Austenitic stainless steels are known for their excellent corrosion resistance, however their hardness and wear resistance are relatively low. These can be improved by proper surface treatment. It has been shown that the plasma nitriding of the stainless steel surface at temperatures below 475^oC leads to improving its wear resistance, tribological and corrosion properties. These positive changes are due to a phase called expanded austenite or S-phase. The temperature of nitriding plays a decisive role in the formation of the nitrogen super-saturated austenite. If it exceeds 500^oC the nitrided layer contains CrN precipitates which results in a decrease of the corrosion resistance. In recent years, many efforts have been made to obtain a metastable nitrogen super-saturated layer on the surface of different stainless steels by various nitriding process, but the formation, structure and properties of the S-phase are still not fully understood. In the present work, XRD and cross-sectional TEM were used to study the structure of the nitrided layer formed during nitriding the AISI 316L stainless steel at temperature of 450^oC. It has been found that the applied treatment led to the formation of the 1-2 mm thick layer of the S-phase. There is no evidence of CrN precipitates. The XRD experiments showed that there occurred the austenite lattice expansion, the value of which depends of the crystallographic direction. The TEM studies confirmed that the layer consists of a single cubic phase. It contains a lot of defects such as dislocations, stacking faults, slip bands and twins. The interface between the layer and the substrate can be clearly visible, it does not create a barrier for the defects propagation. The HREM observations showed that the most prominent feature are nanotwins consisting of a few atomic planes. The outmost region of the nitride layer contained rounded areas of amorphous structure, due to high plastic deformation caused by the cathode sputtering process.

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