Formation of expanded phases in ferritic stainless steel nitrided at low temperatures

Nitriding treatments at low temperatures (<450(degrees)C) are receiving increasing interest for the surface engineering of stainless steels due to the possibility of producing supersaturated solid solutions of nitrogen in the iron-based lattices, known as "expanded" phases. In this study this treatment was applied to AISI 430 ferritic stainless steel with the aim of clarifying the characteristics of the modified surface layer, and in particular the microstructure and phase composition. The formation of expanded ferrite caused high compressive stresses, so that local plastic deformation phenomena occurred, observable as microstructural changes (slip lines, grain swelling) both at the surface and in the section of the samples. Moreover, the formation of a nitrogen-containing hexagonal closepacked phase, dissimilar to epsilon-nitride, was reported for the first time for low-temperature nitrided ferritic stainless steels. It is suggested that this phase might be a N-rich expanded epsilon'-martensite, which formed from expanded ferrite due to a bcc-to-hcp displacive transformation and it remained in the microstructure owing to the residual compressive stresses and nitrogen and chromium solubilization. The presence of this phase contributed to the increase of surface microhardness together with expanded ferrite. The assessment of the corrosion behaviour in 3.5 wt% NaCl aerated solution by means of electrochemical impedance spectroscopy analysis and potentiodynamic measurements showed that, when the modified surface layer consisted only of expanded ferrite, impedance values and corrosion potential had a marked increase and corrosion current density decreased, as compared to those of the untreated steel, suggesting an enhancement of the resistance to general corrosion. On the contrary, when a heterogeneous modified layer formed (expanded ferrite and N-rich expanded epsilon'-martensite), a worsening of corrosion resistance was observed.