Causes and preventive measures of slag inclusion casting defects in nodular cast iron piston rings

The composition of slag inclusions is mainly composed of magnesium silicate, oxide, sulfide and sulfur oxide, which is a very complex non-metallic inclusion.

1. Causes of slag inclusion

a. After the molten iron is treated with magnesium and rare earth, a series of reactions will take place between the sulfur, oxygen, magnesium and rare earth in the molten iron to form sulfide, oxide, magnesium silicate and other non-metallic inclusions. The mass density of these inclusions is smaller than that of molten iron. After spheroidizing treatment, the slagging is not complete or the temperature of molten iron is low. The inclusions remain in the molten iron or the inclusions float up slowly. If it is too late to be removed, slag inclusion will be formed after pouring.

b. After spheroidizing, because magnesium is easy to oxidize, many oxygen atoms are adsorbed on the surface of molten iron, and iron atoms can be activated to oxidize the molten iron and form an oxide film on its surface. The oxide film is in solid-liquid state, so it is difficult to remove it when scraping slag. When the molten iron is left in the ladle, poured, poured and flowed in the cavity, it is oxidized by contacting with the surrounding air and the moisture in the molding sand. When the molten iron rolls, the oxide film is torn into pieces and floats upward, and gathered on the surface of the casting. In addition, during the process of floatation, sulfide particles suspended in molten iron can be adsorbed along the way, which can be brought together to the surface layer of the casting to form slag inclusion.

2. Technological measures to prevent slag inclusion

a. Reduce the mass fraction of residual magnesium and sulfur in molten iron. Because both magnesium and sulfur increase the conjunctival temperature of the oxide film. Therefore, at the same pouring temperature, the higher the mass fraction of residual magnesium and sulfur in molten iron, the more serious the slag inclusion.

b. Ensure enough mass fraction of residual rare earth. Because rare earth can reduce the conjunctival temperature of oxide film. However, excessive residual rare earth content will lead to the deterioration of graphite shape.

c. Increase pouring temperature. As the temperature of hot metal decreases, the oxide film will gradually thicken, resulting in serious slag inclusion.

d. During spheroidizing treatment, the molten iron is added. The formation of secondary slag can be reduced by adding 0.3% cryolite to the surface of molten iron.