The future of heat treating requires new manufacturing solutions like robotics that can work with modular design. Yet so also does temperature monitoring need to be seamless to know how effectively your components are being heat treated — especially through being quenched.In this Technical Tuesday,learn more abouttemperature monitoring through the quench process.
Gas Carburization
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Carburizing has rapidly become one of the most critical heat treatment processes employed in the manufacture of automotive components. Also referred to as case hardening, it provides necessary surface resistance to wear, while maintaining toughness and core strength essential for hardworking automotive parts.
Figure 1. Typical carburizing heat treat temperature profile showing the critical temperature/time steps: (i) carburization, (ii) quench, and (iii) temper. (Source: PhoenixTM)
The carburizing process is achieved by heat treating the product in a carbon rich environment (Figure 1), typically at a temperature of 1562°F–1922°F (850°C–1050°C). The temperature and process time significantly influence the depth of carbon diffusion and other related surface characteristics. Critical to the process is a rapid quenching of the product following the diffusion in which the temperature is rapidly decreased to generate the microstructure, giving the enhanced surface hardness while maintaining a soft and tough product core.
The outer surface becomes hard via the transformation from austenite to martensite while the core remains soft and tough as a ferritic and/or pearlitic microstructure. Normally, carburized microstructures following quench are further tempered at temperatures of about 356°F (180°C) to transform some of the brittle martensite into tempered martensite to enhance ductility and grindability.
Critical Process Temperature Control
As discussed, the success of carburization is dependent on accurate, repeatable control of the product temperature and time at that temperature through the complete heat treatment process. Important to the whole operation is the quench, in which the rate of cooling (product temperature change) is critical to achieve the desired changes in microstructure, creating the surface hardness. It is interesting that the success of the whole heat treat process can rest on a process step which is so short (minutes), in terms of the complete heat teat process (hours). Getting the quench correct is not only essential to achieve the desired metal microstructure, but also to ensure that the physical dimensions and shape of the product are maintained (no distortion/warping) and issues such as quench cracking are eliminated.
Obviously, as the quench is so critical to the whole heat treat process, the correct quench selection needs to be made to achieve the optimum properties with acceptable levels of dimensional change. Many different quenchants can be applied with differing quenching performances. The rate of heat transfer (quench rate) of quench media in general follows this order from slowest to quickest: air, salt, polymer, oil, caustic, and water.
Technology Challenges for Temperature Monitoring
When considering carburization from an industry standpoint, furnace heat treat technology generally falls into one of two camps, embracing either air quench (low pressure carburization) or oil quench (sealed gas carburization/LPC with integral or vacuum oil quench). Although each achieves the same end goal, the heat treat mechanisms and technologies employed are very different, as are the temperature monitoring challenges.
To achieve the desired carburized product, it is necessary to control and hence monitor the product temperature through the three phases of the heat treat process. Conventionally, product temperature monitoring would be attempted using the traditional trailing thermocouple method. For many modern heat treat processes including carburization, the trailing thermocouple method is difficult and often practically impossible.1 The movement of the product or product basket from stage to stage, often from one independent sealed chamber to another (lateral or vertical movement), makes the monitoring of the complete process a significant challenge.
With the industry driving toward fully automated manufacturing, furnace manufacturers are now offering the complete package with full robotic product loading that includes shuttle transfer systems and modular heat treat phases to process both complete product baskets and single piece operations. Although trailing thermocouples may allow individual stages in the process to be measured, they cannot provide monitoring of the complete heat treat journey. Testing is therefore not under true normal production conditions, and therefore is not an accurate record of what happens in normal day to day operation.
Figure 2 shows schematic diagrams of two typical carburizing furnace configurations that would not be possible to monitor using trailing thermocouples. The first shows a modular batch furnace system where the product basket is transferred between each static heat treat operation (preheat, carburizing furnace, cooling station, quench, quench wash, temper furnace) via a charge transfer cart. The second shows the same heat treat operation but performed in a continuous indexed pusher furnace configuration where the product basket moves sequentially through each heat treat operation in a semi-continuous flow.
Thru-process temperature monitoring as a technique overcomes such technical restrictions. The data logger is protected by a specially designed thermal barrier, therefore, can travel with the product through each stage of the process measuring the product/process temperature with short, localized thermocouples that will not hinder travel. The careful design and construction of the monitoring system is important to address the specific challenges that different heat treat technology brings including modular batch and continuous pusher furnace designs (Figure 2).2
The following section will focus specifically on monitoring challenges of the sealed gas carburizing process with integral oil quench. Technical challenges of the alternative low pressure carburizing technology with high pressure gas quench have previously been discussed in an earlier publication.3
Monitoring Challenges of Sealed Gas Carburization — Oil Quench
Figure 3. “Thru-process” temperature monitoring system for use in a sealed carburizing furnace with integral oil quench — (3.1) Monitoring system entering furnace with thermocouple fixed to automotive gears, product test pieces (3.2) System exiting oil quench tank (3.3) System inserted into wash tank with product basket (Source: PhoenixTM)
Presently, the most common traditional method of gas carburizing for automotive steels is often referred to as sealed gas carburizing. In this method, the parts are surrounded by an endothermic gas atmosphere. Carbon is generated by the Boudouard reaction during the carburization process, typically at 1562°F–1832°F (850°C –1000°C). Despite the dramatic appearance of a sealed gas carburizing furnace, with its characteristic belching flames (Figure 3), from a monitoring perspective, the most challenging aspect of the process is not the heating, but the oil quench cooling. For such furnace technology, the historic limitation of “thru-process” temperature profiling has been the need to bypass the oil quench and wash stations, missing a critical process step from the monitoring operation. Obviously, passing a conventional hot barrier through an oil quench creates potential risk of both system damage from oil ingress and barrier distortion, as well as general process safety. However, the need to bypass the quench in certain furnace configurations by removing the hot system from the confined furnace space could create significant operational challenges, from an access and safety perspective.
Monitoring of the quench is important as ageing of the oil results in decomposition (thermal cracking), oxidation, and contamination (e.g. water) of the oil, all of which degrade the viscosity, heat transfer characteristics, and quench efficiency. Control of physical oil temperature and agitation rates is also key to oil quench performance. Quench monitoring allows economic oil replacement schedules to be set, without risk to process performance and product quality.
Figure 4. “Thru-process” temperature monitoring system oil quench compatible thermal barrier design: (1) Robust outer structural frame keeping insulation and inner barrier secure; (2) Internal thermal barrier — completely sealed with integral microporous insulation protecting data logger; (3) Mineral insulated thermocouples sealed in internal thermal barrier with oil tight compression fitting; (4) Multi-channel high temperature data logger; and (5) Sacrificial insulation blocks replaced after each run.
(Source: PhoenixTM)
To address the process challenges, a unique thermal barrier design has been developed that both protects the data logger in the furnace (typically three hours at 1697°F/925°C) and also protects during transfer through the oil quench (typically 15 mins) and final wash station (Figure 3). The key to the barrier design is the encasement of a sealed inner barrier with its own thermal protection with blocks of high-grade sacrificial insulation contained in a robust outer structural frame (Figure 4).
Quench Cooling Phases
Monitoring the oil quench in carburization gives the operator a unique insight into the product’s specific cooling characteristics, which can be critical to allow optimal product loading and process understanding and optimization. From a scientific perspective, the quench temperature profile trace, although only a couple of minutes in duration, is complex and unique. From a zoomed in quench trace (Figure 5) taken from a complete carburizing profile run, the three unique heat transfer phases making up the oil quench cool curve can be clearly identified:
Figure 5. Oil quench temperature profile for different locations on an automotive gear test piece shows the three distinct heat transfer phases: (1) film boiling “vapor blanket”, (2) nucleate boiling, and (3) convective heat transfer. (Source: PhoenixTM)
Film boiling “vapor Blanket”: The oil quenchant creates a layer of vapor (Leidenfrost phenomenon) covering the metal surface. Cooling in this stage is a function of conduction through the vapor envelope. Slow cool rate since the vapor blanket acts as an insulator.
Nucleate boiling: As the part cools, the vapor blanket collapses and nucleate boiling results. Heat transfer is fastest during this phase, typically two orders of magnitude higher than in film boiling.
Convective heat transfer: When the part temperature drops below the oil boiling point. the cooling rate slows significantly. The cooling rate is exponentially dependent on the oil’s viscosity.
From a heat treat perspective, the quench step relative to the whole process (hours) is quick (seconds), but it is probably the most critical to the performance of the metallurgical phase transitions and achieving the desired core microstructure of the product without risk of distortion. By being able to monitor the quench step, the process can be validated for different products with differing size, form, and thermal mass. As shown in Figure 6, the quench curve profile over the three heat transfer phases is very different for two different automotive gear sizes.
Figure 6. Oil quench temperature profile for different automotive gear sizes (20MnCr5 case hardening steel) with different thermal masses: Passenger Car Gear (2.2 lbs) and Commercial Vehicle Gear (17.6 lbs) (Source: PhoenixTM)
Summary
As discussed in this article, one of the key process performance factors associated with gas carburization is the control and monitoring of the product quench step. Employing an oil quench, the measurement of such operation is now very feasible as part of heat treat monitoring. Innovations in thru-process temperature profiling technology offer specific system designs to meet the respective application challenges.
References
[1] Dr. Steve Offley, “The light at the end of the tunnel – Monitoring Mesh Belt Furnaces,” Heat Treat Today, February 2022, https://www.heattreattoday.com/processes/brazing/brazing-technical-content/the-light-at-the-end-of-the-tunnel-monitoring-mesh-belt-furnaces/.
[2] Michael Mouilleseaux, “Heat Treat Radio #102: Lunch & Learn, Batch IQ Vs. Continuous Pusher, Part 1,” interviewed by Doug Glenn, Heat Treat Radio, October 26, 2023, audio, https://www.heattreattoday.com/media-category/heat-treat-radio/heat-treat-radio-102-102-lunch-learn-batch-iq-vs-continuous-pusher-part-1/.
[3] Dr. Steve Offley, “Discover the DNA of Automotive Heat Treat: Thru-process Temperature Monitoring,” Heat Treat Today, August 2023, https://www.heattreattoday.com/discover-the-dna-of-automotive-heat-treat-thru-process-temperature-monitoring/.
About the Author
Dr Steve Offley (“Dr O”), Product Marketing Manager, PhoenixTM
Dr. Steve Offley, “Dr. O,” has been the product marketing manager at PhoenixTM for the last five years after a career of over 25 years in temperature monitoring focusing on the heat treatment, paint, and general manufacturing industries. A key aspect of his role is the product management of the innovative PhoenixTM range of thru-process temperature and optical profiling and TUS monitoring system solutions.
Now that a new year is in full swing, it may be time to consider that all of the heat treating equipment that’s currently in the workplace has aged along with us. Without proper maintenance in place, you may start to see signs of age, wear, and tear on the high output furnaces that this industry relies on.
This Technical Tuesday,was originally published inHeat Treat Today’sJanuary/February 2024 Air and AtmosphereHeat Treatprint edition.
Jacob Laird Mechanical Engineer Premier Furnace Specialists, Inc./BeaverMatic Source: Premier Furnace Specialists, Inc./BeaverMatic
Most companies have a “workhorse” furnace which is run exhaustively, and even new furnaces that run this way can start looking quite worn after just months of use. Yet decades-old equipment remains in regular use across the country, thanks to knowledgeable maintenance personnel. Since there is somewhat of a void in personnel for this position, here are a few ways to make sure your furnaces keep running into old age.
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For roller hearth or belt furnaces with rollers, there can be an extensive number of points in the drive which may facilitate misalignment. Most maintenance crews know to keep chains and sprockets in alignment and to keep bearings well-greased to avoid seizing, but these may not be enough for the high temperatures at which these furnaces typically run. Even though they are turning at slow speeds, the roller’s bearings should be filled with high-temperature grease which is designed not to break down and leak despite the heat constantly being transferred through the roller to the external trunnions (shaft ends). If the bearing already has standard grade grease, it needs to be fully pumped out of the bearing with new high-temperature grease to avoid contamination or reactions between the two which could cause leaking or seizing.
Roller hearth furnace system
For driven rollers, it’s only necessary to “lock-down” the drive side of the roller’s components using cone or dog point set screws (sometimes both) and thread locking compounds. As the furnace heats up, the rollers will expand. By leaving the idle end “free,” it allows a path of least resistance for growth, which allows for the best chance to keep drive mechanisms in-line.
An infrared (IR) thermometer can be a useful tool for diagnosing heat leaks around any furnace and avoiding burns while doing so during operation. It’s important to note that on stainless steel components and the glossy enamel coatings on some furnaces, IR temperature readings likely will not be exact. Quality IR thermometers have adjustable emissivity settings which greatly reduce the error caused by these highly reflective surfaces, but readings still should be used simply as reference points.
FCE insulation
It’s a good idea to occasionally check the furnace case for “hot spots,” and this tool allows it to be done without much effort. These are areas which have a higher than typical temperature compared to the rest of the furnace. This can be one of the earliest signs that insulation quality in that spot has issues. The insulation can be checked and repaired rather than waiting until the furnace’s case steel begins to turn white and burn away, leading to more costly repairs. For brick-lined furnaces in particular, one ideal time to perform this check is during the lengthy dry-out procedure to ramp up to operating temperature after a shutdown. The idle time at low temperatures helps to catch issues before high operating temperatures quickly make them worse. For roller hearth furnaces, simply checking the average temperature of each roller’s exposed trunnions and bearing housings can give insight into potential future issues if individual rollers run hotter than others.
As they say, “The best time to start was yesterday. The next best time is now.” Even a furnace that has seen better days can be maintained, repaired, or rebuilt to keep operations running smoothly and, most importantly, safely.
About the Author
Jacob Laird is a mechanical engineer at Premier Furnace Specialists. Jacob has a BS in both mechanical engineering and physics from South Dakota State University. Among many other things, Jacob is known for his skills in sizing/design of combustion systems, burner assembly, and electrical heating systems.
