There’s always more to learn, and at Heat Treat Today, we strive to help you be well informed. Thanks to our Heat TreatRadio guests, we are happy to offer much more expertise on all things vacuum processing, including hardening, ion nitriding, and the stainless steel materials in vacuum furnaces.
This Technical Tuesday article was written by the Heat Treat Today Editorial Team for theNovember 2023 Vacuum Heat Treating print edition.
Heat Treat Radio #93: Why Ion Nitride? An Exploration with Gary Sharp
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Perhaps the most visual of all vacuum processes, ion nitriding is known for its unique purple glow, caused by nitrogen diffusing into the surface of the workpiece. Gary Sharp gives this glowing summary of ion nitriding, including the development of vacuum equipment to fit the process and special challenges like loading vacuum chambers and parts cleaning in this recent episode.
“[Ion nitriding is] a diffusion process. If you look at a piece of equipment, a hearth plate is a cathode in a DC circuit. The vessel wall is the anode, and the gas is your carrier.
“Through the transfer of energy, you bombard the part with ions and neutral atoms. They transfer their kinetic energy, and that is what actually heats up the parts. In the early years, that was the only way you could heat the parts. Later came more developed equipment.”
“Now, you have auxiliary heating in the walls, which adds some advantage but also a little more complexity in terms of keeping and maintaining a current density on the part adequate to diffuse into the metal itself. Sometimes you put it in a vessel, and you turn on the power supply. All the energy is coming from somewhere else, and you don’t actually diffuse or harden the part itself. It’s been solved, obviously, over the years.”
As Sharp further explains, some advantages of ion (or plasma) nitriding overlap with other forms of nitriding. But specific to this purple heat treatment under vacuum are its great masking abilities.
Heat Treat Radio #82: Gun Part Treatments, Turning Up the Heat with Steve Kowalski
When it comes to stainless steels for gun components, autonomy is the name of the game. Vacuum processing is “critical” in the gun part industry, says Steve Kowalski, president of Kowalski Heat Treating, for this reason of control over the part:
“With stainlesses and the various materials that we’re currently using, having high-pressure quench vacuums/high pressure quench allows for a significant amount of flexibility. We can finetune a recipe, or cycle, to achieve properties that the customer needs where it makes it repeatable.”
Rolls, slitter blades racked, and SS vavle seats for vacuum processing
Source: Kowalski Heat Treating
The means to reach the end of perfectly heat treated components are many, the critical step being defining the heat treat process itself. “The majority of what we process for the gun world (for the firearms world) would be either salt to salt rack austempering or marquenching, vacuum and actually plasma processing, whether it’s FNC or nitriding. We’re involved with those three areas in the gun world.” He later expands upon this list noting, “salt neutral hardening, salt hardening, and then marquenching or austempering, depending on the component.”
Heat Treat Radio #54: Metal Hardening 101 with Mark Hemsath, Part 2 of 3
“Hardening” is a broad thermal processing term that generally means increasing a material’s strength and toughness and minimizing distortion. Hardening can be done in a simple tip-up or bell furnace. So, which hardening processes require vacuum furnaces?
In addition to plasma nitriding as mentioned above, Mark Hemsath of NITREX sheds some light on carburizing as done in a vacuum furnace. He says, “Caburizing is the addition of carbon, right? So, the difference here is that when we talk low pressure . . . it’s done at a negative pressure, less so than atmosphere. We call this either low pressure carburizing or vacuum carburizing; it’s the same process. This takes place at pressures typically in the 1–15 torr range, which is about 1–20 millibar range of pressure. If you know one atmosphere is 760 torr, when we’re going down to 1–15 torr, we’re at pretty good vacuum. Just like with gas carburizing, the higher the temperature, the faster the process. What’s unique with vacuum equipment is that vacuum equipment is typically capable of going to higher temperatures which adds to the speed of carburizing.”
