GMAW Welding Technology And Experimental Study On Large Thick Plate H-shaped Steel Structure

In view of the complicated process,variable parameters and welding cracks in the welding process of large thick plate bridge steel components,optimizing the welding process and its parameters,improving the welding quality,and preventing welding cracks have become the focus of this type of large-scale steel welding research.Based on the Xiaoshunjiang Large bridge project,a key construction project in Zhejiang province,this paper takes large thick-plate bridge steel as the research object,then conducts research on the welding process design and quality control methods of section steel structures.The specific research content are as follows:(1)Based on the welding requirements of the large thick plate bridge steel Q345 qD used in the project,the welding process is improved according to the material characteristics,the simulation analysis route with the core concerns of welding c urrent,welding voltage,welding speed and welding sequence as the core focus is defined.Combined with the characteristics of welding numerical simulation,the thermal-elasticplastic coupling principle analysis is carried out.According to the quality requirement and standard of welding,design welding experiment and sample testing verification analysis route.Based on theoretical simulation and experimental test,the welding quality method and improvement scheme of large thick plate parts are put forward.(2)According to the material characteristics and welding process optimization requirements,the thermo-elastic-plastic method is used to construct a three-dimensional welding seam model,the secondary development of Fortran heat source subroutine is carried out based on the double ellipsoid heat source model.According to the actual welding process and welding standard,the welding process parameters are preset,the welding model simulation analysis step is established,the mesh transition method is adopted to refine the weld area mesh,the standard boundary conditions and unit attributes are set,the complete thermal-mechanical coupling simulation is realized by combining the life-and-death unit.(3)According to the simulation analysis route of the welding process,a simulation study is carried out on the optimal selection of single-pass welding process parameters and the optimal selection of multi-pass welding sequence schemes,obtaining the influence of welding process parameters and welding seam sequence arrangement on weld quality.The simulation results show that welding current and voltage have significant influence on the depth with width of the weld pool,welding speed will affect the temperature distribution of the weldment,the welding sequence will affect the stress and strain distribution of the weldment.According to the welding process simulation results,design the welding process optimization plan,and obtain the corresponding welding temperature field,stress and strain field.The results show that there is no crack in the model,the front welding is preheating in the back welding,and the cooling speed is slow.The residual stress at the starting end of welding with the weld area is concentrated,its transverse residual stress is the l argest in the weld center,reaching345 MPa.Longitudinal residual stress is compressed at the initial end,the weld zone is tensile,with little change.The strain gradually increases from the edge to the weld center,reaching the maximum value of 0.52 mm at the center,which less than the specified error value and meets the welding design requirements.(4)According to the simulation results of welding optimization scheme,design welding experiment,carry out trial production of welding samples and performance with quality inspection,including tensile strength,bending strength,hardness,low temperature impact performance,weld pool size and metallographic microstructure.The test shows that there is no large deformation in the welded plate,the maximum deformation is 0.6mm,which error between the simulation results is very small.The welded joint is defect-free,which has good fusion and no cracks.The comprehensive average value of tensile strength is 565.5MPa,the comprehensive average value of yield strength is 404 MPa.The comprehensive average value of low temperature impact brittleness of welded joints is139 J,the highest hardness of welding heat affected zone is 194.8HV.The microstructure of welded joint has no micro cracks,its metallographic micr ostructure presents acicular ferrite grains,which can improve dislocation rate and plastic toughness.The depth of the experimental molten pool is highly consistent with the depth of the simulated molten pool,and the overall mechanical properties are well matched with the microstructure detection,which meets the design requirements of welding scheme of the project.The welding quality control method and optimization scheme proposed in this paper,which can effectively improve the welding quality,prevent welding cracks,meet the welding performance requirements of the project,have good social and economic benefits,and provide specific project experience to improving the welding performance of thick plate steel.