Gasbarre Thermal Processing System

Adapting to new processing demands puts traditional equipment to the test. Can single-chamber solutions keep up, or will applications require different equipment options for efficient processing? In today’s Technical Tuesday, Bryan Stern, product development manager at Gasbarre Thermal Processing Systems, addresses the advantages multi-chamber isolated heat vacuum furnaces bring to the floor.

This informative piece was first released in Heat Treat Today’s November 2024 Vacuum print edition.

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Do You Hear That? It’s the Sound of Change . . .

In the evolving landscape of vacuum heat treatment, single-chamber batch furnaces have long been the cornerstone of material processing. However, with more traditional processes shifting to vacuum, rising energy costs, and increasing environmental pressure, the disadvantages of that approach are emphasized, enhancing the appeal of alternative technologies. Multi-chamber vacuum equipment, while not new to the industry, offers significant solutions to inefficiencies and challenges faced by single-chamber systems. With advances in technology, improved operational planning, and an increasingly competitive market, multi-chamber isolated heat furnaces are becoming a more viable choice.

What Is an Isolated Heat Vacuum Furnace?

An isolated heat vacuum furnace keeps the heat chamber separate from the ambient atmosphere throughout the process, including loading and unloading. This allows the heated zone to maintain a stable temperature and vacuum between cycles, unlike single-chamber furnaces, which must heat up and cool down for each new load. Key components of this furnace type include an additional evacuation chamber, a dynamic sealing door, and a mechanism for moving the workload between chambers. While multi-chamber isolated heat furnaces may be batch or continuous, the above features fundamentally distinguish them from single-chamber batch equipment. This difference is more than just a technical nuance; it has profound implications for operations and efficiency.

The widespread use of single-chamber vacuum furnaces has significantly shaped the design and operation of vacuum furnaces today. But it is important to remember some of the challenges to this approach that we often take for granted.

Energy Efficiency Has Entered the Chat

Single-Chamber Challenge

In single-chamber systems, the entire furnace must go through a full cycle of loading, evacuation, ramping, soaking, cooling, and unloading for every batch of parts. This adds significant “dead time” on either side of the thermal process. In addition to pump-down time, ramping from room temperature typically adds 1–2 hours to the cycle time before soaking which creates a barrier to throughput. Another drawback is that the energy required to heat the furnace is thrown away after every cycle. Due to the high thermal capacity of materials like graphite and molybdenum, this is not inconsequential. With 100% thermal efficiency defined as only consuming the energy required to heat the work and fixturing, single-chamber batch furnaces typically operate in a thermal efficiency range of around 30%–50%.

Isolated Heat Advantage

In an isolated heat furnace, the work zone remains at temperature and the energy required to heat the furnace is not thrown away. Additionally, the introduction of work to a preheated work zone allows the load to be heated more quickly, reducing the time required to achieve temperature and reducing holding losses. While multi-chamber batch furnaces experience some savings, they still consume excess energy since the heat cage is empty during unloading, loading, and evacuation. Continuous configurations, however, see significant improvement with only holding losses and the energy required to heat the work and fixturing being consumed. These advantages mean that continuous furnaces typically operate in a thermal efficiency range of 45%–65%. The result is a 15%–35% energy efficiency improvement over the majority of existing equipment.

Design Optimization: Do I Detect Some Tension?

Single-Chamber Challenge

The tension of designing a single-chamber furnace to handle both heating and cooling in the same space presents substantial challenges. Insulation pack thickness is often limited to balance the need for quick pump-down. Gas nozzle penetrations through the insulation pack create direct radiation losses. This erodes thermal efficiency, adds thermal mass, and restricts gas flow during cooling. These conflicting design priorities often lead to unsatisfactory compromises and fluctuating designs. Between the additional energy to heat and cool and increase power demand at temperature, there are a lot of energy savings being left on the table.

Isolated Heat Advantage

Because the heating and cooling take place in separate locations, multi-chamber isolated heat equipment benefits from the ability to have dedicated designs tailored at each work position. More insulation can be used as conditioning time is not a significant consideration. Additionally, the insulation can be designed without penetrations, further reducing losses. Moving the work to a dedicated cooling position removes restrictions to gas flow and allows the work to radiate directly to the cold wall. This is especially beneficial at the beginning of a quench when the work is at high temperature. This can allow cooling rates to be achieved with lower quench pressures and smaller quench motors.

Thermal Cycling: Here We Go Again . . .

Single-Chamber Challenge

A single-chamber furnace must be built to endure extreme thermal cycling again . . . and again. This requires detailed design consideration to account for thermal shock, expansion, ratcheting, creep, and low-grade oxidation — all of which contribute to maintenance and replacement cost for expensive, long lead refractory components.

Isolated Heat Advantage

Since the heated portion of the furnace remains at stable temperature and vacuum, internal components are not subject to the same destructive forces. An isolated heat cage can remain in service much longer before requiring service or replacement. It also decreases the likelihood of sudden and unexpected equipment failure. Increasing the lifespan of the most expensive consumable assembly in the furnace is an incredibly valuable advantage that is frequently overlooked.

Throughput and Load Size: Can They Help?