Roller hearth furnaces are known as the work horses of the heat treating industry. Though they may be common, these furnaces still hold some surprises — namely, their diverse applications, potential to be fully automated, and long life span. Five industry leaders provide insight into the current furnace features and how to optimize them for annealing heat treat. As you read, notice the different emphases each expert addresses.
This Technical Tuesday was originally published inHeat Treat Today’sJanuary/February 2024 Air and AtmosphereHeat Treatprint edition.
Application Determines Customizable Furnace Features
This type of furnace is highly customizable, and, as Tim Donofrio, VP of Sales at Can-Eng Furnaces International explains, knowing the application will determine furnace features.
What Features Do You Offer on Your Roller Hearth Furnace for Annealing Various Materials?
Tim Donofrio Vice President of Sales Can-Eng Furnaces International, Ltd. Source: Can-Eng Furnaces International, Ltd.
The following is based on roller hearth furnaces operating at or above 1400°F.
Annealing furnace features depend upon the material being processed and the metallurgical process being carried out. They can be provided with a wide variety of features for preheating, annealing, slow cooling, oxidizing or bluing and accelerated cooling.
Preheating features include direct or indirect heating applications, under air or controlled atmospheres. Preheating in some cases requires features for burn-off of residual blanking fluids prior to entry into the critical anneal chamber and as such, the off -gas must be appropriately addressed.
Annealing can be a direct or indirect heating application using natural gas, blended hydrogen/natural gas, and electrical energy sources. Process protective atmospheres include N2, Exothermic gas, Endothermic gas, N2 + H2, and H2. In some cases, process atmospheres must be carefully controlled and monitored to very low O2 PPM levels to ensure correct oxides are formed or, alternatively, a bright oxide-free finish is achieved, something very critical when annealing electrical steels for transformer core and motor annealing. Today we see a rise in the use of roller hearth furnaces for the manufacture of transformer core and motor cores, processing electrical steels and amorphous metals. This is largely a result of the electrification of the world.
Post-annealing cooling and bluing are paramount to the process success. In most cases, cooling and soaking stages are provided through the integration of direct and indirect cooling methods that include air, water, and externally chilled atmospheres that can be directly injected into the furnace system and recirculated.
How Is Your Roller Hearth System Unique?
Roller hearth furnaces are the work horse of the industry; they are used for a variety of other heat treating applications. For example, roller hearth furnaces can also be used for low temperature curing, tempering, and aluminum heat treating applications. These lower-temperature roller hearth furnaces do not operate above 1400°F and are built with different fabrication and refractory standards. Of course, additional high temperature applications include neutral hardening, case hardening, carbonitriding, isothermal, and spheroidizing annealing and normalizing.
Roller hearth furnace
Can-Eng Furnaces International offers roller hearth annealing furnaces that meet the stringent demands of today’s manufacturers where safety, product quality, and equipment reliability are at the top of our engineers’ minds during design and development. Can-Eng has developed a strong user base that has benefited from design features that ensure tight temperature control and repeatable thermal profiles while also tightly controlling process atmospheres. This is achieved by integrating some of the best available heating and atmosphere technologies while being combined with state-of-the art automation and robotics to deliver the lowest cost of ownership processing systems to our clients.
What Are Best Practices for Training In-House Operators on Roller Hearth Annealing?
Our company promotes hands-on and classroom multi-level training of operators, maintenance staff, and engineers. This provides a complete and full understanding of the equipment and the opportunity to train within the company for future talent development.
What Are the Furnace’s Operational Advantages?
Can-Eng integrates operator-friendly features that contribute to the reduction in energy and atmosphere consumption while minimizing the carbon footprint and emission levels. Combined with on-board system diagnostics, monitoring, and data collection, these allow for minimal operator involvement.
What Is the Most Common Heating Method?
Today, the most common methods are both natural gas and electric. However, Can- Eng works to integrate features that provide our partners with the benefits of reusing waste heat sources such as flue gases within the system or facility to improve operating efficiencies.
What Are the Challenges in Operating This Type of Furnace?
These (atmosphere control, maintenance, uptime, and temperature control) are all typical challenges that are addressed in design. The key is to design flexibility, ease of use, and operator-friendly features to avoid problems in the future when the client wants to process different products in a different way.
Training and Maintenance To Optimize Uptime
Bob Brock, sales engineer at AFC-Holcroft, emphasizes the importance of training and maintenance to best operate the roller hearth system.
What Features Do You Offer on Your Roller Hearth Furnace for Annealing Various Materials?
Bob Brock, Sales Engineer, AFC-Holcroft
Roller hearth furnaces are designed to provide greater uptime, ease of maintenance, and trouble-free operation and are always custom-designed to meet our clients’ specific processing requirements.
Modular designs are available to accomplish basic heating, holding, and cooling segments but also to incorporate burn off, cyclic spheroidizing, bluing, fast cooling, and blast cooling under controlled atmospheres ranging from Endothermic to Exothermic gases, nitrogen, hydrogen, and products of combustion. A broad range of material handling automation and control platforms providing total data monitoring, controlling, tracking, and acquisition capability are also offered.
How Is Your Roller Hearth System Unique?
AFC-Holcroft has designed, built, and commissioned hundreds of roller hearth manufacturing, and fi eld support teams have extensive knowledge and experience with annealing, isothermal annealing, normalizing, carburizing, and solution and aging processes for ferrous and nonferrous applications. This expertise has well positioned us in the roller hearth furnace market.
What Are Best Practices for Training In-House Operators on Roller Hearth Annealing?
3D image of annealing roller hearth furnace Source: AFC-Holcroft
We use a two-step approach when training operators on our equipment. First, hands-on training provides the best opportunity to learn equipment operation, startup and shutdown procedures, as well as maintenance tasks. Second, we follow up hands-on training with classroom discussions to further the operator’s knowledge of equipment and the use of our operating manual. Our two-step approach arms our client operational team with the confidence and knowledge they need to be successful from day one.
What Are the Furnace’s Operational Advantages?
Roller hearth furnaces are designed and built with longevity and uptime in mind. From our integrated preventative maintenance reminders, robust construction, and user-friendly controls, it’s not uncommon to see our equipment still in operation for 50 years or longer.
What Is the Most Common Heating Method?
Natural gas continues to be the predominate heating source in North America, although we have seen an increased interest for alternate heating sources like electric, hydrogen, and bio over the last several years. We anticipate this trend to continue as companies invest in minimizing their carbon footprint, and can provide clients with carbon footprint analysis and operational costs on our equipment.
What Are the Challenges in Operating This Type of Furnace?
Routine equipment maintenance is key to operational uptime. AFC-Holcroft offers a wide range of preventative maintenance programs for our clients. The services can be customized to include hot and cold inspections, thermal imaging, burner tuning, and equipment optimization analysis.
Consistency Is Key To Increase Furnace Life
Given that the roller hearth furnace is a continuous system, understanding how the system works and operating at the correct, consistent rate is crucial for success. Jacob Laird, mechanical engineer at Premier Furnace Specialists, dives in deeper.
What Features Do You Offer on Your Roller Hearth Furnace for Annealing Various Materials?
Premier Furnace Specialists (PFS) is capable of building annealing furnaces for a wide range of workloads. We have built small batch normalizing furnaces with simple manual roller hearths, as well as 120+ foot long fully automated annealing roller hearths with multi-zone control and automatic load staging. We also offer a variety of controlled heating/cooling systems and atmosphere generators and gas dryers to provide optimal annealed part quality. Controlled cooling systems may include: radiant tube indirect cooling, atmosphere forced convective cooling, and post-process forced convective cooling with ambient air.
How Is Your Roller Hearth System Unique?
Jacob Laird Mechanical Engineer Premier Furnace Specialists, Inc./BeaverMatic Source: Premier Furnace Specialists, Inc./BeaverMatic
One of the unique uses for annealing furnaces is for soft magnetic steel alloys aft er they have been cold worked or formed. This is often used for products inside electrical equipment such as electric motors or transformers where grain growth and residual stresses may affect the magnetic properties of the material. The most cost-effective process for this heat treatment is through a continuous atmosphere with a reducing atmosphere (often provided by a lean Exothermic gas atmosphere). For this process, the atmosphere requires a specific range of hydrogen alongside a controlled heating and cooling recipe with multiple stages.
Premier Furnace Specialists also provides the accessory equipment that can be required for a complete annealing operation. We will build the Exothermic gas generators (rich and lean), Exothermic gas dryers (air and water cooled), nitrogen/methanol/hydrogen (or other bulk atmosphere) gas trains/delivery systems, water recirculation and convective cooling systems, load management equipment/software, and any other required pre/post processing equipment right here at our facility in Farmington Hills, MI. By building all of the ancillary equipment alongside the annealing furnace, it allows the client to benefit from installation of the entire system at once, identical spare parts across all pieces for easier maintenance, identical control systems with consistent terminology for ease of operator training, a single contact source for all engineering assistance and troubleshooting, as well as a service department capable of quickly responding to requests for both our equipment and any other equipment the customer may already have.
A 16 ½ ft. wide x 9 ft. high x 125 ft. long roller hearth furnace with four
heating zones and two cooling zones. Maximum temperature of 1500°F,
nitrogen gas atmosphere.
Concerning efficiency, combustion heating systems can be customized with preheat and recuperation systems, recuperative or regenerative burners, or multi-legged radiant tubes to minimize gas train complexity and NOx emissions while maximizing efficiency and profitability. Electrically heated systems can be equipped with SCR power controls which minimize temperature swings at setpoint, provide optimum work chamber uniformity by eliminating heat surges, and conserve energy by reducing current draw at operating temperature. Processes can also be equipped with digital atmosphere analyzers, flowmeters, and gauges capable of displaying the remote equipment conditions at localized control stations or on mobile devices.
What Are Best Practices for Training In-House Operators on Roller Hearth Annealing?
Specifically for roller hearth furnaces, operators and maintenance personnel must understand the rollers and drive systems to ensure products continue processing at a correct rate. For continuous systems in particular, drive failures may result in the loss of large volumes of product that often cannot be recycled as well as lengthy purge/shutdown/ startup times.
As an example, chain and sprocket driven rollers just only be locked down on the drive side of the furnace so that thermal expansion allows them to grow on the idle side. Otherwise, the sprockets may walk out of alignment and cause a multitude of long and short-term issues such as rollers seizing and warping, drive faults, load crashes, and timing issues between multiple driven segments.
Th e best practice would be for operators to be trained to understand how major components of the furnace may affect the part quality. This knowledge will also assist in troubleshooting issues that may arise and correcting them before they become worse.
What Are the Furnace’s Operational Advantages?
Roller hearth furnaces can handle a large assortment of part sizes by varying the roller diameters and spacing between them. For small parts, the rollers can be used to drive a mesh/cast belt or convey trays. For long parts, they can rest on the rollers with multiple support points. The bar, pipe, and tubing industries use incredibly long roller hearth furnaces while many industries process heavy wire coils in them. By segmenting the roller drives and utilizing VFDs or servomotors, roller hearth furnaces become capable of staging loads, customizing processing times, and oscillating at temperature to prevent rollers from warping under heavy loads.
What Is the Most Common Heating Method?
Premier has seen a steady demand for gas fired roller hearth equipment, but most quotes nowadays also request pricing for an electric alternative to compare against. The end user’s facility location and local utility regulations are typically the deciding factor.
What Are the Challenges in Operating This Type of Furnace?
A common challenge for any continuous furnace is maintaining consistent production and limiting shutdowns or idle periods. Large continuous furnaces burn up a significant amount of energy even when idling, so any time spent not in production becomes costly. Even when the equipment sits powered down, start-up procedures including insulation dry-outs, inert gas purge requirements, and atmosphere seasoning can take days until production can resume.
However, once consistent production is maintained, part quality, part consistency, and energy efficiency can be noticeably better than batch equivalents.
Issues can be avoided by noting areas of concern as they arise and following routine maintenance procedures until scheduled annual or biannual shutdowns (often around holiday breaks). Then additional time can be given to address potentially major issues with service visits and inspections by OEM service teams.
An Eye on Energy
Reiterating the customizable nature of this style furnace, Ryan Sybo, project manager at SECO/WARWICK USA, comments on the attention on energy usage that clients and suppliers share.
What Features Do You Offer on Your Roller Hearth Furnace for Annealing Various Materials?
Ryan Sybo, Project Manager,
SECO/WARWICK USA
We offer a wide variety of options as a custom furnace company. We can tailor the furnace to meet the unique needs of individual clients. On annealing furnaces specifically, we offer a controlled cool chamber and a steam blue chamber.
Individual roll sections can be started, stopped, reversed, oscillated, and run at the same speed or at different speeds for maximum process versatility.
Atmosphere integrity is assured through welded gas-tight shells, sealing doors, and pressure control systems.
Fast and slow heating and cooling rates are possible. Plus, pre-heating can be employed.
Post-heat treating processes like steam blue are possible.
Furnace doors are specially constructed and insulated for operation within the temperature zones in which they are located, minimizing stress and warpage caused by temperature differences.
Heating and cooling sections incorporate dependable, high-quality components for long-life operation.
High-speed transfer between sections allows closely spaced workloads or work trays with separation during transfer through doors, assures optimum use of hearth space, and minimizes atmosphere mixing. All door openings can be adjusted to workload heights, permitting faster operation and minimizing atmosphere mixing.
How Is Your Roller Hearth System Unique?
The controlled cool chamber offers precise control of the cooling rate. The steam blue chamber is used to develop a blue oxide, Fe3O4, for electrical insulation characteristics.
Our company has been designing and manufacturing furnaces for over 123 years, and we have been exploring new refractory materials and more energy-efficient burners and recuperators, as well as offering state-of-the-art atmosphere controls.
What Are Best Practices for Training In-House Operators on Roller Hearth Annealing?
Our furnaces are all built to the latest NFPA 86 and OSHA standards, however, safety training like HMI is also important.
What Are the Furnace’s Operational Advantages?
Several of this furnace’s operational advantages include:
Continuous Unlimited Work Flow: Provides better efficiency than batch processing since the workload can continuously feed into the furnace.
Quick, Easy Installation: For SECO/WARWICK USA, these are normally built at our manufacturing facility and tested, then disassembled into sections to fit on a truck or shipping container.
