ATMOSPHERE AIR FURNACES TECHNICAL CONTENT

Case Study: Waste Heat Recovery & Digital Innovation Cut Cycle Times

/ ANNEALING TECHNICAL CONTENT, ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, BURNERS & COMBUSTION SYSTEMS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH

What if your furnace could run faster, cheaper, and cleaner — without major capital investment? Carl Nicolia, president at PSNERGY, LLC, discusses how using waste heat recovery and smart combustion monitoring can cut cycle times in half, reducing gas consumption, and eliminating zone temperature variations.

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

Optimizing combustion and reclaiming waste heat can dramatically improve furnace performance. A real-world bar and coil annealing case study shows how simple retrofits reduced ramp cycle time, cut gas consumption, and eliminated zone temperature variation. The results demonstrate how heat treaters can boost throughput, lower costs, and improve quality without major capital investment.

The Challenge of Industrial Furnace Efficiency

Industrial furnaces are the backbone of metals processing, enabling heat treatment, annealing, forging, and countless other applications. Despite their importance, these furnaces are inherently inefficient. In most cases, less than half of the energy generated by burning natural gas actually reaches the load. Energy is continuously lost through exhaust gases, radiant losses, opening losses, and the heating of fixtures and refractory walls.

On top of this inefficiency, combustion ratios drift over time. Burners fall out of tune, air-to-fuel ratios shift, and temperature distributions across zones become imbalanced. Even with regular maintenance, most furnaces run well below their optimal performance for a significant portion of their operating lives. See figures 1a and 1b, which illustrate how quickly furnaces drift out of tune. Therefore, regular monitoring and adjustment are essential to avoid energy losses and reoccurring performance issues.

This raises a critical question for heat treaters and metal processors: how much efficiency is being left on the table? And more importantly, what would it mean for throughput, energy costs, and product quality if some of that efficiency could be reclaimed?

The following case study of a bar and coil annealing furnace provides a compelling answer.

Figure 1a, 1b. A demonstration of temperature drift that happened in a furnace that was serviced in August 2018 and then again in May 2019. The red points represent oxygen levels measured at each burner when the PSNERGY team arrived on site, while the blue points show oxygen levels immediately after tuning. Although the furnace was optimized during the August 2018 service, the system had already shifted far from optimal conditions within a few months (May 2019). This highlights the inherent inefficiency and constant variability of combustion systems. Source: PSNERGY, LCC

The Application

The facility in this example operates a batch furnace dedicated to bar and coil annealing. The furnace is equipped with 14 non-recuperated U-tube burners across two heating zones.

While reliable, the furnace faced two persistent challenges: long cycle times and inconsistent temperature uniformity across the two zones. Both issues reduced throughput and posed risks to product quality and delivery while also driving up energy costs.

The Problem

The problems facing this manufacturer were not unusual. Long cycle times limited furnace productivity, creating bottlenecks in meeting customer demand. At the same time, uneven zone temperatures made it difficult to maintain uniform metallurgical properties in the product.

With natural gas prices trending upward, energy costs compounded the problem. Every additional hour in the cycle not only resulted in lost throughput, but also higher gas consumption.

The Objective

The project set out with three clear objectives:

  1. Reduce total cycle time: By shortening ramp-up time, the furnace could complete more loads per month, increasing throughput.
  2. Improve zone uniformity: Temperature variation between zones not only affected quality but also required longer soak times to ensure the coldest parts of the load met specifications. Eliminating this variation would allow for both higher quality and shorter cycles.
  3. Lower gas consumption: With energy representing a major portion of operating costs, reducing fuel usage was essential to improving competitiveness and profitability.

The Solution

This improvement method went beyond the traditional practice of tuning a furnace every six to twelve months. Instead, it involved a broader approach utilizing waste heat recovery and digital monitoring tools to achieve optimal combustion at every burner.

The process involved:

  • Installing ceramic radiant tube insert assemblies into the U-tubes
  • Utilizing a combustion monitoring and alerting system to measure air-to-fuel ratio at all burners on the furnace
  • Adjusting all burners to operate within an optimal excess oxygen window (typically between 2.8% and 3.2%) and maintaining those settings over time
  • Ensuring balance between zones allowing the furnace to deliver uniform heating to the load

Figure 2. Before vs. after RIT installation. Source: PSNERGY, LLC

The project began with installing waste heat recovery on all 14 of the non-recuperated U-tubes. In this case, ceramic radiant tube inserts (RTIs) were used because they are quickly and easily installed and capture waste heat normally lost out the exhaust, keeping the energy inside the furnace. Additionally, the RTIs improve temperature uniformity, and reduce gas consumption (see Figure 2).

Installing combustion monitoring at each burner is key to keeping the improvements in place. Instead of waiting for issues to show up in product quality, operators can see what is happening at the burners in real time. When a burner starts drifting out of balance or tune, they have the data to correct it immediately. Constant visibility helps the furnace stay efficient and consistent.

Precision is important when considering the physics of combustion. Measuring excess oxygen at less than 1% (running rich) indicates incomplete combustion is occurring, leading to carbon monoxide and soot formation. At the other extreme, running with too much excess air (running lean) wastes energy. Even 5% excess oxygen results in roughly 13% less energy to the load, while 7% excess oxygen increases those losses to 21%, all while burning the same amount of natural gas.

The Results

The outcomes of this project were dramatic.

  • Ramp cycle reduced by 53%. Prior to any improvements, the furnace cycle time was 30 hours, with ramp-up time accounting for a major portion of the overall cycle. After optimization, ramp-up time was reduced by 8 hours, enabling faster turnaround and greater throughput.
  • Gas consumption reduced by 59% per load. Improved combustion efficiency means that less fuel is required to reach the same metallurgical results. This reduction directly lowers operating costs and CO2 emissions per ton.
  • Zone temperature variation eliminated. By balancing combustion across zones, the furnace achieves uniform heating, reducing the risk of quality issues and minimizing the need for extended soak times.

Figure 3. Graph shows Zone 1 and Zone 2 uniformity (identical curves depicted by yellow and green lines) after the combustion monitoring improvements. Source: PSNERGY, LLC

For the manufacturer, these results translated into both immediate savings and long-term operational advantages. Throughput increased while emissions and quality risks were reduced (see Figure 3).

Broader Implications for Industry

While this case study focuses on a single bar and coil annealing furnace, its implications extend across the heat treat and metals industries.

Most industrial furnaces, regardless of size or application, experience similar inefficiencies. Over time, combustion drifts away from optimal conditions, often unnoticed until performance or quality issues arise. Standard practice, tuning once or twice a year, is rarely enough to maintain proper function.

Capturing waste heat and utilizing technology to monitor and maintain combustion represent major opportunities for manufacturers. By reclaiming even a portion of the 10–30% efficiency losses that occur between tunings, facilities can realize double-digit improvements in throughput and energy consumption.

The return on investment can be substantial. In most cases for these improvements, it’s months. Additional throughput alone will often justify the investment. In many locations, natural gas providers have incentives in place for these projects as they are proven to make substantial reductions in energy use. Just as important, optimizing combustion extends the life of burners and tubes, reduces maintenance emergencies, and stabilizes furnace operation; again, reducing cost and improving efficiency.

Conclusion

Industrial furnaces are indispensable, but they do not have to be inefficient. This bar and coil annealing case study demonstrates that even established furnace systems can achieve impactful performance gains through retrofit combustion optimization.

By focusing on cycle time, energy use, and zone uniformity, manufacturers can unlock faster throughput, lower costs, and higher product quality, while also reducing emissions and operating stress.

The lesson for heat treaters is clear: combustion is not just a background process, it is the heartbeat of the operation. Maintaining combustion properly through the use of easily implemented technology can turn a productivity drain into a competitive advantage.

About The Author:

Carl Nicolia
President
PSNERGY, LLC

Carl Nicolia is president of PSNERGY, LLC, which provides modern solutions to combustion problems, improving equipment life, enhancing productivity, and reducing emissions through smart application of proprietary products, services, and technology.

For more information: Contact Carl Nicolia at cnicolia@psnergy.com.

