This chapter provides a brief review of the scientific and technological developments leading to the widespread use of induction heat treating and its many applications in industry.

An induction heating system consists of a source of alternating current (ac), an induction coil, and the workpiece to be heated. This chapter describes the basic phenomena underlying induction heating with respect to the interactions between the coil and the workpiece. The chapter reviews the mechanistic basis for induction heating and provides an example of eddy-current distribution in a solid bar. The chapter defines two important concepts in the technology of induction heating: equivalent resistance and electrical efficiency. The chapter concludes with a discussion of methods for determination of power requirements for a given application.

This chapter focuses on the transfer of energy between the power supply and the induction heating coil. The most efficient transfer requires that the induction heated load and coil be matched to the power supply and that the electrical circuit containing these elements be properly tuned. The chapter describes these procedures, including the processes involved in tuning induction heating circuits and load matching, impedance matching by means of a transformer, and tuning used for specific types of power supplies.

Besides the induction coil and workpiece, the induction generator (source of ac power) is probably the most important component of an overall induction heating system. Such equipment is typically rated in terms of its frequency and maximum output power (in kilowatts). This chapter addresses the selection of power supplies in terms of these two factors as well as the operational features of different types of sources. The six different types of power supplies for induction heating applications covered in this chapter are line-frequency supplies, frequency multipliers, motor-generators, solid-state (static) inverters, spark-gap converters, and radio-frequency power supplies. The chapter discusses the design and characteristics of each of the various types of power supplies.

This chapter describes two types of auxiliary equipment required in most induction heating installations: cooling systems and device timers. Water- and vapor-based systems used for cooling the power supply and the induction coil are described. The chapter concludes with a brief discussion of timers, with emphasis on open-loop timing systems.

Induction heating has found widespread use as a method to raise the temperature of a metal prior to forming or joining, or to change its metallurgical structure. However, induction heating has specialized capabilities that make it suitable for applications outside of metal treatment and fabrication. This chapter summarizes some of the special applications of induction heating, including those in the plastics, packaging, electronics, glass, chemical, and metal-finishing industries. The chapter concludes with a discussion of the application of induction heating for vacuum processes.

This chapter discusses the basic principles of induction heating and related engineering considerations. It describes the design and operation of induction coils, the magnitude and distribution of magnetic fields, and the forces that generate eddy currents in metals. It explains how induced electrical current causes metal to heat in proportion to their electrical resistance and how it affects temperature dependent properties such as resistivity and specific heat and, in turn, heating rates and efficiencies. It also discusses the effect of hysteresis and explains why eddy currents tend to be confined to the outer surface of the workpiece, a phenomenon known as the skin effect. The chapter includes several data plots showing how the depth of heating varies with frequency and how heating time, power density, and thermal conduction rate correspond with hardening depth.