Long Life: A 40-year lifespan is typical. Less stress on furnace components from faster or constant temperature recycling when compared to belt, chain, or pusher units.
Smaller Factory Footprint: Manufacturers can save about half of the floor space than with multiple batch units.
Flexible Operation: Roller drives can be slowed, sped up, or stopped. Process parameters can be changed, any atmospheres can be used from H2 to air, plus door separations can be used between sections for better separate processing functions.
Lower Production Costs: Each furnace is custom-designed for continuous operation at the desired operating temperatures. Less waste from heat-up and cool-down cycles used in batch systems and in-line processing makes energy recuperation easier to integrate.
What Is the Most Common Heating Method?
Gas fired is the most common, however, we have been seeing a lot of inquiries for electrically heated roller hearth furnaces.
Geographic location is also a big determining factor because some areas have more access to natural gas that can offer reduced operating costs.
What Are the Challenges in Operating This Type of Furnace?
There are no challenges in operating this type of furnace due to our custom-engineered, user-friendly automatic furnace controls. Preventative maintenance can be included in our control systems to remind operators and maintenance personnel to service the equipment. Furnace data and alarms are logged and ready for download and review.
Leveraging Efficient Designs To Process Heavy Workloads
Kelley Shreve, general manager at Lindberg/MPH, hones in on the significance of roller hearth furnace workload capacity as a lynchpin to heat treat operations.
What Features Do You Offer on Your Roller Hearth Furnace for Annealing Various Materials?
Kelley Shreve General Manager Lindberg/MPH Source: Lindberg/MPH
Our roller hearth furnaces are designed to meet the need for accurate, consistent, and efficient processing of heavy workloads. Features include a sturdy roll design for smooth load motion, high-efficiency heating systems for rapid heat transfer, integrated control systems for accuracy of operation and ease of troubleshooting, and material handling systems that simplify operation. Together, these features provide furnaces that will make operations more competitive.
How Is Your Roller Hearth System Unique?
What separates Lindberg/MPH from competitors is our ability to take standard designs and customize them so they are tailor-suited to meet the exact client specifications and floor plans.
Extensive experience in ultra-clean heat treating helped us improve roller hearth equipment as well. Traditionally designed furnaces have transfer sections open to air, which allows rapid heat loss and causes scaling or discoloration of the work. Our proven design shields the work with a directed flow of protective atmosphere through double-door transfer sections. This also ensures isolation of furnace zones that must not be cross-contaminated. An independent, high-speed roll system minimizes transfer time and heat loss. The sight-ports allow direct viewing of work-in-process for easy troubleshooting. The result is clean, consistent work.
What Are Best Practices for Training In-House Operators on Roller Hearth Annealing?
Lindberg/MPH offers complete installation packages which include installation, startup, and training. In-house operators are fully trained on all aspects of operations while our service technician is present.
What Are the Furnace’s Operational Advantages?
Our roller hearth furnaces combine the latest technology in process controls, atmosphere systems, and material handling systems. These furnaces are designed to carry very heavy workloads at high production rates at the lowest possible operating cost. Other advantages are that a roller hearth can be designed to run a multitude of different processes as required.
What Is the Most Common Heating Method?
Roller hearth furnaces can be supplied with either gas fired or electric heating. Gas fired Single End Recuperated Tubes (SERT) provide economical, rapid heating. Electric heating offers reliable, low-maintenance operation using elements tailored to atmosphere application.
What Are the Challenges in Operating This Type of Furnace?
Annealing produces
parts with reduced
hardness and a uniform
microstructure as a
preparation for further
processing. The furnace
has a high-heat section
followed by a controlled
cooling module.
Endothermic, Exothermic,
and nitrogen-methanol
atmospheres are typically
used.
A challenge for this type of furnace is proper maintenance. Operators should manage this challenge by monitoring the preventative maintenance (PM) features and indicators that are available. Setting in place and following a regular PM schedule is going to help ensure the equipment operates dependably and problem free.
How often do you think about the intelligent designs controlling the thermal loop system behind your heat treat operations? With ever-advancing abilities to integrate and manage data for temperature measurement and power usage, the ability of heat treat operations to make practical, efficient, and energy-conscious change is stronger than ever. In part 1, understand several benefits of thermal loop systems and how they are leveraged to comply with industry regulations, like Nadcap.
This Technical Tuesday article by Peter Sherwin, global business development manager – Heat Treatment, and Thomas Ruecker, senior business development manager, at Watlowwas originally published inHeat Treat Today’sJanuary/February 2024 Air & Atmosphere Heat Treat print edition.
Introduction
Heat treatment processes are a crucial component of many manufacturing industries, and thermal loop solutions have become increasingly popular for achieving improved temperature control and consistent outcomes.
A thermal loop solution is a closed loop system with several essential components, including an electrical power supply, power controller, heating element, temperature sensor, and process controller. The electrical power supply provides the energy needed for heating, the power controller regulates the power output to the heating element, the heating element heats the material, and the temperature sensor measures the temperature. Finally, the process controller adjusts the power output to maintain the desired temperature for the specified duration, providing better temperature control and consistent outcomes.
Performance Benefits
Heat treatment thermal loop solutions offer several advantages over traditional heat treatment methods, including improved temperature control and increased efficiency. The thermal loop system provides precise temperature control, enabling faster heating and cooling and optimized soak times. In addition, the complete design of modern thermal loop solutions includes energy-efficient heating and overall ease of use.
Figure 1. Watlow Industry 4.0 solution (Source: Watlow)
Heat treatment thermal loop solutions are integrated with Industry 4.0 frameworks and data management systems to provide real-time information on performance. Combining artificial intelligence and machine learning algorithms can also provide additional performance benefits, such as the ability to analyze data and identify patterns for further optimization. Ongoing performance losses in a heat treatment system typically come from process drift s. Industry 4.0 solutions can explore these drift s and provide opportunities to minimize these deviations.
Heat treatment thermal loop solutions can be optimized using Failure Mode and Effects Analysis (FMEA). FMEA is a proactive approach to identifying potential failure modes and their effects, allowing organizations to minimize the risk of process disruptions and improve the overall efficiency of their heat treatment processes. Historically, this was a tabletop exercise conducted once per year with a diverse team from across the organization. Updates to this static document were infrequent and were primarily based on organization memory rather than being automatically populated in real time with actual data. There is a potential to produce “live” FMEAs utilizing today’s technology and leveraging insights for continuous improvement.
Th e effectiveness of heat treatment thermal loop solutions can be measured using metrics such as overall equipment effectiveness (OEE). OEE combines metrics for availability, performance, and quality to provide a comprehensive view of the efficiency of a manufacturing process. By tracking OEE and contextual data, organizations can evaluate the effectiveness of their heat treatment thermal loop solutions and make informed decisions about optimizing their operations.
Regulatory Compliance
Nadcap (National Aerospace and Defense Contractors Accreditation Program) is an industry-driven program that provides accreditation for special processes in the aerospace and defense industries. Heat treatment is considered a “special process” under Nadcap because it has specific characteristics crucial to aerospace and defense components’ quality, safety, and performance. Th ese characteristics include:
Process sensitivity: Heat treatment processes involve precise control of temperature, time, and atmosphere to achieve the desired material properties. Minor variations in these parameters can significantly change the mechanical and metallurgical properties of the treated components. This sensitivity makes heat treatment a critical process in the aerospace and defense industries.
Limited traceability: Heat treatment processes typically result in changes to the material’s microstructure, which are not easily detectable through visual inspection or non-destructive testing methods. Th is limited traceability makes it crucial to have strict process controls to ensure the desired outcome is achieved consistently.
Critical performance requirements: Aerospace and defense components often have strict performance requirements due to the extreme conditions in which they operate, such as high temperatures, high loads, or corrosive environments. The heat treatment process ensures that these components meet the specifications and can withstand these demanding conditions.
High risk: The failure of a critical component in the aerospace or defense sector can result in catastrophic consequences, including loss of life, significant financial loss, and reputational damage. Ensuring that heat treatment processes meet stringent quality and safety standards is essential to mitigate these risks.
Nadcap heat treatment accreditation ensures suppliers meet industry standards January/February and best practices for heat treatment processes. The accreditation process includes rigorous audits, thorough documentation, and ongoing process control monitoring to maintain high quality, safety, and performance levels.
The aerospace industry’s AMS2750G pyrometry specification and the automotive industry’s CQI-9 4th Edition regulations are crucial for ensuring consistent and high-quality heat treated components. Adherence to these regulations is essential for meeting the stringent quality requirements of the aerospace and automotive industries and other industries with demanding specifications.
Temperature uniformity is a crucial requirement of both AMS2750G and CQI-9 4th Edition, mandating specific temperature uniformity requirements for heat treating furnaces to ensure the desired mechanical properties are achieved throughout the treated components. AMS2750G class 1 furnaces with strict uniformity requirements +/-5°F (+/-3°C) provide both quality output and predictable energy use. However, maintaining this uniformity requires significant maintenance oversight due to all the components involved in the thermal loop.
Calibration and testing procedures are specified in the standards to help ensure the accuracy and reliability of the temperature control systems used in heat treat processes.
Detailed process documentation is required by AMS2750G and CQI-9 4th Edition, including temperature uniformity surveys, calibration records, and furnace classifications. This documentation ensures traceability, enabling manufacturers to verify that the heat treat process is consistently controlled and meets the required specifications.
Figure 2. Eurotherm data reviewer (Source: Watlow)
Modern data platforms enable the efficient collection of secure raw data (tamper-evident) and provide the replay and reporting necessary to meet the standards.
Th e newer platforms also off er the latest industry communication protocols – like MQTT and OPC UA (Open Platform Communications Unifi ed Architecture) – to ease data transfer across enterprise systems.
MQTT is a lightweight, publish-subscribe- based messaging protocol for resource-constrained devices and low-bandwidth, high-latency, or unreliable networks. IBM developed it in the late 1990s, and it has become a popular choice for IoT applications due to its simplicity and efficiency. MQTT uses a central broker to manage the communication between devices, which publish data to “topics,” and subscribe to topics that they want to receive updates on.
OPC UA is a platform-independent, service-oriented architecture (SOA) developed by the OPC Foundation. It provides a unified framework for industrial automation and facilitates secure, reliable, and efficient communication between devices, controllers, and software applications. OPC UA is designed to be interoperable across multiple platforms and operating systems, allowing for seamless integration of devices and systems from different vendors.
The importance of personnel and training is emphasized by CQI-9 4th Edition, which requires manufacturers to establish training programs and maintain records of personnel qualifications to ensure that individuals responsible for heat treat processes are knowledgeable and competent. With touchscreen and mobile integration, a significant development in process controls has occurred over the
last decade.
Figure 3. Watlow F4T® touchscreen and Watlow PM PLUS™ EZ-LINK®
mobile application
By integrating these regulations into a precision control loop, heat treatment thermal loop solutions can provide the necessary level of control and ensure compliance with AMS2750G and CQI-9 4th Edition, leading to the production of high-quality heat treated components that meet performance requirements and safety standards.
Continuous improvement is also emphasized by both AMS2750G and CQI-9 4th Edition, requiring manufacturers to establish a system for monitoring, measuring, and analyzing the performance of their heat treatment systems. This development enables manufacturers to identify areas for improvement and implement corrective actions, ensuring that heat treat processes are continuously improving and meeting the necessary performance and safety standards.
To Be Continued in Part 2
In part 2 of this article, we’ll consider the improved sustainability outcomes, potential challenges and limitations, and the promising future this technology offers to the heat treat industry.
About the Authors
Peter Sherwin, Global Business Development Manager – Heat Treatment, WatlowThomas Ruecker, Senior Business Development Manager, Watlow
Peter Sherwin is a global business development manager of Heat Treatment for Watlow and is passionate about offering best-in-class solutions to the heat treatment industry. He is a chartered engineer and a recognized expert in heat treatment control and data solutions.
Thomas Ruecker is the business development manager of Heat Treatment at Eurotherm Germany, a Watlow company. His expertise includes concept development for the automation of heat treatment plants, with a focus on aerospace and automotive industry according to existing regulations (AMS2750, CQI-9).
For more information: Contact peter.sherwin@watlow.com or thomas.ruecker@watlow.com.
This article content is used with the permission of heat processing, which published this article in 2023.
Find Heat Treating Products And Services When You Search On Heat Treat Buyers Guide.Com
In Part 1, the author underscored the importance of understanding the changes in gas composition through three steps of its production: first, the production in the combustion chamber; second, the cool down of gas to bring the Exothermic gas (Exo gas) to below the ambient temperature; and third, the introduction of the gas to the heat treat furnace. Read Part 1, published in Heat Treat Today’sAugust 2023Automotive Heat Treat print edition, to understand what Exo gas is and to learn about the composition of gas in the first step.
Harb Nayar
Founder and President TAT Technologies LLC Source: TAT
As the author demonstrated in Part 1, Exo gas composition changes in its chemistry for heat treatment; this first step is how the gas composition changes when it is produced in the combustion chamber. The composition of reaction products, temperature, Exothermic energy released, various ratios, and final dew point are all factors that need to be considered to protect metal parts that will be heat treated in the resulting atmosphere.
Now, we’ll turn to Steps 2 and 3.
Step 2: Composition of Exo Gas after Exiting the Reaction Chamber Being Cooled Down
The two examples that follow demonstrate how lean and rich Exo under equilibrium conditions change as they are cooled from peak equilibrium temperature in the combustion chamber down to different lower temperatures (Table B). This cool down brings the Exo down to below ambient temperatures to avoid water condensation.
Example 1: Lean Exo Gas with a 9:1 Air to CH₄ Ratio
The first column highlighted in blue shows the composition of the lean Exo gas as generated in the reaction chamber with an air to natural gas ratio of 9:1. The peak temperature as generated in the combustion chamber is 3721°F. The next four columns show how the composition changes when the lean Exo gas is slowly cooled from 3721°F to 2000°F, 1500°F, 1000°F, and 500°F under equilibrium condition. The following key changes take place as the temperature of the lean Exo is lowered from the peak temperature to 500°F:
Hydrogen volume almost triples from 0.67% to 1.97%.
