Influence of contaminants on gas nitriding

Abstract

Machining of steels not only affects the shape of the workpiece, but also influences surface structure and composition. At elevated temperatures and pressures during machining or the subsequent cleaning process cooling lubricants, hydraulic oils, machine greases, and cleaning residues can form stable adsorption or reaction layers. These layers can work as barriers and therefore inhibit surface modification reactions, as can be seen in the example of gas nitriding. The aim of the present investigation was to explore possible mechanisms of reaction inhibition and develop a nitriding process more reproducible in terms of reaction temperature and residence time. Samples prepared with residues of cleaning and machining processes on different alloy surfaces were nitrided under normal conditions and with extremely short residence times (1 < t < 20 min), in order to detect the effect of reaction and barrier layers on nitrogen acceptance during the initial stages of the nitriding process. For all alloys examined the samples with adsorbed cleaning agents showed the worst nitriding result. Volatile contaminations of industrial cleaners such as tens ides desorbed from the surface during gas nitriding. Non-volatile contamination and reaction layers such as phosphates and silicates are stable and hindered gas nitriding during the entire process period. The nitriding hindrance of non-volatile contaminants was found to increase with increasing chromium and aluminium content of the steel and decreasing nitriding time.