Effect Of Rare Earth QPQ Nitriding Treatment On Microstructure And Properties Of Steel

In recent industrial production,most of the metal parts have incurred huge economic losses due to corrosion and wear loss.Some gear parts and mechanical workpieces applied in harsh environments have high surface performance requirements.Therefore,it is of practical importance to find a metal surface strengthening treatment technology for surface strengthening of different types of steel parts in order to avoid causing a lot of waste and to achieve a green,environmentally friendly and economical modern industry in China.In view of the above problems,in this paper,self-made base salt as nitriding agent,yttrium carbonate,cerium carbonate and lanthanum carbonate as rare earth infiltration agent,potassium nitrate as oxidant,with three different carbon content,low carbon steel,medium carbon steel and high carbon steel as nitriding workpiece,to improve the surface performance of the metal working in some harsh environment.Therefore,the surface microhardness,corrosion resistance and wear resistance were used as the performance basis,and the morphology of infiltration layer was observed and the nitriding mechanism was explored by means of SEM and XRD.The results are as follows:（1）Q235B low carbon steel by rare earth QPQ nitriding treatment,the composition of the physical phase composition of the seepage layer are mainly composed of Fe₃O₄,Li₂Fe₃O₄,Li₅Fe₅O₈ and Fe₃N;under the QPQ process conditions of 2%rare earth and580℃,the thickest seepage layer can be obtained,which is about 27.5μm;under the QPQ process conditions of 2%rare earth and 565℃,the highest The surface microhardness and the minimum corrosion current density were 545 HV and 3.35×10^-6A·cm^-2,respectively,and less rust layer was generated on the surface under the salt spray test.After the excess of rare earth,there is a decrease in performance,which has an obvious effect on the thickness of the seepage layer and microhardness,and a smaller effect on corrosion resistance.（2）42CrMo steel after rare earth QPQ nitriding treatment,surface properties are enhanced,42CrMo steel infiltration layer phase composition are mainly composed of Li₂Fe₃O4,Fe₃O₄,Fe₂N,Fe₃N and Fe₄N;The maximum microhardness of 42CrMo steel can be 780HV at 550℃,and the average microhardness of 42CrMo steel can be increased by about 60HV,and the effective penetration depth can be increased by 40μm under the QPQ process condition of adding 3%rare earth.Under the condition of 3%rare earth and 590℃,the corrosion current density can be reduced by 7 times,which is about 9.103×10^-7A·cm^-2.Under the salt spray test,less rust layer is formed.The average friction coefficient of 42CrMo steel with rare earth added QPQ process is about 0.2lower than that without rare earth added QPQ process,and the minimum average friction coefficient of 0.51 can be obtained under the condition of 3%rare earth added and 550℃QPQ process,and the friction curve is more stable.（3）After rare earth QPQ nitriding treatment,the permeability layer structure of9Cr18Mo high carbon steel is mainly composed of Li₂Fe₃O₄,Fe₃O₄,Fe_2-3N,Fe₄N and Cr N.The average hardness of 9Cr18Mo steel increased by about 100HV,and the highest hardness of 1371HV was obtained with 6%rare earth addition.In the experiment with rare earth content as a variable,although Cr N was precipitated,there was no"Cr poverty"which resulted in reduced corrosion resistance after nitriding,and the performance was good in the salt spray test.The corrosion potential of 9Cr18Mo steel with 3%rare earth QPQ nitrided is-0.09V and the corrosion current density is4.1×10^-7A·cm^-2.The corrosion resistance is the best under this condition.In the wear resistance test,the minimum average friction coefficient is 0.61 under the condition of3%rare earth,and the change of rare earth content has little effect on the wear resistance of 9Cr18Mo steel.