H₂O volume decreases slightly from 19.1% to 17.5%, but still is very high at all temperatures.
Oxidation-reduction potential (ORP) changes as the H₂ to H₂O ratio increases from 0.035 to 0.111. At all temperatures, it is very low.
CO and the CO to CO₂ ratio drop in a big way, making lean Exo from being decarburizing at higher temperatures to being highly decarburizing at lower temperatures.
The percentage of N₂ remains at 70.34 at all temperatures.
There is no C (carbon, i.e., soot) or residual CH₄ at all temperatures.
For all practical purposes, at an air to natural gas ratio of 9:1, the Exo gas as generated is predominantly an N₂ and H₂ (steam) atmosphere with some CO₂ and small amounts of H₂ and CO.
Table B. Air to Natural Gas at 9:1 and 7:1, cooled to various temperatures
Example 2: Rich Exo Gas with a 7:1 Air to CH₄
The column under ratio of seven is highlighted as red to show the composition of the rich Exo gas as generated in the reaction chamber with an air to CH₄ ratio of seven. The peak temperature is 3182°F — significantly lower than that for lean Exo. The next four columns show how the composition changes when the rich Exo gas is slowly cooled from 3182°F to 2000°F, 1500°F, 1000°F, and 500°F. The following key changes take place as temperature of the rich Exo is lowered from the peak temperature to 500°F:
Hydrogen volume almost doubles from 5.58% at peak temperature to 9.91% at 1000°F, and then it drops to 5.70% at 500°F. The overall volume of H₂ in rich Exo is significantly higher than in lean Exo.
H₂O volume decreases slightly from 17.9% to 15.1%, but it is still very high at all temperatures.
Oxidation-reduction potential (ORP) changes as the H₂ to H₂O ratio increases from 0.312 at peak temperature to 0.737 at 1000°F before decreasing to 0.377 at 500°F. Overall, ORP in rich Exo is significantly higher than that in lean Exo.
CO and the CO to CO₂ ratio drop in a big way, making it mildly decarburizing to more decarburizing
The percentage of N₂ remains at 65– 67%, which is lower than lean Exo.
There is no C (carbon, i.e., soot) at any temperature. However, there is residual CH₄ at 1000°F and lower. This increases rapidly when cooled slowly below 1000°F.
For all practical purposes, the rich Exo gas (at air to natural gas ratio of 7:1) generated is still predominantly a H₂
and H₂O (steam) atmosphere, but with more H₂; hence, it has somewhat higher oxidation-reduction potential (ORP) than lean Exo and a bit higher CO to CO₂ ratio (less decarburizing than lean Exo).
In summary, rich Exo as generated in the combustion chamber differs from lean Exo as follows:
It has a little less N₂ % as compared to lean Exo.
It has significantly more H₂ , but a little less H₂O than lean Exo. As such, it has a significantly higher H₂ to H₂O ratio (ORP).
It is decarburizing, but less than lean Exo.
It has residual CH₄ at temperatures below 1000°F. Therefore, it must be cooled very quickly to suppress the reaction of developing too much residual CH₄.
Discussion
Let us take the example of rich Exo (an air to natural gas of 7:1) exiting from the reaction chamber in Table B (see column highlighted in red). The total volume is 853.3 SCFH and has H₂O at 152.4 SCFH (17.9% by volume). This is equivalent to dew point of 137°F. Its H₂ content is 47.6 SCFH (5.58% by volume). And the H₂ to H₂O ratio is 0.312.
If this were quenched to close to ambient temperature “instantly,” this composition would be “frozen,” except most of the H₂O vapor will become water. Let us assume the Exo gas was instantly quenched to 80°F (3.6% by volume after condensed water is removed). Rough calculation shows that the final total volume of H₂O vapor has to be reduced from 152.4 SCFH to about 26.0 SCFH in order to meet the 80°F dew point goal. This means 152.4 – 26.0 = 126.4 SCFH of H₂O vapor got condensed to water.
Now the total volume of Exo gas after cooling down to 80°F= 853.35 – 126.4 = 726.95 SCFH, or almost 15% reduction in volume of Exo gas as compared to what was generated in the reaction chamber.
Of course, the composition of Exo gas will not be the same as calculated above. The exact composition after being cooled down depends upon the following:
a. Cooling rate of the reaction products from the peak temperature in the reaction chamber to some intermediate temperature, typically around 1500°F.
b. Cooling rate of the gas from the intermediate temperature to the final (lowest) temperature via water heat exchangers — typically 10–20°F below ambient temperature unless a chiller or dryer is installed on the system.
Depending upon the overall design of the generator, especially how Exo gas coming out of the combustion chamber is cooled and maintained during the period of its use, the expected Exo gas composition should be in the range of the light red columns in Table B — where temperatures are between 1500°F to 1000°F — however:
Total volume closer to 727 SCFH (since a major portion of H₂O was condensed out)
N₂ between 74–77%
Dew point between 80–90°F
CH₄. between 0.1–0.5%
H₂ percentage between 7–9%
Step 3: Composition of Exo Gas after Being Introduced into the Heat Treat Furnace
The cooled down Exo gas will once again change its composition depending upon the temperature inside the furnace where parts are being thermally processed.
As an illustration, let us assume the following composition of the rich Exo gas (with a 7:1 air to natural gas ratio) at ambient temperature just before it enters the furnace:
Total volume: 727 SCFH
H₂: 8% (58.16 SCFH)
Dew Point 86°F or 4.37% (31.77 SCFH)
CO: 6% (43.62 SCFH)
CO₂: 6% (43.62 SCFH)
CH₄ : 0.4% (2.91 SFH)
Balance N₂ (%)
75.23% (546.92 SCFH)
Table C shows how the composition changes once it reaches the high heat section of the furnace where parts are being thermally treated. The column highlighted blue shows the composition of Exo gas as it is about to enter while it is still at the ambient temperature. The next three columns show the composition of the Exo gas in the high heat section of furnaces operating at three different temperatures depending upon the heat treat application — 1100°F like annealing of copper, 1500°F like annealing of steel tubes, and 2000°F like copper brazing of steel products. The H₂ to H₂O ratio decreases as temperature increases.
Other general comments on Exo generators:
Generally, they are horizontal.
Size ranges from 1,000 to 60,000 SCFH.
Rich Exo generators use Ni as a catalyst in the reaction chamber. Lean Exo does not.
Lean Exo generators typically operate at a 9:1 air to natural gas ratio. There is no carbon/soot buildup.
Rich Exo generators typically operate at a 7:1 air to natural gas ratio. Below about 6.8 and lower ratios, soot/carbon deposits start appearing that require carbon burnout as part of the maintenance procedure.
Table C. Exo gas compositions in heat treat furnaces
Conclusions
A walkthrough of the entire cycle of gas production to cool down to use in the high heat section of the furnace clearly shows that as temperature changes, so does the Exo gas composition for any air to natural gas ratio.
Having a well-controlled composition of Exo gas requires the following:
Well-controlled composition of the natural gas used
Air supply with controlled dew point
Highly accurate air and natural gas mixing system
Highly controlled and maintained cooling system
A reliable ORP analyzer or the H₂ to H₂O ratio analyzer as part of the Exo gas delivery system.
Protecting metallic workpieces is paramount in heat treating, and in order to do this, the atmosphere created by Exothermic gas must be understood, both in the cool down phase and within the heat treat furnace. For further understanding of the good progress made in the improvement of Exo generators, see Dan Herring’s work in the reference section below.
Harb Nayar is the founder and president of TAT Technologies LLC. Harb is both an inquisitive learner and dynamic entrepreneur who will share his current interests in the powder metal industry and what he anticipates for the future of the industry, especially where it bisects with heat treating.
Have you decided to purchase batch or continuous furnace system equipment? Today's episode is part 2 of the Heat Treat Radio lunch & learn episode begun with Michael Mouilleseaux of Erie Steel. Preceding this episode were Part 1 (episode #102) and a Technical Tuesdaypiece, so listen to the history of these systems, equipment and processing differences, and maintenance concerns before jumping into this episode about capability and throughput.
Doug Glenn,Heat Treat Todaypublisher and Heat Treat Radio host; Karen Gantzer, associate publisher/editor-in-chief; and Bethany Leone, managing editor, join this Heat Treat Today lunch & learn.
Below, you can watch the video, listen to the podcast by clicking on the audio play button, or read an edited transcript.
The following transcript has been edited for your reading enjoyment.
An Example: Carburizing (00:52)
Michael Mouilleseaux: What we want to do here is just compare the same part, the same heat treating process, processed in a batch furnace and processed in a pusher.
Figure 1: Carburizing Load Example (Source: Erie Steel)
Here we’re just going to make an example:
Pusher Load Description (00:58)
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I’m going to take a fictious gear: it’s 2 ¾ inch in diameter, it’s got an inside diameter of an inch and a quarter, it’s an inch and a half tall, and it weighs 1.25 pounds. For our purposes here, we’re going to put these in a cast basket. For the furnace that we’re going to put them in, the basket size is 36 inches square — so, it’s 36 x 36. The height in this pusher furnace is going to be 24 inches; the inside dimensions of a 36-inch basket (actually it’s a 35-inch basket that sits on a 36-inch tray) is 32 ½ inches.
Michael Mouilleseaux General Manager at Erie Steel, Ltd. Sourced from the author
We’re going to say that this basket is 18 inches tall, so we’re going to get 7 layers of parts so that there’s approximately 1 inch between each layer of parts. This loading scheme gets us 700 pieces in a basket; it gets us 875 pounds net.
So the 36-inch basket that’s 18 inches tall and we’ve got 10 rows of 10 pieces, and we’ve got 7 layers of these things, so we have some room in between them. The reason for that is circulation of atmosphere and quenchant. This is what’s going to constitute the pusher load.
Batch Load Description (03:09)
Now, when we go to the batch load, we’re going to take four of these, because the batch furnace that we’re going to compare this to is going to be 36 inches wide and it’s going to be 72 inches long. We have two baskets on the bottom, 36, and then two of them is 72, and two on top. They’re 18 inches high, so 18 and 18 is 36 — a standard 36 x 72. It’s got 40 inches of height on it. I can take that 36 inches, put it on a 2 ½-inch tray and I can get it in and out of the furnace.
What is this four baskets? 2800 pieces in a load and 3500 pounds. That’s the difference. I’m comparing one basket, 700 pieces and 875 pounds and we’re going to compare that to what we would do if we ran a batch load, which is significantly more. It’s 2800 pieces and 3500 pounds.
What do we want to do with this?
Let’s say that we’re going to carburize this, and we want 50 thousandths case (total case depth of 0/050”). Now, I will show you very soon why we’ve chosen 50 thousandths case. Because at 1700°F (which is what we’re going to carburize at), the diffusion rate is 25 thousandths of an inch times the square root of time.
Now, I can do that math in my head. 25 thousandths times 2 is 50 thousandths. That means we need four hours. So, the part would have to be in the furnace for four hours, at temperature, carburizing, in order to achieve 50 thousandths case.
Figure 2: Batch IQ Carburizing Load (Source: Erie Steel)
Batch Furnace Time (04:59)
Let’s look at the next section. As we said, the furnace is 36 x 72 x 36 and we have 2800 pieces in the load. So, that is 1700°F. We’re going to say that there is 3500 pounds and there is probably another 800 or 900 pounds in fixturing so that’s about 4500 pounds. It’s very conservative; in a 36 x 72 furnace, you could probably get away with running 6,000 pounds. This is just a load that is well within the capability of that.
Furnace recovery is going to take two hours.
Doug Glenn: Meaning, it’s going to take you two hours to get up to temperature.
Mike Mouilleseaux: Until the entirety of the load is at 1700°F, that’s right. Inside, outside, top to bottom.
We’re going to carburize this at four hours, as we described previously; we calculated that and we need four hours to get our 50 thousandths case. Then we’re going to reduce the temperature in the furnace to 1550°F so that we can quench it.
So, we have two hours of furnace recovery, four hours at carburizing, two hours to reduce the temperature and attain a uniform 1550°F. That’s eight hours, and that’s what you would term an 8-hour furnace cycle.
We know that we have 2800 pieces in the load. In eight hours (2800 divided by 8) you’ve got 350 pieces/hour. That’s what the hourly productivity would be in this load.
We won’t talk about “what could we do.” There’s a lot of things that we could do. This is simply an example.
Pusher Furnace Time (07:05)
Now, in the pusher load, as previously described, it’s 36 x 36 and it’s 24 inches high. Now, we know that we have a basket that’s 18 inches high. Again, it’s going to sit on a 2-inch tray, so we’ve got 21 inches of the top of the basket that is going to fit in the furnace; there are going to be no issues with that whatsoever.
The controlling factor is that we want four hours at temperature. In the boost and diffuse, we have four positions. The furnace cycles once per hour.
We get one load size (700 pieces, 875 pounds) every hour. So, in this example (an 8-position, 36-square pusher) this process would yield 700 pieces an hour, and a batch furnace loaded as we described (same exact loading and number of pieces/basket) would yield 350 pieces/hour. In this scenario, the pusher furnace is going to produce twice the number of parts/hour that the batch would.
So, you would say, “Well, let’s just do that.” What you have to understand is that every hour, you are going to produce 700 pieces. If we went back and we looked at that description of what that pusher system looked like, you would see there are 23 positions in that. When I load a load, it’s going to be 23 hours before the first load comes out.
What we’re talking about is whether or not there were 700 pieces and 800 pounds, 23 of those[ET10][BL11] load.
The point would be, you either have to have enough of the same product or enough of similar product that can be processed to the same process to justify using something like this. Because if we want to change the cycle in the furnace. So, can we do that? The answer is absolutely, yes.
The preheat there, that stays at relatively the same temperature. The first zone in the furnace where we’re preheating the load, that temperature can be changed, as can the temperature in the boost diffuse and/or cycle time.
Figure 3: Pusher Furnace System (Source: Erie Steel)
So, in our example, we used an hour. What if you wanted 40 thousandths case and you’re going to be closer to 45 minutes or 50 minutes of time, how would you accomplish that? That can be done.
