Introduction

Chapter 1

Introduction

Electromagnetic induction, or simply "induction," is a method of heating electrically conductive materials such as metals. It is commonly used in process heating prior to metalworking, and in heat treating, welding, and melting (Table 1.1). This technique also lends itself to various other applications involving packaging and curing. The number of industrial and consumer items which undergo induction heating during some stage of their production is very large and rapidly expanding. As its name implies, induction heating relies on electrical currents that are induced internally in the material to be heated-i.e., the workpiece. These

Table 1,1. Induction heating applications and typical products

Preheating prior to metalworking Heat treating Welding Melting Forging Surface Hardening, Seam Welding Air Melting of Steels Gears Tempering Oil-country Ingots Shafts Gears tubular Billets Hand tools Shafts products Castings Ordnance Valves Refrigeration Vacuum Induction Machine tools tubing Melting Extrusion Hand tools Line pipe Structural Ingots members Through Hardening, Billets Shafts Tempering Castings Structural "Clean" steels Heading members Nickel-base Bolts Spring steel superalloys Other fasteners Chain links Titanium alloys Rolling Slab Annealing Aluminum strip Sheet (can, ap- pliance, and Steel strip automotive industries) 2 Elements of Induction Heating: Design, Control, and Applications so-called eddy currents dissipate energy and bring about heating. The basic components of an induction heating system are an induction coil, an alternating-current (ac) power supply, and the workpiece itself. The coil, which may take different shapes depending on the required heating pattern, is con- nected to the power supply. The flow of ac current through the coil gener- ates an alternating magnetic field which cuts through the workpiece. It is this alternating magnetic field which induces the eddy currents that heat the workpiece. Because the magnitude of the eddy currents decreases with distance from the workpiece surface, induction can be used for surface heating and heat treating. In contrast, if sufficient time is allowed for heat conduction, rela- tively uniform heating patterns can be obtained for purposes of through heat treating, heating prior to metalworking, and so forth. Careful attention to coil design and selection of power-supply frequency and rating ensures close control of the heating rate and pattern. A common analogy used to explain the phenomenon of electromagnetic induction makes use of the transformer effect. A transformer consists of two coils placed in close proximity to each other. When a voltage is impressed across one of the coils, known as the primary winding or simply the "primary," an ac voltage is induced across the other coil, known as the "secondary." In induction heating, the induction coil, which is energized by the ac power supply, serves as the primary, and the workpiece is analogous to the secondary. The mathematical analysis of induction heating processes can be quite com- plex for all but the simplest of workpiece geometries. This is because of the coupled effects of nonuniform heat generation through the workpiece, heat transfer, and the fact that the electrical, thermal, and metallurgical properties of most materials exhibit a strong dependence on temperature. For this reason, quantitative solutions exist for the most part only for the heating of round bars or tubes and rectangular slabs and sheets. Nevertheless, such treat- ments do provide useful insights into the effects of coil design and equipment characteristics on heating patterns in irregularly shaped parts. This informa- tion, coupled with knowledge generated through years of experimentation in both laboratory and production environments, serves as the basis for the practical design of induction heating processes. This book focuses on the practical aspects of process design and control, an understanding of which is required for the implementation of actual induction heating operations. The treatment here is by and large of the "hands-on" type as opposed to an extended theoretical discussion of induction heating or equipment design. Chapters 2 and 3 deal with the basics of induction heating and circuit theory only to the degree that is required in design work. With this as a background, subsequent chapters address the questions of equipment selection (Chapter 4), auxiliary equipment (Chapter 5), process design for common applications (Chapter 6), control systems (Chapter 7), and coil design and fabrication (Chapter 8). The concluding chapters address the ques- Introduction 3 tions of special design features (Chapter 9), materials-handling systems (Chap- ter 10), process design for special applications (Chapter 11), and economic considerations (Chapter 12). To introduce the subject, a brief review of the history, applications, and advantages of induction heating is given next.

