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 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.

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.

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 introduces the principal heat treating processes, namely normalizing, annealing, stress relieving, surface hardening, quenching, and tempering. An overview of four of the more popular surface hardening treatments, namely carburizing, carbonitriding, nitriding, and nitrocarburizing, is provided.

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 results of certain heat treating processes must be verified for case quality and case depth by destructively sectioning a part or parts that were subjected to the process. Test coupons or test pins are often used for diffusion processes such as carburizing, carbonitriding, nitriding, and ferritic nitrocarburizing to provide an accurate heat treating process evaluation. This appendix briefly describes the advantages and selection and design considerations of test coupons. A typical example of the use of test pins for monitoring carburizing and hardening of gears is provided.

Carbonitriding is a modified form of gas carburizing. It is performed in a closed furnace chamber with an atmosphere enriched with a gaseous compound of carbon and nitrogen. This chapter provides information on the carbonitriding of steels, the applications of carbonitriding, the typical case depths, and the hardenability of carbonitrided parts. In addition, the chapter also discusses the processes involved in quenching, tempering, and distortion of carbonitrided steels.

Carbonitriding is a process in which carbon and alloy steel gears are held at a temperature above the transformation range in a gaseous atmosphere of such composition that the steel absorbs carbon and nitrogen simultaneously. The gears are then cooled at a specific rate to room temperature that produces the desired properties. Carbonitriding is generally regarded as a modified gas carburizing process, rather than a form of nitriding. This chapter briefly describes the case depth that can be achieved with carbonitriding, how case depth is measured, and the materials and applications that are suitable for carbonitriding.