On the S-phase formation and the balanced plasma nitriding of austenitic-ferritic super duplex stainless steel

摘要

Graphical abstractDisplay OmittedHighlights•At same temperatures, nitrogen PIII energies ruled the structure of α-γ grains.•The γN(or S) phase mediated nitriding in both austenite (γ) and ferrite (α) grains.•Balanced structure (γN) and mechanical behavior in both α-γ were attained at ∼401 °C.•Ion energy per pulse and the γNphase formation correlated in a linear-like fashion.•This correlation can be employed as a fine-tune control in the α-γ steel nitriding.AbstractThe different physical responses of austenite (γ) and ferrite (α) iron structures upon nitriding result in technical challenges to the uniform modification of α-γ materials, as the super duplex stainless steel (SDSS). The effects of voltage (7–10 kV), frequency and pulse width on the nitrogen plasma immersion ion implantation of SDSS (α∼56%, γ∼44%) were investigated, correlated with structural, morphological and mechanical analyses. By controlling the treatment power, temperatures ranged from 292 °C to 401 °C. Despite the overall increase in hardness for any of the employed parameters (from ∼6 GPa to ∼15 GPa), the structure of individual grains was strikingly dissimilar at the same temperatures, depending on the energetic conditions of implantation. Modified-α grains containing iron nitrides (ε-Fe2-3N, γ′ -Fe4N) presented intense brittleness, whereas the expanded phase γN(S-phase) laid principally in modified-γ grains, exhibiting ductile-like deformation features and thicker layers. The γNwas the dominant phase in both α-γ grains at ∼401 °C, providing them with balanced structure and mechanical behavior. These phenomena corroborate with γNas mediator of the process, through a mechanism involving the nitrogen-promoted ferrite to austenite conversion and nitrides dissolution at high temperatures. An approximately linear correlation of the γNcontent with respect to the ion energy per pulse was demonstrated, which properly embodies limiting effects to the treatment. This can be a parameter for the α-γ steel surface modification, consisting in a better adjustment to obtain more precise control along with temperature.