Thermochemical surface treatments, such as carburizing are widely used methods for case hardening in order to improve wear resistance, impact strength, and fatigue properties in gear applications. Alternatives to carburizing are nitriding or carbonitriding by diffusing nitrogen into the surface of metals for further improvement of the properties, whose physical origins are yet to be established, especially regarding microstructure formation. Previous studies [1] showed that nitrogen enrichment in the austenite has large consequences on its decomposition. The kinetics are largely accelerated and the resulting microstructures are much finer, and associated with stronger properties (hardness) [2]. These effects have been ascribed so far to the formation of nitrides (mostly CrN, MnSiN2) in the austenite. These would provide new nucleation sites for the ferrite. In addition, nitrides could also contribute to austenite grain refinement upon re-austenitization. This could be another origin of the acceleration of kinetics and the microstructure refinement. In this work, new EBSD observations demonstrating the refinement of the austenite grains are presented. A suitable experimental approach is then introduced to study the refinement of the austenite, thanks to in-situ microscopy techniques: high-temperature in-situ EBSD and laser scanning confocal microscopy (LSCM). Previous results from in situ HEXRD showing the acceleration of the austenite decomposition are also recalled.