(1) The influence of chemical composition on carbon equivalent is the biggest. In heavy section, carbon equivalent should be increased as much as possible without graphite floating. The study of magnesium treatment shows that the change of carbon equivalent will obviously affect the shape of graphite. With the increase of carbon equivalent, the number of graphite spheres increases, while the number of non spherical graphite decreases. Therefore, for the hypoeutectic nodular iron, the slow cooling rate makes the spherical graphite distorted; for the eutectic nodular iron, the graphite ball remains spherical even if the cooling rate is slow; for the hypereutectic nodular iron, the graphite is not only round, but also small, but also easy to form fragmented graphite when the nodulizer contains rare earth. Therefore, it is suggested that the carbon equivalent mass fraction be between 4.2% and 4.4%.
In addition, the fragmentary graphite is closely related to the silicon content. The increase of silicon content will promote the formation of fragmentary graphite. Therefore, in the heavy section ductile iron, the silicon content should be as low as possible. For example, for the cast smooth ductile iron, the highest silicon content should not exceed 2.4%.
Excessive rare earth will lead to the increase of fragmentary graphite. The production practice shows that if the residual amount of rare earth exceeds the residual amount of magnesium, the fragmentary graphite will inevitably appear in the heavy section ductile iron. Therefore, the mass fraction of residual rare earth should not exceed 0.03%. In addition, in the heavy section ductile iron, due to the slow solidification process, the evaporation loss of magnesium is caused. Therefore, the residual magnesium content should be controlled at a higher level, and the mass fraction of residual magnesium should not be less than 0.05%.
For Austenitic nodular iron with nickel content more than 20%, fragment graphite often appears. For this reason, antimony can be added with the mass fraction of 0.002% – 0.008% to overcome the appearance of fragmentary graphite.
(2) Inoculation: for heavy section ductile iron, when the number of graphite spheres per 1mm2 area is more than 60, there is no broken graphite.
It is also effective for the number of graphite spheres in heavy section ductile iron to adopt late inoculation. With in mold inoculation, the number of graphite spheres can be increased by 2 times.
Using long-term and high-efficiency inoculants, such as those containing barium and zirconium, the fine and uniform spheroidal graphite can be kept in the heavy section ductile iron with solidification time up to 3H. 75sife with the mass fraction of barium of 1% – 2% can be used for long-term inoculation.
For the heavy section ductile iron, it is advantageous to adopt the inoculant with coarse particles (such as 3-5mm) or agglomerates to overcome the fragmentary graphite. However, excessive inoculation will also lead to fragmentary graphite. Therefore, the mass fraction of inoculant used for later inoculation shall not exceed 0.1%.
(3) In the heavy section ductile iron, some trace elements such as antimony and bismuth are added together with cerium (antimony and bismuth were originally interfering with the spheroidization of trace elements). At this time, they will not interfere with the spheroidization of graphite, but can eliminate the fragmentary graphite.
Without cerium and other elements, adding 0.002% antimony can make the graphite in the center of 200 mm section ductile iron very round. The recovery rate of antimony is 80-85%, and most of antimony in the material can also be recovered. It is easy to make the mass fraction of antimony exceed 0.005% when using the return charge and unknown composition charge. Therefore, when antimony is added, cerium with mass fraction of 0.01-0.05% should be added to counteract the destructive effect of antimony and other elements.
(4) The most effective process measure is to use metal mold or cold iron. The practice shows that the solidification time can be significantly shortened by using the metal mold, so the probability of the occurrence of fragmentary graphite can be reduced. For the cylindrical casting with ￠ 300 mm, the solidification time in the sand mold is 120 min, while that in the metal mold can be shortened to 60 min; for the cylindrical casting with ￠ 200 mm, the solidification time in the sand mold is 60 min, while that in the metal mold is 30 min. At this time, there will be no broken graphite.