Induction furnace melting operation, molten iron characteristics and troubleshooting (power frequency coreless)

1) Smelting operation of power frequency coreless induction furnace. According to the state at the beginning of smelting, the melting operation of the power frequency coreless induction furnace is divided into "cold smelting" method in which the frit is placed and then the metal charge is added, and "cold smelting" method in which the frit is not removed but a part of the molten iron is retained in the furnace. There are two methods of "residual molten iron smelting"

①Cold material melting

a. Production and selection of frit. When there is no residual molten iron in the power frequency coreless induction electric furnace, the frit must be used. The frit is made by using the residual molten iron during the pouring process to pour it into a metal mold. It is easy to take off the shape and there is no need to worry about damage to the lower inclined part when placed in the furnace. From the perspective of electrification efficiency, it is better to have a larger diameter of frit. However, when it is too large, it will damage the furnace wall. When the frit is heated, it will expand and pressurize the furnace wall. Since the penetration depth of the induction current of the T-type induction furnace is large, there is no need to be so large that there is no certain gap between the furnace diameter and the furnace diameter. But for large furnaces of several tons or more, 2 to 3 small-sized frit starters can be used.

b. Loading of frit and metal charge. When lifting the frit with a crane, be careful not to damage the furnace wall. Pre-laying a small amount of steel plate (for example, about 50kg for a 5t furnace) or iron filings at the bottom of the furnace can prevent damage to the furnace bottom. In addition, the starting frit can be preheated by loading the starting frit the day before each continuous melting and allowing the power supply to automatically set the temperature to 900°C. This shortens the time for the first melt the next day. To add metal charge, it is usually put into a bucket and loaded using a crane or an electromagnetic magnet. After the furnace material is filled, it can be energized for smelting. However, when using furnace materials that are easy to build tents, you should wait until the frit melts before loading.

c. Charge ratio, composition adjustment, and temperature rise. Commonly used charge materials include recycled materials, scrap steel, pig iron and iron filings. The general charge ratio for gray cast iron is: 30% recycled iron, 50% scrap steel, and 10% each of pig iron and iron filings. The charge ratio of black-core malleable iron is 50% to 55% of returned iron and 45% to 20% of scrap steel. Except for special circumstances, pig iron is generally not used (or only 5% to 10% pig iron is used). When batching, calculate the contents of C, Si, Mn, etc. in advance based on the charge ratio and material composition; the insufficient parts are adjusted with carburizing agents and ferroalloys. For the sake of simplicity, the element burning loss rate is calculated as 0 for the main metal charges such as returned iron. The carburizing agent is 10% to 20%. The final composition adjustment uses analytical instruments such as thermal analyzers and direct-reading spectrometers to quickly analyze the C, Si, Mn and other contents, and then add carburizing agents, ferroalloys, scrap steel, etc. for adjustment. The heating rate is usually 7~8℃/min, and the maximum power should be used as much as possible. The maximum melting temperature and tapping temperature are determined according to the process requirements of the casting, and are generally around 1500°C.

② Residual molten iron smelting. For power frequency induction electric furnaces, the residual molten iron smelting method should be selected, which can not only improve production efficiency, but also avoid abnormal properties of the molten iron. The main reasons are:

a. Add additional metal charge to the remaining molten iron to lower the temperature of the molten iron in the furnace.

b. The metal charge can increase the generation of graphite core during the melting process

c. When solid metal charge is melted in molten iron, it can inhibit oxidation before melting.

For medium frequency induction furnaces, it is not meaningless to use residual molten iron for smelting. If there is residual molten iron in the furnace, even in the initial stage of power supply, the load fluctuation is small and high power can be input from the beginning, so at least the melting time of the metal charge can be shortened. On the premise of accurately grasping the quality, composition and temperature of the residual molten iron, the operation methods such as charge ratio, composition adjustment, and temperature rise when using residual molten iron to smelt are the same as those for cold material smelting. The amount of residual molten iron is about 1/2 of the furnace capacity. However, in power frequency cored induction furnaces or duplex melting, the amount of molten iron remaining in the furnace is sometimes as little as 1/10, and sometimes as much as 9/10, depending on the production conditions. The cycle time from adding the metal charge to tapping varies depending on the amount of residual molten iron, but 1 to 1.5 hours is the best. Therefore, the smelting cycle of 8 hours of smelting per day is 5 to 8 times. The insulation of molten iron at night or between shifts of two-shift operations requires a full furnace and as little as 1/4~1/2 of the molten iron. The optimal insulation temperature at this time is 1300~1400°C.

