Porosity casting defects and prevention methods

Porosity casting defect refers to the defect caused by the bubbles left in the casting before the liquid metal solidifies completely in the mold. There are three main sources of gases in castings: metal melting process, mold and pouring process. The source of gas in the melting process is different from the specific source of gas in the mold. The characteristic of porosity casting defect is that the porosity is smooth and has metal or oxide skin color. According to the different gas sources of porosity casting defects, porosity casting defects can be divided into three types: invasive porosity casting defects, precipitated porosity casting defects and reactive porosity casting defects.

1. Invasive porosity casting defect

Invasive porosity casting defect refers to the porosity casting defect formed by the invasion of gas from the outside of the metal. It is usually large in shape of round, flat or water drop. This kind of porosity casting defect is mostly due to the water or inclusions in the sand mold or core invading the metal due to high-temperature vaporization and decomposition. According to statistics, about half of the castings in wet mold casting are scrapped due to porosity casting defects, and 50% of the porosity casting defects are invasive porosity casting defects.

The occurrence of invasive porosity casting defects also depends on the gassing and air permeability of molding sand, pouring temperature and speed of molten metal. Therefore, the following measures are usually required to prevent invasive porosity casting defects. The first is to control the gassing property of molding sand, select appropriate adhesive materials and strictly control the addition amount to prevent too much water in molding sand from evaporating into a large amount of gas during pouring; Secondly, the air permeability of molding sand should be ensured. Porosity can be pierced and thin-walled or hollow sand cores can be used to promote the elimination of water vapor in the mold; From the perspective of casting technology, the pouring position should be avoided as far as possible in the horizontal position of the large plane, the pouring temperature and speed should be appropriately increased, or the overflow riser should be set at the highest position of the pouring position, so as to bring out the floating bubbles and impurities.

2. Precipitation porosity casting defect

The porosity casting defect formed by the gas precipitated in the liquid metal is called the precipitated porosity casting defect. For example, if hydrogen, oxygen, nitrogen and other elements are mixed into the molten metal during metal smelting, porosity casting defects are easy to form due to the decrease of gas solubility during cooling and solidification. Unlike invasive porosity casting defects, precipitated porosity casting defects are usually small and spherical.

The main factors affecting the formation of precipitation porosity casting defects are the original gas content of liquid metal, the cooling rate of casting, alloy composition and the properties of gas contained. However, the root cause of precipitation porosity casting defects is that the original gas content of molten metal is too high. Therefore, the fundamental way to prevent precipitation porosity casting defects is to reduce the original gas content. Firstly, in the smelting step, the dissolution of gas elements should be minimized. The specific measures include: using clean and dry furnace charge to reduce the introduction of impurities such as moisture, grease and rust; Reduce the air humidity of smelting environment and control the hydrogen content of molten iron; Due to the high nitrogen content of scrap steel, the amount of scrap steel needs to be controlled. For example, when smelting low-carbon equivalent cast iron, the amount of scrap steel should be controlled within 25%; In addition to the above preventive measures, degassing treatment can also be carried out during liquid metal subcontracting to reduce the gas content of liquid metal and remove the inclusions in liquid metal, so as to achieve the purpose of refining liquid metal before pouring.

3. Reactive porosity casting defect

Reactive Porosity casting defect is the Porosity casting defect caused by the chemical reaction between liquid metal and scum, cold iron, etc. during pouring, or the reaction between liquid metal and water in sand mold to produce gas. Because its distribution position is mostly under the surface of the casting, it can also be called subcutaneous Porosity casting defect or subcutaneous pinhole.

Reactive porosity casting defects can be divided into endogenous and exogenous types according to the different reaction substances with liquid metal. They can be divided into many types according to the different gas components generated by the reaction. Therefore, in actual production, the types of reactive porosity casting defects should be judged in combination with production conditions.

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