VACUUM FURNACES TECHNICAL CONTENT Archives - Page 2 of 8

CFC Fixture Advantages and Challenges, Part 2

What are the factors that lead to carburization and carbon transmission? How can heat treater avoid these unwanted reactions? Discover the challenges of CFC fixtures and the steps heat treaters can take to mitigate these challenges.

This Technical Tuesday article, written by Dr. Jorg Demmel, founder, 0wner, and president, High Temperature Concept, was first published in Heat Treat Today's March 2023 Aerospace Heat Treating print edition.

Introduction

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

The main advantages of CFC fixtures were introduced in “CFC Fixture Advantages and Challenges in Vacuum Heat Treatment, Part 1,” which was released in Heat Treat Today’s November 2022 publication. This included a discussion of the limits of CFC in vacuum and protective atmosphere heat treatment. Successful applications of CFC workpiece carriers in heat treatment were presented along with field test results that included a brief discussion of undesired contact reactions (i.e., carburization and melting of parts). In Part 2 of this paper, the mechanisms involved with carburization and carbon transmission due to direct contact of parts with CFC fixtures will be further explained.

Mass Transfer from CFC Fixtures

The mass transport of carbon from CFC fixtures into steel parts at high temperatures will be examined in the following areas:

Reactions in oxygen (i.e., the reaction medium)
Transport of carbon in CFC during exposure to oxygen
Transfer mechanism into the steel parts
Diffusion of carbon into the steel parts
Part reactions (melting, carbide formation)

Figure 1: 1.6582 steel samples and GDEOS depth profile analysis
Source: Dr. Jorg Demmel, High Temperature Concept

CFC samples were tested in contact with steel samples under laboratory conditions in a vacuum of 7.5 x 10-7 Torr (1 x 10-6mbar). Results of the contact with CFC for steel samples at different temperatures are presented to the left (Figure 1). It is important to note that:

Sample (0) is the reference sample and had no exposure to the contact test.
Sample (0’) is the back side of Sample (0).
Sample (1) is the contact side at 1922°F (1050°C).

All three samples are visually identical, therefore only one is shown. Sample (2) at 1967°F (1075°C) and Sample (3) at 2012°F (1100°C) exhibited a distinct visual surface pattern after CFC contact. This was analyzed by Glow Discharge Optical Emission Spectroscopy (GDOES) and the test location (gray spot) clearly observed on Samples (2) and (3). For Sample (4) run at 2057°F (1125°C), the CFC was found to have adhered to the steel surface.

The carbon content in 10mm depth measured with GDOES (see the profiles in Figure 1) increased from initially 0.29 weight-% for the 1922°F (1050°C) test, although nothing was visible on metal surfaces. For carbon contents, see Table 1.

Table 1. Carburizing of 1.6582-samples in 10 µm depth after CX-27C1-contact (GDOES)
Source: Dr. Jorg Demmel, High Temperature Concept

CFC Reactions with Oxygen

The chemical reactions of CFC with various gases are essential in Step 1 (referenced in Part 1 of this article) and an indicator of chemical thermal suitability.

In the case of the unwanted contact carburization considered above is similar, in a sense, to carburization of steel in contact with carbon powder or granulate. However, the actual carburization mechanism, which occurs between approximately 1616°F and 1697°F (880°C and 925°C), does not take place directly via the carbon contact but is based on the fact that solid carbon reacts with atmospheric oxygen according to the Equation Table to form carbon dioxide (CO_2).

Equation Table. Reaction rates and activation energies for graphite (800°C; 0.1 bar)
Source: Dr. Jorg Demmel, High Temperature Concept

Carbon monoxide (CO) is then formed from CO₂by the Boudouard reaction (Equation 3). At high temperatures and low pressures (see Figure 2), almost only CO is present.

Figure 2. Boudouard equilibrium
Source: Dr. Jorg Demmel, High Temperature Concept

Transport of Carbon

The carbon carrier must be transported to the surface of the parts.

