Environment Conscious Gas Nitriding of Austenitic Stainless Steels

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Three types of austenitic stainless steels, SUS304, SUS316 and SUS310, were preheated in air at various temperatures. Depthwise changes in the surface film composition were analyzed by X-ray photoelectron spectroscopy. Gas nitriding was conducted in a controlled-atmosphere furnace by changing the nitriding temperature and time. Nitriding behavior were examined in connection with the kinetics of the nitriding reaction.The nitriding reaction occurs locally at an early stage of the nitriding treatment. The whole surface becomes covered with a nitrided layer by coalescence of nitrided regions. Thereafter, nitrided layer with a uniform thickness is formed by the growth in a depth direction. Diffusion of nitrogen through the nitrided layer is the rate-determining process in the growth of the nitrided layer. Nucleation of the nitriding reaction is greatly accelerated by atmospheric preheating. The nitrided layer with a uniform thickness and sufficiently high hardness for practical use is obtained when appropriate preheating in air is applied to produce a surface film consisting mainly of iron oxides at the surface and containing a low concentration of CrィイD22ィエD2OィイD23ィエD2 and a high concentration of FeO in the interior. A hardness of the nitrided layer decreases with increasing nitriding temperature.It is confirmed from the present results that gas nitriding of austenitic stainless steels is possible without any chemical pretreatment.