VACUUM FURNACES TECHNICAL CONTENT

CFC Fixture Advantages and Challenges in Vacuum Heat Treatment, Part 1

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OCWhat happens when a lead engineer sticks his head in new advancements in materials from NASA? For the author of this article, it means the successful research and development of a new generation of workpiece carriers and fixtures made from “a high-tech ceramic matrix composite of very strong carbon fiber,” that is, CFC.

This Technical Tuesday article, written by Dr. Jorg Demmel, founder, 0wner, and President, High Temperature Concept, was first published in Heat Treat Today's November 2022 Vacuum print edition.

Introduction: From NASA to Industrial Heat Treatment

Dr. Jorg Demmel
Founder, Owner, President
High Temperature Concept

In the mid-1990s, a development in materials from NASA moved into my focus. I was an associate and lead engineer at the Fraunhofer Institute in Stuttgart, Germany, so I posed the question: Could CFC material (carbon fiber-reinforced carbon) substitute for non-abrasion-resistant and brittle graphite as the material used for workpiece carriers in the soldering process of drills? The answer: yes. The story did not end here. This project, which included the automated handling of the drills in some continuous furnaces, was just the first accomplishment. What ensued was a successful research and development of a new generation of workpiece carriers and fixtures made from CFC (“Carbon Fiber Carbon”).

Material Properties and Main Advantages of CFC

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CFC (aka, CFRC, or C/C), which stands for carbon fiber-reinforced carbon, is a high-tech ceramic matrix composite of very strong carbon fibers (or fiber rovings) in a compensative carbon (graphite) matrix. Material properties of some relevant heat treatment fixture materials were evaluated, and some are shown in Figure 1. These CFC properties have the following positive effects when used as CFC fixtures for heat treatment:

Figure 1. Left to right for 2D CFC SGL Sigrabond Performance, heat resistant austenitic cast alloy steel ASTM A297-HK (ISO G-X 40 CrNiSi 25-20; 1.4848), wrought and annealed Ni alloy Inconel 601 UNS N06601 (NiCr23Fe15Al; 2.4851) and mechanically alloyed Fe alloy, oxide dispersion strengthened Plansee PM ODS 2000 (Cr Al 21 6; 1.4768).

  • Because of their low density, CFC fixtures have a lower weight than their steel alloy counterparts (about five times), which reduces the efforts for manual handling.
  • Because of the increased strength of CFC at high temperature, the fixture weight can be reduced further. Additionally, fixture volume can be reduced — in some applications dramatically — so that, when combined with a specific CFC fixture design, furnace capacities can be increased up to 100%.
  • The following characteristics of CFC fixtures are responsible for the longer fixture life cycles (up to greater than five times), less workpiece distortion and rework, and make an automatic workpiece handling possible for the first time ever: the low CTE (coeffcient of thermal expansion) value for CFC in the direction of the fiber, the fact that CFC is chemically inert in vacuum or
    certain protective atmospheres, has an excellent thermal shock resistance, and it doesn’t grow, creep, or age like metals.
  • Although the specific heat of CFC is higher, the energy consumption can be reduced and shorter heating up and cooling down times can be reached, resulting in up to 30% shorter process cycle times for the same workpieces.

Figure 2. CFC fixture advantages in heat treatment

Figure 2 shows all potential advantages of CFC fixtures compared to state-of the- art steel alloy; a short payback time of the investment with high profitability are possible.

CFC Fixture Suitability in Vacuum Heat Treatment

Since CFC is made of carbon, it is not made for high temperatures above 752°F (400°C) in air or atmosphere with high percentages of oxygen, water vapor, hydrogen, or carbon dioxide for long periods of time. Therefore, vacuum or protective gas atmospheres are, in general, a suitable environment for CFC fixtures.

Table 1. Reaction rates and activation energies for graphite (800 °C; 0.1 bar). Equation (1) is the main combustion reaction, which has the strongest effect and is strongly exothermic (negative change of reaction enthalpies Δ"H). Reaction (2) is the so-called water gas reaction which shows the endothermic oxidation of carbon with vapor. Equation (3) is the Boudouard reaction which occurs endothermic above 700 °C. According to the Boudouard equilibrium the C0/CO2 ratio increases with increasing temperatures and decreasing pressures. Reaction (4) is the methane formation reaction: hydrogen reacts with carbon at temperatures above about 700 °C to CH4. Below 546 °C methane decomposes in carbon and hydrogen.

Table 1 shows the relative reaction rates for graphite according to H. Marsh in Introduction to Carbon Science, 1989 in the “reaction controlled” Zone I up to 1472°F (800°C) under oxygen, steam (H2O) Figure 3. Burning rates of graphite as a function of temperature

Industrial experience shows that CFC under vacuum of < 10-2 mbar at 1472°F or 1832°F (800°C or 1000°C) at a low dew point < -4°F (-20°C) (< 0.1 % vapor content) lasts at least 5,000 hours (real process time). At 3632°F (2000°C), the life is about 2,000 hours. Dew points of about 0°C (about 0.6 % vapor) cause higher reaction rates and reduce lifetime to about 800 to 1,000 hours.

Unwanted Contact Reactions

Contact reactions between the CFC fixtures and the workpieces, primarily made of steel, can lead to changes in the workpieces: for example, carburization of the workpiece in contact with the CFC. It is important to avoid these contact reactions since the properties of the workpieces must under no circumstances be changed in an uncontrolled manner. Neither the chemical composition nor mechanical properties nor the surface may change beyond the permissible tolerance limits. The CFC fixture should also not be subject to any changes that could adversely affect its properties and, above all, its service life.

The following materials, consisting of mainly workpiece materials made of steel, were used in direct contact with CFC, especially in heat treatment and brazing. CFC 1501G (SGL), CF222 (Schunk), or CX-27C1 (GTD, Toyo Tanso) were used as CFC workpiece carrier materials. Table 2 gives an overview of the results. The symptoms columns with “none” indicate no problems. The colored cells showed problems. The last column references the application or the results.

Table 2. Contact/carburization test results from field trial, updated 2022

Figure 4a. CFC level grid (case hardening, oil quench)

Figure 4b: CFC-Rack CVD-Coating

The contact partners and processes in which unwanted contact reactions occurred in the field test (colored in Table 2) and which are not confidential (bold font) are examined more closely in Part 2. See Figure 5 which shows some contact reactions on tempered steel drills after vacuum hardening at 2066°F (1130°C) under vacuum of 0.3 mbar (0.3 hPa or 225 mm Hg or “micron”).

Figure 5. Contact reactions on drill blanks (1.6582) with SiC-coated CFC (Schunk CF222P75 and SGL 1601YI); Scale left about 2:1 and right microsection about 400:1

Figure 6 shows some heavy melting reactions of high-speed steel after vacuum hardening at 2264°F (1240°C) under vacuum of 0.1 mbar (0.1 hPa or 75 mm Hg or “micron”).

