Reasons For Graphite Floating Phenomenon in Ductile Iron Process

(1) Carbon equivalent: The carbon equivalent is too high, so that a large amount of graphite is precipitated in the molten iron at high temperature. Since the density of graphite is smaller than that of molten iron, the graphite floats to the upper part of the casting under the drive of magnesium vapor. The higher the carbon equivalent, the more serious the graphite flotation phenomenon. It should be pointed out that too high carbon equivalent is the main reason for graphite flotation, but it is not the only reason. Casting size and wall thickness are also important factors affecting graphite flotation.

(2) Silicon: Under the condition that the carbon equivalent remains unchanged, appropriately reducing the silicon content will help reduce the tendency of graphite to float.

(3) Rare earth: When the rare earth content is too small, the solubility of carbon in the molten iron will decrease, and the molten iron will precipitate a large amount of graphite, which will increase the floating of graphite.

(4) Spheroidizing temperature and incubation temperature: In order to improve the absorption rate of magnesium and rare earth elements, domestic experimental studies have shown that the most appropriate molten iron temperature during spheroidizing treatment is 1380-1450 °C. In this temperature range, the absorption rate of magnesium and rare earths increases with increasing temperature.

(5) Pouring temperature: In general, the higher the pouring temperature, the greater the tendency of graphite to float. This is because the casting is in a liquid state for a long time, which is conducive to the precipitation of graphite. A.P.Druschitz and W.W.Chaput found that if the solidification time is shortened, the tendency of graphite to float decreases with the increase of pouring temperature.

(6) Retention time: The residence time between the inoculation treatment and the pouring is too long, which provides conditions for the precipitation of graphite. Generally, this time should be controlled within 10 minutes.