The cases considered in Part 1 of this article were conducted in vacuum, that is in the absence of a carburizing atmosphere. The laboratory tests were even carried out in a vacuum as low as 7.5 x 10-7 Torr (1 x 10-6mbar). Nevertheless, part surface reactions were observed.

Transfer Mechanism into the Steel Parts

Theoretically, carbon from the CFC fixtures can be transferred into the steel via solid phase (as opposed to gaseous phase) reactions. Gas particles can be adsorbed by surfaces via physisorption and/or chemisorption. The author’s personal research experience has shown that metal samples usually oxidize after a short time, even in a high vacuum of 7.5 x 10-7 Torr (1 x 10-6mbar). In particular, elements such as iron, molybdenum, and chromium have a strong ability to chemically adsorb oxygen or CO.

Furthermore, there is a disproportionately large amount of adsorbed oxygen in the CFC samples. CFC has open porosities as high as 30%. CFC in industrial practice is never completely evacuated. So, there is a disproportionately large amount of oxygen present in CFC fixtures.

It can be assumed that oxygen repeatedly escapes from the CFC and is initially available in the contact area. Proof of this can be provided by the GDOES analysis. Outside the contact areas, no (gas) carburization took place (as evidenced by the non-contact side of steel samples).

The oxygen and carbon surplus combined with close contact lead to complete reaction of oxygen creating carbon dioxide as in Equation (1). Because of the carbon surplus, almost only carbon monoxide is produced as shown in Equation (2). Because of the very close contact between CFC and steel, C-adsorption by gamma iron and desorption of carbon dioxide as in Equation (5) takes place:

Equation 5
Source: Dr. Jorg Demmel, High Temperature Concept

Since carbon dioxide immediately comes in contact with carbon in the CFC again, carbon monoxide is produced according to Equation (3). In other words, carbon dioxide regenerates immediately and the reaction starts again.

Direct carbon transfer from CFC to metal via solid phase is very unlikely since carbon atoms in CFC are firmly bound in rings.

Diffusion of Carbon in the Steel Parts

In solids, the surface diffusion usually takes place at significantly higher diffusion rates than in the bulk material. The thermodynamic driving force of diffusion or carburizing reactions is the difference in carbon activity for a specific concentration in the austenite to that of the reaction medium. The carbon activity is the ratio of the vapor pressure of the carbon in state under consideration to vapor pressure of pure carbon (graphite/CFC). Alloying elements of the steel influence the activity of the carbon.

Part Reactions (Melting and Carbide Formation)

Steel can begin to melt if, at the given values for temperature and pressure, a partially liquid phase is reached, that is, the solidus line in the phase diagram is exceeded. At even higher temperatures, the liquidus temperature can be reached and steel is completely liquid.

According to metastable iron-carbon diagram phase diagram (Figure 3), a steel such as SAE/ AISI 4340 (34CrNiMo6) alloy (DIN 1.6582) with around 0.47% by weight percent carbon does not begin to melt at 1922°F (1050°C), the exposure temperature for Sample (1), or Sample (2) at 0.56% and 1967°F (1050°C) for Sample (3) with 0.67% for 2012°F (1100°C). The iron-iron carbide phase diagram applies to steels with less than 5% (by mass) of alloying elements and thermodynamic equilibrium, so it is an accurate representation for a SAE/AISI 4340 (34CrNiMo6) alloy.

Figure 3. Metastable equilibrium diagram Fe-Fe3C for steel (good fit for 1.6582)
Source: Dr. Jorg Demmel, High Temperature Concept

A calculation of the solidus temperature shown on the iron-iron carbide diagram (Figure 3), which is dependent on the carbon content and alloying elements, yields a value of 2703.2°F (1,484°C) (J’).