Unveiling the inner workings of a vacuum furnace, he adds, “Now, we didn’t discuss the design of gas carburizing furnaces that much, but typically they’re gas fired and they have radiant tubes. In the interior of the furnace, the higher temperature you go with the really nasty carburizing atmosphere, it reduces the life of those furnaces substantially, so the people that own the furnaces don’t want to go to high temperature. If you can go 100 degrees higher in temperature, like you can with the vacuum carburizing furnace, the process gets much faster. That means higher productivity.”
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Let’s discover new tricks and old tips on how to best serve vacuum furnace systems. In this print edition, Heat Treat Today compiled top tips from experts around the industry for optimal furnace maintenance, monitoring procedures, controls, testing, and more.
This Technical Tuesday article was written by the Heat Treat Today Editorial Team for the November 2023 Vacuum Heat Treating print edition.
#1 Three in One: Control Your Vacuum Furnaces
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Vacuum furnaces are an essential piece of equipment for a variety of industrial applications. They operate in a controlled environment with low pressure, high temperatures, and controlled atmospheres, making them ideal for processing high-quality materials. Here are three tips to guarantee the best results:
1. Understand your furnace’s capabilities and operating parameters:
It’s crucial to know your furnace’s design and its operating parameters — temperature range, pressure range, and cycle time, to name a few. This knowledge will help you determine the optimal setpoints for your process and ensure that you stay within a safe operating range.
2. Monitor process parameters:
To control your furnace, you need to monitor process parameters such as temperature, pressure, gas flow rate, and vacuum level. Using an automated control system like Gefran’s power controller with ethernet communication will help ensure you maintain the desired process conditions throughout the run. You should also regularly check the accuracy and age of your thermocouples and calibrate the system if necessary.
Vacuum furnaces are complex systems, and the process can be hazardous if not done correctly. Train all personnel on proper furnace use to ensure they understand the hazards associated with the process as well as know the SOPs to ensure safe and repeatable results. Your SOPs must cover all aspects of the process, including loading and unloading the furnace, start-up, shutdown, and emergency procedures. In addition, Gefran’s power controllers offer predictive maintenance functions, such as heater diagnostics and constant temperature monitoring of the power cable connection to give you advance notice before issues develop and the line goes down.
By following these tips, vacuum furnace users will improve process control, optimize performance, and reduce energy consumption and downtime. They will also see increased productivity and improved product quality.
Seals are everywhere on any furnace. Do you know where all the seals and leak points are? Door O-rings and rope gaskets are obvious examples. O-rings need to be clean and protected from abrasion. High temperature gaskets need to be flat, smooth, and unbroken. Almost every item of your furnace is sealed in some manner. It is best to replace seals as part of a preventative maintenance program. While your nose can detect ammonia, vacuum leaks require special helium leak detectors and a lot of training. Your furnace manufacturer’s service technician can assist in identifying problem areas and developing a maintenance routine to keep your furnace running. And a simple electronic manometer is great to have handy for running leak-down tests using positive pressures. Auto supply stores sell inexpensive halogen detectors, and some people use smoke bombs to detect leaks.
Table A. Force-diameter indexes for different materials (Source: Foundrax Engineering Products Ltd.)
Use the correct force-diameter index (F/D²) for the material being tested. Apply the test force in accordance with ISO 6506 or ASTM E10, as appropriate. While the indenter is in downward motion and in contact with the material, avoid doing anything that might create vibrations that could reach the machine. When the indenter has withdrawn, measure the resulting indentation in a minimum of two diameters perpendicular to each other and convert the mean measurement into an HBW number. Note: if using a portable Brinell hardness tester, caution should be exercised when removing the machine from the component so that the edge of the indentation is not accidentally damaged when the machine is released.
#4: Preparation Steps When Carrying Out Your Brinell Test
Make sure the test equipment is properly set up. In most instances, this involves keeping the test machine serviced and calibrated in accordance with the international standards (that’s ASTM E10 for Brinell and ASTM E18 for Rockwell) and/or the manufacturer’s instructions (whichever are the stricter) along with mounting it on a level, vibration-free surface. The absence of vibration is crucial if you’re using a lever and weight machine but still desirable for hydraulic and motor-driven types, and it is mandated by the standards.