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Answers in the Atmosphere: Nitrogen — The Swiss Army Knife for Thermal Processors

/ ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, BULK GASES & SYSTEMS TECHNICAL CONTENT, OP-ED

In this installment of Answers in the Atmosphere, David (Dave) Wolff, an independent expert focusing on industrial atmospheres for heat treat applications, explores the versatile role of nitrogen gas in thermal processing.

This informative piece on nitrogen’s critical functions in safety, as a diluent, and as an atmosphere component — including production methods and purity requirements — was first released in Heat Treat Today’s November 2025 Annual Vacuum Heat Treating print edition.

As discussed in the introduction for this series of gas-focused columns, nitrogen gas is ubiquitous in thermal processing — by far the most-used delivered or generated gas in secondary metallurgy. This column covers many important considerations for the use and availability of nitrogen gas, featuring the insights from my recent interview with Air Products experts: John Dwyer, principal engineer; Bryan Hernandez, commercial technology sales engineer; and Emily Phipps, strategic marketing manager. Because of its key role in thermal processing, we expect to have additional columns on nitrogen gas in this series.

Nitrogen serves three important purposes in secondary metallurgy:

  1. Safety
  2. Diluent
  3. Atmosphere

Regarding safety, the Air Products experts shared important attributes of nitrogen and several applications it is most often used in. According to them, nitrogen:

  • will not react with most metals used in fabrication applications until reaching extremely high temperatures
  • will not support combustion or oxidation
  • has about the same density as air (which is 78% nitrogen)
  • is the least expensive industrial gas on a volumetric basis.

For those reasons, nitrogen is used as a purging and inerting gas in metallurgical applications, such as inerting the furnace in preparation for a flammable atmosphere to be introduced, as well as expelling flammable atmosphere at the end of a furnace cycle. They further noted that the National Fire Protection Association (NFPA) Standard 86 for Ovens and Furnaces mandates that nitrogen be always available for furnace inerting except for very specific exceptions where alternative approaches are used (burn in and burn out). Beyond the strict safety considerations, nitrogen protects furnace linings and components from high temperature oxidation.

Dwyer, Hernandez, and Phipps emphasized that when used as a diluent, nitrogen makes it possible to use relatively small volumes of a more expensive reactive gas or gas blend and ensure that the diluted active gas can provide benefits for an entire furnace load of parts. Examples include nitrogen/hydrogen atmospheres where nitrogen gas can enable a relatively small volume of very powerful reducing gas hydrogen to be mixed with a higher volume of nitrogen to fill the furnace interior. I would add that a blended atmosphere of nitrogen/hydrogen will have a higher density than hydrogen alone, and hence may distribute more widely in the furnace rather than just pooling at the ceiling level.

They further discussed how nitrogen can be used as a sole constituent in a furnace atmosphere in many cases, especially at lower temperature ranges, such as tempering and stress relief. In situations where surface finish is a secondary consideration, or where additional operations are going to be performed, they note that the part lower finish quality provided under inert nitrogen alone might be acceptable.

The team then reported that nitrogen forms the bulk of the atmosphere and cryogenic air separation is now available virtually worldwide; because of this, liquified or gaseous compressed nitrogen can also be delivered to clients virtually worldwide. Cryogenically separated nitrogen is, by the nature of the process, extremely pure, and can be assumed to be 99.999% or purer as delivered into the client’s storage vessel. Nitrogen can also be made at the client’s site, using non-cryogenic or cryogenic air separation techniques. For secondary metallurgy, non-cryogenic techniques are the most common because the volumes of nitrogen required are too low for a dedicated cryogenic air separation unit.

Continuing along this line, they explained that while both pressure swing adsorption (PSA) and hollow fiber membrane techniques can be employed to generate nitrogen for a single customer site, the PSA technology is the one primarily used to supply generated nitrogen for thermal processes. This is because the membrane technique for non-cryogenic nitrogen generation makes relatively impure nitrogen, with too much oxygen to achieve the desired surface properties sought by heat treaters. As such, membrane generated nitrogen is primarily used for chemical blanketing and similar low temperature air displacement applications.

The final discussion point I will share from the interview today is about the variability in accepted purity based on the planned usage of nitrogen. The three Air Products experts pointed out that NFPA86 mandates that the atmosphere in a furnace must be below 1.0% oxygen before any flammable gas species can be introduced. So, they continued, nitrogen used solely for safety purging can be relatively impure and still achieve the 1.0% maximum oxygen allowed. When used as the sole atmosphere component (i.e., 100% N₂), or as a carrier gas blended with an active gas like hydrogen, they explained that nitrogen purity must be much higher in order to achieve acceptable surface quality. In general, for atmosphere uses, it should be assumed as a general rule that the purer the nitrogen is, the easier it is to achieve satisfactory heat treat results. The three concluded this thought noting that in blended atmospheres it may be possible to use slightly higher levels of active gases (like hydrogen) to react with excess oxygen in the nitrogen supply, but that approach is unlikely to make sense economically since nitrogen is typically far less expensive than an active gas.

In the December 2025 installment of Answers in the Atmosphere, I’ll share further insights that my interview uncovered. Until then, consider your unique nitrogen needs and therefore whether having direct access to this gas for the benefit of your heat treat operations is essential.

About The Author:

David (Dave) Wolff
Independent expert focusing on industrial atmospheres for heat treat applications

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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Answers in the Atmosphere: Successful Thermal Processing of Metals Requires Atmosphere Savvy

/ ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, ATMOSPHERE GENERATORS TECHNICAL CONTENT, BULK GASES & SYSTEMS TECHNICAL CONTENT, OP-ED

Heat Treat Today is pleased to welcome this regular column spot, Answers in the Atmosphere, to David (Dave) Wolff, an independent expert focusing on industrial atmospheres for heat treat applications. This column explores various atmospheres with Dave and different industry specialists.

This informative piece on the critical role of atmosphere control in metal thermal processing was first released in Heat Treat Today’s October 2025 Ferrous & NonFerrous Heat Treatments/Mill Processing print edition.

Thermal processing of metals is critical to successful production of fabricated metal parts and assembled systems. Characteristics of parts and devices, including blades, springs, wire and cable, medical implants, and electric motors, all depend on successful thermal processing to produce metallic components with specific properties to meet the requirements of the part, assembly, or device. What is sometimes overlooked, however, is that atmosphere is as critical as the heat itself. The wrong furnace atmosphere can undo the best processing recipe, while the right one ensures that parts achieve their intended properties consistently.

Tune into the news, and you will find stories about metal parts incorrectly handled during thermal processing: gears that degrade to powder, camshafts that were too soft, electric switches that fail, materials with the wrong magnetic properties, knives that cannot hold an edge, and so on. These are all problems that occur too frequently and are expensive to resolve, because metal parts are often components in a more complex and expensive assembly. (Imagine the responsibility of parts-making for military jet engines or body-implanted parts. You do not want to be the shop supplying inadequate parts!) It is imperative that heat treating and sintering processes are completed correctly the first time.

Metals thermal processing requires more than just heat. As indicated above, atmosphere is essential to the heat treating process, coming alongside temperature, time, and a specific sequence of operations in a recipe that will ensure the material yields the desired performance. Much like baking bread, thermal processing of metals requires equipment, materials, conditions, and recipes. The furnace is the main equipment (other operations may be performed in a less expensive thermal processing oven). Then there are the materials — the parts being heat treated — which may be bulk metals, alloys, or compacted powder parts with unique blends and surface morphology. The conditions of time, temperature, atmospheres, and perhaps a quenching step come together in a specified recipe. Properly done, heat treating and sintering operations will yield parts that meet the hardness, toughness, appearance, surface finish, shape, dimensions, and other specialized and specified properties.

Since cost is an important driver, metals thermal processors strive to produce compliant parts in as few steps as possible. Innovations can assist in making it possible to consolidate steps, too. But mistakes in thermal processing may result in defective parts or require expensive rework or even additional (secondary) operations to correct deficiencies.

