Metallographic examination shall be carried out in accordance with GB / t9411. The graphite morphology and matrix structure at different positions of 45 ° sector block are shown in Figure 1. When the sampling position moves from the edge to the final solidification position, the size of graphite balls increases gradually, and the number of graphite balls decreases obviously; when the inner quarter position finally solidifies, the size of graphite balls is the largest, and the number of graphite balls is the least; when the distance from the center is the same, the size of graphite balls on the outer side is smaller than that on the inner side, and the number of graphite balls is more than that on the inner side; on the whole section of 45 ° fan-shaped block, the matrix is smaller The microstructure is all ferrite with good spheroidization and no graphite distortion.
According to the statistics of the number of graphite balls, spheroidization grade and spheroidization rate at different positions of the 45 ° fan-shaped block, the spheroidization rate is the lowest (85.4%) at the inner quarter, and the spheroidization grade and graphite ball size are similar. The number of graphite spheres varies greatly. The number of graphite balls in the center and 1 / 4 of the inner part of the 45 ° fan-shaped test block is the worst, so the heat released from solidification can not be effectively transferred out. Therefore, the number of graphite balls in this part is the least, which is 50 / mm2, and the outer part is close to the metal mold. The metal mold has good thermal conductivity and heat storage capacity, which can timely remove the latent heat of solidification, so the number of graphite balls in this part is the most, The number of graphite balls is 100 / mm2. The number of graphite balls at different positions of the 45 ° sector block is shown in Figure 2.
It is found that the microstructure of the 45 ° fan-shaped block is related to the solidification time: on the whole, the spheroidization rate decreases with the increase of solidification time, and the variation trend of the number of graphite balls is similar to that of spheroidization rate. The inner quarter of the block has the longest solidification time, and the number of graphite balls and spheroidization rate are the lowest at this place. The spheroidization rate and the number of graphite balls on both sides of the block are generally higher than those in the center. The number of graphite balls decreases with the increase of solidification time. The change of the number of graphite balls on the whole section of the 45 ° fan-shaped block is consistent with the temperature measurement results, that is, the number of graphite balls is in the order of outer side (125 / mm2), inner side (100 / mm2), outer L / 8 (100 / mm2), inner L / 8 (75 / mm2) and outer L / 4 (7 / mm2) The relationship between solidification time and spheroidization rate and the number of graphite balls is shown in Fig. 3.
