Alloying Effect on Nitrided Case Characteristics of Nitralloy 135M and AISI 4140 Steel

Nitriding surface hardening is commonly used on steel components for high wear, fatigue and corrosion applications. Case hardening results from white layer formation and coherent alloy nitride precipitates in the diffusion zone. This paper evaluates the microstructure development in the nitrided case and its effects on the hardness in both the white layer and the substrate for two industry nitriding materials, Nitralloy 135M and AISI 4140. Computational thermodynamic calculations were used to identify the type and amount of stable alloy nitrides precipitation and helped explain the differences in the white layer hardness, degree of porosity at the surface, and the hardening effect within the substrate. Some initial insights toward designing nitriding alloys are shown.

Influence of Hydrogen on the Plasmas Nitriding Process of 35CrMo Steel

The mechanism of the formation of active nitrogen atoms and the function of hydrogen molecules for the dissociation of the mixture of nitrogen and hydrogen in the plasma nitriding process are discussed in this paper. The results show that the nitrided case thickness of nitrided samples become thinner and the surface hardness decreases with the increase of cathode voltage. Active nitrogen atoms mainly result from non-elastic collision between positive ions H2+ and neutral particles N2 for N2-H2 mixture plasma nitriding at 550V cathode voltage, the volume percent of e-Fe3N and the superficial nitrogen concentration of plasma nitrided case decrease with the increase of the mixture ratio of N2/H2. However, active nitrogen atoms mainly result from non-elastic collision between energetic positive ions H2+, N2+and neutral particles N2 for N2-H2 mixture plasma nitriding at 650V cathode voltage. The existence of a few of nitrogen molecules makes critical energy of positive ions H2+ dissociating neutral N2 molecules drop to 26.06eV and the cathode voltage threshold value become 490V.

Control of the Compound Zone or White Layer

The compound zone is more commonly known as the white layer, simply because 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 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 the methods for controlling compound zone formation, and, finally, reviews the factors related to the determination of case depth nitriding.

Examination of the Nitrided Case

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 discussion covers the process, advantages, and disadvantages of these methods. The chapter also discusses the processes involved in the etching of the sample after microhardness testing and provides some practices that contribute to the safe preparation of specimens. Examples of nitrided case microstructures are also presented.

Microstructures of Nitrided Iron and Steel

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.