Effect of Electrolytic-Plasma Hardening Parameters on the Microstructure and Mechanical Behavior of 20X Steel

This study investigates the effects of applied voltage and treatment duration during electrolytic-plasma hardening (EPH) on the microstructure, phase composition, hardness, surface condition, and tribological behavior of 20X steel. EPH was performed in a 12 wt.% Na₂CO₃ aqueous electrolyte at voltages of 280, 300, and 320 V for 4 and 6 s. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, instrumented indentation, surface profilometry, and ball-on-disk tribological testing were used to characterize the treated specimens. The 280 V/4 s, 300 V/4 s, 320 V/4 s, and 280 V/6 s conditions produced thermally modified surface layers without visible surface damage, whereas treatment at 300 and 320 V for 6 s resulted in localized surface melting. Among the undamaged specimens, the highest hardness was obtained at 280 V, reaching 329.6 ± 15.8 HV after 4 s and 332.1 ± 26.3 HV after 6 s, corresponding to an approximately 1.83-1.85-fold increase relative to the initial value of 180 HV. XRD revealed a predominantly α-Fe-based matrix, while weak secondary reflections could not be assigned reliably to specific phases. The minimum steady-state coefficient of friction was obtained at 300 V/4 s (0.444 ± 0.054), whereas the narrowest wear track was observed at 320 V/4 s (379.48 ± 36.24 μm). No direct correlation was found between hardness and tribological response, indicating that surface roughness and microstructural state should also be considered when selecting treatment parameters. The results define a stable EPH processing window for 20X steel and demonstrate that parameter selection should be based on a combined assessment of surface integrity, hardness, and tribological performance.