For an SAE/AISI 4340 (34CrNiMo6) steel (DIN 1.6582) with 0.3% C and one for 0.5% C, the calculated solidus temperature is 2640°F (1449°C). This is shown on the J’-E’ blue dotted line in Figure 3. In other words, a lower solidus line (cf. dashed blue line in Figure 3) and thus a slight reduction in austenite phase region.

The iron-carbon diagram also indicates that melting of surfaces that have absorbed carbon (e.g., Sample No. 2) will occur at 1967°F (1075°C). This value is within approximately 90°F (50°C) of the temperature used (dotted line E’-C’-F’). From this information we can conclude that the observations seen in Figure 1 are not the result of melting, but rather imprints due to surface softening.

The melting (c.f., Figure 1) observed in Test No. 4, which occurred at 2057°F (1125°C) is likely due to partial carburization of the steel surface and exceeding the solidus temperature. A micrograph confirms eutectic melting and high carbon content, which could also be indirectly confirmed by hardness measurement.

Carbide Formation

Additional reactions can occur between carbon absorbed from the CFC fixtures and the steel parts due to either separation of carbides (e.g., iron carbide in the form of secondary cementite) or carbide formation with alloying elements such as Ti, V, Mo, W, Cr, or Mn (listed in decreasing tendency to form carbides).

Table 2. Reactions between C and metal
Source: Dr. Jorg Demmel, High Temperature Concept

Table 2 lists various elements in alphabetical order that react with carbon above the specified temperatures to form reaction products mentioned, primarily carbides. It should be noted that the temperatures listed apply only to pure metals and pure carbon. As such, they provide only rough approximations of a temperature at which a reaction might begin.

Countermeasures

There are several measures to avoid these unwanted reactions:

Ceramic oxide coatings such as aluminum oxide (Al2O3) or zirconium oxide (ZrO₂) layers placed onto the CFC
Hybrid CFC fixtures having ceramics in key areas to avoid direct contact with metal workpieces
Alumina composite sheets
Boron nitride sprays
Special fixtures made of oxide ceramics

An yttrium-stabilized zirconium oxide layer (93/7) was applied to CF222 by thermal plasma spray and tested successfully (see Figure 4).

Figure 4. Yttrium-stabilized zirconium oxide layer with an average layer thickness of 110µm on CF222 material. The photograph on the right shows a hybrid CFC fixture.
Source: GTD Technologie Deutschland

Summary

It is important to consider the specific process conditions in advance so that unwanted reactions — from carburization to catastrophic melting of the workpieces — can be avoided. Effective countermeasures can be taken.

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-Werksto technik: Grundlagen, Werksto bean-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

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

CFC Fixture Advantages and Challenges, Part 2 Read More »

Energy Efficiency Through Combustion Monitoring

February 21, 2023 / FEATURED NEWS, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

With energy costs soaring and environmental commitments expanding across the industry, is it enough to just tune your industrial combustion burners, or can IIoT devices provide greater insight to achieve burner energy efficiency?

This Technical Tuesday article, written by Taylor Smith, technical sales and marketing specialist, PSNERGY, LLC, was first published in Heat Treat Today's February 2023 Air & Atmosphere Furnace Systems print edition.

Introduction

Taylor Smith
Specialist of Technical Sales and Marketing at PSNERGY
Source: PSNERGY

Industrial furnaces are inherently inefficient and constantly degrading due to high operating temperatures. In most cases, less than 50% of the energy generated through combustion goes to heating the load, while most energy is lost through the exhaust stack or is used to heat the atmosphere, fixtures, and walls of the furnace. An improperly tuned furnace loses 10-30% efficiency on top of the energy losses previously mentioned. This is why keeping industrial furnace combustion systems in tune is critical to performance. This was recently highlighted in John Clarke’s featured article, “How To Make $17,792.00 in a Couple of Hours.”

Continuous Monitoring Is Key

Built on years of experience and field data, combustion engineers at PSNERGY know that only tuning combustion systems annually, or semi-annually, is a good start, but it is not enough. Customer case studies led the team to recognize the importance of frequent combustion monitoring to achieve optimal performance, and ultimately drove the design of their proprietary IIoT monitoring system: Combustion Monitoring and Alerting (CMA).

