Knowledge

The casting process is complex, and there are many factors that can affect the quality of castings. Often, due to poor control of raw materials, unreasonable process plans, improper production operations, and incomplete management systems, various quality defects may occur in castings, such as porosity, sand holes, slag holes, residual slag, shrinkage cavities, shrinkage porosity, cracks, uneven hardness, and poor spheroidization of ductile iron parts. This article mainly analyzes the causes of common casting defects and proposes quality control measures.

1. Porosity

Characteristics: Porosity is often seen as smooth holes of various sizes on the surface and inside the casting. Different types of porosity, due to their different causes and sources, present in various forms. Common types include invasive porosity, precipitative porosity, and subcutaneous porosity.

Causes of porosity: There are many reasons for porosity, such as damp or oxide-rich raw materials, excessive impurities; un-dried pouring tools or pre-pouring additives; excessive moisture in the molding sand or excessive water application during pattern removal and mold repair; insufficient drying of cores or blocked core vent holes; too low pouring temperature or too fast pouring speed, etc.

Quality control measures: For invasive porosity, control the content of gas-generating substances in the molding sand or core sand to reduce gas generation, and lower the moisture content of the wet molding sand. Do not apply excessive water during mold making and repair, and ensure the drying of the core. For precipitative porosity, which is often found on the surface of castings in the same batch, ensure clean and dry raw materials, and avoid using raw materials with high gas content. Also, ensure the dryness of additives. For subcutaneous porosity, measures include appropriately increasing the pouring temperature, reducing the dosage of various additives, and minimizing the pouring time. The addition of inoculant should be controlled at 0.4% to 0.6% (by mass fraction), and the Al content in the inoculant should not exceed 1.5%.

2. Sand holes and slag holes

Characteristics: Sand holes are small holes filled with molding (core) sand at the defect areas on the surface or inside the material. If the defect shape is irregular and filled with inclusions, it is called a slag hole.

Causes of sand holes and slag holes: Low strength of molding sand, insufficient strength of the mold and core, local damage to the mold during box closing, unreasonable pouring system, incorrect direction of the inner gate, metal liquid damaging the mold; residual loose sand in the mold cavity or gate not cleaned up before closing the mold.

Quality control measures for sand holes: First, increase the strength and firmness of the molding sand and the mold, and reduce the burrs on the core to prevent sand washing. Second, clean the loose sand on the mold cavity and core surface before closing the mold and pour as soon as possible. Third, design a scientific and effective pouring sequence to strictly avoid excessive scouring of the mold wall by the molten metal. Quality control measures for slag holes: First, prevent oxidation of the molten iron, strictly control the addition amount of the inoculant and modifier, and be cautious when using in-stream inoculation for ductile iron. Second, design a scientific and effective pouring sequence, install filter screens during pouring, and improve the slag-blocking capacity of the filter screens. Pouring should not be interrupted. Third, strictly control the residual magnesium content in the molten iron for secondary slag. Reduce the sulfur content in the original molten iron, increase the processing and pouring temperatures, and appropriately increase the rare earth content in the inoculant while reducing the magnesium content in the material.

3. Residual slag

Residual slag refers to the wrinkled defects concentrated on the surface of the casting. Residual slag is composed of many substances, with the main components being EPS pattern residues and relatively bright wrinkled substances. Analysis of the causes of residue defects: As the main components of the residue are the remnants of EPS patterns, in terms of permeability, the gas released from the decomposition of EPS patterns escapes very slowly, thus the process of its decomposition and disappearance is very long.

Quality control measures for residue defects: First, ensure good coating permeability and good molding sand permeability; second, appropriately increase the pouring temperature; third, appropriately increase the negative pressure; fourth, appropriately reduce the density and compactness of EPS materials; fifth, appropriately increase the pouring range.

4. Shrinkage Cavities and Porosity

Feature Description: In the thicker sections or the core of the casting, some rough-surfaced holes often occur due to solidification. Some of these holes may have dendritic crystals. The larger and more concentrated holes are called shrinkage cavities, while the smaller and more dispersed ones are called porosity.

Analysis of the Causes of Shrinkage Cavities and Porosity Defects: When the molten metal cools and solidifies, the liquid shrinkage and solidification shrinkage produced during this process are much greater than the solid shrinkage. Moreover, the last solidified areas on the surface of the casting often cannot receive sufficient molten metal.

