The design requirements for mechanical shafts, pinions, and gears often call for features with very hard surfaces (to resist wear) based on a softer core (to avoid brittle fracture). This chapter explains how to selectively harden steel by diffusing carbon and nitrogen atoms into the outer surface layers. It discusses several such surface-hardening processes, including carburizing, nitriding, carbonitriding, and nitrocarburizing.

This chapter discusses the process characteristics, advantages, disadvantages, and applications of various processes involved in surface hardening of steel. These include pack carburizing, liquid carburizing, gas carburizing, vacuum carburizing, plasma carburizing, gas nitriding, liquid nitriding, carbonitriding, and hardfacing. The chapter describes two surface hardening processes by localized heat treatment: flame hardening and induction hardening. It also briefly summarizes other surface hardening processes, namely, aluminizing, siliconizing, chromizing, titanium carbide coatings, and boronizing.

This chapter describes important requirements for ferrous and nonferrous alloys used for gears. Wrought surface-hardening and through-hardening carbon and alloy steels are the most widely used of all gear materials and are emphasized in this chapter. The processing characteristics of gear steels and the bending fatigue strength and properties of carburized steels are reviewed. In addition to wrought steels, the chapter provides information on the other iron-base alloys that are used for gears, namely cast carbon and alloy steels, gray and ductile cast irons, powder metallurgy irons and steels, stainless steels, and tool steels. In terms of nonferrous alloys, the chapter addresses copper-base alloys, die cast aluminum alloys, zinc alloys, and magnesium alloys.

Induction heating, in most applications, is used to selectively heat only a portion of the workpiece that requires treatment. This chapter covers the basic principles, features, and metallurgical aspects of induction heating. The discussion includes the conditions required for induction heating and quenching, the use of magnetic flux concentrators to improve the efficiency of surface heating, and the quenching systems used for induction hardening. The discussion also provides information on time-temperature dependence in induction heating, workpiece distortion in induction surface hardening, residual stresses after induction surface hardening and finish grinding, and input and output control of steel for induction surface hardening of gears.

This chapter discusses the composition and classification of stainless steels and focuses on the processes involved in heat treatment and applications of these steels. The wrought and the cast stainless steels covered are ferritic, austenitic, duplex (ferritic-austenitic), martensitic, and precipitation-hardening. In addition, information on special considerations for stainless steel castings is also provided. The heat treatment processes explained in the chapter are preheating, annealing, stress relieving, hardening, tempering, austenite conditioning, heat aging, and nitride surface hardening. Finally, some special considerations for stainless steel castings are discussed.

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.

Cast irons, like steels, are iron-carbon alloys but with higher carbon levels than steels to take advantage of eutectic solidification in the binary iron-carbon system. Like steel, heat treatment of cast iron includes stress relieving, annealing, normalizing, through hardening, and surface hardening. This chapter introduces solid-state heat treatment of iron castings, covering general considerations for heat treatment and discussing the processes, advantages, and disadvantages of heat treatment of cast iron.

This chapter discusses the processes used in manufacturing to thermally alter the properties of metals and alloys. It begins with a review of the iron-carbon system, the factors that affect hardenability, and the use of continuous cooling transformation diagrams. It then explains how various steels respond to heat treatments, such as annealing, normalizing, spheroidizing, tempering, and direct and interrupted quenching, and surface-hardening processes, such as flame and induction hardening, carburizing, nitriding, and carbonitriding. It also addresses the issue of temper embrittlement and discusses the effect of precipitation hardening on aluminum and other alloys.

The primary objective of carburizing and hardening gears is to secure a hard case and a relatively soft but tough core. For this process, low-carbon steels (up to a maximum of approximately 0.30% carbon), either with or without alloying elements (nickel, chromium, manganese, molybdenum), normally are used. The processes involved in hardening, tempering, recarburizing, and cold treatment of carburized and quenched gears are discussed. Next, the chapter reviews the selection of materials for carburized gears and considerations related to carbon content, core hardness, and microstructure. This is followed by sections discussing some problems that can be experienced in the carburizing process and how these can be addressed, including a section on shot peening to induce compressive residual stress at and below the surface. It then discusses the applications of carburized gears and finally presents a case history of distortion control of carburized and hardened gears.

