【Haichuan Chemical Valve Management】Common internal defects in valve castings and their characteristics
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Proper management of valves is the foundation for stable operation. Everyone is welcome to engage in discussions and share insights on this series of posts: 【HaiChuan Chemical Valve Management】 series of posts: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719309 ---------------------------------------------------------------- Disclaimer: The content contained in this article is intended solely for technical exchange and reference purposes only; it does not constitute any form of professional engineering advice, design basis, or operational guidance. -----------------------------------------------------------------------------------1. Porosity: These are small voids that form within metal as a result of gases that failed to escape during the metal’s solidification process. Their inner walls are smooth, they contain gas, and they exhibit a high reflectivity for ultrasonic waves. However, since it is essentially spherical or ellipsoidal in shape and belongs to a point defect, its reflected wave amplitude has a certain directionality (or: the reflected wave amplitude is less stable than that of area defects). The pores in the steel ingot are flattened into area defects after forging or rolling, which facilitates their detection by ultrasonic testing. 2. Shrinkage cavities and porosity: As castings or ingots cool and solidify, their volume decreases; the areas that solidify later fail to receive replenishment from the liquid metal, resulting in hollow defects. Large and concentrated voids are called shrinkage cavities, while small and scattered pores are known as porosity. They are generally located in the parts of the steel ingot or casting that solidify later at the center; their inner surfaces are rough, and they are surrounded by many impurities and tiny pores. Due to the laws of metal solidification shrinkage, shrinkage cavities are inevitable; their shape, size, and location vary depending on the processing methods used. When it extends into the machined surface of the casting or ingot, it becomes a defect. If the shrinkage cavities are not completely removed during bloom forging and remain in the forged piece, they become residual shrinkage cavities (shrinkage cavity residues, residual shrinkage tubes).
3. Inclusions: Slag formed during the melting process, or refractory materials from the furnace walls that break off and enter the liquid metal, can be incorporated into the casting or ingot during pouring, thus creating inclusion defects. Inclusions usually do not exist alone; they tend to be present in large numbers or scattered at different depths. It is usually distributed in a volumetric form, but it often also possesses certain linear dimensions (or: planar characteristics). 4. Inclusions: Reaction products formed during the melting process (such as oxides, sulfides, etc.) create non-metallic inclusions ； Or certain high-density, high-melting-point components in the metal composition (such as tungsten, molybdenum, etc.) fail to melt completely and remain behind, forming metal inclusions.
5. Segregation: In castings or ingots, segregation refers primarily to the uneven distribution of components that occurs during the smelting process or the melting of the metal. In areas where segregation occurs, the mechanical properties differ from those of the entire metal matrix. If this difference exceeds the allowable range specified by the relevant standards or technical requirements, it is considered a defect. 6. Casting cracks: Cracks in castings are primarily caused by the shrinkage stresses that occur during the cooling and solidification of the metal, when these stresses exceed the material’s ultimate strength. These cracks are related to the design of the casting’s shape and the casting process, as well as to the high levels of certain impurity elements in the metal, which can increase the susceptibility to cracking (for example, high sulfur content leads to thermal brittleness, while high phosphorus content leads to cold brittleness). Axial intergranular cracks also occur in the ingot; if these cannot be closed during subsequent rolling and forging, they remain in the forged part as internal cracks.
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