Single-Chamber Challenge

Single-chamber batch vacuum processing is notorious for the long cycle times and resulting limited throughput. One way to reduce the costs of the wasted energy and dead time is to increase the load size to distribute the cost over more work. While this can increase capacity and reduce the cost per part, it is counterproductive to many objectives of the heat treating process. As the load size increases, it becomes more difficult to maintain thermal and process uniformity across parts at the surface versus the center of the load. This is especially problematic for densely packed loads. Loads take longer to soak out to a uniform temperature, extending cycle times. Similarly, it is difficult to achieve rapid and uniform cooling rates which can lead to higher quench pressures, larger cooling motors, or underutilizing the work envelope.

Isolated Heat Advantage

While multi-chamber batch isolated heat furnaces experience many of the other advantages discussed in this article, throughput is where continuous configurations really shine. Because separate loads are being processed simultaneously, similar or greater throughputs can be achieved with much smaller load sizes. For instance, a process with a two-hour soak would typically require around a five-hour total cycle time in a single-chamber furnace. That same process could be segmented in a continuous furnace indexing loads in as little as 15 minutes, depending on the configuration of the equipment (see Figure 3). With a throughput ratio of 20:1, each load would only need to be 1/20^th of the batch load to achieve the same throughput. With these mechanics, it quickly becomes apparent how continuous processing is capable of achieving much greater throughput while benefiting from the uniformity of smaller load sizes as well as the other advantages discussed.

Scalability: And Another and Another . . .

Single-Chamber Challenge

Increasing the capacity of a single-chamber production line necessitates adding additional discrete furnaces. This means that all of the equipment systems are duplicated. Each furnace means another chamber, pumping system, manifolds, quench motor, VFD, control cabinet, certifications, instrument calibrations, etc. There really is no economy of scale available to help facilitate high volume production.

Isolated Heat Advantage

For most processes, increasing the capacity of a continuous multi-chamber furnace only requires adding additional heated work positions to shorten the index rate. All other auxiliary equipment and infrastructure can serve double-duty, and redundant systems and maintenance are avoided. This applies the cost directly to the necessary equipment (heat cage, elements, power supply, etc.). The resulting economy of scale often makes continuous equipment a far greater value proposition for high-volume applications that would otherwise require multiple furnaces.

Vacuum Performance: Don’t Reduce Me Like That!

Single-Chamber Challenge

Because single-chamber batch furnaces are exposed to air and humidity between each cycle, they require a higher vacuum (i.e., lower pressure) to achieve the purity required for a given process. This is because even though the furnace is evacuated to a low pressure, the remaining atmosphere is still primarily comprised of oxidizers in the form of residual air and water molecules desorbing from the internal surfaces of the furnace. Achieving the high vacuum levels required to achieve the necessary reducing atmosphere in a reasonable time can result in additional pumping equipment such as a booster or diffusion pump. This adds to system complexity, upfront cost, maintenance, and operating cost. Unfortunately, vacuum processes are often developed in, and organized around, single-chamber batch processing, so the actual purity requirement often gets distilled into an ultra-low vacuum level on the process specification. Consequently, these aggressive vacuum specifications are carried over to other types of equipment where they may not be necessary to achieve the same results.

Isolated Heat Advantage

Because the heat cage remains under vacuum throughout the process, there is less exposure to atmospheric contaminants. This allows oxidizing constituents to decay to very low levels leading to improved vacuum purity. Even though the absolute pressure is higher, the makeup of the remaining atmosphere is primarily inert. Given time for desorption to decay, it is entirely possible to have a purer environment at a higher pressure without requiring the complex pumping systems necessary in a single-chamber batch furnace. Reduction levels associated with diffusion pumping in single-chamber furnaces can be achieved at higher pressures with a two-stage or even single-stage pumping systems in an isolated heat furnace. This is one of the most overlooked and misunderstood advantages of isolated heat processing.

The Shift Toward Isolated Heat Furnaces

Despite the many challenges associated with single-chamber batch processing, the prevalence of these furnaces has remained high due to their simplicity and familiarity. So, why are multi-chamber furnaces gaining traction now?

“There is a pending perfect storm of market conditions poised to tip the scales.”

There is a pending perfect storm of market conditions poised to tip the scales. More and more traditional processes are shifting to vacuum for its long list of advantages, including tighter process control, flexibility, safety, insurance liability, and improved working environment, just to name a few. This push to convert more processes is driving a need to optimize efficiency and improve cost. The existing approach has known intrinsic inefficiencies and a limited growth path for improvement.

As more heat treaters either experience or compete with the benefits of multi-chamber isolated heat equipment, adoption will continue to accelerate.

Challenges and Considerations

While isolated heat furnaces offer numerous advantages, they are not without challenges. These systems are more complex, require a detailed specification process, and may not be suitable for very large components, intermittent operations, or applications requiring a high degree of flexibility. Many of the advantages of multi-chamber equipment show up in operating and maintenance costs. These benefits can be missed if these costs are not properly accounted for in the ROI analysis phase. Overemphasizing upfront costs can mean missing out on a much better return on investment for equipment with installation life in the range of 20–30 years.

