Thermal stability, mechanical properties and corrosion resistance of the nitrogen expanded phase produced on the superferritic stainless steel

Full text: The correct analysis of phases formed by plasma-based nitriding of ferritic stainless steels at low temperatures is still a controversial issue. Some authors report that metal nitrides such as M_2-3N and M₄N, where M is an alloying element, are the main product on the N rich layer [1], a result of the low nitrogen solubility in the body-centered cubic lattice. Others report that nitrogen diffuses fast in the matrix, remaining in solid solution [2]. In the latter situation, the properties of such "expanded ferrite" or αN (named in analogy with the well-known γN phase) has been little studied yet. The plasma immersion ion implantation (PI3) is a suitable technique for investigating the near surface modifications on the Fe-Cr alloys. Superferritic stainless steel, a special alloy developed for extreme corrosion environments, were PI3 nitrided at different temperatures from 300-400 °C, resulting in αN-containing phases. The surface hardness and elastic modulus were compatible with modified layers reaching ~2 μm thick. The nitrided surfaces also presented improvement in corrosion resistance properties inferred by tests in NaCl electrolyte, in comparison to reference sample. The evolution of the crystalline phases with temperature and time was monitored using in situ synch roton X-ray diffraction analyses in controlled atmospheres. The expanded phases disclosed a remarkable stability at temperatures above the nitriding ones, a phenomenon that was correlated with thermodynamic conditions for the nitrides precipitation and the thermally activated diffusion of nitrogen in the matrix. References: [1] Blawert, C. et al. "Nitriding response of chromium containing ferritic steels on plasma immersion ion implantation at elevated temperature", Surface Engineering, 18(4), p. 249–254 (2002); 2] Gontijo, L. C. et al. "Characterization of plasma-nitrided iron by XRD, SEM and XPS", Surface and Coatings Technology, 183(1), p. 10–17 (2004). (author)