Study On Plasma Sprayed ZrO₂+WC Hybrid Thermal Barrier Coating For Continuous Casting Machine

As an important part of the continuous casting machine,the draw straightening roller is exposed to the high temperature about 1000°C and water-cooled alternating working environment for a long time,so its thermal fatigue life will reduce and the wear will increase.In addition,due to contact heat conduction,lubrication will fail and the life of the roller end bearing will be reduced it is time-consuming and laborious to replace straightening roller and its supporting parts,which seriously affects the production efficiency of the billet and the economic benefits of the enterprise.Therefore,it is very important to improve the service life of straightening roller and its related accessories.Heat resistance and wear resistance is an important way to improve the life of the roller and related accessories.Plasma spraying technology is a widely used surface strengthening technology.The coating prepared by plasma spraying technology can effectively improve the wear resistance,corrosion resistance and high temperature resistance of the substrate.Zirconia is a ceramic coating material with low thermal conductivity,high temperature resistance and oxidation resistance,which is widely used in aerospace,turbine blades and other fields.Tungsten carbide is a highly wear-resistant alloy material.In order to improve the wear resistance and heat resistance and extend the service life of the straightening roller,a composite coating of zirconia and tungsten carbide was prepared on the surface of the roller.the feasibility of plasma spraying ZrO₂+WC hybrid thermal barrier coating on the straightening roller is studied through the following experiments.In this paper,ZrO₂-8%Y₂O₃?8YSZ?and WC20Cr7Ni powders with a mass ratio of1:1 were prepared by mechanical mixing.The mixed coating of nickel-clad aluminum primer was prepared on the surface of straightening roller and H13 steel by plasma spraying technology.The element distribution in the cross-section of specimen was analyzed by scanning electron microscope.The heat resistance experiment was designed to analyze the heat resistance of H13 steel samples sprayed with mixed coating,uncoated coating,zirconia coating and tungsten carbide coating.The results show that heat resistance of the mixed coating which is close to the zirconia coated specimens.is better than that of the uncoated and WC coated specimens.Then,the hardness of the mixed coating samples was characterized,and rolling experiments were designed to test the wear resistance and bonding strength of the mixed coating.The results show that the hardness of the mixed coating?over 1200HV?is much better than that of the zirconia coating,but it is worse than the tungsten carbide coating whose hardness is 1400HV.The wear resistance of the mixed coating is 5-6 times that of the zirconia coating,and the bonding strength meets the requirements of the actual working condition of the straightening roller.Finally,the thermal shock properties of the mixed coating were studied,and the thermal fatigue resistance of the mixed coating was tested under the severe temperature change environment of the straightening roller.The thermal shock test device was designed to carry out the thermal shock test.The mixed coating before and after the thermal shock was analyzed by metallography,EDS,XRD,etc.It was found that there was no macroscopic peeling off of the mixed coating after 100 times of thermal shock,and the mixed coating has good phase stability.However,there is a certain amount of pores in the mixed coating,which will accelerate the local peeling off of the coating.Moreover,the failure of the coating will be caused by the long-term work under the joint action of expansion mismatch and other factors.