Today's shared content is provided by the global information partnership between leading European heat treat news provider heatprocessing and the team at Heat TreatToday.
What's next in heat treating carbon materials? In this best of the web feature from our European industry partner, heatprocessing, take a moment to see how computer modelling demonstrates the technical feasibility and the efficiency of this dynamic combination of induction heating and radiation heat transfer. Could this method be a practical integration in your heat treating process needs? Would adopting this method save you energy? Take a read and let us know!
An excerpt:
"This dynamic combination of induction heating and radiation during the baking process improves greatly the energy efficiency and permits a very precise control of the temperature profile in the carbon."
While we typically try to send our readers to free content, this article requires a nominal fee to access. We hope that you will find this content beneficial.
Induction is a curious member in the family of heat treating. Its presence is valuable, yet there’s a mystery surrounding it that has even veteran heat treaters exploring it to gain understanding. Journey through this induction hardening primer to learn about this important misfit of the heat treating world.
This Heat Treat Today Technical Tuesday original content feature, written by Kyle Hummel, P.E., COO at Contour Hardening, first appeared in Heat Treat Today's May 2021 Induction print edition. Feel free to contact Karen Gantzer at karen@heattreattoday.com if you have a question, comment, or any editorial contribution you’d like to submit.
In the world of heat treat, induction hardening just doesn’t fit in. There is no big furnace, cycle times are a matter seconds, and the entire process takes place right before your eyes rather than behind the walls of a furnace chamber. Many heat treaters have one old induction machine sitting in the corner of the shop floor, with one remaining employee who knows how to operate it.
Induction is different than all other types of heat treatment, and even many metallurgists shy away from the "black magic" that occurs during the process. When I ask customers how familiar they are with induction hardening, they usually state that they have seen it before, mention something about a coil, but that’s about the extent of their knowledge.
The purpose of this article is to give readers, who are not familiar with the induction hardening process, some background on the fundamental aspects and terminology of the process. The information encompasses the most common questions I am asked by new customers as well as information I would provide in training new employees. My hope is that it will give you enough familiarity with the process to become more comfortable engaging in a conversation about induction hardening.
Why Use Induction?
Selective hardening – Induction allows you to harden only the desired portion of a part, whereas most furnace-based heat treat processes treat the entire component. This means you can harden the particular area that you want to harden, while leaving the rest of the component soft enough to machine further.
Strength – Not only does the part become harder, but the stress (called residual compressive stress) that is induced into the part will make it stronger. Other processes can meet the improved wear resistance of the added hardness but fail to strengthen the part at all, or not as much as induction hardening.
Single piece flow– Because induction hardening is not a batch process (typically one part is hardened at a time), induction machines can be placed in a manufacturing cell, allowing the process flow to be uninterrupted.
Induction hardening in action
Equipment and Tooling
Induction Hardening Machine – Systems will vary significantly in size and complexity depending on the components they are hardening. The primary components of the machine consist of a power supply, heat station (transformer), workstation, and HMI. The fluids system is composed of quenchant to cool the part being hardened and distilled water to cool the internal components of the machine. Heat time, power supply output, part rotation, and quenchant parameters should be controlled, monitored, and logged for each part.
Power Supplies – Power supplies are the most important component of the induction hardener. For the purpose of this article, we will discuss the two most important outputs of the power supply, frequency and power.
Frequency is important because it will help determine the depth of heating. Lower frequencies heat deeper into the part, and higher frequencies heat closer to the surface. To remember this, I like to use the analogy of whales using very low frequency calls to communicate over miles and miles of ocean, whereas the high-pitched squeak of a mouse can only be heard several feet away. For induction hardening, frequencies are split into two groups: medium frequency (MF) and radio frequency (RF). The MF range is typically from 3-50kHz, and RF is from 100-400kHz.
Power is important because it will determine how large of a part you can harden, and how long the heat time will need to be. The more power that a machine can output, the larger the part it can harden and the faster it can harden to a specified case depth. Typical power supply outputs for induction hardening range from 25kW to 1MW.
Coils – 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.
Modern induction coils are water cooled and can be made of tubing or machined copper pieces that are brazed together to make a particular shape to fit the part. They are frequently equipped with sections of a material called flux intensifier, which helps to drive the magnetic field in a certain direction in order to intensify heating in that area and make the coil more efficient.
