Optimization Of Plasma Arc Surfacing Process For T800 Cobalt Based Alloy And Study On Microstructure And Properties Of Surfacing Laser

The working environment of the high temperature valve is very harsh.It often works in high temperature,fluid medium erosion,corrosion and wear conditions,which result in the leakage of valve sealing surface due to corrosion and abrasion.Once the valve sealing surface leaked,it often bring significant economic losses and catastrophic accidents.It can greatly improve the valve sealing surface reliability and service life by using plasma arc surfacing technology surface a layer of T800 cobalt based alloy which has good heat resistance,corrosion resistance,wear resistance and erosion resistance.But when overlaying T800 cobalt base alloy,the surfacing layer is prone to defects and low hardness,because of improper process parameters.In this paper,orthogonal test and comprehensive test were used to optimize the process of plasma arc surfacing for T800 cobalt based alloy and a group of best surfacing parameters would be determined.The microstructure and composition phase of the surfacing layer of comprehensive test were studied,and the effect of dilution rate on the microstructure and properties of the surfacing layer was analyzed.Through the optimization of powder plasma arc surfacing process,a group of optimum surfacing parameters was obtained.The optimum parameters are:500?preheating temperature,4L/min plasma gas flow rate,180A transferred arc current,25mm/min transverse velocity,120mm/min swing speed,28g/min powder feeding rate.When the welding parameters are adopted,the surface of the surfacing layer is well formed and has no defects.The XRD result of different dilution surfacing layer shows that the main composition phases of the surfacing layer are?Co and Co₃Mo₂Si.By observing the microstructure of different dilution rate,it found that with the increasing of the dilution rate,the microstructure of the surfacing layer is changed from the hypereutectic structure to the hypoeutectic structure,and the first precipitated phase is changed from block Co₃Mo₂Si to?Co dendrite.The hardness test result of different dilution layer shows that the increasing of dilution rate will lead to the decrease of the hardness of the surfacing layer.The friction and wear test at 600?result shows that the high temperature wear resistance of the surfacing layer decreases with the increasing of dilution rate.This is because Co₃Mo₂Si has perfect strength and hardness at high temperature,what can greatly improve the high temperature wear resistance of the surfacing layer.However the increasing of dilution rate will lead to the reduction of the bulk Co₃Mo₂Si.The result of hot dip coating?70%Na₂SO₄+30%NaCl?test at 800?shows that the hot corrosion performance of the surfacing layer decreases with the increasing of dilution rate.In the high temperature and salts environment,the deposited layer with different dilution rate can produce a protective oxide film?CoCr₂O₄?,which will be destroyed by the cyclic oxidation and sulfidation of FeS.The increasing of dilution rate can make Fe element in the matrix gets into the surfacing layer increased.It will lead to the formation of FeS which can reduce the thermal corrosion performance of the surfacing layer.