Effect of D.C. electric field on salt bath nitriding for 35 steel and kinetics analysis

A rapid salt bath nitriding technology was primarily developed by additionally applying direct current (D.C.) electric field on the basis of traditional technique (NM). Characterization of the modified surface layers was made by means of optical microscopy, Vickers micro-hardness test and X-ray diffraction analysis. The results showed that D.C. electric field could significantly enhance the nitriding efficiency and the enhancement effect was closely related to the intensity of D.C. electric field. By applying D.C electric field of 7.5 V, even a little thicker compound layer could be obtained at only half duration of that in traditional technique, and the thickness of compound layer increased more than 60%, from 18 μm up to 29 μm at the same treating temperature of 848 K and holding duration of 100 min. Meanwhile, higher surface hardness, modestly higher sub-surface hardness and superior hardness profile were obtained assisted by D.C. electric field. It was also found that the diffusion coefficient of nitrogen was increased more than 1.9 times and activation energy was decreased from 184 kJ/mol to 159 kJ/mol enhanced by D.C. electric field. The possible enhancement mechanism is that D.C. electric field can promote chemical reactions and produce more active nitrogen atoms in the salt bath, positively charge the active atoms and force them diffuse directionally toward the surface of the treated specimen, and hence significantly improve the efficiency of the active atoms in the salt bath.