Effect of nitriding time on microstructural characteristics, hardness, corrosion resistance and surface roughness of EN8 Steel during QPQ Salt Bath Nitriding

The paper examines how QPQ salt bath nitriding affects surface modification and performance improvement of EN8 steel, which is widely used in the piston rod. Specimens were nitrided in a salt bath at temperatures of 570°C, 60, 120 and 180 minutes, and polished at 800 grid and post-oxidised at 380°C, 15 minutes. Scanning electron microscopy (SEM), microhardness testing, X-ray diffraction (XRD), corrosion resistance analysis, surface roughness analysis and endurance testing were used to characterise the modified surface layers. The conclusions proved the appearance of a compound layer of nitride and diffusion zone on the treated surface. The length of time of nitriding affected the surface hardness, with the highest hardness of 559 HV0.01 at 180 minutes as opposed to 482 HV0.01 and 528 HV0.01 at 60 and 120 minutes, respectively. The layer thickness of the compound layer grew to 18.48 µm as opposed to the initial thickness of 10.14 µm, which shows an increase in nitrogen diffusion. There was an increase in corrosion resistance, and red rust resistance rose to 96 hours in salt spray tests. The surface roughness was reduced to 0.75 µm, though not significantly compared to 0.81 µm, and the endurance life was raised by 895 min as compared to 722.9 min. These findings indicate that the QPQ salt bath nitriding is an effective process in increasing the mechanical and corrosion properties of EN8 steel, and the duration of 180 min is the most effective in nitriding EN8 steel.