Effect of direct current field on salt bath nitriding kinetics for 35 steel

The effect of direct current field (DCF) on the kinetics of salt bath nitriding was investigated for 35 steel. This technology was carried out by applying appropriate DCF of 7.5 V between the treated samples at temperature of 545 to 575 for 60 min to 120 min in the nitriding salt bath. The microstructure, nitriding thickness and phases of the treated samples were characterized by optical microscopy and X-ray diffraction (XRD). The results show that DCF can significantly promote the diffusion of active nitrogen atoms, thus enhance the nitriding efficiency, shorten the holding duration and decrease the treating temperature comparing with the traditional salt bath nitriding technique (NM). The diffusion coefficient of nitrogen atoms enhanced by DCF is twice higher than that in NM, and the activation energy for the nitrogen diffusion is decreased from 220 kJ/mol in NM to 181 kJ/mol. The main phases of compound layer are composed of Ε-Fe3N and a little γ'-Fe4N at each case. ©, 2015, Editorial Office of Transactions of Materials and Heat Treatment. All right reserved.