Examining and evaluating the nitrided case is generally accomplished by hardness testing and microscopic examination. This chapter discusses both characterization methods, as well as sample preparation. The chapter also discusses the processes involved in the etching of the sample after microhardness testing and provides practices that contribute to the safe preparation of specimens. Examples of nitrided case microstructures, using optical light microscopy, are also presented.

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.

This chapter first lists the compositions of typical steels that are suitable for nitriding. It then presents considerations for steel selection. The chapter also shows the influence of alloying elements on hardness after nitriding and the depth of nitriding. It provides a detailed discussion on plasma nitriding of type 422 stainless steel, nitriding of type 440A and type 630 (17-4 PH) stainless steel. The chapter also discusses plasma nitride case depths.

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.

Nitriding is a case-hardening process used for alloy steel gears and is quite similar to case carburizing. Nitriding of gears can be done in either a gas or liquid medium containing nitrogen. This chapter discusses the processes involved in gas nitriding. It reviews the effects of white layer formation in nitrided gears and presents general recommendations for nitrided gears. The chapter describes the microstructure, overload and fatigue damage, bending-fatigue life, cost, and distortion of nitrided gears. Information on nitriding steels used in Europe and the applications of nitrided gears are also provided. The chapter presents case studies on successful nitriding of a gear and on the failure of nitrided gears used in a gearbox subjected to a load with wide fluctuations.

Nitriding is a surface hardening heat treatment that introduces nitrogen into the surface of steel while it is in the ferritic condition. Gas nitriding using ammonia as the nitrogen-carrying species is the most commonly employed process and is emphasized in this chapter. Nitriding produces a wear- and fatigue-resistant surface on gear teeth and is used in applications where gears are not subjected to high shock loads or contact stress. It is useful for gears that need to maintain their surface hardness at elevated temperatures. Gears used in industrial, automotive, and aerospace applications are commonly nitrided. This chapter discusses the processes involved in gas, controlled, and ion nitriding.