Abstract: Using α-Al₂O₃ powder as boriding filler and 2% activated carbon as additive, the variation of boriding current was investigated under a constant AC electric field assisted pack-boriding carried out on 40CrNiMo steel. By adjusting the AC electric field voltage to obtain stable low temperature boriding temperature, a borided layer was prepared on the surface of 40CrNiMo steel and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and microhardness tester. The results show that a stable conductor is created as a result of activated carbon adhering to the Al₂O₃ powder's surface, which stabilizes the boriding current at different stages of the electric field assisted pack-boriding, resulting in a relatively stable boriding temperature. A dense single-phase Fe₂B borided layer on the surface of 40CrNiMo steel can be prepared by adjusting the electric field voltage to control the actual boriding temperature at a lower temperature (650, 680, 700 ℃). The SEM observation shows that the thickness of the borided layer is ≥13 μm. The surface hardness of the borided layer is relatively high and can reach about 1200 HV0.05 even at 650 ℃.

Key words: variable electric field, low temperature boriding, hardness, Fe₂B