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Low temperature surface engineering of stainless steels by dissolving large quantities of nitrogen and carbon has become a favorable technological process. Such treatments usually yield the so-called expanded austenite, which is a super-saturated diffusion zone and can be produced by a salt, gas, or plasma-based processes. The present manuscript addressed the production of expanded austenite on a superaustenitic stainless steel at low temperature. Gas mixtures with nitrogen or both nitrogen and carbon bearing gases were applied in a plasma atmosphere at 400°C for 5 h. Microstructural characterization was conducted applying light optical microscopy, X-ray diffraction, and transmission electron microscopy. In addition, microhardness measurements were performed at the surface of the samples. Plasma nitriding and nitrocarburising at 400°C resulted in a homogeneous case composed solely by expanded austenite as detected by X-ray diffraction. Similar microhardness was observed for both processes, whereas nitrocarburising provided a thicker diffusion zone when compared to nitriding. Fine iron-chromium nitride precipitation was only identified by transmission electron microscopy (TEM). Selected area electron diffraction yields similar lattice parameters for both processes.