The bad spheroidization means that the spheroidization does not achieve the expected effect. The metallographic structure of poor spheroidization is: thick flake graphite and a small amount of globular and nodular graphite; sometimes, there is water grass graphite. With the increase of the degree of bad spheroidization, the number and area of the concentrated thick flake graphite increase gradually. The bad spheroidization will make the mechanical properties of nodular cast iron fail to meet the requirements of the brand. The causes and preventive measures of poor spheroidization are as follows:
1.High sulfur content is the main anti spheroidizing element in the original molten iron, which will seriously affect the spheroidization. Generally, the sulfur content of the original molten iron is less than or equal to 0.06%.
In order to ensure spheroidization, when the sulfur content of the original liquid iron is high, the amount of spheroidizing agent must be increased correspondingly. The higher the sulfur content, the more consumption of spheroidizing agent.In order to make the graphite spheroidized well, the nodular iron must contain a certain amount of residual magnesium and rare earth. Under the current production conditions in China, the mass fraction of residual magnesium shall not be less than 0.03%, and the mass fraction of residual rare earth shall not be less than 0.02%.
2.The rust and pollution in the raw material of iron melt oxidation and the oxidation of iron melt in the process of melting and overheating lead to the increase of FeO content in the iron melt, so more magnesium will be consumed in the process of spheroidization, resulting in the low residual magnesium content.
3.The furnace charge contains anti spheroidizing elements. When the anti spheroidizing elements are beyond the allowable range, the spheroidizing effect will be affected. It should be noted that titanium may be contained in scrap steel, and that electroplating materials, aluminum pins and lead series coatings enter the cupola.
4.Rare earth has the ability of neutralizing and anti spheroidizing elements. According to the situation that there are more anti spheroidizing elements in primary iron in China, the amount of residual rare earth in nodular iron in China is more than that in foreign countries.
5.Due to the poor inoculation effect, or incubation decline, the number of graphite balls will be small, making the graphite balls not round.
6.Due to the interface reaction, the magnesium in the molten iron and the oxygen and sulfur on the surface of the mold react, resulting in the deficiency of the residual magnesium on the surface of the casting, forming a thin layer of flake graphite. The solution is to increase the residual magnesium content and reduce the moisture content of molding sand. The sulfur content of molding sand should be less than 0.1%, or the coating which can obtain reducing atmosphere should be used. The coating containing MgO and Cao can be used in resin sand mold with sulfur hardener.