Challenges of fitting a Nitrided austenitic stainless steel Mössbauer Spectrum

Nitriding of austenitic stainless steel (ASS) causes changes to its surface, and it is widely used in technological applications. The extreme sensitivity of the formed layer not only to the nitriding process but also to its parameters (or nitriding conditions) leads to an infinite number of samples, making it difficult to compare it with results from different authors. The nitrided layer has a gradient of the nitrogen content profile. This, added to the presence of non-stoichiometric phases and the presence of induced residual stress, forms a very heterogenous probed area. Another issue is that despite being relatively well studied, the crystallographic structure of the expanded austenite, which is the main structure formed, is not well established. Considering this, many challenges remain concerning the characterization of the microstructure of the formed layer, mainly because of the limitations of the local structure characterization tools, whose results can be extremely difficult to interpret. Moreover, recent results suggest that the surface proximity may substantially affect the structure in this outermost region, and therefore even a few tenths of a micron must be carefully characterized, which increases the complexity of this system. In this context, Mössbauer spectroscopy (MS) is an interesting exploratory technique, which, if correlated with other techniques, can support this challenging task. However, regardless of the intrinsic complexity of this spectroscopy, interpreting the Mössbauer spectrum is not straightforward, requiring not only a solid understanding of the nitrided ASS system, but also a careful correlation with the results of other techniques. In this study, these points are cautiously detailed and the implications in the MS analyses are widely discussed. Finally, some ways of improving the analyses are outlined and discussed.