Causes of casting defects during the valve manufacturing process
This is an analysis of causes of casting defects during the valve manufacturing process.
Gas Bubbles
These are small cavities that cause by residual gas during the metal solidification process. Their smooth inner surface and contain gas will bring a high reflectivity to ultrasonic wave. Because the shapes of gas bubbles are usually sphere or ellipsoid, i.e., dot-shape defect, which will affect the reflection amplitude, the area-type defects sprung from gas bubbles in the ingot after being forged and rolled are noticeable to be detected in ultrasonic testing.
Shrinkage and Porosity
The volume of the ingot or casting will contract during cooling and solidifying processes, and some cavity defects will form in the final solidified part (generally in the center of the casting or ingot) because the liquid metal is not replenished, of which the bigger cluster are called shrinkage cavities and the smaller, scattered cavities are called porosity. The inner surface is rough, with plenty of impurities and fine cavities. Due to the phenomenon of thermal expansion and contraction, the appearing of shrinkage cavities in various kinds of shapes, inches, positions depending on different processing techniques, are inevitable and become defects when extend to the body of casting or ingot. If those shrinkage cavities are not removed before cogging and forging processes, they will remain as holes in the forging pieces.
Inclusions
• Slag Inclusions - During the smelting process, slag inclusions may occur if slag or refractory materials on the furnace are peeled off into the liquid metal, and interfuse into the body of casting or ingot. Slag inclusions usually tend to be dense or dispersed at different depths, which are similar to volumetric defects but generally have a certain linearity.
• Reaction products, such as oxides, sulfides, etc.
• Metal materials with high density, high melting point that cannot be completely melted in the smelting process, such as tungsten, molybdenum, etc.
Segregation
Segregation in castings or ingots mainly refers to segregation that causes by uneven distribution of components during smelting or melting process. The mechanical properties of areas where segregation exists are different from that of the whole metallic matrix, and the segregation will become defect if the discrepancy is out of the allowable range.
Casting Cracks
The cracks in the casting are related to the shape design, casting process as well as cracking sensitivity of metal materials with relatively high content of impurities (For example, high content of sulfur will cause hot brittleness and that of phosphorus will cause cold brittleness.), and mostly caused if the shrinkage stress of the metal during cooling and solidification process exceeds the ultimate strength of the materials. Axial intercrystalline cracks generated in the ingots will remain as inner cracks of forged pieces if they cannot be healed in the subsequent cogging and forging processes.
Cold Shut
This is a lamination, diaphragm-like, area-type defect largely related to the process and design of casting and caused by factors such as splashing, tumbling, discontinuous pouring, metal flows in two or more directions confronting when casting liquid metal. Because the semi-solid film formed when the surface of metal is cooling remains in the body of the casting.
Overlapping Surfaces
When the steel ingot is poured from the ladle to the ingot mold, if the pouring is discontinued, an oxidation film will rapidly formed on the surface of liquid metal previously poured, and the liquid metal later poured will break into the body of ingot and generates a lamination defect, which cannot be removed in the subsequent cogging and forging processes.
Aeolotropism
During the cooling and solidifying processes of casting or ingot, the cooling rates from the surface to the center are different and thus will generate different crystal structures, which brings about the aeolotropism of mechanical properties and that of acoustic properties, namely, from the center to the surface there will be different sound speeds and attenuation. This will adversely affect the ultrasonic testing of the size and position of defects.
Gas Bubbles
These are small cavities that cause by residual gas during the metal solidification process. Their smooth inner surface and contain gas will bring a high reflectivity to ultrasonic wave. Because the shapes of gas bubbles are usually sphere or ellipsoid, i.e., dot-shape defect, which will affect the reflection amplitude, the area-type defects sprung from gas bubbles in the ingot after being forged and rolled are noticeable to be detected in ultrasonic testing.
Shrinkage and Porosity
The volume of the ingot or casting will contract during cooling and solidifying processes, and some cavity defects will form in the final solidified part (generally in the center of the casting or ingot) because the liquid metal is not replenished, of which the bigger cluster are called shrinkage cavities and the smaller, scattered cavities are called porosity. The inner surface is rough, with plenty of impurities and fine cavities. Due to the phenomenon of thermal expansion and contraction, the appearing of shrinkage cavities in various kinds of shapes, inches, positions depending on different processing techniques, are inevitable and become defects when extend to the body of casting or ingot. If those shrinkage cavities are not removed before cogging and forging processes, they will remain as holes in the forging pieces.
Inclusions
• Slag Inclusions - During the smelting process, slag inclusions may occur if slag or refractory materials on the furnace are peeled off into the liquid metal, and interfuse into the body of casting or ingot. Slag inclusions usually tend to be dense or dispersed at different depths, which are similar to volumetric defects but generally have a certain linearity.
• Reaction products, such as oxides, sulfides, etc.
• Metal materials with high density, high melting point that cannot be completely melted in the smelting process, such as tungsten, molybdenum, etc.
Segregation
Segregation in castings or ingots mainly refers to segregation that causes by uneven distribution of components during smelting or melting process. The mechanical properties of areas where segregation exists are different from that of the whole metallic matrix, and the segregation will become defect if the discrepancy is out of the allowable range.
Casting Cracks
The cracks in the casting are related to the shape design, casting process as well as cracking sensitivity of metal materials with relatively high content of impurities (For example, high content of sulfur will cause hot brittleness and that of phosphorus will cause cold brittleness.), and mostly caused if the shrinkage stress of the metal during cooling and solidification process exceeds the ultimate strength of the materials. Axial intercrystalline cracks generated in the ingots will remain as inner cracks of forged pieces if they cannot be healed in the subsequent cogging and forging processes.
Cold Shut
This is a lamination, diaphragm-like, area-type defect largely related to the process and design of casting and caused by factors such as splashing, tumbling, discontinuous pouring, metal flows in two or more directions confronting when casting liquid metal. Because the semi-solid film formed when the surface of metal is cooling remains in the body of the casting.
Overlapping Surfaces
When the steel ingot is poured from the ladle to the ingot mold, if the pouring is discontinued, an oxidation film will rapidly formed on the surface of liquid metal previously poured, and the liquid metal later poured will break into the body of ingot and generates a lamination defect, which cannot be removed in the subsequent cogging and forging processes.
Aeolotropism
During the cooling and solidifying processes of casting or ingot, the cooling rates from the surface to the center are different and thus will generate different crystal structures, which brings about the aeolotropism of mechanical properties and that of acoustic properties, namely, from the center to the surface there will be different sound speeds and attenuation. This will adversely affect the ultrasonic testing of the size and position of defects.