Applications and Future Prospects

Isolated heat vacuum furnaces are not industry specific; rather, they offer advantages across a wide range of applications. Processes characterized by short cycle times benefit because a greater percentage of the floor-to-floor time is dead time and can be recovered, improving equipment utilization. Processes characterized by long cycle times benefit because they can be segmented and indexed at much faster rates, increasing throughput. Surface treatments can benefit from the process uniformity of smaller load sizes without sacrificing throughput. High-volume production environments, in particular, stand to gain the most. Whenever there is a need for more than one batch furnace or where there are numerous small parts in a large work zone, the efficiency and cost savings of continuous isolated heat furnaces truly stand out.

Conclusion

The industry’s focus on efficiency, reduced emissions, and lower operating costs makes isolated heat vacuum furnaces a promising direction for the future. While single-chamber furnaces will still have their place, isolated heat furnaces are becoming more prevalent for many heat treatment processes. Offering superior energy efficiency, better process control, and a more sustainable approach to thermal processing, these furnaces will enable manufacturers to provide high quality, cost-effective solutions that meet today’s market demands and future challenges.

About the Author:

Bryan Stern has been involved in the development of vacuum furnace systems for the past eight years and is passionate about technical education and bringing value to the end-user. Currently product development manager at Gasbarre Thermal Processing Systems, Bryan holds a B.S. in Mechanical Engineering from Georgia Institute of Technology and a B.A. in Natural Science from Covenant College. In addition to being a member of ASM, ASME, and a former committee member for NFPA, Bryan is a graduate of the MTI YES program and recognized in Heat Treat Today’s 40 Under 40 Class of 2020.

For more information: Contact Bryan at bstern@gasbarre.com. 

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All the buzz in our industry seems to indicate that additive manufacturing (AM) and 3D printing are the next hot topics in heat treat, particularly in vacuum heat treat. Heat Treat Today decided to find out how these new technologies are shaping the industry. Read what five heat treat industry leaders had to say about how their companies are preparing for the next generation of AM and 3D printing.

This Technical Tuesday article bringing together the responses from these five companies was first published in Heat Treat Today‘s November 2022 Vacuum print edition.

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What changes have you made to accommodate the AM/3D printing marketplace?

The most important changes relate to the build plate size and how it connects to our standard size systems. Build plates are ever-changing, it seems, as customers have new applications and mostly larger build plates are being requested. In addition, the process parameters – such as temperature and time at temperature and quantity of material – are important. These two items have the most to do with reconfiguring equipment for the AM market. We have also been able to implement our wide range of automation and robotics skills into this equipment as the market scales up for high production.

How will your products and/or services change to accommodate this marketplace?

We are/will be introducing equipment that is in line with standard-build plate dimensions along with reducing operating costs.

Share how 3D printing or AM products/services help heat treaters.

Recent debind and sinter applications have involved, as previously mentioned, complete robotics to handle parts after printing, to debind, to sinter, and then to process specialized by ECM, such as low-pressure carburizing. ECM has also provided equipment to provide all three processes in the same furnace without moving the load or requiring the furnace to cool and reheat. This reduces work processing time along with less handling and less utility cost.

What changes have you made to accommodate the AM/3D printing marketplace?

Nitrex Vacuum Furnaces, through its GM Enterprises acquisition, has moved heavily into additive manufacturing via large production MIM furnaces, which are able to both remove large amounts of powder binders and sinter the parts in the same process. We are in the process of installing and/or starting up five furnaces for these markets, and we have recently employed even more advanced concepts on high volume wax removal. A further trend is on higher value materials, like nickel and cobalt alloys and titanium, necessitating diffusion vacuum levels for processing. Nitrex Vacuum has had this experience already for many years, so moving to smaller scale 3D designs comes with years of experience.

How might your products and/or services change to accommodate this marketplace?

Smaller units are a trend to keep an eye on. We have over a decade of learning from the large units we offer, and this will allow us to compete in these lower volume markets (i.e., 3D) via our proven expertise. Several facts/ideas that we are keeping top of mind are:

• Large potential in the future (whole new market starting to evolve)
• Redesign the product to meet the new needs
• Good for rapid prototyping and quick low volume parts

Furnaces need to be available with fast delivery 3D printing is finding a tremendous niche in fast part production, sourced internally or sourced quickly. These parts may cost more per piece, but having them fast is often more important, and 3D offers this ability to cut weeks or months off of supply chain sourcing.

Share how 3D printing or AM products/services help heat treaters.

The AM sector is still in growth mode. How we help is to give a full-service solution to those customers who want to really increase their volume yet use vacuum in the process. Vacuum helps to transport the binder vapors away from the parts and into the traps for removal. Full binder removal adds to the quality of the parts, as does vacuum sintering of the final parts. We have supplied a few systems over the years with higher, diffusion vacuum levels. As powder materials evolve to higher value materials, there is more interest in diffusion vacuum, and we recently supplied such a system.

What do readers need to know about AM/3D to make decisions today?

Vacuum is the proper way to debind and sinter. Additionally, 3D printing started slow and there were many technologies evolving. Now, it has started to really grow, and the need for smaller furnaces that can offer the same quality as MIM parts produced in high volumes will be a need for 3D part makers, in medium to low volume parts. This may involve furnaces for sinter only, debind and sinter, or even sinter and heat treat. We can see the need to both sinter 3D parts in a small furnace and also heat treat them with special added processes and surface treatments.

