The figure shows the statistical results of the number of graphite balls in three parts of lzqt600-3profile. By processing the images of lzqt600-3 ductile iron profile section edge, 1 / 2R and center, three groups of different graphite spheres with an area of 2259950.861 mm2 can be obtained. The number of graphite spheres at the edge, 1 / 2R and center decreased gradually, which were 318, 312 and 303 / mm2, respectively.
The reason for this phenomenon is that the cooling of the surface layer is fast, the probability of concentration fluctuation is high, there are more heterogeneous nucleation cores, the nucleation rate of graphite ball is large, and the distance from carbon to graphite core is short. In addition, due to the fast cooling and short diffusion time, the solidification of carbon atoms may end before they diffuse to the graphite ball. Therefore, the number of graphite spheres in the surface layer is large and the diameter of graphite balls is small. On the contrary, the cooling rate of hot metal in the core is slow, the fluctuation of concentration is reduced, and the number of heterogeneous nucleation cores decreases with the extension of solidification time, so the number of graphite balls decreases. However, due to the long solidification time, carbon atoms can fully diffuse into the graphite core, which is beneficial to the growth of graphite balls. Therefore, there are more graphite balls in the surface layer, smaller diameter of graphite balls and better roundness; 1 / 2R and the center, due to relatively long solidification time, have less graphite balls, larger ball diameter and lower roundness.