The specificity of induction furnace and countermeasures

　　 Due to the improvement of power supply conditions and the improvement of casting quality requirements in the machinery manufacturing equipment industry, induction castings have been increasingly used in casting production in recent years. In particular, medium frequency induction furnaces have become new foundry and smelters. The equipment selected in the technical transformation is preferred. At present, China's medium frequency induction furnace power supply has been developed to a relatively high level, the protection of the control system is more perfect and reasonable; the application of the computer on the induction furnace makes the metallurgical quality monitoring of the furnace condition monitoring, fault diagnosis and even the melting process internationally modern. level. Induction furnaces are increasingly being widely used. The rapid development of modern machinery manufacturing and metallurgical industry will definitely play a vital role in steadily improving the quality of castings, reducing waste and reducing the energy consumption in the foundry, and improving the working environment of the workshop. In order to make good use and utilization of advanced smelting equipment, it is necessary for our engineering and technical personnel engaged in the foundry industry to study and discuss the characteristics of induction furnaces, and to find suitable technical countermeasures according to specific application conditions.

　　 1. The specificity of induction furnace melting smelting cast iron

Induction furnace is relatively stable in the process of melting cast iron, convenient to adjust the composition, easy to control the temperature, less burning in the smelting process, especially in the production of ductile iron compared with cupola, which is beneficial to obtain low-sulfur iron, etc., and can reduce environmental pollution and improve Labor conditions, casting quality is relatively stable, and can also use cheap raw materials. But everything is always two-fold. In the practice of using induction furnace to smelt cast iron, there are also some casting defects. This is the most common and most prone specificity of induction furnaces.

(1) The white mouth tends to increase, and the degree of subcooling increases. The induction furnace electric iron has a smaller number of crystal nuclei than the cupola iron. The conclusion is: with the increase of holding time and the increase of melting temperature (overheating), the basic structure of cast iron, the number of eutectic groups decreases, and the depth of white mouth increases.

(2) D and E type graphite structures appear. When the molten iron is superheated to a higher temperature and kept for a long time, the amount of type A graphite is easily reduced in obtaining the hypoeutectic gray cast iron, and the obtained cast iron structure, D, The proportion of E-type graphite increases, and the degree of supercooling increases. This supercooled graphite affects the machinability of machining. When the amount of ferrite folded in the basic structure of cast iron increases, when the D and E type graphite appears, the strength and hardness of the cast iron will be affected as the holding time is prolonged.

　　 (3) Shrinkage and shrinkage of cast iron parts. As the molten iron is kept for a long time, cast iron casting defects may occur, in addition to the above-mentioned increase in white mouth tendency and dissimilarity of the matrix structure, it also causes shrinkage and shrinkage defects during solidification, thin-walled castings (single-casting pistons) Rings are prone to excessive corner hardness or white mouth defects; castings with thick wall thickness differences are prone to casting defects such as looseness, shrinkage or shrinkage. The reason for this, in addition to the long-term insulation effect of the molten iron, is of course related to the addition of rusted severely oxidized metal charge.

　 2. Eliminate the technical measures specific in induction furnace casting

　　 (1) Do not excessively increase the temperature of the furnace, do not store or insulate the molten iron at high temperature, adhere to the "smelting and quick-release", avoid high-temperature insulation and long-term "overheating". From the metallurgical point of view, it is necessary to "overheat" as much as possible instead of Multiple overheating can prevent excessive consumption of nucleation products and avoid excessively severe deoxygenation.

As far as the production site management of the foundry is concerned, the phenomenon of smelting molten iron in the induction furnace should be avoided, and the molding speed should be matched with the melting rate of the electric furnace as much as possible. The holding time of the molten iron should be shortened as much as possible. The overheating of the liquid leads to the adverse effects of poor nucleation ability.

　　 (2) Strengthen the inoculation treatment measures. Inoculation of iron liquid before iron can prevent and inhibit the tendency of white mouth. The effect of gestation is best, that is, the method of gestation in the ladle flow nozzle during casting (requires uniform sprinkling on the molten iron through the pouring cup into the mold, the inoculant particles can not be large), so that Promoting the change of graphite toward the A-type direction can effectively suppress or eliminate the formation of supercooled graphite (D-type and E-type). The choice of long-acting inoculants (such as the silicon germanium inoculant FeSi70B5) can significantly prevent the formation of E-type graphite.

　　 (3) Select and use a low-nitrogen recarburizer. The use of a carbonaceous agent made of bitumen coke having a higher nitrogen content causes needle-like pores and shrinkage defects in thick-walled castings. Therefore, a recarburizing agent made of a waste electrode graphite having a low nitrogen content should be used as much as possible, and such casting defects can be effectively reduced. The production of ductile iron castings must also use low-sulfur graphite-type recarburizers, because high sulfur content not only hinders graphitization, but also hinders the absorption of recarburizers, thereby affecting the number of graphite balls and reducing the grade of ductile iron castings.

