Research On Stochastic Characteristics And Evaluation Method Of Casting Defects In Cast Steel Joints

With the development of the spatial structure,the span of the structure is getting larger and larger,and the shape is also diverse.The continuous emergence of new structural systems has made the connection of nodes between the components increasingly complicated.In addition to the traditional welding ball joints,steel pipe joints and other node forms,cast steel joints have been widely used in stadiums,railway stations,large squares and other buildings.In the cast steel structure specification,the cast steel is regarded as the ideal continuous homogeneous material to calculate its bearing capacity,but in fact,due to the limitation of the casting process level and environmental factors,the cast steel joints will inevitably have defects,such as cracks during the production process.Internal or external defects such as inclusions,pores,shrinkage,etc.The existence of defects will affect the mechanical properties of the cast steel joints and even the overall structure to some extent,and may even threaten the structural safety.Therefore,as a key part of the engineering structure,the research on the defect distribution law and evaluation method of the cast steel joint and the accuracy of the nondestructive flaw detection method have important practical significance.In this thesis,the casting defects in GS20Mn5 cast steel joints are taken as research objects.The non-destructive testing and slicing tests of large sample steel castings are used to study the generation and distribution of casting defects in cast steel joints.The data of size,distribution and so on can optimized the design of the structure containing cast steel joints,improve the mechanical properties of the joints,and provide important reference for structural engineers.The research contents and main results are as follows:(1)Non-destructive testing of 24 cast steel joints of a foundry found that the distribution of defects in cast steel joints seems to be random and regular: R zone is extremely prone to cracks;the distribution of inclusions is random.With the flow of molten steel,there is no obvious rule to follow;the pores and inclusions are the main defects of the steel castings,and the fusion of the core supports is common,and most of the cast steel joints need to be repaired.(2)On the basis of learning main types and distribution of casting defects of a typical fourtube cast steel joint by ultrasonic non-destructive testing technology,the cast steel joint is also sectioned by sawing equipment such as sawing machine and water cutting machine,which further clarifies the types and distribution of casting defects.The results of the slicing test are compared with the results of non-destructive testing.The accuracy of ultrasonic flaw detection is verified,which provides suggestions for non-destructive testing of cast steel joints.Probability theory and mathematical statistics are used to estimate the distribution of random variables of casting defect size.The exponential distribution function of the defect size provides a theoretical basis and data support for the reliability study of the cast steel joint,which provides a reference for the establishment of the cast steel node defect random field.(3)The casting simulation of the four-tube cast steel joint with Gs20Mn5 is carried out according to the actual process conditions by using the Pro-CAST software;the casting simulation results show that the vicinity of the riser,the stiffeners and the R zone are the concentrated areas of defects,and the defect concentration areas found in the section test in the chapter3 are basically the same;on this basis,the casting process is improved,and the distribution of defects is further analyzed.(4)Compare and analyze the defect evaluation methods of steel castings home and abroad,and provide some suggestions for the defect evaluation of the current cast steel joints.The structural calculation of the cast steel joints was carried out by ANSYS software found that the area with more defects is also the stress concentration area;this thesis finally discusses the modeling method and evaluation method of defects,in order to provide some reference for the design of cast steel joints.