Each issue, this column will focus on the atmospheres component of heat treating. You’ll read interviews with industry experts focused on the atmospheres used in thermal processing — from relatively inert atmospheres, such as vacuum, nitrogen, and argon, to chemically active atmospheres used for annealing, hardening, and sintering. We will assist thermal processors by explaining how various atmospheres work, what the key properties are that determine successful results, how to buy and utilize the atmospheres, and precautions and alternatives for that atmosphere.

My hope is that this column will help Heat Treat Today readers become better buyers and users of atmospheres, so that you can run a smoother, more reliable, and more profitable operation.

About The Author:

David (Dave) Wolff
Independent expert focusing on industrial atmospheres for heat treat applications

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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Refractory Insulation: The Quiet Defender of Furnace Longevity

/ ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, FOUNDRY CASTING TECHNICAL CONTENT, INSULATION TECHNICAL CONTENT

When it comes to furnace linings, most heat treaters focus on the hot-face materials, the heavy-duty refractories taking the brunt of molten metal, corrosive slags, and extreme heat. And just behind that armor lies a quiet defender: the refractory insulation layer. This layer is often the last line of defense between a functioning furnace and a costly, catastrophic failure. In this Technical Tuesday installment, Roger M. Smith, director of technical services for Plibrico Company, LLC helps readers understand the valuable role of refractory insulation for thermal stability.

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

Why Refractory Insulation Matters

Refractory insulation is more than a buffer or a back-up. It provides structural support to the working lining, maintains shell temperatures within safe limits, and cushions the entire structure against the stresses of expansion and contraction. When this layer fails, you don’t just lose insulation, you risk cracks, shell overheating, and lining collapse. In other words, it can turn a maintenance project into a full-blown emergency.

The Strength Factor: Why Compressive Strength Counts

If there’s one property that deserves special attention, it’s compressive strength. The insulation layer is like the foundation of a house: if it cannot support the load above it, the whole structure suffers. Insufficient compressive strength can lead to creeping, crushing, and distortion, all of which compromise the stability of the hot-face refractory.

At green or ambient conditions, most types of insulating refractories, including monolithic, mineral wool, and ceramic fiber boards, exhibit similar compressive resistance, typically in the range of 40−50 psi at 10% deformation, but strength changes significantly once the furnace heats up.

For example, most mineral wool and ceramic fiber boards contain organic binders that burn off at around 475°F, reducing their compressive strength by roughly 50% at furnace operating temperatures (based on the board manufacturer’s technical data sheets, see Table A). Over time, this can increase thermal conductivity through the reduced thickness of the insulating layer.

In contrast, monolithic lightweight insulating castables, like Plibrico’s Plicast Airlite 25 C/G, not only retain their compressive integrity as the temperatures rise, but they actually gain strength, according to ASTM C165 test data as the material fully sets and stabilizes under heat.

Figure 1. Monolithic insulation, gunned in place, stays strong and gains compressive strength during heat-up.

This difference matters: compressive strength is not static. It changes as the material heats up and insulating products that hold their strength at service temperatures provide a more stable, safe, and reliable support for the hot-face lining.

The takeaway? Stronger, more stable insulation is not just filler. It’s an active structural layer that helps prevent hot-face sagging, cracking, and premature failure, directly contributing to longer furnace life.

Thermal Stability: More Than Just Heat Resistance

Figure 2. Confirmed anchor layout and fully prepped furnace wall ready for monolithic insulation installation.

Compressive strength plays a direct role in thermal stability. Denser, stronger castables with lower porosity are far better at resisting gas penetration, chemical attack, and erosion than lightweight, weaker alternatives.

When insulation loses stability, it can create voids, cracks, and hot spots, risks that threaten not only the hot-face layer but the furnace shell itself. This is why density and porosity are critical: denser insulating castables maintain their structure under load, resist infiltration, and provide reliable support for the hot face.

By contrast, mineral wool and board products often weaken as their organic binders burn off at service temperatures, leading to deformation and unpredictable thermal gradients.

Monolithic lightweight insulating castables can offer a more robust alternative. They retain their integrity as temperatures climb and can even gain compressive strength as they fully set and sinter during heat-up. This added stability reinforces the hot-face layer and helps prevent failures during thermal cycling.

There’s another layer to this: long-term thermal cycling. Furnaces rarely stay at one steady temperature; they ramp up, cool down, and undergo countless micro-cycles during operation. Insulation that can absorb these changes without cracking or delaminating is critical for avoiding premature lining failures.

In short, thermal stability is structural stability — the better your insulation performs under heat and cycling, the longer your furnace lining will last.

Designing for Expansion: Building Flexibility Into the System

Here’s where many lining failures start: in the different layers of lining expanding at different rates.

Figure 3. Completed furnace wall insulation installation, finished in half the time required for board installation.

Hot-face refractories, often dense high-alumina castables, have significantly different thermal expansion coefficients compared to the lighter, more porous insulating castables behind them. The hot face may swell aggressively under load, while the insulation expands far less. If those differences are not accounted for, the result is tensile stresses, delamination, and cracking at the interface between layers. Over time, those cracks can grow, creating pathways for heat and corrosive agents to reach deeper into the lining.

This is where thoughtful design makes all the difference:

  • Anchor systems must hold both layers securely but flexibly, allowing each to expand without transferring destructive stresses. Using materials like monolithic refractories adds another advantage: their insulating properties help better protect the base of refractory anchors, reducing localized heat buildup and minimizing stress concentrations that can lead to cracking or premature anchor failure.
  • Installation sequencing should avoid locking the hot-face layer too tightly to the insulation, preventing “shear failure” during heat-up.
  • Layer composition must be selected so the expansion mismatch is minimized, balancing mechanical stability with thermal shock resistance.

When expansion is designed for, rather than ignored, the entire lining behaves like a single, flexible system instead of two incompatible parts competing for space.

Practical Tips for Getting It Right

Prioritize compressive strength. Choose insulation with enough strength to support the hot-face lining under load. Materials like monolithic lightweight insulating castables maintain or even increase compressive integrity at service temperatures, improving overall lining stability.

Figure 4. Gunned to the ideal thickness, insulation built with compressive strength can handle stress, prevent cracking, and maintain shape under heat.

Pick the right material for the zone. Not every insulating castable is created equal. Match density, chemistry, and expansion to the application.

Control the install. Low-water mixes, vibration placement, and proper curing are non-negotiables if you want consistent density and strength.

Don’t skip the heat-up schedule. Rushing dry-out or startup is one of the fastest ways to ruin a lining before it is even in service.

Revisit your anchor design and how you install around it. Poorly designed anchor layouts can lead to stress points and premature lining failures, so reviewing and optimizing the design is one of the cheapest ways to prevent costly mechanical issues. Plus, consider the installation method: board insulation requires time-consuming cutting and fitting around each anchor, while monolithic insulating refractories like Plicast Airlite 25 C/G can be installed around anchors in less than half the time, reducing labor while improving performance.

Built to Last

Insulation is not just about slowing heat loss, it is also about standing firm when your furnace is under its heaviest load. The right refractory insulation, engineered with compressive strength as a priority, gives your lining the backbone to absorb mechanical stresses, resist cracking, and maintain its shape through the punishing cycles of heat-up and cool-down. It does not just protect the shell; it supports the hot face, prevents hot spots, and preserves the entire system’s structural integrity.

Compression Recovery Data

ProductTemperature (ºF)Compressive Resistance at 10% Deformation (psi)
Monolithic Insulation
Lightweight Castable/Gunite
(Plibrico Plicast Airlite 25 C/G)		23040
65041
100047
Mineral Wool Board
(published)		ambient38
Ceramic Fiber Board
(published)		ambient50
200023
3rd Party Test Lab – Orton Labs
ASTM C165 Measuring Compressive Properties of Thermal Insulations

Note: Mineral Wool and Ceramic Fiber Boards contain organic binders that burn off by 475ºF. This reduces the strength of the board by 50% along with decreasing other important properties including thermal conductivity.
Table A. Compressive strength at service temperatures: comparison of compressive resistance (10% deformation) between monolithic insulation lightweight castable/gunite materials such as Plibrico’s Plicast Airlite 25 C/G and mineral wool and ceramic fiber boards at increasing temperatures.