What changes have you made to accommodate the AM/3D printing marketplace?

Adding a hot isostatic press has been the most notable change Paulo has made to serve the growing AM market. It goes a step further than that though; heat treatment of AM parts has rapidly evolved, and the desire for custom cycles and more data has caused us to make instrumentation changes and do more R&D type work. Understanding the full production path of the parts and doing our part to reduce the time parts are spending in post-processing steps, including offering stress relief, HIP, EDM, and vacuum heat treatment in a one-stop-shop.

How might your products and/or services change to accommodate this marketplace?

As trials continue and boundaries are pushed for both additive and the accompanying thermal processing, we’re constantly keeping an eye on what’s next. Investing in equipment that’s capable while maintaining and instrumenting it to provide the data and reliability the market needs is the name of the game. Of course, open communication with additive manufacturers and printer designers makes this far easier. We value communication with printer manufacturers as it helps us understand demand for our services in terms of build plate size, since, as we all know, furnaces and HIP vessels aren’t one size fits all!

Share how 3D printing or AM products/services help heat treaters.

Additive parts have become commonplace and we’re now regularly providing HIP, stress relief, and solution treating for them. A more interesting example is for parts printed in Inconel 718; we’ve developed a combined HIP and heat treat (or High Pressure Heat Treat) cycle which was able meet material properties specifications when the traditional processing techniques were not. This is where we feel the real cutting edge is when it comes to heat treatment of additive parts; the slow cooling HIP cycles developed for casting decades ago aren’t always optimal for today’s additive parts.

What changes have you made to accommodate the AM/3D printing marketplace?

There are several methods for 3D printing and we as heat treaters and vacuum furnace manufacturers generally classify those methods into two basic groups: those that use liquid binding polymers and those that do not.

For the group who does not use liquid binding polymers, there are no changes thus far to the design of the vacuum furnace that must be made. One significant caution is insuring there is no loose powder on the surface or cavities of the parts. Residual powder on or in the parts could have adverse effects on the parts themselves and to the vacuum furnace. The loose powder can liberate from the part during the heat treat or quench steps during the process and contaminate the vacuum furnace. The powder in the furnace is then considered FOD (foreign object debris) for subsequent heat treatments processed in that furnace. The powder could also accumulate over time and cause an electrical ground the heating elements or the quench motor, clog the heat exchanger, contaminate vacuum gauges and hot zone insulation, among other issues.

For the group that does contain liquid binding polymers, in addition to the comments about avoiding loose powder on or in the parts, care must also be taken to accommodate for the vaporization of the binder that occurs during heating of the parts. The binder, in its vapor form, will condense at cooler areas in the vacuum furnace. The condensed areas are potential contamination points and could have all the same issues and concerns of loose powder as described above. The binder collection locations, whether at intentional or non-intentional places, will also have to be routinely cleaned to maintain ideal binder collection, optimum vacuum pumping, and overall furnace performance.

How might your products and/or services change to accommodate this marketplace?

With the growth of 3D printing using liquid binder polymers, Solar Manufacturing has taken what was learned from the furnace modified at Solar Atmospheres of Western PA for MIM and AM processing and applied it to a new furnace product line specific for the debind and sinter applications. Solar Manufacturing collaborated with our affiliate company, Solar Atmospheres of Western PA, in modifying an existing vacuum furnace to accommodate the debind and sintering processes. A modified hot zone was installed, and a dedicated binder pumping port was added that helps minimize and target the condensation of detrimental binders evaporating out of parts containing binders. The modified Solar Atmospheres furnace is extremely valuable in gaining knowledge about various aspects of the process and learning what works, and what does not work, in furnace and recipe design. Combining the knowledge and experience of process development of Solar Atmospheres with the advanced Engineering Design Team at Solar Manufacturing, we believe we have a furnace design that modernizes and simplifies the debinding process while minimizing traditional maintenance issues.

Share how 3D printing or AM products/services help heat treaters.

We developed a process of debinding and sintering stainless steel parts with our affiliate company Solar Atmospheres in Souderton PA. The project started out with our Research and Development group to develop the process for the client’s parts. As the trials scaled up, test coupons became test parts, eventually full-size loads. There are always challenges to scaling up from test parts to production loads and we were able to provide the support the customer needed through that transition. The R&D eff orts were successful, and the client ended up purchasing multiple furnaces, which was the end goal for both parties.

Additionally, Solar Atmospheres is currently vacuum stress relieving a 3D component for a major U.S.-based aerospace company that is in use in aircraft today. Also, numerous large-scale components destined for deep space.

What do readers need to know about AM/3D to make decisions today?