Typically, commercial heat treaters would come up with a strategy on how to cycle parts in and hold the furnace, or how many empties you would put in the furnace before you would change the furnace cycle.
Obviously, in the last two positions, where you’re reducing temperature, you could change the temperature in either the first two positions, where you’re preheating the load, or you could change the carburizing temperature, because when we’re dropping the temperature, it doesn’t have a material effect upon that.
Typically, in an in-house operation, you would not do that kind of thing, for a couple of reasons, not the least of which would be considering the type of people that you have operating these furnaces. They come in and out from other departments, and this is the kind of thing that you would want someone experientially understanding the instructions that you’ve given them. The furnace operator is not necessarily going to be the one to do it; this may be a pre-established methodology. You want them to execute that. But if you have somebody that is running a grinder and then they’re running a plating line and then they’re coming and working in the heat treat, that would not be the recipe for trying to make these kinds of changes.
As I described to you before, I worked in another life where we had 15 pushers. They were multiple-row pushers. We made 10,000 transfer cases a day. The furnace cycle on every furnace was established on the 1st of January, and on the 31st of December it was still running the same furnace cycle. You never changed what you were doing. The same parts went into the same furnaces and that’s how they were able to achieve the uniform results they were looking for.
Pusher Furnaces and Flexibility (12:45)
So, the longer the pusher furnace is, the less flexible it is.
In this example, you have eight. You know, there are pusher furnaces that have four positions. If you think about it, in a 4-position furnace, you could empty it out pretty quickly and change the cycle.
There are a lot of 6-position pusher furnaces in the commercial heat treating industry; that seems to be a good balance. The number of multiple-row pushers in the commercial industry, they’re fewer and far between. I’m not going to say they’re nonexistent, but enough of the same kind of product to justify that is difficult.
I think the bottom line here is, for companies that are having high variability, low quantity, low volume loads, generally speaking, your batch is going to be good because it’s very flexible, you can change quickly.
However, with a company like the one you were describing where there is low variability and very high volume, pushers are obviously going to make sense. But there is a whole spectrum in between there where you’re going to have to figure out which one makes more sense — whether you’re going to go with a batch or a continuous.
Mike Mouilleseaux: Possibly underappreciated is the aspect of distortion.
In that carburizing example, you’d say, “We have an alloy steel, we’re aiming for 50 thousandths case — what’s the variation within a load?” And I’m going to say that it is going to be less than 5 thousandths, less than 10%. From the top to the bottom, the inside to the outside, it’s going to be less than 5 thousandths. That same process, in the pusher furnace is going to be less than 3 thousandths.
That’s one aspect of the metallurgy. The other aspect is quenching.
Doug Glenn: 5 thousandths versus 3 thousandths — 3 thousandths is much more uniform, right?
Mike Mouilleseaux: Correct.
Doug Glenn: And that’s good because that way the entire load is more consistent (in the continuous unit, let’s say).
Mike Mouilleseaux: That is correct.
Then there is the consistency in quenching. In the batch furnace, you’re quenching 36 inches of the parts. If we had seven layers in the pusher, we have 14 layers of parts in the batch. What are the dynamics involved in that?
We have experience that the ID of a gear (it’s a splined gear) in a batch furnace, we were able to maintain less than 50 microns of distortion. There is a lot involved in that, that’s not for free; there’s a fair amount involved in that and it’s a sophisticated cycle, if you will. That same cycle in a pusher furnace, same case depth, similar quenching strategy, will give you less than half that amount of distortion.
To the heat treater, where we’re talking about the metallurgy of this, you’re going to think 5 thousandths or 3 thousandths is not a big deal.
To the end-user, that reduction in distortion all of a sudden starts paying a number of benefits. The amount of hard finishing that has to be done or honing or hard broaching or something of that nature suddenly becomes far more important.
Doug Glenn: Yes. That adds a lot of money to the total process, if you’ve got to do any of those post heat treat processes.
Mike Mouilleseaux: To a large extent, that is due to the fact that you have a smaller load. If you have a smaller load, you have less opportunity for variation — it’s not that it’s all of a sudden magic.
Doug Glenn: And for the people that don’t understand exactly what that means, think about a single basket that goes into a quench tank and four baskets, arranged two on top and two on bottom. The parts in the middle of that are going to be quenched more slowly because the quench is not hitting it as much.
So, the cooling rates on a stacked load are going to be substantially different than for a single basket, and that’s where distortion can happen.
Mike Mouilleseaux: There are a tremendous number of components that are running batch furnaces successfully. The transportation industry, medical, aerospace, military — are all examples. I’m simply pointing out the fact that there is an opportunity to do something but what we have to keep in mind is — how many of those somethings are there available?
The one thing you would not want to do is try to run four loads in a pusher furnace that could hold 10 because the conditions are not going to be consistent. The front end (the first load) has nothing in front of it so it’s heating at a different rate than the loads in the center, and the last load is cooling at a different rate than the loads that were in the center. That which I just described to you about the potential improvement in distortion, that would be negated in that circumstance.
Doug Glenn: If you’re running a continuous system at full bore and you’re running a batch system at full capacity, especially when you get to the quench, there are a lot of other variables you need to consider in the batch.
This is simply because of the load configuration, and the rates of cooling from the outer parts — top, bottom, sides, as opposed to the ones in the middle. Whereas with a single basket, you still have to worry about the parts on the outside as they’re going to cool quicker than the parts on the inside, but it’s less so, by a significant degree.
Mike Mouilleseaux: Something that I have learned — which is totally counterintuitive to everything that I was educated with and everything that I was ever told— we’d always thought that it was the parts in the top of the load where the oil had gone through and had an opportunity to vaporize and you weren’t getting the same uniform quench—those were the parts that you had the highest distortion.
Counterintuitively, it’s the parts in the bottom of the load that have the greatest degree of distortion. It has very little to do with vaporizing the oil and it has everything to do with laminar flow versus turbulent flow.
Doug Glenn: In the quench tank, is the oil being circulated up through the load?
Mike Mouilleseaux: Yes.
Doug Glenn: So, supposedly, the coolest oil is hitting the bottom first.
Mike Mouilleseaux: Yes.
Thoughts on the Future of Furnace Improvement (22:20)
Doug Glenn: What about the future on these things?
Mike Mouilleseaux: Where do we think this thing is going? Obviously, you’re going to continue to see incremental improvement in furnace hardware: in burners, in controllers, in insulation, in alloys. These things will be more robust; they’re going to last longer. If we looked at a furnace today and we looked at a furnace that was made 50 years ago, and we stood back a hundred yards, almost no one could tell what the difference was, and yet, it would perform demonstrably different. They are far more precise and accurate than ever.
For the process control systems, we’re going to see real-time analysis of process parameters. We don’t have that now. I think that machine learning is going to come into play, to optimize and predict issues and prevent catastrophic things.
In terms of atmosphere usage, if you’re running the same load, and you run it a number of times, the heating rate should be the same, and the amount of gas that you use to carburize that load should be exactly the same. But if you have a problem with atmosphere integrity — you got a door leak, you got a fan leak, or you got a water leak on a bearing — those things are going to change. Now, by the time it gets your attention, you could’ve dealt with that much sooner and prevented other things from happening.
"For the process control systems, we’re going to see real-time analysis of process parameters. We don’t have that now. I think that machine learning is going to come into play, to optimize and predict issues and prevent catastrophic things."
So, did it cause a problem with the part? By the time it causes a problem with the part, it’s really serious. The point is that there is something between when it initiated and when it’s really serious. With the right kind of analysis, that could be prevented. I think that that kind of thing is coming.
Motor outputs, transfer times — I see all of those things being incorporated into a very comprehensive system whereby you’re going to understand what’s happening with the process in real-time. If you make adjustments, you’re going to know why. Then you’re going to know where you need to go and look to fix it.
The other thing I see happening in the future is all about energy and greenhouse gases. Our Department of Energy has an industrial decarbonization roadmap today, and it’s being implemented, and we don’t even know it. One of the targets in this industrial decarburization roadmap is reduction in greenhouse gases: 85% by 2035, net zero by 2050.
So, what does that mean? I’ve listened to the symposiums that they have put on. There are three things that they’re looking for and one is energy efficiency. I’m going to say that we’ve been down that road and we’ve beat that dog already. Are there going to be other opportunities? Sure. It’s these incremental things, like burner efficiency. But there is no low hanging fruit in energy efficiency.
The other thing is going to be innovative use of hydrogen instead of natural gas because the CO₂ footprint of hydrogen is much lower than that of natural gas. If you look at how the majority of hydrogen is generated today, it’s generated from natural gas. How do you strip hydrogen out of there? You heat it up with natural gas or you heat it up with electricity. Hydrogen is four times the cost of natural gas as a heating source.
The other thing that they’re talking about is electrifying. It’s electrify, electrify, electrify. The electricity has to be generated by clean energy. So, does that mean that we run our furnaces when the wind is blowing or the sun is out, or we’re using peaker plants that are run off hydrogen, and the hydrogen is generated when the sun is shining or the wind is blowing, and we’re stripping out the natural gas?
From what I, personally, have seen with these things, these are absolutely noble goals. You could not disagree with them whatsoever. The way that they want to go about accomplishing it, and the timeline that they wish to accomplish that in, is unrealistic.
If you look at how the majority of hydrogen is generated today, it’s generated from natural gas. How do you strip hydrogen out of there? You heat it up with natural gas or you heat it up with electricity. Hydrogen is four times the cost of natural gas as a heating source.
Doug Glenn: Well, Michael, don’t even get me going on this! There are a lot of different things that are going on here but it’s good to hear you say this stuff. I agree with you on a lot of this stuff. They are noble goals; there is absolutely nothing wrong with electrifying.
Now, I do know some people — and even I would probably fall into the camp of one of those guys — that questions the premise behind the whole decarbonization movement. I mean, is CO₂ really not our friend? There’s that whole question. But, even if you grant that, I agree with you that the timeframe in which they’re wanting to do some of these things is, I think, fairly unrealistic.
It’s always good to know the reality of the world, whether you agree with it or not. It’s there, it’s happening, so you’ve got to go in with eyes wide open.
Safety Concerns (29:41)
Mike Mouilleseaux: The safety concerns on these are all very similar. You know, the MTI (Metal Treating Institute) has some pretty good safety courses on these things, and I think there are a lot of people who have taken advantage of that. The fact that it’s been formalized is much better.
When I grew up in this, it was something that you learned empirically, and making a mistake in learning it, although the learning situation is embedded in you, sometimes the cost of that is just too great, so that the probability of being hurt or burnt or causing damage to a facility, is just too great.
There are definitely things that need to be addressed with that, and there are some very basic things that need to be done.
Doug Glenn: Michael, thanks a lot. I appreciate your expertise in all these areas, you are a wealth of knowledge.
Michael Mouilleseaux is general manager atErie Steel LTD. Mike has been at Erie Steel in Toledo, OH since 2006 with previous metallurgical experience at New Process Gear in Syracuse, NY and as the Director of Technology in Marketing at FPM Heat Treating LLC in Elk Grove, IL. Having graduated from the University of Michigan with a degree in Metallurgical Engineering, Mike has proved his expertise in the field of heat treat, co-presenting at the 2019 Heat Treat show and currently serving on the Board of Trustees at the Metal Treating Institute.
Our readers and Heat Treat Radiolisteners will remember a recent episode entitled "Heat Treat Radio #102: Lunch & Learn, Batch IQ Vs. Continuous Pusher, Part 1." Today's Technical Tuesday article is a continuation of this dialog, with Michael Mouilleseaux, a boot-on-the-ground North American heat treat expert from Erie Steel here to answer your questions on the maintenance of batch and continuous pusher furnace systems.
Doug Glenn, Heat TreatToday's publisher,Karen Gantzer, associate publisher/editor-in-chief, join in this Technical Tuesday article.
Stay tuned for a Part 2 continuation of the Lunch and LearnHeat Treat Radio episode, coming to Heat Treat Radio in a couple weeks.
Below, you can watch the video or read from an edited transcript.
Michael Mouilleseaux
General Manager at Erie Steel, Ltd.
Sourced from the author
Introduction to Maintenance
Doug Glenn: We would like to move on to maintenance of the batch furnace and the continuous furnace. What is the cost of maintaining and operating these furnaces?
Michael Mouilleseaux: When they are utilized in a carburizing environment, there is always excess carbon that falls out or precipitates out of the atmosphere, and it ends up as elemental carbon in the bottom of the furnace.
What do you do with that? In furnaces that are using a carburizing environment, the burnout of the furnace is easily the single most important piece of preventative maintenance that you can perform. How is that performed? First, the furnace is vacated; there is no product in the furnace, the temperature is reduced — typically, you want it down around 1500°F or 1550°F — and you introduce room air into the furnace. The room air ignites the carbon. It’s a very primitive operation.
So, what temperature does carbon burn at? It burns at 3000°F.
You need to be very careful. It’s a controlled burn because you can actually damage the furnace through refractory, through the alloy that’s in the furnace, or it can get away. How do you do control it? On one level, you’re just looking at the temperature control. If you have it set at 1550, you’re going to say, “I’m only going to put air as long as the temperature of the furnace does not go up more than 25°F or 50°F.” It’s somewhat dependent upon the piece of equipment and is one of those things that you learn empirically; there is not a hard and fast rule for it.
Then, you can shut off the air. If there is no oxygen, then the source for combustion is taken away and you stop that operation. If you need to do it more rapidly than that, you may need to flood the furnace with nitrogen. Typically, if you have to flood the furnace with nitrogen to do it, you’ve been a little too aggressive in your burnout.
How long do you perform that? The great thing with oxygen probes is that you can utilize your oxygen probe to help you learn when you have burnt out the furnace. You’re not getting an actual carbon atmosphere, but what you do get is a readout from the probe. What you can do is perform the burnout operation until you attain that level and then you know that you’ve done a sufficient job in burning it out. That’s the single most important piece of preventative maintenance that’s done on a furnace used for carburizing.
Doug Glenn: Is that both in batch and in continuous?
