In recent years, more and more factories use the induction electric furnace for cast iron, but usually only the metal material can not keep the carbon content in iron, so that's why it need the recarburizer. For the induction electric furnace, special the intermediate frequency furnace, adding the carbon material is important link for smelting.
1. Shoul choose the carbon additive(calcined petroleum coke) with low nitrogen
The usual nitrogen content in cast iron liquid is below 100 ppm. If the nitrogen content exceeds this concentration (150-200 ppm or higher), it is easy to cause cracks, shrinkage or loose defects in the casting, and thick-walled castings are more likely to occur. This is caused by an increase in the amount of recarburizer added when the proportion of scrap steel increases. Coke-based recarburants, especially bitumen coke, contain large amounts of nitrogen. The graphite graphite has a nitrogen mass fraction of 0.1% or less or a very small amount, and the pitch pyrolysis mass fraction is about 0.6%. If 2% of the recarburizer with a mass fraction of 0.6% nitrogen is added, this increases the nitrogen content by 120 ppm. A large amount of nitrogen is not only liable to cause casting defects, but also nitrogen can promote pearlite compaction, ferrite hardening, and strongly increase strength.
2. Ways to adding the recarburizer
The stirring of molten iron can promote carbon addition. Therefore, the medium frequency induction electric furnace with weak stirring force is much more difficult to increase carbon than the power frequency induction electric furnace with strong stirring power. Therefore, the medium frequency induction electric furnace has the carbonization speed and the melting rate of the metal charge The possibility.
Even in a power-frequency induction furnace with high stirring power, the carbon-increasing operation cannot be ignored. This is because, from the schematic diagram of the induction furnace melting, there is a stirring iron flow separated vertically in the induction furnace, and there is a dead angle near the furnace wall at the boundary. The graphite cluster which stays on the furnace wall and adheres cannot be melted into the molten iron without excessive temperature rise and long-term iron liquid heat preservation. Excessive heating of the molten iron and long-term heat preservation will increase the degree of supercooling of the molten iron and increase the tendency of the cast iron to become white. In addition, for medium frequency induction furnaces that generate a strong induced current near the furnace wall, if molten iron is drilled between the graphite clusters attached to the furnace wall, the drilled metal is melted during the next furnace smelting, resulting in erosion. And damage the furnace wall. Therefore, in the case where the proportion of scrap steel is high and the amount of recarburizing agent is added, it is necessary to pay more attention to the addition of the recarburizing agent.
The addition time of the recarburizer cannot be ignored. If the addition time of the recarburizing agent is too early, it is easy to adhere to the vicinity of the bottom of the furnace, and the recarburizing agent adhering to the furnace wall is not easily melted into the molten iron. On the contrary, if the joining time is too late, the timing of carbon increase will be lost, causing the smelting and heating time to be slow. This not only delays the time of chemical composition analysis and adjustment, but also may cause harm due to excessive temperature rise. Therefore, the recarburizing agent is preferably added little by little during the process of adding the metal charge.