It is also common to have the quenching designed into the coil (machine integral quench, or MIQ) so that quenchant can be applied immediately after heating without the need to move the part to an auxiliary quench mechanism.
Process Basics
Single Shot– Single shot hardening is the most common method of induction hardening where the part and coil remain in the same spot during the heating process. Typically, the part is brought into proximity of the coil, the heating and quenching processes are applied to the part, and then the part is removed from the coil.
Scanning – Scanning involves heating and quenching a small portion of the part while moving either the coil or the part until the desired area is hardened. Quench is directionally applied to the part so that as a new portion of the part is heated, the previously heated section is being quenched appropriately. Scanning is frequently used to harden shafts because heating the entire shaft at once would require too much power.
Dual Frequency – Dual frequency hardening combines the benefits of the deeper heating of the lower MFs with the surface heating capabilities of higher RFs. By utilizing two different frequencies, it is possible to contour the hardening pattern more effectively on gear-like components, which further improves the strength of the part. The frequencies can either be applied consecutively (low frequency preheat followed by a high frequency final heat) or simultaneously.
Induction Tempering – Induction can also be used to complete the temper process in a few seconds rather than furnace tempering which could take hours. Induction tempering takes place after the hardening process and involves heating the part to a much lower temperature than is required during hardening. The targeted temperature for induction temper is higher than that of furnace tempering due to the decreased temper time. This softens the hardened area slightly in order to increase the toughness of the part and improve crack susceptibility.
Quenching – The quench process is just as important as the heating process with induction hardening. Almost all modern systems use a water/polymer quenchant mixture in the range of 5-20% polymer instead of using oils. The quench media is typically sprayed on to the part rather than submerging it into a bath. Quench concentration, temperature, flow, and pressure must all be monitored closely for a robust process. These parameters all function to guarantee that the part is quenched properly and consistently to ensure the correct hardness is achieved and crack susceptibility is minimized. Quench media must also be filtered to remove any process waste that could potentially clog the quench spray holes.
Inspection – Like most other forms of heat treatment, the two most common specifications with induction hardening are case depth and hardness. Most specifications will require surface hardness measurements along with effective case depths to determine the depth of hardening.
Materials – The most common materials to be induction hardened are medium to high carbon and alloy steels, cast irons, and powder metal. Induction is also becoming a popular heat treat method on certain stainless steels in different industries.
Induction hardening in action
What to Look Out For
Cracking – The rapid expansion of the part during heating followed by shrinkage from the accelerated cooldown during quenching increases crack susceptibility of induction hardened parts. Not all parts have a high risk of cracking, but part characteristics such as internal holes, sharp edges, and certain higher carbon materials will require more consideration. If cracking is an issue, the first two areas to investigate are overheating and quench severity. Reducing the quench severity (increasing quench temperature and concentration, reducing flow and pressure) is typically the most effective means of reducing cracking within an induction hardened part.
Distortion – Another side effect of the rapid expansion and contraction is part distortion. It is impossible to not distort the part with induction hardening due to the phase changes in the metal. However, with a robust and carefully monitored process, it is possible to minimize and accurately predict process distortion. Faster heating times and technical expertise in fixturing methods are two common methods to reduce distortion.
Conclusion
Although this information just begins to scratch the surface of the terminology and fundamentals of the process, hopefully it provides a starting point to those with limited experience. Like many other forms of heat treatment, it can take years to develop the knowledge and skills to gain expertise in induction hardening. I have been involved in induction for almost fifteen years, and I find there is always a new application that gives me the opportunity to learn even more.
About the Author: Kyle Hummel is a licensed Professional Engineer who has worked for Contour Hardening for 15 years as a metallurgical engineer and currently manages operations of Contour’s Indianapolis location.
For more information, contact Kyle at khummel@contourhardening.com or 317.876.1530 ext. 333
Ever wonder what the status of induction heat treating is in North America? Well you can stop wondering: Check out these Induction Heating Survey results that represent approximately 450 induction units.
This original content article was written by Karen Gantzer, editor of Heat TreatToday, forHeat TreatToday'sMay 2021 Induction print edition. Feel free to contact Karen Gantzer at karen@heattreattoday.com if you have a question, comment, or any editorial contribution you’d like to submit.
