Degradation of gaseous nitriding of steel by lubricant contamination - Effect of in-situ pre-treatments

The present work focuses on the industrial inhibition of gaseous nitrided parts and on the industrial in-situ pre-treatment that can be used to avoid these inhibitions. The first objective is to obtain a repeatable defective nitriding process on carbon iron-based alloys, while the second is to determine the ability of determined in-situ pre-treatments to counter the previously mastered inhibition. Machining oil is used in order to obtain an industrially consistent inhibition. Its presence before chemical treatment strongly inhibits the adsorption of nitrogen atoms at the other surface, leading to heterogeneous nitriding properties of the treated samples. EDS analysis of oil residues indicates the presence of several elements, such as sulphur and carbon. As several studies already focused on the influence of sulphur on nitriding properties, experimentations on the influence of carbon are detailed. Contamination by carbon deposit leads to similar inhibition as in the case of a machining oil contamination. Literature analyses lead to the determination of three in-situ pre-treatments, namely urea (CH4N2O), pre-oxidation and ammonium chloride (NH4Cl), to counter the detrimental effects such contaminations. Although all the processes enable to suppress the nitriding inhibition due to the presence of machining oil, ammonium chloride indicates the best capability to activate the adsorption of nitrogen of machining oil contaminated surfaces.