up.

Figure 5. TTT curve for sigma phase formation

Figure 6. RA 253 MA grain structures with and without sigma phase

Corrosion Resistance in Salt Bath Applications

As shown in Table C, RA 253 MA may be comparable to alloy 600 when exposed to sodium and potassium salts for heat treating high speed steel.

Table C. Intergranular attack based on exposure to sodium and potassium salts

In this trial, plate samples were exposed to 210–252 cycles in preheat salts at 1300°F–1500°F (700°C–820°C), high heat salt at 2200°F (1200°C), and then quenched in 1100°F (590°C) salt. Table C shows that RA 253 MA has the potential to perform well in a salt bath environment due to its high silicon and chromium levels. While alloy selection is essential, regular maintenance and cleaning of the salt bath and surrounding areas are the most crucial factors.

In salt bath heat treating, the service life of the pot is primarily determined by maintenance not the alloy. Pots must be desludged regularly, and all old, spilled salt must be removed from the furnace refractory when changing pots

Corrosion Resistance

Table D. Sulfidation attack after exposure to an atmosphere containing 13.6% SO2 at 1850°F (1010°C) for 1,860 hours

This alloy performs well, even in hot environments with sulfur in the presence of oxygen. However, it is not resistant to environments with reducing sulfur. Even in the presence of oxygen, the partial pressure of oxygen can be very low while stainless steel is in use. This low pressure can lead to a local sulfidation attack, even in what is considered an oxidizing atmosphere.

Table D displays the depth of intergranular oxidation and sulfidation in test samples exposed to an atmosphere containing 13.6% SO2 at 1850°F (1010°C) for 1,860 hours.

Microstructure

Table E. Charpy v-notch impact results as annealed and after exposure (ft-lb)

The microstructure of RA 253 MA in the annealed and long-term exposure states is shown in Figure 6. In addition, Table E provides the Charpy impact values for the annealed state and at temperatures of 1292°F, 1472°F, and 1652°F (700°C, 800°C, and 900°C) over a long period of exposure.

Based on the microstructure and Charpy impact data, it is clear that sigma phase precipitation is almost non-existent at 1650°F. Moreover, the TTT diagram in Figure 5 indicates that RA 253 MA requires significantly more time to initiate sigma precipitation compared to 310 and 310S stainless steel.

Applications for Use

Given the above capabilities, RA 253 MA can be and has been successfully utilized in a variety of applications. From bell annealing furnace covers, muffle belt conveyors, car exhaust manifolds and exhaust gas flexible tubes to hot air ducts, cooling tower tubes in sulfite process pulp mills, and heat treatment trays for neutral hardening, its abilities can cover a widescope of applications throughout in-house heat treat operations.

References

Andersson, T. and T. Odelstam. “Sandvik 253MA (UNS S30815) — The Problem Solver for High Temperature Applications.” A Sandvik Publication, October 1984.

Kelly, J. Rolled Alloys. Rolled Alloys Bulletin 100. Revised September 2001.

Kelly, J. Rolled Alloys. Rolled Alloys Bulletin 401, Heat Resistant Alloys©. Revised June 2006.

Manwell, C. Rolled Alloys. Rolled Alloys Internal Report, Summary of Cyclic Oxidation Testing at 2000°F, August 2005.

Proprietary Report on the MA Heat Resistant Material Series.

Saum, W. Rolled Alloys. Rolled Alloys Internal Report, Summary of Oxidation Testing at 2000°F, August 2002.

About The Author:

Hugh Thompson
Applications Engineer
Rolled Alloys

Hugh Thompson is a metallurgical engineer at Rolled Alloys, leveraging his expertise from The University of Toledo College of Engineering to drive innovation in specialty alloy solutions. Based in Toledo, he combines deep technical knowledge with industry leadership.

For more information: Contact Hugh Thompson at Hthompson@rolledalloys.com.

The content of this article was initially published by Industrial Heating. All content here presented is original from the author.

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How To Stay Cool This Summer: 3 Technical Cooling Resources for Heat Treat Improvement

July 1, 2025 / COOLING SYSTEMS TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content

We all know that cooling off the right way matters. Your friend may be hot, but dumping a bucket of ice on them just might cause your friendship to crack. The same applies to heat treating. The methods and modes of cooling operations can make or break our bank and equipment.

Heat Treat Today has coalesced technical information across articles and podcast episodes from key experts, including a case study comparing the efficiency of different cooling technologies, a Heat Treat Radio episode full of purchasing guidance and the updates on the latest technologies, and finally a helpful comparative of cooling systems for the automotive industry.

Discover more about these three topics in today’s Technical Tuesday original content feature.

Intelligent Cooling System Improves Operations for Alloy Manufacturer: A Case Study

There’s only one constant about technology: It’s always evolving — revealing new innovations and opportunities. And as these new technologies come to light, heat treating operations have new opportunities to reduce cost, increase efficiency, and ensure consistent, optimized part quality, regardless of the job parameters. With the introduction of new process cooling technologies to the heat treating market, previously unexplored systems become viable solutions for unanswered operating challenges. Gary Burdardt, market development manager with Frigel North America, authored a case study to explore new technologies in cooling operations.

“Located on the East Coast, the manufacturer needed to find an alternative process cooling solution for its vacuum furnace cooling operation. It had been using air-cooled chillers, but the costs of continuous operation were too high. Operating as a batch furnace, the heat load of this particular application was specified to be approximately 200 tons, and process cooling water temperature, which was specified at 70°F, presented a significant challenge.”

Read the full article here: Intelligent Cooling System Improves Operations for Alloy Manufacturer: A Case Study

Heat Treat Radio #100: Cooling Off the Heat (Treat)!

