Research On The Alkali Corrosion Resistance Of Water Wall Composites Prepared By Plasma And Laser Cladding Techniques

High temperature alkali corrosion is one of the main reasons for the failure of metal components such as water wall tubes and primary air nozzles in alkali recovery boilers.The water wall tubes used nowadays are mainly imported explosion composite steel tubes with high industrial costs.Therefore,it is of great significance to study new materials and adopt new technologies to solve such high temperature molten alkali salts and alkaline solution corrosion problems.The plasma surfacing layers and the laser cladding layers are fabricated on the surface of 20G boiler steel using FSAN38 self-fluxing alloy powder customized according to the composition of the composite tube SAN38,310 stainless steel alloy powder,310+10%WC alloy powder and 310+20%WC alloy powder,respectively.The microstructures,phases and chemical compositions of all the layers are observed and analyzed.The basic properties such as hardness,friction and thermal conductivity are evaluated,and the properties of alkali corrosion resistance and its essence are studied compared with the SAN38 composite steel.The following conclusions are obtained:The plasma surfacing layers form a great metallurgical bond with the base metal with small degree element dilution.The stratified organization appears in the welding layer,including equiaxed crystal,cell crystal,columnar crystal,dendritic crystal,and interdendritic multi-element eutectic from the fusion line to the surface.The tungsten carbide particles dissolve,diffuse and associate with Fe,Cr,Ni elements to form metal compounds.The microstructure become denser because that the addition of WC increases the nucleation sites.However,the addition amount is limited.In this experiment,the amount of 10%is approprite.The microstructure of FSAN38 plasma surfacing layer is quite different,which consists of white bright color equiaxed crystals and needle-like regions.Some tungsten carbide particles are deposited in the deposited layers next to the fusion line,and there are halos around the tungsten carbide particles.The hardness of the plasma surfacing layers is higher than that of the SAN38 composite steel,and the friction and wear properties are more stable.The addition of tungsten carbide can greatly improve the properties.The thermal diffusion coefficient of the plasma surfacing layer is less than that of the SAN38 composite layer,which is mainly related to the orderliness of the organization.The corrosion rate of plasma surfacing layer in the electrochemical corrosion of alkali solution and the rate in the high temperature molten alkali salt are all lower than that of SAN38 composite steel.In the corrosion experiment,the integrity of the surface of metal materials mainly depends on the protection of oxides such as Cr203 and NiO.There are no obvious structural defects in the plasma surfacing layers,and there are small holes in the SAN38 steel,which easily becomes a weak zone during the corrosion process,thereby accelerating the corrosion.The 310+10%WC plasma surfacing layer has the best performance in all aspects.The laser cladding layer consists of equiaxed crystals,cellular crystals,columnar crystals,dendrites and interdendritic multicomponent eutectics.Tungsten carbide particles fully dissolve and diffuse under high energy irradiation and convection stirring,which makes the structures become denser and more uniform.However,when the content of tungsten carbide increases to 20%,significant and deep cracks occurred.Some tungsten carbide deposits with halos around and metal compounds inside.The FSAN38 laser cladding layer consists of a white bright area,a wide columnar crystal area,and an uniformly dense.mixed needle area.The laser cladding layers are superior to the plasma surfacing layers in terms of hardness,friction and wear resistance,and own excellent electrochemical corrosion resistance and high temperature molten alkali salt resistance.The thermal diffusivity of the laser cladding layer is slightly higher than that of the plasma surfacing layer,but it is still inferior to the SAN38 blasted composite steel plate.The principle of laser cladding and plasma cladding is similar,and the mechanism of various performance tests is basically the same.But because of the higher energy of laser cladding and the property of quicker heating and faster cooling,the microstructures of the laser cladding layer are better and the performances are much better.Among all the laser cladding samples,the 310+10%WC laser cladding layer has the best overall performance and is the ideal material and process alternative to the explosive composite tube.Although its thermal diffusion capacity is slightly lower than that of the explosive composite steel SAN38,the thickness of the composite layer is only 1.5-2mm,which has little effect on heat transfer,and the internal 20G substrate plays a major role in thermal diffusion.