INDUCTION HEATING EQUIPMENT TECHNICAL CONTENT

Dr. Valery Rudnev on . . . Equipment Selection for Induction Hardening: Continuous and Progressive Hardening, Part 2

This article continues the ongoing discussion on Equipment Selection for Induction Hardening by Dr. Valery Rudnev, FASM, IFHTSE Fellow. Dr. Rudnev previously reviewed equipment selection for scan hardening in three parts. The first part on equipment selection for continuous and progressive hardening is here; the third part is here. To see the earlier articles in the Induction Hardening series at Heat Treat Today as well as other news about Dr. Rudnev, click here


Frequency Selection

Depending on the application specifics, continuous and progressive hardening lines may use the same frequency for various in-line coils. In other cases, power levels and frequencies may be different at different heating positions. The presence of three general process stages (described in Part 1) makes a marked impact on a selection of process parameters and the design of an induction system.

When using different frequencies for the various heating stages, the coil design may need to change as well (e.g., a number of coil turns may need to be adjusted for load matching purpose). Just as the eddy current penetration depth in the heated part is affected by the frequency, the current flow in the inductor is affected as well. The wall thickness of the inductor turns (i.e., copper tubing wall) might need to be adjusted to accommodate different frequencies to maximize the coil electrical efficiency.¹

The wall thickness of an inductor’s heating face should be increased as frequency decreases. It is highly desirable for the current-carrying copper wall thickness to be 1.6 times greater than the current penetration depth in the copper (δCu). Increased kilowatt losses in the copper, which are associated with reduced electrical efficiency and greater water-cooling requirements, will occur if the wall is thinner than 1.6∙δCu. 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.).

As an example, Figure 1 shows a number of continuous in-line multi-coil systems for induction heat treating wire products.²

Several continuous in-line systems for heat treating wire products (Courtesy of Radyne Corp., and Inductotherm Heating & Welding, UK. Both are Inductotherm Group companies.)

There are noticeable benefits of compact induction systems compared to fluidized beds, infrared heaters, and gas furnaces, such as quick response and the ability to provide a rapid change in the process operating parameters to accommodate the required temperature of the wire/cable being processed at speeds up to 5 mps. Frequencies that are in the range of 10 to 800 kHz are commonly applied. A dual-frequency concept can be beneficial to enhance electrical efficiency of while heating different diameters/thicknesses or it can be advantageous for through heating of metallic alloys that exhibit low toughness/high brittleness.

According to the dual-frequency concept, a lower frequency is used during the initial heating stage when the steel is magnetic. In the final heating stage, when the steel becomes nonmagnetic with significantly increased current penetration depth δsteel and becomes substantially more ductile, it is beneficial to use a higher frequency.

Case study¹:

As an example, consider the induction heating of a 1/8 inch-diameter (3.2 mm-diameter) steel rod from ambient to 2000°F (1100°C) using both a single 10-kHz frequency and dual 10-kHz/200-kHz frequencies (see Figure 2). When using the single frequency of 10 kHz (Figure 2, left), the rod’s final temperature experiences very little change regardless of the coil power that is increased more than fivefold (from 17 to 90 kW). The only noticeable difference is related to the initial slope of the temperature-time curve, where the steel is ferromagnetic. Upon reaching the Curie point, there is no noticeable temperature rise. This is the result of severe eddy current cancellation making the steel rod transparent (practically speaking) to the electromagnetic field of the induction coil.

Illustration of the dual-frequency concept when induction heating a 1/8 inch-diameter (3.2 mm-diameter) carbon steel rod from room temperature to 2012°F (1100°C) using both a single frequency of 10 kHz (a) and dual frequencies of 10 kHz/200 kHz (b). (Source: V.Rudnev, Systematic analysis of induction coil failures, Part 11c: Frequency selection, Heat Treating Progress, January/February, ASM Intl., 2008, pp. 27–29.)

In contrast, Figure 2, right, shows that a dual-frequency approach provides a remarkable improvement in the ability to heat the rod above the Curie temperature. A power of 14 kW/10 kHz was used to heat the rod below the Curie point and a power of 19 kW/200 kHz was used above it. The total required power is only 33 kW, compared with 90 kW using just 10 kHz, which was still unable to provide the required temperature rise.

Note: The target temperature of 2000°F (1100°C) is above typical target temperatures when hardening plain carbon or low alloy steels and it is more suitable for hot forming applications. This temperature was selected here to better illustrate a dual-frequency concept and the importance of avoiding eddy current cancellation when choosing operating electrical frequencies. It should be noted though that it is not unusual that the heat treating protocols/recipes for some alloyed steels and stainless steels may require target temperatures of 1900°F to 2100°F (1050°C to 1150°C) range.

In some not too often cases, three frequencies may be used. Lower frequency is applied for preheating inductors, a medium frequency is used for mid-heat inductors, and a high frequency is used for final heat inductors.

Sometimes, it is required that the induction system should be able to heat a variety of sizes using a single frequency. In these cases, in order to provide efficient steel heating, it is necessary to choose a frequency that will guarantee that the “diameter-to-current penetration depth (δsteel)” ratio exceeds 3.6 for any workpiece diameter or heating stage. Thus, it is important to remember that when calculating δsteel, the values of electrical resistivity and relative magnetic permeability of the heated material should correspond to their values at the highest temperature that occurs during the entire heating cycle.

The next installment of this column will review a variety of styles of inductors used in continuous and progressive induction hardening applications.

 

 

References

  1. V.Rudnev, D.Loveless, R.Cook, Handbook of Induction Heating, 2nd Edition, CRC Press, 2017.
  2. J.Mortimer, V.Rudnev, D.Clowes, B.Shaw, “Intricacies of Induction Heating of Wires, Rods, Ropes, and Cables”, Wire Forming, Winter, 2019, p.46-50

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.

