Influence of discharge time on properties of plasma electrolytic borocarburized layers on Q235 low-carbon steel

The plasma electrolytic borocarburizing (PEB/C) process on Q235 low-carbon steel was carried out at 330 V in 30% borax solution with glycerine additive at 5 min, 15 min and 30 min. The influence of plasma discharge time on microstructure, phase constituent, hardness, corrosion behaviors and tribological properties of PEB/C hardening layers were evaluated. Plasma discharge behaviors were investigated by the optical emission spectroscopy (OES) method. The surface loose layer of the PEB/C samples mainly consists of Fe2B, Fe2O3, Fe3C, and FeB phases, but the Fe2B phase is a dominant phase in boride layer. It is found that a Fe2B layer about 20 μm thick for 30 min or 8 μm for 15 min PEB/C treatment was formed on the Q235 low-carbon steel. The PEB/C treatment could significantly improve the hardness, corrosion and wear resistance of Q235 low-carbon steel, which was related to the formation of Fe2B phase. By increasing the borocarburizing time, the corrosion potential shifted toward the noble direction, the corrosion current gradually decreased to 1.049 × 10−6 A cm−2. The wear rate of PEB/C steel reduced with increasing the discharge time, and the PEB/C sample for 30 min against Si3N4 ball had the lowest wear rate of 5.43 × 10−7 mm3/N·m, which was only 1/673 of the bare Q235 steel substrate.