Discover expert tips, tricks, and resources for sustainable heat treating methods Heat Treat Today's recent series. And, if you're looking for tips on combustion, controls systems, or induction in general, you'll find that too! Part 1, today's tips, digs into cleaning and maintenance
This Technical Tuesday article is compiled from tips in Heat Treat Today's May Focus on Sustainable Heat Treat Technologiesprint edition. If you have any tips of your own about induction and sustainability, our editors would be interested in sharing them online at www.heattreattoday.com. Email Bethany Leone at bethany@heattreattoday.com with your own ideas!
1. Maintenance of Induction Coils Used in Hardening Applications
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Soap and hot water will remove sticky quench and debris. Source: Induction Tooling, Inc.
How should you maintain induction coils used in hardening applications? Elbow grease — a little goes a long way. After each use, a simple solution of soap and hot water will remove sticky quench and debris. Scrub hardened dirt with a Scotch-Brite pad. Check for pitting, arcing, and insulator damage. If all is good, use a hot water rinse, and it’s ready for use. If the inductor is to remain on the machine for an extended period, it is advised to wash it and the associated bus daily. Check for damage. Following this simple procedure will reduce business waste.
As industry tries to become more “green,” a number of companies are switching from lubricants that are petroleum or mineral oil-based to water-based (“aqueous”) lubricants instead. However, some of these companies then make the mistake of not changing their degreasing fluids that they use to remove these lubricants prior to their next processing operations, and stay with their standard degreasing fluids, such as acetone or alcohol, which are not effective at fully removing water-based lubricants. Instead, they need to run tests to find an appropriate alkaline-based degreasing fluid for such water-based lubricants, since alkaline-based degreasers will be effective at removing such lubricants. Commonly available dish-detergents (alkaline-based) have been shown to be highly effective for such use.
Induction heat treating: no harsh chemicals, gases, or even CO2 emissions. But to get there, heat treaters should first understand how to plan for an induction design and fabrication project upfront. Consider these five important factors before you dive into induction.
This Technical Tuesday article was composed by John Chesna, general manager at Induction Tooling, Inc. and honoree in Heat Treat Today's 40 Under 40Class of 2022. It appears in Heat Treat Today's May 2023 Sustainable Heat Treat Technologiesprint edition.
Introduction
John Chesna General Manager at Induction Tooling Source: Induction Tooling, Inc.
There are many less than obvious factors to consider when preparing and planning for induction. So where to start? There are five important factors that manufacturers with in-house heat treat operations should understand in order to successfully prepare an induction heating project and design.
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But first, what is induction heating? Induction heat treating is the process in which a high frequency conductor (induction tool) induces currents (eddy currents) into an electrically conductive workpiece. Without ever touching the work-piece, the current generated and the resistance causes heating. Ever since its proven usefulness around the time of World War II, induction has been chosen as the go-to heat treatment for a variety of applications across many industries including agricultural, medical, and transportation. Now, it seems that most industries have taken advantage of induction heat treating, and its popularity will likely only continue to increase with the push for the use of “clean” and “green” energy.
#1 Plan for Inductor Wear
One of the most important factors to an induction project is realizing the inductor/ coil is a wear item. It can be highly engineered, hand fabricated, machined, or even 3D printed. Yet, in the overall process, it is still a wear item: an item that will eventually require replacement or repair. The inductor is exposed to the worst of the elements during the induction process and can fail from standard use, accidents, or unforeseen circumstances. Inductor designers are constantly being challenged to create tools that will last longer, require less maintenance, or run more cycles. All of those can be achieved, but the inductor will eventually require replacing and that is not a bad thing!
A properly serviced and maintained inductor will ensure quality parts are being produced. As the inductor wears, the efficacy degrades, leading to undesirable results. Repair of the inductor will correct this issue and ensure the parameters required for the desired heat treat pattern are restored. Depending on production needs, a good principle is to have more than one inductor on hand so that while one is being repaired the spare inductor can remain on the machine to keep up with manufacturing demand. Planning for this is important for the project’s timing and budget.
