Study On Plasma Electrolytic Carbonitriding Of Medium Carbon Steel

Plasma electrolysis carbonitriding (PEC/N) technology is a new emerging surface technology that could achieve rapid surface modification. In this study, a carbonitrided layer was produced on 5140 steel by plasma electrolytic carbonitriding at room temperature and atmospheric pressure. And the extraction process was chosen by orthogonal test. Surface and cross-section morphology was investigated by optical microcopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the surface morphology is characteristic of pores and spheroids. Particle size of the surface is about 30nm when the sample was processed for 20min. Energy dispersive spectroscopy (EDS) analysis reveals the presence of nitrogen and carbon in the carbonitrided layer. The corrosion inhibition performance of carbonitrided layer is studied by polarization curves.Through the statistical analysis of the test, it is found that the best condition to obtain thick carbonitrided layer is at 125V in the solution composed of water, potassium chloride and formamide accounting for a proportion of 80 percentages. And the distance between should be within 10mm while surface roughness Ra should be 0.13μm. Thickness of the layer increases fast at the beginning but slows down accompanied by the increase of the processing time. When the sample was treated for 20min, a 58μm thick carbonitrided layer formed, with a corresponding Vickers hardness of 650HV0.05. The carbonitrided layer consists of a top white layer, an inner white layer and a diffusion layer. XRD study combined with selected area electron diffraction (SAED) analysis confirms that the top white layer is mainly composed of compounds including s-Fe2-3N, Fe5C2 and Fe3C, while the inner layer is verified, for the first time, to be made ofα-Fe(N) martensite. Study of polarization curves indicates that carbonitrided layer greatly improves the corrosion resistance of 40Cr.