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.

Formation of the nitrided case begins through a series of nucleated growth areas on the steel surface. These nucleating growth areas will eventually become what is known as the compound layer or, more commonly, the white layer. This chapter discusses the influence of carbon on the compound zone. It explains how to control and calculate compound zone thickness. Compound zone thickness can be controlled by dilution, the two-stage Floe process, or by ion nitriding. The chapter describes the factors affecting surface case formation.

The compound zone that forms on the surface of nitrided steels is often called the white layer. When the nitrided sample is sectioned through the case, and then polished and etched with a standard solution of nital (2 to 5% nitric acid and alcohol), the immediate surface etches out as white in appearance above the nitrided case. This chapter focuses on the methods to control the compound zone, or white layer. It first provides information on a test to determine the presence of the white layer, and discusses the processes involved in the reduction of the compound zone by the two-stage process. Next, it describes other methods for controlling compound zone formation, and, finally, reviews the factors related to the determination of case depth in nitriding.