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.