Figure 6. 1.3343 after contact with CFC CF222 at 2282°F (1250 °C) (left approx. 25:1; right detail 100:1)

The carbon transmission mechanism with unwanted carburization, along with eutectic reaction of some workpieces made of steel with CFC, and some technical solutions will be explained in Part 2 of this article.

References

Atkins, P. W.: Physikalische Chemie. 1. vollst. durechges. u. berichtigter Nachdr.d. 1. Aufl., Weinheim, VCHVerlag, 1988 – ISBN 3-527-25913-9.

Bürgel, R.: Handbuch Hochtemperatur-Werkstofftechnik: Grundlagen, Werkstoffbean-spruchungen, Hochtemperaturlegierungen. Braunschweig, Wiesbaden: Vieweg, 1998. ISBN 3-528-03107-7.

Demmel, J.: Advanced CFC-Fixture Applications, their scientific challenges and economic benefits, In: 30th Heat Treating Society Conference & Exposition, Detroit, MI, USA, 15th Oct. 2019.

Demmel, J.: Werkstoffwissenschaftliche Aspekte der Entwicklung neuartiger Werkstückträger für Hochtemperaturprozesse aus Faserverbundkeramik C/C und weiteren Hochtemperaturwerkstoffen, Dissertation, TU Freiberg, Germany, 2003.

Demmel, J.: Why CFC-Fixtures are a Must for Modern Heat Treaters, FNA 2020 Technical Session Processes & Quality, USA, 30th Sept. 2020.

Demmel, J., et al: Applications of CMC-racks for high temperature processes. In: 4th Int. Conf. on High-Temperature Ceramic Matrix Composites, 3.10.2001, p. A-17.

Demmel, J. und J. Esch: Handhabungs-Roboter sorgt für Wettbewerbsvorsprung. Härterei: Symbiose von neuen Werkstoffen und Automatisierung. In: Produktion (1996), No. 16, p. 9.

Demmel, J. und U. Nägele: CFC revolutioniert die Wärmebehandlung. In: 53. Härterei-Kolloquium, Wiesbaden, 10.10.97. Vortrag und Tagungsbericht.

Demmel, J., Lallinger, H.: CFC-Werkstückträger revolutionieren die Wärmebehandlung. In: Härtereitechnische Mitteilungen 54, No. 5, p. 289-294, 1999.

Eckstein, H.-J., et al: Technologie der Wärmebehandlung von Stahl. 2nd Edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1987. ISBN 3-342-00220-4.

Godziemba-Maliszewski, J.; Batfalsky, P.: Herstellung von Keramik-Metall-Verbindungen mit Diffusionsschweißverfahren. In: Technische Keramik, Jahrbuch, Essen, 1 (1988), S. 162-172. ISBN 3-80272141-1.

Grosch, J.: Grundlagen-Verfahren-Anwendungen-Eigenschaften einsatzgehärteter Gefüge und Bauteile, ExpertVerlag, 1994, ISBN 3-8169-0739-3.

Hollemann, A.F.; Wiberg, E.: Lehrbuch der anorganischen Chemie / Hollemann-Wiberg. 91.-100. Aufl ., de Druyter Verlag, 1985 – ISBN 3-11-007511-3.

Kriegesmann, J.: Technische Keramische Werkstoffe. Loseblattwerk mit 6 Ergänzungslieferungen pro Jahr.

Kussmaul, K.: Werkstoffkunde II. Stuttgart, Universität, Lehrstuhl für Materialprüfung, Werkstoffkunde und Festigkeitslehre, Vorlesungsmanuskript, 1993.

Lay, L.: Corrosion Resistance of Technical Ceramics. 1. Aufl ., Teddington, Middlesex, Crown-Verlag, 1983 – ISBN 0-11-480051-0.

Marsh, H.; u.a.: Introduction to Carbon Science. 1. Aufl ., London, Butterworths-Verlag, 1989 – ISBN 0-40803837-3.

Spur, G.: Wärmebehandeln. Berlin, 1987, ISBN 3-446-14954-6.

Samsonow, G.V.: Handbook of refractory compounds. New York, 1980.

Schulten, R.: Untersuchungen zum Kohlenstofftransportmit Carbidbildung in Nickelbasis-legierungen. RWTH Aachen, Fakultät für Maschinenbau, Diss., 1988 Deutsche Keramische Gesellschaft, 1990 following. ISBN 3-87156-091-X.

 

About the Author: Dr. Jorg Demmel is the founder, owner, and president of High Temperature Concept. He received his Engineering Doctorate in the field of CFC workpiece carriers for heat treatment and served in different leading positions for Volkswagen before moving to the U.S. In this article, Demmel draws on his dissertation, “Material scientific aspects of the development of new Fixtures for high temperature processes made of fiber-composite ceramics C/C and other high temperature materials” (Technical University Mining Academy Freiberg, Germany, 2002/3), and his personal experiences. For more information Contact Jorg at jorg.demmel@high-temperature-concept.com

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Heat Treat Future with AM and 3D Printing

/ FEATURED NEWS, SINTERING POWDER/METAL TECHNICAL CONTENT, VACUUM FURNACES TECHNICAL CONTENT
OC

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.

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

Dennis Beauchesne
General Manager
ECM USA, Inc.

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.

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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?

Mark Hemsath
Vice President
of Sales, Americas
Nitrex Heat
Treating Services

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?

Phil Harris
Marketing Manager
Paulo

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?

Trevor Jones
President
Solar Manufacturing, Inc.
Source: Solar Manufacturing, Inc.

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?

Ben Gasbarre
Executive Vice President
Sales & Marketing
Gasbarre Thermal Processing Systems

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:

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Not To Be Neglected: Heat Treat Furnace Maintenance Tips

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OCFundamentals of furnace maintenance sometimes fall between that tricky area of realizing their importance and getting pushed to the end of the to-do list. This original content piece shares tips to bring the fundamentals back to where they belong: at the top of the to-do list. 

3 Tips From "Effective Integral Quench Furnace Maintenance" Article

Ben Gasbarre
President, Industrial Furnace Systems
Gasbarre Thermal Processing Systems

  1.  Safety First | Whether the furnace is in operation, or it is having down time, proper safety measures must be in place. Personal protective equipment, proper shut down of power sources, and even the buddy system are topics taken in to consideration.
  2. Asset Management System | Have up-to-date maintenance records available to any and all employees. "Ensuring important information, such as alloy replacements, burner tuning, or control calibration information, can help operations and maintenance personnel as they plan and assess future equipment needs," comments Ben Gasbarre, president industrial furnace systems at Gasbarre Thermal Processing Systems.
  3.  Cleaning | Reminders include: change filters on combustion blowers, clean things like burners and flame curtains, clean out endothermic gas lines, burn off manual probes at least once a week, etc.