Keeping your heat treat equipment cool is as critical as it is an oxymoron. If you have old cooling systems or are looking to purchase new ones, hear from Matt Reed, director of Sales and Technologies at Dry Coolers, as he shares purchasing considerations, maintenance, and latest technologies with Heat Treat Radio host, Doug Glenn. Learn about the importance of flow, sediment build up, hot spots, and more!

“Vacuum furnaces, around the 1960s and 1970s, when they were being developed, focused on heat treating materials. Cooling is required because you’ve got these inner walled jackets in the furnace, jackets in the heads, you’ve got diffusion pumps, mechanical pumps — all these ancillary pieces of equipment that require cooling. Originally, you could use city water and flow city water right through the furnace. Customers soon find out that that’s a lot of water consumption, so the next step was to look at an evaporative cooling tower. You start recirculating evaporative cooling tower water directly through the furnaces.”

Read the full article here: Heat Treat Radio #100: Cooling Off the Heat (Treat)!

Choosing the Right Cooling System

Deciding on a process cooling system for your automotive heat treat requires intentional consideration. In this article Bob Smith, director of product management at Thermal Care, offers practical and valuable guidance on three options: fluid coolers, cooling towers, and chillers.

“When considering which type of process fluid cooling system is best for your automotive heat treat application, it is important to determine the process fluid medium, desired temperature, and the significance of operating cost versus initial investment. There are often multiple solutions to a process cooling application, and the following is intended to provide a basic outline of the types of systems available and where they are best used.”

Read the full article here: Choosing the Right Cooling System

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Ask The Heat Treat Doctor®: What Are the Differences Between Intergranular Oxidation (IGO) and Intergranular Attack (IGA)?

June 24, 2025 / HARDENING TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content

The Heat Treat Doctor® has returned to offer sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues.

Today’s Technical Tuesday is a pre-release foretaste of the great content you can find in Heat Treat Today’s July 2025 Super Brands print edition.

Heat treaters and metallurgists speak a language unique to our industry and it can be confusing at times; terms like intergranular oxidation (IGO) and intergranular attack (IGA) are good examples, as these terms are often (incorrectly) used interchangeably. While these two phenomena sound similar, they have distinct mechanisms, causes, and impacts on material properties. Expert Dan Herring explores them more below.

What is IGO?

IGO is a process by which oxygen preferentially reacts with the metal surface at the grain boundaries, creating oxides (Figure 1). It is not uncommon, for example, to see IGO present after a hardening process run in an Endothermic or nitrogen/methanol atmosphere. As parts are heated to austenitizing temperature, oxygen present due to minute air leaks or produced during the various chemical reactions with the atmosphere results in IGO. The grain boundaries are highly susceptible to oxidation because these are areas where different crystallographic grains meet and are areas of high energy due to the atomic mismatch and disruption of the regular crystal lattice structure. 

Figure 1. Intergranular oxidation (IGO) along the surface of a heat treated chromium steel component. 1000X – As Polished
Source: The HERRING GROUP, Inc.

What is IGA?

Figure 2. Intergranular attack (IGA) along the surface of a martensitic steel component caused by excessive submersion time in a citric acid solution. 1000X – As Polished
Source: The HERRING GROUP, Inc.

IGA, on the other hand, is a broader term that refers to a corrosion phenomenon (aka chemical attack) that specifically targets the grain boundaries of a material. Unlike IGO, intergranular attack (Figure 2) is not limited to oxidation reactions but encompasses a variety of forms of attack involving such things as the formation of precipitates, the dissolution of material at grain boundaries, or the creation of corrosion cracks. Common forms of IGA include stress corrosion cracking (SCC) or sensitization in stainless steel.

In stainless steels, IGA is often triggered by high-temperature environments, usually in the range of 840º – 1560ºF (450 – 850°C) where carbon reacts with chromium to form chromium carbides at the grain boundaries, thus reducing the material’s resistance to corrosion in localized regions. In other alloys, factors like pH, chloride concentration, and temperature can lead to IGA.

Both IGO and IGA weaken the material’s structural integrity or lead to embrittlement compromising the material’s integrity.

Effect on Material Properties

The main effect of intergranular oxidation is the degradation of the mechanical properties, particularly a reduction in both ductility and toughness. As oxidation progresses along the grain boundaries, the material tends to become brittle, which can lead to premature failure under certain types of stress or thermal cycling. IGO often appears visually as a uniform discoloration or thin oxide layer on the surface. Surface pitting is not typically observed.

By contrast, IGA often appears as visible cracks, pits, or localized regions where the metal has been attacked (along the grain boundaries). This leads to a reduction in mechanical strength and can lead to SCC under certain circumstances. IGA can severely compromise the integrity of the material, particularly in critical applications like pipelines, pressure vessels, and nuclear reactors.

Materials Involved

IGO is most commonly observed in steel, aluminum, titanium, and nickel-based alloys, not only during heat treatment but when exposed to oxidizing environments in high-temperature applications, which also result in degradation and loss of material strength and other properties.

IGA tends to be more prevalent in stainless steels, corrosion-resistant alloys, and aluminum alloys. It is especially noticeable in alloys that are susceptible to sensitization (where chromium carbides precipitate at grain boundaries), leading to localized corrosion and cracks. Alloys that form a passivating oxide layer can be more susceptible to IGA if that layer is disrupted.

Principal Concerns

The main concern with intergranular oxidation is material embrittlement, leading to reduced ductility and potential failure under mechanical stress, especially in high-temperature applications. It can also affect the integrity of critical components, such as those used in aerospace or power generation industries.

By contrast, the primary impact of intergranular attack is loss of material strength, leading to structural failure, often without any clear outward signs (e.g., under chloride-induced SCC). It is more likely to cause immediate failure or a dramatic loss in performance, especially in structures exposed to corrosive environments.

