This chapter provides information on various contributors to failure of carburized and carbonitrided components, with the primary focus on carburized components. The most common contributors covered include component design, selection of proper hardenability, increased residual stress, dimensional stability, and generation of quenching and grinding cracks. They also include insufficient case hardness and improper core hardness, influence of surface carbon content and grain size, internal oxidation, structure of carbides, and inclusion of noncarbide. Details on micropitting, macropitting, case crushing, pitting corrosion, and partial melting are also provided.

This chapter discusses the various factors influencing the evaluation of fatigue fracture of nitrided layers. It begins by describing the problems of enhancing the fatigue resistance of machine components. The significance and detailed assessment of the effect of a structural flaw are then explained, using investigations of the effect of variable core conditions on fatigue resistance as an example. This is followed by a discussion on the processes involved in the evaluation of fatigue properties of nitrided steels. The chapter also describes the determination of the fatigue characteristics of nitrided steels after the carbonitriding treatment.

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.

Gaseous ferritic nitrocarburizing, like salt bath nitrocarburizing, involves the introduction of carbon and nitrogen into steel in order to produce a thin layer of iron carbonitride and nitrides, the "white layer" or compound layer, with an underlying diffusion zone containing dissolved nitrogen and iron (or alloy) nitrides. This chapter first presents the development and principles of the process. It then discusses the properties of gaseous ferritic nitrocarburized components. The chapter also presents the applications for the ferritic nitrocarburizing process. It provides an overview of the safety considerations.

Carbonitriding introduces and diffuses atomic nitrogen into the surface steel during carburization. This chapter focuses on case composition of a carbonitrided case, case depth, case hardenability, hardness gradients, void formation, and applications of carbonitriding. The chapter discusses furnaces suitable for carbonitriding, atmosphere constituents, batch furnace atmospheres, continuous furnace atmospheres, safety, temperature selection, quenching, tempering, and hardness testing of a carbonitrided case.

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 reviews failure aspects of structural ferrous powder metallurgy (PM) parts, which form the bulk of the PM industry. The focus is on conventional PM technology of parts in the density range of 6 to 7.2 g/cc. The chapter briefly introduces the processing steps that are essential to understanding failure analysis of PM parts. This is followed by a section on case hardening of PM parts. The methods used for analyzing the failures are then discussed. Some case studies are given that illustrate different failures and the methods of prevention of these failures.

Medium-carbon steels are typically hardened for high-strength, high-fatigue-resistant applications by austenitizing, quenching to martensite, and tempering. This chapter explains how microalloying with vanadium, niobium, and/or titanium provides an alternate way to improve the mechanical properties of such steels. It also addresses microalloyed forging steels and explains how nontraditional bainitic microstructures can be produced by direct cooling after forging.

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 contains a list of technical papers and books related to the heat treatment of gears.

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 details suitable steels for gas nitriding and discusses conventional gas nitriding, plasma (Ion) nitriding, the ferritic nitrocarburizing processes, gaseous ferritic nitrocarburizing, plasma nitrocarburizing, and the salt-bath ferritic nitrocarburizing processes.

This article discusses the applications, compositions, and properties of cemented carbides and cermets. It explains how alloying elements, grain size, and binder content influence the properties and behaviors of cemented carbides. It also discusses the properties of steel-bonded carbides, or cermets, the various grades available, and the types of applications for which they are suited.

This chapter provides practical information on surface treatments that work by altering the surface chemistry of metals and alloys. It discusses the use of phosphate and chromate conversion coatings as well as anodizing, steam oxidation, diffusion coatings, and pack cementation. The chapter also covers ion implantation and laser alloying.

Heat treatment of steel involves a number of processes to condition the microstructure and obtain desired properties. This includes various methods namely, annealing, normalizing, and hardening by quenching and tempering. This chapter focuses on general heat treatment procedures and the applications of particular types or grades of carbon and low-alloy steels. The discussion covers carbon steel classification for heat treating, tempering of quenched carbon steels, and austempering of steel. In addition, the chapter discusses the effects of alloying and hardenability on steel and provides information on martempering of steel.

A working knowledge of diffusion is necessary to understand and predict the behavior of metals and alloys during manufacturing and in certain types of service. This chapter covers the fundamentals of diffusion in solids and some of the applications in which diffusion plays a role. It discusses the mechanisms behind interstitial, substitutional, grain boundary, and surface diffusion, the derivation and use of Fick’s laws, and the basic principles of diffusion coating processes, including carburizing, nitriding, nitrocarburizing, cyaniding, carbonitriding, boriding, aluminizing, siliconizing, chromizing, vanadizing, and titanizing. It also discusses diffusion bonding and presents several approaches for dealing with oxide barrier problems.

This chapter discusses the thermally induced changes that occur on the surface of steel exposed to different environments. It explains how oxide scales form during heat treating and how factors such as temperature, composition, and surface finish affect growth rates, grain structure, and uniformity. It provides examples of oxides that form beneath the surface of steel and explains why it occurs. It describes the conditions associated with decarburization and explains how to determine the depth of decarburized layers in eutectoid, hypoeutectoid, and hypereutectoid steels. It also discusses the carburizing process, the factors that determine the depth and gradient of the carburized case, the effect of post-process treatments, and a variation on the process known as ferritic carbonitriding.

Distortion is defined as an irreversible and usually unpredictable dimensional change in a component due to thermal processing or temperature variations and loading in service. This chapter describes two types of distortion: size distortion and shape distortion. It addresses how distortion can be managed by controlling certain factors. The chapter discusses the cause and effect of distortion during nitriding, the processes involved in stock removal prior to nitriding, and the criteria for post-machining operations.

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.