Microstructure And Properties Of High Velocity Air Fuel Cr₃C₂/Fe Composite Coating

In this paper,High Velocity air-fuel Spray（HVAF）technology was used to prepare Cr₃C₂/Fe composite coating on the surface of D2 steel cold roll.Firstly,the microstructure and properties of supersonic flame sprayed Cr₃C₂/Fe composite coating were investigated by adding micron Cr₃C₂and nano-Cr₃C₂powders（0wt.%,25wt.%,50wt.%,75wt.%,100wt.%,respectively）at the total content of 10wt.%Cr₃C₂ceramic powders To optimize the ratio of micron and nano Cr₃C₂of Cr₃C₂/Fe composite coating;On this basis,the effects of different total Cr₃C₂content（5wt.%,10wt.%,15wt.%）on the microstructure and properties of Cr₃C₂/Fe composite coatings were studied.The above optimized Cr₃C₂/Fe composite coating was annealed to study the effect of annealing process on the microstructure improvement and performance improvement of Cr₃C₂/Fe composite coating.The microstructure,mechanical properties and wear properties of Cr₃C₂/Fe composite coating were tested and analyzed by XRD analyzer,optical microscope（OM）,scanning electron microscope（SEM）,Vickers hardness tester,nano indentation tester,tensile testing machine and friction and wear testing machine.The experimental results show that:（1）Cr₃C₂/Fe composite coating was prepared by supersonic flame spraying HVAF technology.XRD results show that it mainly consists of Fe phase,Cr₃C₂phase,Cr₂O₃phase and Cr₇C₃phase.（2）Cr₃C₂powders with different micron-nano ratios were added（total Cr₃C₂was10wt.%）.The results showed that:When the nano-micron Cr₃C₂ratio is 50wt.%,the microstructure of Cr₃C₂/Fe composite coating is the tightest,and the microhardness,bonding property and fracture toughness are also the best.The microhardness is450HV_0.3(matrix hardness is 260HV_0.3),and the bonding strength is 42.8MPa.The fracture toughness is 2.26MPa·M^1/2.Friction and wear tests show that the friction coefficient of the five groups of Cr₃C₂/Fe composite coatings is stable at about 0.6.Compared with other ratios of Cr₃C₂/Fe composite coatings,the nano-micron Cr₃C₂/Fe composite coatings with 50wt.%Cr₃C₂have better friction and wear properties,and fewer furrows and spalling pits in wear marks.The lowest wear rate is only 5.4×10^-14m³/N·m.（3）Cr₃C₂powders with different proportions（Cr₃C₂nano-micron ratio of50wt.%）were added,and the results showed that:The bonding strength and hardness of Cr₃C₂/Fe composite coating with 10wt.%Cr₃C₂powder ratio are higher than that of5wt.%and 15wt.%composite coating,while the porosity of Cr₃C₂/Fe composite coating with 10wt.%Cr₃C₂powder ratio is lower than that of 5wt.%and 15wt.%composite coating.Friction and wear tests at room temperature show that the wear rate of Cr₃C₂/Fe composite coating with the ratio of Cr₃C₂powder to 10wt.%is lower than that of 5wt.%and 15wt.%,and the bonding strength is also better than that of5wt.%and 15wt.%composite coating.The Cr₃C₂/Fe composite coating with nano-micron Cr₃C₂powder containing50wt.%each（the total amount of Cr₃C₂is 10wt.%）was annealed.The results show that after annealing,the Cr₃C₂/Fe composite coating formed by Cr₃C₂powder and Fe-based powder and D2 steel matrix formed a micro-metallurgical bonding region.With the increase of annealing temperature,the element diffusion between Cr₃C₂/Fe composite coating and D2 steel matrix is intensified,which promotes the metallurgical bonding at the interface and improves the bonding strength.The Cr₃C₂/Fe composite coating annealed at 700℃and held for 3h has the best wear resistance and fracture toughness.After annealing,secondary carbides are precipitated in the Cr₃C₂/Fe composite coating,and the size of these secondary carbides increases with the increase of annealing temperature,which makes the wear resistance,bonding strength and hardness of the coating increase and then decrease.