Numerical Simulation Of Casting Process Of Cast Steel And Mechanical Analysis Of Cast Steel Nodes With Initial Defects

Cast steel nodes have beautiful appearance,flexible sculpt and desigin,each branch transits smoothly.Cast steel nodes are completed by one time casting,with the advantage of isotropic effect and good homogeneity,which can prevent heat-affected zone caused by weld,relieve stress concentration,and improve the mechanical properties of components.Because of the advantages of its own performance,in recent years,cast steel nodes have gradually become a new node form in engineering field.However,due to the influence of alloy composition,casting technics,shape of casting and mold,heat radiation conditions and other reason,It is inevitable that there will be some initial defects such as cracks,cold cracks and other non planar defects such as porosity,sand inclusion,shrinkage porosity,pore,etc.It is more dangerous for the cast steel members subjected to large internal force and alternating load.Although making great efforts to eliminate all kinds of casting defects in production,it is very difficult to achieve no defect with existing technological conditions.And the existing quality assessment standards for cast steel nodes are just divided into two categories:planar and non planar defects,which are too indistinct.The location of defects in cast steel node is not considered in the high stress,and failing to develop a detailed assessment method for specific defect types.In this paper,the numerical simulation of casting process,distribution of casting defects and mechanical properties of cast steel nodes with initial defects are studied.The first step is studying the influence of casting parameters,such as thickness-to-width ratio,casting speed,on casting process by using casting process simulation software.The second step is designing the casting system of K cast steel node,simulating the casting process,analyzing the initial defect location.The third step is investigating the modeling method of initial defects in finite element,and the effect of initial defects on the static and fatigue properties of K cast steel nodes.Lastly,giving some suggestions on the limitation of existing quality assessment standands for cast steel nodes.The main research contents are as follows:(1)Describing the production technology of stell casting,the theory of flowing and solidification of molten metal during casting process,briefly introducing the casting simulation software and initial defects criterion.Studying the size,position and distribution information of initial defects through the simulation of the cross plate casting.The results are compared with literature to verify the applicability of software in this paper.Studying two kinds of random factors on shrinkage defects,such as thickness-to-width ratio and casting speed.(2)Desiging the casting system,rising and casting mold of typical K cast steel nodes in engineering.According to the actual casting requirements and examples,ensuring intial conditions,such as casting method,casting time,casting speed,temperature,cooling method,etc.Studying the size,postion and distribution information of initial defects of K cast steel nodes through software.(3)Discussing the simulation methods of shrinkafe defect field,thermal crack field and sand defect field in finite element model.Like:According to the change of velocity field with time,analyzing of the location of the sand defects,metal flow rate and sand washing time to judge the severity of sand defect.The region with thermal crack index more than 0.035 is considered as the true thermal crack,and cracks are established in finite element model.The shrinkage defect in K cast steel joint presents a kind of field distribution,due to the low shrinkage rate,the elastic modulus is reduced in the finite element model so that elastic modulus field can be used to express the shrinkage defect field.(4)Comparing the static of the K cast steel nodes with no defects and the K cast steel nodes with initial defects through finite element analysis.Analysis found that:The damage forms of the K cast steel node are always because the stress of pipe wall reaches the yield point,Node itself has a high degree of security.Because the initial defects are small,they have little influence on the node stiffness,and have certain influence on the strength,which will cause the local stress concentration.(5)Comparing the local fatigue properties of the K cast steel nodes with no defects and the K cast steel nodes with initial defects through finite element analysis.Analysis found that:Because of different loading conditions,the effect of initial defects on the fatigue behavior of cast steel joints is not same,the reason is that the stress field is different in different loading ways.Initial defects have great influence on the local fatigue behavior of cast steel nodes,especially in the area of high stress,the safety factor is less than the allowable safety factor.In the low stress region of branch pipe,the initial defects have some influence,but the safety factor is still higher than the allowable safety factor.(6)It is found that the existing methods for evaluating the quality of cast steel nodes are not comprehensive enough,it is a limitation to evaluate the quality of steel castings only according to the defect size.The type,size and position of the defects have different influences on mechanical properties of the nodes.Therefore,the quality evaluation of cast steel nodes in engineering should be based on the type,size and location information,combined with the results of mechanical analysis.