Floating defect of graphite in ductile iron casting

Graphite floating means that there are a large number of graphite balls on the upper surface of the casting, and at this time, the graphite shape, both the original compact ball into the flowering shape. As a result, the surface quality and mechanical properties of castings are deteriorated.

The floating of graphite is related to the carbon equivalent of the molten iron, the geometry of the casting and the cooling rate. In addition, it may also be related to the nucleation degree of the molten iron. The effect of cooling rate is more obvious. For general ductile iron castings, the mass fraction of carbon equivalent is 4.3-4.7%, which is applicable for small and medium-sized parts. However, for heavy section ductile iron castings, the mass fraction range of carbon equivalent should be reduced to 4.3-4.4%, otherwise, graphite floating will occur. If the heavy section ductile iron is cooled by metal mold, the carbon equivalent will be increased to 4.6-4.7%, and graphite will not float.

The phenomenon of graphite floating is also related to pouring temperature. With the increase of pouring temperature, the possibility of graphite floating increases. The appearance of graphite floating phenomenon is related to the wall thickness of casting, the composition of molten iron (expressed as liquid eutectic composition, cel (mass fraction,%) = C + 1 / 4Si + 1 / 2P) and pouring temperature (as shown in the table in the article “properties and chemical composition of common cast iron”). With the increase of wall thickness of casting and the increase of pouring temperature, the critical maximum liquid eutectic composition decreases That’s the allowable reduction in carbon equivalent.