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.

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

“…The work carried out by Irretier et al also showed that a pre-oxidation of 30 min in air improved the mechanical properties after nitriding in the case of 42CrMo4-grade steel, and inhibited these mechanical properties in the case of C15 and X155CrMoV12.1 grade steels [24]. Neither of these results is in agreements with the present work.…”

“…It should be considered that the pretreatment parameters can influence the layer formed. Irretier et al prepared the samples with a very low partial oxygen pressure (5.7 10 -23 Pa) [24], while the oxidation process was carried out, in this work, with a partial oxygen pressure of approximately 13 10 3 Pa. As a consequence, it can be assumed that the layer formed during the present work is thicker than the one obtained in the reference, leading to a non-negligible reduction duration, and so, a…”

“…Pre-oxidation also modifies the iron nitride nucleation localization from grain boundaries to homogeneously on the whole surface. [24]. The impact of the pre-treatment varies according to the contaminant nature, and in the case of machining oil, 30 min of pre-oxidation had an even higher detrimental effect on final surface hardness.…”

“…19). Hardness-depth profiles of these samples are indicated in nitriding at 520 °C and K N = 10 atm -1/2 [24]. However, the criterion of evaluation of the nitriding quality was chosen to be the surface hardness.…”