#2 Types of Inductor Designs
Determining a specific inductor design will be necessary to properly heat parts. The inductor creates the magnetic field in the workpiece, and typically the inductor is shaped to couple closely where heat treatment of the part is desired. Additionally, if quenching is required for the heating application, this function will be considered in the inductor’s design. The inductor’s design must deliver the electrical energy and quench medium to the workpiece while allowing accessibility for material handling purposes. For this reason, inductors take on many different designs.
Six turn multi-turn inductor Photo Source: Induction Tooling, Inc.
Pancake inductor with strap supports Photo Source: Induction Tooling, Inc.
MIQ (Machined Integral Quench) scanning inductor with removable quench plate Photo Source: Induction Tooling, Inc.
Common inductor designs include:
Pancake: used for heating flat surfaces
Single turn or multi-turn: commonly shown as copper tubing wrapped around cylindrically around the workpiece
Hairpin: typically, a simple back and forth loop used to heat long lengths internally or externally on the workpiece
Split return: used to focus the energy in particular areas of the workpiece
MIQ (machined integral quench) paddle: the most commonly used design for scanning applications
#3 Power and Frequency
Know the power supply and/or work-head power and frequency. Depending on the composition of the part that requires processing, the power and frequency of the equipment will help estimate the depth of the pattern that can be achieved, as well as help determine how successful the part will be for induction heating. Irregularly shaped geometries with points, holes, or sharp edges sometimes cause difficulty establishing eddy currents where the induction pattern is desired. Some parts, after review, are good candidates for induction heat treatment but cannot be processed with the existing power supply and/or work-head setup.
If an inductor is being built to mount to existing induction equipment, it is important to know the scope of parts that are currently being processed or expected to be processed on the machine. The electrical circuit of the power supply, work-head, and inductor must load match to the part. If a variety of parts are being run then multiple styles of inductors may exist or will be required to be used. Different designs of inductors, e.g., single-turn, multi-turn, or split return used on the machine will change the transformer effect and capacitor requirements of the system. Availability to tune the system capacitance and inductance becomes vitally important for operation. Please note that adjusting capacitance can be dangerous and should only be done by a trained technician. Newer power supplies function differently than older models, yet load tuning needs to be considered.
#4 Part Details
A detailed pre-induction print is needed. The print should list the material as well as the desired heat treatment pattern to determine the inductor design. As the print specifies the pattern, it should also provide limits. Inductors are then typically designed to the shape of the part. The inductor may require an integrated quench, electrically insulating protective coating, locators, or additional assembly fixturing depending on the part’s size. An inductor built for one part may be used or tried on a similar part. However, the same results cannot be expected to render on the part for which it was not designed. If the manufacturer knows that a family of parts will be run, the full scope should be presented to inductor designers for consideration before the build.
#5 Material Handler
Ideally an inductor supplier would be contacted to develop the induction heating process for a part; then, that information should be shared with the material handling designer. That would be the ideal, but that’s not the way it usually happens. Sometimes, a machine is built to process a part that no longer is in use, so the machine is now being retrofitted to process different parts. The design of a new inductor is needed to accommodate this existing machine which may create size constraints to the inductor’s design.
The contact style, how the inductor mounts to the work-head, will need to be determined. There are a variety of commonly used power supplies and work-heads available from OEMs in the market. As each OEM keeps their contacts standard to their equipment, there is no singular standard footprint in the market. Once the contact style has been determined, the inductor can be designed for maximum power delivery efficiency. How the part and inductor are presented to each other is important. The centerline distance, a measurement from where the inductor mounts to where the part will be processed, needs to be known. The centerline determines the required length of the inductor and indirectly how much room is available for the inductor’s design.
Conclusion
Due to the variety of factors, no two projects are ever the same. Induction heating is an exciting technology, and I encourage everyone to learn more about it.
About the Author: John Chesna is the general manager of Induction Tooling, Inc. and has been involved with the induction heat treating industry for over 8 years. He is a graduate of the University of Akron with a Bachelor of Science in Mechanical Engineering Technology. His responsibilities include overseeing day-to-day operations including the design, manufacturing, and testing of induction heat treating inductors. Additionally, John was a recipient of Heat Treat Today's 40 Under 40 award in 2022.
