Thermal stability of stainless steel surfaces nitrided by plasma immersionion implantation

The thermal stability and the magnetic nature of metastable nitrided surfac e layers produced by plasma immersion ion implantation at 300 degrees C on an austenitic X6CrNiTi1810 (f.c.c.) and a ferritic X10CrAl18 (b.c.c) stainl ess steels were examined. The surface phase analysis was performed by the C onversion Electron Mossbauer Spectroscopy (CEMS) and the results were corre lated with X-ray diffraction and microhardness measurements. The dominant p hase in the as-nitrided surfaces of both steels was found to be expanded au stenite (S-phase). The first structure changes of phase composition of the surface of the ferritic X10CrAl18 steel were detected after an annealing at 200 degrees C while in the case of the austenitic X6CrNiTi1810 steel at 25 0 degrees C. The lower stability of the ferritic steel is ascribed to highe r diffusion of nitrogen atoms in b.c.c. structure and to a lack of other au stenite stabilising elements besides nitrogen.