The nature of expanded austenite

This paper attempts to reduce some of the confusion that exists over the nature of the nitrogen-rich layer produced by nitriding austenitic stainless steel at temperatures below 500°C. Cross-sectional transmission electron microscopy shows that the modified layer is dominated by a cubic phase with considerable expansion of the austenite lattice. In some cases, a thin (<0.1 μm) layer of CrN and α-Fe precipitates occurs right at the surface. There is an increase in dislocation density near the interface with the underlying material. These sublayers are not observed in X-ray diffraction, which confirms the dominant phase as an expansion of the austenite lattice and indicates a slight triclinic distortion, although a tetragonal lattice is not completely ruled out. Magnetic-force microscopy shows that the expanded austenite is ferromagnetic over most of the layer but becomes paramagnetic as the nitrogen concentration drops towards the interface with the underlying material. These features are common to austenitic stainless steel nitrided by low-pressure rf plasmas, plasma-immersion ion implantation and pulsed dc-glow discharges.