ABSTRACT
It has been established that plasma thermochemical surface engineering at sufficiently low temperatures can produce a precipitation-free hardened layer on austenitic stainless steels. Both nitrogen and more recently, carbon can be used as the alloying species. Such a low temperature nitrided or carburised layer not only has a high hardness, but more importantly has a good corrosion resistance. The enhanced mechanical and chemical properties of the surface engineered layer are derived from the supersaturation of the alloying element (N or C) in the austenite lattice and the formation of a tetragonally distorted face-centred phase (i.e. the S phase). The origin of the formation of such a metastable face centred tetragonal (fct) phase during both low temperature plasma nitriding and carburising has been investigated by X-ray and electron diffraction analysis. It was found that tetragonal distortion in the nitrided layer is mainly caused by the evolution of a high compressive residual stress in the layer; whilst the formation of high density line and planar lattice defects such as dislocations and stacking faults is responsible for the tetragonal distortion in the carburised layer. The degree of distortion is greater in the nitrided layer than in the carburised layer, leading to a more significant hardening effect in the former. The stability of the nitrogen and carbon fct phase has also been investigated. At elevated temperatures, the metastable fct phase tends to decompose into thermodynamically stable chromium nitrides (or carbides) and fcc austenite.