Choosing monolithic insulating castables that gain strength at operating temperatures, instead of mineral wool or ceramic fiber boards that lose half their capacity as binders burn away, is an investment in lining longevity. Get this layer right, and you secure longer campaigns, lower maintenance costs, and the confidence that your furnace can keep pace with production demands. Get it wrong, and you risk premature failures, costly outages, and avoidable downtime. In the end, refractory insulation built for compressive strength and stability is not just a detail, it is what keeps your furnace, and your operation, running at its best.

About The Author:

Roger Smith
Director, Technical Services
Plibrico

Roger Smith is a seasoned professional in the refractory industry. With Master of Science in Ceramic Engineering from the University of Missouri – Rolla, Roger has over 15 years of experience in the processing, development, and quality assurance of both traditional and advanced ceramics. He has a proven track record in developing innovative ceramic formulations, scaling up processes for commercial production, and optimizing manufacturing operations.

For more information: Visit www.plibrico.com.

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What’s the Rebricking and Relining Process for My Atmosphere Furnace?

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

This Technical Tuesday installment is part of the Maintenance Message column series. In today’s edition Nate Sroka, quality assurance engineer for Ipsen, provides a complete maintenance guide to rebricking and relining atmospheric furnaces. Keep this one bookmarked for a quick reference to components, the rebricking process, expectations, and project timeline questions!

This informative piece was first released in Heat Treat Today’s August 2025 Automotive Heat Treating print edition.

Introduction

The interior brick walls of an atmosphere furnace endure extreme temperatures — sometimes reaching 2200°F — every hour of every day. Over time, the bricks become brittle, crack, and experience thermal expansion, which can open seams in the mortar.

After years of continuous operation, users may notice exterior walls becoming hot enough to melt insulated cables or components attached to the furnace. When bricks start falling out of place or insulation begins to sag, it’s time to shut down the furnace, assess damage, and plan for repairs. Typically, furnaces operating for five to ten years since installation or their last major overhaul require rebricking or relining.

Understanding the Components: Bricks and Boards

Knowing the key components used in the rebricking and relining process prepares you for discussions about repairs.

Insulated Fire Bricks (IFB) come in various temperature ratings. A 2300°F brick is less efficient and durable under extreme heat than a 2600°F brick but is often more cost-effective. High-rated bricks typically line the interior, while lower-rated bricks provide an additional insulation layer.

Insulating boards made from calcium silicate form the thermal barrier between the heating chamber and external components. They can withstand temperatures from 1000°F to 1800°F and are commonly used in lower-temperature furnaces.

Mineral wool is a fibrous insulating material used to fill gaps around furnace entry points and seams. Made from volcanic rock, ceramic, or slag, it allows for expansion and contraction due to temperature changes.

One key thing to know about atmosphere furnaces is that they are almost always “on.” In a vacuum furnace, recipes use electric elements that shut off after every cycle, and quenching often happens within the same chamber. However, in an atmosphere furnace, turning off the burners and then restarting the furnace from room temperature the next day is much less energy efficient than running the burners and holding a consistent temperature, even when the furnace is empty. Parts from an atmosphere furnace are typically quenched in an oil or salt bath, separate from the heating chamber.

Figure 1. Debris from a damaged atmosphere furnace that collapsed on a hearth
Source: Ipsen

Starting the Rebricking Process: What You Need to Know

Prepare for a Quote

First, review the pre-quote checklist to make sure you have the right information to get an accurate quote.

Pre-Quote Checklist: Important information to have on hand when getting a quote for rebricking or relining your atmosphere furnace

  • Furnace model number and serial number
  • Heating chamber dimensions (w/d/h)
  • Archway dimensions from base to top
  • Door dimensions and condition
  • Drawings or engineering plans outlining any aftermarket modifications
  • Typical operating temperatures
  • Any additional materials installed to prevent heat penetration
  • Photos from as many angles of the furnace as possible
  • A list of any consumable or heavy-wear components that also need to be replaced

Build a Timeline

Start collecting quotes at least a year in advance and place a purchase order no less than six months before the planned shutdown.

Consider Scheduling Factors

Many furnace operators are looking to have work like this completed during a summer or winter holiday shutdown period. Advanced planning improves scheduling flexibility.

Site Preparations

Before the service team arrives, ensure the workspace is ready:

  • Clear space for staging new materials and removing old bricks.
  • Provide access to a forklift, a durable waste collection container, a dumpster, and other required tools and resources.
  • Confirm power connections near the site for welding and other power tools.
  • Review lockout/tagout procedures with maintenance and operations teams.
  • Determine the required furnace cool-down time before disassembly and plan furnace shutdown accordingly.
  • Identify and disconnect any electrical, process gas, or water-cooling lines as outlined prior to service.

Rebricking Day: What You Should Expect

During disassembly, consider performing additional maintenance, such as:

  • Inspecting and rotating or replacing burner tubes
  • Inspecting and replacing pusher chains and skid hearth section
  • Checking doors, door hoods, and fan bungs
  • Conducting leak tests and changing the quench oil

Coordinate these tasks with the service team to avoid disruptions. The rebricking process spans several days, allowing time for concurrent inspections and repairs.

Final Inspection and Testing

Upon project completion:

Figure 2. A newly installed furnace, bricks properly installed, before furnace ignition
Source: Ipsen
  • Inspect the furnace with the installation team to ensure all work aligns with project specifications.
  • Document any changes as a reference for future maintenance.
  • Perform a “dry out” procedure and clean the quench tank before refilling the tank. See “Example Dry Out Procedure” break-out box.
  • Run the furnace without parts to test for temperature uniformity.
  • Carburizing furnaces may need extra burn-in time to “season” the bricks:
    • Fresh bricks absorb free carbon until fully saturated.
    • When bricks are properly saturated, furnace atmospheres stabilize.
    • Time of burn-in is dependent on the percentage of carbon-level the system needs to achieve.
  • Identify potential hot or cold spots that may require further insulation adjustments.

Post-Installation Best Practices

A rebrick or reline of a furnace is a significant investment. To get the most from your furnace, make the time to take a proactive approach:

  • Establish a daily maintenance inspection for the first week, followed by weekly checks for the first month.
  • Resume regular maintenance schedules if no issues arise.
  • Schedule independent inspections with a field service engineer at three, five, seven, and nine years to proactively assess furnace condition and secure preferred maintenance dates.

By following these steps, atmosphere furnace operators can maximize uptime, streamline service quoting, optimize downtime usage, and ensure efficient future rebuilds.

This article was originally published on ipsenusa.com.

About The Author:

Nate Sroka
Quality Assurance Engineer
Ipsen

Nate Sroka has been with Ipsen since March 2014. He holds a bachelor’s degree in mechanical engineering and a master’s degree in engineering and industrial management from Northern Illinois University. Nate oversees the Quality/Documentation/Warranty (QDW) department, ensuring that Ipsen maintains ISO 9001 and ISO/IEC 17025 standards. He is also responsible for documentation related to installation and operations manuals, regulatory certificates, and managing warranty programs.

For more information: Contact Nate Sroka at nate.sroka@ipsenusa.com.

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What’s the Rebricking and Relining Process for My Atmosphere Furnace? Read More »

Is It Stuffy in Here? Exhaust Systems

/ ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, BURNERS & COMBUSTION SYSTEMS TECHNICAL CONTENT, GUEST COLUMN, Manufacturing Heat Treat Technical Content, OP-ED

In each installment of Combustion Corner, Jim Roberts, president of U.S. Ignition, reinforces the goal of the series: providing informative content to “furnace guys” about the world of combustion. The previous column examined the air supply inlet — the inhale, and this month, Jim is examining the exhaust system — the exhale, and how to inspect it, maintain it, and manage it.

This informative piece was first released in Heat Treat Today’s March 2025 Aerospace print edition.

A guy walks into a room full of furnace guys and says, “Is it just me, or is it a tad stuffy in here?”

