Fig. 4 Schematic diagram of EUTECTIC TRANSFORMATION OF GRAY CAST IRON
During the EUTECTIC transformation of gray cast iron, graphite and austenite are symbiotic and have the characteristics of co-existence of graphite, austenite and liquid phase. Even so, because graphite is in the leading position in the transformation process, the synergistic growth of graphite and austenite is not so close, and the interface of symbiotic crystals is uneven. It is also considered that the EUTECTIC transformation of gray cast iron should be regarded as ‘abnormal eutectic transformation’.

In Gray cast iron, the paragenetic crystal composed of graphite and austenite is usually called “eutectic group”. EUTECTIC and EUTECTIC, and EUTECTIC and primary Austenite, grow together to join each other, the liquid phase disappears, and the eutectic transformation process is finished.

2.GRAPHITE NUCLEI IN GRAY IRON

The heterogeneous nucleation of graphite in gray cast iron can be divided into two stages.

The first stage: Some strong deoxidization elements form micro-oxides in the molten iron, in which Al and SI are the main elements and MN, TI and Zr are the cores.

The second stage is to form the outer layer of (MN, x) s series sulfide on the micro-scale oxide, which is the heterogeneous crystal nucleus of graphite precipitation, its size is less than 5 m, generally 0.4 ~ 2.0 m.

When the cast iron is not inoculated, the X in (MN, x) s is mainly Fe, and the sulfides contain few elements such as CA, AL, and TI.

After inoculation treatment, x contains CA, AL, Ti, SR, and RE. The sulfides have good compatibility with graphite lattice and small particles, so they are suitable for graphite nucleation. If adequately prepared, a thin layer of silicate can be formed on the surface of Sulfides (MN, x) s to further improve its compatibility with graphite lattice.

Therefore, in order to get a good effect of inoculation treatment, the content of oxygen and sulfur should be kept in the original iron melt of gray cast iron. Generally speaking, sulfur content should not be less than 0.06 %, oxygen content should be about 0,003 %.

It is generally accepted that AL plays no inoculation role in gray iron. Moreover, if the AL content in gray iron is above 0.02 %, the surface tension of molten metal will be reduced. It is a common understanding in the foundry industry that the pinhole defect is natural to appear when the clay green sand mold casting process is used. Therefore, it is generally desirable that the AL content in cast iron be lower or that the AL content be less of a concern.

In fact, AL plays an important role in the precipitation and growth of graphite in gray cast iron, which can decrease the undercooling degree of EUTECTIC transformation and increase the number of eutectic groups and is beneficial to the formation of a-type graphite. Generally, the content of Al should be controlled between 0.005% and 0.01 %. Keeping such AL content can not only have the positive effect mentioned above but also can not induce pinhole defect.

Therefore, the content of CA and AL in inoculant is significant.

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