Silicon carbide is a compound consisting of one carbon atom and one silicon atom, with silicon accounting for 70% and carbon occupied by 30% by weight. It was discovered by Edward G Acheson when he made synthetic diamonds. Because it is very hard and can cut glass, metal and other materials, its original use is as an abrasive. Since it does not oxidize at almost any temperature, it can be used as a refractory material.
Because it is also very stable at high temperatures, it has been widely used as a filler for kiln. Because it can resist the erosion of slag, it can be used as a slag brick for aluminum smelting furnaces and blast furnaces. Since it dissolves in the presence of slag, carbon atoms and silicon atoms are released as charged ions (C-4 and Si +4), and thus are an effective deoxidizer widely used in electric furnace steelmaking. . When it is added to gray iron, ductile iron or malleable cast iron, it is not only very soluble, but also allows carbon and silicon to enter the melt as an alloy.
When the temperature is lower than 1620 ° C, its carbon will act as a deoxidizer, so that less stable oxides such as FeO and MnO are reduced by the reaction of SiC + FeO = Si + Fe + CO. When the temperature is higher than 1620 ° C (such as steelmaking), silicon will take on all the deoxidation tasks, while carbon acts as a recarburizer and its yield can reach 100%. Coreless induction melting of iron is the main application area of silicon carbide. In the United States, about 95% of coreless induction furnaces use SiC as the primary source of silicon. In gray iron, ductile iron and malleable cast iron, the reaction of SiC+FeO=Si+Fe+CO is used to reduce the content of FeO and MnO in the slag by SiC.
Since the presence of FeO can lower the melting point of any slag, at any given temperature, more slag becomes liquid because of the decrease in the melting point of the slag. For example, when the FeO content in the slag is 10%, its melting point will be 1350-1400 ° C, and in addition to the strong agitation of the coreless induction furnace, the liquid slag will be "homogenized" in the melt. Thus, thousands of very small slag particles are left in the melt, and many surface defects of the casting are such high flowability of high FeO and MnO slag (commonly referred to as manganese silicate slag) being brought into the mold. Caused.
If SiC is added to reduce the FeO content of the slag to 1 or 2%, the melting point will increase to 1500-1550 ° C. Then, at the usual tapping temperature (1500-1550 ° C), the slag or Still remain solid, or only a small amount becomes liquid, thus retaining a larger monomer in the furnace, which makes the slag particles easily removed due to the higher floating speed and is taken The chance of casting into the mold and causing casting defects is greatly reduced.
The presence of very small FeO-SiO2 inclusions (fayalite) in molten iron is the main reason for the decrease in fluidity of molten iron, an increase in the tendency to shrink, and an increase in white mouth, which is especially true for ductile iron. Therefore, reducing the amount in the molten iron can eliminate the tendency to increase shrinkage and white mouth.
Since silicon carbide dissolves in molten iron rather than melts, it takes longer to enter the molten iron than FeSi. Due to the effect
The time is longer, so the recession time increases. Therefore, in the case of ductile iron, although the furnace often does not leave room for silicon or only leaves little room for it, many foundries have found that it is economically advantageous to incorporate at least 3-4 kg/T of SiC into the charge. of.
The metallurgical effect they saw was that the white mouth was reduced, the defects caused by slag were reduced, the number of graphite balls increased, the tendency to shrink was reduced, and the decay time increased. This is partly due to the replacement of the ferrosilicon containing aluminum and the recarburizer having a higher N and S content with SiC having a low residual content.
Another use of SiC in ductile iron is the "pre-growth" effect of pure magnesium treatment. One of the disadvantages of performing pure magnesium treatment is that it increases the tendency to produce shrinkage and carbide. Foreign studies have shown that adding 2kg/TSiC to the treatment package is the most effective way to eliminate this metallurgical problem.
Since the content of FeO in the ductile iron slag is higher than that in the gray iron or the vermicular iron slag, the problem caused by the fluid slag to the ductile iron is more serious than that caused by the gray iron or the vermiculite, therefore, It is better to add SiC to the ductile iron.
In terms of gray iron and vermicular iron, the metallurgical effect observed by foreign foundries is basically the same as that observed in ductile iron: the amount and fluidity of slag are reduced, the number of eutectic groups is increased, and the tendency of white mouth is reduced. The recession has grown. In addition, foreign gray iron and vermicular iron foundries often have enough room for a certain amount of SiC in the charge. The most common addition amounts are: grey iron 10-15kg/T and vermiculite 5-10kg/T. China is a large country of silicon carbide production, with an annual output of more than 200,000 tons, of which metallurgical grade silicon carbide production accounts for about 1/3. Currently, it is mainly used as a deoxidizer for electric furnace steelmaking in China. less.
Because many iron foundries in China, especially ductile iron mills, have problems such as defects caused by slag, and they all face the problem of how to meet higher and higher quality standards and increasingly fierce cost competition. The combination of deoxidizers and recarburizers and resource-rich silicon carbide will become a very important tool for many iron foundries in China to reduce slag defects, improve casting quality and reduce costs.
In addition, with the improvement and promotion of the technology of producing ductile iron with high-magnesium alloy cored wire, the work of partially or completely replacing ferrosilicon with carbonizing, silicon-increasing and pre-incubation functions will also be Many iron foundries mention the agenda because it not only improves the quality of ductile iron, but also further reduces production costs.
In short, the application of silicon carbide in today's iron foundry in China is both imperative and the trend of the times.
Features:
1. Increase the core of graphite nucleation and enhance the ability of graphite nucleation. Reduce the degree of subcooling of molten iron and reduce the tendency of castings to whiten.
2, good deoxidation effect, purification of molten iron.
3. Extend the service life of the lining.
4. Reduce super-cooled graphite in gray cast iron to promote the formation of type A graphite.
5. Adding in the production of ductile iron can increase the number of graphite balls, increase the spheroidization rate, make the graphite balls more round and fine, and reduce the amount of spheroidizing agent added.
6. Instead of ferrosilicon smelting, it can reduce the aluminum content and reduce the subcutaneous pores.