Preparation And Performance Study Of Vanadium-niobium Permeated Layer Based On Thermal Diffusion Method

With the continuous development of the national economy,more stringent requirements are raised for mechanical parts or steel components in the operating and work conditions.So the materials are required to have higher performance to meet the working requirements of different environments.At present,there are many new methods for the surface modification of materials.The preparation of thermo-reactive diffusion layers on the surface of the material is one of the common treatments that can effectively replace the alloying of the material.Due to the performance advantages of the vanadium carbides(VC)and niobium carbide(NbC)layer,the hardness,wear resistance and corrosion resistance of the steel substrate can be effectively improved.In this dissertation,diffusion pack cementation coatings via Nb and V-Nb reactive source with an NH4Cl chloride activator have been studied in order to investigate on the surface of two steel substrates of 40Cr steel and 45 steel.The preparation process was investigated,the best treatment time and temperature were obtained,and then the pre-nitriding treatment was performed.The effect of niobium and the formation of vanadium-niobium layers and their properties were studied.The conclusions are as follow:1)By setting different time and temperature process parameters,a diffusion carbide infiltration layer was prepared on these two steel substrates.The comprehensive consideration of the formation of the infiltration layer and the preparation of the technical strip resulted in:1000℃×6h is the optimum technological parameters.The surface of the prepared(V,Nb)C layer is uniform and dense without obvious pores or cracks.For the 40Cr substrate,the thickness of of the single NbC diffusion layer is 9μm,and the thickness of the(V-Nb)C layer is 4μm.For the 45 substrate,the thickness of the single NbC layer is 8 νm,and the thickness of the(V-Nb)C layer is 3.5 μm.2)The thickness of the single NbC layer formed on the 40Cr steel is higher than that of(V-Nb)C diffusion layer.A dense NbC layer was formed on the surface of the 40Cr steel,and the peak of NbC was stronger than that of the vanadium-niobium layer by the XRD.The non-corrosive NbC phase was formed in the single-permeability sample of 45 steel,and the content of NbC was higher than that of the(V,Nb)C layer.At the same time,a large amount of elemental niobium and FeNb phases existed.The V,Nb carbides,Fe5C2 and were formed in the vanadium-niobium infiltrated layer on the surface of 45 steel.3)The wear resistance of the single niobium and vanadium-niobium coated 40Cr steel substrate is greatly improved compared with that of the substrate.The samples showed different wear resistance improving along the row 40Cr steel substrate →Niobium coated sample →Vanadium and Niobium coated sample.According to the corrosion test,the corrosion resistance is improving along the row 40Cr steel substrate→Niobium coated sample→Vanadium and Niobium coated sample.For the 45 steel subatrate,the wear resistance that was exhibited by the Vanadium and Niobium layer sample was the highest,followed by,Niobium coated and 45 steel substrate.The results of the corrosion resistance are similar and Vanadium and Niobium layer sample were higher than others.4)After pre-nitriding of the 40Cr steel and the 45 steel,the single Nb is diffused to form a niobium carbonitride and other metal compounds on the surface.For the V-Nb treatment,the vanadium-niobium carbonitride(V,Nb)CN is formed in the surface of the substrate.The singled diffusion layer thicknesses of the 40Cr steel and the 45 steel were 10 μm and 13 μm,and the thickness of the vanadium-niobium carbonitride layers were 10 μm.Compared with the samples that were not pre-nitriding,the infiltration of nitrogen contributed to the formation of the thicker reactive-diffusion layer.5)After the pre-nitriding of 40Cr steel,the micro-hardness of the infiltrated layer is greatly improved compared with that of the no nitriding coated samples,respectively.The micro-hardness is maxium in the the surface of the diffusion layer,and then is reduced along side the surface to the steel substrate.This distribution is useful for improving the adhesion between the diffusion layer and the substrate.In the meanwhile,it can improve the sample properties.The wear-resistance of 40Cr steel substrate was improved after the pre-nitriding treatment,and the wear resistance of V-Nb coated sample after pre-nitriding of 40Cr steel was better than that of single Nb coated samples,and the dominate wear mechanism was scuffing mechanism.Similarly,the corrosion resistance of the samples after pre-nitriding and diffused treatment are better than that of the bare substrate.The sample treated with V-Nb diffusion layer after pre-nitriding of 40Cr steel contains more NbC and Fe3N corrosion-resistant phases,making the corrosion resistance higher than that of the matrix.6)The wear resistance is in the order of vanadium and niobium co-alloying after prenitriding of the steel substrate of:vanadium-niobium carbonitride layers after prenitriding>single nitriding after prenitriding>45 steel substrate.The corresponding main wear mechanisms are:abrasion mechanism,adhesion wear and abrasive wear,respectively.The diffusion of active N atoms into the matrix improves the corrosion resistance of the nitriding layer during adhesion wear and abrasive wear of steel 45.The order of corrosion resistance of the sample is:vanadium-niobium carbonitride layers after prenitriding>single nitriding after prenitriding>45 steel substrate.