The figure below shows the graphite morphology of as cast ductile iron. It can be seen from figure a that in the nodular cast iron with a treatment temperature of 1560 ° C, the graphite is spherical and there is a small amount of nodular graphite. According to the formula, the spheroidization rate of nodular cast iron is 0.80 and the graphite grade is grade 3; When the treatment temperature is reduced to 1510 ° C, the graphite in the nodular cast iron sample is mainly spherical, and the content of nodular graphite is less, its spheroidization rate is 0.90, and the graphite grade is grade 2; When the treatment temperature is continuously reduced to 1460 ° C, there are more nodular and flocculent graphite in nodular cast iron, the nodularization rate of nodular cast iron is reduced to 0.70, and the nodularization grade of graphite is reduced to grade 4.
In addition, it can be seen from the figure that the number of graphite balls in nodular cast iron with a treatment temperature of 1560 ° C is the least. This is because the treatment temperature of 1560 ° C is higher, the nodularizing agent is burned more, resulting in serious failure of the nodularizing agent, and the effective graphite core formed in the metal body is reduced. Therefore, the number of graphite balls generated after solidification is the least. However, in the nodular cast iron with the treatment temperature of 1510 ° C, due to the good spheroidization of graphite and sufficient time for growth, more graphite balls are generated, which increases the spheroidization rate and stone plastic grade of nodular cast iron. Further reduce the treatment temperature. When the treatment temperature is 1460 ° C, due to the low treatment temperature and insufficient spheroidization treatment, the growth time of graphite balls in nodular cast iron is shortened, the number of spherical graphite in nodular cast iron is reduced, and the number of flocculent and nodular graphite is increased. Therefore, the spheroidization rate and graphite grade of nodular cast iron treated at 1460 ° C are the lowest.