This chapter describes surface modification processes that go beyond conventional heat treatments, including plasma nitriding, plasma carburizing, low-pressure carburizing, ion implantation, physical and chemical vapor deposition, salt bath coating, and transformation hardening via high-energy laser and electron beams. The chapter compares methods and includes several example applications.

Gas (atmosphere) carburizing is the de facto standard by which all other surface hardening techniques are measured and is the emphasis of this chapter. Initially, the chapter describes the process and equipment for gas carburizing. This is followed by sections discussing the processes involved in quenching, hardening, tempering, recarburizing, and cold treatment of carburized and quenched gears. Next, the chapter reviews the selection process of materials for carburized gears and provides information on carbon content, properties, and core hardness of gear teeth. The problems associated with carburizing are then covered, followed by the processes involved in heat treat distortion and shot peening of carburized and hardened gears. Information on grinding stock allowance on tooth flanks to compensate for distortion is also provided. The chapter further discusses the applications of carburized and hardened gears. Finally, it reviews vacuum carburizing and compares the attributes of conventional gas carburizing and vacuum carburizing.

This chapter discusses vacuum furnace processes and their design. The chapter focuses on the pressure levels, heating elements, pumping systems, temperature control systems, and quenching systems in vacuum furnaces. It then details specific applications of vacuum heat treating.

This chapter focuses on resistance characteristics and methods of protecting different forms of high-temperature gaseous corrosion, namely high-temperature oxidation, sulfidation, carburization, hydrogen effects, and hot corrosion.

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.

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 focuses on gas carburizing. It includes a history of carburizing and explains the objectives of the carburizing process. The chapter discusses atmospheres, sources of carbon, reactions in the gas carburizing process, and effects of process variables such as temperature and time. Furnaces for gas carburizing are described and potential microstructure problems are introduced.

This chapter discusses the conditions under which carburization and metal dusting occur. It describes the chemical reactions and thermodynamic relationships that drive carburization and metal dusting attack and the factors that determine the amount of damage that metals and alloys are likely to sustain. The chapter also explains how carburization affects creep strength and fracture toughness, and how surface conditions and finish and the presence of sulfur affect metal dusting behaviors.

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.

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.

This chapter begins with a review of some of the terms used in the gear industry to describe the design of gears and gear geometries. It then discusses the types of gears that operate on parallel shafts, intersecting shafts, and nonparallel and nonintersecting shafts. Next, the processes involved in the selection of gear are discussed, followed by information on the basic stresses applied to a gear tooth, the strength of a gear tooth, and the most widely used gear materials. Further, the chapter briefly reviews gear manufacturing methods and the heat treating processing steps including prehardening processes, through hardening, and case hardening processes.

This chapter discusses furnace atmospheres. It describes how furnace atmospheres protect metals, transfer heat, and supply alloying elements (carbon and nitrogen). The chapter focuses on the different types of atmospheres that are available to the heat treater: combustion products, air, exothermic, salt, nitrogen, endothermic, ammonia, hydrogen, inert gas, and vacuum.

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.

Mold steels are used for plastic molding and certain die-casting applications and are designated as group P steels in the AISI classification system. The fabrication and performance requirements that differentiate them from other types of tool steels are described in this chapter. It provides information on hubbing and machined cavity grades of mold steels and describes the performance of the corrosion-resistant mold steels. The chapter discusses the processes involved in forging, annealing, stress relieving, carburizing, hardening, and tempering of mold steels. It presents the selection criteria and applications of mold steels.

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 reviews the knowledge of the field of gear tribology and is intended for both gear designers and gear operators. Gear tooth failure modes are discussed with emphasis on lubrication-related failures. The chapter is concerned with gear tooth failures that are influenced by friction, lubrication, and wear. Equations for calculating lubricant film thickness, which determines whether the gears operate in the boundary, elastohydrodynamic, or full-film lubrication range, are given. Also, given is an equation for Blok's flash temperature, which is used for predicting the risk of scuffing. In addition, recommendations for lubricant selection, viscosity, and method of application are discussed. The chapter discusses in greater detail the applications of oil lubricant. Finally, a case history demonstrates how the tribological principles discussed in the chapter can be applied practically to avoid gear failure.

This chapter familiarizes readers with tempering and refrigeration treatments and their effect on case-carburized parts. It explains how tempering makes such parts easier to machine, more structurally and dimensionally stable, and more durable in certain applications. It identifies key process parameters and provides test data showing how they affect hardness, yield strength, bending and contact fatigue, and fracture toughness. It also addresses potential problems stemming from process-related factors such as the presence of hydrogen and the effects of aging and grinding. In regard to refrigeration, the chapter explains that it is not uncommon for subzero treatments to be included in the production of carburized parts whether as a standard procedure or optional step. Subzero cooling promotes the transformation of retained austenite to martensite, thereby increasing surface hardness and reducing the propensity of quenched carburized steels to burn and crack during surface grinding. The chapter includes numerous data plots and tables showing how the various treatments influence hardness, wear resistance, tensile properties, and fatigue and fracture behaviors.

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.

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.

The appendix provides a glossary of practical heat treating terms.