2) Precautions for induction furnace melting operations

①The addition of pig iron. At present, for the smelting of gray cast iron, not only the medium frequency induction electric furnace, but also the pig iron ratio of the power frequency induction electric furnace has increased. For power frequency induction furnaces, pig iron is usually added in the later stages of smelting. This is considered from the fact that pig iron contains primary crystal graphite and has the function of generating graphite core. Production practice also shows that adding a small amount of pig iron and pouring it immediately can reduce the tendency of whitening. But on the contrary, if the pig iron ratio is too large, the strength will decrease. From the perspective that melting part of the metal charge earlier can improve the efficiency of electricity supply, it cannot be said that it is an appropriate method to add all the pig iron with a low melting point later. Therefore, when to add pig iron should be considered comprehensively based on the actual production conditions. In addition, large-scale furnaces are mostly power frequency induction furnaces, and hoppers are commonly used to add metal charges. If pig iron with rust and moisture is added to a large amount of heated molten iron, it will explode when immersed in the depth of the molten iron, causing the molten iron to splash. From a safety perspective, adding pig iron in the later stages of melting is not a good idea. As for the medium frequency induction furnace which has the advantage of fast melting, it is unanimously believed that it is better to add pig iron in the early stage of melting. This is because the shape of pig iron is relatively consistent and it is easy to obtain high induced current, which is beneficial to increasing the smelting speed.

②Selection and addition of carburizing agent. The nitrogen content in cast iron is generally below 100X.lO-6. When the nitrogen content is (150~200)×10-6 or higher, cracks, shrinkage, and fissure-like subcutaneous pores are likely to occur. This is more likely to occur for thick-walled parts. The reason is that as the scrap steel ratio increases, the amount of carburizing agent added increases. Coke-based carburizers, especially pitch coke, have high nitrogen content. For example: the content of electrode graphite (N) is less than 0.1-% or very trace, but the content of pitch coke (N) is 0.6%. If the carburizer containing o.6% nitrogen is added in an amount of 2%, 120×10-6 nitrogen will be added. For this reason, as a carburizing agent, it is better to choose electrode graphite with low nitrogen content. As for the method of adding carburizer, for medium frequency induction furnaces, it is best to add it gradually when the metal charge is added. If it is added too early, it will easily stick to the bottom of the furnace; if it is added too late, it will not only cause a delay in ingredient adjustment, but may also cause errors in excessive heating.

③ Addition of ferrosilicon. Tests have shown that the higher the Si content in the molten iron, the slower the carburization rate will be when carburizer is added. Therefore, for medium frequency induction furnaces with weak stirring force, it is better to add ferrosilicon later. However, adding ferrosilicon too late will cause a delay in the analysis and adjustment of the molten iron composition in the furnace, just like the carburizing agent. For power frequency induction electric furnaces with strong stirring, there is no need to worry about the slow carburization rate caused by high Si. Ferrosilicon can be added at an early stage. However, if it is added too early and the metal charge has not yet melted, it will also have the opposite effect of promoting oxidation.

④Add metal charge. When induction furnace is used for melting, the phenomenon of shedding of furnace materials rarely occurs. However, when using loose charge in power frequency electric furnaces, multi-branch charge such as lateral runner, charge with sticky sand and rust that easily produces slag, or when all iron filings are used for smelting, scaffolding is likely to occur, so attention should be paid to it. . A better way to prevent scaffolding is to control the amount of material added so that part of the molten metal level can be seen frequently. Thin steel plates and iron filings should be added to the clean molten iron to prevent them from being pulled toward the furnace wall by electromagnetic force and causing slag to be involved, making it difficult to melt. For medium frequency induction furnaces, although there is no need to remove frit, if the furnace is filled with charge from the beginning of power on, the load variation will increase and frequency conversion will be hindered. In addition, the parallel variable frequency medium frequency induction electric furnace will produce irregular frequency changes when it is under high load. For this reason, it is better to add the metal charge gradually.

⑤ Precautions when heating up. Unlike medium frequency induction electric furnaces, power frequency induction electric furnaces heat up slowly. In order to avoid changes in the properties of the molten iron during this period, the furnace lid should be closed as much as possible to prevent heat dissipation on the surface of the molten iron and achieve rapid temperature rise. A radiation thermometer can be installed on the furnace cover, and the furnace cover needs to be opened to facilitate temperature measurement.