Effectiveness of surface activation in correlation with surface finishing condition for expanded austenite formation during plasma nitrocarburizing of AISI 316L

Jafarpour Saeed M. - Technische Universität Bergakademie Freiberg (Germany)

The focus of the current study is one of the most important challenges during thermochemical diffusion treatment of austenitic stainless steel with different surface finishing conditions, namely the necessary surface de-passivation initiated by different types of in-situ surface activations.

This study then addresses the effectiveness of three types of surface activations of (i) ion sputtering of the surface, (ii) the exposure of surface to reactive gas species (as HCN) and (iii) simultaneous application of (i) and (ii). These three-surface activation types were applied to polished and to ground surfaces of AISI 316L during a 30 min activation step at 400 °C followed by an afterglow plasma nitrocarburizing (PNC) treatment for 3 hours at 430 °C. Both surface activations and afterglow PNC treatments were conducted in a modified hot-wall reactor using separately plasma-activated graphite electrodes, respectively. The reactor was equipped with a laser-based absorption spectroscopy sensor for real-time monitoring and measurement of the gas composition resulting from the discharge at the graphite electrodes during the treatments. The generated expanded austenite layers were analyzed by cross-sectional light optical microscopy, glow discharge optical emission spectroscopy (GDOES), and X-ray diffraction (XRD).

It was shown that a polished surface was activated more effectively than a ground surface by applying a comparable activation type. For a ground surface treated without surface sputtering, no expanded austenite formed whereas for a polished surface, an expanded austenite layer formed under the same condition. Additionally, depending on the applied activation type, the characteristics of the resulting expanded austenite layer varied with respect to the maximum C and/or N uptake as well as the developed microstructures. If the surface activation and the subsequent afterglow PNC treatment allow a strong uptake of N, the C uptake reduced, likely due to competition of these two elements for uptake into the steel.

Keywords: austenitic stainless steel, surface activation, expanded austenite, hot-wall reactor, laser absorption spectroscopy