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3 Tips From "Furnace Diagnostics for Validation, Preventative Maintenance, and R&M" Article

Daniel Hill, PE
Sales Engineer
AFC-Holcroft
Source: AFC-Holcroft

  1.  Rules and Regulations | The military and energy industries are sectors that have strict standards to follow. Different heat treating shops are using a software module to maintain furnace data, looking at data reports to make sure the furnace systems are running properly.
  2. Timely Maintenance | Making a maintenance plan and then following it means that no tasks are overlooked or forgotten.
  3. After Repairs and Adjustment | Make sure that after trouble shooting and performing repairs, the software generated reports are examined and that furnaces continue to be maintained. Daniel Hill, PE, sales engineer at AFC-Holcroft says, "This saves valuable time and resources, improves availability, and likely increases profitability."

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3 Tips From "How CQI-9 Compliant Quench Oil Analysis Can Aid in Proper Care of Quench Oil" Article

Greg Steiger
Senior Key Account Manager
Idemitsu Lubricants America

  1. Proper Levels of Sludge and Water Quench | Failing to keep the quench oil clean results in problems on surface finish. Maintain the quench from the start by filtering, cleaning, and replenishing to keep end product surfaces more acceptable.
  2. Frequency of Sampling | "[The] more often a quench oil is analyzed, the easier it is to use the quench oil analysis as a tool in the proper care of a quench oil," explains Greg Steiger, senior key account manager at Idemitsu Lubricants America.
  3. Regular Addition of Fresh Oil | Proper maintenance of quench oil will result in some loss through filtration. Be sure to replenish.

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Case Study: Heat Treat Equipment Meets the Future Industry Today

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OCModern industry trends and expectations pose new challenges to heat treating equipment; in addition to the expected requirements (e.g., safety, quality, economy, reliability, and efficiency), factors like availability, flexibility, energy efficiency, environmental, and the surrounding carbon neutrality are becoming increasingly important.

Maciej Korecki, vice president of Business Development and R&D at SECO/WARWICK, presents this special Technical Wednesday case study for the last day of FNA 2022 to focus on an equipment solution that meets these modern industry demands: a semi-continuous vacuum furnace for low-pressure carburizing (LPC) and high-pressure gas quenching (HPGQ).

Maciej Korecki
Vice President of Business of the Vacuum Furnace Segment
SECO/WARWICK

Introduction

At least 60 years ago, vacuum furnaces first appeared in the most demanding industries (i.e., space and aerospace), then spread to other industrial branches, and are now widely implemented in both mass production and service plants. Use of vacuum technology does not look like it is slowing down anytime soon.

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The driving forces behind this growth in vacuum technology are two-fold: first, the increasing heat treatment requirements that result from the directions of industrial development and production systems, and second, environmental protection, where the advantages of vacuum technologies are undeniable.

 

Traditional Atmospheric Technology

Case hardening by carburizing is one of the most widely used heat treatment technologies. It consists in carburizing (introducing carbon to the surface) followed by quenching of the carburized layer. Typically, the work is carburized in a mixture of flammable gases (CO, H2), and quenched in oil in an atmosphere furnace, using methods developed in the 1960s.

These methods have a history of development, though the question remains if the technological developments can keep up with the requirements of modern industry. Safety is an issue with this method due to the use of flammable (and poisonous) gases and flammable oil, as well as open flame, which in the absence of complete separation from the air can lead to fire, or poisoning.

In addition, they affect their environment by releasing significant amounts of heat, polluting the surroundings with quenching oil and its vapors. They require the use of washers and cleaning chemicals, emit annually tens or even hundreds of tons of CO2 (greenhouse gas, the main culprit of global warming and dynamic climate change) coming from the carburizing atmosphere, and for these reasons, they need to be installed in dedicated so-called “dirty halls” separated from other production departments.

The resulting requirement to limit the temperature of the processes to 1688-1706 oF (920-930oC) is also not without importance, as it blocks the possibility of accelerating carburization and increasing production efficiency (due to the use of metal alloys in the construction, the service life of which drops dramatically at higher temperatures) and the formation of unfavorable intergranular oxidation (IGO), which is a characteristic feature of the atmospheric carburizing method.

Quenching in oil is effective, but it does not have precise controllable, repeatable, and ecological features that heat treaters may need. Due to the multiphase nature of oil quenching (steam, bubble, and convection phase) and the associated extremely different cooling rates, it is characterized by large and unpredictable deformations within a single part and the entire load. Furthermore, there is no practical method to influence and control the quench process.

Modern Vacuum Technology with LPC and HPGQ

Vacuum carburizing appeared as early as the 1970s, but it could not break through for a long time due to the inability to control and predict the results of the process, and heavy contamination of the furnaces with reaction products.

The breakthrough came in the 1990s, when acetylene began to be used as a carbon-bearing gas and computers were employed to control and simulate the process. Since the beginning of the 21st century, there has been a rapid development of the low pressure carburizing (LPC) technology and an increase in its industrial demand, which continues today with an upturn.

Vacuum carburizing occurs with the aid of hydrocarbons (usually acetylene), which catalytically decompose at the surface, providing carbon that diffuses into the material. The process is carried out under negative pressure (hundreds of times less than atmospheric pressure) and is very precise, efficient, and uniform due to the very high velocity and penetration capacity of the gas molecules, allowing the carburizing of large and densely packed loads and hard-to-reach surfaces such as holes.

In addition, the use of non-oxygen-containing hydrocarbon atoms eliminates the qualitative problem of intergranular oxidation (IGO). The process is completely safe, there is no flammable or poisonous atmosphere in the furnace and no open flame, and the furnace can work unattended and is fully available and flexible, i.e., it can be turned on and off on demand, which does not require any preparation. Similarly, changing the carburizing parameters takes place efficiently.

Due to the design of the vacuum furnace and the use of materials with high resistance to temperature, i.e., graphite — the only limitation for the temperature of the carburizing process is the steel from which the parts are made — it is possible to carburize at higher temperatures than traditional methods allow. The result is a significantly shorter carburizing time and increased furnace efficiency versus what can be achieved in an atmosphere furnace.

Neutral gas cooling was included with the vacuum furnaces. Initially, engineers used a cooling gas (nitrogen or argon) at near ambient pressure and natural convection. Subsequent solutions introduced fan-forced gas flow in a closed circuit. The cooling efficiency under such conditions was hundreds of times lower compared to that of oil, allowing only high-alloy steels and parts with very limited cross-sections to be hardened. Over the following decades, the development of HPGQ was focused on improving cooling efficiency by increasing pressure and velocity and using different types of gas and their mixtures. Current systems have cooling efficiencies on a par with oil-based systems and enable the same types of steel and parts to be hardened, with the advantage that deformation can be greatly reduced and reproducible, and the process is completely controllable (through pressure and gas velocity) allowing any cooling curve to be executed.

Vacuum technologies have an ecological edge. Because of their design and processes, vacuum furnaces do not interfere with the immediate surroundings and are environmentally friendly, so they can be installed in clean halls, directly in the production chain (in-line). They emit negligible amounts of heat and post-process gases which are not poisonous and contain no CO 2 at all. Gas quenching eliminates harmful quenching oil and the associated risk of fire and contamination of the immediate environment, as well as the need for equipment and chemicals for its removal and neutralization. Nitrogen used for cooling is obtained from the air and returned to it in a clean state, creating an ideal environmentally friendly solution.

