Medium frequency power supply manufacturer talks about the quality control of cast iron smelting in large and medium-tonnage variable frequency electric furnaces

A company produces various castings for domestic and foreign customers, with an annual output of more than 20,000 tons, mainly iron castings, which are molded using an IMF furan resin sand molding line and melted by two 6t variable frequency electric furnaces from the American PIL-LAR company. There are many types of castings, small batches, and high requirements. The requirements for various castings on molten iron vary greatly, which brings certain difficulties to smelting and quality control of molten iron. The following are some of the author’s experiences and opinions.

(1) Smelting production arrangement. Arrange the smelting grade, tonnage and smelting time according to the daily molding completion status, and found the following problems.

① Sometimes it is difficult to coordinate the demand for the grade of molten iron and the smelting schedule, resulting in either the mold on the production line not being poured completely, affecting the operation of the molding line; or there is no mold for pouring the molten iron in the furnace, causing the molten iron to be kept warm in the furnace for a long time, which affects the quality It is not good for production and operation.

② The electric furnace has a large capacity. Pouring multiple grades of castings in the same furnace will increase energy consumption, reduce productivity, and is not conducive to quality assurance.

③ Castings of the same brand with different sizes and thicknesses require pouring of molten iron with different compositions, and it is impossible to meet the requirements by organizing melting only by brand.

④The same brand of molten iron can have different formulas with different costs, and each formula is often only suitable for certain castings. Therefore, it is recommended to make various pre-arrangement plans based on the needs of casting production, integrating melting and molding capabilities, minimizing the number of component adjustments in the melting process, and making the melting production as stable and normal as possible to improve production efficiency and quality.

(2) Quality control of the smelting process

①Molten iron is kept warm. Keeping molten iron warm at high temperatures will cause temperature changes, element burning loss, dissolved gases and other changes in the state of the molten iron. Practice has proved that the following countermeasures can be taken. For long-term insulation, the insulation temperature should be reduced as much as possible, such as adding a small amount of clean and dry recycled materials to cool down, and using glass and other materials to make slag to cover the surface of the molten iron. After the heat preservation is completed, use carburizer and various alloys to adjust the composition. After these additives are completely melted, iron cannot be tapped within 20 minutes to ensure that the molten iron can be fully absorbed. When adding a large amount of ductile iron carburizer, the molten iron should be heated to above 1550°C and kept warm for at least 30 minutes to fully absorb the molten iron. Otherwise, there will be a layer of graphite floating on the surface of the macro section of the thick part of the casting. The metallographic examination shows that the graphite layer is mainly strips of different thicknesses and lengths, with some lumps of graphite sandwiched between them. If gray cast iron is tapped after the carburizing agent and alloy steel are completely melted, the incubation effect will be poor, and the thick parts of the casting will have loose tissue.

②Selection of molten iron components. When several grades of castings are to be poured into the same molten iron furnace, the ingredients should be proportioned according to the middle composition, the batch size is larger, or the more important grade, and slightly higher. Use different inoculants (such as carbon) when tapping the iron. Silicon inoculants, barium-silicon alloys, rare earth calcium-barium alloys, etc., and alloys (electrolytic copper or other alloys) can be adjusted in grade to achieve better results. When smelting ductile iron, it is advisable to use high-carbon grade ingredients because during the heat preservation and tapping processes Medium carbon will burn out. After the high-carbon grade iron is tapped, scrap steel can be added to reduce carbon. This has less impact on the quality of the molten iron and is faster. If the opposite method is used, that is, using the carburization method to adjust the composition, it will It causes large fluctuations in the quality of the molten iron and takes a long time to adjust. It is easy to cause poor absorption of the carburizer and poor inoculation effect. During the iron tapping process, the amount of carbon in the furnace will continue to decrease. It is better to use an inoculant for adjustment. Only after add carbon only when the carbon content has dropped significantly.

③The variety of molten iron changes. When there is still ductile iron molten iron left in the furnace and it is necessary to melt gray cast iron molten iron, if there is a lot of remaining molten iron, it is best to raise the temperature to 1550°C and keep it warm for a period of time, otherwise the casting structure will be messy and broken. Even after incubation, it is difficult to obtain type A graphite from graphite strips and blocks, which affects the performance of castings. If the remaining liquid iron in the furnace is gray cast iron and it is necessary to smelt ferrite ductile iron, you should consider whether the manganese content and anti-spheroidizing elements are too high, and you should pay attention when selecting pig iron.

④The problem of dissolved gas in molten iron insulation. If the molten iron is kept at high temperature for a long time (for example, more than 1 hour), the oxidation and dissolved gas problems of the molten iron should be considered. Especially in cylinder and box castings with more resin sand cores, nitrogen pores must be prevented from being generated. According to experience, when the (Ti) content in molten iron is as high as 0.04%, the formation of nitrogen pores can be effectively prevented.

⑤Carbonization problem. The use of coke powder to increase carbon is cheap, but if the amount is large, it will cause problems: first, there is a lot of smoke and dust, which pollutes the environment; second, the amount of sulfur increase is large; third, the burnt loss is large. The electrode fragments contain high carbon and low sulfur, and can be used in industrial applications. Frequency furnaces are used as carburizers, but for variable frequency electric furnaces above 500kW, due to the short melting time and fast heating rate, the electrode fragments are difficult to melt and absorb in a short time, "especially when carburizing is required during the tapping process. After comparative tests on several materials, we chose to use carburizers. The raw materials and manufacturing processes of carburizers are different, and the use processes are also different. The tests must be carried out carefully, and we have paid a heavy price for this. Molten iron After adding the carburizing agent, there must be sufficient holding time. The length of time is related to the type of carburizing agent, production process and molten iron temperature. Otherwise, it will cause abnormal graphite shape and material abnormality in the casting. Generally speaking, if you add people For large quantities, it is recommended that the melting temperature should reach or exceed 1520°C and the holding time should be above 15rnin. When adding carburizer during the smelting process, it is advisable to add the carburizer in batches when the charge has just begun to melt and the molten iron is visible, not all at once. Add too much. 90% to 95% of the total amount can be added during the melting period, and the remaining small amount can be added when adjusting the ingredients.

⑥Oxidation problem of molten iron. Dirty furnace charge or poor smelting control will lead to severe oxidation of molten iron, affecting the matrix structure and graphite morphology of the casting, causing problems such as poor incubation and spheroidization, white spots and shrinkage, large shrinkage tendencies, and many pores and slag holes. . For this reason, clean and dry furnace materials must be selected. Carburizing and siliconizing treatments have a certain deoxidation effect, so when the liquid iron is severely oxidized, you can consider using silicon carbide, calcium silicon alloy, aluminum ferrosilicon, and rare earth magnesium iron. Consider using carbide, calcium silicon alloy.

(3) Reasonably optimize production according to quality and cost. Production practice shows that it is difficult to take into account production, quality and cost issues when considering production arrangements purely from the perspective of modeling or smelting. Therefore, only by carrying out a dynamic and overall balance can we achieve better economic benefits. But the following conditions are required.

① Smelting technology has strong adaptability.

②The styling ability is highly adjustable and can quickly switch styles.

③Coordinate and stimulate the enthusiasm, creativity and sense of responsibility of production personnel.

④The production line should be guaranteed to be put into production at a lower cost.