How to Detect IGO and IGA

IGO is typically detected by examining the material’s surface using optical or scanning electron microscopy (SEM). Non-destructive techniques, such as X-ray diffraction (XRD), can also be used.

IGA is usually detected through methods like microstructural examination, electrochemical testing, or failure analysis. Techniques, such as SEM or energy-dispersive X-ray spectroscopy (EDS), can be used to examine the grain boundary regions for signs of corrosion.

How to Avoid IGO and IGA

IGO can be avoided by one or more of the following:

Environmental control: Making sure the heat treat furnace has no leaks, reducing oxygen partial pressure or controlling the furnace atmosphere in high-temperature heat treat operations.
Alloy design: The use of materials with stable oxide-forming elements (e.g., chromium, titanium and aluminum) or alloys with high resistance to oxidation (e.g., nickel-based superalloys).
Temperature control: Maintaining lower process temperatures and shorter times where possible to prevent oxidation at the grain boundaries.
Coatings and surface treatments: Application of protective coatings, such as copper plating, post-heat treatment aluminizing, or chrome plating, to reduce oxygen interaction with the grain boundaries during service.

IGA can be avoided by one or more of the following:

Environmental control: Reducing exposure to aggressive chemicals (e.g., chloride ions) by maintaining proper pH levels or using inhibitors in post-cleaning processes.
Proper alloy selection: Selecting materials resistant to intergranular corrosion (e.g., low carbon “L” grades of stainless steel or alloys with improved grain boundary stability).
Heat treatment: Avoiding sensitization of stainless steel by proper heat treatment methods that prevent the formation of chromium carbides at grain boundaries.
Stress relief: Reducing the likelihood of stress corrosion cracking by managing internal stresses during manufacturing and in-service conditions.

Key Differences

The differences between these phenomena are summarized in Table 1.

Summing Up

While both IGO and IGA involve attack at the grain boundaries, they differ in their mechanisms, causes, and effects. From a heat treater’s perspective, IGO most often results at high temperature in oxygen-bearing furnace atmospheres, while IGA often results from pre- or post-heat treatment processing (cleaning, passivation, plating, etc.). Proper material selection, furnace and environmental control, awareness of what can happen, and inspection for these effects are key to preventing them from occurring.

References

Roberge, Pierre R., Corrosion Engineering: Principles and Practice, Mc-Graw Hill LLC, 2008.

Stene, Einar S., Fundamentals of Corrosion: Mechanisms, Causes, and Monitoring.

Schweitzer, Philip A., Fundamentals of Corrosion: Mechanisms, Causes and Preventative Methods, CRC Press, 2009.

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 Herring 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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The Cost of Furnace Insulation Failure

June 23, 2025 / BURNERS & COMBUSTION SYSTEMS TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content, OP-ED

You see a little orange light coming from your furnace while it’s operating. What if that was a clue that you were losing over $7,000 annually on one furnace? In today’s Combustion Corner installment Jim Roberts, president of US Ignition, shares more details about the long term costs of furnace insulation failure.

This informative piece was first released in Heat Treat Today’s May 2025 Sustainable Heat Treat Technologies print edition.

A furnace guy walks into a bar and smells burning hair! A sure indication of wasted resources…

Normally, I would not concern myself, as a burner guy, with heat loss issues. But as a furnace guy, this is one of the biggest culprits when it comes to running an energy-efficient operation. Burner guys take it as an affront when the burners get blamed for being inefficient or hard to keep balanced. It’s the ultimate slap in the face when the burners (and sometimes the whole furnace) get labeled as a “gas hog.” The seasoned furnace guys who just read that are shrinking back in horror at the mention of a gas hog because they know there are many ways to waste fuel, and some of them are hard to rectify if equipment is not up to snuff.

This installment will provide an example of what can be done to avoid wasting fuel and why you should prioritize this problem.

Insulation and Energy Loss

The aforementioned smell of burning hair, of course, was rather dramatic and hopefully unlikely, but we have all walked into a heat treat facility and been hit in the face with some sort of otherworldly blast of heat. I know, you’re thinking, “Well, duh, Captain Obvious, we are in the business of making things really hot in here.” I get it. However, we all know that if the furnace insulation has broken down, or worse yet, failed completely in spots, energy loss is imminent and will affect the bottom line. And it never seems to be one big issue, but it’s a compounded effect that will add up to serious energy dollar loss.

A Tale of Two Furnaces

Our example today is the retelling of my own experience. I got called to a shop in the Northwest geo-zone a while ago (okay, a long while ago). There were two furnaces sitting side by side with matching load profiles. The manager of the operation walked me out into the work area, and staring at a pair of furnaces said, “One is using almost twice as much fuel. Same everything from an equipment standpoint but almost double the fuel usage.” I looked and observed that the furnace in question had visible orange around the door seams, around the burner flanges, and around the flue. The other furnace had a completely dark exterior. The work associates in this plant were all suffering from radiation blindness — they could not see this very visible damage because the insulation on this furnace had deteriorated slowly enough they were accepting it as normal. Only, it’s not.

Let’s Run the Numbers

If you can see any type of color around doors, the energy loss is massive. At 2000°F Flue gas temperatures, the heat loss from radiation alone is already around 40,000 BTU/hr per square foot of visible radiation.

If you consider that there are probably outside air ingresses through these gaps as well, you can estimate that will result in 10,000 BTU/hr per square foot of additional loss. Those numbers combine for a 50,000 BTU/hr per square foot of loss from the big orange leaks. That’s 50 cubic feet of natural gas every hour for every square foot. You might say, “Well, nobody would have a square foot of glowing furnace shell.” However, if you take it a 10-foot door opening, and the gap is 1 inch all the way around, the square foot of exposed area is leaking heat off at 4 times that square footage because it’s really just a ribbon of heat pouring out.

