Orientation dependence of nitrogen supersaturation in austenitic stainless steel during low-temperature gas-phase nitriding

A hardened surface layer was produced on 316L austenitic stainless steel via low-temperature gas-phase nitriding. The effects of nitriding temperature, nitrogen activity and processing time were systematically studied. Concentration depth profiling by Auger electron spectroscopy revealed very high levels (up to 25 at.%) of interstitial nitrogen in solid solution. This causes a lattice parameter expansion of up to 10% at the alloy surface. X-ray diffractometry revealed that the expansion and thus the level of nitrogen supersaturation strongly depends on the crystallographic orientation. We attribute this effect to the elastic anisotropy of austenite, exacerbated by a nitrogen-induced paramagnetic-to-ferromagnetic transition. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.