The presented advantages of vacuum technologies influence its dynamic development and increase the demand of modern industry, and the gradual replacement of atmospheric technologies.

Vacuum furnaces are available in virtually any configuration: horizontal, vertical, single, double, or multi-chambered, tailored to the process and production requirements. In light of recent global changes, requirements, and industrial trends, special attention should be paid to disposable, flexible, and rapidly variable production and process systems, as well as independent and autonomous systems, which include a three-chamber vacuum furnace for semi- continuous heat treatment, equipped with LPC and HPGQ.

Three-Chamber Vacuum Furnace — CaseMaster Evolution Type CMe-T6810-25

This is a compact, versatile, and flexible system designed for vacuum heat treatment processes for in-house and commercial plants, dedicated to fast-changing and demanding conditions in large-scale and individual production (Fig. 1). It enables the implementation of case hardening by LPC and HPGQ processes and quenching of typical types of oil and gas hardened steels and allows for annealing and brazing. It is characterized by the following data:

  • working space 610x750x1000 mm (WxHxL)
  • load capacity 1000 kg gross
  • temperature 2282oF (1250oC)
  • vacuum range 10-2 mbar
  • cooling pressure 25 bar abs
  • LPC acetylene gas
  • Installation area 8x7m

The furnace is built with three thermally and pressure-separated chambers (Fig. 2.), and operates in a pass-through mode, loaded on one side and unloaded on the other, simultaneously processing three loads, hence its high efficiency. The load is put into the pre-heating chamber, where it is pre-heated to the temperature of 1382oF (750oC), depending on the requirements: in air (pre-oxidation), nitrogen or vacuum atmosphere. It is then transferred to the main heating chamber, where it reaches process temperature and where the process is carried out (e.g., LPC).

In the next step, the charge is transported to the quenching chamber, where it is quenched in nitrogen under high pressure. All operations are automatic and synchronized without the need for operator intervention or supervision.

Fig. 2. Construction and schematic furnace cross-section CMe-T6810-25.
Source: SECO/WARWICK

Particularly noteworthy is the gas cooling chamber, which in nitrogen (rather than helium) achieves cooling efficiencies comparable to oil (heat transfer coefficient >> 1000 W/m2K), thanks to the use of 25 bar abs pressure and hurricane gas velocities in a highly efficient closed loop system. The cooling system is based on two side-mounted fans with a capacity of 220 kW each, forcing with nozzles an intensive cooling nitrogen flow from above onto the load, then through the heat exchanger (gas-water), where the nitrogen is cooled and further sucked in by the fan (Fig. 3). The cooling process is controllable, repeatable, and programmable by gas pressure, fan speed and time. An intense and even cooling is achieved. The result is the achievement of appropriate mechanical properties of parts with minimal hardening deformations, without the use of environmentally unfriendly oil or very expensive helium.

Fig. 3. Cross-section of the furnace CMe-T6810-25 cooling chamber.
Source: SECO/WARWICK

An integral part of the furnace system is the SimVaC carburizing process simulator, which enables the design of furnace recipes without conducting proof tests.

Distinctive Features of the CMe-T6810-25 Furnace

The advantages of this type of furnace — versus more traditional or past forms — can be demonstrated in a number of usability and functional aspects, the most important of which are the following:

Safety:

  • Safe, no flammable and poisonous atmosphere
  • No open fire

Production and installation:

  • Intended for high volume production (two to three times higher output when compared to single- and double-chamber furnaces)
  • Effective and efficient LPC (even five times faster than traditional carburizing)
  • Total process automation & integration
  • Clean room installation
  • Operator-free
  • Compact footprint

Quality:

  • High precision and repeatability of results
  • Uniform carburizing of densely pack loads and difficult shapes (holes)
  • No decarburization or oxidation
  • Elimination of IGO
  • Ideal protection and cleanliness of part surfaces
  • Accurate and precise LPC process simulator (SimVaC)

Quenching:

  • Powerful nitrogen quenching (neither oil nor helium is needed)
  • Reduction of distortion
  • Elimination of quenching oil and contamination
  • Elimination of washing and cleaning chemicals

Operational:

  • Flexible, on-demand operation
  • No conditioning time
  • No human involvement and impact
  • High lifespan of hot zone components — i.e., graphite
  • No moving components in the process chamber

Ecology:

  • Safe and environmentally friendly processes and equipment
  • No emission of harmful gases (CO, NOx, SOx)
  • No emission of climate-warming gas CO2

Based on the CMe-T6810-25 furnace performance, it is rational and reasonable to build heat treatment systems for high-efficiency and developmental production in a distributed system by multiplying and integrating further autonomous and independent units. The reasons for doing so are because the furnace design affords:

  • No risk of production total breakdown
  • Unlimited operational flexibility
  • Less initial investment cost
  • Unlimited multiplication
  • No downtime while expansion
  • Independent quenching chamber
  • Independent transportation
  • Independent control system

The characteristics, capabilities and functionalities of the CMe-T6810-25 furnace fit very well with the current and developmental expectations of modern industry and ecological requirements, which is confirmed by specific implementation cases.

Case Study

The three-chamber CaseMaster Evolution CMe-T6810-25 vacuum furnace was installed and implemented for production at the commercial heat treatment plant at the Polish branch of the renowned Aalberts surface technologies Group in 2020.

Fig. 4. Gearwheel used in the case hardening process.
Source: SECO/WARWICK

The CMe furnace, together with the washer and tempering furnace, forms the core of the department's production, which is why the furnace is operated continuously. Last year, the furnace performed over 2000 processes and showed very high quality (100%) and reliability (> 99%) indicators. The very high efficiency of the furnace was also confirmed, which, with relatively low production costs, contributes to a very good economic result.

The case hardening process on gearwheels used in industrial gearboxes was taken as an example. The wheel had an outer diameter of about 80 mm and a mass of 0.52 kg (Fig. 4), and the load consisted of 1344 pieces densely packed in the working space (Fig. 5) with a total net weight of 700 kg (920 kg gross) and 25 m2 surface to be carburized. The aim of the process was to obtain an effective layer thickness from 0.4 – 0.6 mm with the criterion of 550 HV, surface hardness from 58 – 62 HRC (Rockwell Hardness C), core hardness at the gear tooth base above 300 HV10 and the correct structure with retained austenite below 15%.

Fig. 5. A photograph of the arrangement of gearwheels in the load.
Source: SECO/WARWICK

The LPC process was designed using the SimVaC® simulator at a temperature of 1724oF (940oC) and a time of 45 min, with 3 stages of introducing carburizing gas (acetylene), obtaining the appropriate profile of carbon concentration in the carburized layer, with a content of 0.76% C on the surface (Fig. 6).

