Study On Salt Bath Chromium And Vanadium-nitrocarburizing Of TA2 Titanium Alloy

TA2 titanium alloy has the characteristics of high specific strength,corrosion resistance and weldability.It is widely used in shipping,chemical industry,marine and other fields,but its hardness and wear resistance are poor.The material is prone to wear and cavitation,which seriously affects the service life and safety of the product.In this project,a new surface treatment process of titanium alloy is studied,which is called salt bath chromium and vanadium-nitrocarburizing(or compound treatment).This process generates a high hardness compound Chromizing Layer on the surface of the material to improve the wear resistance and cavitation resistance,and further expand the application of TA2 alloy.The main research results are as follows:(1)Based on the TA2 alloy compound treatment test,the effect of treatment time and treatment temperature on the hardness of the compound layer was studied,and the hardness was used as an index to conduct a four-factor and three-level orthogonal experiment.The study found that as the treatment time and treatment temperature increase,the hardness of the carburized layer shows an upward trend.The surface microhardness can reach 1408HV,which is 604%higher than that of the untreated sample(200HV)and 124%higher than that of the carbonitriding sample(627HV).The typical process parameters obtained by orthogonal test are:co-permeation temperature 850?,co-permeation time 4h,chromium supply agent 7.5%,vanadium supply agent 10%.(2)Using optical microscope and white light interferometer to observe the micromorphology of the surface and cross-section of the compound layer,it can be observed that the surface of the compound layer is very dense and uniform,and the compound layer covers the scratch,pit and other defects on the surface of the sample before compound treatment;the cross-section can be divided into compound layer,transition layer,matrix,and there are no loose layer,bubble,crack and other defects;The thickness of the layer can reach77?m,which is much thicker than that of carbonitriding and TD treatment.(3)The results showed that Ti N,Ti₈C₅,VC,Cr₇C₃,V₈C₇ and other high hardness compounds were the main phases in the compound layer by XRD.Combined with microhardness and EDS analysis,the mechanism of treatment was studied.In the early stage of CO infiltration,C and N atoms penetrated rapidly through gap diffusion,while Cr and V atoms diffusion mechanism was displacement diffusion or vacancy diffusion,which required that the co-infiltration temperature was more than 700?and the co-infiltration time was more than 2 hours before a large amount of infiltration could be carried out.According to the second law of Fick diffusion and Arenius formula,the diffusion activation energy of the compound treatment is calculated,and the diffusion equation of the treatment is derived:l²=1.897?10^-9exp(-9533.68/T)t.(4)The two-dimensional and three-dimensional morphology of wear marks were observed by ultra-depth of field microscope and white light interferometer,and then the wear mechanism was analyzed.Under the conditions of dry friction,Na Cl solution lubrication and oil lubrication,the wear rates of the compound treated samples are 50%,42%and 47%lower than those of the untreated samples,respectively,indicating that the compound treatment can effectively improve the wear resistance of TA2 titanium alloy.There are a lot of furrows and adhesion pits on the surface of the three samples,so the main wear mechanisms are abrasive wear and adhesion wear.With the increase of the experimental load,the adhesion wear and plastic deformation of the samples increase,while the fatigue wear phenomenon appears on the surface of the samples at a higher sliding speed.(5)Ultrasonic vibration cavitation erosion test,electrochemical corrosion test and static tensile test were carried out on the compound treated sample,carbonitriding sample and untreated sample to study the effect of compound treatment on cavitation erosion resistance,corrosion resistance and matrix strength of TA2 alloy.The experimental results show that the weight loss of cavitation erosion of the compound treated sample is 50%and 43%lower than that of the untreated sample and the carbonitriding sample respectively,and the depth and number of cavitation pits on the surface of the compound treated sample are much lower than that of the untreated sample and the carbonitriding sample,which indicates that the compound treatment can significantly improve the cavitation erosion resistance of TA2 alloy;the self-corrosion potential of the compound treated sample is higher than that of the untreated sample The corrosion current density decreases by 37%and the tensile strength decreases by15%and 19%respectively,while the reduction of area increases by 31%and 42%respectively after the compound treatment at 800?and 900?.The reason is that the grain size increases,the strength decreases and the plasticity increases after long-term storage at high temperature.