Effect of Nitrogen Concentration Profile on the Wear Rate of Expanded Austenite Phase

Abstract

The wear rate of the expanded austenite phase formed by low-temperature plasma nitriding treatment determines the lifetime of the treated stainless steel components. In this research, austenitic stainless steel (AISI304) samples were plasma nitrided between 1 h and 24 h at a temperature of 400℃, a pressure of 1.0 Pa (25％ N₂ and 75％ H₂) gas ambiance. The expanded austenite phase was characterized in thin layers with about the same nitrogen concentration. Characterization of the samples was performed by wavelength dispersive X-ray analysis (WDS) using Electron Probe Micro Analyzer (EPMA), X-ray diffraction (XRD), laser microscopy, and micro-Vickers hardness. The results of our experiments confirm that the wear rate of the expanded austenite phase varies with the depth from the surface. Thin layers of expanded austenitic phase with the same nitrogen concentration but formed under different treatment times showed the same wear rate. As the nitrogen concentration increased, the lattice parameters and compressive residual stresses calculated from the XRD pattern showed a logarithmic increase. The wear rate, on the other hand, decreased exponentially and reached 35 × 10⁻⁶ mm³/min at 25 at％. Finally, from the data obtained for the wear rate against the nitrogen concentration, an exponential fitting curve was established to determine the wear rate at any specific depth of the austenitic expanded phase.

Fig. 7　(a)Wear rate of different treatment time with distance from surface, (b) the relationship between wear rate and hardness of expanded austenite phase with nitrogen concentration of expanded austenite phase.