A brief note for tests made using portable Brinell hardness testers that apply the full test load (albeit without the ability to maintain it uninterrupted for the full 10 seconds): While it might not always be possible to mount the machine on a solid and level surface, the rest of the above still applies.
If the anvil is mounted on a leadscrew, ensure that it is properly secured. Similarly, jigs should be in good condition, correctly mounted and holding the test piece securely. It is easy to become very relaxed about the amount of energy that goes into applying 3,000 kg to a 10 mm ball, but if the component shatters under the load, the results can be dramatic and, potentially, very dangerous. Don’t forget your safety boots! Also, as fingerprint residue is corrosive, gloves should always be worn.
“There are many factors to consider when thinking about the right vacuum level for vacuum brazing. Foremost among these is the ability to ‘wet’ the surface so that the braze filler metal will flow freely and be drawn into the braze joint by capillary action. To secure good wetting, the parts must be clean, the vacuum furnace well conditioned and leak free, and the proper level maintained.”
Source: Dan Herring, The Heat Treat Doctor®, Vacuum Heat Treatment, vol II, 2016 pp.283
#brazing #vacuum level #leakfree
#6: Voyaging Vacuum Furnace Maintenance
"[If] a vacuum furnace is to be moved from one location to another, a careful inspection and close monitoring of the water system should be done in the months that follow the move. Dislodged scale can clog cooling paths and create hot spots. Corrosion effects can be accelerated, and the integrity of connections can be compromised. Older equipment that has not been on a treated water system of some type is especially vulnerable.”
Source: Dan Herring, The Heat Treat Doctor®, Vacuum Heat Treatment, vol II, 2016 pp.283
#inspection #corrosion #movingequipment
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The question in many heat treaters’ minds is, “Why would I want more documentation on my furnaces?” TempTABs can act as an early warning sign that further temperature monitoring is necessary.
This Technical Tuesday article was written by Thomas McInnerney and Garrick Ackart of The Edward Orton Jr. Ceramic Foundation, for Heat Treat Today'sNovember 2023 Vacuum Heat Treatingprint edition.
The Need for User-Friendly Documentation
Thomas McInnerney
Engineering Manager of Pyrometric Products
Edward Orton Jr. Ceramic Foundation.
Source: Edward Orton Jr. Ceramic Foundation
Increased regulations called for in AMS2750G and CQI-9 were, for the most part, driven by the client purchasing the items. With a business climate that can generate a product-liability lawsuit quicker than a rapid quench, clients are trying to protect themselves.
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Consequently, most heat treating facilities will perform the necessary and required temperature uniformity surveys (TUSs) as well as thermocouple calibrations. Once the formal TUS is complete, other than the information generated from the control thermocouples, the challenge still exists for the furnace operators to ascertain what happens throughout the furnace between surveys. By passing the last survey but failing the next one, how do you detect that something changed two days after the good survey or two days before the failed survey?
It is true you can run a temperature data logger with an array of thermocouples attached through the furnace to get a complete picture of the furnace performance, but that process includes production interruption, an expenditure of precious manpower, and significant expense in maintaining the data logger. Essentially, we have just defined the need for a cost-effective, user-friendly device to monitor the day-to-day repeatability of the performance of the furnace.
Metals Industry Demands
Garrick Ackart
Marketing and Business Development Manager
Edward Orton Jr. Ceramic Foundation.
Source: Edward Orton Jr. Ceramic Foundation
Driven by the question raised above, The Edward Orton Jr. Ceramic Foundation initiated a development project to provide such a product for the metals industry.
Demands of the metals industry are quite different from those of the ceramic industry. The detection device would have to be able to withstand rapid heat-up schedules, rapid quench, a wide variety of furnace atmospheres (air, nitrogen, hydrogen), and no atmosphere (vacuum), and do all this without introducing contaminants to the products being heat treated — no small challenge for an engineered ceramic product. Following a great deal of consultation and experimentation, Orton developed a product, the TempTAB, that can be used to benchmark and monitor furnace performance in most heat treating applications.