We have all been able to imagine that it is hard to focus and do your job in an environment where it seems like it’s hard to breathe. Well, our hard workin’ buddy, the furnace, is continually stuck in a cycle of trying to breathe in, breathe out — and then somewhere in between, the magic of combustion and heat happens! We talked last month about the “breathe in” part of the combustion process. This month, we are going to remind you that if you take a really good, productive, inhaled, life giving breath, you are probably going to want to exhale at some point, too!

Tip 2: Ensure Exhaust Systems Are Properly Functioning and Clean

Inhale, exhale. It makes sense that if we were earlier having issues with the air supply inlet, the exhaust should also be checked. Today’s combustion equipment is sophisticated and sensitive to pressure fluctuations. If the exhaust is restricted, the burners will struggle to get the proper input to the process. I used to use the example of trying to spit into a soda bottle. Try it. It’s tough to do and invariably will not leave you happy. Clean exhaust also minimizes any chance of fire. Read on for three examples.

A. Check the Flues and Exhausts for Soot

If you are responsible for burners that are delivering indirect heat (in other words, radiant tubes), you have a relatively easy task ahead to check the flues/exhausts. Each burner usually has its own exhaust, and one can see if the burners are running with fuel-rich condition (soot/carbon). Soot is not a sign of properly running burners and will signal trouble ahead. Soot can degrade the alloys at a chemical level. Soot can catch fire and create a hot spot in the tubes. Soot obviously signals you are using more fuel than needed (or your combustion blower is blocked, see the first column in this series).

As a furnace operator or floor person, it should be normal operating procedure to look for leakage around door seals.

Here’s a sub tip: If you cannot see the exhaust outlets directly, look around the floor and on the roof of the furnace up by the exhaust outlets. Light chunks of black stuff is what is being ejected into the room when it breaks free from the burner guts (if it can). That will tell you it’s time to tune those burners. If you do not have a good oxygen/flue gas analyzer, get one. It can be pricey, but it will pay for itself in a matter of months in both maintenance and fuel savings.

B. Seriously … Check the Flues and Leakage Around Door Seals

If you are running direct-fired furnace equipment, or furnaces that have the flue gases mixed from multiple burners, it gets a little trickier. All the same rules apply for not wanting soot. Only now, it can actually get exposure to your product, it can saturate your refractory, and it can clog a flue to the point that furnace pressure is affected. An increase in furnace pressure can test the integrity of your door seals. It can back up into the burners and put undue and untimely wear and tear on burner nozzles, ignitors, flame safety equipment, etc. As a furnace operator or floor person, it should be normal operating procedure to look for leakage around door seals.

C. Utilize Combustion Service Companies

Ask the wizards. Combustion service companies can usually help you diagnose and verify flue issues if you suspect they exist. It’s always a great idea to set a baseline for your combustion settings. Service companies can help you establish the optimum running conditions. Again, money well spent to optimize the performance of your furnaces. I’m sure you already have a combustion service team; some are listed in this publication. Otherwise, consult the trade groups like MTI and IHEA for recommended suppliers of that valuable service.

Check flues monthly. It should be a regular walk around maintenance check.

Don’t let the next headline be your plant. See you next issue.

About The Author:

Jim Roberts
President
US Ignition

Jim Roberts, president at US Ignition, began his 45-year career in the burner and heat recovery industry directed for heat treating specifically in 1979. He worked for and helped start up WB Combustion in Hales Corners, Wisconsin. In 1985 he joined Eclipse Engineering in Rockford, IL, specializing in heat treating-related combustion equipment/burners. Inducted into the American Gas Association’s Hall of Flame for service in training gas company field managers, Jim is a former president of MTI and has contributed to countless seminars on fuel reduction and combustion-related practices.

Contact Jim Roberts at jim@usignition.com.

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Why Normalize, and Is a ‘Still Air’ Cool Really Important? Part 2

/ ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, HARDENING TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content, VACUUM FURNACES 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.

This informative piece was first released in Heat Treat Today’s March 2025 Aerospace Heat Treating print edition.

Last time (Air & Atmosphere Heat Treating, February 2025) we addressed the question of why normalizing is necessary. Here we look at the importance of a “still air” cool on the final result. Let’s learn more.

What Is a “Still Air” Cool?

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As we learned last month, the term “cooling in air” is associated with normalizing but poorly defined in the literature or in practice, either in terms of cooling rate or microstructural outcome. This lack of specificity has resulted not only in many different interpretations of what is needed, but in a great deal of variability in the final part microstructure.

By way of example, this writer has on multiple occasions asked what changes are made to car bottom furnace cycles where cars are pulled outside of the plant for “air cooling” (Figure 1). Questions such as, is the furnace opened and the car pulled out in inclement weather? And, is this practice done on a particularly windy day, or in a rain or snowstorm or when the temperature is below zero? An all-too-common response is, “Only if it isn’t raining ‘too hard’ or snowing ‘too much’; then, we wait a while.” No wonder part microstructures are often found to vary from part to part and load to load!

Most heat treaters agree, however, that normalizing is optimized by a cooling in “still air.” This term also hasn’t been clearly defined, but it will be here based on both an extensive survey of the literature and the most common heat treat practices. In Vacuum Heat Treatment, Volume II, I define a still air cool as: “Cooling at a rate of 40°F (22°C) per minute … to 1100°F (593°C) and then at a rate of 15°F–25°F (8°C–14°C) per minute from 1100°F (593°C) to 300°F (150°C). Any cooling rate can be used below 300°F (150°C).”

Typical car bottom normalizing furnace opening to the outside environment

In addition, many consider nitrogen gas quenching in a vacuum furnace at 1–2 bar pressure to be equivalent to a still air cool. But again, so many factors are involved that only properly positioned workload thermocouples can confirm the above cooling rates are being achieved.

Also, many use the term “air cooling” to differentiate the process from “air quenching,” “controlled cooling,” and “fan cooling.”

Recall from the previous installment of this column that any ambiguity with respect to cooling rate ought to be defined in engineering specifications and/or heat treat instructions so that the desired outcome of the process can be firmly established.

From the literature, several important observations will serve as cautionary reminders. In STEELS, George Krauss points out that: “Air cooling associated with normalizing produces a range of cooling rates depending on section size [and to some extent, load mass]. Heavier sections air cool at much lower cooling rates than do light sections because of the added time required for thermal conductivity to lower temperatures of central portions of the workpiece.”

George Totten’s work in Steel Heat Treatment indicates: “Cooling … usually occurs in air, and the actual cooling rate depends on the mass which is cooled.” He goes on to state:

After metalworking, forgings and rolled products are often given an annealing or normalizing heat treatment to reduce hardness so that the steel may be in the best condition for machining. These processes also reduce residual stress in the steel. Annealing and normalizing are terms used interchangeably, but they do have specific meaning. Both terms imply heating the steel above the transformation range. The difference lies in the cooling method. Annealing requires a slow [furnace] cooling rate, whereas normalized parts are cooled faster in still, room-temperature air. Annealing can be a lengthy process but produces relatively consistent results, where normalizing is much faster (and therefore favored from a cost point of view) but can lead to variable results depending on the position of the part in the batch and the variation of the section thickness in the part that is stress-relieved.

In “The Importance of Normalizing,” this writer offers the following caution: “It is important to remember that the mass of the part or the workload can have a significant influence on the cooling rate and thus on the resulting microstructure.”

Finally, Krauss again observes: “The British Steel Corporation atlas for cooling transformation (Ref. 13.7) establishes directly for many steels the effect of section size on microstructures produced by air cooling.” (Note: Interpretation of continuous cooling transformation (CCT) curves will be the subject of a future “Ask The Heat Treat Doctor” column.)

Since hardness is one of the most commonly used criteria to determine if a heat treat process has been successful, it should also be noted that one can usually predict the hardness of a properly normalized part by looking at the J40 value when Jominy data is available.

The Metallus (formerly TimkenSteel) “Practical Data for Metallurgists” provides an example of the type of data available to metallurgists and engineers to help define a required cooling rate for normalizing (Figure 2).

All literature references to normalizing agree (or infer) that the resultant microstructure produced plays a significant role in both the properties developed and their impact on subsequent operations.