The process was carried out in the CMe-T6810-25 furnace and had the following course from the perspective of a single load (Fig. 7):

  1. Loading into a pre-heating chamber, heating and temperature equalization in 1382oF (750oC) (100 min in total).
  2. Reloading to the main heating chamber, heating and temperature equalization in 1724oF (940oC), LPC, lowering and equalizing the temperature before quenching in 1580oF (860oC), reloading to the cooling chamber (total 180 min).
  3. Gradual quenching at a pressure of 24, then 12 and 5 bar, discharge of the load from a quenching chamber (total of 25 min).

The load stayed the longest in the main heating chamber – for 180 minutes. This means that with the continuous operation of the furnace in this process, the cycle will be just 180 minutes, i.e., once every three hours the raw load will be loaded, and the processed load will be removed from the furnace.

In the next step, the parts underwent tempering at a temperature of 160oC.

The result of the process was tested on ten parts taken from the reference corners and from the inside of the load. The correct layer structure (Fig. 8) and hardness profile (Fig. 9) were achieved, and all the requirements of the technical specification were met (Tab. 1).

Tab. 1. Comparison of the parameters required and obtained in the process.
Source: SECO/WARWICK

During the process, the consumption of the costliest energy factors was monitored and calculated, and the results per one load are as follows:

  • Electricity – 550 kWh
  • Liquid nitrogen – 160 kg
  • Acetylene – 1.5 kg
  • CO2 emissions – 0 kg

Cooling water and compressed air consumption have not been included as they have a negligible impact on process costs.

Summary: Efficiency and Economy

As a result of the process, all technological requirements have been met, obtaining the following indicators of efficiency and consumption of energy factors calculated for the entire load and per unit net weight of the load (700 kg):

  • Capacity: 3h/load; 233 kg/h; 5 thousand tons/year (6500 h)
  • Electricity: 550 kWh; 0.79 kWh/kg
  • Liquid nitrogen: 160 kg; 0,23 kg/kg
  • Acetylene: 1,5 kg; 2,1 g/kg
  • CO2 emissions : 0 kg; 0 kg/kg

On this basis, it is possible to estimate the total cost of energy factors in the amount of approximately EUR 100 per load or approximately EUR 0.14/kg of net load (assuming European unit costs of 2021). It is important that these costs are not burdened by CO2 emission penalties, as can happen with more traditional furnaces.

To sum up the economic aspect, based on an example process, a CMe furnace capacity of 1,500 net tons of parts per year was achieved for 6500 hours of annual furnace operation, at a cost of energy factors of about 100 EUR per load, or 0.14 EUR per kg of parts. The economic calculation is very attractive, and the return on investment (ROI) is estimated at just a few years.

Conclusion

While the advantages of this type of vacuum application are clear from this case study, the example discussed here does not represent the full capabilities of the CMe-T6810-25 furnace, even this process can be optimized and shortened, thereby increasing the furnace's efficiency, and reducing costs. It is possible to carry out carburizing processes (LPC) or hardening alone in a 1.5 h cycle, which would double the capacity of the furnace and similarly reduce the cost of energy factors and shorten the ROI time.

 

For more information:

Go to www.secowarwick.com

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Bundle Up for Fall with 4 Vacuum Heat Treat Articles

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OCGetting excited for the November print edition? In 2021, Heat Treat Today released the inaugural Vacuum Heat Treating print edition. This edition is set to release every November to help heat treaters better work their vacuum furnaces and vacuum heat treat processes. 

This Technical Tuesday original content round-up shares the hottest vacuum heat treating articles from this past year as you bundle up for the cool weather this fall. Enjoy!

Graphite in Vacuum Furnace Fixturing

The Use of Graphite for Vacuum Furnace Fixturing

Let's talk about carbon/carbon composite --- C/C.

Why is the vacuum furnace industry excited about its use in graphite vacuum furnace fixtures, grids, and leveling components? Because it can be readily machined for special shapes and applications. The lighter-weight material is mostly composed of carbon fibers and a carbon matrix (or binder).

Contact us with your Reader Feedback!

As the authors of this article explain, "They are among the strongest and lightest high temperature engineered materials in the world compared to other materials such as basic graphite, ceramics, metal, or plastic. C/C composites are lightweight, strong, and can withstand temperatures of over 3632°F (2000°C) without any loss in performance." Intrigued, are you not?

Read the article by Solar Atmosphere's Roger Jones and Real J. Fradette, "The Use of Graphite for Vacuum Furnace Fixturing."

 

Step-by-Step Guide To Choose Heat Treating Equipment (English / Español)

If it's time to choose an industrial furnace, let's break it down step by step:

Step One: Quote Request

Step Two: Supplier Selection

Step Three: Study and Evaluation of Offers

Step Four: The Price

Follow this guide and avoid saying things like "The substation and/or the cooling tower did not have the capacity"; "The equipment is not what we expected"; or “They never told us that the furnace needed gas in those capabilities." If there are steps you take when selecting an industrial furnace, let us know in a Reader Feedback note here.

Read the article by Carrasco Hornos Industriales' Carlos Carrasco, "A Guide to Selecting Heat Treating Equipment" or "Guía para la Selección de Equipos para Tratamiento Térmico."

 

Pressure vs. Velocity and the Size of Your Furnace

Vacuum Gas Cooling: Pressure vs. Velocity, Part 1 of 2

If you like the R&D world of heat treat, but also like to be grounded in practical heat treat solutions, this is the article for you. Read about what this commercial heat treat found out about how size relates to the pressure and velocity of vacuum furnace cooling rates. Here are the facts you will learn:

  1. The greatest impact on the cooling performance in a vacuum furnace is to increase the___ ______ within ___ _____.
  2. This is achieved by ______ __ ______ of the ______ ____.

Read the article by Solar Atmosphere's Robert Hill and Gregory Scheuring, "Vacuum Gas Cooling: Pressure vs. Velocity, Part 1 of 2."

 

Energy at Large: A Heat Treat Vacuum Furnace Case Study

Harnessing the Sun: A Heat Treat Case Study with General AtomicsIf you like to read about how heat treaters can be game changers in multinational science projects, this is the article for you. A specially designed vacuum heat treat furnace was commissioned to heat treat critical components in a large energy generator. The heat treating of these components takes 5 weeks to complete; talk about a long, uniform heat treat period.

Read about the energy experiment, the heat treat furnace, and the heat treating process in this technical feature.

Read the article by SECO/VACUUM's Rafal Walczak, "Harnessing the Sun: A Heat Treat Case Study with General Atomics."

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

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Start Your Week with 4 Heat Treat Articles in Spanish

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, THERMOCOUPLES TECHNICAL CONTENT, VACUUM FURNACES TECHNICAL CONTENT

OC

At Heat Treat Today, we want to make sure everyone in the North American heat treat industry is well informed so they can be happier and make better decisions. With that in mind, we have been growing our library of Spanish heat treating content.

