Research On Preparation Technology Of Laser Cladding Cobalt-Based Alloy Erosion Resistant Coating

The cooling circulating water pump is the key auxiliary equipment of the water-cooled unit of the coastal nuclear power plant,which has a great impact on the normal operation of the nuclear power plant.The cooling circulating water pump is subject to long-term seawater corrosion and sediment erosion,resulting in equipment failure or structural incompleteness,which directly affects the safe and reliable operation of the nuclear power plant.The failure is generally in the form of surface defects such as pitting and packing wear,and from the economic aspect,this kind of large parts are generally treated by surface repair.Laser cladding has the advantages of small heat input and distortion,good controllability and low dilution rate,etc.It is the preferred method to realize the repair of failed pumps and improve the corrosion resistance and erosion resistance of pump surfaces,prolong service time,improve safety and reliability,and reduce operation and maintenance costs.In this thesis,G-X5Cr Ni13-4 super martensitic stainless steel for cooling circulating water pump was used as the base material,and 2540-00 cobalt-based alloy powder produced by H?gan?s was selected as the base powder,to which different ratios of CeO₂ rare earth oxides and Ta were added as the clad material to prepare laser clad coating.The effects of process parameters on macroscopic forming and hardness were studied,and the laser cladding process parameters were optimized.The effects of CeO₂ and Ta on the macroscopic morphology,microstructure,mechanical properties,residual stresses,corrosion performance and erosion properties of the cobalt-based alloy clad coating were investigated.（1）The process parameters selected in this thesis are:laser power 2500W,scanning speed350mm/min,powder feeding rate 1.32r/min,protective gas flow rate 40L/min,multi-pass fusion lap rate 30%.The metallographic organization of the cobalt-based clad layer was divided into four different regions,namely:the plane grain near the fusion line,the columnar grain in the lower part of the clad layer,the cellular dendrite in the middle of the clad layer and the equiaxed grain in the top of the clad layer.The phase of the clad layer is mainly composed ofγ-Co,γ-Ni,M₇C₃ and M₂₃C₆（M=Cr,W,Fe）metal carbides with face-centered cubic（FCC）structure.The microstructure of the clad layer shows a cellular Co-rich solid solution grains with eutectic carbides distributed in a network on the grain boundaries.（2）The addition of CeO₂ and Ta does not introduce defects into the clad layer.CeO₂improves the wettability of the clad layer and has a significant effect on the macroscopic formation of the clad layer of cobalt-based alloy.The addition of Ta does not have a significant effect on the macroscopic formation of the clad layer.The addition of CeO₂ did not affect the type of phase of the clad layer,but the net-like carbides in the CeO₂-modified cobalt-based complex clad layer showed a more obvious phenomenon of continuous sheet aggregation.When the addition of CeO₂ exceeds 2%,obvious microscopic porosity can be found in the clad layer.Diffusely distributed Ta C particles were generated in the Ta-reinforced cobalt composite clad layer,and the in situ generated Ta C was in the form of regular geometry particles,mainly distributed in the reticulated carbides between the solid solution grains.The addition of the rare-earth oxide CeO₂ to the Ta-enhanced cobalt-based composite clad layer was able to change the shape of the in situ generated Ta C particles,resulting in faster growth and larger overall size at specific grain planes.（3）When laser cladding of cobalt-based self-fusing alloys is carried out on the surface of martensitic stainless steel,the residual stress on the surface of the clad layer is residual tensile stress,and the maximum principal stress direction is nearly the same as the laser scan,and the residual stress parallel to the laser scan direction is much larger than that perpendicular to the laser scan direction.The addition of CeO₂ to the clad material increases the residual stress in the clad layer.When the content of CeO₂ reaches 3%,the residual stress in the clad layer reaches668.24 MPa,which is 30.48%higher than that of the clad layer without addition,and reaches50.59%of the tensile strength of the clad layer.The addition of Ta to the clad layer of cobalt-based alloy can significantly reduce the residual stress in the clad layer,and the maximum residual principal stresses in the clad layer are 198.16 MPa,248.82 MPa and 209.13 MPa when the Ta content is 10%,20%and 30%,respectively,which are 61.3%,51.42%and 59.17%lower than the clad layer without addition.（4）The corrosion resistance of each component clad layer in both 3.5 wt.%Na Cl solution and 0.5 mol/L H₂SO₄ solution is significantly better than the martensitic stainless steel substrate,and the 3 wt.%CeO₂ cobalt-based composite clad layer has the strongest corrosion resistance in3.5 wt.%Na Cl solution and 0.5 mol/L H₂SO₄ solution.The corrosion failure of the martensitic stainless steel substrate in 3.5 wt.%Na Cl solution was in the form of pitting corrosion,and in0.5 mol/L H₂SO₄ solution was in the form of selective corrosion.Each different composition of the fused cladding layer in the 3.5wt.%Na Cl solution and 0.5mol/L H₂SO₄ solution in both environments corrosion failure form of selective corrosion,in the corrosion reaction,the carbide phase and solid solution phase between the formation of a microcell,solid solution as the anode oxidation reaction continues to dissolve and generate Cr₂O₃ film,the carbide phase as the cathode reduction reaction is protected exposure precipitation.（5）The erosion resistance of each component of the clad layer is much higher than the martensitic stainless steel substrate.Among them,the cobalt-based composite clad layer with30 wt.%Ta has the strongest erosion resistance at 90°impact angle,with a mass loss of 32.37mg,which is 21%lower than the non-added clad layer and 76.86%lower than the substrate.The cobalt-based composite clad layer with 10 wt.%Ta showed the highest resistance to erosion at 30°impact angle with a mass loss of 14.06 mg,which was 27.93%lower than the no-addition clad layer and 80.64%lower than the substrate.The erosion effect on the substrate and clad layer is produced by a combination of three mechanisms:cutting,ploughing,substrate deformation and cracking.At 90°impact angle,the substrate deformation and cracking mechanisms dominate the erosion of the substrate,while the ploughing and cutting mechanisms dominate the erosion of the cladding.At 30°impact angle,the erosion loss mechanism was dominated by ploughing for all samples.