Figure 2. Combined hardenability chart for normalized and austenitized SAE 4140 steel showing approximate still air cooling rates and resultant hardness (data based on a thermocouple located in the center of the bar diameter indicated)

Final Thoughts — The State of the Industry

It is all too common within the industry for some companies who wish to have normalizing performed on their products to specify only a hardness range on the engineering drawing or purchase order callout that is given to the heat treater.

Industry normalizing practice here in North America varies considerably from company to company. Normalizing instructions are sometimes, but not often enough, provided on either purchase orders, engineering drawings, or in specifications (industry standards or company-specific documents). These instructions range from, in the case of certain weldments, absolutely nothing (i.e., no hardness, microstructure, or mechanical properties) to referencing industry specifications (e.g., AMS2759/1) or specifying complete metallurgical and mechanical testing including hardness and microstructure.

Most commercial heat treaters often perform normalizing to client or industry specifications provided to them. Others prefer so-called “flow down” instructions in which the process recipe is provided to them. It is a common (and mistaken) belief that this removes the obligation of achieving a given set of mechanical or metallurgical properties even if they are called out by specification, drawing, or purchase order.

Also, the final mechanical properties that result from normalizing are seldom verified by the heat treater. Rather, a hardness value (or range) is reported, but hardness is not a fundamental material property, rather a composite value, one which is influenced by, for example, the yield strength, work hardening, true tensile strength, and modulus of elasticity of the material.

References

ASM International. “ASM Handbook, vol. 4, Heat Treating,” 1991.

ASM International. “ASM Handbook Volume 4A, Steel Heat Treating, Fundamentals and Processes,” 2013.

Chandler, Harry, ed. Heat Treater’s Guide: Practices and Procedures for Irons and Steels. 2nd ed, ASM International, 1995.

Grossman, M. A., and E. C. Bain. Principles of Heat Treatment, 5th ed, ASM International, 1935.

Herring, Daniel H. Atmosphere Heat Treatment, vol. I, BNP Media, 2014.

Herring, Daniel H. Atmosphere Heat Treatment, vol. II, BNP Media, 2015.

Herring, Daniel H. Vacuum Heat Treatment, vol. I, BNP Media, 2012.

Herring, Daniel H. Vacuum Heat Treatment, vol. II, BNP Media, 2016.

Herring, Daniel H. “The Importance of Normalizing,” Industrial Heating April 2008.

Krauss, George. STEELS: Heat Treatment and Processing Principles, ASM International, 1990. 463.

Krauss, George. STEELS: Processing, Structures, and Performance, ASM International, 2005.

Practical Data for Metallurgists, 17th ed. TimkenSteel, 2011

Totten, George E., ed. Steel Heat Treatment Handbook, vol. 2, 2nd ed., CRC Press, 2007.

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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‘Furnace Guys’ and Filtration Systems

/ ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, BURNERS & COMBUSTION SYSTEMS TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content, OP-ED

Jim Roberts, president of U.S. Ignition, joins us in the renewal of the Combustion Corner column. In this installment, Jim establishes that the goal of the series is to provide informative content to “furnace guys” about the world of combustion, furthering the spirit of the Heat Treat Today motto: “We believe people are happier and make better decisions when they are well informed.”

This informative piece was first released in Heat Treat Today’s February 2025 Air/Atmosphere Furnace Systems print edition.

Contact us with your Reader Feedback!

So … A guy walks into a room full of furnace guys …

And the story (or joke) begins again. I used to be one of the furnace guys. It’s a really niche group of strange, unique, and sometimes knowing people, who, by the way, are not gender specific. To me, “a guy” is a moniker as specific as saying that person over there is a swimmer.

But as furnace guys, those same individuals have a peek at the stuff that normal planet walkers don’t. They — or rather WE — know how to almost tame the beast. We have learned what it means to control temperatures that can crack stone. We can bend metal and make it do what we want at temperatures that the human eye cannot gaze upon without safety filters between us and the beast.

And what is this beast? It’s called combustion. It’s a phenomenon that allows the very air around us and anciently sourced resources to burn like hellfire and yet still do our bidding. But there are fewer and fewer guys who manage the beast these days. And that is how a column like this takes launch.

This publication, and its talented editorial staff, have always been driven to provide information that, in their own words, will allow the greater masses this privilege: “We believe people are happier and make better decisions when they are well informed.”

It was not lost on the staff that with dwindling numbers of longtime combustion people some of the benefits of being “well informed” were needed. They felt information could be presented in such a fashion that old-timers like me could share some of the tried-and-true techniques that we have used over the years. The hope is to not only make the workplace safer, but also to increase efficiency and performance in the processes that utilize combustion.

When we walk into almost any facility and go over to the underperforming furnaces, we can bet part of the problem will be inlet air source or exhaust outlet issues.

To some, this will seem like remedial information. That is GREAT. Because that means that you already understand a fair portion of the pathway to combustion performance. You can be the lead in your facility on combustion safety and understanding. Yay!

We are going to start with a visit to an article I wrote some time ago that then later became a pamphlet called “10 Combustion Tips.” It was written with plant maintenance guys in mind as they traveled the factories and facilities that they had responsibility for. We’ll turn this into a series of tips that are really intended for those less experienced to start. We’ll continue in upcoming editions of Heat Treat Today, and hopefully, everyone will feel like this was beneficial when cruising the aisles of your factories.

Tip 1: Keep the Process Air Filters Clean

I know, this seems so obvious, doesn’t it? Utilities tell us over and over to keep your home furnace filters clean. But I would be willing to bet that almost 30% of all furnace issues that we see in the field start at the blower supplying our combustion air. It’s the lungs for your burners! Any filter blockage will result in serious problems. As the system impedes under a clogged filter, your process may not get the required input. Clogged filters put undue strain on the combustion air blowers over time, so your electrical and motor maintenance costs may escalate. Additionally, the burners may go fuel rich. This wastes fuel and can create carbon, which at its best is an insulator. At its worst, it is a fire hazard.

Tip Solutions

A. Check the filters monthly: It is pretty easy to see if a filter is dirty. Your production folks may have even told you the furnace is slowing down. Less air, less heat. Take a peek … you will know. If it’s a fiber-based filter, replace it. Better yet, make it a habit to check filters every month.

B. Clean the screen: If not a replaceable filter, clean the metallic/plastic screen type with some solvent that will cut the machine/quench oil that’s probably the clog culprit. DO NOT put the filter back on dripping wet with solvent. I apologize to furnace guys out there for having to explain that, but it’s the new world, right? If you didn’t understand why, please refer to the movie “Back Draft.”

C. Get outside: Consider ducting an outside air source to the combustion air blower. Fresh air delivered at a stable temp will always help with furnace and burner performance.

So there, was that so hard? Nope, almost simple. And yet when we walk into almost any facility and go over to the underperforming furnaces, we can bet part of the problem will be inlet air source or exhaust outlet issues.

Don’t let it be your plant. See you next issue.

About the Author

Jim Roberts
President
US Ignition

Jim Roberts, president at US Ignition, began his 45-year career in the burner and heat recovery industry directed for heat treating specifically in 1979. He worked for and helped start up WB Combustion in Hales Corners, Wisconsin. In 1985 he joined Eclipse Engineering in Rockford, IL, specializing in heat treating-related combustion equipment/burners. Inducted into the American Gas Association’s Hall of Flame for service in training gas company field managers, Jim is a former president of MTI and has contributed to countless seminars on fuel reduction and combustion-related practices.

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

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A Better Way To Get Things Done: Refractory Insulation

/ ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, INSULATION TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content

The faster the refractory installation, maintenance or repair, the more efficient and, by extension, profitable it is to the company, as savings fall to the bottom line. In this Technical Tuesday installment, Roger Smith, director of technical services at Plibrico Company, LLC, examines the challenges of insulation systems, taking a closer look at ultra-lightweight refractory gunite as a fast, flexible solution to controlling heat.

This informative piece was first released in Heat Treat Today’s February 2025 Air/Atmosphere Furnace Systems print edition.