Below, click the blue headings to learn from Víctor Zacarías about pyrometry standards in the aerospace and automotive industries, from Carlos Carrasco about selecting heat treating equipment, from Bill Munn about leadership and motivation, and from Erika Zarazúa about CQI-9's probe method A. If you'd like to view this content in English, click the America flag icon. 

Pirometría: la normativa para un Procesamiento Térmico confiable en la Industria Automotriz y Aeroespacial

Víctor Zacarías
General Director
Global Thermal Solutions Mexico

"Las operaciones de tratamiento térmico son percibidas generalmente como cajas negras cuyos resultados son poco predecibles. Si bien, entendemos los mecanismos físicos involucrados para modificar las propiedades de un material, los hornos de tratamiento térmico son sistemas termodinámicamente imperfectos, y por ende los resultados finales en ocasiones también lo son."

Contact us with your Reader Feedback!

"Heat treatment operations are generally perceived as black boxes whose results are not very predictable. Although we understand the physical mechanisms involved in modifying the properties of a certain material, heat treatment furnaces are thermodynamically imperfect, and sometimes the final results are too."

This article first appeared Heat Treat Today's March 2022 Vacuum Furnace print edition.

Guía para la Selección de Equipos para Tratamiento Térmico

Carlos Carrasco
Founder
Carrasco Hornos Industriales

"Este artículo ayuda a los ingenieros a comprar equipos de tratamiento térmico. Hay muchas razones para seleccionar cuidadosamente los hornos industriales. Uno, es el costo del horno en sí y otro, es que el producto que se está tratando térmicamente afectará los resultados de su empresa."

"There are many reasons to select industrial furnaces carefully. One is the cost of the furnace. Another is realizing heat treating will affect the product and the bottom line. There is more specialized engineering in heat treating equipment than is apparent from the outside."

This article first appeared in Heat Treat Today's November 2021 Vacuum Furnace print edition.

Guía para el líder que quiere motivar

Bill Munn
Leadership Coach
Bill Munn
Source: Bill Munn Coaching

Por definición, de aquí se desprende que no se desempeña solo; es más, está posicionado no solo para trabajar en conjunto con otros seres humanos sino también para liderarlos; si su potencial se ha de realizar, puede que ya haya entendido bien que en esas personas reposa la verdadera clave del éxito que a futuro pudiera conseguir ¿Cómo, pues, lograr su compromiso?

By definition, this means that you are not functioning alone. Moreover, you are positioned not only to work with fellow human beings, but to lead them. And if your potential is to be realized, you may already know well that those people are the true key to your future success. So how will you engage them?

This article first appeared in Heat Treat Today's September 2022 print edition.

Guía para conducir pruebas System Accuracy Tests conforme a CQI-9 4ta. Edición

Erika Zarazúa
Regional Purchasing Manager 
Global Thermal Solutions México
Source: Global Thermal Solutions México

"Las pruebas SAT deben realizarse a todos los sistemas de control, monitoreo y registro de los equipos de procesamiento térmico. Esto no aplica para los sistemas de ‘alto-límite” cuya única función es la de proteger al horno de un sobre calentamiento."

"System Accuracy Tests (SATs) must be performed on all control, monitoring, and recording systems of thermal processing equipment. This does not apply to “high limit” systems, whose sole function is to protect the furnace from overheating."

This article first appeared in Heat Treat Today's August 2022 Automotive print edition.

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

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Harnessing the Sun: A Heat Treat Case Study with General Atomics

/ ENERGY HEAT TREAT, ENERGY HEAT TREAT TECHNICAL CONTENT, FEATURED NEWS, VACUUM FURNACES TECHNICAL CONTENT

OC Imagine this: A huge lab facility nestled in the south of France . . . teams of scientists and technicians striving to bring carbon-free energy solutions to the world . . . “replicating the high-energy fusion reaction that powers the sun and stars.” To complete the project, what heat treat solution is needed? Read more in this Technical Tuesday to find out.

This article by Rafal Walczak, product manager at SECO/VACUUM, will be published in Heat Treat Today’s December 2022 Medical & Energy print edition.

Introduction

For this case study, we will discuss how SECO/VACUUM built a highly specialized custom heat treating furnace used in the construction of the central component of a large, multinational science experiment.

The Experiment

ITER (standing for International Thermonuclear Experimental Reactor and meaning “the way” in Latin) is the largest high-energy science experiment ever conducted. At a giant lab facility in southern France 35 countries, hundreds of vendors, and thousands of scientists and technicians are collaborating on a device to demonstrate the feasibility of clean, safe, carbon-free energy production by replicating the high-energy fusion reaction that powers the sun and stars.

Figure 1. ITER Laboratory at the Cadarache research center in southern France
Source: ITER Organization

There are no solid materials that can touch, much less contain, such a high-energy reaction without immediately vaporizing. Instead, this super-hot cloud of plasma must be contained by a special configuration of magnets called a tokamak, which can trap charged particles in a toroidal or donut-shape cloud. This tokamak has 10 times more plasma containment volume than any other tokamak ever built.

The term “tokamak” comes to us from a Russian acronym that stands for “toroidal chamber with magnetic coils” (тороидальная камера с магнитными катушками).

The Magnet

Figure 2. ITER central solenoid and one isolated solenoid module
Source: General Atomics ITER Manufacturing

General Atomics’ Magnet Technologies Center near San Diego, CA was contracted to build the ITER tokamak’s large central magnet, the most powerful superconducting magnet ever built, strong enough to lift an aircraft carrier. Other magnets in the tokamak serve to contain the plasma. The central solenoid is an oscillating magnet responsible for inducing current in the plasma cloud similar to how an induction stove heats a pan, except it is heating the plasma to 15 times the temperature of the surface of the sun. Far too large to be constructed and transported in one piece, the 12-meter-tall, 4-meter-wide coil of wires must be built in six 2-meter-tall modules to be joined once they are all on site at the lab. A seventh module will be built as a spare.

Kenneth Khumthong, technical lead for final testing and fabrication certification for ITER Central Solenoid at GA, described the tests on each module of the magnet, saying, “We run a battery of tests on each and every module subjecting them to voltages as high as 30,000 volts and powering them with as much current as 40,000 amps. This is done to ensure that every module meets all of ITER’s specifications prior to shipping them out to France.”

Embrittlement vs. Field Strength Tradeoff

Other superconducting electromagnets in the ITER tokamak will be made using coils of relatively durable niobium-titanium alloy. Past experiments have demonstrated that magnetic fields greater than 12 Tesla disrupt the superconducting properties of Nb3Ti. The ITER central solenoid, however, must sustain magnetic field strengths above 13 Tesla. For this reason, the central solenoid coils must instead use niobium-tin as its superconducting wire, which more reliably maintains superconducting properties in such high magnetic fields but is also more brittle and too fragile to bend after reaction to Nb3Sn. In order to accommodate for the brittle wire, General Atomics had to first coil the wire and jacket into their final shape before heat treating the metals into their superconducting, albeit brittle, alloy Nb3Sn.

