3. EUTECTIC TRANSFORMATION AND NUCLEATION OF CAST IRON
Both gray cast iron and ductile iron are EUTECTIC FE-C alloys, and eutectic transformation is the most important step in the solidification process.
Although primary austenite is precipitated in hypoeutectic cast iron, eutectic cast iron, and hypereutectic cast iron, however, the eutectic transformation does not rely on austenite nucleation and crystallization But in the molten iron with eutectic component between the primary austenite Dendrites, the nucleation of graphite starts alone.
Gray cast iron and nodular cast iron, the eutectic transformation of the formation of the structure, are co-formed by graphite and austenite eutectic crystals but formed differently.
The graphite precipitation in cast iron is mainly promoted by heterogeneous nucleation. The Heterogeneous Crystal Nucleus supported by precipitated graphite is composed of many kinds of oxides, many kinds of Sulfides, many kinds of silicate and other non-metallic inclusions. Due to the different composition of various cast irons, the treatment methods are different, and the actual composition of the graphite crystal nucleus is also different.
Based on the considerable research work done in this area in recent years in a number of industrialized countries, a consensus has emerged that goes something like this:
1. EUTECTIC TRANSFORMATION OF GRAY CAST IRON
Commonly known as ‘eutectic transformation’ , it refers to the crystallization of two (binary) or more (multicomponent) solid phases in a liquid alloy of a certain composition at a certain temperature And also has the liquid phase and the precipitation of a variety of solid phase coexistence.
As far as the stable system of FE-C alloy is concerned, two solid phases, graphite and austenite, are precipitated during eutectic transformation, and graphite, austenite, and liquid phase coexist during eutectic transformation until the end of the eutectic transformation.
The leading phase of EUTECTIC transformation of gray cast iron is graphite. After graphite precipitation, austenite precipitates between the branches of graphite, and the two grow together to form a kind of near-spherical co-crystallization and growth symbiosis crystal. The leading edge of the EUTECTIC crystal in contact with the liquid phase is uneven, and the tip of the graphite sheet is always protruded out of the EUTECTIC crystal and extended into the liquid phase, maintaining the state of leading growth and branching in the liquid phase. The transformation of the eutectic crystal is shown in figure 4.
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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