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What are the causes of defects in the production process of valve castings?

1.Porosity

This is the metal solidification process fails to escape the gas left inside the metal to form a small cavity, its inner wall is smooth, containing gas, ultrasound has a high reflectivity, but also because of its basically spherical or ellipsoidal, that is, for the point defects, affecting the reflected wave amplitude. The porosity in the ingot after forging or rolling is flattened into an area-type defect and is favourable to be detected by ultrasonic testing.

2.Shrinkage and loosening

When the casting or ingot is cooled and solidified, the volume should be contracted, in the last solidified part because of the lack of liquid metal supplementation and will form a cavity-like defect. Large and concentrated voids called shrinkage, small and scattered voids are called loose, they are generally located in the ingot or casting centre of the last solidified part of the wall is rough, surrounded by many impurities and small pores. Due to the law of thermal expansion and contraction, shrinkage holes are bound to exist, just with the processing technology to deal with different methods and have different forms, sizes and locations, when it extends to the casting or ingot body when it becomes defective. Ingot in the open billet forging if the shrinkage hole is not removed cleanly and brought into the forging becomes residual shrinkage hole (shrinkage hole residual, residual shrinkage tube).

3.Slag entrapment     

Slag in the melting process or the refractory material on the furnace body flaking into the liquid metal, in the pouring was involved in the casting or ingot body, the formation of slag defects. Slag is usually not a single presence, often dense state or scattered in different depths, it is similar to volume-type defects, but often have a certain degree of line.

4. Inclusions     

Melting process reaction products (such as oxides, sulphides, etc.) - non-metallic inclusions, or metal components of certain components of the additive material is not completely melted and remain to form metal inclusions, such as high-density, high melting point components - tungsten, molybdenum and so on.

5.Segregation     

Casting or ingot in the segregation mainly refers to the smelting process or the melting process of the metal because of the uneven distribution of the composition of the composition of segregation, segregation exists in the region of the mechanical properties are different from the mechanical properties of the entire metal matrix, the difference exceeds the permissible range of standards will become defects.

6.Casting cracks     

Cracks in the casting is mainly due to the metal cooling and solidification of the contraction stress exceeds the ultimate strength of the material and caused by the casting shape design and casting process, but also with the high content of some of the impurities in the metal material caused by the sensitivity of the cracking (for example, high sulphur content has a hot embrittlement, high phosphorus content has a cold embrittlement, etc.). Axial intergranular cracks can also occur in ingots, and if they cannot be forged together in subsequent open billet forging, they will remain in the forging and become internal cracks in the forging.

7.Cold segregation     

This is a unique casting of a layered defect, mainly with the casting of the casting process design, it is in the pouring of liquid metal, due to splashing, wave, pouring interruptions, or from different directions of the two (or more) metal flow encounter, etc., because of the liquid metal surface cooling the formation of semi-solid film to stay in the casting in vivo and the formation of a diaphragm-like area type defects.

8. Turning skin     

This is the steelmaking from the ladle to ingot mould pouring ingot, because of the pouring interruption, pause and other reasons, the first pouring into the liquid metal surface in the air quickly cooled to form an oxide film, in the continued pouring of the new pouring of liquid metal will be broken into the ingot turned into the body of the formation of a layered (area type) defects, which in the subsequent ingot forging in the open forging can not be forged together to eliminate.

9. Anisotropy     

Casting or ingot cooling and solidification, from the surface to the centre of the cooling rate is different, and thus will form different crystalline organization, manifested in the mechanical properties of the anisotropy, but also led to the acoustic properties of the anisotropy, that is, from the centre to the surface of the sound velocity and sound attenuation is different. The existence of this anisotropy will have an adverse effect on the size and location of the defects assessed during ultrasonic testing of castings.