Quality Control Measures for Shrinkage Cavities and Porosity Defects: First, different castings have different wall thicknesses and corresponding chemical compositions. The content of magnesium should be strictly controlled. Second, design a scientific gating system, appropriately reduce the pouring temperature, and ensure that all parts of the casting surface can be adequately compensated. Third, strictly control the content of the inoculant before the furnace, generally 0.3% to 0.5% is optimal. Fourth, avoid oxidation of the molten iron.

5. Cracks

Feature Description: Cracks on the surface of the castings are usually curved. Cracks are divided into hot cracks and cold cracks. The crack shape of hot cracks is irregular, and the color of the crack end surface is often dark black with obvious oxidation and a tortuous shape. In contrast, the crack shape of cold cracks is relatively straight and regular, and the oxidation on the surface of the casting fracture is relatively mild, showing a metallic luster.

Analysis of the Causes of Cracks: When the casting cools and solidifies, internal stress is generated. Once the stress exceeds the yield limit of the material, cracks will occur.

Quality Control Measures to Prevent Cracks: First, the chemical composition of the molten iron must be strictly controlled. The sulfur content can affect the occurrence of "hot brittleness". In gray cast iron, the sulfur content (w(S)) should not exceed 0.11%, but it should not be too low either, as too low a content will affect the inoculation effect. The phosphorus content is a factor affecting the occurrence of "cold brittleness" in castings, leading to cold cracks. Therefore, in gray cast iron, the phosphorus content (w(P)) should be less than 0.13%. Second, control the cooling rate of different parts of the casting to prevent local overheating. Third, do not open the mold too early after pouring. The mold can only be opened when the temperature of the casting inside the mold is below 550°C. Fourth, try to change the structure of the casting to avoid stress concentration.

6. Inconsistent Hardness

Description of inconsistent hardness phenomenon: After processing, the surface of the castings often shows microscopic concavities and convexities, and there may be hard spots in some areas. Due to the significant difference in surface hardness of the castings, the hardness of the hard spot areas may exceed the standard.

Quality control measures to avoid inconsistent hardness: First, increase the pouring temperature of the molten iron; second, strictly control the addition amount of inoculant; third, strictly prohibit the use of alloy steel; fourth, the design of the gating system should be scientific to ensure uniform cooling of the castings.

7. Incomplete spheroidization and poor spheroidization

Characteristics of incomplete spheroidization and poor spheroidization: After oxidation, the fracture surface of the casting appears grayish black, with reduced mechanical properties and a significant decrease in the residual magnesium content in the casting. This is the phenomenon of incomplete spheroidization. If the fracture surface of the casting appears silvery gray with relatively scattered black spots, and a small portion of graphite is found to be in flake or vermicular form upon metallographic examination, this is the phenomenon of poor spheroidization.

Quality control measures for incomplete spheroidization and poor spheroidization: First, strictly control the addition ratio of the spheroidizing agent, which is directly related to the sulfur content of the molten iron and the magnesium content of the spheroidizing agent; second, appropriately extend the spheroidizing treatment time to ensure sufficient chemical reaction time between the spheroidizing agent and the molten iron, generally within the range of 75 to 95 seconds; third, strive to perform desulfurization treatment to effectively reduce the sulfur content of the original molten iron; fourth, strictly control the content of anti-spheroidizing elements in pig iron (such as arsenic, lead, titanium, bismuth, aluminum, etc.); fifth, prevent oxidation of the molten iron, and maintain an appropriate temperature when processing ductile iron, choosing the chemical composition of the spheroidizing agent based on the temperature of the molten iron; sixth, for large-section castings, try to reduce the rare earth content as much as possible, and if necessary, add a small amount of antimony to neutralize the effect of rare earth on the distortion of spheroidal graphite. Conclusion

Casting is one of the main methods to obtain the blanks of mechanical products and an important basic process in the machinery industry. The qualification rate of castings and the repair rate of defective castings are the bottlenecks restricting the development of China's casting industry. Therefore, analyzing and researching the causes of common casting defects and proposing quality control measures to avoid defects are of great significance for improving the product quality of casting enterprises and enhancing the mechanization level of our country.

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