So now, I was witnessing 200 cubic feet of fuel leaking out every hour that this furnace was heating all day, every day. That is 200 cubic feet × 24 hours/day × 6 days/week × 50 weeks/year = 1,440,000 cubic feet of gas wasted on a single door.

If we estimate that gas is averaging around $5.25/1,000 cubic feet of industrial grid price, that leaky door costs $7,560.00 per year in fuel. If we consider that the gas that was being blown into the room was really intended to heat the load, we can argue there are production losses as well.

Become an Energy Hero

In the case of the client I was helping, I recommended refractory repairs to ensure there was no orange showing outside the furnace. The manager thought I had invented heat — I was his energy hero — and all of a sudden, the burners weren’t gas hogs, and the furnace was up to speed with its twin.

You, too, can be a burner/furnace/energy hero for your facility by not allowing yourself to become radiation blind. Look around, feel the heat that is there, and don’t accept it as the norm. When you see it, fix it. The money you save will almost always pay for the repairs many times over.

Be safe always, and we’ll chat more next month.

About The Author:

For More Information: Contact Jim Roberts at jim@usignition.com.

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Smarter Furnace Systems: Engineering Energy for Sustainability from the Ground Up

June 17, 2025 / CONTROLS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT, Manufacturing Heat Treat Technical Content

There are many avenues for achieving new feats in sustainability. One foundational method of pursuing sustainability is efficient furnace design. In this Control’s Corner installment of Technical Tuesday, Stanley Rutkowski III, senior applications engineer, RoMan Manufacturing, Inc., explores ways to design furnace electrical power systems for efficiency.

This informative piece was first released in Heat Treat Today’s June 2025 Buyers Guide print edition.

In the world of industrial heat treating, sustainability isn’t just a buzzword; it’s a measurable outcome of engineering decisions. While discussions around energy usage often focus on renewable sources or carbon offsets, the path to sustainability begins much earlier — with the design of the furnace’s electrical power system.

From transformers and load configurations to modern control technologies, every part of the furnace power pathway affects how efficiently energy is used — and how much of it is wasted. A well-designed system doesn’t just heat effectively; it does so with less resistance, fewer losses, and minimal disruption to the power grid.

The Power Triangle: Real, Reactive, and Apparent

Understanding sustainability starts with understanding how energy is consumed. Utility companies bill based on more than just energy (kWh). They measure and potentially build via:

Real power (kW): the usable energy
Reactive power (kVAR): the energy lost due to inductance and system inefficiencies
Apparent power (kVA): the total power delivered, including losses
Power factor: the ratio of real power to apparent power, indicating system efficiency
Peak demand: the highest level of power drawn during a billing period

Furnace systems with poor power factor or high reactive power incur more cost, even if their real energy usage is low. That’s why electrical design is so critical.

Control Systems: The Shift to Digital

Legacy systems, such as tubes or saturable reactor-based VRTs, have largely given way to more efficient SCR- (silicon controlled rectifier) and IGBT- (insulated-gate bipolar transistor) based controls. IGBT technology, in particular, offers high-frequency switching, reduced losses, and excellent power factor performance. These systems also provide communication protocols — giving real-time insight into power draw, voltage stability, control temperatures, and even predictive maintenance alerts.

Digital communication allows users to evaluate trends over time. For example, changes in DC bus voltage or output current may signal a degrading heating element, enabling early intervention. Smart controls also help avoid peak demand charges by shifting high-load operations to off-peak hours or adjusting recipes to consume less total power.

Load Configurations and Layout

Load configuration is equally as important: single-phase, Scott-T two-phase, or balanced three-phase arrangements. Poorly balanced systems place stress on utility infrastructure and reduce power factor. Balanced loads, especially when combined with IGBT control, lower disturbances to the grid and increase efficiency.

Physical layout also plays a key role. Long conductor runs increase resistance and inductive reactance, which raises energy consumption and heat loss. “Close coupling” the transformer and conductors near the furnace feedthrough reduces losses and improves power delivery, which is important for sustainability and cost savings.

AC vs. Rectified DC Power

Finally, consider how power is delivered. While AC remains common and easy to install, rectified DC systems eliminate voltage zero-crossings, resulting in more stable heating and reduced thermal stress on elements. For high-precision applications like carburizing or annealing, DC systems can extend equipment life and improve thermal uniformity.

Conclusion

Energy sustainability in heat treating isn’t just about switching to greener sources — it starts with how power is delivered, controlled, and consumed. Getting a power conversion expert involved early in the planning and system design process ensures that every component is optimized for efficiency, reliability, and long-term performance. This early collaboration helps manufacturers reduce energy costs, extend equipment life, and achieve more sustainable operations without compromising results.

About The Author:

Stanley F. Rutkowski III
Senior Applications Engineer
RoMan Manufacturing, Inc.

Stanley F. Rutkowski III is the senior applications engineer at RoMan Manufacturing, Inc., working on electrical energy savings in resistance heating applications. Stanley has experience in welding, glass and furnace industries from R&D, design, and application standpoints. For more than 15 years, his focus has been on energy savings applications in industrial heating applications.

For more information: Contact Stanley at srutkowski@romanmfg.com.

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Development of an Advanced Silicon Carbide Heating Element

June 3, 2025 / Manufacturing Heat Treat Technical Content

In this Technical Tuesday installment, Kazunori Hokaku, business director/general manager/sales engineering dept. at Tokai Konetsu Kogyo shares the sustainability benefits of SiC heating elements.

This informative piece was first released in Heat Treat Today’s May 2025 Sustainable Heat Treat Technologies print edition.