Independence Day is right around the corner, and Heat TreatToday wanted to share some red, white, and blue processes from across the heat treating industry. We're highlighting induction hardening, gas nitriding, and hydrogen generation. Curious? Get ready for Independence Day with this red, white, and blue Technical Tuesday.
Induction hardening in action Photo Credit: Contour Hardening
Induction hardening is a bit out of place in the heat treating world. There are no huge pieces of equipment, long wait times, or mysterious happenings behind furnace walls. But using those red hot coils for hardening can be a game-changer, because induction hardening offers selective hardening, increased part strength, and uninterrupted process flow.
"The induction coil is a copper conductor that is shaped in order to harden the specified area of the part. The current that flows through the coil is what produces the magnetic field, which in turn heats the part. Coils are typically part specific, since they need to be precisely constructed to heat a particular portion of the part."
White layer from nitriding Photo Credit: SECO/VACUUM
Gas nitriding is a valuable case hardening process. In gas nitriding, a white layer made up of a nitrogen-rich compound is formed. This white layer is hard and wear-resistant, but is also very brittle.
"This compound layer depth is dependent on processing time. In the more traditional two-stage process, the case depth produces a gradient of hardness from surface to core that commonly ranges from 0.010-0.025”, with minimal white layer, typically between 0-0.0005”."
Water and electricity: that's all the materials that are needed to generate hydrogen on site. Water electrolyzers for hydrogen generation are compact, portable, and reliable, as well as being safer than storing gases. Could the future of heat treating — and perhaps the end of natural gas — be "blue"? Now, unless you live on the beach in the Bahamas, the water you're used to probably isn't blue, but you catch our drift.
"Electricity and water come into a plant in pipes and wires and are highly reliable. Additionally, there are no hydrogen storage tanks taking up a large amount of unusable space."
Find heat treating products and services when you search on Heat Treat Buyers Guide.com
During the day-to-day operation of heat treat departments, many habits are formed and procedures followed that sometimes are done simply because that’s the way they’ve always been done. One of the great benefits of having a community of heat treaters is to challenge those habits and look at new ways of doing things. Heat TreatToday’s101 Heat TreatTips, tips and tricks that come from some of the industry’s foremost experts, were initially published in the FNA 2018 Special Print Edition, as a way to make the benefits of that community available to as many people as possible. This special edition is available in a digital format here.
Today we continue an intermittent series of posts drawn from the 101 tips. The tips for this post can be found in the FNA edition under Hardness Testing, CQI-9 Compliance, and Hardening/Tempering.
Heat TreatTip #22
Properly preparing a hardness sample can save time and money.
Inspection Mistakes That Cost
Rockwell hardness testing requires adherence to strict procedures for accurate results. Try this exercise to prove the importance of proper test procedures.
A certified Rc 54.3 +/- 1 test block was tested three times and the average of the readings was Rc 54 utilizing a flat anvil. Water was put on the anvil under the test block and the next three readings averaged Rc 52.1.
Why is it so important that samples are clean, dry, and properly prepared?
If your process test samples are actually one point above the high spec limit but you are reading two points lower, you will ship hard parts that your customer can reject.
If your process test samples are one point above the low spec limit but you are reading two points lower, you may reprocess parts that are actually within specification.
It is imperative that your personnel are trained in proper sample preparation and hardness testing procedures to maximize your quality results and minimize reprocessing.
Whether you need to meet rigid CQI-9 standards or not, what are the top 3, nay 4 best practices that nearly every in-house heat treat department ought to follow to make sure their pyrometer stuff is together?
Daily furnace atmosphere checks. Use an alternative method to verify your controls and sensors are operating properly and that there are no issue with your furnace or furnace gases.
Daily endothermic generator checks. Using an alternate method to verify your control parameter (dew point typically) or the gas composition is accurate will alleviate furnace control issues caused by bad endothermic gas.
Verify/validate your heat treat process every 2 hours OR make sure process deviations are automatically alarmed. this is a solid practice to ensure your controls and processes are running properly. This practice can help ensure that parts are being heat treated to the proper specification intended.
