Nine factors of casting porosity formation and 5 improvement measures of process (2)
2.Technological measures to prevent subcutaneous air hole
1) The technological measures to prevent subcutaneous stomata mainly include:
2) Desulfurization treatment. The so-called desulphurization, that is, as far as possible, the sulfur in the molten iron is transferred to the slag, and then eliminated, the less FeS in the molten iron, the higher the desulphurization efficiency, appropriately increasing the basicity of the desulphurized slag and the temperature of the molten iron, and reducing the content of ferrous oxide in the slag, therefore, increasing the slag quantity, improving the fluidity of slag, strengthening stirring and raising the temperature of molten iron and slag are beneficial to the full desulphurization, timely slag removal, and prevention of sulfur recovery. The special industrial calcium carbide, which is composed of calcium carbide and calcium oxide, is used as a desulphurizing agent to remove sulfur. The amount of addition is about 0.5% ~ 1.5% of liquid iron.
3) To improve the purity of molten iron in order to improve the purity of molten iron, reduce slag.
4) The measure of adding a slag-collecting agent to the small ladle to remove slag for the second time has reduced the number of subcutaneous air holes but has not been eradicated. The mold’s surface can be buffered with cryolite powder to eliminate the subcutaneous pores, but the application shows that it is not ideal. After the casting is cleaned, some small slag holes will still appear on the surface, sometimes very similar to the small pits formed by the subcutaneous air holes, and the surface quality still does not meet the requirements of product quality, this may be the reason that cryolite reacts with inclusions in Molten Iron to form liquid slag, which remains on the surface of the casting and can not be removed when designing the GATING system for ductile iron castings, filter screen, and slag stop device should be added as far as possible. Adjusting the coke-iron ratio to optimize the smelting process.
5) Raising the tapping temperature of Cupola is beneficial to eliminating and reducing heredity and improving casting quality. Therefore, it can be said that the tapping temperature of Cupola reflects the production level of modern cast iron. The temperature measured in front of the Cupola Reaches Over 1470 °C, at the same time, the measures of desulphurization and slag-collecting, especially when the commercial slag-collecting agent is added, the quantitative cryolite powder (Trisodium hexafluoroaluminate Na3AlP6, melting point 994 °c) is added as a flux on the surface of the molten iron, and the slag-collecting effect is improved by properly stirring and standing. Therefore, the hydrogen absorption of the interface liquid iron layer is prevented, and the formation of subsurface blowhole of ductile iron castings can be effectively suppressed.
6) Control of chemical composition.
Although the chemical composition is not the basis of acceptance, it is the premise of producing as-cast ferritic ductile iron castings with high toughness, strictly controlling and calculating the burden ratio, controlling the final silicon content (SI < 3.0%) and keeping the ferritic content as low as possible. Also, when the content of MN is too high (MN > 0.65%), the slag porosity will be formed. Controlling Mn <, 0.30% can avoid the formation of cementite. At the same time, the use of low sulfur, low phosphorus clean charge, strictly limit the element content of white and anti-spheroidization, it is also must follow the principle.
7) Improving the permeability of the mold is beneficial to reducing the subcutaneous air hole.
8) A 5t medium-frequency coreless induction furnace was used to melt the ductile iron, and the subcutaneous blowhole was controlled. The composition is easy to control, and the molten iron temperature can be raised to 1530 °C ~ 1550 °C. Because no flux is used, the purity of molten iron is ensured, no coke is used in an electric furnace (especially high sulfur coke), no sulfur is added, the metallurgical quality of molten iron is greatly improved and stabilized, which creates hardware facilities for the production of High-Quality Ductile Iron Castings.
Under the conditions that the water content of the mold, the gas content in the molten iron, the residual amount of spheroidizing elements and other harmful elements are effectively controlled, the desulphurization and slag removal effects are improved at the same time to ensure the high purity of the molten iron, with the aid of higher melting treatment and pouring temperature, improving the metallurgical quality of molten iron is the only effective measure to prevent the subcutaneous air hole.