Michael Mouilleseaux: Identical, yes.
Doug Glenn: I’ve got a couple other questions about furnace burnouts as someone who’s not a furnace operator. You said that there’s “carbon dropout” in the furnace. I know that in some furnaces, parts of the atmosphere may precipitate onto the coolest part of the furnace. Is that what is happening, or are we talking about carbon powder at the bottom of a furnace?
Michael Mouilleseaux: It is carbon powder, and it becomes more egregious. The powder then begins to accumulate into pebbles, nuggets, and larger size pieces. That’s more problematic. When it is in a powdered form, that is the best.
The question will be: How often do you have to do this? As with everything, the answer is — it depends. It depends on what you’re doing; it depends on how aggressive you are in your carburizing.
In the boost phase, we talked about carburizing at upwards of 1%. As soon as you exceed the saturation level of carbon, you’re going to precipitate out the excess carbon. What is that number? It’s different for every temperature. At 1500°F, it’s .9 or .85; at 1750°F, it’s 1.25. But to attain that, you’re actually putting natural gas into the furnace, and the amount of natural gas that you put into the furnace and its dissociation rate — the rate that it breaks down — can then subsequently be diffused into the parts; all of that comes into play.
With saturation levels of carburizing, there is always some residual carbon that’s in the furnace.
Doug Glenn: You mentioned that carbon burns at around 3,000 degrees. Are you taking the furnace up to that temperature?
The great thing with oxygen probes is that you can utilize your oxygen probe to help you learn when you have burnt out the furnace. You’re not getting an actual carbon atmosphere, but what you do get is a readout from the probe. What you can do is perform the burnout operation until you attain that level and then you know that you’ve done a sufficient job in burning it out. That’s the single most important piece of preventative maintenance that’s done on a furnace used for carburizing
Michael Mouilleseaux: No. The burnout cycle is at 1500 or 1550. You raise that carbon to that level and introduce oxygen, and what you want is a slow burn.
We next think about the systems involved in the furnace. First there is the heating system. In a gas-fired furnace, some critical things to consider are burner recovery, burner adjustment, and the amount of excess air that results in that burner adjustment. That’s a preventative maintenance operation that needs to be performed on a regular basis. It probably doesn’t need to be done daily, but monthly is optimal. If everything is very steady, including the barometric pressure, then you don’t need to do all of those adjustments.
Now, electric furnaces have SCRs that fire the elements, and you have to pay attention to the tuning of those things to make sure that they’re operating at optimum performance. One of the ways that you can do that, in a batch furnace, is if you look at the recovery time.
For example, if you have a load that weighs 4000 lbs. and you put it in the furnace and you know that it takes an hour and a half for the furnace to recover to temperature, but then all of a sudden, it takes an hour and 45 minutes, or an hour and 50 minutes, or two hours, obviously the burners are not producing the same amount of heat. The burners are not pumping the requisite amount of BTUs to achieve that recovery time. Could that be related furnace circulation? Could it be related to the insulation in the furnace? At an extreme, it could. Typically, though, it’s related to burner or SCR tuning.
Those are the kinds of things that are very easy to pay attention to.
"Electric furnaces have SCRs that fire the elements, and you have to pay attention to the tuning of those things to make sure that they’re operating at optimum performance. One of the ways that you can do that, in a batch furnace, is if you look at the recovery time."
Setting up PM Through Controls System
The control schemes in the PLC are typically very robust. So, you can establish a program and the PLC is going to say, “I want to heat it at this rate, I want the carbon potential to be .4%, I want to hold this at two hours at temperature, and then I want to initiate a quenching cycle.” Typically, PLCs are quite robust.
The thing you have to be careful with is obviously not just power outages, but brownouts. Brownouts are when you don’t quite lose all voltage, but you lose some of it. If you don’t have some kind of a filter on the power you can mitigate with, or have an uninterruptable power supply for the PLC, you can damage those things, resulting in some major work on the PLC.
The other part of that is the furnace circulation. We’ve got fans in these furnaces, and we circulate the atmosphere. The primary stages of heating in the furnace are convection, until we get to 1200 degrees. How do we convect the heat? We have the atmosphere in the furnace, the fan circulates, it washes the atmosphere down the radiant tubes, it heats up the atmosphere, the atmosphere comes into contact with the components, and we’re convection-heating the parts.
Once we get to 1200 degrees or more, then the primary method of heating becomes radiant heating. That’s where the radiant tubes are then the primary means of transferring energy. But the fans become very important. Are they balanced? Is the RPM correct? Is the amp reading on the fan? Those are areas to look at.
You have to understand how the furnace operates when it’s healthy — the furnace manufacturer can help you and/or you just learn empirically. For instance, what would it mean if, all of a sudden, I’m drawing much fewer amps on a circulating fan and it’s running very rough? Quite possibly, we’ve lost a fan blade.
Then there is the atmosphere control system. All that we just described is applicable to both continuous and batch furnaces. The furnace needs to be sealed and you want a couple inches of water column pressure — excess pressure — in the furnace relative to atmosphere pressure, since safety is the number one concern.
The atmosphere that we’re talking about in most of these furnaces is endothermic atmosphere. It’s a reducing atmosphere, meaning that it’s combustible. If, of course, we have combustion in a closed vessel, that’s called an explosion.
The reducing atmosphere, in and of itself, is if you look in a furnace that is at anything above 1200 degrees where it’s red, up to 1700–1800 degrees where it’s going to be yellow to white — and there is no flame . . . . People are absolutely amazed when they look in an atmosphere furnace and they see no flame. What you should see is everything in a relative, uniform color. The parts should be a uniform color. If you look at the tubes, they should be a little lighter because the tubes will always be somewhat above the temperature of the parts . . . .
Back to the atmosphere: We want to be sure that the atmosphere stays in the furnace and that we maintain that pressure in the furnace. So, what would be a cause to lower the pressure in the furnace? A door leak or a leak in a fan. It could be, if you have a mechanical handling system, a leak through that system. Those are all places to look.
The PM on that? For maintaining the level of lubrication in the fan bearings, see that they’re cooled so that the outlet temperature of the coolant — be it air or water — should be higher than the inlet temperature; that shows that they’re being cooled.
I can’t tell you an absolute number, but I can say that for the equipment that we have, we have numbers that we’ve developed; we know that if the outlet temperature of the water is 20 degrees higher than it is going in, we’re doing a good job of cooling the bearings.
The door seals in furnaces, typically, are brick on brick. Typically, they use a wedge system to seal the doors in the furnace. But, of necessity, these are wear items. Therefore, in preventative maintenance, you might notice a burnout around a door where you hadn’t had one before. That tells you that atmosphere is leaking out of that door and so a repair is needed in the near future.
An interesting thing about a batch furnace: Most of them only have one door. So, it’s quite easy — you can open the vestibule and, in a maintenance operation, if you gassed up the furnace, you could see.There is always going to be some atmosphere coming around the door because that’s where the atmosphere goes into the vestibule, but it should be at the top; it shouldn’t be around the sides, and it definitely shouldn’t be at the bottom. It should be very consistent.
That’s one of those things that, again, you empirically learn. You look at it — it’s a visual operation to say what you’re doing.
There are two other systems: First, the quench system. We talked about how critical the quench system is. The RPMs of the prop, the amp draw of the motors for the props — those things should be very consistent. I think they should be monitor and data logged. The reason for that is you want to know when you quench a load that the RPMs of those props are what you have set it for. When you introduce a load into the quench, the amp draw is, of necessity, going to increase. That’s because you’ve put something in the path of the quenchant so, in order to maintain that flow, you’ve increased the amount of work that it takes to rotate those props.
That’s the kind of thing that you want to monitor. If the amp draw is changing, that means that there’s something in the quench system. Could it be the bearings? Could it be the motor? Those are some things that you’d need to take a look at and be certain of. Obviously, the props need to be in balance; you don’t want any vibration in them.
Doug Glenn: This is also true on the continuous furnace. You’ve got three or four green props in the batch furnace, and it would be the same in the continuous furnace.
Source: Erie Steel, Ltd
Maintenance of Quenchant
Michael Mouilleseaux: Also, there is the maintenance of the quenchant. I’m of the belief that the quench should be continuously filtered. I’m not a fan of batch filtering. I’ve been doing this long enough that I’ve done that, and it just isn’t successful. Quite possibly there are operations that allow it.
If you’re carburizing, you’re going to have particulate in the quenchant because that same atmosphere precipitation of carbon finds its way into the quench. It’s going to be on the parts, it’s going to be on the trays, it’s going to be dragged in there. So, you have this particulate carbon in the quench and it acts as a catalyst to break down the oil.
One way to extend the life of the oil is to make sure that you’re continuously filtering that out. People say 50 microns or 100 microns or 25 microns. Experientially, I’m going to say that it’s going to be 25 microns. If you have a 100-micron filter, that’s great for getting the pebbles out of the quench or the scale, if that were to be an issue with your customer’s parts, but that’s not sufficient to filter out the particulate that’s going to be of the size that’s going to catalyze the breakdown of your quenchant.
Doug Glenn: I assume that if you’re providing for some sort of continuous filtering of your quench, that’s built into the quench structure. The quench tank is built for that, right, and you’re continually flowing it through this filter?
Michael Mouilleseaux: I’m not going to say that no manufacturers offer sufficient quench filtering, but I am not aware of anyone that offers a quench filtration system that’s sufficient. Most of these things end up being standalone. You want to draw the quenchant from the bottom of the tank in one quarter, you want to put it through a series of filters, and you want to put it back into the furnace at the opposite end of the quench tank.
I can say with certainty, that a batch furnace which has not been filtered well, if you remove the quenchant from the furnace after six months — definitely after 12 months — of using it in daily carburizing, you’re going to take 55-gallon drums of sludge out of the furnace, and the sludge is essentially carbon that’s mixed in with the oil.
For that same furnace, with a sufficient quench filtration system, there will be little pockets in the four corners of the quench tank, but that’s about it.
CQI-9, Nadcap and all of those standards have a requirement for monitoring of quenchant. One of the monitors should be particulate because that lets you know how good a job you’re doing in filtering.
Having done it properly, one can say, “Well, I have to replace my quench oil,” — fill in the blank — “once a year, once every six months, once every two years.” Properly maintained and filtered, the quenchant does not have to be replaced very often.
You’re going to drag out a little oil on every load. You want to let the load drip so that you’re not taking that precious quench oil and just putting it in the wash and washing it off. But in a batch furnace, you could have a couple hundred gallons a month to four hundred gallons, depending on the size of the furnace, of add-back that you’re putting in there. Is that sufficient to maintain all of the additives that are in the quenchant? Is that something that you need to monitor? Typically, the manufacturer can do that for you. You get monitoring and you see what the quench speed is, what is the viscosity, flash – all of those important pieces of information.
Now, it doesn’t come for free. A filtration system is costly, and the filters are costly. A year’s worth of quenchant is five years’ worth of filters. In my mind, that’s a good tradeoff.
Karen Gantzer: So, Michael, when the process is filtering the quench, does this happen during production downtime?
Karen Gantzer
Associate Publisher/ Editor in Chief
Heat Treat Today
Michael Mouilleseaux: No, it’s done continuously. Even when the furnace is not running on the weekend, you’re still filtering the oil. You’re going to be taking 20-50 gallons out of the quench tank but you’re putting it right back in. It just passes through filters.
Some people have utilized centrifuges. It’s a very successful way of filtering out carbon particles in oil. The caveat on that is you don’t want the oil above 140 degrees. If you get the oil above 140 degrees and for every 20 degrees you go up, you start doubling the oxidation rate of the oil.
In high-temperature oil, we do a fair amount of modified marquenching. We do it in closed canisters. The seals must be temperature-tolerant, but it is very successful.
The last part is going to be the quench heating and cooling. Typically, at the first part of the week when you’re starting up the furnace or if you’re going from operation A to operation B and it requires a higher temperature quenchant, you’re going to use either gas or electric elements that are going to heat it. Those things need to be monitored so that they’re available when you need them. The last thing that you want to do is start out the week and find out that the quench heaters don’t work; then, you’re trying to find a couple of dummy loads that you can heat up to put into the quench to heat up the quenchant before proceeding with operations.
Then, of great, importance is quench cooling. In petroleum-based quenchants, you’ve got a flashpoint of 400 degrees plus or minus — could be 350, could be 450, depending upon the quenchant that you’re using. You don’t want the temperature of that oil to approach that flashpoint. You do that by using a quench-cooling system. It’s a big radiator. You’ve got a pump, and you set it when you want the pump to go on. You pump the oil out to the quench coolant, and when it comes back, once you’ve attained what your temperature is, then you stop.
Doug Glenn: I’ve got a couple quick questions on this. First, is the quench heater an immersion tube?
Michael Mouilleseaux: Yes. Gas-fired tubes and gas-fired units are very small u-tubes that go into the quench tank. Electrical units have got elements that are tolerant to that.
Doug Glenn: Typically, you’re using those because you’re actually using the quenchant and always putting hot things into it, so once the quench fluid is up to temperature, it’s not a problem. You’re using that quench heater just to get the thing up to temperature. So after that, most of the time, you’re using the cooler to keep it cool, correct?
Michael Mouilleseaux: Absolutely. That’s a control scheme. The last thing that you want to do is set the quench heater so that it’s within five degrees of setpoint and set the quench cooling so that it’s within five degrees of setpoint — then, the temperature just sits there, with heating and cooling fighting each other. You’re heating and cooling oil unnecessarily. You want to give yourself some bandwidth on that.
Material Handling System
Last is going to be the material handling system. In the batch furnace, many have what we call a “rear handler.” We saw the cart and it would push the load into the vestibule, the inner door would open, and it would push the load into the furnace. It’s always preferable to push hot loads, not to pull on them. The reason is that the base trays are alloy and the compressive strength is much higher than the tensile strength is. If you’re pulling on loads, you’re going to break trays.
Once the load is in the furnace, you would have a rear handler so when the cycle is terminated and the inner door opens, you would have a mechanism — it may have a flat bar that’s half the width of the tray — that actually pushes the load into the quench vestibule.
