In the solidification process of metal nonmetal system, nonmetal matter can be the core of metal crystallization, but metal can not be the core of nonmetal matter crystallization, which is called unidirectional nucleation.
In the process of solidification, graphite is the core of austenite precipitation, but austenite cannot be the core of graphite precipitation. Similarly, in the eutectic solidification process of Al Si alloy, Si can be the core of Al precipitation, but al cannot be the core of Si precipitation.
When primary graphite is precipitated in hypereutectic cast iron and eutectic transformation occurs in hypoeutectic cast iron, graphite is precipitated first and then austenite is precipitated with graphite as the core. In order to better control the structure of cast iron, it is very important that there are a lot of nuclei in the molten iron which match the graphite lattice well.
It is difficult to achieve homogeneous nucleation in the crystallization and solidification of molten metal. Considering that the melting point of graphite is much higher than that of iron, it is very ideal to achieve homogeneous nucleation if there is fine graphite left in molten iron. However, due to the following reasons, the feasibility of this method has not been recognized up to now
(1) The solubility of C in molten iron is very high, so it is difficult to control the number and size of residual graphite particles in molten iron, so it is difficult to control the microstructure and metallurgical quality of cast iron;
(2) When smelting gray cast iron, if the size of residual graphite particles in molten iron is slightly larger, it is very conducive to the precipitation of graphite based on it, which will lead to the appearance of coarse “C-type graphite” in the structure. When smelting gray cast iron in induction furnace, because there is no high temperature superheated zone in cupola, the graphite with larger particle size is not easy to completely dissolve into molten iron, which is easy to cause “C-type graphite” in the structure. For example, when smelting gray cast iron in induction furnace, a large number of pig iron ingots (more than 15%) are used in the charge, which often occurs.
Some people have also proposed the idea of graphitization nucleation: the ability of liquid iron to dissolve C is much stronger than that of solid iron, and the solubility of C will drop sharply when the liquid iron solidifies. If the graphite crystal nucleus can be precipitated by itself, it is certainly very conducive to the precipitation of graphite. However, many experimental studies have shown that the nucleation in cast iron by graphitization itself requires about 250 ℃ undercooling, which is far lower than the metastable equilibrium temperature in the Fe-C equilibrium diagram. Under this condition, only carbides can be produced and graphite can not be precipitated.
In cast iron, the nucleation of graphite also depends on heterogeneous nucleation
(1) The nucleation process can be divided into two stages.
The first stage: some strong deoxidizing elements form fine oxides in molten iron, mainly Si and Al, and also Mn, Ti, Zr, etc., which are the core of graphite nucleus.
In the second stage, the outer layer of (Mn, x) s sulfide is formed on the fine oxide, which is the heterogeneous crystal nucleus attached to the graphite precipitation, and its size is less than 5 μ m. Generally 0.4 ~ 2.0 μ m。
(2) The treatment method of spheroidal graphite cast iron is different from that of gray cast iron, and the heterogeneous nucleus on which graphite precipitates is also different from that of gray cast iron. The purity of molten iron treated by spheroidization is high, and the contents of W (s) and w (o) are significantly reduced. From the point of view of thermodynamic energy potential, sulfides of some elements are more stable than oxides, so sulfides such as MGS, CAS and MNS are formed first as the core of crystal nucleus.
Then, a variety of oxides are formed on the fine sulfide, and these oxides react with SiO2 to form a composite silicate outer layer, which has a good match with the graphite lattice, which is the heterogeneous nucleus precipitated by spherical graphite.
If the original molten iron is inoculated with inoculant containing s and O after spheroidizing treatment, it should have good effect. This idea has been confirmed by the research work of European counterparts more than ten years ago. The use of inoculants containing s and O can improve the nodularization rate, increase the number of graphite balls and reduce the size of graphite balls. Therefore, the quality of ductile iron castings can be improved in many ways.
