Visual observation of the test block section with a thickness of 400mm shows that there are dense distribution of suspected fragment shaped abnormal graphite with different severity in the center of test block section of e3-6, e3-8 and e3-9, and the fragment graphite aggregates to form gray spots with large color difference with the casting body color (as shown in Fig. 1).

Take samples from the area with color spots in the center and observe the metallographic structure under the microscope. It is found that the graphite balls are irregular wormlike, flocculent and fragmentary, and the graphite is not evenly distributed in the form of scattered or aggregated accumulation, and the sizes of the spheres are different (as shown in Fig. 2). According to the standard evaluation, the spheroidization grade is basically 5 or below, which belongs to the typical deformed graphite and poor graphite morphology of ductile iron.

The metallographic structure of the corresponding spot free area is observed. Under the same conditions and magnification, the graphite ball is round and round, and the graphite ball is evenly distributed, and the diameter of the graphite ball is small (as shown in Fig. 3). According to the standard evaluation, the spheroidizing grade can reach 1-2, which belongs to the typical graphite morphology of high-quality nodular cast iron.

Core samples were taken from three blocks of e3-2, e3-5 and e3-8 schemes with a carbon content of 3.70 wt.% (core sampling refers to sampling at 2 / 3 of the wall thickness direction of the test block). Three groups of samples were taken from each test block to test the tensile strength RM, yield strength Rp0.2 and spheroidization grade.