Manufacturers that rely on industrial grade furnaces, boilers and incinerators to produce their quality products are always looking for ways to improve. It is how they stay relevant and, more importantly, profitable. But you don’t get better just by desiring it. You need to identify better ways to get things done and introduce risk-neutral change to current operational processes. By some estimates, inefficient processes can reduce a company’s profitability by as much as one third.

Given refractories’ importance in safeguarding an operation’s multimillion-dollar thermal-processing equipment, and to avoid unscheduled downtime, it is smart business to have a sustainable maintenance and repair process in place. When a refractory situation does arise, the more proficient the process solution the better.

Controlling the Heat

Click the image above to read Roger Smith’s column on extending the life of refractory linings.

Furnace design is largely about controlling heat to maximize energy efficiency. An energy source — whether that is gas, coal, wood or electricity — is used to heat the furnace, and the furnace lining is designed to keep that heat inside the furnace. There are other factors to be considered, such as the environment inside the furnace, whether there is any abrasion or chemical interactions, or whether the furnace maintains a steady state temperature or undergoes temperature cycles. Regardless of what considerations have to be made for the hot-face lining, an insulation package must be used to reduce fuel consumption and control the cold-face temperature.

There are a large variety of insulation packages and materials that can be used in furnace design. Insulation comes in the form of board, fiber, brick and castables. Each type of insulation comes with its own sets of considerations, such as insulation value, installation method and cost. When considering the insulation package for the vertical wall of a furnace, support must also be considered because the insulation is expected to stay where it is placed and not slump over time. There also must be a means of connecting the hot-face working lining to the furnace structure to provide support. This is accomplished with an anchoring system that connects to the furnace shell and penetrates some distance into the dense hot-face working lining.

Anchoring Systems Challenge Insulation Installations

Anchors are considered to be the bones of a refractory installation and have several functions. They hold the refractory to the wall to keep it from falling in. They also prevent wall buckling due to the internal thermal stresses created by high temperatures. And, to a lesser degree, anchors can also help support the load of the refractory weight.

The anchoring system, however, can present big challenges when installing or maintaining the insulation. In most furnace applications, anchors are first welded directly to the furnace shell. Next, the insulation package is installed and finally the working lining. With anchors sticking off the furnace shell, installing insulation can become a challenge.

Fiber insulation in the form of blanket can be pressed into the gaps between the anchors, but it is important that the insulation remains in place during the life of the furnace. Industrial furnaces tend to vibrate, either from use of combustion or exhaust blowers or other process equipment. This constant vibration can cause fiber insulation to slump and lead to hot spots in the furnace wall due to the lack of insulation.

Figure 1. Anchoring systems are installed before refractory insulation and can pose challenges.

Insulation board is rigid enough to support itself on its end and can be found in a variety of densities and thicknesses to obtain the required insulation value. However, insulation board typically comes in sheets that will have to be cut to fit around the anchors. This can result in a significant amount of manpower and a significant amount of time in a furnace installation. The downtime of an industrial furnace can be costly, which often results in tens of thousands of dollars per hour in lost profits. For this reason, companies try to minimize the time spent rebuilding a furnace. Fewer man hours on a rebuild also tends to reduce the overall cost of the project.

Ultra-lightweight refractory gunites offer a means of installing a large amount of insulation in a relatively short period of time. A gunite is a monolithic refractory castable that is pumped dry through a hose under pressure and is mixed with water at the nozzle. Once the wet castable impacts the surface, it stiffens quickly to avoid slumping and hardens as it dries. This means that the gunite could be installed over the anchors with minimal time. The installer only needs to wrap the end anchors with masking tape to keep them clean for the working lining.

Figure 2. Cold-face and heat storage/loss graph for a production furnace

Distinct Differences in Refractory Gunites

Ultra-lightweight castables are a sub-set of the lightweight castables category but with a very important difference: density. For example, the average lightweight castable with a maximum service limit of 2400°F typically has a density of about 80–90 pcf (pounds per cubic foot). By comparison, ultra-lightweight castables with a maximum service limit of 2400°F will have a density of about 25–30 pcf.

This important distinction comes into play when looking at insulation thickness and calculating cold-face temperature. At the stated densities in a furnace operating at 2000°F, it would take nearly three times more lightweight castable than an ultra lightweight castable to achieve the same cold-face temperature — making many ultra-lightweight castables perfect for insulation and most lightweight castable refractories impractical to use as part of the total insulation package.

Ultra-lightweight castables that achieve final densities of 25–30 pcf while offering service temperatures above 2400°F are available through various refractory manufacturers. One such product, Plicast Airlite 25 C/G (aka Liquid Board) from the Plibrico Company, is designed to be installed via casting or gunite using conventional gunite equipment. With low thermal conductivity and thermal-shock resistance, this material is durable and quick to install. It also has advantages over insulation board, which has a labor intensive installation process of cutting around all the welded anchors, and fiber insulation, which can experience frequent hot spots due to slumping insulation. With an ultra-lightweight, Liquid Board-type of castable, it is possible to attain required insulation values and extended lining life with the installation speed of a refractory gunite.

Working With, Not Against, the Anchoring System

Let’s consider a real-life production furnace operating at 2000°F with a simple 9-inch refractory lining consisting of six inches of dense refractory and three inches of insulation. For comparison, we will assume an ambient air temperature of 81°F and eliminate any effects of exterior wind velocity. The dense refractory working lining for these examples is Pligun Fast Track 50, a 50% alumina, 3000°F-rated refractory gunite.

As seen in Figure 2:

  • Using three inches of ceramic fiber blanket at a density of 6 pcf, a cold face temperature of 252°F can be achieved.
  • Using three inches of insulation board at a density of 26 pcf, a cold face temperature of 247°F can be achieved.
  • Using three inches of an ultra lightweight gunite such as Plicast Airlite 25 C/G with a maximum service temperature of 2500°F and assumed density of 25 pcf, a cold-face temperature of 262°F is expected.

The calculated difference in cold-face temperature between insulation board and the ultra-lightweight gunite is 15°F, but the difference in installation time savings could be multiple shifts.

Figure 3. Ultra-lightweight gunite is quickly applied over anchors with standard equipment.

The cost of downtime can be incredibly high for any manufacturer, especially since downtime can result in a series of costs and losses (both tangible and intangible), including production, labor, replacement costs, product losses and, if unexpected, reputation damage. Industry resources estimate downtime can cost thermal processing companies between $250,000 and $1 million per hour. When multiplied over several shifts, this could mean millions of dollars in downtime costs. Not to mention that labor is a major contributor to the overall cost of a refractory project. The quicker the refractory installation, the less downtime and the more profitable the company.

For example, in an approximately 750-square-foot round duct application (cylinder) with anchors already installed, on average, installation of four inches of the different insulation types can be estimated at:

  • Fiber Insulation — 137 total labor hours, or ~5.5 square feet/hour
  • Insulation board — 288 total labor hours, or ~2.6 square feet/hour
  • Ultra-light gunite/Liquid Board — 80 total labor hours, or ~9.4 square feet/hour

The quick and easy installation of the ultra-light gunite/Liquid Board represents an average estimated financial savings in downtime of between $35 million and $130 million — savings that drops directly to a company’s bottom line. The time compression of installing gunite also holds an added advantage for the insulation installer because labor hours can come with a premium price tag and can sometimes be in short supply. All of this makes the ultra-lightweight gunite solutions an excellent choice to minimize downtime and rebuild costs while meeting the furnace design criteria.

Conclusion

Manufacturers that rely on industrial-grade furnaces, boilers and incinerators to produce their quality products are constantly looking for ways to reduce costs, increase profits and improve efficiencies by looking at and introducing risk-neutral change to current processes. Maintaining efficiency and avoiding unscheduled shutdowns of heat processing equipment requires maintenance. Selecting quality materials and risk neutral installation processes that minimizes maintenance completion times can help companies become more efficient.

