The influence of plasma nitriding on the resistance of X20Cr13 steel to cavitation erosion

Abstract

The effect of plasma nitriding on the cavitation erosion resistance of X20Cr13 steel was studied. Nitriding was performed using the active screen plasma nitriding method at a temperature of 400 °C, with a gas mixture of 60 % N₂ and 40 % H₂, and nitriding durations of 5 and 10 h. The nitriding time influenced the thickness of the nitrided layer, which measured 26 μm and 39 μm, respectively. X-ray diffraction revealed the presence of γ′-Fe₄N and ε-Fe_2-3N phases in the surface zone of the nitrided layer. The maximum hardness of the nitrided layer was similar for both durations (1352 HV for 5 h and 1362 HV for 10 h), more than tripling the hardness of the untreated steel (420 ± 6 HV), and was located 15 μm below the surface, irrespective of nitriding time. Cavitation erosion tests conducted in a cavitation tunnel showed that, despite the increase in surface hardness, nitriding reduced cavitation erosion resistance. A thicker nitrided layer resulted in lower resistance to cavitation erosion, primarily due to brittle fracture in the nitrided steel, with cracks propagating along cleavage planes. In contrast, the non-nitrided X20Cr13 steel exhibited ductile fracture behaviour. Roughness parameters of the surface profiles were similar for both nitriding durations, regardless of the erosion rate or testing time, indicating a consistent degradation mode, which differed from the untreated steel's degradation behaviour. Testing time had more impact on the size of the eroded area than mass loss.