Research Of Welding Residual Stresses And The Fracture Properties Of The Dissimilar Girth Welded Joint Of Steel Castings

Circular hollow section(CHS)structure has been widely used in large-space steel structure and steel-concrete road bridge,because it brings together architecture aesthetics with sustainable mechanical behavior.Welded joints of tubular structure have been shown to be insufficient to the development of modern steel structure.The safety and reliability are significantly affected due to the complicated welded geometry and severe stress concentration.As a new type of joint,the application of cast steel node remove the complex intersecting curve,severe stress concentration and potential welded defect in connection area.Conventional directly welded joint is replaced by cast steel node;Weld seam along the intersecting curves of directly welded joints is simplified as a butt girth welding detail.Furtherly,any complicated joint types can be easily manufactured and the shape can be optimized for the distribution of forces,which improves the fatigue behavior of the whole structure.However,the welded connection between the casting stubs and tubular structure becomes the vulnerable region that determines the safety of the structure.Since the requirement of casting and design techniques,the dissimilar problem exists in such welded joint,that is the discontinuity of geometry and mismatch of applied base materials.The difference of pipe wall-thickness and material properties are the significant content of those problems respectively.The topic of this research is concentrated on the problems of residual stresses profiles subjected to the effect of asymmetrical geometry of welded joint,mismatch of base materials and metallurgy transformation in heat-affected zone,the fracture properties and coupling analysis technology of welding simulation.Three kinds of dissimilar welded joints with different engineering background are applied to analyze the corresponding sub-problem,respectively.The objectives of this paper is to explore the welding residual stress(RS)profiles and fracture properties in such dissimilar welded joint as well as the simulation technology of computational welding mechanics.Therefore,the main research works and conclusions of this paper can be expressed as follow:1.The welded material deposition technique is improved in the weld simulation model.Based on the established simulation model,the residual stress profiles and mechanical behaviour in the dissimilar thickness butt-welded joints is explored.“full-elastic element” with low material properties is introduced in welded area and included in mechanical calculation.The filler element with real material properties will be reactived with the moving of weld arc in the thermalmechanical analysis.The calculation problem due to the element zero volume distortion since the low material parameters in high temperature.Developed welding simulation model is applied to comparatively analyse the mechanical behaviours and residual stresses in the butt-welded joints with similar and dissimilar thicknesses of steel plates.The results explored the effect of weld geomtry on the mechanical behaviour of welded joint.It conducts that asymmetrical welded geomtry in such dissimilar welded joint leads the variation of RS distribution,and causes the change of mechanical properties weld connection.Lower RS occurs in the thicker steel plates in heat-affected zone.2.The analysis of RS profiles and welding technique in multi-pass girth-welded joint with different base materials.Througthing the adjustment of filler element reaction and definition of welding arc movement with time and space,the analysis of weld technique in multi-pass girth joint is carried out in this work.Based on the simulation model of multi-pass welded joint,it concludes the RS distribtuion and variation mechanism in such dissimilar welded joint.The results reveal that asymmetrical RS distribution is found since the mismatch of base materials.The lower RS occurrs on the welded components with smaller yield strength.The results of weld sequence analysis convinced that the RS in such dissimilar welded joint is definitely influenced by the welding sequences and weld start/stop positions.The welding stress and deformation can be controlled by optimizing the welding techniques.Since the constrained boundary conditions may cause the extral stress in welding,the variation regularity of RS under the influence of external constraints is different due to the mismatch of base materials.3.The formation and variation mechanism of RS in girth-welded joint between steel castings and circular hollow section structure.Such kinds of dissimilar welded joint is subjected to the combined effects of asymmetrical geometry and mismatch of base materials.Although the cast steel normally applied with lower yield strength and thicker wall-thckness than the selected CHS structure,the results convince that larger axial RS and local deformation are found in steel castings due to the effect of asymmetrical weld geometry.The formation and variation mechanism of RS in such dissimilar welded joint is definitely different from the ordinary welded structure.The mechanical behaviour in welding is more complicated since the combined influence of asymmetrical geometry and dissimilar base material properties.The conlusion is also verified by the exploration of different weld joints.It reveals that single V type welded geometry contributes to decrease the RS and the range of heat-affected zone.4.A new procedure of thermo-metallurgical-mechanical couple analysis in welding is developed.It reveals the main interaction relationship of welding phenomena.Based on the theory KoistinenMarburger model,Leblond phase evaluation model and the continuous cooling transformation(CCT)diagram,the problem of solid-state transformation analysis in wedling metallgy phenomena is solved by the proposed method.Furtherly,a coupling thermo-metallurgical-mechanical analysis procedure in welding is developed.The proposed method considers the continuous heating transformation and continuous cooling transformation behavior of materials.The influence of transformation latent heat,transformation plastic deformation and volume expansion are also employed in the proposed couple analysis model.The established model based on S355J2 steel reveals that it can predict the type of metallurgy transformation and phase proportion of materials in weld zone and heat-affected zone in welding phenomena,based on the temperature,heating and cooling conditions in real time.the developed procedure is convinced by corresponding research work.The advantage of the proposed method is that there is no need to predefine the type of microstructural behaviour and phase transformation,and predict the phase proportion merely by interpolating the temperature acroding to the phase transformation model,which ignores the non-uniform and time varying of metallurgy phanomena in welding.It also makes contribution to the existing method that neglects the effect of phase transformation and provides a newly kind of coupling analysis method for the interation of welding metallurgy phenomena,temperature and stress field in welding.5.Analysis of internal surface crack propagation behaviour in RS field and fracture properties of girth-welded joint between steel castings and CHS structure.The stress intensity factors for internal surface circumferential crack and crack propagation behavior in weld-induced RS field are explored to comprehensively understand the failure mode and fracture resistance of such dissimilar welded joint of steel castings.The results convinced that asymmetrical welds geometry of such dissimilar welded joint leads to weaker fracture resistance than the ordinary weld structure with equal wall-thickness.Both the asymmetrical weld geometry and tensile residual stress contribute to the growth of internal surface crack at welded root in the circumferential direction and radially from inside to outside of the pipe welded joint.The fracture resistance is decreased since the combined effects of residual stress and asymmetrical welds geometry.