The Wire 

Figure 3. A dissection of the central solenoid conductor strands, central spiral, and structural jacket
Source: ITER Organization
  • Niobium-tin wire strands react to become Nb3
  • Copper strands serve as traditional conductors to safely dissipate stored energy when the superconductivity experiences a disruption. The copper strands do not react with the niobium-tin.
  • A central spiral maintains a hollow channel to circulate liquid helium to chill the Nb3Sn wires to 4°K, below their superconducting temperature of 12°
  • Creating such strong magnetic fields inside a coil of wire will also tear apart the coil of wire itself if that wire is not supported inside a high strength jacket. The ITER central solenoid wire bundle is about 38.5 mm diameter, housed inside a 50 x 50 mm stainless steel jacket.
  • Total maximum current in the superconductor wire is 48,000 amps.
  • Worldwide niobium production increased six-fold for several years just to meet the niobium demands of the ITER project.

The Heat Treating Furnace

Figure 4. Technicians ensure proper placement before lowering heat treat furnace
Source: General Atomics ITER Manufacturing

In order to convert the niobium-tin metal conductors into superconductors, each of these 4 meter by 2 meter 110 ton solenoid sections must be heat treated for five weeks, exceeding 1200°F (650°C) at its peak. The heat treatment serves to alloy the niobium and tin together into Nb3Sn, which becomes a superconductor when chilled with liquid helium to 4°Kelvin. No such heat treating furnaces existed, so General Atomics turned to SECO/VACUUM to build a custom heat treating furnace large enough to fit these solenoids and packed with all the technology needed to meet the strict quality control standards of this monumental experiment.

Five inch wide metal band heaters ring around the walls of the furnace with nearly 900kW of heating power. Covering 50% of the walls, they provide a very uniform heat. This is brought about by the following seven steps.

The Heat Treating Sequence

In addition to alloying the niobium-tin wires, the furnace also serves to remove the stresses in the stainless steel jacket housing the superconducting wire and to bake off any residual contaminants prior to reaching reaction temperature.

1. Complete a quality control test: Vacuum seal the untreated solenoid coil in the room temperature furnace and charge the inside of the conductor jacket with 30 bar high pressure helium to test for leaks after forming and welding.

  • Monitor furnace atmosphere with ultra-high sensitivity mass-spectrometer helium detectors.

2. Purge with argon gas while slowly ramping up heat.

  • This drives off hydrocarbons and oxygen before system reaches reaction temperatures.
  • Monitor furnace atmosphere with gas chromatograph to find impurities from residual oils and lubricants leftover from manufacturing process.
  • Monitor and control argon circulation and exchange with mass flow sensors and circulation blowers that penetrate the furnace lid with ferrofluidic feedthrough seals around the blower motor shafts.

3. Maintain at 1058°F (570°C) for about 10 days. Confirm stabilized temperature and pure atmosphere.

4. Proceed to 1202°F (650°C) for four days. This is the actual reaction phase that achieves the primary objective of converting the niobium-tin into the superconducting alloy Nb3

5. Very slowly and uniformly ramp back down to room temperature to avoid additional stresses in the coil.

6. Complete another quality control test: Evacuate the argon and once again vacuum seal the solenoid coil in the room temperature furnace and recharge with 30 bar high pressure helium to test for leaks after heat treating. Monitor atmosphere for the presence of helium, which would indicate a leak in the coil.

7. Only then is it ready for the post-heat treating stages of wrapping with insulation and encasing in epoxy resin for rigidity.

Options, Upgrades, Special Features

Figure 5. Cutaway illustration showing the furnace construction
Source: SECO/VACUUM

There was no room for error. SECO/VACUUM collaborated with the engineers at General Atomic to create a heat treat furnace that can assure temperature variation within the coil never varies by more than 18°F (10°C) anywhere in the furnace at any time in the five-week cycle and achieves near-perfect repeatability for all seven modules.

They accomplished this with quadruple-redundant control thermocouples and feeding temperature data from 150 points in the coil into the control computers. To shield against impurities, the furnace is first evacuated to a vacuum pressure of 0.001 Torr, and then purged with pure argon to drive out any residual oxygen or hydrocarbons that could contaminate the purity of the superconductor. Monitoring the argon atmosphere for impurities are redundant mass spectrometers. The argon is circulated by seven convection fans to heat the solenoid assembly evenly. Each of these fans must be driven through ferrofluidic feedthrough seals which allow the rotating shafts to operate through the furnace walls without compromising the vacuum seal of the furnace.

Consult, Collaborate, and Partner with SECO/VISORY

General Atomics first began discussing this project with Rafał Walczak, the product manager at SECO/VACUUM, in early 2010. Both teams spent over two years on conceptual discussions, preliminary designs, and process simulations before SECO was even awarded the contract. Once SECO was on board, it took another two years of design, fabrication, and installation before the furnace could be put into operation. SECO/VACUUM built it to handle a lifetime of use without error so they could be sure that it would work flawlessly for the seven cycles that it actually had to run.

The SECO/VISORY Heat Treat Advisory Council is a team of SECO/VACUUM heat treat experts and consultants with diverse thermal experience and process knowledge who are available to help companies solve their specific heat treat equipment challenges.

Rafal Walczak
Product Manager
SECO/VACUUM
Source: Rafal Walczak

About the Author: Rafal Walczak is the product manager at SECO/VACUUM. Rafal joined SECO/WARWICK Group as a service engineer in Vacuum Furnaces Division soon after graduation from Technical University of Zielona Góra in 2002. Since 2008, he has been involved in vacuum furnace sales in Europe and the USA. The combination of his technical background and field service experience help him provide outstanding support to his SECO/VACUUM customers. For more information, contact Rafal at Rafal.Walczak@SecoVacUSA.com.

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Getting to the “Hearth” of It: 5 Hearth Tips

/ FEATURED NEWS, FIXTURE RACKING SYSTEMS TECHNICAL CONTENT, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

OC

Want a free tip? Read some of the top 101 Heat Treat Tips that heat treating professionals submitted over the last few years. These handy technical words of wisdom will keep your furnaces in optimum operation and keep you in compliance. If you want more, search for "101 heat treat tips" on the website! This selection features 5 tips all about the hearth of your furnace!

Also, check out Heat Treat Resources in the September 2021 magazine to check it out yourself!

Hacksaw Your Hearth!

When loading parts, carefully place the workload on the center of the hearth (front-to-back and side-to-side). Make sure it is stable and no part of the load is close to or touching the heating elements. This can create arcing and damage your parts.

Tip: Once the load is in place, mark the hearth posts with a hacksaw to quickly find the front and back measurements each time.

(Ipsen USA)

TZM Moly Grids

A very commonly observed failure mechanism with a moly post hearth assembly is bending of the moly posts. They will stay fairly straight at the center of the hearth area, but they can distort badly toward the outer sides of the work zone. The outer rows of vertical posts end up leaning away from each other. This is due to the very high linear thermal expansion coefficient of nickel-iron alloy grids (usually 330 SS or Inconel). With a high load on the nickel alloy grid, it is not able to slide on the perpendicular hearth beams as the temperature rises. The outer hearth post rows are forced in an outward direction. The quenching of the furnace load does not reverse all of this effect and over time results in the severe bending of the hearth posts.

