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

- May 27, 2018-

1.Air hole

This is a small cavity formed by the gas that cannot be escaped during metal solidification and remains inside the metal. Its inner wall is smooth, contains gas, and has high reflectivity for ultrasonic waves, but because it is basically spherical or ellipsoidal, It is a point defect that affects its reflection amplitude. The pores in the steel ingot are flattened into area-type defects after being forged or rolled to facilitate detection by ultrasonic testing.

2. Shrinkage and looseness

When the casting or ingot cools and solidifies, the volume shrinks, and in the final solidification part, void-like defects are formed due to lack of complement of the liquid metal. Large and concentrated voids are referred to as shrinkage pores. Small and scattered voids are referred to as loose pores. They are generally located in the final solidified part of the center of the ingot or casting. The inner wall is rough, surrounded by many impurities and tiny pores. Due to the law of thermal expansion and contraction, the shrinkage cavity is inevitably present. However, the shape, size and position of the shrinkage hole are different depending on the processing method of the processing. When it extends to the casting or ingot body, it becomes a defect. When the billet is forged, it will become residual shrinkage (remaining shrinkage, residual shrinkage) if it is not cut clean and brought into the forging.

3. slag inclusion

The slag in the smelting process or the refractories on the furnace body is peeled off into the liquid metal, and is entrained into the casting or the steel ingot body during pouring, thus forming a slag inclusion defect. Slag inclusions do not usually exist singly, they tend to be dense or disperse at different depths. They are similar to volumetric defects but often have a certain degree of linearity.

4. Inclusion

The reaction products in the smelting process (such as oxides, sulfides, etc.) - non-metallic inclusions, or the addition of certain components in the metal components are not completely melted and remain to form metal inclusions, such as high density, high melting point component - tungsten Mo, etc.

5. Segregation

Segregation in castings or ingots mainly refers to the segregation of components formed during the smelting process or the melting process of metals due to uneven composition distribution. The mechanical properties of the areas where segregation is present are different from the mechanical properties of the entire metal matrix, and the differences exceed the allowable standards. The scope becomes a defect.

7. Cold isolation

This is a peculiar delamination defect in the casting, which is mainly related to the casting process design of the casting. It is when the liquid metal is poured, due to splashing, wave breaking, pouring interruption, or two or more strands coming from different directions. ) Metal flow meets and other reasons, because the semi-solid film formed by the cooling of the liquid metal surface remains in the casting body and forms a diaphragm-like area-type defect.

8. Peeling

This is when the steel ingot is poured from the ladle to the ingot mold during steelmaking. Because of casting interruptions, pauses, etc., the surface of the liquid metal poured first is quickly cooled in the air to form an oxide film, and the newly poured liquid metal will be A delamination (area type) defect formed by breaking into the steel ingot is not eliminated by the subsequent forging of the steel ingot.

9. Anisotropy

When the casting or ingot cools and solidifies, the cooling rate from the surface to the center is different. Therefore, different crystalline structures will be formed. The anisotropy of the mechanical properties will also lead to the anisotropy of the acoustic properties, that is, from the center to the The surface has different sound speed and sound attenuation. The presence of this anisotropy can have an adverse effect on the size and position of flaws during the ultrasonic testing of castings.