Measuring Dimension: How a TempTAB Works
How does the device work, and how is it made and controlled? The device depends on a constant slope curve of shrinkage versus temperature. When the device is exposed to more temperature and for longer periods of time at peak temperature, the amount of shrinkage increases.
Figure 1. The temperature monitoring system consists of ceramic sensors, a
measuring gauge, and software to convert dimension to temperature.
Source: Edward Orton Jr. Ceramic Foundation
TempTABs are small disks made from exact blends of select ceramic materials prepared in an environment where the processing variables are tightly controlled. The ceramic material is selected based on its predictable shrinkage, which is affected more by temperature than time; even so, holding at or near the peak temperature will have an impact on the final dimension.
Once the TempTAB is out of the furnace, its diameter is measured with a micrometer. The dimension, in millimeters, is entered into an Excel workbook that automatically looks up the equivalent temperature inside the furnace based on the furnace cycle time.
Temperature conversion charts are available with each batch of TempTABs for converting the diameter measurement to temperature. The charts have several columns of data which allow the user to find the data that is best associated with their final furnace cycle hold times (temperatures available for 10-, 30-, 60-, 120-, and 240-minute hold times). The charts are built into the software to allow you to monitor up to nine different locations inside the furnace for up to 360 runs (Figure 2). The software is available free from Orton’s website.
The resulting temperature data generated by the software is graphically displayed in both table and numerical format for easy interpretation. The data can also be copied into other Excel spreadsheets and SPC (Statistical Process Control) programs to be incorporated into existing quality programs.
Figure 2. Orton TempTAB software allows process temperature tracking at a glance.
Source: Edward Orton Jr. Ceramic Foundation
Primary Uses: Early Warning Device & Quality Control
Heat treat companies use these disks as an early warning device and to document that their processes are under control. First, they benchmark their thermal process by running several TempTABs through the heat treat furnace.
After establishing a benchmark with upper and lower control limits, the company will run the disks on a regular schedule, placing them in the same location alongside the parts being treated in the furnace (see process temperatures graphed with TempTABs in Figure 3).
Figure 3. Temperature data is displayed by location and can be copied into existing SPC software.
Source: Edward Orton Jr. Ceramic Foundation
At a glance, the furnace operator, the quality manager, or the general manager can see if the process is under control. The size of each disk indicates if the thermal process is, or is not, within the established control limits.
The case studies that follow demonstrate these primary uses in real-world heat treat.
Case Study #1: Furnace Documentation When You Need It
A manufacturer with in-house heat treating ran TempTABs alongside the thermocouples in one of its required nine-point uniformity surveys with a data logger. After the formal survey, they continued to run disks in each load, monitoring shrinkage of the disks. The heat treating operations wanted to document the thermal treatment of the product in every load. If something did change inside their furnace before the next required survey, the TempTABs would act as an early warning system alerting them that a formal survey may be necessary.
Case Study #2: Developing Backup Facilities/Preparing for Increased Demand
A company specializing in powder metal sintering wanted to duplicate a sintering process of one of their products, currently only being done at a single manufacturing site, at a second location. Initially, they duplicated all the settings in the new location (temperature settings and belt speed) and found that the resultant parts differed from those of the original site.
The company began to consider TempTABs. They liked the idea of having a device that could provide them with furnace temperature readings since they knew that it was an important variable to the quality of their parts. For one year, TempTABs were used daily for process control of the furnace. This use proved that the furnace was consistent and stable.
Since they had developed a benchmark of the disk dimensions yielding good parts, they were able to adjust the new facility settings so their process could be duplicated in the second facility. Within weeks, the powder metal sintering experts could produce products in the new facility consistent with the original facility.