Bob Hill, president of Solar Atmospheres of Western PA, reminded us to “realize and acknowledge that AM is still in its infancy stage. Therefore, many metallurgical uncertainties still exist for the multiple printing processes that exist. Understanding this new kind of metallurgy for each printing process, while developing standards and specifications unique to additive manufacturing, is still a huge obstacle. Until this is accomplished, AM will not be the ‘disruptive’ technology that all the experts predict it will be.” If your business is printing parts with liquid polymer binders, you should seriously consider how you plan on debinding and sintering the parts ahead of time. Printed parts in the “Green” or even “Brown” state are fragile and if you are going to ship the parts somewhere else for the debind and sinter steps, extreme care must be taken to prevent the parts from fracturing during transit. Although the shipping can be safely and successfully accomplished, ideally a furnace is available at the print shop to immediately perform the debind and sinter process to avoid those potential shipping difficulties. The other forms of 3D printing that do not contain liquid polymers generally do have this issue.

What changes have you made to accommodate the AM/3D printing marketplace?

From our inception, Gasbarre has had expertise in the powder metallurgy industry, which requires debind and sinter applications similar to that in the AM and 3D printing markets. Our ability to supply equipment for both powder and parts producers has set us up for quick adoption into this market. While considerations need to be made specific to AM, our focus has been on technical support and helping the market grow to higher volume applications.

How might your products and/or services change to accommodate this marketplace?

As adoption of these technologies grow, the volume at which parts need to be produced will grow. Our line of continuous processing equipment in both vacuum and atmosphere applications are well suited. Whether it be debind and sinter, annealing, or stress relieving, we have equipment and expertise that can grow from early production to high volumes.

Share how 3D printing or AM products/services help heat treaters.

Overall, Gasbarre is here to be a resource and support the growth of the additive market. Whether that be through new equipment, servicing existing equipment, or involvement in the industry organizations, we have the expertise to drive success today and into the future!

What do readers need to know about AM/3D to make decisions today?

Additive manufacturing is such a dynamic technology, it is difficult to state one specific item. There is the potential for significant growth opportunities for new applications, but also the potential replacement of traditional manufacturing methods. We also know there is substantial backing for the technology by both private industry and government entities. Like other emerging technologies in the automotive and energies sectors, additive manufacturing isn’t a matter of if, but when it’ll achieve wide scale adoption and high-volume applications.

It is amazing how the list of materials being utilized with this technology is growing. While metals and alloys have not been the majority of the market, it is rapidly growing. With that growth, there is a wide variety of applications and thermal processing requirements for those materials. As well, the different additive and 3D printing processing methods (i.e., binder jetting, powder bed fusion, etc.) leads to a similar diversity in thermal processing requirements.

For more information, contact the leaders:

• Dennis Beauchesne, General Manager, ECM USA, Inc. DennisBeauchesne@ECM-USA.COM
• Mark Hemsath, Vice President Sales, Furnaces and Heat Treating Services, Nitrex. mark.hemsath@nitrex.com
• Phil Harris, Marketing Manager, Paulo. pharris@paulo.com
• Trevor Jones, President, Solar Manufacturing, Inc. trevor@solarmfg.com
• Ben Gasbarre, Executive Vice President, Sales & Marketing, Gasbarre Thermal Processing Systems. ben.gasbarre@gasbarre.com

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ThermTech, heat treat service provider in Waukesha, WI, has increased their capabilities to provide services for the medical, aerospace, mining and oil, nuclear, and agricultural industries.

Jason Kupkovits, vice president of Sales & Strategic Direction at the company, commented on that ThermTech will be continuing their 40 years of quality assurance, turnaround time, on-site engineering, and customer service standards.

Partnering with Gasbarre Thermal Processing Systems, ThermTech significantly increased their normalizing, annealing, stress relieving, tempering, and neutral hardening capacity through the acquisition of three new furnaces. These three furnaces — now fully operational — include: a dual zone, direct-fired box austenitizing furnace; a large batch tempering furnace; and an additional tempering furnace. These furnaces are compliant with AMS2750 at different class certifications.

ThermTech has also added two additional vacuum furnaces from Ipsen, USA. The furnaces have dimensions of 36″ wide x 36″ tall x 48″ long with capabilities of quenching up to 6 bars of pressure utilizing nitrogen or argon gas as the quench medium. These large vacuum furnaces are AMS class 3 (+/-15°F) certified capable of AMS2750.

ThermTech added a solution annealing furnace from Williams Industrial Service to give their operational aluminum line additional heat treat capabilities. This line is capable of a sub-15 second transfer to air blast quench, a water quench range of 55°F up to boiling, a sub-7 second transfer to water quench which exceeds AMS 2770/AMS2771 specifications, as well as load thermocouple monitoring during the solution treatment, quenching, and aging.

Another recent acquisition includes a new austempering/marquenching furnace from Michigan based AFC-Holcroft. This furnace can handle a single part racked in the vertical orientation up to 56″ long. The working dimension of the furnace is 36″ W x 72″ L x 56″ H and is capable of operating with salt temperatures ranging from 350°F — 750°F. “The UBQA system is an environmentally friendly ‘green technology,'” commented Dan Hill, sales engineer at AFC-Holcroft, “which can be used to impart resistance to distorting, cracking or warping of heat-treated components.” Applicable processes include marquenching, austempering, and carburizing with additional washing and tempering capacity accompanying the new marquenching/austempering furnace. Installation is expected in early 2023.

