Abstract
Steel manufacturing affects not only the shape of a workpiece but also its surface structure and composition. Cooling lubricants, hydraulic oils, and machine greases form stable adsorption or reaction layers at the elevated temperatures and pressures which occur during the manufacturing process. Adsorption or reaction layers can work as barrier layers and inhibit surface modification reactions, as in gas nitriding. Surface layers containing oxygen, sulphur, and phosphorus have been prepared on different alloy surfaces. The aim of the investigation was to explore possible mechanisms of reaction inhibition and develop a more reproducible nitriding process in terms of reaction temperature and furnace residence time. In this study, samples were nitrided at extremely short residence times (1 < t < 20 min) to detect the effects of reaction and barrier layers on nitrogen acceptance during the initial stages of the nitriding process. Surprisingly, for all alloys examined, the technically clean reference samples never showed the highest nitrogen concentrations. On the other hand, nitride layer growth rates were increased when surfaces were covered with a thin oxide or phosphate layer before nitriding.
Get full access to this article
View all access options for this article.