HISTORY

The birth of electromagnetic induction technology dates back to 1831. In November of that year, Michael Faraday wound two coils of wire onto an iron ring and noted that when an alternating current was passed through one of the coils, a voltage was induced in the other. Recognizing the potential applications of transformers based on this effect, researchers working over the next several decades concentrated on the development of equipment for generating high-frequency alternating current. It was not until the latter part of the 19th century that the practical application of induction to heating of electrical conductors was realized. The first major application was melting of metals. Initially, this was done using metal or electrically conducting crucibles. Later, Ferranti, Colby, and Kjellin devel- oped induction melting furnaces which made use of nonconducting crucibles. In these designs, electric currents were induced directly into the charge, usually at simple line frequency, or 60 Hz. It should be noted that these early induction melting furnaces all utilized hearths that held the melt in the form of a ring. This fundamental practice had inherent difficulties brought about by the mechanical forces set up in the molten charge due to the interaction between the eddy currents in the charge and the currents flowing in the primary, or induction coil. In extreme cases, a "pinch" effect caused the melt to separate and thus break the complete electrical path required for induction, and induction heating, to occur. Problems of this type were most severe in melting of nonferrous metals. Ring melting furnaces were all but superseded in the early 1900's by the work of Northrup, who designed and built equipment consisting of a cylin- drical crucible and a high-frequency spark-gap power supply. This equipment was first used by Baker and Company to melt platinum and by American Brass Company to melt other nonferrous alloys. However, extensive application of such "coreless" induction furnaces was limited by the power attain- able from spark-gap generators. This limitation was alleviated to a certain extent in 1922 by the development of motor-generator sets which could supply power levels of several hundred kilowatts at frequencies up to 960 Hz. It was not until the late 1960's that motor-generators were replaced by solid-state converters for frequencies now considered to be in the "medium-frequency" rather than the high-frequency range.*

*Modern induction power supplies are classified as low frequency (less than approximately 1 kHz), medium frequency (1 to 50 kHz), or high or radio frequency (greater than 50 kHz). 4 Elements of Induction Heating: Design, Control, and Applications

Following the acceptance of induction heating for metal melting, other applications of this promising technology were vigorously sought and devel- oped. These included induction surface hardening of steels, introduced by Midvale Steel (1927) and the Ohio Crankshaft Company (mid-1930's). The former company used a motor-generator for surface heating and hardening of rolling-mill rolls, a practice still followed almost universally today to enhance the wear and fatigue resistance of such parts. The Ohio Crankshaft Company, one of the largest manufacturers of diesel-engine crankshafts, also took advantage of the surface-heating effect of high ac frequencies and used motor-generators at 1920 and 3000 Hz in surface hardening of crankshaft bearings. This was the first high-production application of induction heating for surface heat treating of metals. The wider application to a multiplicity of other parts was an obvious step. For example, the Budd Wheel Company became interested in induction surface hardening of the internal bores of tubular sections and applied this technique to automotive axle hubs and later to cylinder liners. World War II provided a great impetus to the use of induction heating technology, particularly in heat treating of ordnance components such as armor- piercing projectiles and shot. The ability to use induction for local as well as surface hardening was also called upon to salvage over a million projectiles which had been improperly heat treated, yielding local soft spots. In addition, it was found that tank-track components, pins, links, and sprockets could be hardened in large quantities most effectively by high-frequency induction. In a different area, induction heating was applied to preheating of steel blanks prior to hot forging of parts such as gun barrels. In recent years, the application of induction heating and melting has increased to the point where most engineers in the metalworking industries are familiar with existing applications and have some ideas for potential uses. In addition, various nonmetals industries are now beginning to develop a famil- iarity with induction heating principles as they find and develop uses in mak- ing their products. Many of the recent developments have been promoted by the development of high-efficiency solid-state power supplies, introduced in 1967. Over the last several decades, the efficiency of these units has increased to almost 95% in terms of the percentage of line-frequency energy converted to the higher out- put frequency (Fig. 1.1). In terms of equipment cost per kilowatt available for heating, this has actually resulted in a decrease in cost after adjustment for inflation (Fig. 1.2).

APPLICATIONS OF INDUCTION HEATING

As can be surmised from the above discussion, induction heating finds its greatest application in the metals-processing industries (Table 1.1). Primary Introduction 5

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Fig. 1.1. Conversion efficiency of induction heating power supplies (from R. W. Sundeen, Proceedings, 39th Electric Furnace Conference, Houston, TX, AIME, New York, 1982, p. 8)

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Motor- Integral generator motor- generator

540 Hz Early solid state "10 Solid C cO state Variable- O (3 tuned (/) solid O state Present solid state

1948 1953 1958 1963 1968 1973 1978 1981 Year Fig. 1.2. Change in cost of induction heating power supplies since 1948 (from R. W. Sundeen, Proceedings, 3gth Electric Furnace Conference, Houston, TX, AIME, New York, 1982, p. 8) 6 Elements of Induction Heating: Design, Control, and Applications uses fall into the major categories of heating prior to metalworking, heat treating, welding, and metal melting. While these are the most common uses, a variety of other operations, such as paint curing, adhesive bonding, and zone refining of semiconductors, are also amenable to induction heating meth- ods. Each of these applications is briefly discussed below.

