Effects of the Hydrogen-to-Nitrogen Ratio in Plasma Nitriding on the Surface Properties of Cold Work Tool Steels

This study systematically investigates the effect of hydrogen flow rate (100, 200, 300, and 400 sccm) on the properties of DC53 steel during a 4 h plasma nitriding process conducted at 400 °C in an asymmetric bipolar pulsed reactor. A comprehensive characterisation approach was employed. X-ray diffraction (XRD) was used to identify the phase composition, revealing the formation of a compound layer consisting of ε-Fe2–3N (identified by its (100), (101), and (102) planes) and γ’-Fe4N (identified by its (220) plane). Mechanical properties were assessed using Vickers microhardness for surface measurements and nanoindentation for depth profiling. Glow discharge optical emission spectroscopy (GD-OES) provided elemental depth analysis, while a ball-on-disk tribometer evaluated the tribological performance. The optimal treatment was achieved at a hydrogen flow rate of 200 sccm. This condition yielded a peak surface hardness of 1121.5 ± 69.2 HV0.2. GD-OES analysis directly correlated this mechanical enhancement to a high surface nitrogen content of approximately 8.5% and an effective diffusion depth of about 50 µm.