Case Study #3: High-Value Heat Treating
A heat treating facility serving the aerospace industry historically ran nine thermocouples in every load of their batch furnace for bright annealing stainless steels to document furnace performance. The method required using many type S thermocouples and a data collection unit. Labor costs included setting up the thermocouple array and replacing the certified thermocouples. It was expensive and disruptive; they wanted an alternative.
The time needed to replace the TempTABs was minutes and only required one person’s time to place and gather them. After doing a correlation study of at least five runs over a week, the heat treat facility replaced the thermocouples with TempTAB disks. Now, a single operator places TempTABs inside every load so they can gather information at a lower cost. If they see any change in the amount of TempTAB shrinkage, they will run the thermocouple array to see precisely how the temperature profile has changed.
Figure 4. TempTAB wired in place during daily monitoring.
Source: Edward Orton Jr. Ceramic Foundation
About the authors:
Thomas McInnerney is the engineering manager of Pyrometric Products at The Edward Orton Jr. Ceramic Foundation. He received his BS in Ceramic Engineering at The Ohio State University and has been a key leader in the development and application of TempTABs for 22 years.
Garrick Ackart is the Marketing and Business Development manager at The Edward Orton Jr. Ceramic Foundation. He received a Bachelor of Science degree from Alfred University in Ceramic Engineering, an MBA from The Ohio State University, and has more than 25 years of experience in the ceramic and glass industry.
Reducing the industrial carbon footprint has been at the forefront of much discussion, heat treat industry-specific or otherwise. How can heat treaters dealing with vacuum operations consider sustainability in a carbon-conscious market?
This Technical TuesdaySustainability Insight article was written by Bryan Stern, the product development manager at Gasbarre Thermal Processing Systems, for Heat TreatToday'sNovember 2023 Vacuum Heat Treating print edition.
Bryan Stern
Product development manager
Gasbarre Thermal Processing System
Source: Gasbarre
There is a growing understanding that changes in environmental policy and corporate initiatives will have an increasing impact on the landscape of domestic processing and manufacturing industries in the near future. This is of particular interest to the heat treating industry as thermal processing intrinsically consumes large amounts of energy.
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Energy has always been a financial reality for heat treaters, but the impact of transitioning environmental reform will reach beyond monthly utility bills. This is because large players in primary heat treat markets will seek to integrate low-carbon service and equipment suppliers into their direct and indirect supply chains to meet decarbonization objectives.
As a result of this impending trajectory, there has been more attention on furnace design and energy sources within the thermal processing industry. One topic that has received a great deal of focus is the potential benefit of vacuum furnaces as a less emissions-intensive approach to heat treating. Although fundamentally based on electrification, it would be difficult to argue that at least some of the interest in vacuum does not stem from a reactionary desire to distance thermal processing from the image of fire-breathing fossil fuel furnaces given the current political environment.
But beyond the undeniably more marketable aesthetic, the legitimate question remains: Does vacuum heat treating provide tangible environmental advantages over combustion-fired atmosphere alternatives?
Atmosphere integral quench furnace
The soundness of the argument for electrification and vacuum is not as obvious as it might first appear. To start, eliminating on-site combustion does not eliminate CO2 emissions. Electrical utilities still have emissions factors (reported in CO2 equivalent emissions per kWh) that must be accounted for as part of Scope 2 supply emissions. Counterintuitively, the national average emissions factor for electric power is 2.2 times that of natural gas to produce an equivalent amount of thermal energy.1,2 This is primarily due to the inefficiencies associated with generating and transporting electricity versus converting fossil fuels directly to thermal energy on site.
In addition to having higher emissions, electricity is 3.6 times the cost of natural gas for an equivalent amount of energy based on national averages for 2022.3,4
The cost effectiveness of gas fired atmosphere furnaces historically has been the motivator behind their use, unless the process benefitted in some other way from vacuum processing.
If electricity has a greater carbon footprint and is more expensive per unit of energy than fossil fuels, why is the industry transitioning to electrification and increasingly favoring vacuum processing? The answer lies with several factors both internal and external to the equipment itself.
