Cause Analysis of Defects in Valve Castings During Production

1. Blowholes

These are small cavities formed in the metal by gas that fails to escape during the metal solidification process. Their inner walls are smooth with gas trapped inside, exhibiting high ultrasonic reflectivity. However, as they are basically spherical or ellipsoidal (i.e., point defects), the amplitude of their reflected waves is affected. Blowholes in steel ingots are flattened into area-type defects after forging or rolling, making them easier to detect by ultrasonic testing.

2. Shrinkage Cavity and Shrinkage Porosity

Castings or steel ingots undergo volume contraction during cooling and solidification, and cavity-like defects form in the last solidified areas because they fail to be replenished with liquid metal. Large and concentrated cavities are called shrinkage cavities, while small and scattered voids are known as shrinkage porosity. They are generally located in the final solidified central parts of steel ingots or castings, with rough inner walls and often accompanied by numerous impurities and small blowholes around them. Due to the law of thermal expansion and contraction, shrinkage cavities are inevitable, only varying in morphology, size and position with different processing techniques. They become defects when extending to the main body of castings or steel ingots. If shrinkage cavities are not completely cut off during the blooming forging of steel ingots and are carried into forgings, they become residual shrinkage cavities (shrinkage cavity remnants, residual shrinkage pipes).

3. Slag Inclusions

Slag Inclusions form when smelting slag or spalling of refractory materials from the furnace lining enters the liquid metal and is entrapped in the main body of castings or steel ingots during pouring. Slag inclusions usually do not exist alone; they tend to be dense or scattered at different depths. They are similar to volume-type defects but often have a certain linear dimension.

4. Inclusions

These refer to non-metallic inclusions which are reaction products during smelting (such as oxides, sulfides, etc.), or metallic inclusions formed by the unmolten residues of additives for certain components in the metal composition, such as high-density and high-melting-point components - tungsten, molybdenum, and so on.

5. Segregation

Segregation in castings or steel ingots mainly refers to compositional segregation formed by uneven component distribution during smelting or metal melting. The mechanical properties of areas with segregation are different from those of the entire metallic matrix, and it becomes a defect when the difference exceeds the allowable standard range.

6. Casting Cracks

Cracks in castings are mainly caused by the shrinkage stress during metal cooling and solidification exceeding the ultimate strength of the material. They are related to the shape design and casting process of castings, as well as the cracking sensitivity caused by high content of certain impurities in the metal material (e.g., hot brittleness with high sulfur content, cold brittleness with high phosphorus content, etc.). Axial intergranular cracks can also form in steel ingots; if they cannot be forge-welded during subsequent blooming forging, they will remain in the forgings and become internal cracks of the forgings.

7. Cold Shuts

This is a unique layered defect in castings, mainly related to the pouring process design of castings. During the pouring of liquid metal, it is formed when a semi-solid film formed by the cooling of the liquid metal surface is trapped in the casting body due to splashing, surging, interrupted pouring, or the meeting of two (or more) metal streams from different directions, resulting in a diaphragm-like area-type defect.

8. Skin Folds

This defect occurs when pouring molten steel from the ladle into the ingot mold during steelmaking. Due to interrupted or paused pouring, the surface of the first poured liquid metal cools rapidly in the air to form an oxide film. During subsequent pouring, the newly poured liquid metal breaks through the oxide film and turns it into the steel ingot body, forming a layered (area-type) defect. This defect cannot be eliminated by forge-welding during the subsequent blooming forging of steel ingots.

9. Anisotropy

When castings or steel ingots cool and solidify, the cooling rate from the surface to the center is different, thus forming different crystalline structures, which manifests as anisotropy of mechanical properties and also leads to anisotropy of acoustic properties - that is, different sound velocities and acoustic attenuation from the center to the surface. The existence of such anisotropy will have an adverse effect on evaluating the size and position of defects during ultrasonic testing of castings.