Since the shrinkage of stainless steel precision casting is much greater than that of cast iron, to prevent shrinkage cavities and shrinkage porosity defects in castings, risers, chillers, and subsidies are mostly used in the casting process to achieve sequential solidification.
To avoid the defects of shrinkage cavities, shrinkage porosity, pores, and cracks in stainless steel castings, the wall thickness should be uniform, sharp corners and right-angle structures should be avoided, sawdust should be added to the molding sand, coke should be added to the core, and hollow molds should be used. Cores and oil sand cores are used to improve the flexibility and air permeability of sand molds or cores.
Due to the poor fluidity of molten steel, to prevent cold insulation and insufficient pouring of steel castings, the wall thickness of steel castings should not be less than 8mm; dry casting or hot casting should be used; the pouring temperature should be appropriately increased, generally 1520° to 1600°C, due to the high pouring temperature, the superheat of the molten steel is large, the liquid remains in the liquid state for a long time, and the fluidity can be improved. However, if the pouring temperature is too high, it will cause defects such as coarse grains, thermal cracks, pores, and sand sticking.
Therefore, for general small, thin-walled, and complex-shaped precision castings, the pouring temperature is about the melting point temperature of steel + 150°C; the structure of the pouring system should be simple, and the cross-sectional size should be larger than that of cast iron; the pouring temperature of large, thick-walled castings About 100℃ higher than its melting point.
The main reasons for pores in stainless steel castings:
For box-type and trolley-type resistance furnace plates, high-temperature tempering must be performed before use, and the temperature should be controlled above 950 degrees as much as possible. The purpose of this is to eliminate the internal stress during the casting process. Moreover, during the use of the furnace plate, it is necessary to ensure that the workpieces are placed evenly and that the work cannot be piled up in a certain local position. Otherwise, the furnace plate will be uneven in the heat dissipation process during the heating process, and it will easily cause the furnace plate to deform and crack. , reduce the service life of the furnace floor.
During the stainless steel casting process, pore problems often occur, which brings a lot of trouble to the casting process. The pores are bubble defects formed by the gas precipitated in the ingot during the cooling and solidification process of the metal liquid. Let’s analyze the main causes of pores:
01: The paint has poor air permeability or insufficient negative pressure, and the filling sand has poor air permeability. The gas and residue in the mold cavity cannot be discharged in time, and pores are formed under the filling pressure.
02: The pouring speed is too slow and fails to fill the sprue cup. The sprue is exposed, air is drawn in, and slag is sucked in, forming pores and slag holes.
03: The foam model gasifies and decomposes to generate a large amount of gas and residue that cannot be discharged from the mold in time. The dry sand filled with foam and coating layers is poorly dried. Surrounded by the high temperature of the liquid alloy, a large amount of hydrogen and oxygen are cracked and invade the casting. The main cause of stomata.
04: Due to the unreasonable design of the gating system, the filling speed of the metal liquid is greater than the foam gasification and gas discharge speed, causing the gasification residue to be trapped in the metal liquid at the filling front and vaporized again to form smoke-black decomposition pores on the inner wall.
05: The connection between the sprue cup and the sprue and the pouring system is not well sealed, especially the connection between the sprue and the sprue cup. Sand inclusions and pores are easily formed under the action of negative pressure. This phenomenon can be calculated and explained using Bernoulli’s equation.
06: The particle size of the molding sand is too fine, the dust content is high, and the air permeability is poor. The internal blockage of the negative pressure pipe causes negative pressure distortion, making the negative pressure value around the cavity far lower than the indicated negative pressure, and the vaporized matter cannot be discharged from the coating in time. Formation of pores or wrinkles.
07: The pouring temperature is low, and the molten metal at the filling front cannot fully vaporize the foam. The undecomposed residual material has no time to float to the riser and solidifies to form pores in the casting.
08: Poor deoxidation of molten steel, unclean slag removal on the furnace table, furnace, and ladle, too short sedation time, ineffective slag blocking during the pouring process, and unreasonable pouring technology causing slag holes.
09: The location of the gate opening is unreasonable, forming a dead corner area during mold filling. Due to the gas pressure in the mold cavity, the gasification residue accumulates in the dead corner to form pores. The cross-sectional area of the gate is too large, causing the filling speed to be greater than The foam gasification slows down, swallows the foam, decomposes, and gasifies inside the alloy, and the gas cannot be discharged to form pores.
10: The capacity of the sprue cup is too small, and the molten metal forms a vortex and intrudes into the air to form pores.
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