There it’s pushed by the charge car and the inner door is open. That same handler, from the charge car, pushes it into the furnace. Now, when the cycle is terminated, there is a handler in the rear of the furnace that pushes it into the vestibule for quenching.
The exception is right here: When it’s taken out of the vestibule, typically the charge car goes in and grabs it and pulls it out. But, at that point, you’re at 100 or 200 degrees so, at that temperature, you have no material effect upon the strength of the alloy.
Doug Glenn: Okay, the motion it took it from the tray on the left inside is going to push it in and then the next step it’s also going to push it into this “hot zone,” correct?
Michael Mouilleseaux: Yes.
Doug Glenn: But what you’re saying is, when it’s coming out of the hot zone, there’s probably a mechanism on the far righthand side of the hot zone that’s going to push it back. Nothing is going in to pull it out because it’s hot.
Michael Mouilleseaux: Extended reach cars put the load into the vestibule and then put it into the hot zone.
There are some rear handlers that, rather than being a simple push function, have a dog mechanism that allows them to go and get the load in the vestibule and pull it into the furnace. Personally, I am not a fan of that; I like the extended reach car because when you’re pushing something, it is very easy to determine if you’ve put it in the right location. If you grab a load and pull it, you could lose the attachment on that load and then it’s not put exactly where you want it to be.
You can put amp meters on these things so that the amount of force that the motors require to pull in or push out a load. The one thing you need to be cognizant of is that it’s going to take more power — a higher amp draw — to push a 4000-pound load than it is a 2000-pound load. Once you understand what that is, you can monitor these furnaces and then they start making sense to you.
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Explore this innovative technology in this article composed by Aravind Jonnalagadda (AJ), CTO and co-founder of Synergy Additive Manufacturing LLC. This article offers helpful information on the acceleration of EV dies, possible heat treatable materials, and the process of laser heat treating itself. Read more to assess the current state of laser heat treating, as well as the future potential of this innovative technology.
When the Rubber Meets the Road, How Confident Are You?
Reliable and repeatable heat treatment of automotive parts. Without these two principles, it’s hard to guarantee that a minivan’s heat treated engine components will carry the family to grandma’s house this Thanksgiving as usual. Steve Offley rightly asserts that regardless of heat treat method, "the product material [must achieve] the required temperature, time, and processing atmosphere to achieve the desired metallurgical transitions (internal microstructure) to give the product the material properties to perform it’s intended function."
TUS surveys and CQI-9 regulations guide this process, though this is particularly tricky in cases like continuous furnace operations or in carburizing operations. But perhaps, by leveraging automation and thru-process product temperature profiling, data collection and processing can become more seamless, allowing you better control of your auto parts. Explore case studies that apply these two new methods for heat treaters in this article.
Batch or continuous — which equipment is better for your operations? Today’s Heat Treat Radio episode is a lunch & learn to answer your burning question about batch IQ vs. continuous pusher furnace systems. Michael Mouilleseaux of Erie Steel is a boots-on-the-ground expert in North American heat treat, and he’ll share a bit about the history of these systems before getting into the equipment and heat treat processing differences.
Doug Glenn, Heat TreatToday's publisher and the Heat TreatRadio host, Karen Gantzer, associate publisher/editor-in-chief, and Bethany Leone join this Heat TreatToday lunch & learn.
Below, you can watch the video, listen to the podcast by clicking on the audio play button, or read an edited transcript.
The following transcript has been edited for your reading enjoyment.
The History of Batch and Pusher Furnaces (00:52)
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DOUG GLENN: Can you talk with us a little bit about the whole history of batch furnace versus pusher furnace?
MICHAEL MOUILLESEAUX: Sure. And thank you for having me!
Interestingly enough, the pusher furnace — which we might say is a more complex piece of equipment than a batch integral quench furnace — preceded the batch furnace. Atmosphere pushers were around prior to World War II. I spoke with a number of folks in the industry and asked, “How could that possibly be, given the level of complexity?” Interestingly, pushers were available because the atmosphere was generated by a charcoal generator.
If you think back to pack carburizing, we used charcoal and some accelerator. You would put it in a closed container, you’d heat it up, and that’s how you carburized things before you had atmosphere furnaces. Utilizing that same concept, they generated an atmosphere, put it in a furnace, and pushers were the first ones to do that.
So, who were the folks who did that? They were AFC-Holcroft, Surface Combustion, and Ipsen, all the usual characters and suspects there.
Pusher furnaces were available in single row and multiple row configurations.
They were heated with gas or electricity. I have to think that the earlier ones were heated by gas. Typically, they employed oil quenching. Although atmosphere cooling could be in the works, to find something of that vintage is very difficult. Maybe someone listening to this will weigh in and say, “Well, let me help you with that.”
The batch integral quench furnace is post World War II. What precipitated the development of the batch integral quench furnace was the development of the atmosphere generator, and that’s thanks to and around 1941 he actually published a book on atmosphere generators. I’m not sure where to find documentation of the patent he was granted for this generator. It might be interesting to discover. But again, Lindberg, Surface, Ipsen, — all these folks had these furnaces in the late 40s/early 50s.
When they started out, these furnaces were relatively small. The furnace might have had a tray that was 12 inches x 12 inches x 8 inches tall. You’d struggle to fit a hundred pounds into something like that.
But the batch furnace is by far the most popular atmosphere furnace that is available. You’ve got a variety of processing capabilities, which makes very flexible. There are a wide variety of sizes, even today; it can be heated with electricity or gas (we’ll talk about that a little bit later). You can have an oil quench furnace, you can use a polymer quench, and you can have a furnace where you atmosphere-cooled the load after it was processed in the primary furnace.
During this discussion, I’m going to use “batch,” “batch IQ,” and “batch integral quench” semi-interchangeably. So, if I say “batch” and I forget the “IQ” or if I say “batch integral quench” — these are all the same pieces of equipment. We have numerous names for the same thing.
DOUG GLENN: Gotcha. You said the continuous furnace came first because the atmosphere was being created by burning charcoal inside the furnace, that created a carbon rich environment?
MICHAEL MOUILLESEAUX: Actually, it was a generator that was pumped into the furnace.
DOUG GLENN: Got it. That was confusing; I was wondering how they were burning charcoal inside a furnace.
MICHAEL MOUILLESEAUX: Actually, it was explained to me that because the pusher furnace was so much larger, when you would open the doors to place or extract a load, the relative pressure drop of opening a door wasn’t that great. So, these primitive charcoal generators could accommodate that.
But in a batch furnace, arguably, the door is one wall of the furnace, and you couldn’t create a sufficient amount of pressure in the furnace. So, it had to wait until we had endothermic generators so that we could establish a furnace pressure higher than atmospheric pressure to make batch furnaces. It’s fascinating.
Basics of the Batch Furnace (05:41)
DOUG GLENN: And as you said, it is probably the most popular furnace used today by many, many heat treaters. Let’s talk about batch furnaces, here we go.
MICHAEL MOUILLESEAUX: Let’s look at the CAD drawing for a batch furnace. The batch furnace is primarily two components. You can see the hot zone — that is the furnace proper. It’s highly insulated, it has radiant tubes in it (so we can put atmosphere in the furnace), and the heating portion does not affect the atmosphere.
It is loaded through a vestibule, and the vestibule is pressurized as well. A load can go into a vestibule, you can close the door, you open the inner door, the load goes into the furnace, you can process it and then, as you can see, you can either quench the load or you can top cool the load.
CAD drawing of a batch furnace.
Common Processes in a Batch Furnace (06:31)
What kind of things can we do in an atmosphere furnace? Answer: operations that do not require quenching. We could stress relieve, we could subcritically anneal, we could supercritically anneal (so, above and below 1350/1400 Fahrenheit), and then we can normalize.
Normalizing is utilized for products like forgings or castings which are made at a very high temperature. You’ve got a number of structures in the component and what you want is a “normal” structure. You want a uniform structure throughout the part so that it can be machined.
Normalizing is typically performed at a high temperature, and it’s put into this top cooled/atmosphere cooled chamber. In the old days, that was termed “air cooling” — it was a rate equivalent if you just set it out in air. These top cooled chambers are somewhat insulated; they have cooling jackets that are in the side, and there is a fan in them so you can circulate the atmosphere through it so that you get uniform cooling throughout the load.
DOUG GLENN: Michael, this isn’t considered high pressure gas quenching though, right?
MICHAEL MOUILLESEAUX: Not even close.
In this animation, we have the load going into the furnace, the vestibule door closes, the furnace door opens, the furnace door closes, we perform whatever process we want, we extract the load out of the furnace, and it goes up into the top cool chamber. It’s then atmosphere cooled. When that is completed, we take the load out.
The time in the furnace could be four hours (plus or minus). The time up in the top cool chamber would probably be an hour or two. Once the load is extracted from the furnace and is put into the top cool chamber, and you reestablish pressure in the vestibule, you actually open the outer door, put another load in and start processing the next load while the initial load is being cooled.
Then, there are processes that require quenching. In degree of simplicity, first there is neutral hardening. Neutral hardening implies that the atmosphere in the furnace is neutral with the carbon content of the steel. So, for a 30-carbon steel, you’d want a 30-carbon atmosphere; for a 40-carbon steel, you’d want a 40-carbon atmosphere. The optimum is to neither enrich nor to deplete the surface carbon; you don’t want to change the chemistry. Typically, neutral-hardened parts are subsequently oil-quenched.
Then, there is carbonitriding. In carbonitriding, you have a high carbon atmosphere. You also introduce ammonia into the furnace. The ammonia dissociates right in the furnace. The carbon and nitrogen diffuse into the surface of the component. is held at a sufficient temperature to attain the case step that is desired, then, again, it is extracted into the vestibule, and it is quenched.
Carburizing would be another process. It’s similar to carbonitriding, except there is no ammonia. It’s only carbon that’s diffused into the surface of the part. Typically, parts that are carburized are oil-quenched.
There are, however, strategies and components where you would carburize, and then you would take the part and you would top cool it. You might take the part out of the furnace, and you may reorient it. So, parts that are distortion-critical may be oriented in one direction for carburizing, and then reoriented for hardening. You may carburize twice as many parts as you harden, so the hardening load would be half the size.
A low temperature process which is more complex is ferritic nitrocarburizing. That, typically, is performed around 1000°F. It is performed in batch furnaces, as well. Typical process cycles for that are going to be, at temperature, an hour and a half/two hours. That process can either be atmosphere cooled or it can be quenched; it depends on whether you’re looking for solid solution hardening or if you’re just looking for the nitrided layer and you’re not trying to do anything to the substrate.
I think that we have an animation for this.
Diagram of a batch system load.
Again, the load is loaded in the vestibule, the vestibule pressure is reestablished, the load is put into the furnace and, at that point, we perform whatever operation it is that we want to do of those previously described operations.
In the animation, you can see that the load is immersed in the quench. Following the quenching operation, it’s withdrawn from the furnace.
The total time for quenching is 10 minutes. When the load is brought up out of the oil, typically you let it sit there and allow it to drip so the precious quench oil you’ve paid your money for can go back into the quench. You’re washing and removing as little quenchant as is possible. In the heat treating operation, quenching is the single most critical portion of the operation.
A Note on Quenching (12:32)
MICHAEL MOUILLESEAUX: When we’re carburizing, we have a portion of an hour where there would be no significant change in the case depth of the part. When we temper the parts, we have hours. You could temper it for three hours, you could temper it for five hours, and you’re not going to have a material change in what’s performed. In the quenching operation, the latitude that you have in quenching is in seconds.
Our typical protocol is that when a load is extracted from the furnace, from the time that the furnace door opens into the vestibule to when the load bottoms out at the bottom of the quench, in a batch furnace, must be 40 seconds maximum.
DOUG GLENN: 40?
MICHAEL MOUILLESEAUX: 40 seconds maximum. Typically, it’s done in 20 or 25 seconds. But it’s 40 seconds maximum. In a pusher, that number is 30 seconds maximum. This is something that you track; it’s data logged. If it exceeds that, at that point, you’re going to have to perform some inspection on that load that is much higher and much more intense had it not taken that much longer.
DOUG GLENN: Can you, very briefly, explain why is it so important? I’m assuming it has something to do with martensite start and martensite finish and all that good stuff, but is there a layman’s way of explaining why the time to quench is so important?
MICHAEL MOUILLESEAUX: Essentially, you want to have the load at a uniform temperature when it goes into the quenchant. If we have a significant variation in the low temperature, from the top to the bottom or the front to the back — even if the quenching operation is completely uniform — we’re going to have a variation in properties, variation in hardness, and certainly the probability of variation in core hardness.
For those things that are distortion-critical, it is absolutely important that the load has a similar temperature, across the load, top to bottom, inside to out, when it’s quenched.
Batch Furnace Systems (15:00)
MICHAEL MOUILLESEAUX: You typically don’t have a singular furnace, you have a system. What’s involved in a system?
What we’re looking at here is a relatively simple system. You have a loading operation. Obviously, the parts need to be loaded in baskets or fixtures. In some way, the load needs to be built. Typically, there is a station for that.
Diagram of a batch system furnace line.
Following loading, it’s put into a preheat furnace. A preheat furnace is identical to what we would call a “temper” or a “draw.” You can thermally clean the parts by heating them up to 800°F. The other thing is that those that you put into that part are 20% the cost of getting those BTUs when you’re putting it in the high heat furnace, so it just makes economic sense. You’re cleaning the parts and you’re preheating the parts.
Then you’re going to put it into the furnace to perform the furnace operation; it’s either going to be top cooled or quenched. If it’s top cooled, you’re going to stop that top cooling operation at 300°F or 400°F. You’re going to put it in a cooling station and allow it to cool to room temperature. If you quench the part, if you’re modified marquenching it, it’s 250°F plus; if it’s quenched in regular oil, it could be 150–180°F.
The next operation is to wash the part. Typically, you don’t want to wash hot parts; you want to allow them to cool to room temperature. Sometimes you do, but more often than not, no.
Then there’s the wash station; you’re washing the parts. Then, you’re taking them out of the washing station and allowing them to drip. Then, you’re going to put them into a temper and you’re going to temper it for three to seven or eight hours, or something of that nature. You extract the load from the tempering furnace, put it in a cooling station, and allow it to cool down to room temperature so you can then unload it.
