Research On Microstructure And Properties Of Q345/316L Dissimilar Steel Laser Welded Joint

Q345/316L dissimilar steel welded joints can take advantage of inherent properties of different material,and are widely used in offshore wind power,oil and gas transportation pipelines,boilers power plants and other fields.Laser beam welding has the advantages of high energy density and welding productivity,small heat input and welding deformation,and good flexibility.It is one of the effective methods for dissimilar steel welding.Dissimilar steel joint of Q345/316 is welded by laser beam welding.It can take advantage of laser welding,and excellent welded joints can not only reduce the cost of welding,but also improve the service life of welded structure.Therefore,in this paper,the laser welded joint of Q345/316L dissimilar steel is chosen as the research object.The welding imperfection,microstructure and properties of welded joints are mainly studied,and the mechanism of the welded joints is analyzed.The concepts of convective interface and convective mixing zone were established by analyzing the characteristics of the convective and macroscopic topography of the penetration keyhole laser welding pool,and the generation mechanism of pores and macrosegregation was analyzed.The mechanism of stomata formation is that change of the laser incident angle due to the change of local convection direction or flow characteristics,resulting in instability of vapor pressure inside the keyhole,and local shrinkage or bulges of the keyhole wall.After bulges'falling from wall,the bubbles captured by the solid-liquid interface before escaping formed pores.They are obtained spontaneously by the thermodynamic calculation.Based on the characteristics of heat transfer,momentum transfer,and mass transfer in the convective mixing zone are easier to satisfy the thermodynamic and kinetic conditions of crystallization.The formation process of macrosegregation in the cross-section of the laser welded joint is established.It is considered that crystallization occurs first in the convection mixing zone.Under the action of the convection of molten pool,the non-mixed boundary layer which is brought into the weld is super-cooled,forming an"island-like structure";or the weld metal intrudes into the non-mixed boundary layer to form"beach structure".The formation mechanism of macrosegregation is further verified by OM and EPMA analysis.Through the microstructure of the weld analyzed by OM,the solidification process of the weld is L?L+A?L.However,in the incomplete mixing zone near 316L,the solidification characteristic of L?L+A?L+A+?A+F?_eutectic?A+F_eutectic,which is mainly related to the cooling rate of the molten pool and the supercooling degree of the local dendrite tip,the rapid cooling rate and the local undercooling induce austenite formation.The results of EPMA point analysis and XRD diffraction showed that the main precipitated phases of the welded joints were carbides Cr₂₃C₆,Cr₇C₃ and?phases Fe-Ni-Cr-Mo,FeCr,etc.Based on the element segregation coefficient K?,the element migration process model was established.Cr migrated from the 316L alloy to the molten pool,Ni migrated from the molten pool to the Q345 alloy,Mo precipitated at the grain boundary,and C migration was a short-range migration mechanism.C enriches the martensite layer which generally occurs in fusion line.Increasing the laser power and welding speed can reduce the tendency of pore formation,but that will aggravate the microsegregation of alloy elements,and will also promote the growth of martensite and austenite in the heat affected zone.The fracture locations of fully penetration welded joints are located at the base metal far away from the heat-affected zone of Q345.The welding speed and laser power have little effect on the hardness,but the hardness of the equiaxed crystal area is lower than that of the columnar crystal area,so that the hardness of the center of the weld is slightly lower than the two sides of the weld.The corrosion resistance of the welded joint in 0.25mol/L NaHCO₃ solution is better than that in 3.5%NaCl solution.The self-corrosion potential and corrosion current density of the welded joint are between the two kinds of base metals.When the laser power and welding speed are increased,the number of carbide and intermetallic phases also increases.The carbide and intermetallic phases can be used as cathodes to form micro-corrosion galvanic couples with the base metal,which adversely affects the corrosion resistance.