The cast iron manhole frame & cover is made of nodular cast iron. Nodular cast iron is its main raw material. The quality of this manhole cover is directly related to the spheroidization rate. Generally, the coating used to treat castings is antirust asphalt paint, so our nodular cast iron manhole frame and manhole cover should be sprayed with antirust asphalt paint after molding, so as to have better antirust performance.
Pig iron is an iron carbon alloy with carbon content greater than 2%. The carbon content of industrial pig iron is generally 2.5% - 4%, and contains C, Si, Mn, s, P and other elements. It is a product smelted by iron ore through blast furnace. According to the different forms of carbon in pig iron, it can be divided into steelmaking pig iron, casting pig iron and nodular cast iron. The precipitated graphite is a spherical cast iron. The splitting effect of spheroidal graphite on metal matrix is smaller than that of flake graphite, so that the strength of cast iron reaches 70 ~ 90% of the strength of matrix structure, the tensile strength reaches 120kgf / mm2, and has good toughness. Except for iron, the chemical composition of nodular cast iron is usually as follows: carbon content of 3.6 ~ 3.8%, silicon content of 2.0 ~ 3.0%, total amount of manganese, phosphorus and sulfur of no more than 1.5%, and an appropriate amount of rare earth, magnesium and other spheroidizing agents.
Fabrication of cast iron manhole frame and manhole cover:
1. The chemical composition is strictly required. The carbon and silicon content of raw molten iron is higher than that of gray cast iron, and the content of manganese, phosphorus and sulfur in nodular cast iron is reduced.
2. The discharging temperature of molten iron is higher than that of gray cast iron to compensate for the loss of molten iron temperature during spheroidization and inoculation treatment.
3. Carry out spheroidizing treatment, that is, add spheroidizing agent to molten iron.
4. Add inoculant for inoculation treatment.
5. Nodular cast iron has poor fluidity and large shrinkage, so it needs higher pouring temperature and larger pouring system size. Riser and chill shall be reasonably applied and the principle of sequential solidification shall be adopted.
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