In cylinder casting, the subcutaneous pores are mainly distributed at the position of L ~ zmrn under the outer circular surface, but also on the inner circular surface, while in double ring casting, they are mainly distributed at the position of 1mm under the upper surface. They are spherical, ellipsoidal and acicular and are exposed after machining.
The formation of subcutaneous stomata is a complex physical and chemical process. The gas in the subcutaneous pores is mainly hydrogen, which comes from both the gas separated from the molten iron and the gas invaded from the outside.
1. Causes of subcutaneous stomata formation
a. The gas inside the molten iron comes from the moisture contained in the wet charge, rusted scrap and other materials; the furnace lining, spheroidizing treatment package, pouring package, spheroidizing agent, inoculant and covering agent of the smelting furnace are not dried, and all contain moisture. During smelting and pouring, these water will decompose and produce hydrogen.
b. Hydrogen is also the main source of subcutaneous pores in the gas invaded by molten iron. According to the data, during the filling process of molten iron, the magnesium vapor in molten iron reacts with the water in molding sand to produce hydrogen.
Mg + Hz – MgO + 2 H
Magnesium as catalyst promotes the reaction of carbon and water in molding sand to produce hydrogen.
C ten HZO ~ co ten H: PCs. (2)
The carbides of water and magnesium react with iron to decompose to produce acetylene gas, which then decomposes to produce hydrogen.
[Fe · mg] C + HZO (gas) ~ (Fe · mg) O + czh: PCs. (3)
C: Hz ~ ZC + Hz (4)
c. In the production of nodular iron, a layer of oxide film will be formed on the surface of molten iron due to the activation of magnesium after spheroidizing treatment, and the formation temperature of the oxide film is about 1230c. The formation of oxide film is closely related to the existence of subcutaneous stomata. The formation of subcutaneous stomata is effected by oxide film. Because this layer of oxide film increases the interface pressure between the casting and the mold, which prevents the gas inside the molten iron and the gas invaded from escaping as soon as possible, and then the surface of the casting solidifies gradually, and the gas stays under the surface of the casting to form subcutaneous pores.
d. Due to the solidification characteristics of nodular iron, the passage of gas escaping is blocked too early, which makes the gas stay under the surface of casting to form subcutaneous porosity.
2. Technological measures to prevent subcutaneous pores
a. Strictly control the moisture content of molding sand to reduce the amount of gas produced by the reaction between molten iron and mold interface. In cylinder casting, the controlled moisture content of molding sand is 3.6% – 4.5%, and the air permeability of molding sand is more than 80 units; in double ring casting, the moisture content of molding sand is 3.6% – 4.1%, and the air permeability of molding sand is more than 90 units.
b. The spheroidizing agent, inoculant, spheroidizing treatment package and pouring package must be preheated and baked to above 200C before use to reduce the amount of hydrogen brought in. Especially, the lining of smelting furnace must be baked according to the process requirements after overhaul. .
C magnesium and sulfur both increase the formation temperature of oxide film. The residual w (mg) increased every time. The formation temperature of oxide film should be increased by 15 ℃ for 0.01%, and 10 ℃ for 0.001% increase of sulfur content in molten iron. Therefore, on the premise of ensuring the spheroidization of molten iron, the residual w (mg) should not exceed 0.05%, and the lower the mass fraction of sulfur in molten iron, the better.
d. Because the pouring temperature is related to the residual w (mg) and the formation temperature of oxide film, the pouring temperature of molten iron should not be lower than 1350 ℃, which is one of the effective measures to prevent the formation of subcutaneous pores. At the same time, pouring at high temperature can prolong the time when molten iron is in liquid state, and is also conducive to the escape of gas.
e. The designed pouring system should ensure the smooth flow of molten iron when it is injected into the mold cavity, avoid the overturning of molten iron in the mold cavity as much as possible, so as to reduce the oxidation of molten iron and facilitate the escape of magnesium vapor, which is conducive to prevent the generation of subcutaneous pores. Open gating system can also be used.