Regularity Of Corrosion Resistance And Mechanism Of45Steel After Deep-layer QPQ Treatment

QPQ technology is a new metal surface modification technology in recent years and amajor breakthrough in the field of metallic materials.The formation of QPQ technology can betraced back to cyanide salt nitriding method in1929has gone through a long stage ofdevelopment through gas nitriding method,cyanide salt nitriding method,reactive cyanide saltnitriding method,gas soft nitriding, pollution-free salt bath compesite processing method. QPQtechonology not only inhertits the advantages of exisiting technology,but also overcome thedisadvantages of existing technology.QPQ technology has high wear resistance,highcorrosion resistance,fatigue resistance, micro-deformation,energy saving,environmentalprotection.Because of the thinner layer,the workpieces after QPQ treatment do not have thebetter wear resistance and corrosion resistance,so deep-layer QPQ technology has become anew topic.Deep-layer QPQ technology is a new technology which increase the nitridingtemperature above600℃.Quenched and tempered state of45steel was treated by deep-layer QPQ technology andQPQ technology this test.Corrosion resistance of45steel after QPQ technology and deep-layerQPQ technology was studied by salt spray test and electrochemical test,and the regularity ofcorrosion resistance by deep-QPQ technology was investigated.Microstructure of the nitridelayer was observed by metallographic microscope and EDS.Elements distribution ofnitrogen,oxygen and carbon and phase in nitride layer were detected by EDS andXRD,meanwhile the mechanism of corrosion resistance was studied.The hardness curve of thenitride layer was analyzing by using hardness gradient method.The results show that corrosion resistance of the nitride layer after deep-layer QPQtreatment is better than the one after QPQ technology.Increasing the nitriding temperature andextending the nitriding time will make the compound layer deeper and corrosion resistancebetter.When the nitriding temperature between640and660℃, corrosion time is more than1000h. Corrosion potential of nitride layer at660℃for2h is positive,corrosion current densityis smallest.With the extension of the nitriding time and the increase of the nitridingtemperature,corrosion potential will remove to cathode,corrosion current densitydecrease.Phase and elements distribution of the nitrided layer of different process parametersare the same.In general,the hardness gradient trend first rise then decline and level off atlast.Corrosion resistance of45steel depends on the depth of the compound layer.