Evaluation Of Welding Technology And Numerical Simulation Of Welding Joint For CO₂ Welding Of Q345R Steel

Welding is the main connection for pressure vessel manufacturing.Failure analysis results show that many accidents are related to the welded joints with weak quality of pressure vessel shell.Therefore,the safety assessment of welded joints is very important,and the welding process parameters,welding passes and welding sequences are the key factors.According to that,the related test equipments were used to test the mechanical properties,micro structure and hardness of welded joints obtained by Gas Metal Arc Welding?GMAW?and Flux Cored Arc Welding?FCAW?of Q345R steel.And then a welding process evaluation was going to be carried out to provide reference for production practices.Besides,in order to provide theoretical bases for the optimization of the actual welding process parameters,the relevant software is used to simulate the temperature field,the residual stress as well as strain field of CO₂ welded joints.At the same time,it is found that there are many documents about the effect of heat input change on welded joints.However,based on the constant heat input,there is few studies on the influence of different welding speeds towards welding joints.According to that,in order to further improve the applicability of the numerical simulation in the process of production,we studied whether the double ellipsoid heat source size parameters of CO₂ single pass welding will affect the molten pool,and we also studied whether four kinds welding sequences will affect on different lengths'welded joins of residual stress and its deformation.The results show as follows:1.The process assessment of joints by welding 26 mm Q345R steel using GAMW and FCAW:1)The macroscopic observation,X-ray radio-graphic inspection,micro-structure and hardness test showed that:Two kinds of weld seam appearances were well formed,and weld seam appearance of FCAW was more uniform,beautiful,shiny and hard to rust.The two kinds of welds were all qualified.The micro-structure of the two joints were composed of ferrite and pearlite.Hardness test results showed a similar distribution pattern,with high hardness in the fusion zone,away from the area,the value rapidly decreased to190HV-240HV.2)Tensile,bending and impact tests showed that the average tensile strength of the two joints was 578.1MPa and 555.1MPa,which were higher than those of the base material550MPa.The strength of joint obtained by FCAW was higher,but having a lower toughness.3)The two joints both satisfied the welding process evaluation requirements of pressure equipment of NBT47014-2011.2.The study of numerical simulation on heat source parameters of Q345R steel CO₂single pass welding showed that the changing of heat source parameters will have some influence on the weld pool temperature,weld length,width and depth.3.The numerical simulation studies of Q345R steel CO2 single pass welding with different welding sequences showed that,the back-step welding,skip welding,Symmetrical welding and continuous welding were suit for 400⁶00 mm,600¹000 mm,more than 1000mm,less than 400 mm,repectively.4.The numerical simulation comparative analysis of 26 mm Q345R steel two kinds of welding processes showed that:The simulation results of weld pool shape was consistented with the actual condition;The residual stress distributions of the same joint regions were similar;The residual stress was related to the solder,and the value of the GMAW is larger,with the maximum 465MPa.5.The numerical simulation comparative analysis of the 26 mm Q345R steel by FCAW with the same heat input and different welding speeds showed that welding temperature gradient and residual stress decreased with reducing welding speeds,and increased with raising welding speeds when heat input was constant.Considering the residual stress and strain,simulation results of cooling time t_8/5,and welding efficiency,it was concluded that the optimal welding speed of FCAW was 5 mm/s.