Conduct periodic system accuracy tests (SATs) per pre-defined timelines in CQI-9. Good pyrometry practices are an essential part of heat treatment. Because of the importance of temperature in heat treatment, ensure timeliness of all pyrometry practices addressing thermocouple usages, system accuracy tests, calibrations, and temperature uniformity surveys.
Control of Back Tempering With Induction Heat Treating
Induction heat treating is a selective hardening process. When hardening an induction path close to an area that had previously hardened, the heat from the hardening the second path tempers back the area that was previously hardened. This is a particularly common issue when tooth by tooth hardening of small gear teeth. Back tempering will reduce the hardness on the adjacent area and this effect may range from a few to over 10 HRC points.
Factors to Minimize Back Tempering
Process Issue
Questions to ask
Correct & repeatable placement of quenches
Can quench position be verified and set up repeatedly in the same position?
Verification of quench flow
Is the quench flowing freely through the quench system? Are the quench holes blocked? Are the flowmeters reading accurately?
Integrity of the quench
Was the percentage polymer measured? Is the quench quality okay? Is the quench contaminated?
Inductor design
Is the inductor designed to minimize heat on the tip? Is the quench effectively cooling the part?
Retained heat
Is a skip tooth hardening pattern being used to minimize residual heat in the induction hardening zone? Is the scan speed appropriate?
This article continues the ongoing discussion on Equipment Selection for Induction Hardening by Dr. Valery Rudnev, FASM, IFHTSE Fellow. Six previous installments in Dr. Rudnev’s series on equipment selection addressed selected aspects of scan hardening and continuous/progressive hardening systems. This post continues a discussion on equipment selection for induction hardening focusing on single-shot hardening systems.
The first part on equipment selection for continuous and progressive hardening is here. The second part in this series on equipment selection for single-shot hardening is here; the third part is here. To see the earlier articles in the Induction Hardening series at Heat TreatTodayas well as other news about Dr. Rudnev, click here. This installment continues a discussion on equipment selection for continuous and progressive hardening applications.
Why Single-Shot Hardening?
With the single-shot method, neither the workpiece (cylinder shaft, for example) nor the coil moves linearly relative to each other; the part typically rotates instead.¹ The entire region that is to be hardened is heated all at once rather than only a short distance, as is done with scan hardening.
With conventional scan hardening of cylindrical parts, induced eddy currents flow circumferentially. In contrast, a single-shot inductor induces eddy currents that primarily flow along the length of the part. An exception to this rule would be the half-moon regions (also called the crossover or bridge sections) of a single-shot inductor, where eddy current flow is circumferential.
Normally the single-shot method is better suited for hardening stepped parts where a relatively short (1.5–2 in. [38–50mm] long heated area is commonly minimum) or moderate length area is to be heat treated. This method is also better suited to cylindrical parts having axial symmetry and complex geometry including various diameters.
When scanning these types of parts, improper austenitization of certain areas may occur due to localized electromagnetic field distortion, for example. Insufficient quenching due to the deflection of quench flow not allowing it to properly impinge on the surface in various diameter regions may also occur. Both factors are considered undesirable and can cause low hardness, spotted hardness, or even cracking. For example, the use of scan hardening on stepped shafts with large shoulders, multiple and sizable diameter changes, and other geometrical irregularities and discontinuities (including fillets, flanges, undercuts, grooves, etc.) may produce severely non-uniform hardened patterns. In cases like this, a scan hardening inductor or progressive/continuous hardening system would be designed around the largest diameter that would have sufficient clearance for safe part processing.¹ However, variations in the shaft’s diameter, to a significant extent, will result in a corresponding substantial deviation in the workpiece-to-coil coupling in different sections of the shaft, potentially causing irregular austenization.
Besides that, sharp corners have a distinct tendency to overheat owing to the buildup of eddy currents, in particular when medium and high frequencies are used. The electromagnetic end and edge effects may also cause the shoulders to severely overheat while the smaller-diameter area near the shoulder (including undercuts and fillets) may have noticeable heat deficit. These factors may produce a hardness pattern that might grossly exceed the required minimum and maximum case depth range, making it unacceptable. Single-shot hardening is usually a better choice in such applications. As an example, Figure 1 shows some examples of components for which single-shot hardening would be a preferable method of heat treating.
