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

Abstract

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), oxynitriding (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·㎠, but it became larger after all nitriding processes. Among the 0.02 wt% Al-added samples, the greatest ASR was approximately 30 Ω·㎠ 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 oxynitrided 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.

Keywords: Nitriding, Bearing steel, Electrical resistivity, Microstructure