The above research on the mechanical properties of cast steel joints and the mechanical properties of cast steel joint welds are two aspects of the research on the properties of cast steel joints. The existence of casting defects is not considered in the relevant research, and the cast steel material is considered to be a uniform and continuous material. In recent years, the influence of casting defects on the mechanical properties of materials and components has attracted scholars’ attention.
In terms of the influence of casting defects on the mechanical properties of cast steel materials, Li Xiaolong of Nanchang University used the three-dimensional reconstruction method of CT scanning to obtain the characteristics of casting hole defects, mapped the casting defect information to the finite element grid, carried out the fatigue analysis of the finite element model with hole defects, and obtained the influence law of casting defect characteristics on the service performance and fatigue performance of castings.
Hadin et al. Carried out experimental research on cast steel materials and found that micro casting defects have a great impact on the fatigue life, fracture toughness and plasticity of the materials; The discreteness of the data obtained from the specimens with macro casting defects is much greater than that of micro casting defects; For casting defects less than 200um, the material strength and elastic modulus are almost the same as those of intact specimens, but the elastic modulus of specimens with macro casting defects is greatly affected by the shrinkage volume fraction, and the functional relationship between the elastic modulus and the local shrinkage volume fraction is given. Zhu Lianbing of Nanchang University also studied the effects of different hole defect characteristics on the mechanical properties and fatigue life of dog bone test bar under tension and bending of long strip by using three-dimensional reconstruction technology. Yan Huadong of Southeast University used the three-dimensional reconstruction method of CT scanning to obtain the characteristics of casting hole defects, improved the GTN damage model based on micro holes, and considered the mixed hardening behavior of GS-20Mn5V cast steel.
In terms of the influence of casting defects on the mechanical properties of cast steel joints, haldiman Sturm of the steel structure laboratory of EPFL (Swiss Federal University of Technology) regards the initial casting defects in cast steel joints as cracks, and believes that the crack propagation life is the fatigue life of cast steel joints. Through design, the crack propagation life on cast steel joints can be equal to the design life of circular butt welds of cast steel joints, In order to optimize the fatigue resistance of cast steel joints. Cai Jianguo and Feng Jian of Southeast University simulated two typical casting defects of cast steel joint crack a (length: 80mm, width and depth: 2mm) and hole B (radius: 15mm) with ANSYS, and conducted finite element analysis. It was found that the maximum stress at the edge of class a casting defect increased by 16.3% compared with that without defect, and the maximum stress at the edge of class B casting defect increased by 21.7% compared with that without defect, It is inferred that the mechanical properties of cast steel joints with defects are greatly weakened, but the crack is not a real crack model. Zhao Jing of Southeast University used the equivalent elastic modulus random field to characterize the randomness of casting defects, and conducted probabilistic finite element simulation on the steel pipe truss structure with cast steel joints. The research shows that the sensitivity of structural performance to the randomness of elastic modulus of cast steel material is much greater than that of section steel, and the material at the intersection of main pipe and branch pipe has a great influence on the structural strength. Gong Weijia of Southeast University used the concept of fatigue safety factor in the mechanical manual to study the influence of casting macro hole defects on the fatigue performance of cast steel joints. He believed that the current evaluation method only based on the size of the most serious casting defects is not comprehensive, and the stress state at the location of casting defects should be considered at the same time. Tang Qi of Southeast University discussed the sand washing defect field, hot crack defect field, shrinkage defect field and the simulation method in the finite element model of cast steel joints, expressed the shrinkage defect field with elastic modulus field, and considered the influence of stress concentration caused by some casting defects on the fatigue performance of cast steel joints.
The above studies show that casting defects have a very significant impact on the mechanical properties of materials and components. In terms of the impact of casting defects on the mechanical properties of components, the location and size of casting defects in components are assumed, and the distribution and size of casting defects in nodes are not simulated. Therefore, it is of great significance to study the true distribution of defects in cast steel joints, which is the key problem to study the influence of casting defects on the mechanical properties of joints. At present, there are few studies on the distribution law of macro casting defects in cast steel joint casting, and the macro casting defects in the literature are arbitrarily assumed casting defects, which are lack of specific data support. In terms of the distribution law of macro casting defects, only one foreign document mentioned the flaw detection and slicing of a typical X-type cast steel joint. As shown in Figure 1, it divides the cast steel joint into four areas, namely, the welding preparation area, the area near the intersection line, the high stress area and other areas. 24 samples were taken from the cast steel joint by mechanical cutting, one of which is shown in Figure 2. The flaw detection results show that there are few casting defects near the nozzle, the size of casting defects is about 1mm, and there are many shrinkage defects near the intersection area. The research shows that there is a certain law in the distribution of casting defects, but the research sample size of this literature is small, and there is no detailed description of the randomness of casting defects, nor does it explain the influence of these casting defects on cast steel joints.