Reasons and Measures of Graphite Floating in Ductile Iron

(1) Macroscopic features: Graphite floating often appears on the upper surface of the casting, the edge of the test block and the bottom of the riser, and a layer of black spots with clear boundaries and smoothness appear on the upper surface of the fracture.

(2) Microscopic features: the graphite aggregates, in the shape of flowers and twigs, and some completely burst, as shown in Figure 2. This kind of structure seriously weakens the mechanical properties of ductile iron, so that the strength and toughness index of the material decreases significantly.

    (3) Reason analysis: The main reason for graphite floating is that the carbon-silicon equivalent is too high (carbon equivalent> 4.55%). When the carbon-silicon equivalent exceeds the eutectic composition, due to the high casting temperature, the molten iron will precipitate graphite before solidification. If the liquid stays for a period of time, the graphite will grow and aggregate at this time. Since the density of graphite is much lighter than that of molten iron, the aggregated graphite is easy to float. Sometimes inclusions are also brought to the upper surface of the casting, resulting in graphite floating on the upper surface of the casting; under the same conditions (the same carbon equivalent), the larger the casting, the higher the casting temperature and the slower the cooling rate, the molten iron in the casting mold. If the high temperature liquid is kept in the interior for a long time, the graphite has enough floating time, so the graphite floats more seriously.

(4) Measures to be taken to prevent graphite from floating: ① Strictly control the carbon and silicon equivalent, carbon should not exceed 4%, silicon should not exceed 3%, and the amount of carbon and silicon in thick and large castings should be lower; ② Increase the cooling rate, in the wall Place cold iron in the thick part; ③ Add a small amount of elements that strongly prevent graphitization, such as adding molybdenum to prevent graphite from floating; ④ Strictly control the residual amount of rare earth elements.