In recent years, Silicon Carbide (SiC) heating elements have been increasingly used in demanding applications involving high temperatures and extreme atmospheres. Battery material manufacturing is one of many such applications. Therefore, improved service life contributes to increased productivity and sustainability as well as reduced industrial waste. This article discusses the long service life for recrystallized SiC heating elements having both excellent oxidation and corrosion resistance.

Tokai Konetsu Kogyo in Japan has been manufacturing EREMA® SiC heating elements since 1936. SiC heating elements are categorized as ceramic heating elements, which are widely used in a temperature range between 932°F–2912°F (500°C–1600°C) as shown in Figure 1.

Table 1. Example of applications for SiC heating elements

The heat value per unit area (i.e., watt density) of SiC heating elements is quite high, 5 to 10 times that of metallic Nichrome wire heating elements, for example. SiC heating elements are chemically stable and an environmentally friendly source, free of air and noise pollution compared to gas-fired or liquid fuel systems. As such, they are chosen and used for a variety of applications, such as Lithium-ion battery material (i.e., cathode/anode/solid state battery materials), powder metallurgy, aluminum, hardening and case hardening applications (e.g., carburizing), electronic parts (MLCC, ferrite), and dental materials as shown in Table 1.

SiC heating elements come in a variety of shapes, namely straight rod, U-shaped, and W-shaped designs. They are affordable and easy to handle compared to other ceramic heating elements. It is important to remember, however, that their service life is drastically influenced by high temperatures and the atmosphere.

The failure mechanism of a SiC heating element to its service life is shown in Figure 2. SiC reacts with O2 and creates SiO2, by which the resistance of the heating element increases.

Therefore, increasing bulk density and reducing specific surface area are key to service life longevity. This relationship between bulk density and service life in alkaline atmosphere (Li2CO3) has been explored by Tokai Konetsu Kogyo using scanning electron microscopy (SEM) photos with results as shown in Figure 3.

Figure 2. Mechanism of resistance increase (service life)

Silicon Carbide 101

■ Electric heating elements are a popular choice of many heat treaters. They come in a variety of shapes, sizes, and materials. One of the most common types are silicon carbide (SiC) heating elements, known by several tradenames including Globar™ and StarBar™. They are used extensively throughout the heat treating industry when high temperatures, maximum power, and heavy duty cycles are required.

A SiC heating element is typically, but not always, an extruded tubular rod or cylinder made from high purity grains of silicon carbide that are fused together by either a reaction bonding process or a recrystallization process at temperatures in excess of 3900°F (2150°C). The result is a chemically stable material with a low thermal expansion coefficient and little tendency to deform.

Spiral-cut silicon carbide heating element design provides increased resistance for applications up to 3000°F (1650°C)

Recrystallization forms fine grains of silicon carbide that act as “bridges” or connection points between larger grains thus forming conductive pathways. The number of bridges formed dictates the material’s resistance: the greater the number, the lower the resistance. The secret to the creation of a good heating element is controlling this formation process within the material to develop a consistent electrical resistance

The factors that influence the life of a SiC heating element include the type of furnace atmosphere, watt density, operating temperature, type of service (continuous or intermittent), and maintenance. Furnace type, design, and loading play an important role as well. SiC heating elements are extremely versatile operating, for example, in air up to 3000°F (1650°C).

Finally, the choice of heating element depends on many factors. For example, SiC heating elements are capable of higher operating temperatures and higher watt loadings than say metallic elements; they are self-supporting and can be used in furnaces either too wide or too long to be spanned by other element types and are relatively easy to change while hot. SiC heating elements are used extensively in brazing and sintering furnaces running continuously at or above 2050°F (1120°C) and for other processes where the temperature range lies between 2375°F (1300°C) and 2725°F (1500°C).

With permission from the author, Dan Herring, the information cited has been used in part from, Herring, Daniel H. “Electric Heating Elements Part One: Silicon Carbide.” Industrial Heating, September 2008. ■

SEM photos show the Sustainable Development Goals (SDGs) model observed increased bulk density with low porosity and very thick neck growth of SiC grains. The specific surface area for the SDGs model of 0.03 m2/g by Brunauer–Emmett–Teller (BET) method is smaller than that of the standard high-density grade of 0.05 m2/g.

Figure 3. Life test (resistance increase in alkaline atmosphere)

As a result, the graph in Figure 3 shows that the service life for the EREMA®SDGs model (BD = 2.65) is the longest, which means a reduction not only in downtime of furnace operation but also of industrial waste.

About The Author:

Kazunori Hokaku
Business Director/General Manager/Sales Engineering Dept
Tokai Konetsu Kogyo

Kazunori Hokaku graduated from Kyoto Institute of Technology in 1985 with a major in ceramics. He has been with Tokai Konetsu Kogyo Co., Ltd., since 1985 and is currently the business director/general manager/sales engineering dept.

For more information: Contact Kazunori Hokaku at k.hokaku@tokaikonetsu.co.jp

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Furnace Loads Powered with AC vs. DC

May 28, 2025 / Manufacturing Heat Treat Technical Content

In heat treating, the choice of power supply is a critical decision. Whether you’re using an AC transformer or a rectified DC system (AC transformer rectifier), this decision plays a significant role in process efficiency, equipment longevity, and operational costs. While AC transformers have been the industry standard for decades, rectified DC power is becoming more relevant due to its distinct electrical and thermal characteristics. Understanding the differences between these two power sources helps in-house heat treaters optimize furnace performance based on their specific application needs.

This informative piece was first released in Heat Treat Today’s May 2025 Sustainable Heat Treat Technologies print edition.

Electrical Differences: AC vs. Rectified DC Power

AC Transformer Systems

In a traditional setup, an AC transformer steps down high-voltage grid power to the appropriate level for the heating elements. These elements operate on an alternating current waveform, where voltage and current fluctuate between positive and negative cycles at a standard frequency (typically 50 or 60 Hz).