The heat treat service provider’s long-term strategy is to increase growth in the Midwest and on a national scale. This includes adding more workers and integrating the use of a robotics handling systems, which is expected to be installed in late 2022.

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Your parts need heat treated to herculean surface hardness but with a soft, ductile core. That is to say, you are looking at case hardening processes, most likely one of these: gas carburizing, low-pressure carburizing, carbonitriding, gas nitriding, and ferritic nitrocarburizing.

Mike Harrison at Gasbarre Thermal Processing Systems brings us a Technical Tuesday article about what case hardening is and how five of the most common processes vary by (1) comparing the specific guidelines for each temp and time, (2) identifying equipment used to perform each process, and (3) providing a chart (at the end!) to understand different process considerations.

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Case hardening falls into a class of heat treatment processes that typically involve the addition of carbon and/or nitrogen to the material through solid-gas reactions at the surface followed by diffusion. These processes are performed for any number of reasons that generally include increasing strength and wear resistance, but in all cases the end result is a harder, higher-strength surface with a softer, more ductile core.

Case hardening processes can be divided into two subsets: those that include quenching to harden, such as gas carburizing, low-pressure carburizing (LPC), and carbonitriding; and those that do not include quenching, such as gas nitriding and ferritic nitrocarburizing (FNC). This article will provide a brief look into each process, the types of equipment used, and considerations for implementation.

Diffusion + Quenching Processes

These processes involve heating the workload to austenitizing temperature, which is above the upper critical temperature for the material in question, then supplying and allowing the desired element(s) to diffuse into the part surface, followed by rapid cooling (quenching) to create a phase change to martensite that strengthens the material. Tempering is then performed to create a material that has the desired final strength and ductility properties. The result is a high concentration of added elements on the surface that continually decreases through diffusion until eventually matching the same concentration as the base material; this gradient similarly produces a hardness that is higher at the surface, gradually diminishing until reaching the core. Higher alloyed steels may also see a microstructural change in the core from quenching that produces a core with higher hardness than the previously untreated material, but lower than the surface hardness produced.

Atmosphere Gas Carburizing

Gas carburizing is a process where carbon is added to the material’s surface. The process is typically performed between 1550-1750°F, with carburizing times commonly between 2-8 hours; of course, these values can vary depending on the material, process, and equipment. The most common atmosphere used for atmosphere gas carburizing is endothermic gas with additions of either natural gas or propane to increase the carbon potential of the furnace atmosphere. Common case depths achieved are around 0.005-0.040″, with deeper cases possible through a combination of longer treatment times and/or higher temperatures.

The atmosphere gas carburizing process can be performed both in batch and continuous equipment. On the batch side, traditionally an integral quench (IQ) furnace is used (Fig. 1); it consists of a heating chamber where the workload is heated and exposed to the carburizing atmosphere, then the workload is transferred to an attached quench tank for cooling. The entire furnace system is sealed and under protective atmosphere to preserve the part surface and maintain safe control of any combustible gases. For batches of large product, a pit furnace can be used for carburizing with the workload being transferred via an overhead crane into and out of the furnace to a quench tank.

For continuous processing, a belt furnace can be used. The product is placed on a belt and then progresses through the furnace at the desired temperature and atmosphere composition; the carburizing time can be varied by adjusting the belt speed through the furnace. At the end of the furnace, the parts drop off the belt into the quench tank. Then, a conveyor pulls the parts out of the tank and drops them on another belt to be washed and tempered. For continuous processing of heavier loads pusher furnaces, rotary retort, rotary hearth, and roller hearth furnaces can be used.

To achieve a carburizing atmosphere endothermic gas is typically used, which is produced by an endothermic gas generator (Fig. 2) that heats a combination of natural gas and air to create a mixture that is approximately 40% hydrogen, 40% nitrogen, and 20% carbon monoxide. This mixture is generally considered carbon-neutral, meaning it will neither add nor deplete carbon from the surface. To increase the carbon concentration the endothermic gas needs to be enriched with a gas (typically natural gas or propane) that will help produce additional carbon monoxide, which will “boost” the carbon potential and drive carbon diffusion into the material.

A less common carburizing atmosphere comes from a nitrogen-methanol system, where nitrogen gas and liquid methanol are combined and injected into the furnace. Upon exposure to the high furnace temperature the methanol will decompose to hydrogen and carbon monoxide. Natural gas or propane additions are still required in order to provide carbon for absorption into the surface of the steel.

Low-Pressure Carburizing

Low-pressure carburizing (LPC), or vacuum carburizing, is a variation of carburizing performed in a vacuum furnace. Instead of the atmospheres mentioned previously, a partial pressure of hydrocarbon gas (such as propane or acetylene) is used that directly dissociates at the part surface to provide carbon for diffusion. After LPC, the workload is transferred to a quench system that could use oil or high-pressure gas, typically nitrogen. LPC with gas quenching can be an attractive option for distortion prone complex geometries as the cooling rates are slower than oil quenching; however, given the slower cooling rate, it becomes very important to choose a higher alloyed steel that will achieve the desired hardness.