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Dr. Valery Rudnev on . . . Equipment Selection for Induction Hardening: Continuous & Progressive Hardening, Part 1

This article continues the ongoing discussion on Equipment Selection for Induction Hardening by Dr. Valery Rudnev, FASM, IFHTSE Fellow. Previously, Dr. Rudnev reviewed equipment selection for scan hardening in three parts. This first installment in a new sub-series addresses equipment selection for continuous and progressive hardening. The second part in this series on equipment selection for continuous and progressive hardening is here; the third part is here. To see the earlier articles in the Induction Hardening series at Heat Treat Today as well as other news about Dr. Rudnev, click here


Introduction

The hardening of steels, cast irons, and P/M materials represent the most popular application of induction heat treatment. There are four primary methods for induction hardening [1]:

  • Scan hardening,
  • Continuous and progressive hardening,
  • Static hardening, and
  • Single-shot hardening.

These methods are related to the heating mode, essentials of inductor design, part geometry, and processing specifics. The previous three installments of this column, “Dr. Valery Rudnev on …”, discussed select subtleties associated with induction scan hardening. This article is devoted to continuous and progressive induction hardening techniques.

Continuous and Progressive Hardening

This method is commonly applied when heat treating elongated workpieces, such as bars, tubes, rods, wires, plates, beams, pins, and others. Long parts are more readily processed in a horizontal manner and heated as they progressively pass through multiple inductors. Inductors are positioned in-line or side by side. Each inductor may have a different design and power/frequency setting. This type of hardening is not limited to horizontally processed parts; vertical processing and arrangements at certain angles are also possible, if suitable.

There are also cases when a workpiece is statically heated to a certain temperature and then progressively moved to another heating position or static inductor for the next heating stage. These processes are referred to as progressive processing/heat treatment.

Induction practitioners sometimes consider continuous or progressive horizontal hardening systems as horizontal scanners. The difference is vague and it is a matter of terminology. Some heat treaters feel that it would be appropriate to differentiate these systems based on the number of inductors included in the induction machine design. Horizontal systems consisting of a single inductor are commonly referred to as horizontal scanners. In contrast, if a system consists of two or more heat treat inductors, then it might be referred to as a continuous or progressive heat treat system.

With the continuous hardening method, the workpiece is moved in continuous motion through a number of in-line inductors. Multiturn solenoid coils and, to lesser a degree, channel-style inductors and split-return inductors are most typically used in continuous heat treating lines. As an example, Figure 1 shows a side view of a horizontally arranged continuous induction system consisting of three in-line coils. Each coil consists of three turns.

Figure 1

As another example, Figure 2 shows a top view of a continuous heat treating line that comprises four in-line hardening coils and a spray quench device positioned after the last inductor. Workpieces (e.g., bars, shafts, rods, pins, etc.) are processed end-to-end through the inductors in a continuous motion.

Figure 2

Progressive multi-stage hardening is used when multiple workpieces are moved (via a pusher, indexing mechanism, robot, walking beam, etc.) through a number of coils. Therefore, the entire component or its portions are sequentially heated (in a progressive manner) at certain predetermined heating stages inside the in-line horizontal (being more typical) induction heater or a multi-position horizontal or vertical heater where coils are positioned side by side.

Continuous or progressive hardening methods are typically used for through hardening of elongated or moderate-length parts processing end to end and, to a lesser degree, for surface hardening. Outside diameters for case hardening (surface hardening) usually vary from 1/2 in. (12 mm) to 4 in. (100 mm). In through hardening applications of solid cylinders, the diameters may be as small as 1/8 in. (3 mm).

It is possible to recognize three heating stages in through hardening applications [1]:

  1. Initial or magnetic stage,
  2. Interim stage, and
  3. Final heating stage.

Initial or magnetic stage. Temperatures anywhere within the workpiece are below the A2 critical temperature (Curie point); thus, the steel is ferromagnetic and the current penetration depth is typically quite small. Skin effect is fairly pronounced at this stage and the heat source distribution resembles a conventional exponential distribution. The maximum power density is located at the surface and sharply decreases toward subsurface and the core. Heat source generation is localized by the fine surface layer of the workpiece. This leads to a rapid increase in temperature at the surface with a minor change in the core. This stage is characterized by high electrical efficiency often reaching 90% or so.

Interim stage. During this stage, the austenized surface layer and near-surface area is heated above the A2 critical temperature; however, the internal region, having temperatures below the Curie point, retains its ferromagnetic properties. At this stage, the power density distribution along the radius has a unique non-exponential “wave-like” distribution, which is very different from the commonly assumed exponential distribution. The cause for this behavior has been explained in Ref.1.

Final heating stage. The thickness of the austenized surface layer that exhibits nonmagnetic properties becomes greater than the current penetration depth in hot steel at a given frequency, and the “wavelike” distribution disappears. The classical exponential power density distribution will then take place. As expected, heat source generation depth has increased dramatically compared to an initial stage resulting in a more in-depth heating effect. With time, the core temperature exceeds the Curie point and the entire cross section will be nonmagnetic.

In surface hardening applications, there are typically only the first two heating stages.

Depending on the application specifics, the same frequency may be used for various coils or process stages. In other cases, power levels and frequencies may vary at the different heating stages. The presence of above-described process stages makes a marked impact on a selection of process parameters and design of an induction system and will be discussed in the next installment of this column.

References

1. 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.

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Good Design Practices Lengthen Induction Tooling Life

 

Source: Fluxtrol.com

 

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.”

Read more: “Best Practice for Design and Manufacturing of Heat Treating Inductors”

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