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Induction and flame hardening are methods of hardening the surfaces of components, usually in selected areas, by the short-time application of high-intensity heating followed by quenching. These processes are used when gear teeth require high hardness, but size or configuration does not lend itself to carburizing and quenching the entire part. This chapter focuses on the processes involved in the induction and flame hardening, covering the applicable materials, hardening patterns, preheat treatment, quenching, tempering, surface hardness, case depth, hardening problems, dual-frequency process, and applications.

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.

This chapter discusses the quenching process and its adaptation to induction heat treating. It describes the three stages of quenching, the cooling characteristics of various types of quenchants, and the details of nearly a dozen compatible quenching methods. It also explains how to verify whether a quenchant can cool a workpiece fast enough to achieve martensitic transformation without cracking or distortion.

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.

Electromagnetic induction, or simply "induction," is a method of heating electrically conductive materials such as metals. It is commonly used for heating workpieces prior to metalworking and in heat treating, welding, and melting. This technique also lends itself to various other applications involving packaging and curing of resins and coatings. This chapter provides a brief review of the history of induction heating and discusses its applications and advantages.

To a large extent, the induction coil and its coupling to the workpiece determine the precise heating pattern that is developed. However, it is often desirable to modify this pattern in order to produce a special heating distribution or to increase energy efficiency. At other times, the high heating rates of induction are needed for processing nonconductors. This chapter describes broad methods of accomplishing such objectives: modification of the field of magnetic induction, use of devices to prevent auxiliary equipment or certain portions of a workpiece from being heated, and techniques to apply heating to electrically nonconductive materials. These methods make use of devices such as flux concentrators, shields, and susceptors. The chapter provides a description of the materials for these devices and guidelines for their application.

This appendix provides practical information on induction coils and how they are made. It discusses soldering methods, preferred materials, design challenges, and best practices and procedures. It also discusses the design, construction, and application of magnetic flux concentrators and the growing use of computer simulation.

Induction heating is a rapid, efficient technique for producing localized or through heating in a wide range of industries. The economics as well as the technical feasibility of induction heating should be important considerations prior to investing in such a system. A number of cost elements enter into the analysis. These include equipment and energy costs, production lot size and ease of automation, material savings, labor costs, and maintenance requirements. This chapter discusses each of these factors. It compares the cost elements of induction heating with those of its main competitor, gas-fired furnace heating. Several typical examples are provided to illustrate the economic considerations in design and application of induction heating processes.

This chapter discusses the design and operating principles of various types of electromagnetic coils. It explains how induction coils are classified based on the direction of the eddy currents they induce in the workpiece and the corresponding orientation, whether longitudinal or transverse, of the associated magnetic flux. It then discusses the factors that influence coil design and selection, including coupling efficiency, frequency, the number and spacing of turns, and the use of flux intensifiers. It also includes images and illustrations of various types of coils and coil geometries for basic as well as special purpose applications.

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 discusses the selection, use, and integration of methods to control process variables in induction heating, including control of workpiece and processing temperature and materials handling systems. The discussion of temperature control includes a review of proportional controllers and heat-regulating devices. Integration of control functions is illustrated with examples related to heating of steel slabs, surface hardening of steel parts, vacuum induction melting for casting operations, and process optimization for electric-demand control. Distributed control within larger manufacturing systems is discussed. The chapter also covers nondestructive techniques for process control and methods for process simulation.

Because of its speed and ease of control, induction heating can be readily automated and integrated with other processing steps such as forming, quenching, and joining. Completely automated heating/handling/control systems have been developed and are offered by induction equipment manufacturers. This chapter deals with materials handling and automation. First, it summarizes basic considerations such as generic system designs, fixture materials, and special electrical problems to be avoided. Next, it describes and provides examples of materials-handling systems in induction billet heating, bar heating, heat treatment, soldering, brazing, and other induction-based processes. The final section discusses the use of robots for parts handling in induction heating 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.

Mechanical components often require surface treatments to meet application demands. This chapter describes several surface hardening treatments for steel and their effect on microstructure, composition, and properties. It discusses flame hardening, induction heating, carburizing, nitriding, carbonitriding, and nitrocarburizing. The discussion on carburizing addresses several interrelated factors, including processing principles, alloying, surface oxidation, residual stresses, bending fatigue, contact fatigue, and fracture.

This chapter describes case depth and discusses flame hardening, laser heat treatment, electron beam hardening, induction heat treatment, and induction hardening.

Coil design for induction heating has been developed and refined over time based on the theoretical principles applied in practice to several simple inductor geometries such as the classical solenoidal coil. This chapter reviews the fundamental considerations in the design of inductors and describes some of the most widely used coils and common design modifications. Specialty coil designs for specific applications are also discussed. The chapter concludes with sections devoted to coil fabrication and design of power-supply leads.

The detailed heating requirements for specific applications must be considered before construction and implementation of any induction heating process. These requirements may include considerations such as type of heating, throughput and heating time, workpiece material, peak temperature, and so forth. The major applications of induction technology include through heating, surface heating (for surface heat treatment), metal melting, welding, brazing, and soldering. This chapter summarizes the selection of equipment and related design considerations for these applications.