What issues should be paid attention to when using a medium frequency furnace to melt gray cast iron?（一）
- Selection of raw materials and charge ratio
The quality of charge directly affects the quality of molten iron . The cleanliness and dryness of charge are high in medium frequency furnace smelting gray cast iron. If the charge is unclean, contains harmful elements or the smelting is not well controlled, it will lead to oxidation and low purity of molten iron, seriously deteriorating the quality of molten iron, affecting the matrix structure and graphite morphology of cast iron, causing problems such as poor inoculation, white cast iron, large shrinkage tendency and many pores. Therefore, the management of raw and auxiliary materials should be strengthened, and the use of charge with severe rust and oil pollution is strictly prohibited.
In order to improve the purity of molten iron and stabilize its chemical composition, carbon steel scrap should be used and account for more than 50% of the charge ratio; the return charge should be the pouring and riser of the same grade casting, and the attached sand and coating should be removed before use, and the usage should be about 40%; the scrap iron should also be the same grade casting machining scrap; the casting pig iron should be from a stable source, clean and less rusty, and low in harmful elements. The castings produced by this kind of pig iron have good and stable internal quality. The source of pig iron should not be easily changed during the production process. Pig iron should be added at the beginning of smelting, and the ratio can account for 15% to improve the graphite morphology of cast iron; the carburizer should be a commercial graphite carburizer or a carburizer treated with high temperature graphitization. It should be added as early as possible during the smelting process so that the carburizer is in direct contact with the molten iron and has sufficient time to melt and absorb.
- Chemical composition
C and Si are elements that strongly promote graphitization. If the amount of w(C) and w(Si) is too high, it will lead to graphite coarsening, increase the volume fraction of ferrite, and reduce the volume fraction of pearlite, thereby reducing the strength and hardness of cast iron. In high-strength gray cast iron, the amount of w(C) and w(Si) should be appropriately reduced within a certain range. While ensuring the gray cast iron, it is beneficial to refine the graphite, promote the formation of pearlite, and improve the mechanical properties. CE and Si/C ratio can significantly affect the structure and mechanical properties of gray cast iron [8]. Selecting an appropriate CE and Si/C ratio is beneficial to improving the structure and performance of cast iron. CE is the most important factor affecting the intrinsic quality of gray cast iron castings. Improving CE can greatly improve the casting performance of cast iron, reduce white cast iron, shrinkage cavities, shrinkage and leakage defects, and reduce the scrap rate. This is especially important for thin-walled cast iron parts. However, if CE is too high, the amount of graphite precipitation increases, and the ferritization tendency is obvious, which will reduce the tensile strength and hardness of the casting. The thick wall of the casting is prone to coarse grains and loose structure defects due to slow cooling; if CE is too low, local hard areas are easily formed in the thin wall of the casting, resulting in poor processing performance. This is because eutectic ledeburite and D-type and E-type undercooled graphite are prone to appear in the structure of low-CE gray cast iron, resulting in reduced casting performance, increased sensitivity of casting section, increased internal stress, and increased hardness. Properly increasing the Si/C ratio can increase the strength of cast iron and improve the cutting performance of cast iron. Under the same conditions, different Si/C ratios can produce large differences in the structure and mechanical properties of cast iron. When CE is constant, the Si/C ratio increases from 0.6 to 0.8, and the strength and hardness of gray cast iron reach peak values; when the Si/C ratio is constant, the strength and hardness of gray cast iron decrease with the increase of CE. While strictly controlling CE at the production site, an appropriate Si/C ratio should be selected. The CE of gray cast iron melted in medium frequency furnace should be about 0.3% higher than that in cupola, and the w (C) content should be about 0.1% higher than that in cupola, and the Si/C ratio should be controlled at 0.6-0.7, so that the cast iron can maintain appropriate hardness and high tensile strength.
Mn and S are elements that stabilize pearlite and hinder graphitization. Mn can promote and refine pearlite. The increase of w (Mn) content can improve the strength and hardness of cast iron and the volume fraction of pearlite in the organization. Mn can promote the formation and stabilization of carbides and inhibit the generation of FeS. In addition, Mn also forms high-melting-point compounds with S as heterogeneous nuclei to refine grains. Therefore, the amount of Mn used in high-grade gray cast iron should be increased. However, if the w (Mn) content is too high, it will affect the nucleation of molten iron during crystallization, reduce the number of eutectic groups, lead to coarse graphite, and produce supercooled graphite, which will reduce the strength of cast iron. S is a limiting element in gray cast iron. An appropriate amount of S plays a positive and beneficial role in the nucleation and growth of graphite, and can improve the inoculation effect and machining performance of gray cast iron. In order to ensure the inoculation effect, the medium frequency furnace smelting gray cast iron generally requires w (S) ≥ 0.06%. An appropriate increase in w (S) can improve the graphite morphology, refine the eutectic group, shorten the length of flake graphite, bend the shape, and blunt the end, weakening the cutting and damaging effect of graphite on the matrix, thereby improving the performance of cast iron. Therefore, the lower the S content in gray cast iron, the better. P is generally a harmful element in gray cast iron, which is easy to form low-melting-point phosphorus eutectics at the grain boundary, causing cast iron cold cracking. Therefore, in gray cast iron, the lower the w (P) content, the better. For cast iron parts with density requirements, the w (P) content should be less than 0.06%.
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