Elements of Induction Heating Copyright © 1988 ASM International® Stanley Zinn, Lee Semiatin, p 1-8 All rights reserved. DOI: 10.1361/eoih1988p001 www.asminternational.org 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 alter- nating-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 work- piece. 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 con- trol 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 "pri- mary," an ac voltage is induced across the other coil, known as the "second- ary." 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 proper- ties 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 prac- tical 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 induc- tion 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 heat- ing 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 appli- cation 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 induc- tion 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 induc- tion 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 appli- cation 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 tubu- lar 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 tech- nology, 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,