LPC typically provides faster carburizing times when compared to traditional gas carburizing. This can be attributed to a more efficient reaction of the hydrocarbon gas used and to the option of using higher carburizing temperatures, typically up to 1900°F. This is made possible by the type of internal furnace construction of vacuum furnace design, although care must be taken at higher temperatures to avoid undesirable grain growth in the material. LPC also has the benefit of eliminating the potential for intergranular oxidation, since it is running in a vacuum system.

LPC is typically performed in a single-chamber vacuum furnace, with oil quenching or high-pressure gas quenching done in a separate chamber (Fig. 3). Continuous vacuum furnaces can also be used in applications that require increased throughput (Fig. 4).

Carbonitriding

Despite its name, carbonitriding is more closely related to carburizing than it is to nitriding. Carbonitriding is a process where both carbon and nitrogen are added to the material surface. This process is typically performed in a range of 1450-1600°F and generally produces a shallower case depth than carburizing. Carbonitriding is used instead of carburizing for plain carbon steels that do not contain enough alloying content to respond well to quenching, as the added nitrogen can provide a higher hardenability in the case to allow for proper hardness development.

Atmosphere carbonitriding can be performed in the same equipment as is used for carburizing. The furnace atmosphere is still typically endothermic gas-based and includes the addition of ammonia to provide the nitrogen. Vacuum carbonitriding with both hydrocarbon and ammonia additions can also be performed in the same equipment as used for vacuum hardening and low pressure carburizing.

Diffusion Only Processes

These processes involve heating the workload to a temperature below the austenitizing temperature, allowing the desired element(s) to diffuse into the part surface, then slow cooling. The increase in hardness at the material surface comes only from the addition of the diffused element(s), and not from a phase change due to quenching. As these processes are performed below the lower critical temperature (i.e., below the austenitizing range), the desired core hardness and microstructure need to be developed through a separate heat treatment prior to case hardening. Generally, the process temperature selected should be at least 50°F below any prior treatment temperatures to avoid impact to the core properties.

Gas Nitriding

Gas nitriding is a process where nitrogen is added to the material surface. The process is typically performed between 925-1050°F; cycle times can be quite long as the diffusion of the nitrogen is slow at these temperatures, with nitriding times typically ranging from 16 – 96 hours or more depending on the material and case depth required. Nitriding can be performed in either a single or two-stage process and has the potential to produce two types of case, the first being a nitrogen-rich compound layer (or “white layer”) at the surface that is extremely hard and wear-resistant but also very brittle. This compound layer depth is dependent on processing time. In the more traditional two-stage process, the case depth produces a gradient of hardness from surface to core that commonly ranges from 0.010-0.025″, with minimal white layer, typically between 0-0.0005″. Nitriding is typically performed on higher alloyed steels or steels specifically designed for the nitriding process (e.g., Nitralloy®) as it relies on the formation of nitrides to create the increased hardness, which is achieved through the use of nitride-forming alloys such as aluminum, molybdenum and chromium. Pre and post oxidation treatments can be incorporated into the cycle to achieve certain benefits. Since the process does not require quenching to harden, it has the potential of producing a product that is more dimensionally stable and may not require any post-process finishing.

This process is most commonly performed in batch equipment; while it is possible to use a continuous furnace, keeping the ends of furnace sealed to contain the atmosphere can be challenging. Traditionally, pit furnaces have been used for nitriding as they can accommodate larger load sizes and can be easier to seal as gravity helps keep the lid sealed; however, horizontal designs have gained in popularity in recent years (Fig. 5). In either case, the furnaces are usually a single-chamber design with the load sealed inside an Inconel or stainless steel retort.

To achieve a nitriding atmosphere, ammonia (not nitrogen) is used to supply the atomic nitrogen necessary for diffusion. At the process temperatures used, ammonia does not readily dissociate on its own; rather, it dissociates when exposed to a heated steel surface (iron acting as a catalyst) into atomic nitrogen and hydrogen. To control the amount of nitrogen available for nitriding, the dissociation rate of the ammonia can be measured with high dissociation rates (high hydrogen content) providing a lower nitriding potential and low dissociation rates (low hydrogen content) leading to more nitriding potential. The depth of the compound layer can be varied through control of the nitriding potential, with higher nitriding potentials producing a thicker compound layer.

For more precise atmosphere control, an ammonia dissociator can be used to provide gas to the furnace that has already been split to dilute the atmosphere with hydrogen to more quickly achieve a high dissociation rate in the furnace. The ammonia dissociator is a heated box with a small retort inside; the ammonia is passed through this retort that contains a catalyst to promote the dissociation of the ammonia, and the resulting gas mixture is cooled and then injected into the furnace.

Ferritic Nitrocarburizing

In the author’s opinion, just like with carbonitriding, ferritic nitrocarburizing (FNC) is named incorrectly as it is more closely related to nitriding than it is with carburizing. FNC is a process that is still mostly nitrogen-based but with a slight carbon addition as well. The added carbon helps promote compound layer formation, particularly in plain carbon and low alloy steels that do not contain significant nitride-forming alloys. This process is typically performed in a range of 1025-1125°F with cycle times much shorter than nitriding, typically 1-4 hours. The compound layer produced is usually much deeper than nitriding at 0.0005-0.0012″, with case depths reaching up to 0.025″, although in many applications a case depth may be difficult to measure. FNC is usually performed instead of nitriding in applications where the deeper compound layer is needed to increase wear resistance, but the added strength of a deep case depth is not as critical.

