Austenitic nickel-free nitrogen-alloyed stainless steels (ANFSS) are increasingly favoured in biomedical and structural applications due to their biocompatibility, good corrosion resistance, and cost-effectiveness. When fabricated via Laser Powder Bed Fusion (L-PBF), these materials exhibit unique microstructural features that can be further optimized through post-processing. This study explores the influence of Ultrasonic Nanocrystal Surface Modification (UNSM) on the microstructure and surface characteristics of L-PBF-produced Fe-16Mn-14Cr-0.27C-0.35N Ni-free stainless steel ANSFF. Single and double UNSM scans were employed to assess their effects on surface roughness and microstructural features, using an optical 3D surface profilometer and optical microscopy, respectively. Surface roughness analyses revealed that a single UNSM scan reduced the arithmetic average roughness (Ra) of as-built samples by approximately 85% (from ~10 µm to ~1.5 µm), and the average peak-to-valley height (Rz) by over 80% (from ~54 µm to ~8.6 µm). A second UNSM scan yielded marginal improvements in Ra but introduced variability in Rz. For polished samples, UNSM slightly increased the surface roughness. Surface characterization through hardness measurements revealed a significant increase in surface hardness, attributed to refined grain structures from 15 µm to 3 µm and induced compressive residual stresses. These enhancements underscore the potential of UNSM as a viable post-processing strategy for tailoring the surface properties of advanced stainless steels, particularly for applications in biomedical implants and high-performance engineering components.

Keywords: Ni-free N-alloyed stainless steel; Additive manufacturing; L-PBF; Grain refinement; Ultrasonic Nanocrystal Surface Modification