| Concrete pump truck is an important part of today's construction machinery.However the piston rods of concrete pump truck is serviced in high impact or alternating load conditions,which makes the surface of piston rod fallen off to failure.The piston rod can be remanufactured by the high velocity arc spraying.But due to lamellar structure of spraying coating,mechanical bonding with substrate,existing pores,oxides and unmelted particles,it affected the performance of remanufacturing piston rod surface in a certain.Remelting technology could effectively improve the microstructure and mechanical properties of the original spraying coating.In this thesis,the as for object,the spraying coating of remanufacturing concrete pump piston rod was remelted by plasma remelting technology,and the microstructure,microhardness,wear resistance and corrosion resistance of remelting coating was studied in this thesis.The spraying coating was remelted by plasma remelting,and orthogonal experiment was designed to determine the major or minor factor.It was found that the m,ajor factor is arc current,the minor factor is plasma gas flow,the middle factor is moving speed.Optimum parameters were obtained for plasma remelting,which were current 80A,plasma gas flow rate 3L/min,moving speed 2mm/s,respectively.The microhardness and microstructure of different overlapping ratio area were analyzed and obtained the best overlapping ratio 30%through multitrack remelting overlapping experiment.The microstructure and phase composition of spraying coating and remelting coating under different technical parameters were compared by using the optical microscope,scanning electrical microscope(SEM),energy dispersive spectroscopy(EDS),X-ray diffractometer(XRD),and microhardness tester.The plasma remelting could eliminate the original pores,unmelted particles,and a little oxides Al2O3 and Cr2O3.At the same time,remelting coating generated solid solution(Fe,Cr),high hard phase carbides including(Fe,Cr)7C3 and Cr23C6.The coating became homogeneous after remelting,metallurgical bonding with the substrate.The remelting coating was composed of bottom column crystals,middle exquiaxed crystals and top cellular crystals structure in honey distribution.Compared with spraying coating,the porosity of remelting coating was 1.6%,which increased 69.5%,and the microhardness of remelting coating was 1.4 times more than spraying coating.The wear resistance of high velocity arc spraying coating,chromium plating coating and remelting coating were tested to analyze the friction and wear characteristics of three coating under different loads by CETR sliding friction and wear testing machine.The result showed that the wear mechanism of spraying coating was fatigue wear and adhesive wear,the wear mechanism of chromium plating coating was slight abrasive wear,and the wear mechanism of remelting coating was mainly plow wear.In remelting coating there existed hard phase carbide as "skeleton" effect,which obviously improved the bearing capacity and wear resistance.To sum up,the wear resistance of remelting coating was better than that of spraying coating,which reached the performance level of new piston rod.The Tafel polarization curve measured through electrochemical experiments was fitted to corrosion resistance analysis.The corrosion potential of remelting coating was higher than that of spraying coating and chromium plating coating,which were 20.2mV,12.5mV respectively.And the corrosion current density of spraying coating was 8.21E-6,which was much larger than that of remelting coating and chromium plating coating,which are 3.29E-7 and 9.53E-8 respectively.So it could be concluded that the corrosion speed of spraying coating is obviously higher than that of remelting coating and chromium coating,which proved that the corrosion resistance of plasma remelting coating had been significantly improved.The spraying path on the piston rod was planned and verified reasonably through analyzing the distribution of temperature.And the remelting coating was prepared by optical parameters.Finally,the remanufacturing process of piston rod was evaluated about economics,and it revealed that the cost of remanufacturing products was much lower than that of new products. |
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