Rectified DC Systems

In a rectified DC system setup, an AC transformer is combined with a rectifier to convert the stepped-down AC voltage into a pulsating or smoothed DC supply. This provides a continuous electrical current rather than an alternating waveform, changing how heat is delivered to the furnace.

Table 1. Benefits of AC vs. rectified DC power

Heat Distribution and Process Stability

One of the key differences between AC and rectified DC power in furnace heating is how each affects heat distribution within heating elements.

AC Heating

The alternating nature of AC power means the voltage crosses zero multiple times per second, leading to cyclic fluctuations in power delivery. In heat treating, this can create small but notable variations in temperature stability, especially in high-precision applications where uniform heating is critical.

Rectified DC Heating

Because DC power provides a continuous voltage and current, it eliminates these fluctuations. This results in more stable and consistent heating, which can be beneficial in processes that require tight thermal tolerances, such as carburizing, nitriding, and annealing.

Heating Element Performance and Longevity

The type of power supply can also impact the lifespan and efficiency of heating
elements.

AC-Powered Heating Elements

Alternating current can cause thermal cycling effects within the heating elements, leading to mechanical stress over time. This may contribute to faster wear, increased oxidation, and potential premature failure of heating elements in some applications. However, for many general heat treating processes, AC remains a cost-effective and widely accepted solution.

Rectified DC-Powered Heating Elements

The stable power flow of rectified DC reduces thermal cycling stress on heating elements, allowing for longer operating life and more efficient heat transfer. This is particularly relevant for graphite and silicon carbide heating elements, which perform better with steady-state power input.

Power System Efficiency and Infrastructure Considerations

When integrating AC vs. rectified DC systems into a heat treating operation, several infrastructure and efficiency factors come into play.

AC Transformer Systems:
- Standard in industrial settings and require minimal modification to existing electrical infrastructure
- Simpler and often lower cost installation compared to rectified DC systems
- More efficient for long-distance power transmission, which can be a factor in large industrial operations with multiple furnaces
Rectified DC Systems:
- Require a rectifier in addition to the transformer, adding to equipment costs and complexity
- Potentially higher efficiency in localized heating applications by reducing resistive losses in certain furnace designs
- Reduced electromagnetic interference (EMI) compared to AC, which can be beneficial in sensitive heat treating processes

Process Suitability: When to Use AC vs. Rectified DC Power

Temperature stability, heating uniformity, and process sensitivity are important factors to consider when choosing between AC and rectified DC power for different heat treatment processes.

Conclusion

Both AC and rectified DC power play important roles in heat treating, and the choice depends on process requirements, equipment lifespan, and infrastructure considerations. AC transformer-powered systems remain the standard due to their low cost, compatibility with existing grids, and simpler installation. They are ideal for general heat treating applications that do not require extreme precision in thermal control. However, rectified DC systems offer more stable heating, reducing wear on heating elements and improving temperature uniformity. While they require additional equipment, they can be beneficial in high-precision applications or when maximizing furnace efficiency is a priority.

Understanding the strengths and limitations of both AC and rectified DC power sources enables heat treaters to
select the optimal system for their specific production needs, balancing cost, efficiency, and process performance.

About The Author:

Brian Turner
Sales Applications Engineer
RoMan Manufacturing, Inc.

For more information: Contact Brian Turner at bturner@romanmfg.com.

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Gladwell vs. Einstein: Thin-Slicing PIDs

May 20, 2025 / Manufacturing Heat Treat Technical Content

By Steven Christopher, senior engineer at Super Systems Inc., and Katie Bastine, former quality manager at ThermoFusion.

The three letters, P-I-D, send shivers down most spines; tuning may induce an actual headache. This has proven true for decades, but why is the concept so overwhelming? This article will attempt to answer that difficult question with simple considerations.

This informative piece was first released in Heat Treat Today’s May 2025 Sustainable Heat Treat Technologies print edition.

PIDs (Proportional-Integral-Derivative) need not be anxiety-producing. Let’s break it down to reduce the stress, but first, let’s credit one of the world’s deepest thinkers. Einstein defined insanity as, “Repeating the same thing over and over and expecting different results.”

Is this happening in our industry? Why does every Proportional-Integral-Derivative (PID) article begin with tuning a new controller? In reality, very few new controllers are installed. Instead, a failing TUS dictates re-tuning an existing controller. Let’s start with an existing controller, and then intentionally RUIN our PIDs.

Katie Bastine, quality manager of Nadcap-certified Thermo-Fusion of Hayward, CA, agreed to help and offered one of the company’s many Endothermic nitriding or batch furnaces. We settled on a vacuum furnace with graphite heating elements, which is a perfect candidate because it was relatively quick to respond and easy to manipulate.

What Are PIDs?

According to Blink author Malcolm Gladwell, we need a basic understanding of PIDs, and perhaps no more. His fantastic book details the brain’s ability to thin-slice situations — meaning the ability to make quick, often correct, decisions. He discusses in great depth the importance of data for experts, but he hypothesizes that too much data for the average person negatively influences the decision-making process. Gladwell claims, “The role of those other factors is so small … that extra information is more than useless. It’s harmful. It confuses the issues.”

This may be our mistake: too much information freezing our ability to act rather than empowering us. The goal of this article is not to train experts but rather to help with that often-paralyzing first step.

Consider the following definitions:

Proportional band (Pb) compares the error at a single moment or the difference between the set-point and control TC. Adjusting the Pb parameter determines how large an error is required to reduce/increase output.
Integral (or reset) compiles this same error over a period of time, appropriately adjusting to the Pb’s output.
Derivative (or rate) monitors the rate of change, estimating future error and “applying the brakes” when necessary. The remainder of this article limits theory and focuses on observations.