　　 (4) Before the iron water is discharged, the pre-heated and rust-free new iron block is added to the furnace, which is an effective measure to eliminate the excessively cold graphite and the white corner of the thin-walled casting (such as the single-cast piston ring). In the later stage of smelting, a certain proportion of new iron (commonly known as bread iron) is added, because the primary graphite contained in the new iron can act as a nucleus. Therefore, before sampling before tapping (about 1420 ° C), adding a small amount (ie 5% to 10% of the total amount of new iron in the batch) of new iron can effectively prevent the formation of supercooled graphite and sharp white spots in thin-wall castings. The basis is "undissolved graphite dot theory", which effectively avoids white-wall defects in thin-walled castings and effectively improves the cutting performance of thin-walled castings. Standing in the safety production requirements: the new iron pieces must be baked, preheating is essential to prevent splashing, so as not to burn the smelting operators.

　　 (5) In principle, the amount of CE in the electric furnace smelting molten iron should be slightly higher than that of the cupola. The use of induction furnaces must also control the chemical composition of the molten iron. Although chemical composition is not an important basis for the acceptance of castings, the stability of chemical composition is the basis for the production of cast iron. Some units and smelters do not pay much attention to the control of the chemical composition of molten iron. In addition, some smelting operators have a partial understanding that sulfur is a harmful element. The lower the sulfur content, the higher the quality of the castings produced. I don't know, when using the induction furnace to smelt cast iron, in order to ensure good results, the w _S = 0.045% ~ 0.065% in the molten iron should not exceed w _S = 0.08%. Practice has proved that at this time, the metallographic structure of the casting is almost all A-type graphite, and the length is 2 to 3, and it can reach 4 to 5 after being inoculated. The pearlite content is ≥90%. As the composition is relatively stable, the matrix structure and graphite morphology are improved, and the mechanical properties are correspondingly improved, and casting defects such as shrinkage, shrinkage, and microcracking are reduced.

Control the Si content more accurately. Taking the production of the QT450-12 ductile iron Air Compressor crankcase and gearbox housing as an example, due to the large difference in wall thickness, difficult machining and micro-crack defects often appear in thick walls. After microscopic examination, the metallographic structure is needle-like cementite. When the spheroidal graphite cast iron is smelted by an induction furnace, the temperature is generally above 1520 ° C. The purpose is to eliminate various impurities and compounds and improve the spheroidization rate. Since the solidification of the spheroidal graphite cast iron is porridge-like solidification, the surface of the above two products is solidified first under the negative pressure state at the wall thickness of the cast iron, and the surface is solidified first, and Si segregation is likely to occur. We sampled the defect and chemically analyzed it. The content of the outer surface and the internal Si were very different, and it was judged that the processing was difficult due to severe Si segregation. Since the influence of nucleation ability is determined by the amount of Si, the Si content of the original molten iron should be controlled. Usually =1.2%~1.4%, gestation twice, and after pouring in the pouring iron mouth after the inoculation treatment, the final wSi should be controlled within 2.65%. The higher the Si content, the easier it is to form segregation. As long as Si segregation occurs, the graphite sphere is not rounded and deformed, so controlling the Si content is particularly important. Furthermore, the design of the gating system is re-examined to make the temperature field uniform as much as possible, to reduce the temperature difference of the molten iron in the cavity, and to suppress the formation of Si segregation. After taking the above countermeasures, the smelted ductile cast iron has improved elongation, improved toughness, and significantly reduced casting defects such as hard edges and cracks in the casting window.

　　 (6) Strengthen the management of charge materials and strengthen the concept of “fine materials and fine products”. Pay attention to the cleaning of the charge. The raw and auxiliary materials in the furnace should be clean and free of rust. The material must be cleaned by the drum. The scrap is cut into a moderately sized block (2/3 of the inner diameter of the electric furnace lining), and it is cleaned without rust. The fine selection is free of impurities (such as aluminum, resulting in subcutaneous pores); iron filings should be cleaned, remove impurities, and then pressed into a cake and then added.

　　 3. Carbonation technology for induction furnace melting of cast iron

　　 The recarburizing agent is used in casting, which can greatly increase the amount of scrap steel, reduce the amount of pig iron or use no pig iron. At present, most of the recarburizers are suitable for electric furnace melting. Usually, we increase or decrease the carbon by adding a recarburizer or adding scrap.