FNC can be performed in the same equipment used for nitriding, as long as a hydrocarbon gas is available to the furnace such as carbon dioxide or endothermic gas. FNC can also be performed in an IQ furnace using a mixture of ammonia and endothermic gas; for cooling, the parts can be oil quenched or slow cooled in a top cool chamber (if equipped).

Considerations

Case hardening processes are some of the most common heat treatments performed, but each process has its own unique needs. The table below provides a summary of the considerations that need to be made when selecting the optimum process. This list is by no means exhaustive; it is encouraged to work with a furnace manufacturer familiar with each process to help select the correct process and equipment needed.

About the Author: Mike Harrison is the engineering manager of the Industrial Furnace Systems division at Gasbarre. Mike has a materials science and engineering degree from the University of Michigan and received his M.B.A. from Walsh College. Prior to joining Gasbarre, Mike had roles in metallurgy, quality, and management at both captive and commercial heat treat facilities, gaining nearly 20 years of experience in the thermal processing industry. Gasbarre provides thermal processing equipment solutions for both atmosphere and vacuum furnace applications, as well as associated auxiliary equipment and aftermarket parts & service.

For more information: Contact Mike at mharrison@gasbarre.com

An automotive parts supplier in Mexico will receive a rebuilt 24 inch, four zone continuous mesh belt brazing furnace.

The rebuild included a new 330 stainless steel muffle, new silicon carbide heating elements, new cooling sections, and new furnace controls to meet CQI-9 requirements. The CQI-9 controls package includes data acquisition, preventative maintenance alerts, remote connectivity, furnace parameter trending, and temperature deviation alarms.

The partner chosen for this rebuild, Gasbarre Thermal Processing Systems, designs, manufactures, and services a full line of industrial thermal processing equipment, offering batch and continuous thermal processing equipment for both atmosphere and vacuum applications as well as a full line of alloy fabrications, replacement parts and auxiliary equipment.

Heat Treat Today offers News Chatter, a feature highlighting representative moves, transactions, and kudos from around the industry.

Personnel & Company Chatter

• Bill Gornicki was recently appointed Director of Sales at ECM-USA, Inc. in Pleasant Prairie, WI.
• AFC-Holcroft, in Wixom, MI, recently moved its European satellite office from Delémont, Switzerland, to Swiebodzin, Poland, as necessitated by the retirement of their Director of European Operations. The new director, Marek Kedzierzynski, will be based out of Poland.
• Wire Experts Group, the parent company to Pelican Wire and Rubadue Wire, recently announced the newest members of their leadership team and their respective roles: Brinson White will now lead the Engineering & Maintenance teams at both Pelican and Rubadue as WEG Director of Engineering; Mike Skorupa has been named Director of Continuous Improvement across all business units; and Kevin Clements has been named Global Supply Chain Manager. 
  
• RETECH Systems, LLC, a SECO/WARWICK Group company, has finalized plans to relocate its headquarters from Ukiah, CA, to Buffalo, NY.
• Charlie Li, of DANTE Solutions, began teaching a new master-level Mechanical Engineering class entitled “Advanced Manufacturing Processes: Heat Treatment of Steels” at Cleveland State University.

Equipment Chatter

• Solar Atmospheres has purchased two microscopes, one a ZEISS AxioVert A1 Inverted Materials Microscope and the other a a Hitachi smart Scanning Electron Microscope, to enable them to better serve the needs of their customers.
• Magnetic Specialties, Inc. recently shipped two 510KVA, three phase step down 6-pulse rectifier transformers and DC inductors for use in industrial rectifier applications.
• The Grieve Corporation recently installed their new electrically-heated 2000°F inert atmosphere heavy-duty box furnace to be used for heat treating titanium at a customer’s facility.
• Gasbarre Thermal Processing Systems recently commissioned a model CVPQ Continuous Vacuum Furnace with 5 BAR pressure quench capabilities, and a precision gas nitriding and ferritic nitrocarburizing furnace, in the Midwestern United States.
• Ipsen USA offers free evaluations of any brand of vacuum heat-treating system in the United States. An Ipsen Customer Service team member will check all major components of the furnace and provide a written health report with a suggested 18-month maintenance plan.
• Tenova recently received the official notice to proceed with the new Hot Dip Galvanizing (HDG) line for NLMK Group in Lipetsk, Russia.
• Pries Enterprises finished a 50,000 sq ft expansion and installation of a state-of-the-art anodizing line, making them the only vertically integrated extruder-anodizer fabricator in their immediate area.

Kudos Chatter

• Grupo Mess was recently named an exclusive Buehler distributor of metallographic and hardness equipment in Mexico.
• Aerospace Testing & Pyrometry recently announced the opening of their newest regional office in Greenville, SC. The territory will include North Carolina, South Carolina, Virginia, Georgia, Tennessee and Alabama.
• Constellium SE was recently recognized with the “Best Performer Award” by Airbus.
• Advanced Heat Treat Corp. recently announced that it has added gas nitriding to its Nadcap® accreditation.

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