Preparation And Performance Of The Carbonitriding Layers By Plasma Electrolytic Carbonitriding On The Steel Surface

Plasma electrolytic carbonitriding(PECN) technology, also being called micro-arc carbonitriding(MAC/N) is a special surface treatment process for rapidly carbonitriding on various metal surfaces. In this study, the micro-arc carbonnitriding process on 45 steel was investigated. The discharge characteristic in the electrolyte solution was concerned. Moreover, the effects of electrolyte composition, frequency and duty cycle of the pulse powers on the conditions for sparking and the microstructure, and properties of the formed carbonitriding layers were studied as well. The reaction mechanism in the electrolyte was also discussed.The results related to the discharge characteristic in the electrolyte solution indicated that the sparking voltage decreased with the increase of the conductivity of the electrolyte, frequency and duty cycle of the pulse power. It increased firstly and then decreased with the increase of the urea concentration in the electrolyte. On the other hand, the discharged current increased with increasing the conductivity, and decreased with the increase of the duty cycle. It decreased firstly and then increases with the increase of the frequency. The current and voltage characteristic curve demonstrated that the discharge process could be divided into four stages and most of the voltage drop was on the cathode under a constant pressure mode,. The generated inert N2 on anode could not provide active nitrogen particles for carbonitriding while only the NH3 generated from cathode reaction provided active nitrogen to form cabonitrided layers on the steel surface. The temperature of the sample surface was always higher than that of the electrolyte during the discharge process,The study on the effect of the solution compositions on the carbonitriding treatment demonstrated that the thickness of the carbonitriding layer increased with the increase of the urea content under constant voltage and treatment time. The content of KCl in the electrolyte could affect the quality of the layer through affecting the electrical parameters during the PECN process. It was found that the phases of the layer were mainly composed of martensite, iron compounds, iron compounds. The surface morphology was porous and the surface roughness reduced by increasing the frequency or decreasing duty ratio. Increasing the frequency and the duty ratio could lead to the increase of the hardness on the sample surface. However, the hardness on the section of the sample gradually decreased from the top surface to the substrate. Increasing the treatment time could effectively increase the thickness of the layer.The wear resistance test indicated that the surface wear resistance of PECN layer was significantly improved when compared to that of the substrate. The wear mechanism was microcutting accompanied with a small amount of fatigue spalling. Increasing the duty cycle and the frequency of the pulse power would reduce the friction coefficient and the worn scar became narrower by reducing duty cycle of the power.