Study On Microstructure And Properties Of Cobalt-based Composite Powder Plasma-arc Surfacing Layers

The valves are fluid control equipment in the application of chemicals,machinery and other industries.Because of the mutual friction,extrusion and shear between the sealing surface,and erosion and corrosion effect of fluid,the valves are frequently in high temperature and high hydrodynamic pressure during industrial use and the valves are vulnerable to be damaged.Cobalt-based alloy is widely used in repairing and strengthening the sealing surface of the valve for its high temperature resistance,wear resistance and corrosion resistance.This paper adopts plasma-arc surfacing welding technology,adding a certain amount of WC particles respectively in the Co-Mo-Cr,Co-Cr-W,Co-Cr-Ni three different series of cobalt-based alloy which are selected as surfacing material.After the preparation of cobalt-based composite powder surfacing layers on the surface of 304 stainless steel,the influence of WC on the different series cobalt-based alloy surfacing layers of the microstructure and mechanical properties can be found in this research.The test indicates that the optimal process parameters of three kinds of cobalt-based alloy plasma-arc surfacing.The welding current is 99 A and the powder feeding rate is 35 g/min for Co-Mo-Cr cobalt-based alloy;the welding current is 90 A,the powder feeding rate is 35 g/min for Co-Cr-W cobalt-based alloy;Co-Cr-Ni cobalt-based alloy,the welding current is 99 A and the powder feeding rate is 38 g/min.Three kinds of cobalt-based alloy surfacing layers are mainly divided into four layers,fusion zone,equiaxed dendrite near the fusion zone,columnar crystal in certain direction between the near fusion zone and near surface and fine equiaxed grains near the surface from the bottom to the top of surfacing layers successively.The addition of WC to Co-Mo-Cr series alloy has a great influence on the microstructure.With the increase of WC content,the amount of carbides precipitated in the Co-based solid solution increases,the carbides size gets larger and the change can be noticed in the shape of the carbides,from irregular blocks to cruciform and "x".The dilution rate of cobalt-based alloy surfacing layer becomes greater with the increasing addition of WC.When 10%WC is added to the layers,the surfacing layer was composed of γ-Co,CoSi,Co7Mo6,CrSi2,CoWSi,Mo3Co3C,M23C6 and so on.With the increase of WC content,the matrix phase was reduced and the carbide phase increased.There are a number of carbides like Fe3W3C and Fe3Mo3C.With the addition of WC is 40%,the WC phase appears in the surfacing layer.After added 10%～30%WC in Co-Cr-W series cobalt-based alloy,the grains become finer.Dendrites cobalt-based solid solution precipitates first,then interdendrite eutectic structure can be formed.When the content of WC is 40%,the carbon content in the weld pool is in high level,flower-like carbide precipitines first,then eutectic structure forms.When 10%WC is added,the surfacing layer is composed of γ-Co,CoCx,Cr7C3 and CrSi2,and the M6C type carbide Co3W3C is formed as 20%WC and 30%WC are added in.When the WC addition is 40%,the dilution rate of cobalt-based alloy surfacing layer becomes larger,and the content of Fe raises,and the Fe3W3C phase is formed.After the addition of 10%～20%WC into the Co-Cr-Ni series cobalt-based alloy,the microstructure of the surfacing layers after solidification not change,and the grains become finer.When 30%WC is added,the globular precipitates appear on the upper part of the surfacing layer.When WC content is 40%,the microstructure of the surfacing layer changes evidently,and there appears multi-morphology structure,flower-like carbide precipitines in the cobalt-based solid solution.According to experimental result,the surfacing layers are composed of γ-Co,Ni-Cr-Fe-C,CrSi2,Co3W3C as 10%WC and 20%WC are added.While the addition of WC increases to 30%,surfacing layer appears Fe3W3C and W2C phase.In this paper,after added the WC,the Rockwell hardness and Vivtorinox microhardness of the surfacing layers of three series of cobalt-based alloy are remarkably boosted with the increase of WC content.In normal temperature wear tests,the wear resistance of cobalt-based alloy surfacing layers above-mentioned can be enhanced and the wear mechanism changed from abrasive wear to adhesive wear and abrasive wear together with the increasing addition of WC.In the high temperature wear test,the friction coefficient of Co-Mo-Cr series cobalt-based alloy surfacing layers with WC particles is reduced to one-third of the original.In the electrochemical corrosion test,after adding WC,the self-corrosion potential of the Co-Mo-Cr series cobalt-based alloy surfacing layers is increased.However,the corrosion current density is increased greatly and the corrosion speed is accelerated,which makes the corrosion resistance worse.After the addition of WC into Co-Cr-W series cobalt-based alloy,the self-corrosion potential is increased,reduced the corrosion tendency.The corrosion current density is increased slightly.And the corrosion resistance of the 10%WC surfacing layer was better than that of Co-Cr-W series cobalt-based alloy.After added the WC,the corrosion current density of Co-Cr-Ni cobalt-based alloy surfacing layers was sharply reduced,and the corrosion resistance of the 30%WC surfacing layer is the best.