Microstructure And Properties Of TD Salt-Bath Vanadizing Layer On The Surface Of 40Cr Steel

40Cr steel is a kind of common gear steel,and it has a good mechanical processing performance.Now,in order to break through the traditional gear surface strengthening method,TD salt-bath vanadizing technology is a good choice because of its characteristics including simple operation,low cost,obvious strengthening effect.This technology,by the high temperature diffusion,can form vanadium carbide layer with good stability and excellent wear resistance on the sample surface to prolong the service life of the gear,which has extremely important research value.However,TD salt-bath vanadizing technology has carbon content requirements on matrix material selection,and technical aspects to solve the problem of reducing deformation.The carbon content of 40Cr steel is higher than 0.35%,and the quenching degree of 40Cr steel is good,which can be effectively solved with quenching and slowly cooling operation.The selection of 40Cr steel on the matrix material of the surface reinforcement of gear steel can meet the technical requirements.This experiment,after setting the reasonable process parameters,chooses the anhydrous borax salt（Na₂B₄O₇）as the matrix,which takes full advantage of the characteristic of borax molten in high temperature can clean the surface with the products of matrix surface oxide reaction and form larger thickness of vanadizing layer,chooses NaF as the activator with better fluidity and B₄C as the reductor because it can substantially reduce the production of the dope to further improve the salt-bath fluidity,adds V₂O₅ as vanadium agent,according to the salt-bath formula 77%Na₂B₄O₇+4%B₄C+9%NaF+10%V₂O₅（wt.%）,selects the temperature of 900℃,950℃,1000℃and 1050℃vanadizing 4h,6h and 8h to prepare the vanadizing layer,and the main contents and the results of the research are following:（1）After setting the reasonable process parameters,the matrix surface can form the vanadizing layer with the certain thickness.The results of metallographic microscope show that the continuity and the compactness of the vanadizing layer are different in different process parameters.Meanwhile the continuity and the compactness of the vanadizing layer at 900℃and 1050℃are poor,and their grains are equiaxial.The continuity and the compactness of the vanadizing layer are improved at 950℃and 1000℃,and thier grains are columnar.（2）X-ray diffraction（XRD）is used to detect and analyze the phase of the vanadizing layer.The results show that the growth of VC grains in the vanadizing layer of different technological parameters has the preferred orientation.The growth of VC grains in vanadizing layer at 900℃has two preferred orientations.The vanadizing layer at 950℃and 1000℃shows that the mutual transformation of VC phase with different crystal surface index.The vanadizing layer at 1050℃has partly fallen off,which results the phase of VC reducing.（3）Scanning electron microscope（SEM）is used to observe the microstructure of the vanadizing layer section and energy disperse spectroscopy（EDS）is used to detect and analyze the composition of the vanadizing layer section.The results show that there is the transition zone in a few vanadizing layers with different process parameters.The surface decarburization of the vanadizing layer at 950℃and 1000℃causes the vanadizing layer migration,and the content of C element in the surface decreased.The vanadizing layer at1050℃has the common phenomenon of the surface falling off.（4）Analyzing and discussing the formation mechanism,growth law and grain growth mechanism of the vanadizing layer,and studying the influence of different vanadizing temperature and treatment time on the vanadizing layer thickness.The results show that the growth rate of the vanadizing layer is increasing with the increase of temperature,but the decarburization of the vanadizing layer causes the oozing layer falling off and the thickness decreasing.The growth kinetics curve of the thickness of the vanadizing layer and the treatment time approximately satisfies X^0.544=0.375t.The grain growth speed at different direction of vanadium carbide determines the grain growth pattern.V_l≈V_h in the vanadizing layer at 900℃forms isoaxial crystals.V_l<V_h in the vanadizing layer at 950℃and 1000℃forms columnar crystals.V_l>V_h in the vanadizing layer at 1050℃forms the grain direction being parallel to the surface of the matrix.（5）Microhardness of the vanadizing layer is measured and analyzed by microhardness tester.The results show that the vanadizing layer greatly increases the matrix hardness.Meanwhile the hardness of the vanadizing layer is increasing with the thickness increasing.However,the grain size of the ooze layer at 1050℃is large and the hardness is decreased.（6）Conducting friction and wear experiments on the vanadizing layer,and testing and analyzing the wear resistance and wear mechanism of the vanadizing layer.The results show that the wear quality of the vanadizing layer is less with different technological parameters.The vanadizing layer at 950℃has good anti-friction property,relatively less wear quality and better wear resistance.The vanadizing layer at 900℃is adhesive wear mechanism.The abrasion mechanism of the vanadizing layer at 950℃is common.Most of the other process parameters are adhesive wear with fatigue wear mechanism.（7）Testing the corrosion resistance of 3.5%NaCl solution on the vanadizing layer,and comparing and analyzing the corrosion resistance of the vanadizing layer with different parameters.It shows that the surface integrity affects the corrosion resistance of the vanadizing layer.When the density and the continuity of the vanadizing layer are poor,and the corrosion resistance is relatively poor.The decarburization of the vanadizing layer surface does not weaken the corrosion resistance,but when there are holes and cracks on the vanadizing surface,and the corrosion resistance will be seriously reduced.