key points in solidification process of gray cast iron and nodular cast iron (2)
2. UNIDIRECTIONAL NUCLEATION
In the solidification process of the metal-nonmetal system, the nonmetallic substance can be the core of metal solidification, while metal cannot be the core of nonmetallic substance solidification, which is called One-way nucleation.
The structure of cast iron, consisting primarily of a metallic Matrix and a carbonaceous component (graphite and / or carbide). In addition to all kinds of white cast iron, cast iron contains free graphite. Graphite can be the core of austenite precipitation, and 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, and Al cannot be the core of SI precipitation.
When primary graphite is precipitated from hypereutectic cast iron and hypoeutectic cast iron is transformed into eutectic graphite, and then austenite is precipitated with graphite as the core. In order to control the structure of cast iron better, it is very important to make the molten iron contain a lot of nuclei which match the graphite crystal lattice well.
3. GRAPHITE NUCLEUS AND HETEROGENEOUS NUCLEUS
The crystallization and solidification of molten metal are difficult to achieve homogeneous nucleation. What is the situation of graphite precipitation from Molten Iron? Given that the melting point of graphite is much higher than that of iron, it is of course ideal to achieve homogeneous nucleation if there is micro-graphite remaining in the liquid iron. However, the feasibility of this method has not been accepted for the following reasons.
The solubility of carbon in molten iron is very high, so it is difficult to control the quantity and size of graphite particles in molten iron, so it is difficult to control the structure and metallurgical quality of CAST IRON
When smelting gray cast iron, if the size of the residual graphite particles in the molten iron is slightly larger, which is very favorable for the graphite to be precipitated on the basis of it, it will lead to the appearance of large ‘c-type graphite’ in the microstructure. When gray iron is smelted in an induction furnace, because there is no high-temperature superheating zone like in Cupola, the larger graphite is not easy to dissolve into molten iron, so it is easy to produce c-type graphite in the structure. For example, this is often the case when a large number of pig iron ingots (more than 15%) are used in the charge.
It has also been proposed that graphitization nucleation: the ability of liquid iron to dissolve carbon is much stronger than that of solid iron. The carbon solubility will drop sharply when the liquid iron solidifies. However, many experiments and researches show that the sub-stable equilibrium temperature in Fe-C diagram is much lower than that in the Fe-C diagram. In this condition, crystallization, solidification, can only produce carbide, it is impossible to precipitate graphite. The nucleation of graphite in cast iron must also be assisted by heterogeneous nucleation.
In the early days, powder crystalline graphite was added to the inoculant for gray cast iron.
In order to further improve the metallurgical quality, both gray cast iron and nodular cast iron are increasingly being preconditioned with silicon carbide or crystalline graphite. The mechanism of the action of crystalline graphite remains to be further explored.