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The existence and formation of expanded austenite in ferritic stainless steels remains a subject of debate. This research article aims to provide comprehensive insights into the formation and decomposition of expanded austenite through in situ structure analyses during thermal treatments of ferritic steels. To achieve this objective, we employed the Plasma Immersion Ion Implantation (PIII) technique for nitriding in conjunction with in situ synchrotron X-ray diffraction (ISS-XRD) for microstructural analyses during the thermal treatment of the samples. The PIII was carried out at a low temperature (300–400 °C) to promote the formation of metastable phases. The ISS-XRD analyses were carried out at 450 °C, which is in the working temperature range of the ferritic steel UNS S44400, which has applications, for instance, in the coating of petroleum distillation towers. Nitrogen-expanded ferrite (αN) and nitrogen-expanded austenite (γN) metastable phases were formed by nitriding in the modified layers. The production of the αN or γN phase in a ferritic matrix during nitriding has a direct relationship with the nitrogen concentration attained on the treated surfaces, which depends on the ion fluence imposed during the PIII treatment. During the thermal evolution of crystallographic phase analyses by ISS-XRD, after nitriding, structure evolution occurs mainly by nitrogen diffusion. In the nitrided samples prepared under the highest ion fluences—longer treatment times and frequencies (PIII 300 °C 6 h and PIII 400 °C 3 h) containing a significant amount of γN—a transition from the γN phase to the α and CrN phases and the formation of oxides occurred.

A comparative study on the mechanical properties, scratch resistance, and localized corrosion (pitting and crevice) of plasma-nitrided Inconel alloy 718 (UNS NO7718: IN 718) was carried out. Thermochemical treatment was performed at low temperatures (400 and 450 °C) for 4 h. The treatment formed layers with thicknesses of 7.17 ± 0.89 µm (400 °C) and 7.96 ± 0.48 µm (450 °C). The XRD and nanohardness analyses indicated the formation of a hard layer composed of the expanded austenite phase (γN), CrN at 400 °C, and CrN + γ at 450 °C, with a maximum indentation hardness of 12 and 12.5 GPa, respectively, when compared to the 5 GPa substrate hardness. The scratching tests (2–8 N) showed that with increasing load, the nitrided surfaces had a transition from 100% microcutting to a combination of microplowing/cutting, with the presence of cracks. The critical load of the nitrided surfaces was 3 N for 400 °C and 4 N for 450 °C. The untreated condition maintained a crack-free combined mechanism regardless of the load. For the same load, the nitrided surfaces held lower coefficient of friction values and higher scratch resistance values, which were more pronounced at 450 °C. The linear polarization tests (3.56 wt.% NaCl) showed pitting corrosion in all samples, with the 450 °C condition being less resistant. Nitriding at 400 °C increased the crevice corrosion resistance of Inconel, while at 450 °C, it severely damaged it. Nitriding at 400 °C brought concomitant gains in hardness and scratch and crevice corrosion resistance when compared to the as-received IN 718.