Within the scope of the equipment, gas fired furnaces are intrinsically inefficient. Burners exhaust hot gas which continuously siphons energy away from the process. Although less significant for direct fired burners, this effect is amplified for indirect burners, which are commonly used. Recuperators and regenerators can dramatically improve efficiencies by recycling exhaust to pre-heat combustion air, but additional energy is always required for burner systems beyond what is needed to heat the work and overcome losses through insulation. Electric furnaces, on the other hand, have no such additional demand, and the energy they consume is more directly applied to the process. Although the type of energy used is more financially and environmentally costly per unit, electric vacuum equipment uses that energy more efficiently.
In addition to the demands from the burner exhaust, gas fired furnaces usually depend on a blanketing atmosphere to protect the work from oxidation. Endothermic gas is commonly used for this purpose, and in addition to the heat input required for endothermic gas generation, CO and CO2 are products of the reaction. Although it is an objective of endothermic gas generation to minimize the amount of CO2 present in the furnace, the CO exhausted to the atmosphere eventually reacts to form CO2, leading to a higher effective emissions rate. The use of a vacuum as a protective atmosphere is less carbon-intensive as it relies primarily on the power required to operate the vacuum pumps. This leads to much lower emissions to create the processing atmosphere.
Looking outside of the equipment at the overall manufacturing process, heat treating in vacuum can often eliminate post processing steps required when using other types of equipment. This may come in the form of less oxidation or scale, meaning less part cleanup, or low distortion gas quenching, allowing final machining to be moved forward in the manufacturing process or removed altogether. These potential production cost savings are not new, but the value of eliminating the emissions associated with additional manufacturing steps will only serve to further incentivize vacuum equipment moving forward.
There is one final dynamic outside the scope of the equipment that contributes to the explanation of the industry’s push toward vacuum. The emissions factors associated with electric power generation are decreasing, a trend which is expected to continue. The contribution of renewable energy to the domestic power grid is projected to more than double in the next seven years.5
Single chamber vacuum furnace
Although the contribution from renewable sources is still significantly less than fossil fuels, changes in generation are not the only factors at play. Significant efforts are being made to develop grid-scale energy storage solutions. Although most often associated as a prerequisite for intermittent production from renewables, these storage solutions serve an important function for the existing infrastructure. By storing excess power during low demand and releasing it during peak hours, grid scale energy storage would allow fossil fuel power plants to run at more optimized efficiencies without having to ramp up and down to match demand.
Beyond the process efficiencies of vacuum discussed above, investing in electric fired equipment is the only way to capture the benefits of ongoing improvements to electric supply and generation infrastructure. While the benefits of electrification may currently depend on contextual variables such as geographic location and equipment design, natural gas fired processing has a relatively fixed ceiling for future improvement. As an added advantage of electrification, the carbon accounting reductions from the improvement in emissions factors can be captured passively after the initial investment.
While the above advantages of electrification and vacuum do help explain the industry’s push in that direction, it is worth considering how vacuum equipment will continue to evolve to maximize energy efficiency and reduce emissions. Historically, the majority of vacuum furnaces have been single chamber batch style pieces of equipment. This configuration usually requires that loading and unloading occur at, or near, room temperature to avoid oxidation of sensitive materials. In addition to longer floor-to-floor times, this means that the energy required to heat the furnace is thrown away at the end of each cycle.
The competitive demand for low-carbon solutions will drive the use of multi-chamber batch and continuous style furnaces that allow stored energy to be conserved between cycles. This will be especially true as we see more high-volume manufacturing shift away from traditional continuous atmosphere heat treating. In the past, batch vacuum processing has been too restrictive to both part cost and throughput to be competitive. As emissions concerns gain prominence, vacuum furnace configurations that offer higher energy efficiencies and throughput will begin to close that gap.
The processing and energy advantages of electric vacuum furnaces have positioned them well to meet the low-carbon demands of an increasingly emissions-conscious market. It will be exciting to see how the equipment continues to develop to meet those needs in the future.
