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.

Steels are over 95% Fe, so a good starting point for understanding steel is to study the nature of solid iron. This chapter provides information on the composition, phase transformation, and associated changes in the crystal structure of pure iron at varying temperatures.

Steel is an important material because of its tremendous flexibility in metal working and heat treating to produce a variety of mechanical, physical, and chemical properties. The purpose of this chapter is to present the metallurgical principles of heat treatment of steel in a generalized manner. The chapter provides a discussion on the constitution of commercially pure iron, subsequently leading to discussion on the iron-carbon alloy system. The chapter also describes the effect of carbon on the constitution of iron and of the solubility of carbon in iron. It provides information on transformations and on the classification of steels by carbon content. The chapter ends with a discussion on the effect of time on transformation and on the use of time-temperature-transformation diagrams.

The nitriding process can be applied to various materials and part geometries. This chapter focuses on tool steels, pure irons, low-alloy steels, and maraging steels. Various considerations such as the surface metallurgy requirements of the die, including case depth, compound layer formation, and temperature, are also discussed in this chapter. The chapter also addresses steel selection and surface metallurgy of gears.