Study On Vanadium Carbide Coating On Die Steels By Thermal Diffusion Process

Salt bath vanadising belongs to salt bath surface alloying by TD process.Vanadium carbide coating formed by this process is very stable at room temperatureand shows excellent wear property. The process has been given more and moreattention since it was put forward by Japanese Toyota research center. In abroad, theprocess has become a mature technology and been widely used in the field of coldworking die surface hardening. However, some technology difficulties, such as shortservice life of salt bath, severe erosion of device, large deformation of work pieceand difficult cleaning of remained salt agent, cause the process has not been usedwidely at home. Therefore, the device, vanadizing technology, microstructure,properties and mechanism have not been adequately studied at home.Aiming at such actuality, the initial behavior, growth course and microstructurehas been systematically studied by applying SEM/EDS, XRD and EPMA on thebase of successful development of device with industrial value for TD process andTD process technology. The micro-hardness of coating surface and cross-sectionwere determined by Victor hardness tester. Abrasion resistance of vanadium carbidecoating was tested by MM200 abrasion tester and the wear mechanism wasdiscussed according to the observation results of wear morphologies. The effects ofrare earths on vanadizing were deeply examined. Based on the experimental results,the forming mechanism of coating was analyzed from the points of thermodynamicsand kinetic and the growth model was established. The main and valuableconclusions are as followings:1. By studying the effects of main technological parameters and thecomposition of substrate on the thickness of coating, it is found that the increase ofimmersion temperature can evidently increase vanadizing rate. The calculationresults show that the thickness of coating is nearly parabolic in relation toimmersion temperature and linear in relation to dissolved carbon content ofsubstrate.2. The research results of initial behavior and growth course lead to the conclusion that the microstructure of substrate influences initial behavior of coatingformation and the initial behavior and growth pattern are similar at variousimmersion temperatures. However, the influence of immersion temperature ongrowth rate of coating can lead to spend various times in the same stage.3. By analyzing microstructure of vanadium carbide coatings formed onseveral carbon steels and alloying steels at different technology parameters, theconclusions can be drawn that immersion temperature influences the shape and sizeof grains and temperature and substrate effect the preferred orientation of coating.Moreover, the analysis results of EPMA and EDS show that the concentrations ofvanadium and carbon slightly change from interface to coating surface andimmersion temperature influences the concentrations of C and V.4. The testing results of surface hardness of coating by Victor hardness testerindicate that surface hardness of coating is lower than the ideal hardness ofvanadium carbide and hardness increases with the increase of substrate hardness andthe thickness of coating. The testing results of cross-sectional micro-hardness showthat hardness gradually increases from coating surface to interface and appearsbreak at interface. Moreover, the comparison of hardness of coatings formed atvarious temperature lead to the conclusion that low temperature process canincrease the hardness of coating.5. Vanadium carbide coating can greatly increase abrasion resistance. Itsabrasion mechanism is fatigue peeling abrasion.6. According to the experimental and above analysis results, themathematical model of kinetics of the coating growth by TD process wasestablished adopting the unsteady diffusion theory and regular solution sub-latticetheory. Based on the mathematical model, the analysis results of effects of differentalloying elements on the thickness of carbide layer lead to the conclusion that whenthe austenitic carbon activity influential factor of an alloy element in alloyed steelsis positive the alloy element will improve the growth of carbide coating, whereasnegative, it will reduce it.7. By studying the effects of rare earths on vanadizing technology andmicrostructure of coating, the conclusions can be drawn that there are an optimal addition amount of rare earths an optimal process temperature for accelerating effectof rare earths and that the accelerating effect of rare earths focuses at the initialstage of TD process and decreases with the increase of the thickness of thevanadium carbide layer. Moreover, the addition of rare earths can decrease the sizeof grain and brittleness of coating and increase the hardness of coating.