Ultrasonic-frequency pulsed DC nitriding: Low-temperature nitriding of AISI 316L stainless steel with enhanced surface properties

Conventional low-temperature (<450 °C) nitriding of austenitic stainless steel, while preserving corrosion resistance by forming the γ_N phase, suffers from prohibitively slow kinetics, posing a major challenge for precision components. This study presents an ultrasonic-frequency pulsed DC nitriding (UFPN) strategy that overcomes this limitation by intensifying nitrogen diffusion via high-frequency ion bombardment-induced vacancies. Remarkably, a 9.5 μm thick γ_N phase layer was achieved at a significantly reduced temperature of 320 °C within only 5.5 h, offering substantially improved efficiency over conventional plasma nitriding methods. The treated surface exhibits excellent properties, including high hardness (>13 GPa) and superior corrosion resistance (I_corr = 4.5 × 10⁻⁷ A/cm²). This technique provides an efficient and reliable solution for low-temperature surface hardening of austenitic stainless steels.