Characteristics of melting molten iron in power frequency induction heating furnace

① Characteristics of melting molten iron in power frequency induction heating furnace. Mass production practice shows that compared with molten iron smelted in a cupola furnace, the gray cast iron melted by a power frequency induction heating furnace is less likely to have molten iron patterns, has a greater tendency to whiten and shrink, and has slightly higher strength and hardness. The properties of the above-mentioned power frequency induction heating furnace for melting molten iron are also shared by the medium frequency induction heating furnace. However, because the medium frequency induction heating furnace heats up and melts faster than the power frequency induction heating furnace and has lower stirring power of molten iron, the tendency of white spots and shrinkage holes is smaller than that of the power frequency induction heating furnace.

②The main reasons for the formation of the properties of molten iron in power frequency induction heating furnaces are as follows.

a. The cupola furnace uses coke as the main source of sulfur, while the power frequency induction heating furnace does not have the opportunity to increase sulfur during smelting. The sulfur content of the molten iron is the total sulfur content in the metal charge, so the sulfur content is low. When the scrap ratio is high, the molten iron is only 0.02% to 0.03%. The sulfur content of molten iron has a great influence on the graphitization of cast iron.

b. The time from melting the metal charge in the cupola furnace to coming out of the furnace is very short, only about 10 minutes. During melting in a power frequency induction heating furnace, the temperature rise during this period must be at least 1 hour. The high temperature of the molten iron is maintained for a long time. In addition, the strong stirring effect of the molten iron unique to induction heating reduces the tendency to form graphite cores. Therefore, the molten iron is supercooled; and as the holding time of the molten iron is extended, the degree of supercooling increases. The smaller the eutectic degree of high-grade cast iron, the more significant the impact of holding time on subcooling. Moreover, regardless of whether it is pregnant or not, the degree of subcooling increases as the heating temperature of the molten iron increases.

c. Effect of nitrogen content. Another difference between the power frequency induction heating furnace and the cupola furnace in cast iron smelting is the large change in the metal charge ratio, that is, the large amount of scrap steel added. For this reason, carburizing agents must be added to meet the requirements of the molten iron composition. When the nitrogen content in the carburizer is high, it will cause the problem of high nitrogen content in the molten iron. The concentration of nitrogen in molten iron in equilibrium with the nitrogen in the air is approximately 100X10 (-6) (calculated value). If it is above this equilibrium concentration [actually (150~200)×10(-6)), cracks, shrinkage or fissure-like subcutaneous pores will occur in the casting. Even the amount of nitrogen below the equilibrium concentration affects the strength and hardness because nitrogen promotes the refinement of pearlite and has a hardening effect on ferrite. Generally, for gray cast iron, thin-walled parts should be controlled below 0.013%, while thick-walled parts should not exceed 0.008%. However, if the nitrogen content is too low, as the wall thickness increases, the strength will obviously decrease, and the hardenability will not be good. Therefore, it is also necessary to regularly analyze the nitrogen content in cast iron.

③Corresponding measures taken to improve the properties of molten iron

a. Appropriately adjust the chemical composition. As the holding time of the molten iron increases, the carbon content in the molten iron fluctuates, so appropriate addition of carbon is required. It is usually 0.05% to 0.1% higher than that in cupola melting, and the Cr content should be strictly controlled to reduce the tendency to whiten.

b. Selection and addition of charge. In order to improve the graphite morphology of gray cast iron, 10% pig iron is usually added and added to the furnace at the later stage of smelting. When producing ductile iron with ferrite matrix, low manganese pig iron should be selected. In order to prevent defects caused by excess nitrogen, try not to use pitch coke as a carburizing agent, and it is better to use electrode graphite. Because the nitrogen content of electrode graphite is low.

c. Melting temperature and holding time. The melting temperature should not be too high, and the heat preservation of the molten iron in the furnace should be avoided as much as possible. If the molten iron must be kept warm, low temperature insulation should be used as much as possible. These are to avoid excessive deoxidation reaction, resulting in a reduction in the graphite core produced.

d. Breeding treatment. The length of the molten iron holding time brings changes in the depth of the white hole. In this regard, adding graphite-based inoculant is an effective method. In summary, in order to normalize the characteristics of the molten iron in the power frequency induction heating furnace, the amount of carbon can be adjusted, the amount of nitrogen can be controlled, the molten iron can be prevented from overheating, the holding time can be shortened, and incubation can be used.