Study On The CMT Surface Surfacing Process And Sequence For Water Wall Tube

Water wall tube is an important cooling device in thermal power plants and industrial boilers.Due to the serious corrosion and wear of pipe row caused by a large number of flue gas and other particles in its service environment,the traditional thermal spraying method can not meet the protection needs,but the coating prepared by surfacing welding method can achieve the protection effect.In this thesis,Ni625,Cr30Ni70 and Cr50Ni50 welding wires are selected.Through the mechanical properties,friction and wear properties and corrosion resistance,the wear resistance and corrosion resistance of three kinds of welding wire cladding layer are compared and analyzed.Then,under three CMT surfacing technology modes（pulse,PMC and CMT）,the influence of welding process parameters（wire feeding speed,welding speed,arc length correction,etc.）on weld forming law was compared and analyzed.Finally,combined with the finite element simulation technology of flat surfacing and pipe surfacing,the influence of different surfacing sequences on deformation was studied.The results show that:The tensile strength,average microhardness and impact strength of Ni625welding wire cladding layer are higher than Cr30Ni70 and Cr50Ni50,but the elongation is slightly lower than Cr50Ni50 and Cr30Ni70.The friction coefficient of three kinds of welding wire cladding layer fluctuates little in the early stage,decreases first and then increases in the late stage,and the overall friction coefficient has little difference.Is about 0.6;Compared with Cr30Ni70 and Cr50Ni50,Ni625 cladding layer has fewer surface wear scratches,and the wear marks and edges are smooth,and the wear is not serious.The wear amount and relative wear rate of the cladding layer of Ni625 are smaller than those of Cr30Ni70 and Cr50Ni50.The corrosion rate of Cr30Ni70 and Cr50Ni50 is five times that of Ni625.With the increase of wire feeding speed,the melting width increases rapidly,but its increasing trend and range are relatively small compared with the melting width.With the increase of welding speed,the weld width and residual height decrease simultaneously.The arc length correction increases,the weld width increases,and the weld residual height decreases.Through the analysis of three surfacing process modes,the heat input and dilution rate are MIG PMC>pulse MIG>MIG CMT at the same wire feeding speed and welding speed,and the microhardness of the surfacing layer is lower than that of the other two processes,and the hardness of the surfacing layer is higher than that of the base material（200～230 HV）.The hardness distribution between surfacing layer and base material is uniform.Ni625 consists ofγ-（Ni,Fe）phase,M₂₃C₆hard phase and Cr₂Ni₃phase,andγ-（Ni,Fe）phase is the majority phase.The structure of the surfacing layer is dendritic austenite,and the matrix structure is composed of ferrite and pearlite.The structure of the surfacing layer and base metal near the fusion line has a certain growth,and the direction is perpendicular to the fusion line,which has little effect on the structure far away from the fusion line.The surfacing layer and matrix interface are well fused without separation.The distribution of elements in the surfacing layer and the matrix is relatively uniform,and the elements near the fusion line are diluted to a small degree,and Fe element does not have a serious transition phenomenon.When welding flat plates,the overlap rate is 40%,indicating a continuous and smooth transition in the overlap area of the weld seam,and the thickness of the overlay layer is about 2.5 mm;The longitudinal shrinkage on both sides is greater than the middle area,and the transverse shrinkage deformation is relatively uniform.The longitudinal shrinkage is greater than the transverse shrinkage,and the deflection is smaller in the middle area than on both sides;There is no significant difference in deformation among the three welding sequences for 8 mm flat plate surfacing,and the deformation of the symmetrical surfacing method from both sides to the middle is slightly smaller than the other two welding methods.Through finite element simulation of three different surfacing sequences for pipe rows,the results show that the maximum deformation from one side to the other is 2.83 mm;When welding from the middle to both sides,the maximum deformation is 3.10 mm;When welding from both sides to the middle,the maximum deformation is 2.77 mm.Through simulation analysis,changing the welding sequence can help reduce welding deformation.