The main defects are: porosity (the small cavity formed by the gas that fails to escape during the solidification of the metal), shrinkage cavities and porosity (when the casting or ingot is cooled and solidified, the volume should shrink, and the part that solidifies later is due to the Without the addition of liquid metal, it will form void-like defects), slag inclusions, inclusions, segregation, casting cracks, cold insulation, skinning, anisotropy.
This is a small cavity formed inside the metal by the gas that failed to escape during the solidification of the metal. Its inner wall is smooth, contains gas, and has a high reflectivity for ultrasonic waves, but because it is basically spherical or ellipsoid, That is, point defects, which affect the reflected amplitude. The pores in the ingot are flattened into area-type defects after forging or rolling, which is beneficial to be found by ultrasonic testing.
2, Shrinkage and porosity
When the casting or ingot is cooled and solidified, the volume will shrink, and the later solidified part will form a hollow defect because the liquid metal cannot be replenished. Large and concentrated voids are called shrinkage cavities, and small and dispersed voids are called porosity. They are generally located in the later solidified part of the center of the ingot or casting. The inner wall is rough, and there are many impurities and small pores around. Due to the law of thermal expansion and contraction, shrinkage holes are bound to exist, but they have different shapes, sizes and positions with different processing methods. When they extend to the casting or ingot body, they become defects. When the ingot is forged, if the shrinkage cavity is not removed cleanly and brought into the forging, it will become a residual shrinkage cavity (shrinkage cavity residual, residual shrinkage tube).
3, Slag inclusion
The slag in the smelting process or the refractory material on the furnace body peels off into the liquid metal, and is involved in the casting or ingot body during pouring, forming a slag inclusion defect. Slag inclusions usually do not exist alone, but are often dense or dispersed at different depths. They are similar to volumetric defects but often have a certain linearity.
Reaction products (such as oxides, sulfides, etc.) during the smelting process - non-metallic inclusions, or some additives in the metal components are not completely melted and remain to form metal inclusions, such as high-density, high-melting components - tungsten , Molybdenum, etc.
Segregation in castings or ingots mainly refers to the segregation of components due to uneven distribution of components during the smelting process or the melting process of metals. The mechanical properties of the areas where segregation exists are different from those of the entire metal matrix, and the difference exceeds the allowable standard. Scope becomes a flaw.
6, Casting cracks
Cracks in castings are mainly caused by the shrinkage stress of the metal during cooling and solidification exceeding the ultimate strength of the material. It is related to the shape design and casting process of the casting, and is also sensitive to cracking caused by the high content of some impurities in the metal material. properties (such as hot brittleness with high sulfur content, cold brittleness with high phosphorus content, etc.). Axial intergranular cracks will also occur in the ingot, and if they cannot be forged in the subsequent billet forging, they will remain in the forging and become the internal crack of the forging.
7, Cold compartment
This is a unique delamination defect in castings, which is mainly related to the casting process design of castings. ) metal flow encounters and other reasons, because the semi-solid film formed by the cooling of the liquid metal surface remains in the casting body to form a diaphragm-like area defect.
8, Flip the skin
This is when the steel ingot is poured from the ladle to the ingot mold during steelmaking. Due to the interruption of pouring, the pause, etc., the surface of the liquid metal poured first is rapidly cooled in the air to form an oxide film, and the newly poured liquid metal will be poured when the pouring continues. A delamination (area-type) defect formed by breaking into the ingot body, which cannot be eliminated by forging in the subsequent ingot forging.
When the casting or ingot is cooled and solidified, the cooling rate from the surface to the center is different, so different crystalline structures will be formed, which are manifested as anisotropy of mechanical properties, and also lead to anisotropy of acoustic properties, that is, from the center to the center. Surfaces have different sound velocities and sound attenuations. The existence of this anisotropy will adversely affect the size and location of defects in the ultrasonic inspection of castings.