Cause Analysis of Defects in Valve Castings During Production

1. porosity
This is a small void formed by the gas which can not escape during the solidification of metal. Its inner wall is smooth and contains gas. It has a high reflectivity to the ultrasonic wave, but because it is basically spherical or ellipsoid, that is to say, it is a dot defect, which affects the reflected wave amplitude. The air holes in ingots are flattened into area defects after forging or rolling, which is beneficial to be detected by ultrasonic testing.
2. Shrinkage and Porosity
When a casting or ingot is cooled and solidified, its volume shrinks. The final solidified part will form a void defect because it can not be supplemented by liquid metal. Large and concentrated voids are called shrinkage voids, while small and dispersed voids are called looseness. They are generally located in the last solidified part of the center of the ingot or castings. Their inner walls are rough, and there are many impurities and small voids around them. Because of the law of thermal expansion and cold contraction, shrinkage holes are inevitable, but they have different shapes, sizes and locations with different processing methods. When they extend to the casting or ingot body, they become defects. Ingot will become residual shrinkage (shrinkage residual, residual shrinkage pipe) if it is not removed and brought into the forging during the ingot forging.
3. slag inclusion
Slag inclusion defects are formed when slag or refractory material on the furnace body is peeled off into liquid metal during smelting and is involved in the casting or ingot body during pouring. Slag inclusion usually does not exist singly, but tends to be concentrated or dispersed in different depths. It is similar to volume defects but often has a certain linearity.
4. inclusions
The reaction products (such as oxides, sulfides, etc.) - Non-metallic inclusions, or the additives of some components in the metal composition are not fully melted and remain to form metal inclusions, such as high density, high melting point components - tungsten, molybdenum and so on.
5. segregation
Segregation in castings or ingots mainly refers to the composition segregation formed during smelting or metal melting due to the uneven distribution of components. The mechanical properties of the segregation area are different from those of the whole metal matrix, and the differences beyond the allowable standard range become defects.
6. Casting Cracks
Cracks in castings are mainly caused by shrinkage stress exceeding the ultimate strength of materials during cooling and solidification, which is related to the shape design of castings and casting process, and also to crack sensitivity caused by high impurity content in metal materials (such as hot brittleness when sulphur content is high, cold brittleness when phosphorus content is high). Axis intergranular cracks also occur in ingots. If they cannot be forged in subsequent open forging, they will remain in the forgings and become internal cracks of the forgings.
7. cold separation
This is a unique layered defect in castings, which is mainly related to the casting process design of castings. It is caused by splashing, surging, interruption of pouring, or the encounter of two (or more) metal streams from different directions when pouring liquid metals, because the semi-solid film formed by cooling the liquid metal surface remains in the body of the castings and forms a diaphragm-like area. Type defects.
8. upset
This is a layered defect formed when ingot is poured from ladle to ingot mould, because of interruption and pause of pouring, the first liquid metal surface is rapidly cooled in air to form oxide film, and the newly poured liquid metal breaks it into ingot body during continuous pouring. It can not be forged and closed in subsequent ingot open forging.
9. Anisotropy
When 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 characterized by the anisotropy of mechanical properties, and also lead to the anisotropy of acoustic properties, i.e. different sound velocity and sound attenuation from the center to the surface. The existence of this anisotropy will have a negative impact on the size and location of defects in ultrasonic inspection of castings.