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

The different physical responses of austenite (gamma) and ferrite (alpha) iron structures upon nitriding result in technical challenges to the uniform modification of alpha-gamma 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 (alpha similar to 56%, gamma similar to 44%) were investigated, correlated with structural, morphological and mechanical analyses. By controlling the treatment power, temperatures ranged from 292 degrees C to 401 degrees C. Despite the overall increase in hardness for any of the employed parameters (from similar to 6 GPa to similar to 15 GPa), the structure of individual grains was strikingly dissimilar at the same temperatures, depending on the energetic conditions of implantation. Modified-alpha grains containing iron nitrides (epsilon-Fe2-3N, gamma' -Fe4N) presented intense brittleness, whereas the expanded phase gamma(N) (S-phase) laid principally in modified-gamma grains, exhibiting ductile-like deformation features and thicker layers. The gamma(N) was the dominant phase in both alpha-gamma grains at similar to 401 degrees C, providing them with balanced structure and mechanical behavior. These phenomena corroborate with gamma(N) as 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 gamma(N) content 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 alpha-gamma steel surface modification, consisting in a better adjustment to obtain more precise control along with temperature. (C) 2017 Elsevier B.V. All rights reserved.