| Salt bath nitriding is a widely used chemical heat treatment technology that can significantly improve surface properties such as corrosion resistance and wear resistance. Salt bath nitriding technology has the following advantages: simple process, less-pollution and convenient handling.Since the production of active nitrogen atoms mainly depends on decomposition of salt bath, the concentration and thermal diffusion of active nitrogen atoms toward the surface of treated specimen mainly depends on the temperature. In normal salt bath nitriding, the temperature is about 570℃, the limited thermal diffusion rate at this temperature results in the shortage of energy consuming and lower efficiency. Therefore, it is necessary to investigate a method to improve the efficiency in salt bath nitriding technology.The effect of direct current field(DCF) in salt bath nitriding for 35 steel and 42 CrMo steel on the efficiency, microstructure and properties was primarily developed with self-designed set-up in this study. Meanwhile, the kinetics of salt bath nitriding for 45 and 42 CrMo steel was investigated and compared each other. OM, XRD, micro-hardness instrument and electrochemical workstation were used for investigating the structure, phase structure and performances.The results showed that DCF could effectively shorten the holding duration or increase the thickness of compound layer comparing with traditional technique(NM) for 35 steel. The holding duration was shortened from 100 min to less than 50 min by applying 7.5V DCF to get a compound layer thickness of 18.4μm at 575℃,and the thickness was increased from 18μm up to 29μm at the same duration of 100 min. The same typical characteristic peaks formed at DCF and NM test conditions, which are mainly composed of ε-Fe3 N and a little γ’-Fe4 N. The cross-section hardness could be improved to 750HV0.01 at the temperature of 575℃ and holding time of 80 min with 7.5V, which is about three times of the matrix. Compared with the conventional salt bath nitriding, the corrosion resistance of 35 steel treated by DCF was also improved. Activation energy(Q) for the nitrogen diffusion in 35 steel assisted by DCF was determined to be 159kJ/mol, which is lower than that of 184 kJ/mol in NM.DCF could also effectively reduce the temperature or increase the thickness of compound layer comparing with NM process for 42 CrMo steel. The temperature was reduced from 560℃ in NM to 530℃by applying 7.5V DCF to get a compound layer thickness of 6.1μm at 80 min, and the thickness was increased from 6.1μm up to 12.1μm at the same temperature of 560℃. The same typical characteristic peaks formed at DCF and NM testing conditions, which were mainly composed of ε-Fe3 N, a little γ’-Fe4 N and CrN. The cross-section hardness could be improved to 1146HV0.01 at the temperature of 575℃ and holding time of 80 min with 7.5V, which was about three times of that of the matrix. Compared with the conventional salt bath nitriding, the corrosion resistance of 42 CrMo steel treated assisted by DCF was also improved.Meanwhile, the results show that the thickness of compound layer of 42 CrMo steel is thinner than that of 45 steel under the same condition, while 42 CrMo steel owns higher surface hardness. Compared with 45 steel, the same typical characteristic peaks are formed, which are mainly composed of ε-Fe3 N, and γ’-Fe4 N for 42 CrMo steel. In addition, CrN phase is existed in 42 CrMo steel. The diffusion coefficient of nitrogen in 45 steels is 1.5 times of that in 42 CrMo steel. Activation energy for the nitrogen diffusion in 45 steels is determined to be 225 kJ/mol, which is lower than that of 184 kJ/mol in 42 CrMo steel. |
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