N 2 /H 2 Non-thermal Transferred arc Plasma Nitriding Treatment of Stainless Steel at Atmospheric Pressure

In the paper, a novel metal surface nitriding method is presented by using an atmospheric pressure non-thermal transferred arc plasma technique. In the N 2 /H 2 plasma nitriding process, the surface of stainless-steel is treated by the arc root, which shows an irregular movement in a circle-like zone with about 5 mm diameter observed by high speed camera. The discharge has characteristics of high voltage and low current, and the gas temperature of the arc column plasma is only about 2900 K determined by the diatomic molecule N 2 + (B 2 Σ u + →X 2 Σ g + ) fitting method. It is found that the NH radical production in plasma plays an important role in the nitriding process. The thickness of nitrided layer can reach 60 μm after processing of 120 min under the optimal experimental conditions. The surface hardness of stainless-steel increased from 220 to about 1200 HV 0.1 (Vickers hardness under the force of 0.1 kg) after plasma treatment. The treating saturation time for stainless-steel surface hardness reaching to 1200 HV 0.1 is around 9 min.