Features: the fracture surface of the casting is gray black, and the mechanical properties are obviously low. Through metallographic examination, it can be found that the graphite is flaky, and the residual magnesium content and rare earth content of the casting are too low, which is generally non spheroidization. The fracture surface of the casting is still silver gray, but there are scattered gray black spots, and the mechanical properties are low. Metallographic examination shows that a small part of graphite is flake or vermicular, and the residual magnesium and rare earth content of the casting are lower than the normal content, which is generally poor spheroidization.
Precautions:
(1) According to the sulfur content of the original liquid iron and the magnesium and rare earth content of the nodulizer, the addition amount of the nodulizer is determined. For example, when the w (mg) is 7% – 9% and the w (RE) is 2% – 5%, and the treatment temperature is not more than 1500 ℃, the addition amount in Table 1 can be used for reference. The specific addition amount shall be adjusted according to the situation of each plant.
(2) The spheroidizing treatment should ensure that there is enough reaction time between the spheroidizing agent and the molten iron. Generally, the reaction time is 80-100 seconds. The treated ductile iron should be poured as soon as possible.
(3) Try to reduce the sulfur content of the original molten iron. If high carbon and low sulfur coke is used, desulfurization treatment can be adopted if conditions permit. When tapping the original molten iron, it is necessary to avoid discharging to the slag (the sulfur in the slag is 3-4 times of that in the molten iron).
(4) Strictly control the content of anti spheroidizing elements (such as arsenic, lead, titanium, bismuth, aluminum, etc.) in pig iron.
(5) In order to prevent the oxidation of molten iron, the temperature should be moderate when processing ductile iron. According to the temperature of molten iron, the chemical composition of the nodulizer should be selected.
(6) If necessary, a small amount of antimony can be added to neutralize the effect of rare earth on the distortion of spheroidal ink.