Influence of Polishing and Post-Oxidation Parameters on Surface Roughness of QPQ-Treated EN8 Steel: A Taguchi Design Study

Gas spring piston rods operate under repetitive sliding contact, fluctuating mechanical loads, and exposure to aggressive environmental conditions, making surface integrity a key determinant of durability and functional reliability. Quench–Polish–Quench (QPQ) salt bath nitriding is an advanced thermochemical treatment widely employed to improve the surface characteristics of steel components through enhanced hardness, wear resistance, and corrosion protection. This study examines the effect of polishing grit size, oxidation duration, and oxidation temperature on the surface roughness of QPQ-treated EN8 steel used for gas spring piston rod applications. All specimens were nitrided at 570°C for 180 min, followed by polishing and post-oxidation treatments designed according to a Taguchi L9 orthogonal array. Surface roughness measurements were carried out using a Mitutoyo SJ-410 profilometer. The experimental data were analyzed using Signal-to-Noise (S/N) ratio methodology and Analysis of Variance (ANOVA) to identify the most significant process variables and determine optimum operating conditions. The results demonstrated that polishing grit size had the greatest influence on surface finish, while oxidation temperature and oxidation time exhibited comparatively lower effects. The lowest average surface roughness of 0.12 μm was achieved using a polishing grit size of 1200, oxidation time of 30 min, and oxidation temperature of 380°C. The optimized treatment parameters produced a smoother and more uniform surface morphology, which is desirable for improving the tribological performance and service life of gas spring piston rods. The outcomes of this investigation provide valuable guidance for the industrial optimization of QPQ salt bath nitriding processes for EN8 steel components.