Thermal Stability of Stainless Steel Surfaces Nitrided by Plasma Immersion Ion Implantation

The thermal stability and the magnetic nature of metastable nitrided surface layers produced by plasma immersion ion implantation at 300 °C on an austenitic X6CrNiTi1810 (f.c.c.) and a ferritic X10CrAl18 (b.c.c.) stainless steels were examined. The surface phase analysis was performed by the Conversion Electron Mîssbauer Spectroscopy (CEMS) and the results were correlated with X-ray diffraction and microhardness measurements. The dominant phase in the as-nitrided surfaces of both steels was found to be expanded austenite (S-phase). The first structure changes of phase composition of the surface of the ferritic X10CrAl18 steel were detected after an annealing at 200 °C while in the case of the austenitic X6CrNiTi1810 steel at 250 °C. The lower stability of the ferritic steel is ascribed to higher diffusion of nitrogen atoms in b.c.c. structure and to a lack of other austenite stabilising elements besides nitrogen.