Study On Electrolytic Plasma Carbonitriding Of20CrMnTi

In urea solution, Plasma Electrolytic Carbonitriding（PEC/N） of pinion steel20CrMnTi had been investigated in this thesis. The effect of different types ofdischarge, on-load voltage and processing time on processing result had beenmeasured by metallographic structure observation, micro-hardness test, XRD phaseanalysis, SEM observation and elemental diffusion analysis et al. Moreover, thestability of urea solution as the electrolyte of PEC/N had been clarified bymulti-redoing test of the same electrolyte.The results shows that: It contains two types of discharge in PES, in whichdischarge A has little effect on how the C/N permeate into the workpiece but showsgreat promise on cleaning material surface or surface activation, and discharge B isthe main factor controlling the PES process but shows great difficulties on having itcontrolled. However, the difficulties were solved with a certain bleeder circuit and asuitable PEC/N technology compounding discharge A and B is fast and effective toget the carbonitriding layer. Using the same type of discharge under different loadingvoltage will lead to approximately the same metallographic structure （lots of needlemartensite and a little carbonitride and retained austenite in near surface and thewhole area martensite in cores） after processing the same time and shows littledifference on the layer thickness. The voltage should be low as the discharge B lightedto get the discharge B controlled. In that case,180V would be enough to get a nicelayer. At the same time, processing time cannot be too long to obtain a qualified layerfor a great many factors on controlling the discharge, for instance, no longer than5min.After processing at240V in different urea concentration for1min, the resultsshowed that the higher the urea concentration, the more thickness the layer gets.However, the layer thickness didn’t increase enough in proportion when the ureaconcentration reached20%wt. To avoid the waste of urea,20%urea were chosen tofeed the process. Potentio-dynamic polarization test indicated the20CrMnTi corrosionresistance was improved after PEC/N treated.The element diffusion test on the species treated after different procession timedemonstrated that a kind of oxide skin occurred on the surface in aqueous electrolyteas the iron oxidated by water vapor. The oxide skin can be cleaned by the discharge A, but the oxygen seep into the10-20μm near surface area which leads to thenon-martansite’s appearance.The stability test of the urea solution electrolyte indicated that urea solutionwould resolve fast after2min treated on180V with concentration lost of nearly50%.Its processing ability nearly lost after4thredoing experiment. The solution depositionis (C_0.4,N_0.6)Ti.