Nitriding is a surface hardening heat treatment that introduces nitrogen into the surface of steel while it is in the ferritic condition. Gas nitriding using ammonia as the nitrogen-carrying species is the most commonly employed process and is emphasized in this chapter. Nitriding produces a wear- and fatigue-resistant surface on gear teeth and is used in applications where gears are not subjected to high shock loads or contact stress. It is useful for gears that need to maintain their surface hardness at elevated temperatures. Gears used in industrial, automotive, and aerospace applications are commonly nitrided. This chapter discusses the processes involved in gas, controlled, and ion nitriding.

Steels that resist corrosive attack from normal atmospheric exposure and contain a minimum of 10.5% Cr and 50% Fe are generally classified as stainless steels. Their special qualities lie in a chromium-rich oxide surface film that quickly regrows when damaged. This chapter discusses the classification, composition, properties, treatments, and applications of austenitic, ferritic, martensitic, duplex, precipitation-hardening, powder metallurgy, and cast stainless steels. It also reviews the history of stainless steels and provides information on alloy designation systems.

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.

This chapter addresses the issue of retained austenite in quenched carburized steels. It explains why retained austenite can be expected at the surface of case-hardened components, how to estimate the amount that will be present, and how to effectively stabilize or otherwise control it. It presents detailed images and data plots showing how retained austenite appears and how it influences hardness, tensile properties, residual stresses, fatigue and fracture behaviors, and wear resistance.

This chapter discusses the factors that affect die steel selection for hot forging, including material properties such as hardenability, heat and wear resistance, toughness, and resistance to plastic deformation and mechanical fatigue. It then describes the relative merits of various materials and the basic requirements for cold forging dies. The chapter also covers die manufacturing processes, such as high-speed and hard machining, electrodischarge machining, and hobbing, and the use of surface treatments.

This chapter discusses hardening processes that involve changes in surface composition. These case hardening treatments are broadly classified into four groups: carburizing, carbonitriding, nitriding, and nitrocarburizing. Key parameters and operating considerations for each treatment are discussed.

The relationship of stress and strength gradients must be considered simultaneously in analysis of a particular type of fracture. This chapter discusses the principal elastic stress distribution in members of various shapes under different types of pure loads. A basic understanding of both the stress and strength gradients of metal parts with and without stress concentrations and under different types of loading is provided. The chapter also describes the effect of service conditions on applied stresses.

This chapters discusses the considerations involved in the qualification and analysis of induction hardening treatments. The discussion covers material selection and prior heat treatment, hardness and case depth, frequency selection, power density and heating time, part and process tolerances, geometrical effects, quenchant selection, coil design, and work-handling equipment. The chapter also presents several examples, walking readers though each step, and discusses the development of setup instructions and operating procedures.

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.

This chapter discusses the processes involved in the heat treatment of steel, namely austenitizing, hardening, quenching, and tempering. It begins with an overview of austenitizing of steels by induction heating, followed by a discussion on the processes involved in transformation of the soft austenite into martensite or lower bainite in the hardening operation. The chapter provides information on various quenching systems and a description of quenching techniques, namely austempering, martempering, and patenting. Difficulties associated with hardening of steel are discussed. Further, the chapter describes the equipment used for and principal variables of tempering. It discusses the causes for various forms of embrittlement due to tempering. Information on multiple tempering, protective-atmosphere tempering, and selective tempering are also provided, along with processes involved in selection of tempering temperature. The chapter ends with a section discussing various effects, advantages, and disadvantages of precipitation hardening.

This chapter discusses surface engineering treatments, including flame hardening, induction hardening, high-energy beam hardening, laser melting, and shot peening. It describes the basic implementation of each method, the materials for which they are suited, and their effect on surface metallurgy.