Initial phase formation during nitriding of austenitic stainless steel

Abstract

Expanded austenite formed by inserting nitrogen into austenitic stainless steel is a puzzling phase since its discovery about 30 years ago: variable nitrogen concentration correlated with a variable lattice constant together with an unusual depth profile normally indicating a compound layer instead of a diffusion layer. While expanded austenite has been described for nitriding experiment durations as low as 5 min and >48 h, the initial phase transition has not been investigated in detail. Here, very low ion current density experiments prolonging the initial phase together with in-situ X-ray diffraction (XRD) experiments during nitriding are presented. Additionally, the formed layers are investigated by depth-resolved in-situ XRD measurements during successive removal of the surface layer. It is shown that two phases, a low expansion phase and a high expansion phase, are formed sequentially over time with the growth rate of the latter phase being much higher, thus obliterating any trace of the initial phase. It is surmised that the driving force is a threshold in the nitrogen concentration.