Bryan Stern is the product development manager at Gasbarre Thermal Processing Systems. He has been involved in the development of vacuum furnace systems for the past 7 years and is passionate about technical education and bringing value to the end-user. Bryan holds a B.S. in Mechanical Engineering from Georgia Institute of Technology and a B.A. in Natural Science from Covenant College. In addition to being a member of ASM, ASME, and a former committee member for NFPA, Bryan is a graduate of the MTI YES program and is proud to have been included in Heat TreatToday's 40 Under 40 Class of 2020.
As we get further into the heart of fall, it’s time to turn up the heat (treat)! – but how can this be done in an optimized and sustainable way?
Today’s Technical Tuesday original content round-up features tips and tricks from our summer print editions on how to optimize and sustain your heat treat operations, even during the chilly months. So, bundle up, grab a hot drink, and review these insightful pieces!
Sustainability Insights Corner
In May, Heat TreatToday began publishing "Sustainability Insights" from the IHEA editorial team. Here's a brief overview of the recent insights all in one place:
June: NEW Sustainability and Carbonization Webinar Series. Although this year's IHEA Webinar series may have come and gone, it's not too late to establish a foundational understanding of carbon and sustainability here!
August: Reducing the Carbon Footprint of Your Heat Treating Operations. Brian Kelly of Rockford Combustion is back with yet another suitability insight, here exploring ways to assess your heat treating operation's carbon footprint, tune your combustion systems, explore renewable fuels, and much more.
September: Process Heating and the Energy-Carbon Connection. Explore the issue of greenhouse gases and how recent conversations are affecting the heat treating industry with Michael Stowe of Advanced Energy.
In Case You Missed the May Issue: Induction and Sustainability Tips
Looking for sustainability tips for your heat treating operation, but lacking in time? Heat TreatToday's May Issue has you covered with a quick read: "13 Induction and Sustainability Tips." We'll highlight a few below which made it into a recent Technical Tuesday feature:
Sustainable Energy for Furnaces? What does the Future Hold?
What will the future run on? With growing discontent around current energy sources like natural gas and other fossil fuels, power sources for furnace equipment are due for a makeover.
Explore the question of sustainable energy for furnaces in the future with industry experts John Clarke of Helios Electric, Philippe Kerbois of Glass, various authors from Watlow, and Stuart Hakes of F.I.C. (UK) Limited.
How much electrical power is being used in the typical heat treatment plant? And how can power (and money) be saved in these operations? If these questions peak your interest, explore further with Roger A. Jones and William Jones of Solar Atmospheres.
Learn about savings in electricity and money in areas of electric motors, high vacuum diffusion pumps, gas blowers, building lighting, AC/heating, and more in this article.
Those familiar with vacuum heat treatments are surely acquainted with the vacuum heat treatment of titanium and how such furnaces create the ideal environment for titanium's heat treatment. However, not all titanium and its alloys are created equal. Enter the beta titanium alloy.
In this best of the web article from TAV Vacuum Furnaces, discover the potential applications for beta titanium alloys, as well as the effects that various vacuum heat treatments can have on the mechanical properties of the alloy. Additive manufacturing (AM) technologies, specifically laser powder bed fusion, are gaining increased interest in the treatment of beta titanium alloys, due to their efficiency and their cost-cutting potential. Learn more about the chemistry and applications of this unique material below.
An excerpt:
Beta titanium alloys have an unique combination of desirable properties: their high specific strengths, creep resistance, oxidation and corrosion resistance, excellent temperature resistance up to 600°C and hardenability, make them very attractive for aerospace applications. On the other hand, the excellent biocompatibility and low elastic modulus, closer to that of human bone compared to other alloys, make Ti beta alloys an excellent material for biomedical applications.
What is the connection between AMS2750 specifications and furnace classifications? With tight specifications, what does the heat treater need to know to be compliant? Follow along as we take a brief look into this often-overlooked topic.
