Intermediate stages of CrN precipitation during PIII nitriding of austenitic stainless steel

The formation of metastable expanded austenite in austenitic stainless steel after energetic nitrogen insertion is a well established fact. While the initial formation of CrN precipitates and the longterm evolution towards a continuous layer containing CrxN and a bcc-phase have been known for several years, no information about the intermediate stage beyond XRD data indicating an X-ray amorphous phase is available. Here, this intermediate stage is investigated with a temperature series of nitrogen plasma immersion ion implantation (PIII) experiments using austenitic steel 316Ti. Scanning electron microscopy (SEM) viewgraphs of cross sections clearly show a very corrugated interface between the already transformed zone and the original expanded austenite. This interface is growing towards the interface between the substrate and the expanded austenite and simultaneously becoming more and more flat. The competition between nucleation of precipitates and growth of these transformed regions is governed by the much faster dynamics of the latter process at higher temperatures (respective longer times).