　　 (1) Factors affecting carbon increase One of the concerns of iron liquid carbonation is the rate of carbon increase; the second is the rate of carbon absorption during carbon addition. The main factors affecting carbon increase are as follows: 1 The composition of molten iron. High-manganese iron solution helps to absorb carbon, and high-silicon and high-phosphorus iron solution hinders carbon absorption. The lower the carbon saturation in the molten iron, the faster the carbon absorption rate; in the high silicon germanium liquid, the burning loss of carbon is as high as 18%; in the high manganese liquid, the burning loss of carbon is almost zero, which is Proof of the above argument: manganese helps to absorb carbon, while silicon blocks carbon absorption. 2 The temperature of the molten iron at the time of carbon addition. 3 The type and quality of the recarburizer. 4 The degree of iron liquid agitation. The agitation of the molten iron can promote carbonation. Therefore, the medium frequency induction electric furnace with weak stirring force is relatively more difficult to absorb carbon in the medium frequency electric furnace than the power frequency induction electric furnace with strong stirring force. Excessively prolonging the agitation time will increase the carbon burnout and reduce the carbon absorption rate.

　　 (2) Induction furnace carbon-increasing method In the electric furnace smelting, the method of adding carbonizer should be carried out with the charge of scrap steel and the like, and the addition of small dose can be added to the surface of the molten iron. However, it is necessary to avoid feeding large quantities into the molten iron to prevent excessive oxidation and the carbonization effect is not obvious and the carbon content of the casting is insufficient. The amount of the recarburizer added is determined according to the ratio of other raw materials and the carbon content.

Different types of cast iron, choose different types of recarburizers according to your needs. The specific method of carbon increase is as follows:

　　 First, when adding the charge through the batching calculation, the recarburizer is added in batches as needed. When the charge is not completely melted, the carbon-reinforcing agent is wrapped and pressed into the unmelted molten iron, so as to avoid the merging; when the desired temperature is reached, the carbon-increasing process ends.

　　 Second, for the medium frequency induction furnace, the addition of the recarburizer is preferably added gradually when the metal charge is added. Premature addition, it is easy to adhere to the bottom of the furnace and the lower part of the furnace wall; too late to join, not only lead to delay in composition adjustment, it is likely to cause excessive temperature rise errors, the use of recarburizer in medium frequency electric furnace smelting, according to the ratio or carbon The equivalent requirement is added to the lower part of the electric furnace according to the material, and the absorption rate of the recarburizer can reach more than 95%.

　　 Third, after the molten iron is melted, if the carbon content is insufficient to adjust the carbon content, the molten iron is heated until the charge is completely melted, and then the surface of the molten iron and the slag in the furnace are cleaned, and the broken glass slag is covered to cover the sputum mirror. After the opening, the recarburizing agent is added, and the molten iron is pressed into the molten iron to prevent it from being exposed on the surface of the molten iron, and the carbon is dissolved and absorbed by electromagnetic stirring or manual stirring, so that it is taken up into the molten iron, and the carbonation operation is completed in about 15 minutes. The carbon absorption rate can be around 90%.

　　 Fourth, when it is necessary to simultaneously increase carbon and increase silicon, it is necessary to follow the principle of “increasing silicon after adding carbon”. After the carbonization is completed, the ferrosilicon can be added to increase the silicon, otherwise the speed of the iron absorption of carbon will be affected.

　　 (3) The method of carbon reduction or decarburization is to reduce the carbon in the preheated scrap steel when the molten iron is not fully melted; the second is to use the iron liquid stirring motion to iron oxide (the blacksmith workshop) The hammer slag is entangled in the molten iron to perform decarburization and oxidative removal of other elements. After practice, the interference is too big and affects each other. The data show that 10% of the hammer slag is added at 1420 °C, and in addition to the sulfur element, the other four elements of the conventional five elements of cast iron are reduced.

　　 (4) Ingredients principle of acid furnace lining of induction furnace When carbon is melted under different temperature conditions, the carbon and silicon with the most content in the molten iron will react with the lining to change the thickness of the lining and the composition of the molten iron. Especially at high temperatures, the SiO2 in the quartz sand lining is reduced by the carbon in the molten iron. As a result, the lining is eroded and thinned, and the composition of the molten iron will reduce carbon and silicon. Therefore, in the casting production, the pre-furnace batching calculation must be based on the principle of “low silicon, high carbon”, so that the casting composition produced will not deviate. This is also a characteristic of melting cast iron using an induction furnace.

　　 4. Conclusion

　　 The use of advanced smelting equipment in the foundry industry is a major advancement. Being able to recognize the performance of the equipment, find the crux of its specificity, take targeted technical measures to weaken and eliminate the adverse effects, and it is also a great progress to fully grasp and utilize such equipment. In order to further improve the economic benefits of induction furnace melting, it is necessary to make full use of its strengths, to change the production methods, improve the technical measures to reduce the defects of the cast iron parts of the induction furnace melting, and there is still a lot of work to be done, still has a long way to go.