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By replacing the stainless steel or nickel alloy grids with a moly or TZM alloy moly grid, which exhibits very low thermal expansion, the hearth life can be increased. For comparison, the figure shows the coefficients of linear thermal expansion for commonly used grid materials. For example, a 36” wide 330 SS grid at 70°F grows to 36.6” wide at 2200°F.

Another significant benefit of TZM moly grids is use at higher furnace process temperatures without the problem of a softened, sagging grid that cannot support the load properly.

(Grammer Vacuum Technologies, Inc.)

How to make thru-process temperature monitoring robot friendly!

In modern rotary hearth furnaces, temperature profiling using trailing thermocouples is impossible as the cables would wind up in the furnace transfer mechanism.

Due to the central robot loading and unloading and elimination of charging racks/baskets the use of a conventional thru-process system would also be a challenge.

Faced with such loading restrictions it is necessary to fit the thermal barrier inside the cavity of the product (engine block shown) and allow automated loading of the complete combined monitoring system and product.

To allow miniaturization of the thermal barrier to fit, but also provide sufficient thermal protection, the use of phased evaporation technology is critical. Such a system allowed BSN Thermoprozesstechnik GmbH in Germany to commission such a furnace accurately and efficiently and thereby optimize settings to not only achieve product quality but ensure energy efficient, cost effective production.

(PhoenixTM)

Hearth Height Adjustment

The available width and height of the work zone in a vacuum furnace with a round hot zone is determined by the elevation placement of the top of the furnace hearth. This distance is determined by the length of the vertical hearth support posts. By having spare, interchangeable hearth post sets of varying lengths, one can extend the work zone width or height as needed. The figure shows a variety of work zone dimensions that are possible with a standard 36” wide x 36” tall typical work zone as an example. The important thing in choosing your work zone shape is to maintain an (approximately) 3” clearance between the elements and the work zone to avoid part to element contact.

Note: With the symmetric shapes of modern, round hot zones there is good reason to expect good temperature uniformity anywhere within the 3” clearance ring shown in Figure 1. If you can build a survey fixture capable of surveying all the space you want to use, you theoretically could use more than just the rectangular space shown in the examples. Getting an auditor to accept the survey is a separate task.

(Grammer Vacuum Technologies, Inc.)

TZM Moly Hearths

In the case of furnaces with all-molybdenum hearths or of graphite hearths with molybdenum (“moly”) support posts, a direct replacement of those moly posts with TZM alloy moly posts will both increase strength of the hearth assembly and eliminate problems with recrystallization-induced embrittlement of the posts. (For an all-moly hearth, replacement of the horizontal load beams with TZM would have a similar benefit.) The comparative strengths vs. temperature of TZM alloy and pure moly are shown in the graph. Whereas at room temperature the strengths are very similar (around 110KSI-120KSI), once you exceed the 2000F recrystallization temperature of pure moly, the difference becomes dramatic. At 2000F the pure moly is about 40% of the strength of TZM alloy. By the time it reaches 2300F the pure moly is only about 25% of the strength of TZM alloy.

Not only is the TZM alloy much stronger than pure moly at temperature, but it also does not suffer from the same embrittlement problems. Pure moly, once it has recrystallized, forms very brittle grain boundaries in its microstructure. Its behavior begins to resemble that of glass. This is the primary mode of failure of moly components in vacuum furnaces – breakage due to intermetallic grain boundary embrittlement. TZM’s recrystallization temperature is around 2500F, and even when it does recrystallize, it forms very fine new grains that still have decent ductility. Hence, we recommend TZM alloy as a replacement for pure moly in all structural applications for vacuum furnaces. It is the “right stuff.”

Note that all metals used in a vacuum furnace, moly and TZM alloy included, will suffer from distortion due to the numerous thermal cycles they experience. Moly hearth beams are a good example. Once distorted moly hearth beams can be very difficult if not impossible to straighten without breaking them. To have any chance at all they must be heated to forging temperatures. TZM hearth beams however, due to their good ductility can often be heated to forging temperatures and successfully straightened. Most heat treating shops scrap out the moly hearth beams rather than even trying to straighten and re-use them. With a TZM hearth the hearth components can typically be re-used with a newly re-lined hot zone saving a large additional expense.

(Grammer Vacuum Technologies, Inc.)

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Getting to the “Hearth” of It: 5 Hearth Tips Read More »

Diagnosing Discolored Work in Vacuum Heat Treating

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

Source: VAC AERO International, Inc.

Part discoloration after vacuum heat treating? What can heat treaters do to prevent this? In this best of the web, Q&A-style article, witness the heat treating industry gather around to exchange ideas and find a solution to the problem. Part position, backfill gas level, contaminated quench gas, or an air leak could all be to blame in this Technical Tuesday.

Dan Herring weighs in on the issue as well. To read The Heat Treat Doctor's® diagnosis, click the link below. Learn how the color and position of the discoloration give clues as to the source of the problem.

An excerpt:

"So, what else could be happening? Let The Doctor add a few thoughts to the discussion. First, the fact that the discoloration (staining) is brown in coloration suggests that the oxide is forming on the part surface during cooling when the temperature is in the range of (approximate) 245ºC – 270ºC (475ºF – 520ºF). This is supported by the fact that the oxidation does not occur “during natural cooling” (which we assume to mean cooling under vacuum). Second, the fact that the discoloration is more evident at the bottom of the load suggests the phenomenon is (gas exposure) time dependent, that is, the longer the parts take to cool through the critical range, the greater the chance for discoloration."

Read more: Discolored Work in a Vacuum Furnace – The Heat Treat Community Answers the Clarion Call

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Sintering and Binder Jetting With Vacuum Furnaces

/ FEATURED NEWS, MANUFACTURING HEAT TREAT NEWS, SINTERING POWDER/METAL TECHNICAL CONTENT, VACUUM FURNACES TECHNICAL CONTENT

Source: TAV Vacuum Furnaces

Ever heard of binder jetting (BJT)? It's an evolving technology that is quickly catching up to metal injection molding (MIM). Compared to MIM, BJT has a lower cost per part rate, produces larger parts, and, because BJT is a cold process, it does not introduce residual stress inside the part.

Even though BJT is a cold process, sintering is a key step in BJT. Read this best of the web article to learn the ins-and-outs of sintering with binder jetting.

An excerpt:

"Vacuum sintering furnaces are usually the go-to choice for sintering of [binder jetting] parts, thanks to the ability to provide bright and shiny sintered parts, the tight process parameters control and the possibility to work with different debinding and sintering atmospheres."

Read more at "Binder Jetting and Vacuum Furnaces: Everything You Need To Know"

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Sintering and Binder Jetting With Vacuum Furnaces Read More »