To get the most BTUs to the load per unit of natural gas purchased, tuning must be combined with continuous combustion monitoring. Tuning without continuously monitoring combustion increases the risk of losing energy to the load, decreasing efficiency, and creating excessive carbon emissions.

Case Studies: Data-Driven Furnace Efficiency

The following case studies represent two examples of data collected throughout the country on furnaces of all sizes and configurations. One thing remains consistent: simply checking combustion once or twice per year does not ensure optimal furnace performance.

These figures show before and after measurements taken on the same forty-four burner radiant tube roller hearth furnace, six months apart. The red points on the graphs represent excess oxygen in each burner’s exhaust when the team arrived on site, while the blue points represent excess oxygen in each burner’s exhaust after tuning the furnace. A significant variance in combustion performance can be observed in the six months between tunings, which means a large portion of the natural gas purchased is being wasted out the stack and creating carbon emissions.

To ensure maximum energy is being applied to the load for every BTU burned, combustion should be tuned to the ratio of 11.5:1. This 11.5:1 ratio of air to gas results in an ideal excess oxygen measurement of 3%. When PSNERGY engineers perform combustion tuning on an industrial furnace, they set the excess oxygen at the burner between 2.8% and 3.2%. This optimal range is marked by the green dashed lines on the graphs.

You may be questioning, “Does too little or too much excess oxygen really affect combustion performance?” Yes! Burners operating above 4% or below 1.5% are considered outside of the control limit range, marked by the red dashed lines on the graphs. With less than 1.5% excess oxygen at the burner, furnaces produce carbon monoxide and soot, which can clog burners, making them even more inefficient. These carbon emissions can also create an unsafe work environment for plant employees. When operating at 5% excess oxygen, 8% of energy to the load is lost. When operating at 7% excess oxygen, 21% of energy to the load is lost. Imagine buying the same amount of natural gas and only getting 79% of the energy!

A few things to notice on these graphs: burners are rarely, if ever, found in the ideal performance zone after six months. There is no way to know when each burner drifted out, because continuous monitoring was not yet implemented. Therefore, this drift in combustion performance, which significantly decreases furnace efficiency, could have happened anytime during the six month period between combustion tunings. Tunings may be scheduled, but combustion does not operate on a fixed schedule. You cannot know when the burners drift out of tune without monitoring. Another point to note is that the burners do not always move in the same direction as they go out of tune. In Figure 1, thirty one out of the forty four burners were burning under 1.5% excess oxygen, which means they were burning rich and creating carbon emissions and soot. The PSNERGY service team tuned all of those burners back into the optimal performance range. As you can see in Figure 2 data, taken six months later, out of the same forty four burners, seven burners were burning rich, while thirty one of the burners were operating lean with over 4% excess oxygen, which significantly decreases the amount of energy to the load. These figures demonstrate why it is crucial to continuously monitor and tune your combustion system as needed based on the data, not the calendar.

Combustion Monitoring and Alerting (CMA)

Circling back to our initial question of, “Can IIoT devices provide greater insight to achieve burner energy efficiency?” the data presented here answers with a resounding YES! In fact, various companies across steel, aluminum, and heat treating industries have already successfully implemented this solution.

Not only does continuous monitoring help achieve burner efficiency, but it also helps bridge the gap in combustion knowledge and manufacturing by making combustion performance easy to see and maintain. With manufacturing leaders facing fourteen-year high natural gas prices and a generational gap in manufacturing expertise, systems like CMA are proving to be crucial to business success. Delivering 10-20% improvement in furnace efficiency, less waste, reduced carbon emissions, and ensured quality, takes your furnaces from being a necessary expense to a strategic asset.

Now the question is: Are you performing combustion maintenance on a fixed schedule or are you trusting real time data?

