The optimization of salt bath nitrocarburizing followed by post-oxidation (Quench-Polish-Quench, QPQ) for austenitic stainless steels remains challenging because improvements in wear resistance are frequently accompanied by deterioration in corrosion resistance. In particular, the coupled effects of cyanate concentration, nitrocarburizing temperature, and treatment duration on the evolution of the modified layer and the corresponding wear-corrosion balance have not been systematically clarified. In this work, a three-factor, three-level orthogonal experimental design (L9) was employed to quantitatively investigate the effect of QPQ processing parameters on the microstructure, phase constitution, hardness, tribological behavior, and electrochemical corrosion performance of 316L stainless steel. The objective was to identify the governing factors controlling comprehensive surface performance and establish an optimized processing window for valve sealing components operating in mildly corrosive environments.
QPQ-treated specimens were characterized through scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), surface roughness measurements, and microhardness testing. Tribological behavior was evaluated by reciprocating sliding wear tests against Al₂O₃ balls under a maximum Hertzian contact pressure of approximately 1.23 GPa, while corrosion behavior was assessed by potentiodynamic polarization in 3.5wt.% NaCl solution. Range analysis and analysis of variance (ANOVA) were further employed to quantify the statistical significance of each processing parameter on wear resistance.
All QPQ-treated specimens developed a typical gradient structure consisting of an outer oxide film, a compound layer, and a diffusion layer, with total modified-layer thicknesses ranging from 41.33 to 74.58 μm. Cross-sectional SEM observations combined with EDS line scanning revealed the formation of a continuous oxygen-enriched surface oxide approximately 7 μm thick. XRD analysis confirmed that the modified layer consisted primarily of Fe₃O₄, ε-Fe_2-3N, γ′-Fe₄N and CrN phases. Nitrogen supersaturation induced the formation of expanded austenite together with lattice expansion and residual compressive stresses, whereas prolonged treatment at elevated temperature promoted partial decomposition of expanded austenite and precipitation of CrN.
The QPQ treatment remarkably enhanced surface hardness. All treated specimens exhibited hardness values exceeding 1 100 HV_0.3, while the maximum hardness reached 1 275.09HV0.3, approximately 4.7 times that of the untreated substrate. Although the average friction coefficients remained within a relatively narrow range (0.58-0.69), the wear volume varied dramatically among different processing conditions. The minimum wear volume, 2.087×10⁶ μm³, was obtained for specimen 6, corresponding to a reduction of 67%-75% compared with untreated 316L. In contrast, specimen 7 exhibited the largest wear volume (1.345×10⁸ μm³), nearly two orders of magnitude higher than specimen 6 despite maintaining a relatively high hardness.
Microstructural observations revealed that this abnormal wear behavior originated from the degradation of modified-layer integrity rather than insufficient hardness. Under the combined condition of high cyanate concentration (38.2wt.%-40.2wt.%) and prolonged nitrocarburizing (4 h), numerous interconnected microcracks developed within the compound layer and propagated preferentially along grain boundaries. These defects acted as stress concentration sites during cyclic Hertzian contact, initiating local spallation. Detached hard fragments subsequently participated in three-body abrasion, accelerating abrasive wear and fatigue-induced delamination, thereby causing a drastic increase in material loss. Correspondingly, SEM examination of worn surfaces demonstrated that specimen 6 exhibited only shallow grooves characterized by mild abrasive wear, whereas specimen 7 presented extensive spallation pits, debris accumulation, and severe material removal.
Electrochemical measurements indicated that high-temperature QPQ treatment generally reduced the corrosion resistance of 316L stainless steel in chloride-containing environments. Compared with the untreated substrate, all QPQ-treated specimens exhibited higher corrosion current densities and lower polarization resistance. Corrosion morphology demonstrated that chloride ions preferentially penetrated through the crack network within the oxide film and subsequently propagated along subsurface microcracks. For specimen 6, corrosion was mainly confined to the crack intersections where fine Fe-rich oxide particles accumulated, while the oxide layer remained largely intact. In contrast, specimen 7 showed extensive agglomerated iron oxide corrosion products together with localized chromium-enriched regions. EDS analysis suggested that these chromium-rich areas resulted primarily from preferential dissolution of iron rather than the formation of a continuous protective Cr₂O₃ film. Consequently, the cracked oxide layer gradually lost its protective capability, allowing localized corrosion to continuously propagate into the modified layer.
Statistical analyses demonstrated excellent agreement between range analysis and ANOVA. Nitrocarburizing time was identified as the most influential parameter affecting wear resistance, followed by nitrocarburizing temperature and cyanate concentration (C>B>A, P< 0.05). Although the theoretical optimum predicted by range analysis was A₁B₃C₁, the experimentally verified optimum within the investigated parameter space was A₂B₃C₁, corresponding to a cyanate concentration of 36.2wt.%-38.2wt.%, a nitrocarburizing temperature of 580 ℃, and a treatment duration of 2 h. This condition simultaneously achieved the highest hardness, the lowest wear volume, and relatively superior corrosion resistance among all tested specimens.
These findings demonstrate that the comprehensive performance of QPQ-treated 316L stainless steel is governed primarily by the structural integrity of the modified layer rather than hardness alone. Excessive nitrocarburizing promotes crack formation within the compound layer, which simultaneously accelerates tribological failure and chloride-induced corrosion. The optimized QPQ processing parameters established in this work provide a practical strategy for balancing wear resistance and corrosion resistance in 316L stainless steel and offer valuable guidance for the surface engineering of critical valve sealing components operating under combined sliding and mildly corrosive service conditions.