Application of Various Nitriding Processes to Improve the Electrical Resistance of Al-Added 52100 Bearing Steel

Two ASTM A295 52100 bearing steels with different concentrations of Al (0.02 and 1.90 wt%) were nitrided by three processes: pure nitriding (gas), oxy-nitrocarburizing (gas) and salt bath nitriding. To assess their electrical performance, the area-specific resistance (ASR) of the surface was measured by the four-point probe method. The microstructure of the surface layer was investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the ASR of the non-nitrided sample was only approximately 0.005 Ω cm², but it became larger after all nitriding processes. Among the 0.02 wt% Al-added samples, the greatest ASR was approximately 30 Ω cm² for the sample nitrided in a salt bath, which can be ascribed to the large amount of oxides (Fe₃O₄) in the compound layer that exhibits higher electrical resistivity than other nitrides (ε-Fe₃N and γ′-Fe₄N) observed in the pure or oxy-nitrocarburized samples. Additionally, the addition of Al significantly increased the ASR in the salt bath nitrided sample, which indicated that the Al₂O₃ formed along the pores in the compound layer played an important role in improving the electrical resistance.