(1) Through the non-destructive testing of 24 cast steel joints, it is found that the distribution of casting defects seems random, but there are rules to follow, and cracks are very easy to appear at the intersection of multiple pipes; The distribution of inclusions is random, and there is no obvious law to follow with the flow of liquid steel; Pores and inclusions are the main defects of cast steel joints. Lack of fusion of core support is very common, and most cast steel joints need repair welding.
(2) The slice test of a typical four branch cast steel joint shows that the hole defects account for the majority of the macro defects of the cast steel joint, and it can be considered that the hole defects are the dominant defects in the cast steel joint; Most defects are distributed in the outer and middle layers of cast steel joints, that is, the outer 2 / 3 wall thickness, and there are few defects on the inner side; Most of the porosity defects are distributed on the outside of the cast steel joint, the shrinkage porosity defects are mainly concentrated in the middle of the wall thickness, and the inclusions basically appear in the outer and inner wall thickness; The most defects on the branch pipe are inclusion defects; The most defects near the intersecting line are shrinkage cavity and porosity defects; The most common defect of the main pipe is shrinkage and porosity. The distribution of defect length tends to be an exponential distribution. Through the analysis of the distribution function and probability density distribution function of defect length, it can be predicted that among the defects greater than 10mm, the defects with length between 10mm and 20mm will account for 95.49% of the total defects, and the defects with length between 20mm and 30mm will account for 4.30%. The comparison between slice test and NDT shows that there will be more defects in the blind area of NDT, and rib plate and R area are the flaw detection areas that should be paid more attention to.
(3) The casting process of the cast steel joint is simulated by ProCAST software, and the causes of shrinkage porosity and crack defects of the cast steel joint are explained. It is found that the riser area, R area and rib area of the cast steel joint will produce shrinkage porosity defects, and the R area is easy to produce crack defects. The results of numerical simulation also show that the position of pouring riser, rib and the shape of cast steel joint should be designed reasonably. Layered casting system and bottom injection casting system have little effect on the distribution of casting defects of the joint, but the combined use of cold iron and riser can greatly reduce the casting defects of cast steel joints, but the use of cold iron may transfer the casting defects to other areas.
(4) More casting defects often occur in the area near R area, and the area with concentrated casting defects is often the area with concentrated stress, which makes the cast steel joints face greater risks. By comparing the specifications of various countries, it is found that the quality evaluation method of steel castings in China mainly draws lessons from Europe, and the evaluation system needs to be further improved. There is almost no impact of defects on the mechanical properties of castings, and only the German specification allows cracks.