About the Author: Taylor Smith is a specialist of Technical Sales and Marketing at PSNERGY, located in Erie, Pennsylvania. Her tenacity and competitiveness as a Division I athlete have helped her quickly gain knowledge and hands-on experience in the heat treating industry. Taylor has a deep passion for manufacturing and works hard to build the next generation of leaders, serving on the board of directors for Women in Manufacturing WPA. For more information: contact Taylor at tsmith@psnergy.com

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

Energy Efficiency Through Combustion Monitoring Read More »

Vacuum Furnaces: Origin, Theory, and Parts

January 24, 2023 / AEROSPACE HEAT TREAT, AEROSPACE HEAT TREAT TECHNICAL CONTENT, AUTOMOTIVE HEAT TREAT, AUTOMOTIVE HEAT TREAT TECHNICAL CONTENT, FEATURED NEWS, MANUFACTURING HEAT TREAT, MANUFACTURING HEAT TREAT TECH, VACUUM FURNACES TECHNICAL CONTENT

Vacuum furnaces are widely used in the aerospace and automotive industries. These furnaces are used for multiple processes including brazing, aging, and solution heat treating for countless materials. Typically, vacuum furnaces are utilized to ensure a lack of oxidation/contamination during heat treatment. This article will talk about the origins, theory, and main parts of vacuum technology and how it is used in both aerospace and automotive industries.

This Technical Tuesday feature was written by Jason Schulze, director of technical services at Conrad Kacsik Instrument Systems, Inc., and was first published in Heat Treat Today's December 2022 print edition.

A Brief History

Vacuum furnaces began to be used in the 1930s for annealing and melting titanium sponge materials. Early vacuum furnaces were hot wall vacuum furnaces, not cold wall vacuum furnaces like we use today. Additionally, most early vacuum furnaces did not utilize diffusion pumps.

Vacuum Heat Treat Theory

Jason Schulze Director of Technical Services Conrad Kacsik Instrument Systems, Inc.

Vacuum technology includes vacuum pumping systems which enable the vessel to be pulled down to different stages through the process. Degrees of vacuum level are expressed opposite of pressure levels: high vacuum means low pressure. In common usage, the levels shown below in Figure 1 correspond to the recommendations of the American Vacuum Society Standards Committee.

Vacuum level will modify vapor pressure in a given material. The vapor pressure of a material is that pressure exerted at a given temperature when a material is in equilibrium with its own vapor. Vapor pressure is a function of both the material and the temperature. Chromium, at 760 torr, has a vapor pressure of ~4,031°F. At 10¯5, the vapor pressure is ~2,201°F. This may cause potential process challenges when processing certain materials in the furnace. As an example, consider a 4-point temperature uniformity survey processed at 1000°F, 1500°F, 1800°F, and 2250°F. This type of TUS will typically take 6-8 hours and, as the furnace heats up through the test temperatures, vacuum readings will most likely increase to a greater vacuum level. If expendable Type K thermocouples are used, there is a fair chance that, at high readings, you may begin to have test thermocouple failure due to vapor pressure.

Figure 1. Vacuum levels corresponding to the recommendations of the American Vacuum Society Standards Committee
Source: Jason Schulze, Conrad Kacsik Instrument Systems, Inc.

Vacuum Furnace Pumping System

Vacuum heat treating is designed to eliminate contact between the product being heat treated and oxidizing elements. This is achieved through the elimination of an atmosphere as the vacuum pumps engage and pulls a vacuum on the vessel. Vacuum furnaces have several stages to the pumping system that must work in sequence to achieve the desired vacuum level. In this section we will examine those states as well as potential troubleshooting methods to identify when one or more of those stages contributes to failure in the system.

Vacuum furnaces have several stages to the pumping system that must work in sequence to achieve the desired vacuum level. Each pump within the system has the capability to pull different vacuum levels. These pumps work in conjunction with each other (see Figure 2).

