Gas nitriding – process control and nitriding non-ferrous alloys

Abstract

This paper, based on the Second Lakhtin Memorial Lecture, gives an overview of the developments in the process control of gas nitriding and describes the current status of nitriding of non-ferrous alloys. Control of the nitriding process involves both thermodynamic and kinetic factors. On this basis, Lakhtin developed concepts that provide the foundations for controlled gas nitriding processes. The progress in the development of sensors in the last few years offers new possibilities for gas nitriding and nitrocarburising process control. The control of the reaction gas composition leads to higher quality assurance. Adjusting defined nitriding and oxidation potentials enables surface activation of components which allows, for example, a uniform nitriding of stainless steels at temperatures of 300°C. There are significant differences between the nitriding behaviour of aluminium, titanium, and iron alloys owing to the different solubilities of nitrogen in the base metal, the differences between the nitriding and solidus temperatures, and the different oxygen affinities of the base metals. Considerable progress has been made in the field of plasma nitriding of aluminium alloys in recent years. An essential part of this is the evidence that commercial plants can be used for the plasma nitriding of aluminium alloys. New possibilities for the production of load specific nitrided layers on titanium alloys allow the nitriding process to be intensified and combined with solution treatment and subsequent aging.

Diffusion Nitride Surface Layers on Aluminum Substrates Produced by Hybrid Method Using Gas Nitriding

“…Other studies showing the effectiveness of similar solutions were recently reported [26]. The beneficial effect of magnesium as an alloying element in an aluminum substrate on the kinetics of nitriding under glow discharge conditions was also earlier described [5,9]. However, an unfavorable phenomenon accompanying glow discharge nitriding related to the formation of a high near-surface concentration of magnesium in the form of zones of magnesium, MgO, which negatively impacts the effects of nitriding machine parts in industrial practice, was also reported [27].…”

“…Early research carried out in the field of plasma-assisted nitriding of aluminum alloys [3] confirmed that nitriding under glow discharge conditions, in particular the phenomenon of cathodic sputtering, creates favorable conditions for the growth of surface layers of aluminum nitride of the AlN type with thickness up to a few µm. It was shown that even in medium vacuum conditions (10 −2 -1 Pa), a typical industrial facility for plasma nitriding steel can be successfully used to carry out the processes [5]. At the same time, the beneficial effect of the AlN layer on tribological resistance was confirmed [3].…”

“…From Arai's work in the mid-1980s [2], it became clear that nitriding of aluminum alloys is in fact possible if it is carried out under glow discharge conditions. Since that discovery, further investigations have been carried out [3][4][5][6][7][8][9] focused on the development of technological solutions based on cathode sputtering performed during the nitriding process and conducted under glow discharge conditions enabling the removal of the passive oxide coating and the control of the aluminum substrate's possible re-oxidation processes.…”

Diffusion induced decomposition of cementite layers during gas nitriding of 4140 steel: thermodynamical and kinetical aspects

“…Underneath the compound layer, a socalled diffusion zone develops in the ferrite matrix, in which nitrogen is interstitially dissolved. The compound layer can improve the wear and corrosion resistance significantly, whereas the diffusion zone can enhance the fatigue strength [5][6][7][8]. Rapid quenching is not required after the conventional gas nitriding treatment; thus, it prevents dimensional distortion [9].…”