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There are many factors that affect the spheroidization rate and roundness of ductile iron, and the following factors are analyzed from the three aspects of chemical composition, process and solidification control:

1. chemical composition

1, carbon equivalent and silicon

Carbon and silicon are the basic components that affect the roundness of ductile iron graphite spheres. Under the relative cooling rate and inoculation conditions, increasing the carbon equivalent can improve the roundness of the graphite ball, increase the self-feeding ability and reduce the carbide. Under the relative carbon equivalent conditions, with the increase of the amount of silicon, especially large cross-section ductile iron, will produce crushed block graphite. Therefore, in the case of no graphite floating, the carbon equivalent should be increased as much as possible; at the same time, the silicon content should be reduced as much as possible under the condition of ensuring incubation.

2. Magnesium

The control of the effective amount of residual magnesium is important to improve the roundness of graphite spheres. When the effective residual magnesium is less than 0.045%, increasing the amount of effective residual magnesium can improve the roundness of graphite spheres. When the effective residual magnesium is more than 0.045%, abnormal graphite will be produced and the roundness of graphite spheres will be reduced.

3. Rare earth

Rare earth has two beneficial effects in molten iron: one is desulfurization to gas, which plays the role of spheroidization and indirect spheroidization; the other is to interact with trace elements to eliminate the harmful side and play a favorable side. Both of these aspects can improve the roundness of graphite spheres. However, excessive residual rare earth, especially large section ductile iron, will make the graphite morphology deterioration, especially easy to produce broken block. The ideal range is 0.010 to 0.019 percent.

4. Trace elements

The relative amount of antimony and bismuth and the right amount of rare earth can improve the roundness of graphite spheres and improve the mechanical properties. Suitable addition range: 0.0025~0.005%.

2. process

1. Sballing treatment

(1) Heavy rare earth magnesium spheroidizing agent can improve the roundness of graphite spheres more than light rare earth magnesium spheroidizing agent. Mainly desulfurization ability, anti-recession ability.

(2) If the process of light rare earth magnesium spheroidizing agent is well controlled and supplemented by trace elements, it can also improve the roundness of graphite spheres.

(3) The flushing method is better than the subcontracting method, so it can also improve the roundness of graphite balls.

2. Inoculation treatment

(1) strengthening inoculation can improve the roundness of graphite ball.

(2) The use of anti-recession inoculant can improve the roundness of graphite balls.

3, with nodular iron filings as the covering agent for spheroidization treatment, it can form a passivation graphite core similar to that of non-dissolved graphite dispersion, resulting in long-term concentration fluctuation, greatly increasing the effective nucleation rate, and improving the roundness of graphite spheres.

4, as far as possible to reduce the pouring temperature can improve the graphite ball roundness.

3. solidification control

1. The use of cold iron can improve the roundness of graphite balls.

2. The use of chromite sand can improve the roundness of graphite balls.

3. Other measures that can speed up cooling and shorten the time during the eutectic transformation of the ball iron liquid can improve the roundness of the graphite ball.