| Valve is an important component in the fluid system for intercepting and reversing the media,are widely used in town water and heating supply systems,petrochemical pipelines,aerospace engines and other key areas.Hard seal ball valve has been wildly used all over the world for the advantages of simple structure,convenient maintenance and reliable performance.In China,hard seal ball valve is developing with a high pace and attaches great importance by the majority of valves manufacturing enterprises.Due to the special structure and the specific way of working of the hard seal ball valve,when it is in the process of opening and closing,the seat sealing surface and the ball will be hard to avoid friction,and then cause wear.This kind of situation will be more serious under thermal state,once the serious wear was happened,it will directly cause the valve leakage and have serious consequences.In order to ensure the quality and safety of the hard seal ball valve,researchers used the thermal spraying technology to prepare the Ni-based wear resistant coating on the surface of the ball and the sealing surface of the valve seat,to enhance the wear resistance of them.But as the result of the limited performance of the Ni-based alloy coating,it has a little improvement of the ball valve's life,and the coating prepared by Ni Cr-Cr3C2,Co Cr-WC and other materials had some new problems,such as high production cost,poor coating processing technology.And in this regard,it is most urgent to explore a new material and process technology to achieve the best balance between the valve product performance and economic benefits.This study put forward a new scheme to improving the properties of Ni-based alloy coating by adding a certain amount of carbides?Cr3C2?.The Ni-based Cr3C2 composite coating with Cr3C2 mass fraction of 0%,12.5%,25%,and 50% respectively and it was prepared on the surface of the test samples by using the high velocity oxy-fuel?HVOF?technique.The hardness,microstructure,phase composition and element distribution of the coating were measured and the wear tests under thermal state and molten salt?Na2SO4+25%wt Na Cl?corrosion tests were carried out to explore the wear mechanism of the coating at 50?,100?,150?,200?,250?,300?,and the mechanism of molten salt corrosion under 800?,the results showed that:1.The coating prepared by HVOF have low porosity and high hardness;2.The addition of Cr3C2 can produce Cr B,Ni B,Cr7C3,Cr23C6 and other hard phases in Ni-based alloy matrix and disperse in it,which significantly improve the hardness of Ni based alloy coatings and enhance the ability to resist plastic deformation under cyclic load of coating to make the coating has good wear resistance,and maintain low friction coefficient and wear rate.3.The temperature has a significant effect on the friction and wear properties of the coating.Due to the formation of oxide film,the friction coefficient and wear rate of the coating decrease with the increase of the temperature in the range of 50? to 250?.When the temperature exceeds250 ?,due to severe adhesive wear,the wear resistance of Ni-based alloy coating decreased with the increase of friction coefficient and wear rate.To add a certain amount of Cr3C2 can reduce the adhesive wear of coating significantly under this temperature.When the amount of Cr3C2 reached about 37.5% and the temperature rised to 300 ?,the coating friction coefficient and the wear rate is still at a low level.4.Under the condition of 800?,the molten salt corrosion has a strong corrosive effect on the coating.The S element in the corrosive salt can penetrate into the coating to weaken the bonding strength between the coating and the substrate,and directly lead to the loss of the Ni-based alloy coating,besides,the Cl element is able to be used to decompose the Cr2O3 protective film,which produced by the coatings at high temperature.The corrosion resistance of the composite coating is better when the mass fraction of Cr3C2 reaches to 37.5%. |
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