Electro-spheroidization effects on the mechanical property and microstructures in S50C medium‑carbon steels: A comparison with conventional subcritical spheroidization

This study systematically investigated the microstructure evolution and mechanical softening of normalized S50C medium‑carbon steels subjected to direct current electro-treatment at current densities of 2.75–4.25 × 10³ A/cm² for 1 h. Compared to conventional subcritical spheroidization (700 °C for 24 h), electro-treatment achieved a 24.1 % micro-hardness reduction to 157.8 ± 0.6 HV at a significantly lower processing temperature (268.8 °C) and 96 % shorter duration. Electron backscatter diffraction analyses revealed complete ferrite recrystallization at high current densities, accompanied by grain boundary rearrangement and dislocation annihilation. Concurrently, the cementite morphology evolved from lamellar to spheroidized, with the spheroidization rate increasing from 3.4 ± 0.8 % to 99.5 ± 3.4 %, and the interlamellar spacing of pearlite expanding from 0.37 ± 0.10 μm to 1.45 ± 0.32 μm. These transformations followed a two-stage mechanism: electro-fragmentation at a moderate current density (2.75 × 10³ A/cm²) and Ostwald ripening at higher levels (3.00–4.25 × 10³ A/cm²). A composite Hall-Petch, Bailey-Hirsch, interlamellar spacing based model quantitatively captured the micro-hardness contributions from grain boundary strengthening, strain hardening, and interlamellar cementite strengthening in dual-phased medium‑carbon steels. A thermal benchmark experiment at 269 °C for 1 h confirmed that Joule heating alone could not trigger the observed changes, underscoring the dominant role of athermal effects in metallurgical behaviors. Literature-based thermodynamic analysis further suggests that electro-treatment introduces additional free energy, promoting interface destabilization and accelerated phase transformations. Overall, this method enables effective spheroidization and recrystallization while preventing severe grain growth, offering a low-temperature and high-efficiency alternative for sustainable processing of medium‑carbon steels.