Features: a layer of clear and dense black spots can be seen on the upper surface of the fracture surface of the casting. Through metallographic examination, it can be found that the graphite balls at the top of the fracture surface gather, and sometimes there are continuous or individual flowering graphite at the lower part of the aggregation layer. Graphite floating seriously weakens the mechanical properties of nodular iron, and reduces the strength, hardness, elongation and impact toughness.
Precautions:
(1) Strict control of carbon equivalent is the fundamental way to solve the problem of graphite floating. In general, carbon equivalent is controlled at 4.3% – 4.7%. Thin and small parts are higher than the upper limit, thick and large parts are lower than the lower limit.
(2) Speed up the cooling speed of the casting and place the cold iron in the thick part. Sometimes some anti graphitization elements (such as molybdenum) can be added.
(3) The content of rare earth should not be too high.
Table 1 the relationship between the amount of sulfur in the original molten iron and the amount of adding nodulizer (w (mg) is 7% – 9%, w (RE) is 2% – 5%)