Effect Of Electrolyte On Structure And Properties Of Plasma Electrolytic Nitriding On38CrMoal Steel

The effect of organization and performance by plasma electrolytic nitriding on38CrMoAl steel under three different systems of the electrolyte proportions had beeninvestigated in this thesis. The microstructure and phase composition after processingby plasma electrolytic nitriding had been measured by OM, SEM, EDS and XRD.Deeper analysis about element distribution in layer had been implemented byRF-GDOES. The layer corrosion resistance had been measured by electrochemicalworkstation. Microhardness of specimens was also evaluated. The stability of optimalproportion of the electrolyte under three kinds of system had been clarified bymulti-redoing test of the same electrolyte.The experiment results show that: The nitriding layer consisted mainly of Fe₃N andFe₂N of iron nitrides with different concentrations of urea, the structure of the matrixis lath martensite. The higher the urea concentration, the more thickness the layer gets,the content of Fe3O4decreased gradually. The distance for the biggest microhardnessposition to the surface increases obviously, and the corrosion resistance trend isdecline to rise,when the urea concentration reached60%wt, the93μm the layer gets,the maximum hardness is897HVand the corrosion potential is most, the corrosionresistance is best. So after treated at60%urea is the optimal matching.To treat the urea electrolyte repetitively, with the increase in number of treated,the trend of conductivity is gradually rised, the thickness of layer, the concentration ofurea and the value of PH are decreased gradually. The diffraction peak of Fe3O4showed a trend of enhanced obviously. After treated six times, AlN phase wereappeared. The concentration of urea is from60%to17%, the thickness of layer falldown to25μm, termination of repeated trials.The phase of layer which is mainly of Fe₃N and Fe₂N of iron nitrides underdifferent concentration of formamidec.With the concentration of formamide increased,the thickness of layer showed downward trend after the first growth, the thickness oflayer reached122μm when the concentration of formamidec is70%, the content ofFeO is less than before, the distance of the maximum hardness position to the surfacefirst increased and then decreased. However, the performance of corrosion resistantfirst declined and then increased. When the concentration of formamidec is80%, theperformance of corrosion resistant is best. Take together, the optimal matching of formamidec is70%.The electrolyte of formamidec with treatment again, with the increase of usedtimes, the trend of conductivity and PH value is increased obviously, the white layergradually covered by needle nitrides, the thickness of the total layer and the diffusionlayer first increase and then decline, the thickness of layer is195μm after5times. Thecrack is increased after4times and the diffraction peaks of FeO increased gradually.The thickness of the layer is decline to173μm, termination of repeated trials.The layer phase which is mainly of Fe₃N and Fe₂N of iron nitrides with differentconcentration of the system of ammonia water. The substrate is lath martensite. Withthe concentration of ammonia water increased, the thickness of layer and the diffusionlayer consist of needle nitrides and small carbide at first increase then decline. Thecontent of FeO is decline. The maximum microhardness first increase and thendecrease and the distance of the maximum hardness position to the surface increasedgradually, when the concentration of the ammonia water is60%, the max thickness oflayer is reached165μm, the maximum hardness is1023HV. The performance ofcorrosion resistant is a trend of decline after the first increase. When the concentrationof the ammonia water is80%, the performance of corrosion resistant is best. So theoptimal matching of ammonia water is60%.The electrolyte of ammonia water with treatment again, with the increase of usedtimes, the crack is increased, the thickness of layer and the diffusion layer firstincrease and then decrease, The maximum of the thickness of layer is215μm after2times, the white layer gradually covered by vein needle nitrides, the crack is increasedafter3times, FeO content increased gradually, PH value and the concentration of NH3are decreased gradually, the trend of conductivity is increased gradua lly, theconcentration of NH3declined to4.03%, termination of repeated trials.In three electrolyte systems, with the concentration of nitriding mediumincreased, the surface roughness showed a descent tendency. The highest hardnessarea is gradually increasing distance from the surface and the maximum hardness isgradually declining. According to the contrast of the electrolyte with treatment again,the thickness of layer is decreased under the electrolyte of urea after2times. Theelectrolyte of ammonia water after treated3times with requiring refueling. However,the thickness of layer is decreased under the electrolyte of formamidec after5times,the stability is the best.