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.

Diffusion is the movement of atoms through the crystalline lattice. This chapter discusses the two main types of diffusion that can occur in solids: interstitial diffusion and substitutional diffusion. It describes Fick's first and second laws of diffusion, with emphasis on several applications of the latter. The chapter also provides information on the temperature dependence of diffusion, intrinsic diffusion coefficients (Kirkendall effect), and high diffusion paths.

This chapter familiarizes readers with the use of Fick’s laws of diffusion in heat treating, coating, and other metallurgical processes. It contains worked solutions to nearly 30 problems requiring the calculation of activation energy, diffusion coefficient, concentration level, surface layer thickness, case depth, and processing time and temperature. The selected problems deal with various types of iron, steel, and nonferrous alloys and processes ranging from aluminizing, chromizing, carburizing, and plasma nitriding to hydrogen dissipation, decarburizing, and oxidation. A few diffusion problems involving single-crystal silicon are also included.

Diffusion is the primary mechanism by which carbon atoms move or migrate in iron. It is driven by concentration gradients and aided by heat. This chapter provides a practical understanding of the diffusion process and its role in the production and treatment of steel. It discusses the factors that determine diffusion rates and distances, including time, temperature, and the relative size of the atoms involved. It also describes two heat treating methods, carburizing and decarburizing, where carbon diffusion plays a central role.

This chapter discusses the process, principles, and modeling of the diffusion brazing system. The applications of diffusion brazing to wide-gap joining and layer manufacturing are also discussed.

The first step in the hardening of steel is getting it hot enough to form austenite, from which martensite can form upon quenching. Not all steels have the same austenitization requirements, however. High-carbon wear-resistant steels, such as bearing and tool steels, require the presence of carbides during austenitization; plain carbon and low-alloy steels do not. This chapter describes the austenitization process used in each of the two cases, namely single-phase austenitization (the accepted method for plain carbon low-alloy steels) and two-phase austenitization (required for high-carbon steels). It also addresses process-specific issues, explaining how the presence of carbides (in the two-phase process) produces significant changes, and how homogenization and austenite grain growth influence the single-phase process.

Several process parameters must be considered to ensure success in achieving desired metallurgical properties and to minimize distortion. This chapter provides a detailed discussion on the liberation of nitrogen, dissociation of the gas at the selected nitriding temperature, why ammonia is used, distortion, and preheat treatment.