What are the roles of uhp graphite in the production of scrap steel castings?
Source: uhp graphite-Industry news<hr /> 1. Application of SiC in charge graphite electrode is a silicon-based nucleating agent with a melting point of 2 700 ℃ and does not melt in molten iron. According to SiC+Fe=FeSi +C (non-equilibrium graphite) melts in molten iron, where Si in SiC is combined with Fe, and the remaining C is non-equilibrium graphite, which serves as the core of graphite precipitation.
In addition, graphite electrode is also a good deoxidizer. Gray cast iron, ductile iron and malleable cast iron all use the reaction of SiC+FeO=Si+Fe+CO to reduce the content of FeO and MnO in the slag, thereby purifying the molten iron and reducing The furnace wall is oxidized to extend the life of the furnace wall, and uhp graphite can achieve the purpose of increasing silicon and carbon.
The graphite electrode is a material with the reputation of “rich core”. As an additive to cast iron alloys, it reduces the tendency of castings to white mouth, enhances the graphite nucleation ability and increases the graphite nucleation center, and obtains a moderate number of eutectic clusters. Meet the required graphite shape and distribution. After the synthetic cast iron melting process is applied, when the proportion of scrap in the charge is too high and the number of cores in the molten iron is too small, the nucleation effect of uhp graphite can play a key role. The metallographic structure of the casting (cylinder body and cylinder) was compared by two melting processes without graphite electrode and 1% graphite electrode. It can be seen that adding a certain amount of uhp graphite during smelting has a significant effect on improving metallurgical quality and improving graphite morphology. (A) Without graphite electrode 100× (b) Add 1% graphite electrode 100× 2. The selection of alloying elements and the amount of added metal charge of gray cast iron is generally composed of pig iron, scrap steel, reheating charge and alloying elements. As we all know, alloying elements It can promote pearlite, strengthen the matrix structure, and improve the mechanical properties and serviceability of castings. It is generally believed that in addition to carbon and iron in cast iron, the intentionally added metal elements are all used as alloying elements. The alloy elements added to the engine block and cylinder head produced by the author are Si, Cu, Mn, Cr, Sn. The addition of alloy elements beyond a certain range will have a negative impact on the quality of molten iron and the strength of the casting, and will increase with the addition of alloy elements. The increase in production cost will increase accordingly. Therefore, based on the use of synthetic cast iron technology and the application of uhp graphite and silicon-barium-zirconium inoculant, appropriately reducing the proportion of alloys is an important way to save materials and reduce production costs. It is also the key to reducing the tendency of molten iron to shrink and improving the processing performance of castings. factor.
The silicon in the silicon cast iron is composed of the original molten iron silicon and the inoculated silicon. The data points out that when producing gray cast iron, the mass fraction of silicon in the molten iron brought by the inoculant is not more than 0.3%. Under certain conditions, each inoculant has its optimal addition amount. Excessive use of inoculants will not bring about greater inoculation effect, but will waste the inoculants, reduce the temperature of molten iron, increase the shrinkage of the castings, pores and slag inclusions and other defects. Insufficient inoculation, there are too few effective nucleation cores in the molten iron, the tendency of the molten iron to be too cold and the too cold graphite to increase. Silicon is an element that strongly promotes graphitization and is the main component in cast iron. Silicon strongly weakens the Fe-C bond, significantly expands the TEG～TEM range, and significantly increases TEG, which affects the precipitation of graphite. Therefore, silicon is more than carbon. Strong graphitization ability. The literature points out that when w(Si)<1.2%, it is difficult to obtain complete ash solidification even if the carbon content is increased to w(C)>3.5%.
Therefore, the raw iron liquid must have a certain silicon content before inoculation, which can come from pig iron, ferrosilicon added with the charge during the smelting process, or graphite electrode. The silicon content in the original molten iron depends on the size and structure of the casting. It is generally believed that the relatively fast cooling rate of the gasoline engine cylinder block and cylinder head castings requires relatively high silicon content in the original molten iron, and the diesel engine block with a relatively slow cooling rate The silicon in the original molten iron of the cylinder head is relatively low. Through experimental comparison, under the same inoculation amount (0.3% silicon barium zirconium inoculation and 0.05% silicon strontium inoculation with flow), the same graphite electrode dosage and the same alloy content, w(C)=3.2%～3.35% , The silicon in the original molten iron increases from 1.4% to 2.0%, the carbide and ferrite are reduced, the length of the graphite becomes longer, and the leakage tendency increases. The use of uhp graphite increases the graphite nucleation ability, reduces the tendency of white mouth, and improves the tensile strength of the casting. Datong graphite electrode (reported on December 8, 2018)