Figure 2. Vacuum pumps work in conjunction with one another
Source: Jason Schulze, Conrad Kacsik Instrument Systems, Inc.

The mechanical pump is the initial stage of vacuum. This pump may pull from 105 to 10. At pressures below 20 torr the efficiency of a mechanical pump begins to decline. This is when the booster pump is initiated.

The booster pump has two double-lobe impellers mounted on parallel shafts which rotate in opposite directions (see Figure 3).

Figure 3. Booster pump positions
Source: Jason Schulze, Conrad Kacsik Instrument Systems, Inc.

The diffusion pump (Figure 4) is activated into the pumping system between 10 and 1 microns. The diffusion pump allows the system to pump down to high vacuum and lower. The diffusion pump has no moving parts.

Figure 4. Diffusion Pump
Source: Jason Schulze, Conrad Kacsik Instrument Systems, Inc.

The pump works based on the vaporization of the oil, condensation as it falls, and the trapping and extraction of gas molecules through the pumping system.

Image 1. Holding Pump
Source: Jason Schulze, Conrad Kacsik Instrument Systems, Inc.

The holding pump (Image 1) creates greater pressure within the fore-line to ensure that, when the crossover valve between the mechanical and diffusion pump is activated, the oil within the diffusion pump will not escape into the vessel.

Vacuum Furnace Hot Zone Design

Image 2. Insulated
Source: Jason Schulze, Conrad Kacsik Instrument Systems, Inc.

The hot zone within a vacuum furnace is where the heating takes place. The hot zone is simply an insulated chamber that is suspended away from the inner cold wall. Vacuum itself is a good insulator so the space between the cold wall and hot zone ensures the flow of heat from the inside to the outside of the furnace can be reduced. There are two types of vacuum furnace hot zones used: insulated (Image 2) and radiation style (Image 3).

The two most common heat shielding materials are molybdenum and graphite. Both have advantages and disadvantages. Below is a comparison (Tables 1 and 2).

Table 1
Source: Jason Schulze, Conrad Kacsik Instrument Systems, Inc.

Table 2
Source: Jason Schulze, Conrad Kacsik Instrument Systems, Inc.

Vacuum Furnace Quenching System

Quenching is defined as the rapid cooling of a metal to obtain desired properties. Different alloys may require different quenching rates to achieve the properties required. Vacuum furnaces use inert gas to quench when quenching is required. As the gas passes over the load, it absorbs the heat which then exits the chamber and travels through quenching piping which cools the gas. The cooled gas is then drawn back into the chamber to repeat the process (see Figure 5).

Figure 5.Diagram of gas quenching
Source: Jason Schulze, Conrad Kacsik Instrument Systems, Inc.

Vacuum Furnace Trouble Shooting

In Table 3 are some helpful suggestions with regard to problems processors may have.

Table 3
Source: Jason Schulze, Conrad Kacsik Instrument Systems, Inc.

Summary

Vacuum furnaces are an essential piece of equipment when materials need to be kept free of contamination. However, there are times when this equipment may not be necessary, and is therefore considered cost prohibitive, although this is something each processor must research. This article is meant to merely touch on vacuum technology and its uses. For additional and more in-depth information regarding vacuum furnaces, I recommend a technical book called Steel Heat Treatment, edited by George E. Totten.

About the Author: Jason Schulze is the director of technical services at Conrad Kacsik Instrument Systems, Inc. As a metallurgical engineer with over 20 years in aerospace, he assists potential and existing Nadcap suppliers in conformance as well as metallurgical consulting. He is contracted by eQuaLearn to teach multiple PRI courses, including pyrometry, RCCA, and Checklists Review for heat treat.

Contact Jason at jschulze@kacsik.com
website: www.kacsik.com

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

Vacuum Furnaces: Origin, Theory, and Parts Read More »

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

November 29, 2022 / FEATURED NEWS, MANUFACTURING HEAT TREAT, VACUUM FURNACES TECHNICAL CONTENT

What 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

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

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)

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.