This defect is the small hole formed by the the air failed to escape and left in the internal metal in the process of the metal solidification. The gas-hole in the steel is crushed into an area defect by forging or rolling， so that it is advantageous for being found by ultrasonic testing.
2.Shrinkage and loose
When castings or steel under the cooling solidification, the volume would shrink and form a hole-like defect without the complement of the liquid metal in the final process of the solidification. Large and concentrated voids are called shrinkage holes, small and scattered spaces are called loose. They are usually located in the steel or castings center under the final solidified process, and its inner wall is thick, surrounded by many impurities and small pores.
The slag in the smelting process or the refractory on the furnace is exfoliated into the liquid metal, and is entrained in the castings or steel during pouring. The the slag defect is formed. The slag is usually present in a dense state or dispersed in different depths, it is similar to the volume type defects but often has a certain degree.
The reaction product (such as oxides, sulfides, etc.) during the smelting process forms the non-metallic inclusion, or the additives of certain components of the metal component are not completely melted and remain to form the metal inclusion (such as high density, high melting point components - Tungsten, molybdenum, etc.).
The casting or ingot segregation mainly refers to the components segregation formed by the uneven distribution in the smelting process or metal melting process. The mechanical properties of segregated areas are different from the mechanical properties of the entire metal matrix, and if the difference is beyond the allowable standard, it is a defect.
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