Generally, high carbon, low silicon, low manganese, low sulfur and phosphorus (i.e. high, all low) are used to strengthen inoculation.
(1) carbon and silicon
In nodular cast iron, the selection of carbon and silicon content is mainly to ensure spheroidization, improve casting performance and eliminate casting defects.
Carbon equivalent Ce: the carbon equivalent of nodular cast iron is generally taken from the eutectic or hypereutectic composition. Improving the carbon equivalent can ensure the need of spheroidization and improve the casting performance (making the liquid iron have good fluidity, little tendency to form shrinkage cavity and porosity). Increase the ferrite content in as cast ductile iron. If the carbon equivalent is too high, graphite will float easily, which will reduce the properties of castings.
C%:
Generally, high carbon and low silicon can be adopted to strengthen inoculation according to the above principles. With high carbon content, the number of graphite precipitated increases, the ball diameter is small and the sphericity is good; with large graphitization expansion, the shrinkage cavity and porosity can be reduced or eliminated under the premise of high mold rigidity, and the dense casting can be obtained. If the carbon content is too high, graphite will float easily.
Si%:
The increase of Si% can increase the content of as cast ferrite and decrease the content of pearlite. On the premise of meeting the requirements of graphitization, the amount of final silicon should be reduced as much as possible. The silicon content of thick and large parts should be lower to prevent graphite from floating; the silicon content of thin and small parts can be higher to prevent large amount of white structure cementite; manganese is an obstacle to graphitization element, which can stabilize and refine P-body. However, manganese is easy to produce segregation in ductile iron, which reduces the plasticity and toughness of castings. Therefore, various grades of ductile iron hope to have low manganese content, especially as cast ductile iron and heavy section ductile iron. Phosphorus segregation easily occurs in nodular cast iron, and phosphorus eutectic structure is formed on the grain boundary, which seriously reduces the plasticity and toughness of nodular cast iron. Phosphorus can improve the hardness and wear resistance, but it is also easy to increase the shrinkage, porosity and cracking tendency of castings.
Sulfur is also a harmful element in ductile iron. High sulfur will inevitably consume more spheroidizing agent, which will seriously affect the spheroidizing effect, and at the same time, it is easy to cause casting defects such as spheroidizing recession, poor spheroidizing, shrinkage cavity and porosity, and subcutaneous porosity.
For castings with high sulfur content or special requirements, necessary desulfurization measures shall be taken before spheroidizing treatment, and the sulfur content in the original molten iron shall be reduced to less than 0.02% after desulfurization.
Magnesium and rare earth elements are both spheroidizing elements, but also anti graphitization elements with strong desulfurization and deoxidization. Therefore, the amount of magnesium and rare earth in molten iron should not be too low or too high.
If the residual is too low, it is easy to cause poor spheroidization and spheroidization recession.
Although high spheroidization can be guaranteed, a large number of cementites are easy to be produced in the matrix structure, and with many casting defects (slag inclusion, subcutaneous porosity, shrinkage porosity and white mouth, etc.), the shape of graphite balls is deteriorated and the performance is reduced.
Because the chemical composition of the molten iron should meet the requirements of high carbon and low silicon, manganese, sulfur and phosphorus, the requirements for raw materials are higher, especially the quality of the used raw pig iron (sulfur and phosphorus content) has a greater impact on the quality of nodular cast iron.
Scrap is generally used as the charge for adjusting the composition of molten iron. Carbon steel is selected and the content of impurity elements is strictly controlled.
Germany stipulates that the total amount of trace elements in pig iron for nodular iron is Σ t ≤ 0.075%, and Ti ≤ 0.038%.
China stipulates that the total amount of trace elements in pig iron for nodular iron is Σ t ≤ 0.1%, and Ti ≤ 0.045%.
ZXB/T0001-2011.
High purity pig iron for casting.
