Graphite morphology and microstructure of lzqt600-3 ductile iron profile

As shown in the figure is the structure diagram of polished state of lzqt600-3 ductile iron section edge, 1 / 2R and center. It can be seen from the figure that the number of graphite balls at the edge is more than the other two parts, the diameter of graphite balls is the smallest and the spheroidization effect of graphite is the best. It can be seen from figures (a) to (c) that the number of graphite balls decreases, the diameter of graphite balls increases and the spheroidizing effect becomes worse, that is, the roundness decreases. According to GB / T 9441-2009 metallographic examination of nodular cast iron, the spheroidizing grading diagram and graphite size grading diagram are directly evaluated.

The results of artificial evaluation of graphite morphology of lzqt600-3 ductile iron profile from the edge to 1 / 2R to the center are as follows: the size grade of graphite ball in three parts is 7, 6 ~ 7 and 6, and the spheroidization grade is I ~ II, I ~ II and III ~ IV. however, this method of direct assessment based on images is greatly affected by subjective factors, and the results of different people’s evaluation will be biased. Therefore, the characterization of materials by this method is not accurate and rigorous, and the characterization results are not convincing. At the same time, this method has less indexes to evaluate, and the description of graphite characteristics is not comprehensive enough. Therefore, Image J software is used to conduct a comprehensive quantitative analysis of graphite characteristics.

From the edge to the center, the size of graphite ball increases, and the roundness decreases. The reason for these phenomena is that the hot metal on the surface of the profile is cooled rapidly. After spheroidizing, the hot metal enters the water-cooled graphite crystallizer and gets rapid cooling. Therefore, the graphite ball is small and round, and the spheroidization level is high. However, the core is finally solidified, even after it is out of the mold, it is solidified in the shell of the profile, so the cooling speed is slow and the solidification time is long. As we all know, with the prolongation of spheroidization and inoculation time, the residual magnesium content may decrease, and the fluctuation degree of inoculant concentration will decrease, which will lead to spheroidization and inoculation decline, so that the spheroidization level will be reduced.

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