Plasma nitriding using high H2 content gas mixtures for a cavitation erosion resistant steel

- Plasma nitriding using high H2 content gas mixtures (for 5, 10, and 20% N2) in CA-6NM steel was studied for short nitriding time.
- The N2 content for high H2 content gas mixtures (>80% H2) strongly influences the phases' formation in the nitrided layer.
- The finer and more homogeneous distribution of the nitrided layer phases obtained at 5% N2 lead to a better cavitation erosion resistance of the CA-6NM steel.
- The worse cavitation erosion behavior is verified for samples nitrided at 20% N2 is supposed to be due to the formation of a multiphase compound layer constituted by Fe4N + Fe2-3N + CrN.

Plasma nitriding using high H2 content gas mixtures in CA-6NM martensitic stainless steel was studied in the present work. Nitriding was performed in H2 + N2 gas mixtures for 5, 10 and 20% N2, in volume, at 773 K (500 °C), during 2 h. Changes in the surface morphology and nitrided layer constitution were characterized by SEM, XRD, roughness analysis, and nanoindentation technique. Cavitation erosion behavior of the nitrided samples was also investigated by means of a 20 kHz ultrasonic vibrator. The study was emphasized for the three first cavitation stages (incubation, acceleration, and maximum erosion rate stage) of the cumulative erosion-time curve. Results indicate that the gas mixture nitrogen content strongly influences the phases' formation and its distribution on the nitrided layer. Better cavitation erosion resistance which was attributed to the finer and more homogeneous distribution of the nitrided layer phases was verified for samples treated at 5% N2. Otherwise, worse cavitation erosion behavior for samples nitrided at 20% N2 is supposed to be due to the formation of multiphase compound layer constituted by Fe4N + Fe2-3N + CrN, which can infer residual stress in treated surface.