This Technical Tuesday article, written by Douglas Shuler, owner and lead auditor, Pyro Consulting LLC, was first published in Heat Treat Today's March 2023 Aerospace Heat Treating print edition.
Doug Shuler Lead Auditor Pyro Consulting
AMS2750 is the specification that covers pyrometric requirements for equipment used for the thermal processing of metallic materials. AMEC (Aerospace Metals Engineering Committee) is one of the committees which oversees the changes and revisions of AMS2750. There are five main sections in the technical requirements of the specification: sensors, instrument calibrations, thermal processing classification, SAT (system accuracy testing), and TUS (temperature uniformity surveys). Additionally, there are quality provisions that detail what happens if a calibration or test is either past due or fails.1
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Revisions to the original requirements have occurred over the years, with the newest being Revision G. The structure of Revision G has carried over from Revision F and has remained the current structure of the AMS2750 specification. This structure includes furnace classes, which are based on the minimum requirements for temperature uniformity.
Furnace classes are defined in Figure A of Revision D Figure 1.
Figure 1. AMS2750G furnace class uniformity tolerances Source: Doug Shuler
Originally, furnace classes were based on temperature uniformity, but also subzero transformation, refrigerated storage of aluminum alloys, and embrittlement relief, Figure 2.
Figure 2. Original AMS2750 instrument accuracy requirements, no class structure Source: Doug Shuler
AMS2750 Revision C was released in May 1990 and started to implement the class and instrumentation type structure and differentiated between furnaces for heat treating parts versus furnaces for heat treating raw materials. Furnaces for heat treating parts were classified based on uniformity, but also on a readability requirement. Furnaces for heat treating raw materials were classified based on a readability requirement alone.
AMS2750 Revision D was released in September 2005 and continued to define equipment class (Figure A)* and instrumentation type (Section 3.3.1.1)*. It also clarified chart recorder resolution (Table 4)*, print and chart speed (Table 5)*, and testing frequencies for SAT (Tables 6, 7)* and TUS (Tables 8, 9)* for the processing of parts versus raw materials.
AMS2750 Revision E was released in July 2012 and continued to build on the clarity presented in Revision D by adding an instrumentation type table (Figure 3)* instead of a simple text description in the body of the specification.
Figure 3. AMS2750 Revision C: distinguishment between furnaces for heat treating parts versus raw materials Source: Doug Shuler
Moving to AMS2750 Revision F, the specification saw a major rewrite and restructuring where the tables were moved from the end of the document to the first area text that called out the specific table. Revision F also put into place a sunset date for analog instruments.
That brings us to the current revision of AMS2750, Revision G, which has carried forward the structure of Revision F and only sought to further clarify the intent of the requirements.
Over the years, the technology of sensor, instrument, and furnace manufacture and capability has continued to produce better and tighter controls for the process of heat treating. The evolution of AMS2750 has recognized these advancements and has kept pace with them in technology. The understanding of the origins of AMS2750 and how it has evolved is vital in understanding its application to today’s heat treat special processes.
*Specified figure, table, or section is associated with the AMS2750 revision being discussed.
About the Author: In 2009, Douglas (Doug) Shuler became the owner of Pyro Consulting LLC and also began working with Performance Review Institute (PRI), first as an instructor and course developer and later as an auditor for the Nadcap program. As a lead auditor for Nadcap, he has conducted over 380 Nadcap special process and aerospace quality management system audits on behalf of the Aerospace Primes over the past 10+ years. Doug continues to focus on instruction, training, and education for the heat treat industry, developing courses, authoring exams, and employing the PIE method: “Procedures that Include all requirements, and Evidence to show compliance.”
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
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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 (CO2).
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 CO2 by 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
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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
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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!
Figure 1 Source: PSNERGY
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.
Figure 2 Source: PRNERGY
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
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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
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.Image 3. Radiation
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.
What is the connection between AMS2750 specifications and furnace classifications? With tight specifications, what does the heat treater need to know to be compliant? Follow along as we take a brief look into this often-overlooked topic.
