Investigation On Welding Parameters Optimization And Numerical Simulation Of Large Complex Components

Due to the large size of steam turbine,large number of welds,heavy welding task and the influence of welding workers on the quality of welds,it is urgent to propose an automatic welding process to ensure the welding stability.This paper studied the narrow gap MAG welding parameter,microstructure and properties of Q235 B steel 30 mm thick plate,revealed the generation mechanism of welding defects,optimized the welding parameters,clarified the relationship and the evolution law between process and microstructure,and simulated the deformation and residual stress of large complex components under different welding sequences to determine the optimal welding sequence.By observing the droplet transfer form and arc shape of normal arc and swing arc narrow gap MAG welding,the mechanism of defects in welding process is revealed,and the main welding parameters are optimized,obtained the best process.The results show that when the wire feeding speed is too low,the droplet transfer is short-circuit transfer,and the spatter is serious.When the welding speed is too high,the weld is convex,and the long dry elongation and the small gas flow rate will produce surface pores.For the swing arc narrow gap MAG welding,changing the welding speed will not change the form of droplet transfer,which is jet transfer,small swing angle and short sidewall residence time will cause the side wall unfused.Based on the above single factor test,the parameter range is determined.Taking the maximum weld width and penetration depth as the optimization conditions,the optimal parameters of normal arc narrow gap MAG welding are determined as welding speed 4mm/s,wire feeding speed 8m/min,dry elongation 15 mm,and the optimal parameters of swing arc narrow gap MAG welding are welding speed 4mm/s,swing angle 54 ° and sidewall residence time 0.4s.For the well formed joints under two kinds of arc forms,the microstructure characteristics of heat affected zone(HAZ)of welded joints are mainly studied,discussed the microstructure transformation law of coarse grain zone in HAZ,analyzed the difference of mechanical properties of each welding layer,and finally selected the best welding method through EBSD verification.The results show that the microstructure of HAZ is pearlite + widmanstatten and bainite,respectively.The coarse-grained region of normal arc presents the periodic distribution of coarse pearlite + widmanstatten ? ferrite ? pearlite + ferrite,while that of swing arc presents the periodic distribution of coarse bainite ? bainite ? coarse bainite.The results show that the hardness of each zone of the swing arc welding joint is higher than that of the normal arc,and the tensile strength is 423.10 MPa and 421.11 MPa respectively,which indicates that the performance of the swing arc welding joint is better than that of the normal arc,and it is consistent with the EBSD results that the grain size of the swing arc welding joint is fine and the grain boundary with large angle is multiphase.After determined the welding method,this paper designed three different welding schemes according to the actual welding sequence to simulate the deformation and residual stress produced by the optimization process in the welding of large and complex components.The results show that different welding sequence has little effect on the residual stress of large and complex components,and the Mises stress of each welding seam of the component remains at about 244 MPa when the welding sequence is changed,but the welding sequence has great influence on the deformation of the component.When the component with less constraints is welded first,the residual stress of the component is small but the overall deformation is large,and the maximum axial deformation is 71.92 mm,which can be reduced to 16.50 mm with gradually increase the constraint,that is to say,the best order of welding is firstly welding the straight weld from the inside to the outside along the radial direction for the internal retaining ring,vertical flange and horizontal flange of the cylinder block,then welding the circumferential welds from the outside to the inside along the axial direction for the cylinder block and stirrup kit,which can ensure the overall deformation of the components.