Numerical Simulation Of Welding Residual Stress And Deformation In Low Alloy High Strength Steel Weldments

Due to the superior comprehensive mechanical properties,low cost and excellent processability,the low alloy high strength structural steel has been widely used in the industry of bridges,building structure,construction machinery,ships,offshore platforms.Fusion welding technology has been commonly used to assemble the parts in low alloy high strength structural.With the metallurgical characteristics of the low alloy high strength structural steel,the solid-state phase transformation(SSPT)will inevitably occur in the welding process.That the change in volume and mechanical properties of materials during the process of SSPT has a significant impact on the formation process of welding residual stress and deformation.The stress produced in welding process was often the main cause of hot crack,cold crack and layered tear crack.Simultaneously,welding residual stress was an important factor leading to the fatigue property decrease,inducing stress corrosion cracking and promoting brittle fracture.Welding deformation was the main reason for the reduction of dimension accuracy of welding structures,which not only directly affected the appearance quality of products,but also increased the cost and delayed of construction.Therefore,the research on welding residual stress and deformation of low alloy high strength structural steel has considerable theoretical significance and engineering application value.However,owing of the limitation of high cost,heavy workload and destructive to material,it was difficultly to directly measure welding residual stress in practical engineering by means of experiment.At the same time,the conventional experimental method can't accurately obtain the residual stress in the welded joint.In the past 20 years,great progress has been made in numerical simulation methods,and it was possible that measuring residual stress can be solved by numerical simulation of welding in the engineering application.Based on the SYSWELD software,a nonlinear finite element method(FEM)considering thermo-metallurgical-mechanical was developed to calculate the temperature field,microstructure,residual stress and welding deformation of typical welded jointed of a series of low alloy high strength structural steel.Based on the developed computational approach,the temperature field,microstructure,hardness and residual stress of single and double remelted welded joints of Q345 steel were simulated.And based on results of the numerical simulation of Satoh model,the formation and evolution mechanism of welding residual stress of Q345 steel under different thermal cycles was clarified.The simulation results show that under the welding conditions in this study,bainite is dominant in welding seam,and hardness value is about 310 HV;the heat-affected zone(HAZ)contains ferrite,pearlite,bainite and s small amount of martensite.With the increase of the distance to the fusion line,the hardness value decreases;The microstructure of base metal is ferrite and pearlite,and hardness value is about 160 HV.Form the results of numerical simulation,we can see that Considering the effect of SSPT,the peak value of longitudinal residual stress in weld seam and HAZ is about 580 MPa,which is higher than the yield strength of base metal at ambient temperature.Without considering the effect of SSPT,the peak value of longitudinal residual stress in weld seam and HAZ is only 460 MPa.While the effect of SSPT on the longitudinal residual stress is not obvious in the area of far from the weld and HAZ.Based on the developed computational approach in this study,the effects of SSPT on welding residual stresses of low alloy high strength structural steels with different strength grades such as Q345,Q420,Q690 and Q960 were studied.The results show that with the increase of material strength(or carbon equivalent),the effect of SSPT on residual stress is more obvious.Especially,for the Q960 steel,the peak value of longitudinal residual stress in weld is only 200 MPa,which is much lower than the yield strength of base metal at ambient temperature.The results show that SSPT has a significant effect on the residual stress of Q960 steel.In this paper,the numerical simulation and experimental verification of welding deformation of SUS304/Q235 dissimilar steel tube-block welded joint were carried out.The effects of different deposition sequence and thickness of tube on welding residual stress and deformation are analyzed,and the distribution characteristics of residual stress at the initial position of welding are further explored.The research show that the residual stress distribution of the tube-plate joint is significantly affected by the deposition sequence,and the deformation distribution pattern is also affected by the deposition sequence to some extent.As the thickness of tube increases,the deformation in the radial direction decreases,and the peak value of the hoop residual stress increases.Meanwhile,this research takes the example of “welding structure of rear axle bridge”,the effects of SSPT,welding sequence and weld direction on residual stress and deformation were taken into account.The research showed that the peak value of longitudinal tensile residual stress of weld seam is significantly reduced after SSPT.Ignoring the influence of the phase transformation factors will make the predicted welding deformation larger.In addition,deformation modes vary with welding sequence.In this paper,the effect of SSPT on welding residual stress and deformation of low alloy high strength structural steels with different strength grades was systematically studied.The result of this thesis can be used to predict the effect of SSPT on welding residual stress and deformation of the grade of low alloy high strength steel,and provide theoretical guidance for establishing finite element model for predicting residual stress and deformation of welding structure in engineering application.