Characterization of surface hardened layers on Q235 low-carbon steel treated by plasma electrolytic borocarburizing

• Prepare hardened layers on steel by plasma electrolytic borocarburizing (PEB/C).
• Characterize morphology, microstructure and hardness of PEB/C steel.
• Evaluate corrosion resistance of PEB/C steel by polarization and EIS.
• The PEB/C treatment greatly improved wear resistance of steel.

The Q235 low-carbon steel was hardened by plasma electrolytic borocarburizing (PEB/C) process. Surface borocarburizing was carried out in 15% borax aqueous solution with glycerin additive at 290 V under different treating time. The hardened layers on Q235 low-carbon steel were analyzed by SEM, XRD and microhardness test. Corrosion behavior of PEB/C layers was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy. Their wear performance under dry sliding was measured using a pin-disc friction and wear tester. It showed that the PEB/C layer consisted of a boride (Fe2B) layer and a transition layer. The thickness of boride layer increased with the borocarburizing time and reached 8 μm after 30 min treatment. The highest microhardness of the boride layer was up to 1450 HV, which was much higher than that of bare Q235 steel with about 170 HV. The PEB/C treatment slightly improved corrosion resistance of Q235 low-carbon steel and the corrosion resistance of the PEB/C treated steel increased with increasing the borocarburizing time, which was ascribed to the formation of boride in the hardened layer. After the PEB/C treatment, the friction coefficient of Q235 low-carbon steel decreased from 0.63 to 0.16. The lowest wear rate of hardened layer under dry sliding against ZrO2 ball was about 1.438 × 10−6 mm3/N m, which was only 1/12 of the bare Q235 steel. The PEB/C treatment is an effective method to improve the wear resistance of carbon steel.