As you can see, the way that is accomplished is with this transfer cart. The transfer cart extracts the load from the loading table, pushes it into the preheat furnace, pulls it out of the preheat furnace, and pushes it into the batch furnace. Then the batch furnace quenches it, but when the outer vestibule door is opened, the transfer car must go in and get the load and pull it back onto the transfer car. The car pushes it across the aisle into the cooling station, picks it up, puts it in the wash, takes it out of the wash, puts it into the temper, takes it across the aisle when the tempering is finished, extracts it from the temper, and puts in on the cooling station. That transfer cart is an important piece of equipment.
But you can see there are a lot of moving parts to this. And you might ask, “Why would you do this?” Well again, because of the flexibility of the batch furnace. In this example, batch furnace #1 can be performing neutral hardening; batch furnace #2, at the same time, can be carbonitriding; the neutral hardening load finishes and the next load that goes in there could be annealed; after the load is annealed, then you could take a load and it could be normalized; then you could go back and you could neutral harden again.
So, if you don’t have multiple loads of the same thing, this offers a degree of flexibility that almost is not available in any other type of atmosphere processing equipment.
DOUG GLENN: Right. And the fact that you have more than one furnace, more than one high heat furnace, more than one temper furnace, gives you almost (not exactly, but closer to) a continuous process even though each furnace is a “batch,” if you will.
MICHAEL MOUILLESEAUX: Correct.
There are charge cars that are automated, so the charge car knows where the loading station is — it goes to that loading station. You could either have a human unload it or, in the highest degree of automation, it gets there and you have a PLC that is overseeing or supervising this entire operation, and it would know to take that load onto the cart, where to take it next, and what to do. It becomes a semi-automated method of heat treating.
Properties of the Pusher Furnace (19:53)
DOUG GLENN: Let’s move on to the pusher furnace, the continuous system.
MICHAEL MOUILLESEAUX: The pusher furnace, as you can see in this description, contains the vestibule, the furnace, and the quench. We’ve just broken it down into the pusher furnace proper.
Diagram of a pusher furnace.
Loads are put into the vestibule and then, sequentially, they move their way through the furnace. The first zone of the furnace would be what we would call the “preheat” and that’s where we bring the part up to temperature.
In this example, we’re showing boost-diffuse. This is an example where we would be carburizing. The first couple of positions would be a boost. We carburize at a higher carbon content because it diffuses more rapidly at the initial point of carburizing. Then, at the tail end of the carburizing cycle, we reduce the carbon content to what our desired surface carbon content would be.
An example might be: We would start out and we’d boost at 1% or 1, and the diffuse cycle would be .8% carbon. You do that for a couple of reasons: You want to mitigate any retained austenite, so the bar is quenched at a higher carbon. You have opportunity for development of an unacceptable amount of retained austenite. At the extreme, you could start developing carbides and those become very difficult to re-solution. That’s the rationale for having a boost-diffuse. You do that same thing in a batch furnace; I just didn’t describe that as such.
And then the drop zone. We want to reduce the temperature prior to quenching so that we have very uniform quenching properties and if the components that we’re heat treating are distortion-critical, it’s very important as to what the temperature is prior to quenching.
So, we carburize at a high temperature (1700 Fahrenheit/1750 Fahrenheit), because the diffusion rate is much higher at that temperature. But we only want to quench these parts at 1500 or 1550 Fahrenheit because we want to have an absolute minimum amount of distortion.
Every hour, the vestibule door to the quench is going to open and you would cross-push that load into the quench vestibule, you would close the door, and just as the animation described in the batch furnace, that load would drop on an elevator into the quench.
Now that we’ve done that, we have an opening. That last position is open. So, we go to the vestibule on the front end of the furnace, we open that door, we put a load in there, we close the door, and we’ll close it long enough for us to reestablish the furnace pressure (no more than 3–5 minutes). Once we’ve established furnace pressure, we can open the door between the vestibule and the first preheat zone, and then to the left of the vestibule is going to be a mechanism for pushing these loads, hence the term “pusher”? Could it be hydraulic? It could. Could be mechanical? Both are employed.
What you’re doing is pushing it further by one position. Because the last position is open, the second to the last load progresses into the last position, the load that you put in the vestibule goes into the first position.
DOUG GLENN: A couple quick questions: Really, the sequence starts with the load being pushed out of the furnace into the quench vestibule and then dropped in. That leaves that last spot open in the furnace. Then everything else starts and we push it all down, correct?
MICHAEL MOUILLESEAUX: You are correct.
DOUG GLENN: In this illustration, it looks like there are divisions between each of these different locations. In the preheat, it looks like there are three or four; in the boost-diffuse, it looks like you’ve got two or three. Those aren’t actually physical barriers; You’re just showing where the load would progress to, correct?
MICHAEL MOUILLESEAUX: You are correct.
DOUG GLENN: Are there any chamber divisions in a pusher furnace?
MICHAEL MOUILLESEAUX: In a pusher furnace, you have arches above the load and that helps to compartmentalize. The key word there is “helps.” You don’t have an actual compartmentalization.
Let’s say that we want to perform a carburize at 1700°F in this furnace. If you had three preheats, you may want to perform these somewhere below the 1700°F with the last position being at 1700°F so that the load that goes into the carburizing zone is at temperature and it’s ready to accept carbon.
The carburizing zone would all be at the same temperature, but you have to understand these parts are all at 1700°F and we want to quench it at 1550°F, let’s say. We have two positions that we are going to allow the load to cool down to 1550°F.
So, would you want a zone arch there? I think that you would, yes. Would you want a fan in those zones? If you had a fan in those zones, and you are circulating the atmosphere through those loads, you have a better opportunity to attain a uniform temperature from the top to the bottom of the load than if you did not.
Diagram of a pusher furnace system.
MICHAEL MOUILLESEAUX: Here’s a pusher furnace system. Typically, but not always, pushers are put into a system because you have multiple operations that you must perform. This example is in a U-shape. The loading and unloading are next to each other. This could be a linear layout.
In another life, I worked for a company in Syracuse, New York that had 14 furnaces that were all linearly oriented. So, the person on the front of the furnace did one thing, the person on the back of the furnace did another thing, and they really didn’t communicate.
I, personally, am not a fan of that. I like this operation because you can have one or two people performing the loading and unloading operation, and you could have a furnace operator who would be responsible for the overall control of this piece of equipment.
You can see that we have four loads here that are in whatever way we chose to fixture them — baskets, fixtures, or whatever it might be. We’ve put a couple of parts in a preheat so we could perform that same cleaning that we talked about in preheating the load with low-cost BTUs. The preheat then goes into the vestibule, the loads progress down through the furnace as we described, you get to the end and that load is quenched. When the load comes out of the quench, just as in the batch furnace, it’s going to be 150–200°F plus. We want that to cool down to room temperature because the next operation is going to be washing.
After the load to cools down to room temperature, we then put it in the wash. Following the wash operation, you might have a drip station. So, whatever it was that you have washed off in the water, you don’t want that to go into the temper. Following the drip station, then you would go into the tempering furnace. Here we’re showing three positions; it could be three, it could be six, it could be nine. This is just an example.
Following the tempering operation, we would go out and in that first position, you might have a blower underneath and you would be circulating, room temperature air through it up into a duct work ahead and that’s how you would cool the room down to low temperature. Those loads would progress down that unload station so, at the very end, you are unloading the parts, perhaps for a subsequent shop blast cleaning operation or development of rust preventative or maybe they’re just put back into the customer’s container.
In a captive operation, they might go into a container where the parts would go on to a subsequent grinding or hard-turning operation.
This can be automated. Here you can see that the loads progress into the preheat, they progress through the furnace, they go into the quench, and they’re put into the wash. It’s quick.
Diagram of a pusher furnace load.
DOUG GLENN: Yes. It doesn’t happen this fast in real life, everyone!
MICHAEL MOUILLESEAUX: In the temper, the load exits the temper and goes into the unloading station. The point of this is to show that it progresses through the furnace.
The advantage is that you have relatively small loads that you’re processing, there is a very consistent process in the pusher furnace, and what you’re on for is that however you’ve designed this system, every load goes through every station. You don’t have an opportunity to easily extract a load as quenched and not wash it. It can be done. You could have a furnace designed to do that, but it’s not easy. After it’s washed, as you can see in this animation, typically it’s going to progress into the temper. Could you design a station that would allow you to offload it? You could, but normally that’s not how that’s done.
So, the load progresses through the temper and then you go in to where it is then subsequently unloaded.
If the batch furnace’s strong suit is the fact that it is extremely flexible — particularly in a “systemic” way — the pusher furnace’s strength is its productivity. °
DOUG GLENN: Yes, higher levels of productivity. But you’ve got to have, if not the same product, at least the same process on whatever it is you’re putting in there.
MICHAEL MOUILLESEAUX: Bingo. That’s exactly what you must have there, yes.
About the expert: Michael Mouilleseaux is general manager at Erie Steel LTD. Mike has been at Erie Steel in Toledo, OH since 2006 with previous metallurgical experience at New Process Gear in Syracuse, NY and as the Director of Technology in Marketing at FPM Heat Treating LLC in Elk Grove, IL. Having graduated from the University of Michigan with a degree in Metallurgical Engineering, Mike has proved his expertise in the field of heat treat, co-presenting at the 2019 Heat Treat show and currently serving on the Board of Trustees at the Metal Treating Institute.
Renewable fuels or hydrogen have entered the scene as these are fuels that contain little or no carbon. So, no carbon in the fuel means no CO2! These fuels present an excellent opportunity to significantly reduce carbon.
This Sustainability Insight article was composed by Brian Kelly, manager of Application Engineering at Honeywell Smart Energy and Thermal Solutions (SETS) and president of the Industrial Heating Equipment Association. It can be found in Heat Treat Today's August 2023 Automotive Heat Treatingprint edition.
The need to understand the impact of greenhouse gases (GHGs), especially carbon-based emissions, on climate change is gaining much more interest recently from organizations that have industrial heating processes. Most industrial heating processes are fueled by carbon-based fossil fuels such as natural gas, propane, fuel oil, diesel, or coal. In basic terms, if you have combustion processes in your organization, you are emitting carbon (CO2). Impacts on climate change due to these carbon emissions have prompted government and corporate actions to reduce carbon. These actions are creating unique new opportunities for more sustainable and lower carbon process heating methods. In this article, we will focus on ways to reduce carbon in typical fossil fuel fired heat treat thermal processes. First step: Figure out where you are today. Do you know?
Assess Your Carbon Footprint
Brian Kelly
Image Source: Honeywell
More and more companies are interested in understanding their GHG/carbon footprints, so they can determine what processes are their biggest CO2 offenders, and on what assets to focus on in order to have the largest impact on reducing carbon. Whether your thermal processes are being heated by fossil fuels (typically natural gas) or electrically, each will have a carbon footprint. Fuel gases are being burned to provide the heat and they produce CO2 as a result. Most electrical power is currently being produced by fossil fuels, so electricity will have a CO2 amount associated per kW. What can be done to burn less fuel in your furnaces or ovens, which directly relates to reducing CO2?
Tune Your Combustion Systems
Radiant tube burner with plug recuperator in a U-tube
Source: Honeywell
Over time combustion systems drift and are not at their optimum air/fuel ratio. By simply tuning your burner system on a routine basis, you can fire at the optimum air/fuel ratio for the process and be as efficient as possible. For example, if a furnace is firing on natural gas, operating at 1800°F, and currently operating at 35% excess air, tuning your burners to 10% excess air could save approximately 15% in fuel consumed. The fuel costs will be reduced, and the resulting CO2 will be reduced by that same percentage!
Maintain Your Furnaces/Ovens
A simple review of your furnaces or ovens to observe any hot spots, openings, faulty seals, or refractory issues will identify areas that will cause your systems to operate less efficiently, thus using more energy. Repairing these problems and consistently maintaining them will have the systems running more efficiently and producing as little carbon as possible.
Upgrade Your Firing Systems To Be More Efficient
Direct fired self-recuperative burner
Source: Honeywell
Incorporating preheated combustion air into furnace combustion systems can significantly reduce fuel consumption and therefore the resulting carbon. The two main methods for introducing hot air into a combustion system are recuperation and regeneration. The most popular air preheating method in heat treating applications is recuperation. For a direct fired furnace, this can take the form of a central stack recuperator or self-recuperative burners. Self-recuperative burners have grown in popularity in recent years as they get rid of the need for hot air piping, recuperator maintenance, and most are often pulse fired, which will not only maximize efficiency but also promote temperature uniformity in the furnace and often be lower in emissions. For indirect fired (radiant tube) furnaces, you can apply/add a plug recuperator to an existing cold air fired burner in a furnace that has a U or W-tube to preheat the combustion air or apply self- recuperative burners installed in Single-Ended Radiant (SER) tubes to optimize your furnace firing. The SER tube material can be upgraded to silicon carbide which allows higher temperatures/flux rates that can provide the opportunity to increase throughput and reduce the possible CO2 per cycle.
Combustion air preheating can result in energy savings of close to 25% over cold air combustion.
Renewable Fuels/Hydrogen
Renewable fuels or hydrogen have entered the scene as these are fuels that contain little or no carbon. So, no carbon in the fuel means no CO2! These fuels present an excellent opportunity to significantly reduce carbon. Hydrogen has been of interest because it has the opportunity to be a zero-carbon industrial fuel when produced with renewable energy such as wind, solar, hydro, or nuclear power. As these methods become more prevalent, they bring down the price of hydrogen and increase its availability. This could be a significant driver to greatly reduce CO2 in thermal processes. These topics as well as many others are being discussed in an on-going Sustainability Webinar series hosted by IHEA to provide education and insight into the ever-changing sustainability landscape.
Single ended self-recuperative radiant tube burner
Source: Honeywell
About the author:
Brian Kelly is manager of Application Engineering for Honeywell Smart Energy and Thermal Solutions (SETS) and current president of the Industrial Heating Equipment Association (IHEA).
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