TIP: Understand the units. Pb can be expressed as percentage, degrees, or gain. Integral and derivative can be expressed as seconds, minutes, or repeats/minute. This article will use percentage and seconds.

Note: An increase to Pb (percentage) will have the same outcome as an opposite decrease to Pb (gain).

Evaluation Criteria

Before evaluating PIDs, it is important to agree what makes a good one. Th at list could be quite long, so this article will intentionally avoid considerations such as repeatability and recovery time. Instead, we will evaluate:

Aggressiveness — rate at which temperature approaches setpoint
Overshoot — both initial overshoot followed by how quickly it returns
Steady-state — oscillations (both period and amplitude) once settled out Aggressiveness is when the output first reduces as temperature approaches the final setpoint. Reducing too early sacrifices heat-up time while simultaneously improving overshoot. Like many PIDs, a delicate balance exists between any two parameters — a constant set of “give and take.” This consideration is less important when ramping to the final setpoint, because the output typically never reaches its maximum.

Are PIDs the Problem?

Algorithms are stable by nature; input data then calculate output. Thus, changes in behavior are rarely “failing PIDs,” but rather some external factor. If ever there was a time to pause, this is it. Before embarking on the time-consuming effort of tuning, evaluate the furnace holistically. What changed? Are PID changes masking a physical issue? Make sure you are fixing the right problem.

Many confuse PIDs with uniformity. While these concepts coexist, uniformity tends to indicate the health of a furnace, which is influenced by such things as heating system design, element/tube/valve condition, insulation, radiant effects, changes to rheostat/trim settings, and convection turbulence. A negative change in one may result in a failed TUS and prove impossible for PIDs alone to overcome.

Remember, a sudden, exaggerated loss in control suggests PIDs are not at fault.

TIP: Forget the TUS. If the control TC is good, then so are the PIDs.

Uniformity is defined by two characteristics: Delta and balance. Delta is the difference between the coldest and hottest temperature. Balance is the relationship between these temperatures and the control TC. Consider Figures 1 and 2 representing an AMS2750F Class 2 furnace with +/-10°F tolerances.

Figure 1 centers around setpoint. With a delta of 21°F, however, no amount of tuning will pass TUS.
Figure 2 reduces delta to 15°F, but the unbalanced nature results in a failing TUS on the lower limit. PIDs will never improve uniformity.

Poor uniformity (Delta) can be overcome by the aforementioned factors and (balance) by adjusting the control TC position or applying an offset (if allowed). The possible combinations are so wide they are beyond the scope of this article.

Pay Attention to Output

Output is an important (and often overlooked) tuning parameter. PID changes are driven by the control TC, but they have practical limits. These limits are often visible in the output well before they are in the control TC. Tuning efforts should always monitor the output for:

Backing off — temperature when output begins to reduce
Stability — ability of output to converge on an appropriate value
Response — noticeable difference in time for the control TC to respond to an output change

TIP: In addition to setpoint and control TC (PV), record output (CV) to better understand tuning. A lot happens in one minute, so try recording every 1–5 seconds if possible.

Healthy PIDs

Thermo-Fusion’s vacuum furnace ramped to 1000°F at 40°F/min with an aggressive approach, minimal overshoot and continued a “straight line” at soak (using PIDs of 2.0/75/150). As the control TC neared soak, the output backed off 60°F before the soak temperature — neither too early nor too late. The output settled down after a few quick oscillations, suggesting the Pb was not too small.

Figure 3 demonstrates what PIDs should look like. Now let’s disrupt these values, learning from the result. We begin by exploring Pb’s effect because it has the most influence on the trio.

TIP: When adjusting parameters, go big! Start with large (40–60%) changes, then fine-tune with smaller (10–20%) adjustments.

Increase Pb

Proportional band influences when output first reduces and how fast it adjusts. The first mostly impacts furnaces experiencing immediate setpoint changes. All furnaces, including those that ramp to final soak, must consider the second — how fast the output adjusts.

A “sweet spot” exists for Pb. Let’s consider the extremes. A Pb of “∞” backs off very early but too slowly. This results in either overshoot followed by slow, rolling oscillations or no overshoot but also failure to reach setpoint. Same cause, but very different outcomes.

Figure 4 demonstrates the first example: reducing early but too slowly to eliminate overshoot.

TIP: Decrease Pb until the output “bounces around”, then slightly increase Pb. This approach offers diminishing returns, with the output eventually becoming unstable.

Decrease Pb

If increasing Pb slows the output, decreasing must offer the opposite effect. A Pb of “0.0” represents on/off control, backing off very late (at setpoint) but quickly (100% to 0% immediately) followed by rapid oscillations. A smaller Pb presents a double-edged sword, which is an advantage to furnaces with an immediate control/output response, but a disadvantage for those with a lagging relationship. This allows the output to wind up or down too much before the control TC responds.

A small Pb minimizes overshoot but sacrifices steady-state control. Pay special attention to the output (specifically the 1/4 Pb line on Figure 5). As the control TC approaches soak, there are tremendous output swings followed by instability — classic signs of too small a Pb.

TIP: Watch the output. If “bouncing around,” increase the Pb, which dampens output.

Increase Integral

Integral considers past error, “winding up” as large error exists and adding to the output. Small error conversely “unwinds” the Integral. A larger Integral parameter adds more to the Pb’s output. This may improve aggressiveness, but it sacrifices other aspects of a healthy PID.

Too large an Integral overemphasizes previous error, potentially resulting in overshoot, then quickly unwinding as the error becomes smaller, flattening the control TC. Integral has a second benefit: reducing “droop” as the control TC approaches soak only to prematurely stall. A third benefit compensates for a furnace that heats faster than it cools (or vice versa).

