Application Of Silicon Carbide in Nowaday's Cast Iron Works

Silicon carbide is a compound consisting of one carbon atom and one silicon atom, with silicon accounting for 70% and carbon occupying 0% (by weight). Since it dissolves in the presence of slag, carbon atoms and silicon atoms will be released as charged ions (C-4 and Si +4), so it is an effective deoxidizer widely used in electric furnace steelmaking. When it is added to grey, ductile or malleable iron, it not only dissolves easily, but also alloys carbon and silicon into the melt. When the temperature is lower than 1620℃, 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 ℃ (such as in steelmaking), silicon will undertake all the deoxidation tasks, while carbon will act as a recarburizer, and its yield can reach 100%. Coreless induction melting of iron is the main field of application for silicon carbide. About 95% of coreless induction furnaces in the United States use SiC as the primary silicon source. In terms of gray iron, ductile iron and malleable iron, SiC is used to reduce the content of FeO and MnO in the slag through the reaction of SiC+FeO=Si+Fe+CO.

Since the presence of FeO can lower the melting point of any slag, more of the slag becomes liquid at any given temperature because of the lowering of the melting point of the slag. For example, when the FeO content in the slag is 10%, its melting point will be 1350-1400 ℃, and under the strong stirring action of the coreless induction furnace, this liquid slag will be "homogenized" in the melt, As a result, thousands of very small slag particles are left in the melt, and many surface defects of the casting are the very fluid FeO and MnO slag (usually called manganese silicate slag) that is brought into the mold caused. If SiC is added to reduce the FeO content of this slag to 1 or 2%, its melting point will be raised to 1500-1550 ° C, then, at the usual tapping temperature (1500-1550 ° C), this slag maybe remains solid, or only a small amount becomes liquid, leaving a larger monomer in the furnace, which makes the slag particles easier to dislodge due to their higher uplift velocity and reduces the chance by carry them into the casting to cause defects .

The presence of very small FeO-SiO2 inclusions (fayalite) in the molten iron is the main reason for the decrease in the fluidity of molten iron, the increase in shrinkage tendency and the increase in white casting, especially for ductile iron. Therefore, reducing its amount in the molten iron can eliminate the tendency to increase shrinkage and white casting.

Since silicon carbide is dissolved in molten iron rather than melted, it takes longer to enter the molten iron than FeSi. Due to the longer action time, the decay time increases. Thus, in the case of ductile iron, many foundries have found that it is economical to add at least 3-4kg/t SiC to the charge.The metallurgical effects they saw were a decrease in the occurrence of white casting, a decrease in defects caused by slag flow, an increase in the number of graphite nodules, a decrease in the tendency to shrink, and an increase in the decay time. This is partly the result of replacing the aluminum-containing ferrosilicon and the higher content N and S recarburizers with SiC with a low residue content.