Microstructural characterisation and properties of thermochemically treated iron-based alloys

Some results of a gaseous thermochemical treatments based on nitrogen, sulphur and oxygen on an Astaloy Mo alloy and AISI 316L austenitic stainless steel are discussed in order to show the importance of the chemical composition and the microstructure of the base material on the wear resistance of the treated specimens. In order to reduce the case depth and to increase the microhardness of the diffusion layers, boron was added to the base material. In this way, it is possible to close porosity, due to the liquid phase sintering. The treated specimens underwent a specific characterisation in order to investigate the microstructure and the constitution of the hardened layers as well as the tribological properties in dry sliding conditions. Both the microhardness measurements and the nitrogen concentration depth profile indicate that boron is able to reduce the nitrogen penetration. Furthermore, the combined surface thermochemical treatments improves the tribological behaviour of the material, and in particular, sulphuration significantly changes the morphology of the treated specimens, and consequently, the dry sliding wear of the sintered material is decreased.