In the cast iron production to improve the metallurgical quality of iron, the production of stable quality of high grade gray cast iron and cast ductile iron parts, to ensure that the ideal metallurgical organization and mechanical properties, to prevent metallurgical deterioration, shrinkage of loose, under the skin porosity and other casting defects, is the casting enterprise efforts to pursue the goal. Silicon carbide through a large number of production practice is an effective way to improve the metallurgical quality of cast iron is one of the important materials.
At present, the application of silicon carbide in the production of cast iron is mainly used in two aspects:
- one is for the carbon and silicon increase agent for synthetic cast iron;
- the second is used as a pretreatment agent for the liquid iron spheroidization and gestation before the liquid iron pretreatment.
Silicon carbide has abrasive grade (SiC≧98%) and metallurgical grade (SiC≦94%).
Abrasive grade silicon carbide added to the furnace is converted into cast iron carbon and silicon, one is to improve the carbon equivalent; the second is to strengthen the ferrofluid reductivity, greatly reducing the adverse effects of rusting furnace charge. Adding silicon carbide can prevent carbide precipitation, increase the amount of ferrite, so that the cast iron organization is dense, significantly improve machining performance and make the cutting surface smooth.
Metallurgical grade silicon carbide in the production of ductile iron is often used in the pretreatment of ferro-liquid, which can form a long-lasting graphite core, increase the number of graphite spheres per unit area of ductile iron, and improve the spheroidization rate. For reducing non-metallic inclusions and slag, eliminating shrinkage, eliminating subcutaneous porosity also has a good effect.
Silicon carbide (≥ 98%) in the synthesis of cast iron applications
SiC added to gray iron can promote the generation of A-type graphite, improve the metallurgical quality, improve the purity of the liquid iron , and SiC responds well to the gestation treatment. The use of silicon carbide can simultaneously achieve the purpose of increasing carbon and silicon, but the main increase in silicon, and scrap steel (ordinary carbon steel) has a low content of carbon and silicon (C <0.4% or so), the organization of the fine, high maturity, so the use of SiC and the use of carbon enhancers with the use of the synthetic cast iron can be obtained to obtain the organization and performance of the more superior.
The mechanism of action of SiC
The mechanism of action of SiC may be related to the melting process of SiC. Unlike ferrosilicon, silicon carbide has a high melting point, in iron water is not melted but slowly dissolved, in the dissolution process not only to the surrounding silicon atoms, but also provide carbon atoms. At the same time, silicon carbide in the process of iron dissolution, the formation of many fine particles around it, these particles and then dissolved, dissolved to form many carbon atom groups, these carbon atom groups as the later graphite nucleation basis. Silicon carbide on the ductile iron raw iron treatment has a significant reduction of white mouth tendency, eliminate anti-white mouth, improve the organization of the role.
For all cast irons (gray cast iron, ductile iron, perforated cast iron), the presence of nuclei in the molten iron helps the molten iron to solidify in accordance with the iron-graphite stabilization system. There are two nucleation conditions for the solidification process of cast iron: austenitic nucleation (for which there is no current application); and graphite nucleation. Current theory suggests that in order to promote the formation of gray cast iron, a suitable and active crystalline core is required, which is a complex compound of oxygen and sulfur containing active elements such as Ca, Ba, Sr, and RE. It is further theorized that a properly sized, undissolved graphite mass in the molten iron can serve to promote the precipitation of pre-eutectic and eutectic graphite cores. In other words, in order to form a spherical graphite core, casting iron silica and silicon carbide, etc. (containing Ca, Ba, Sr, RE, and other reactive elements) are required, and the presence of graphitic dots in the ferrofluid contributes to the increase in the number of spherical graphite cores. Both experiments and production practice have shown that in the presence of heterogeneous cores, silicon carbide can promote nucleation by increasing the number of nucleation points in the ferrofluid. Therefore, in synthetic cast iron, in order to improve the nucleation ability of the ferrofluid, the nucleation ability of the ferrofluid is improved in the presence of silicon carbide, which further affects the solidification process and thus improves the cast iron microstructure.