Examples of components for which a single-shot hardening would be a preferable method of heat treating. (Courtesy of Inductoheat Inc., an Inductotherm Group company)
In some not so frequent cases, when hardening larger parts, there are advantages to the single-shot method over the scanning method, such as the reduction of shape/size distortion, enhanced metallurgical quality, and increased production rate.
Single-shot hardening may also be the preferred choice when shorter heat times/high production rates are desired. For example, in some applications, the time of heating for single-shot hardening can be as short as 2 s, though 4 to 8 s is more typical.
However, the single-shot method has some limitations as well. One of them is cost. Single-shot inductors are typically more expensive to fabricate compared to the coils used for scanning. This is because the single-shot inductor, to some degree, must follow the contour of the entire region required to be heated. Additionally, a single-shot inductor is usually able to harden only one specific part configuration, whereas a coil used for scanning may be able to harden a family of parts.
Besides that, in some case hardening applications using a scanning method, it is possible to apply certain pre-programmed pressure/force on a workpiece during heat treating. This allows distortion to be controlled. Single-shot hardening might also permit applying this technique but there might be some limitations.
Design Features of Single-Shot Inductors
Single-shot inductors are made of tubing, either 3-D printed or CNC-machined from solid copper to conform to the area of the part to be heated. This type of inductor requires the most care in fabrication because it usually has an intricate design and operates at high power densities, and the workpiece’s positioning is critical with respect to the coil copper profiling. Figure 2 shows several examples of induction heating of different components using single-shot inductors.
Several examples of induction heating of different components using single-shot inductors. (Courtesy of Inductoheat Inc., an Inductotherm Group company)
In order to provide the required temperature distribution before quenching, heat is sometimes applied in several short bursts (pulse heating) with a timed delay/soaking between them to allow for thermal conduction toward the areas that might be difficult to heat.
Single-shot inductors typically require higher power levels than used in scan hardening because the entire area of the workpiece that needs to be hardened is austenitized at once. This is the reason why single-shot hardening normally requires having a noticeably larger power supply compared to scan hardening, resulting in increased capital cost of power source. Additionally, the increased power usage and power densities combined with complex geometry can reduce the life of the inductor. For this reason, single-shot inductors often have shorter lives than scan inductors.
It is always important to keep in mind that, electrically speaking, the inductor is typically considered the weakest link in an induction system. For this reason, most single-shot inductors have separate coil-cooling and part-quenching circuits. The inductor will fail if power is increased to the point at which the water cannot adequately cool it. Additional cooling passages may be needed with high-power density, single-shot inductors. A high-pressure booster pump is also frequently required.
The next several installments of Dr. Valery Rudnev on . . . will continue the discussion on design features of single-shot inductors and equipment selection.
Induction Hardening Tips: Equipment Selection for Scan Hardening, Part 3
This is the third installment of a multi-part column on equipment selection for induction heat treatment. Part 1, Dr. Valery Rudnev On . . . Induction Hardening Tips: Equipment Selection for Scan Hardening, covered types of scanners, scan hardening system setup, quenching challenges, maximizing process flexibility, and computer modeling. In Part 2, Dr. Valery Rudnev discussed another critical aspect of induction scan hardening: inductor design subtleties and a comparison of different fabrication techniques (brazing vs. CNC
machining vs. 3D printing).
In this installation, Dr. Rudnev focuses on Moveable Inductor versus Moveable Part.
Moveable Inductor versus Moveable Part
As stated in one of the previous installments of this column, when a scan processing mode is chosen, either the inductor or the part or both may be moved during the heating and quenching. This installment discusses the applicability of those approaches (movable inductor vs. movable part), as well as pros and cons associated with both techniques.
Figure 1. An example of scan hardening of track shoes for earth-moving machines that often specify deep hardness case depths (up to the 24 mm).
The choice to move the inductor or to move the part is primarily based on required production rate as well as on the size, weight, and geometry of the component compared to the size, weight, and geometry of the inductor: in other words, it depends on which of the two is easier to move.
