The floating of graphite and abnormal shape of graphite in the center is a kind of metallographic structure defect associated with shrinkage cavity and porosity tendency, which is very difficult to avoid in heavy section ductile iron casting. In order to avoid shrinkage cavity and porosity, the carbon equivalent CE will be increased. However, the increase of CE value will lead to graphite floating in the upper layer of thick large section of stator blade flange at the top of intake cylinder and abnormal graphite distortion in the center (as shown in Fig. 1).

The graphite floating mostly appears on the upper surface of the casting. From the section observation after processing or grinding, the floating position of graphite is a black spot layer with obvious boundary; the abnormal deformation of graphite mostly occurs in the center and central section of the heavy casting position, and the macroscopic observation shows black or black gray aggregation spots, and the more serious the distortion, the more dense the spots gather.

Graphite morphology, number of graphite balls and roundness of nodular cast iron have a very important impact on its physical and chemical properties. The mechanical properties of nodular cast iron will be decreased if graphite floating and broken graphite defects appear.