Karen Gantzer Managing Editor Heat TreatToday
Heat Treat Today conducted a survey with those companies that perform in-house heat treating as well as commercial heat treaters who use induction heating equipment. The results represent approximately 450 induction units, and we received very interesting and beneficial information from the questions posed. Below is a sampling of the questions and responses.
When asked the number of induction coils owned, 27% have over 100 coils, 16% own 50 to 100, and 13.5% have ownership of between 16 to 30 coils. Interestingly, 50% of respondents design and make the vast majority of their induction coils.
There were eight different power supply and transformer selections noted in the results. 62.5% use IGBT generators, while 33.3% use vacuum tube generators, and tied for third with 25% of respondents using thyristor or MOSFET generators.
Surveys. Polls. While well-designed ones can require time to complete authentically, the effort is worth the data received because it helps many make informed decisions. Heat Treat Todaybelieves that people make better decisions when they are well-informed, and so, with that thought in mind, if you’re interested in seeing the full report of this induction survey, please email me at Karen@heattreattoday.com.
In this month’s column, John Clarke will expand his discussion beyond combustion safety to include the economic issues that are concerns to all equipment owners and operators.
This column appeared in Heat Treat Today’s2021 Induction May print edition.
John Clarke is the technical director at Helios Electric Corporation and is writing about combustion related topics throughout 2021 for Heat Treat Today.
John B. Clarke Technical Director Helios Electric Corporation Source: Helios Electric Corporation
The furnace's or oven’s burner management system (BMS) and its associated components are all that stand between us and an incident. The severity of these incidents ranges from the very expensive — a damaged furnace or oven — to the tragic — loss of a human life. It is a testament to the good work of hundreds of people that combustion system explosions are so rare. That said, the risk to life and property mandates that we revisit this subject frequently, and the risk to profitability dictates we expand our consideration beyond safety to include uptime and quality, as well.
National Fire Protection Association Standard 86 (NFPA 86), or “Standard for Ovens and Furnaces,” provides a standard that is the most common guide to the application of combustion components used in the US. This excellent prescriptive standard reflects the common thinking of people with hundreds of years of combined experience; but it still requires expertise to properly interpret and apply its requirements. It is important to not only understand what component must be provided, but also why.
NFPA 86 is used as a guide for the design of your BMS which includes the various control components to properly monitor the startup and operation of the burner. NFPA 86 also applies to the fuel train, constructed of components that regulate the flow of fuel and air and includes blowers, regulators, valves, filters, and sensors. What BMS and fuel train safety system issues should most concern an end user? An end user must know what it really means when your system is stamped “NFPA 86 Compliant.” To paraphrase Clint Eastwood: The end user needs to know their system’s limitations.
The NFPA 86 standard has been developed to protect life and property, but not production and profits. It is also a prescriptive standard, providing specific guidance to what components need to be applied and in what order. The shortcoming of a prescriptive code is that it must be mostly generic, that is, it applies to types or classes of equipment as opposed to specific applications. Given the variety of burner applications used in industry, it would be impractical to specify every component, order, and wiring for every conceivable process heating application.
Why is this a concern for end users? A specific application may have unforeseen risks or are out of the scope of NFPA 86 . Critical failure modes may be indirectly associated with a burner failure. For example, loss of a process air flow may allow a heat exchanger to overheat before a high temperature limit instrument detects the temperature rise. In this case, the process air flow must be monitored, and the flow or pressure switch monitoring the air flow must be added to the interlock string. This way, the burner will shut off as soon as the air flow failure is detected and not wait for the heat exchanger’s temperature to rise to an unsafe temperature. Another reason to “exceed” the code is that often ovens or furnaces are one element in a much larger manufacturing system. An example would be a continuous paint line, where a failure of the curing oven might shut down an entire facility.
What should an end user do? Ensure the system provided meets the standards and codes, NFPA 86, the Fuel Gas Code (NFPA 54), NEC, etc. This level of compliance is the minimum – and is often not the optimal. Additionally, invite the OEM who built the system to apply their experience and exceed the standards if it provides a more robust system. It may cost a few dollars up front, but it will be pennies when compared to the cost of an incident or, in many cases, an outage.
Encourage your supplier to apply a recognized process to the system review, perhaps a failure mode effects analysis (FMEA) and factor in not only the cost of an incident, but the cost of lost production or quality rejects as well. Consider an independent third-party review – it never hurts to get a second opinion. Review the cost of redundancy, be it online or near online . What is the cost of a second flame rod and flame safeguard when compared to the value of four hours of production?