Weight is an important factor because the movement can occur several hundred times each day and, in some cases of high production, even several thousand times per day. For example, during induction surface hardening of track shoes for earth-moving machines that often specify deep hardness case depths (up to 24 mm), it is much easier to move the inductor around the workpiece instead of moving the track shoes, the weight of which can exceed several thousand pounds. (Figure 1)
When moving the inductor, both flexible cables and hoses are used or the inductor is hard-bused to the transformer and the transformer or heat station moves with the inductor. In some cases, the power supply itself may be moved at a moderate rate to scan a stationary workpiece [1]. Another example of moving the inductor is surface hardening of trailer axles. (Figure 2)
Figure 2. (Left image) Horizontal scanner to induction harden both ends of a trailer axle. A walking beam system was incorporated into the machine for part transfer. At the heating station, the axle is lifted off the beam and the power supply and inductor are indexed to position for scan hardening. After the completion of surface hardening of one end, the axle is then lifted off the transfer mechanism and rotated 180° to induction harden the opposite end. Heavy-duty precision shafting and bearings are used for stability and consistency. (Right image) shows a close-up of a movable inductor to scan harden trailer axle ends. Heating time is less than 8 s per axle end.
The length of the part to be heated is also an important consideration When a component is of moderate weight, it is obviously preferable to move the part rather than the inductor. For example, it is much easier and more cost-effective to design a hardening system that anticipates moving a workpiece that weighs less than 0.25 kg (<0.5 lb) rather than moving an entire power supply, as it is shown in Figure 3.
Figure 3. Horizontal scanner that provides a maximum scan rate up to 200 mm/s (8 in./s). (Courtesy of Inductoheat Inc., an Inductotherm Group company.)
In other cases, it may not be practical to move very large and elongated components. It would consume too much floor space to move the part through a stationary inductor. In the case of low production rates, the best choice might be to move the inductor, but the length of the high-frequency power leads could become a problem with respect to voltage drop and power loss. In this case, it is preferable to move the inductor with the power supply attached. Then, the moving cables are operating at a low frequency (50–60 Hz) with lower voltage drop and power loss. In the case of high production, continuous horizontal systems may be more suitable.
The consideration of the length of the leads (e.g., cables or buses) from the power source to the inductor is important. They should be as short as possible to conserve energy and to allow the power source to operate properly without reaching any limits (for example, voltage limit). If these leads are too long, the inductance increase can be so significant that it may result in a substantial power loss and voltage drop. The voltage drop in the leads may even exceed the voltage at inductor’s terminals. Long leads could net an excessive total needed power, a measurable reduction in energy efficiency, and potential concerns regarding the process repeatability owing to the possibility of an appreciable inductance change of the flexible leads during their motion, that in some cases may negatively impact process repeatability.
Whether moving the inductor or moving the part, the induction system can be designed to be efficient and robust in order to ensure smooth and consistent operation and the production of quality parts.
I recommend Reference #1 to readers interested in further reading on this subject.
References
V. Rudnev, D. Loveless, R. Cook, Handbook of Induction Heating, 2nd Edition, CRC Press, 2017.
Dr. Valery Rudnev, FASM, IFHTSE Fellow, is the Director of Science & Technology, Inductoheat Inc., and a co-author of Handbook of Induction Heating (2nd ed.), along with Don Loveless and Raymond L. Cook. The Handbook of Induction Heating, 2nd ed., is published by CRC Press. For more information click here.
Induction Hardening Tips: Equipment Selection for Scan Hardening, Part 2
This is the second installment of a multi-part column on equipment selection for induction heat treatment. Part 1, Dr. Valery Rudnev On . . . Induction Hardening Tips: Equipment Selection for Scan Hardening, covered types of scanners, scan hardening system setup, quenching challenges, maximizing process flexibility, and computer modeling. In this installment, Dr. Valery Rudnev discusses another critical aspect of induction scan hardening: inductor design subtleties and a comparison of different fabrication techniques (brazing vs. CNC
machining vs. 3D printing).
