AbstractThe compound layer of iron nitrides (white layer) and the diffusion layer formed during the nitriding of an austenitic stainless steel have been investigated by a combination of optical and electron microscopy and X-ray powder diffraction. The mechanical behaviour of the nitrided surface is controlled by the microstructure, which is shown to be dictated by the nitriding temperature and cooling rate from that temperature. The results provide a complete description of the phase distribution in the nitrided case and indicate the nitriding conditions required to achieve a wide range of properties, from surface hardening to deliberate embrittlement.