Common defects in the production of ductile iron by lost foam casting process
Part 2
In terms of molten metal
The molten metal has poor smelting quality and high gas content. If the molten metal is in contact with the air for too long during the smelting process, it will absorb a large amount of gas and form pores in the casting after pouring.
If the pouring temperature of the molten metal is too high, the gas content of the molten metal will increase, and the amount of gas generated by the decomposition of the pattern will also increase, increasing the possibility of pore formation.
In terms of operation
During the pouring process, if the pouring speed is not well controlled, the molten metal will splash, and a large amount of gas will be involved to form pores.
Improper negative pressure control, too low negative pressure, cannot effectively extract the gas generated by the decomposition of the pattern and the gas in the molding sand; too high negative pressure will cause the molten metal to fill the mold too quickly and easily involve gas.
3. Shrinkage and shrinkage
Reason: Ductile iron has liquid and solid shrinkage during solidification. If the casting structure is not designed properly, the riser is not set properly, and the pouring temperature is too high, it will lead to insufficient supply of liquid metal, thus forming shrinkage and shrinkage.
Solution: Reasonably design the casting structure so that the casting can achieve sequential solidification, set a sufficient number and size of risers, control the pouring temperature, and avoid too high.
4. Slag inclusion defect
The main reasons for the gas inclusion defect of ductile iron produced by the lost foam casting process are as follows:
Mold and coating
Mold quality problem: The surface of the mold is not smooth, has impurities or defects, and the impurities will fall off into the molten metal during pouring. At the same time, the residues produced by incomplete gasification of the mold may also form slag inclusions.
Coating problem: The coating has low strength and is easily washed off by the molten metal during pouring, forming slag inclusions; the coating has poor air permeability, which will prevent the gas generated by the gasification of the mold from being discharged smoothly, forming gas inclusions in the molten metal.
Sand aspect
Sand particle size and compactness: Uneven sand particle size or too high compactness will lead to poor air permeability, poor gas discharge, and easy formation of gas inclusions; too much mud in the sand will also increase the possibility of slag inclusions.
Casting system aspect
Irrational design: Improper design of the position, number and shape of the inner runner will cause uneven filling of the molten metal, resulting in excessive or too slow local flow rate. Too fast will wash the surface of the mold to produce slag inclusions, while too slow will make it difficult to discharge gas and inclusions, forming gas inclusions.
Molten metal aspect
Smelting problem: During the molten metal smelting process, if the charge is not clean, contains oil or impurities, the inclusions in the molten metal will increase; too high or too low smelting temperature will affect the fluidity of the molten metal and the floating of inclusions, increasing the risk of gas inclusions.
Pouring temperature: If the pouring temperature is too high, the thermal shock of the molten metal on the mold will increase, causing the mold surface to fall off and form slag inclusions. At the same time, the gasification speed of the mold will be too fast, and the gas will not be discharged in time; if the pouring temperature is too low, the molten metal will have poor fluidity, and inclusions and gases will be difficult to discharge, and gas inclusions will also be easily formed.
Operation
Pouring operation: The pouring speed is unstable, and the molten metal will splash or turbulent, which will involve gas and inclusions; if the flow is interrupted during the pouring process, an oxide film will form on the surface of the molten metal that enters the cavity first, and the subsequent molten metal will involve it to form slag inclusions.
Negative pressure control: If the negative pressure is not appropriate, the negative pressure is too low, and the gas and inclusions cannot be effectively discharged; if the negative pressure is too high, the molten metal will fill the mold too quickly, involving gas and inclusions. Stopping the negative pressure too early after pouring will also cause the gas and inclusions to be unable to be fully discharged.
5. Cold shut
Reason: Insufficient filling capacity of molten metal, too low pouring temperature, too slow filling speed, complex casting structure, resulting in the molten metal not being able to merge smoothly during the filling process.
Solution: Increase pouring temperature and filling speed, optimize casting structure, and improve the filling conditions of molten metal.
6. Sand sticking
Reason: Insufficient refractoriness of molding sand, poor coating performance, too high pouring temperature, chemical reaction between molten metal and molding sand.
Solution: Select molding sand with high refractoriness, improve coating performance, control pouring temperature, and avoid excessive reaction between molten metal and molding sand.