Next, review the steps to service the system. Look at the mean time to replace (MTTR) a failed component. Has the system been designed to be easily serviced? Are there pipe unions on either side of all critical valves? Where are the spare parts located? What skill trades are required to make the repair? Is post replacement calibration or testing required? And if so, has it been documented?
Ask if the BMS provides a clear indication of the reason for a shutdown. The interlock string, a logical series wiring of critical components where any one component indicating a fault will disable the combustion system, should be monitored in a way where the “first out” or component that will shut down the system, is clearly identified.
Lastly, it is the end user’s responsibility for periodic inspections and equipment maintenance. NFPA 86 prescribes that the BMS and fuel train components are inspected per the manufacturer’s recommendation, but at least once a year.
The annual inspection is a critical step for safe operation but is viewed by many end users as simply a cost. Add to this the relative reliability of most components and we are presented with the ironic risk that maintenance personnel may take short cuts during the periodic inspection. One such person may say, “I always check the low gas pressure switches and they always pass, so I thought, what would it hurt if I skipped the test this year?”
For a more robust inspection, consider adding more value to the process. Combine the safety inspection with an extensive equipment calibration and service: Replace the filters, change the thermocouples, calibrate the control instruments, tune the burner, check the fuel-to-air ratio of the burner, and inspect the BMS components. This adds value to the process and makes it more palatable for the maintenance department.
When the cost of downtime of a key piece of equipment is high, practice the repair, at least on paper. However, if a failed burner shuts down an automotive assembly line, isn’t it worth the time to run actual drills?
In general, most burner trips are the result of a failed sensor, a UV scanner, dirty flame rod, an open thermocouple, or the vibration from an unbalanced fan tripping a pressure switch. In other words, when this type of trip occurs, the greatest cost is lost production, followed by the labor to diagnose the problem and then the cost to replace the component. Generally, the purchase price of the component is far less than the other costs associated with the system trip. Do not be penny wise and pound foolish. Spare parts are a pretty good investment.
If you need the heat from a burner to make your product, it makes sense to not only consider safety, but also plan reduced downtime as well. In the coming articles, we will examine these issues in greater detail, so stay tuned.
John Clarke, with over 30 years in the heat processing area, is currently the technical director of Helios Corporation. John’s work includes system efficiency analysis, burner design as well as burner management systems. John was a former president of the Industrial Heating Equipment Association and vice president at Maxon Corporation.
Maciej Korecki Vice President of the Vacuum Furnace Segment SECO/WARWICK (source: SECO/WARWICK)
An international arms and military equipment manufacturer in Brazil needed to quickly expand and was recently able to receive a new vacuum furnace to meet their manufacturing demands.
The solution was provided by the parent company to North American SECO/VACUUM, SECO/WARWICK. Their furnace, the VECTOR®, is a single-chamber vacuum furnace that uses gas quenching and can be used for multiple metal heat treatment applications and processes. In this configuration, equipped with a round graphite heating chamber, it may be used for most standard processes including hardening, tempering, annealing, solutionizing, brazing and sintering.
"A situation where we have a product almost ready to be collected is rare. This time, the customer was indeed looking for a standard solution," said Maciej Korecki, vice president of the Vacuum Furnace Segment at the SECO/WARWICK Group.
Induction heat treaters know that proper coil design is crucial to increasing longevity, improving production quality, and cutting costs. The authors of this paper on Coil Design Techniques (C. Yakey, V. Nemkov, R. Goldstein, J. Jackowski) draw on an extensive library of published case histories in induction coil design and performance evaluations and provide their own case study of an automotive CVJ stem hardening coil in order to demonstrate how the elimination of failure points and application of improved design guidelines can result in increased coil lifetimes, even in an inductor that in some circumstances can have a short lifetime.
An excerpt:
“The quality of an induction coil is a major determinant of the cost to produce induction heat treated components. Oftentimes, the difference between a well designed and manufactured inductor and a poor performing inductor is not readily apparent. However, a high-quality induction coil can lead to substantially lower component manufacturing costs and higher profitability for the induction heat treater.”
“Because precision and stability in temperature control are important when welding P91 pipe, induction heating is well-suited to this application for its control and uniformity of heating.”
Source: heatprocessing
