Comparative Study of Microstructure, Phase Composition, Hardness, and Tribological Behavior of Low-Carbon Steel After Electrolytic-Plasma and Laser Surface Hardening

The aim of this study was to comparatively evaluate the effects of electrolytic-plasma surface hardening (EPSH) and laser surface hardening (LSH) on the microstructure, phase state, microhardness, and friction behavior of low-carbon Steel 20. EPSH was performed using a two-stage regime of 320 V for 3 s followed by 200 V for 40 s and a thermocyclic regime of 320 V for 2 s, 200 V for 2 s, and 50 V for 2 s repeated for five cycles. LSH was carried out at a scanning speed of 7 mm/s and 75% of the maximum laser power (approximately 2250 W) using single- and double-pass treatments. The modified layers were characterized by SEM, XRD, microhardness measurements, and dry sliding tribological tests. EPSH produced the strongest hardening response, reaching approximately 600 HV with a hardened-layer depth of 70–100 μm. Double-pass LSH increased the near-surface microhardness to approximately 460 HV. Phase analysis revealed Fe1−xO after EPSH, Fe3O4 after single-pass LSH, and Fe1−xO, Fe3O4, and Fe2O3 after double-pass LSH. All treatments reduced the coefficient of friction compared with untreated steel. Overall, EPSH provided deeper and stronger hardening, whereas LSH produced more localized surface modification and oxidation.