Introduction
Hardening inductors are often considered the weakest link in an induction hardening system because they may carry significant electrical power and operate in harsh environments exposed to high temperatures, water, and other coolants while being subjected to mechanical movement and potential sudden part contact.
Single-turn or multiturn inductors may be used in scan hardening (Figure 1). Copper profiling and the number of turns is determined by the workpiece geometry, required hardness pattern, and the ability to properly load match the coil to the power supply without reaching the operational limits or by other specific process requirements, such as the production rate or the hardness pattern runout/pattern cutoff. [1]
Figure 1: Single-turn or multiturn inductors may be used in scan hardening.
The longer (in case of horizontal arrangement) or the higher (vertical arrangement) the scan coil is, the faster the scan rate can be. This is due to the simple fact that the longer inductor leads to a longer period when the part will be inside the coil; therefore, the scan rate can be greater. However, limitations on the maximum length of the inductor’s heating face may be associated with the maximum permissible runout.
Hardness Pattern Runout Control
Single-turn inductors with narrow heating faces (3mm-6mm wide) are used where a sharp pattern runout is needed. An example of this would be the case where a pattern must end near a snap ring groove. Inductors with wider heating faces or two-turn coils can be used when a faster scan rate is desired and an extended runout is permitted. The main disadvantage to the excessively wide heating face is that it may result in an unspecified shift of coil current density when hardening complex geometric parts due to an electromagnetic proximity effect. [1]
Inductor Fabrication Techniques
In applications where high process repeatability is critical (including automotive, aerospace, defense and other industries), the great majority of scan hardening inductors are CNC machined from a solid copper block, thus making them rigid, durable, and repeatable. CAD/CAM/CNC software programs are created that provide appropriate cutter-to-copper spatial relationships, which produce inductors of the required shape and precision regardless of complexity. Figure 2 shows a variety of finished and semi-finished CNC-machined hardening inductors. [2]
Figure 2: finished and semi-finished CNC-machined hardening inductors
In other cases, copper tubing (square, rectangular, round, or die-formed shaped tubes) may be used for coil fabrication (Figure 3). Copper tubing is typically annealed to improve its ductility, bending properties, and workability. When sharp bends or complex coil shapes are required, inductor segments made from tubing are assembled by brazing. Joints are often overlapped, creating tongue-and-groove joints. Butt-joints should not be used.
Figure 3: Copper tubing (square, rectangular, round, or die-formed shaped tubes) may be used for coil fabrication.
A complex geometry inductor that contains numerous brazed joints, and elbow-type 90° joints in particular, could experience impeded water flow in the cooling coil turns, shortening coil life. Poor quality brazed joints are prime candidates for water leaks affecting not only the coil life expectancy but also a quality of hardened components due to a potential soft spotting in the areas of water leaks. Eliminating braze joints or dramatically reducing their number, particularly in current-carrying areas, is the key to fabricating durable, reliable, and long-last inductors.
Additive manufacturing (AM), or 3D printing, delivers successful fabrication of fixtures, tooling, holders, etc. Recently, some inductors have been fabricated using 3D printing as well. It is important to keep in mind that AM is not a single technology but it comprises a number of processes including direct metal laser sintering, electron beam melting, directed energy deposition, direct and indirect binder jetting, and others.
Depending upon a particular AM technique used in fabricating hardening inductors, it may face major challenges to match properties of pure copper. This includes (1) obtaining sufficiently high thermal conductivity (2) or low electrical resistivity, (3) ensuring high volumetric density, and (4) having minimum amount of residuals, just to name a few. All these factors affect coil life. Therefore, if you compare 3D printed inductors with brazed coils comprising numerous brazed joints, in the majority of cases, the life of 3D printed coils will surpass life of brazed inductors because of elimination of brazed joints in current-carrying regions. In addition, fabrication accuracy and repeatability of AM inductors typically surpasses the accuracy of brazed or bended coils.
