How does spheroidal graphite cast iron reduce shrinkage?

- Jul 22, 2019-

1. Spheroidal graphite cast iron Because the molten iron contains magnesium, the eutectic point on the state diagram is shifted to the right. When the magnesium content is 0.035-0.045%, the actual eutectic point is about 4.4-4.5%.


2. The composition of the ductile iron is near the eutectic point, and the molten iron has the best fluidity, so the molten iron is easy to replenish when solidified.


3. The sulfur content before and after the spheroidal spheroidization should not change too much. That is, the original molten iron sulfur content should not be too high. High sulfur content, graphite is easy to precipitate too early. It is easy to produce shrinkage.


4. Maximize the carbon content without premature graphite floating and no precipitation of primary graphite. My understanding of this sentence: Generally speaking, the larger the hypereutectic, the more nascent graphite is produced in the liquid state, which is unfavorable for reducing shrinkage.


5. During the solidification of the ductile iron, the time during which the graphite expands is controlled to delay the graphitization expansion. High carbon and low silicon in the case of carbon equivalent selection. The appropriate amount of residual magnesium, the correct gestation and focus on the final flow.


6, hot metal attention to rapid smelting, to avoid the temperature in the furnace, the furnace preservation time is too long, to avoid excessive smelting insulation temperature of more than 1550 degrees, loss of a large amount of carbon and crystal core. Generally, it will be re-processed after more than 10-20 minutes. This kind of molten iron, even after various incubations, produces carbides and shrinkage, which is difficult to eliminate.


7. After the spheroidalization of the molten iron, it is necessary to pour immediately. It is strictly forbidden to wait too long, so that the spheroidization will breed a recession.


8. When using a spheroidizing agent containing cerium rare earth, the graphite crystal at the initial stage of solidification is less, and the appearance of individual large graphite balls is avoided. The number of graphite balls is relatively large, and the size is relatively uniform, indicating that the precipitation time of graphite balls in solidification is relatively uniform, and the expansion is larger in the late solidification stage.


9. The larger the carbon equivalent of ductile iron, the wider the crystallization solidification range and the larger the solid-liquid coexistence interval. During the solidification process, the liquid molten iron flow is affected by the primary dendrites, which hinders the flow of the shrinkage and is easy to form shrinkage. At the same time, the high content of silicon in molten iron is easy to promote graphite nucleation and growth prematurely. At this time, the graphitization expansion in the solid-liquid coexistence period is unfavorable for the reduction of shrinkage. Therefore, through the above-mentioned some technological measures, the graphitization expansion is delayed, and in the actual casting technology work, it is of great guiding significance to solve the shrinkage phenomenon of ductile iron castings.