1. Gas holes
A gas hole is a small cavity formed due to the gas that has not escaped in the solidification process of the metal. It has smooth inner walls and contains gas; there is a high reflectivity to ultrasonic waves. However, it is spherical or ellipsoidal, that is, it is a dot like defect, which affects the reflected wave amplitude. The gas hole in the steel ingot is flattened after forging or rolling, becoming area defects and good for being found by ultrasonic testing.
2. Shrinkage cavities and porosity
When the casting or steel ingot is cooled and solidified, its volume will shrink. The later solidified part will form a cavity because it cannot be supplemented by the liquid metal. Big and concentrated cavities are called shrinkage cavities, while small and scattered holes 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 surrounded by many impurities and small holes. Shrinkage holes are inevitable due to the law of expansion caused by heat and contraction caused by cold. They have different shapes, sizes and are located on different parts depending on the processing methods. When they appear for the casting or ingot, they become defects. When the steel ingot is forged, if the shrinkage cavity is not removed and brought into the forging, it will become a residual shrinkage cavity.
3. Slag inclusions
In the smelting process, the slag or the refractory material on the furnace body peels off and enters the liquid metal, and is drawn into the casting or the steel ingot body in the pouring process, forming a slag inclusion. Slag inclusions usually do not exist alone, which are often dense or scattered at different depths. They are similar to volumetric defects but often have a certain degree of linearity.
4. Inclusions
Reaction products in the smelting process such as oxides and sulfides form non-metallic inclusions, or some of the additives in the metal are not completely melted and form metallic inclusions, such as high-density, high-melting-point tungsten, molybdenum, etc.
5. Segregation
Segregation in castings or steel ingots mainly refers to the composition segregation formed in the smelting process or the melting process of the metal because of uneven composition distribution. The mechanical properties of the segregated area are different from those of the entire metal matrix. If the difference exceeds the allowable standard, it becomes a defect.
6. Casting cracks
The cracks in the casting are mainly caused by the shrinkage stress of the metal in the cooling and solidification process exceeding the ultimate strength of the valve. They are caused by the shape design of the casting and the casting process. The sensibility cracking caused by the high content of some impurities in the metal is also the reason. For example, hot brittleness occurs when sulfur content is high, and cold brittleness happens when phosphorus content is high. Axial intergranular cracks will also occur in the steel ingots. If they cannot be forged in the subsequent forging, they will remain in the forgings and become internal cracks in the forgings.
7. Cold shuts
A cold shut is a unique layering defect in castings, which is mainly related to the casting process design of the castings. It is caused by splashing, tumbling, pouring interruption, or meeting of two strands or multiple strands of molten metal from different directions when liquid metal is poured. The semi-solid film formed by the cooling of the liquid metal surface remains in the casting body and forms a diaphragm like area defect.
8. Flipping
When the steel ingot is poured from the ladle to the ingot mold in the steelmaking process, the surface of the liquid metal firstly poured in the air is rapidly cooled to form an oxide film due to the interruption of the pouring. When the pouring continues, the newly poured liquid metal will break the oxide film, making it get into the steel ingot body and forming a laying defect. It cannot be eliminated by forging in the subsequent forging of the steel ingot.
9. Anisotropy
When a casting or steel ingot is cooled and solidified, the cooling rate from the surface to the center is different. Therefore, different crystalline structures will be formed, which is manifested as anisotropy of mechanical properties, also leads to anisotropy of acoustic properties, that is, there are different speeds of sound and sound attenuation from the center to the surface. The existence of this anisotropy will adversely affect the size and location of defects during ultrasonic testing of castings.