The situation is different when comparing life of 3D printed coils vs. CNC machined inductors. Fabrication accuracy of both processes is very similar, however, in high-power density applications even small degradation of above discussed four factors associated with AM might become essential causing greater probability of stress-fatigue and stress-corrosion copper failure of 3D printed coils compared to CNC machined inductors fabricated from pure copper. Another factor to consider is repairability of 3D printed inductors. If you need to do a revision then it would be most likely required you to re-manufacture 3D printed coils. Regardless of a fabrication method and for quality assurance purposes, it is beneficial to apply computerized 3D metrology laser scanner technology (Figure 4) to verify coil dimensional accuracy and alignment precision after inductor fabrication and assembly.
Figure 4: It may be beneficial to apply computerized 3D metrology laser scanner technology to verify accuracy and alignment after inductor fabrication and assembly.
Material Selection
Copper and copper alloys are almost exclusively used to fabricate induction coils due to their reasonable cost, availability, and a unique combination of electrical, thermal, and mechanical properties. Proper selection of copper grade and its purity is crucial to minimize the deleterious effects of factors that contribute to premature coil failure including stress-corrosion and stress-fatigue cracking, galvanic corrosion, copper erosion, pitting, overheating, and work hardening. Cooling water pH also affects copper susceptibility to cracking.
Oxygen-free high-conductivity (OFHC) copper should be specified for most hardening inductors. In addition to superior electrical and thermal properties, OFHC copper dramatically reduces the risk of hydrogen embrittlement and developing localized “hot” and “cold” spots. The higher ductility of OFHC copper is also important because coil turns are subjected to flexing due to electromagnetic forces. The higher cost of OFHC copper is offset by improved life expectancy of hardening inductor.
For scan inductors that are intended to heat fillets, an appropriate copper heating face region must be focused into the fillet area. Coil copper profiling and the use of flux concentrators (flux intensifiers) are beneficial to focus the magnetic field into the fillet. These applications require careful design because the induced current has a tendency to take the shortest path and stay in the shaft area rather than flowing into the fillet [1]. Therefore, all efforts must be made to focus the heat generation into the fillet. Typically, higher frequencies work better for this purpose.
Copper Wall Thickness
It is important to maintain sufficient wall thickness to carry the electrical currents. The wall thickness of an inductor’s heating face should increase as frequency decreases. This fact is directly related to both the current penetration depth in the copper δCu. [1] It is highly desirable for the current-carrying copper wall thickness to be 1.6 times greater than the δCu calculated at maximum working temperature. Increased kilowatt losses in the copper, which are associated with reduced coil electrical efficiency and greater water-cooling requirements, will occur if the wall is thinner than 1.6∙δCu.
The table below shows the variation of δCu vs. frequency at room temperature (20°C/68°F).
In some cases, the copper wall thickness can be noticeably thicker than the recommended value of 1.6∙δCu. This is because it may be mechanically impractical to use a tubing wall thickness of, for example, 0.25 mm (0.01 in.).
I recommend Reference #1 to readers interested in further discussion on design of hardening inductors.
Dr. Valery Rudnev, FASM, is the Director of Science & Technology, Inductoheat Inc., and a co-author of Handbook of Induction Heating (2nd ed.), along with Don Loveless and Raymond L. Cook. The Handbook of Induction Heating, 2nd ed., is published by CRC Press. For more information click here.
LEAX Group, a Swedish manufacturer of advanced components and subsystems for automotive, commercial vehicles, mining, construction, and general industry sectors, has installed a low pressure carburizing (LPC) furnace at their Brinkmann, Germany, facility (LEAX Brinkmann GmbH) to boost the company’s heat treatment processing capabilities. The extensive installation takes about two months and the first hot load is scheduled for December. Along with the addition of a new induction machine at their Falköping, Sweden, facility, this new LPC furnace serves as the centerpiece of the massive MBS project.
LEAX, which is based in Köping, Sweden, operates heat treatment shops in seven of their twelve production sites, including Latvia, Germany, Hungary, Brazil, and China, and focuses on induction hardening and processing and refining approximately 300,000 parts per year. This added LPC hardening furnace brings a process to LEAX’s manufacturing process that has been a mainstay in the automotive industry. The full transition to the MBS project will take up to two years, but “we [will] switch hardening from the older oven to the new,” said Anders G. Larsson, COO/Heat Treatment for LEAX Brinkmann GmbH.
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