Preheating Prior to Metalworking. Induction heating prior to metalworking is well accepted in the forging and extrusion industries. It is readily adapted to through preheating of steels, aluminum alloys, and specialty metals such as titanium and nickel-base alloys. Frequently, the workpieces in these types of applications consist of round, square, or round-cornered square bar stock. For steels, the high heating rates of induction processes minimize scale and hence material losses. The rapid heating boosts production rates. Induction heating is also useful for selectively preheating bar stock for forming operations such as heading.

Heat Treating. Induction heating is used in surface and through hardening, tempering, and annealing of steels. A primary advantage is the ability to control the area that is heat treated. Induction hardening, the most common induction heat treating operation, improves the strength, wear, and fatigue properties of steels. Steel tubular products, for example, lend themselves quite readily to hardening by induction in continuous-line operations. Tempering of steel by induction, although not as common as induction hardening of steels, restores ductility and improves fracture resistance. Also less commonly applied is induction annealing, which restores softness and ductility-impor- tant properties for forming of steels, aluminum alloys, and other metals.

Melting. Induction processes are frequently used to melt high-quality steels and nonferrous alloys (e.g., aluminum and copper alloys). Advantages specific to induction melting as compared with other melting processes include a natu- ral stirring action (giving a more uniform melt) and long crucible life.

Welding, Brazing, and Soldering. High-frequency induction welding offers sub- stantial energy savings because heat is localized at the weld joint. The most common application of induction welding is welded tube or pipe products that lend themselves to high-speed, high-production automated processing. Induc- tion brazing and soldering also rely on the local heating and control capabil- ities inherent in the induction heating process.

Curing of Organic Coatings. Induction is used to cure organic coatings such as paints on metallic substrates by generating heat within the substrate. By this means, curing occurs from within, minimizing the tendency for formation of coating defects. A typical application is the drying of paint on sheet metal. Introduction 7

Adhesive Bonding. Certain automobile parts, such as clutch plates and brake shoes, make use of thermosetting adhesives. As in paint curing, induction heating of the metal parts to curing temperatures can be an excellent means of achieving rapid bonding. Metal-to-nonmetal seals, widely used in vacuum devices, also rely heavily on induction heating.

Semiconductor Fabrication. The growing of single crystals of germanium and sil- icon often relies on induction heating. Zone refining, zone leveling, doping, and epitaxial deposition of semiconductor materials also make use of the induction process.

Tin Reflow. Electrolytically deposited tin coatings on steel sheet have a dull, matte, nonuniform finish. Heating of the sheet to 230 °C (450 °F) by induction causes reflow of the tin coating and results in a bright appearance and uniform coverage.

Sintering. Induction heating is widely used in sintering of carbide preforms because it can provide the necessary high temperature (2550 °C, or 4620 °F) in a graphite retort or susceptor with atmosphere control. Other preforms of ferrous and nonferrous metals can be sintered in a similar manner with or without atmosphere protection.

ADVANTAGES OF INDUCTION HEATING

Prior to the development of induction heating, gas- and oil-fired furnaces provided the prime means of heating metals and nonmetals. Induction heating offers a number of advantages over furnace techniques, such as:

• Quick heating. Development of heat within the workpiece by induction pro- vides much higher heating rates than the convection and radiation processes that occur in furnaces (Fig. 1.3). • Less scale loss. Rapid heating significantly reduces material loss due to scal- ing (e.g., for steels) relative to slow gas-fired furnace processes. • Fast start-up. Furnaces contain large amounts of refractory materials that must be heated during start-up, resulting in large thermal inertia. The internal heating of the induction process eliminates this problem and allows much quicker start-up. • Energy savings. When not in use, the induction power supply can be turned off because restarting is so quick. With furnaces, energy must be supplied continuously to maintain temperature during delays in processing and to avoid long start-ups. • High production rates. Because heating times are short, induction heating often allows increased production and reduced labor costs. 8 Elements of Induction Heating: Design, Control, and Applications

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Bar section, cm

Fig. 1.3. Comparison of times for through heating by induction and gas-fired furnace techniques as a function of bar diameter (from R. Daugherty and A. A. Huchok, Proceedings, 11th Biennial Conference on Electric Process Heating in Industry, IEEE, New York, 1973)

In addition to those listed above, other advantages that induction heating systems offer include: • Ease of automation and control • Reduced floor-space requirements • Quiet, safe, and clean working conditions • Low maintenance requirements.