About the Author:

Roger M. Smith
Director of Technical Services
Plibrico Company, LLC

Roger M. Smith, a seasoned professional in the refractory industry, is the director of technical services at Plibrico Company, LLC. With a master’s degree in Ceramic Engineering from the University of Missouri — Rolla, Roger has over 15 years of experience in the processing, development and quality assurance of both traditional and advanced ceramics. He has a proven track record in developing innovative ceramic formulations, scaling up processes for commercial production, and optimizing manufacturing operations.

For more information: Visit www.plibrico.com.

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

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Why Normalize, and Is a ‘Still Air’ Cool Really Important? Part 1

/ ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, HARDENING TECHNICAL CONTENT, Manufacturing Heat Treat Technical Content, OP-ED

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.

This informative piece was first released in Heat Treat Today’s February 2025 Air/Atmosphere Furnace Systems print edition.

People often ask two fundamental questions related to normalizing. First, is it necessary? Second, just what and how important is a “still air” cool to the end result? Let’s learn more.

Why Normalize?

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Normalizing is typically performed for one or more of the following reasons:

  • To improve machinability
  • To improve dimensional stability
  • To produce a homogeneous microstructure
  • To reduce banding
  • To improve ductility
  • To modify and/or refine the grain structure
  • To provide a more consistent response when hardening or case hardening

For example, many gear blanks are normalized prior to machining so that during subsequent hardening or case hardening dimensional changes such as growth, shrinkage, or warpage will be better controlled.

Normalizing imparts hardness and strength to both cast iron and steel components. In addition, normalizing helps reduce internal stresses induced by such operations as forging, casting, machining, forming or welding. Normalizing also improves chemical non-homogeneity, improves response to heat treatment (e.g., hardening), and enhances dimensional stability by imparting into the component part a “thermal memory” for subsequent lower temperature processes. Parts that require maximum toughness and those subjected to impact are often normalized. When large cross sections are normalized, they are also tempered to further reduce stress and more closely control mechanical properties.

Large paper roll normalized in a car bottom furnace and cooled (due to its mass) using the assistance of a floor fan.

Soak periods for normalizing are typically one hour per inch of cross-sectional area but not less than two hours at temperature. It is important to remember that the mass of the part or the workload can have a significant influence on the cooling rate and thus on the final microstructure. Thin pieces cool faster and are harder after normalizing than thicker ones. By contrast, after furnace cooling in an annealing process, the hardness of the thin and thicker sections is usually about the same.

Micrograph of medium-carbon AISI/SAE 1040 steel showing ferrite grains (white etching constituent) and pearlite (dark etching constituent). Etched in 4% picral followed by 2% nital. (Bramfitt and Benscoter, 2002, p. 4. Reprinted with permission of ASM International. All rights reserved.)

When people think of normalizing, they often relate it to a microstructure consisting primarily of pearlite and ferrite. However, normalized microstructures can vary and combinations of ferrite, pearlite, bainite, and even martensite for a given alloy grade are not uncommon. The resultant microstructure depends on a multitude of factors including, but not limited to, material composition, part geometry, part section size, part mass, and cooling rate (affected by multiple factors). It is important to remember that the microstructure achieved by any given process sequence may or may not be desirable depending on the design and function of the component part.

The microstructures produced by normalizing can be predicted using appropriate continuous cooling transformation diagrams and this will be the subject of a subsequent “Ask The Heat Treat Doctor” column.

In this writer’s eyes, industry best practice would be to specify the desired microstructure, hardness, and mechanical properties resulting from the normalizing operation. Process parameters can then be established, and testing performed (initially and over time) to confirm/verify results.

In many cases, the failure of the normalizing process to achieve the desired outcome centers around the lack of specificity (e.g., engineering drawing requirements, metallurgical and mechanical property call outs, testing/verification practices, and quality assurance measures). Failure to specify the required microstructure and mechanical properties/characteristics can lead to assumptions on the part of the heat treater, which may or may not influence the end result.

“Normalizing is the heat treatment that is produced by austenitizing and air cooling, to produce uniform, fine ferrite/pearlite microstructures in steel … In light sections, especially in alloy hardenable steels, air cooling may be rapid enough to form bainite or martensite instead of ferrite and pearlite.”

What Is Normalizing?

The normalizing process is often characterized in the following way: “Properly normalized parts follow several simple guidelines, which include heating uniformly to temperature and to a temperature high enough to ensure complete transformation to austenite; soaking at austenitizing temperature long enough to achieve uniform temperature throughout the part mass; and cooling in a uniform manner, typically in still air” (Herring, 2014).

It is also important to remember that normalizing is a long-established heat treatment practice. As far back as 1935, Grossmann and Bain wrote:

Normalizing is the name applied to a heat treatment in which the steel is heated above its critical range (that is, heated to make it wholly austenitic) and is then allowed to cool in air.

Since this is one specific form of heat treatment, it will be realized that the structure and mechanical properties resulting from the normalizing treatment will depend not only on the precise composition of the steel but also on the precise way in which the cooling is carried out.

The term ‘normalizing’ is generally applied to any cooling ‘in air.’ But in reality, this may cover a wide range of cooling conditions, from a single small bar cooled in air (which is fairly rapid cooling) to that of a large number of forgings piled together on a forge shop floor … which is a rather slow cool, approaching an anneal. The resulting properties in the two cases are quite different.

In plain carbon steels and in steel having a small alloy content, the air-cooled (normalized) structure is usually pearlite and ferrite or pearlite alone … More rapid cooling gives fine pearlite, which is harder; slow cooling gives coarse pearlite, which is soft. In some few alloy steels, the normalized structure in part may be bainite.

The hardness of normalized steels will usually range from about 150 to 350 Brinell (10 to 35 Rockwell C), depending on the size of the piece, its composition and hardening characteristics.

Importance of Defining Cooling Rate

In 2005, Krauss underscored the importance of defining cooling rate when he wrote: “Air cooling associated with normalizing produces a range of cooling rates depending on section size [and to some extent, load mass]. Heavier sections [and large loads] air cool at much lower cooling rates than do light sections because of the added time required for thermal conductivity to lower temperatures of central portions of the workpiece.”

Microstructures Created by Normalizing

The microstructural constituents produced by normalizing for a particular steel grade can be ferrite, pearlite, bainite, or martensite. The desired microstructure from normalizing adds an important cautionary note, as addressed by Krauss in STEELS (1990 and 2005), namely: “Normalizing is the heat treatment that is produced by austenitizing and air cooling, to produce uniform, fine ferrite/pearlite microstructures in steel … In light sections, especially in alloy hardenable steels, air cooling may be rapid enough to form bainite or martensite instead of ferrite and pearlite.”

Next time: We define a “still air” cool and look at the state of normalizing in North America.

References

ASM International. “ASM Handbook, vol. 4, Heat Treating,” (1991): 35–41.

ASM International. “ASM Handbook Volume 4A, Steel Heat Treating, Fundamentals and Processes,” (2013): 280–288.

ASM International. “Metals Handbook, 8th ed., vol. 1, Properties and Selection of Metals,” (1961): 26.

ASM International. “Metals Handbook Desk Edition,” (1985): 28-11, 28-12.

Chandler, Harry, ed. Heat Treater’s Guide: Practices and Procedures for Irons and Steels. 2nd ed, ASM International, 1995.

Grossman, M. A., and E. C. Bain. Principles of Heat Treatment, 5th ed, ASM International, 1935, 197–198.

Herring, Daniel H. Atmosphere Heat Treatment, vol. I, BNP Media, 2014.

Herring, Daniel H. Atmosphere Heat Treatment, vol. II, BNP Media, 2015.

Herring, Daniel H. “The Importance of Normalizing,” Industrial Heating April 2008.

Krauss, George. STEELS: Heat Treatment and Processing Principles, ASM International, 1990. 463.

Krauss, George. STEELS: Processing, Structures, and Performance, ASM International, 2005. 253–256, 574.

Lyman, Taylor, ed. Metals Handbook, 1948 ed. ASM International, 1948. 643.

Practical Data for Metallurgists, 17th ed. TimkenSteel.

Totten, George E., ed. Steel Heat Treatment Handbook, vol. 2, 2nd ed., CRC Press, 2007. 612-613.

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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