TIP: With similar overshoot to a large Pb, a large Integral differs with less undershoot before stabilizing.

If the control TC stalls before soak without closing, increase the Integral. If the control TC looks more like a saw tooth than a sine curve, increase the Integral.

Decrease Integral

Too small an Integral eliminates the PIDs knowledge of history, leaving all the work to the Pb. Error can change rapidly at any moment, which results in an equally rapid change in output. An appropriately sized Integral offers a smoothing effect on the system. Too small an Integral disregards previous error, possibly making the system unstable.

Changes to Derivative

Derivative is difficult to simplify, but (channeling our inner Malcolm Gladwell) let’s try. Derivative is perhaps most easily thought of as a counterweight to the actions of P-I alone. Derivative evaluates the error’s current rate of change to estimate future error. This forecasting allows Derivative to prematurely reduce or increase output.

Derivative is frequently overused and often not required. Exceptions must be evaluated on a case-by-case basis. A visible indicator suggesting a benefit from Derivative is a delayed response between the control TC and output. As output increases, does the control TC immediately rise? Or does it take a while to respond?

TIP: P-I alone often can’t overcome a significant lag between the control TC and output. Increasing Derivative will counteract the delay.

Summary

We hope this article provides the confidence to take that difficult first step. The beauty of PIDs is they are free to make and easy to undo. Therefore, do not be intimidated in taking that first step. Worst-case scenario, you revert. Pair these tips with the following guidelines, and you will be fine.

Change only one parameter at a time.
Cool the furnace between tests; don’t increase +100°F only to try again.
If you overshoot, don’t abandon ship — observe steady-state. You often learn more from failure than success.
Document, document, document! Simplify your thinking, and don’t simplify your notes.
If stumped, place the controller in manual output, forget the setpoint but watch for stability. If the furnace can’t control, then how can the PIDs?

About The Authors:

Steven Christopher
Senior Engineer
Super Systems Inc.

Steven Christopher has been involved in countless projects for both captive and commercial heat treaters implementing critical technology applications. He brings a vast knowledge base as it relates to industrial automation, and his experience with heat treat equipment is second to none.

For more information Contact Steven at schristopher@supersystems.com

Katie Bastine
Former Quality Manager
ThermoFusion

Katie Bastine, formerly of ThermoFusion, has 13 years of industry experience and was recognized in Heat Treat Today’s 40 Under 40 Class of 2021 .

For more information: Contact Katie at htt@heattreattoday.com.

This article was initially published in Industrial Heating. All content here presented is original from the author.

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Is There Too Much Air in Here

April 28, 2025 / BURNERS & COMBUSTION SYSTEMS TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content, OP-ED

What’s the relationship between excess air and your bottom-line? In this article, Jim Roberts, President, U.S. Ignition, shares how to increase efficiency and reduce waste in your heat treating operations.

This informative piece was first released in Heat Treat Today’s April 2025 Annual Induction Heating & Melting print edition.

A furnace guy walks into a heat treat shop . . . and notices there is a little bit of a yelp to the burners, or the furnace operator mentions the furnace is slowing down on heat up recovery times from a cold load. Or, if you are responsible for fuel costs and monitoring the gas meters, you might notice that situation is slipping in the wrong direction. Or, the burners seem to be dumping soot on your floor. We discussed that in past columns — remember?

Well, it’s all got to do with air. It may seem odd to talk about air when the objective is to utilize fuel at an optimum efficiency, but that’s how we intend to get combustion under control. Let’s go after the air. You remember that we talked about making sure that combustion air sources (blowers, eductors, etc.) were all operating at optimum performance, so the air remains supplied as engineered when the equipment was new. So, now we have our air being delivered at the peak levels we want, but it looks like one of the air valves has shifted, which we covered in the last column on keeping the air sources clean.

This next little tidbit of information is intended to show us all how much this little-considered entity we call AIR can affect the bottom line. Here’s some info you might find interesting.

Eliminate Excess Air

If controls have moved or another phenomenon has caused the burners to lean out, it could cost you a fortune. Most burners are designed to burn with a small percentage of excess air (less than 15%).

Exceptions would include air heating equipment and low temperature drying operations where the excess air is used to control the temperature of the flame. If you operate a burner that has been designed to run at 10–15% excess air and the burner controls or settings drift into the range of 50% excess air (that is a difference of 2–3% O₂ or 7.5% O₂ in the products of combustion), the difference in an 1800°F oven operation is a calculated 9% loss of fuel efficiency. If you operate a 1 million BTU/hr burner, firing at 75% of the time six days a week for 50 weeks a year, your gas usage would be approx. 5400 therms a year. If we calculate that your gas costs (delivered) are in the range of $4 per 1,000 cu/ft, keeping one burner in tune would save approximately $1,950 per year.

What!!! If you are running a good-sized batch furnace with four burners, that’s a cool $7,800 dollars per year. A ten burner continuous line is going to save almost $20,000 dollars per year. All that just because you cared enough to check excess air levels regularly.

Of course, wasting fuel because you are heating air instead of product is a terrible thing. But don’t forget you can go the other way, too, and go fuel rich with the settings. Then, you take the chance of actually damaging equipment with the carbon you could be producing in a reducing (excess fuel) situation. Carbon can affect all sorts of equipment life, including shortening burner component life and reducing radiant tube and fixture life. It’s not good. Don’t do it. No excess air and no excess fuel will lead you to a happier and more profitable life.

As always, I recommend that you associate your business with the furnace and combustion technicians in your area who can help you make sure everything stays in tune. We’ll chat in the next edition of Heat Treat Today about how to keep a handle on this in-house, so you can tell your experts what you are seeing and start saving yourself gobs of fuel!

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

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