No abstract

This article summarizes the terminology for gas reactions, and discusses low-temperature nitriding and nitrocarburizing of stainless steels. It describes the various nitriding processes, namely, high- and low-pressure nitriding, oxynitriding, sulfonitriding, oxysulfonitriding, ferritic nitrocarburizing and austenitic nitrocarburizing. The article includes a discussion on the difficulties in specimen cleaning, importance of furnace purge, uses of pre and post oxidation, depassivation, or activation, and requirements for perfect nucleation in nitriding process. In nitriding, the successful atmosphere control depends on various potentials. The article summarizes the methods of measuring potentials in nitriding and nitrocarburizing, provides useful information on the furnaces used, and the safety precautions to be followed in the nitriding process. It also describes the sample preparation procedures and testing methods to ensure the quality of the sample.

Controlled nitriding and ferritic nitrocarburizing cycles have become commonly used to improve wear resistance, mechanical properties, and corrosion resistance. The framework for process control is based on standards such as AMS 2759/10 and 2759/12A. In all cases, these standards allow for a tolerance in control parameters. This work presents the influence of standard allowed deviations in control parameters, for example, nitriding potential and temperature for a given furnace class, on actual results when comparing iron, carbon steel and low alloy steel samples. The results point to interesting variations in white layer thickness. We also discuss other potential factors such as the shift of actual results due to carbon and alloying elements, and the subsequent alteration of results between expected and actual as demonstrated by shifting boundaries on the Lehrer diagram. As a conclusion, the study underlines the fact that while tolerances are allowed, precise control in specific furnace classes is necessary to obtain an ideal repetition of results. Additionally, improved control lowers gas usage and gas flows to a strict minimum, with a direct impact on the cost of quality, cost of treatment, and emissions affecting the environment.