Incipient decomposition of nitrogen-expanded Austenite in Si-containing high-Ni alloys during nitriding

In contrast to the nitride formation in steels nitrided at 480–600°C, low-temperature nitriding (LTN) generates a precipitate-free, interstitially-supersaturated nitrogen-expanded Austenite (γ_N) case in Austenitic stainless steels (SSs). However, the temperature/time-dependent Cr-nitride precipitation in γ_N triggers long-range Cr migration, severely deteriorating corrosion resistance. Hence, the upper LTN temperature is often limited to 450°C. Consequently, the diffusion-based γ_N-304/316 cases formed are relatively thin, with poor load-bearing capacity (typically < 15 µm, 500 HV_0.3), and require extended treatment times (e.g. 20–50 hrs). Here, we demonstrate the concomitant formation of ultra-fine Cr/N-rich (1–6 nm) and Si/N-rich (3–30 nm) nanostructures, with short-range Cr segregation, during the early-stage decomposition of the N-modified Austenite cases from a range of Si-containing and high-Ni SSs nitrided at 430 and 480°C. Depending on temperature and Ni/Si concentration, two early γ_N decomposition stages are defined based on the Austenitic matrix, i.e. a N-supersaturated γ_N (Stage I) and a ‘less-expanded’ γ_(N) (Stage II). The Cr/N-rich nanophase initiates via Cr-N nanoclustering along one of the {200} planes. The Cr-containing, Si/N-rich nanostructure is postulated to evolve from rock-salt FCC-structured (Cr_1-xSi_x)N to a nanocomposite consisting of amorphous Si₃N₄ in-between CrN nanocrystallites. The partially decomposed γ_N/γ_(N) case obtained at 480°C/10 hrs presents high case thickness (reaching 26 µm), high hardness (at 1550–1750 HV_0.025), and enhanced load-bearing capacity (up to 1244 HV_0.3), while still providing good corrosion resistance. This points to the development of ‘Nitralloy-like’ Austenitic steels and a time-efficient nitriding approach for partially decomposed nitrogen-expanded Austenite cases.