SAND CASTING EQUIPMENT.

In the solidification process of the metal-nonmetal system, the nonmetal material can be the core of the crystalline precipitation of the metal, while the metal cannot be the core of the crystalline precipitation of the nonmetal material, which is called unidirectional nucleation.

Cast iron contains free graphite in various tissues, and during its solidification process, graphite is the core of austenite precipitation, while austenite cannot be the core of graphite precipitation.

Similarly, during the eutectic solidification of Al-Si alloy, Si can be the core of Al precipitation and Al cannot be the core of Si precipitation.
When peri-eutectic cast iron precipitates incipient graphite, and when sub-eutectic cast iron undergoes eutectic transformation, both precipitate graphite first and then austenite with graphite as the core. In order to better control the organization of cast iron, xinyuanzhu group improves the quality of cast iron parts by controlling the perfect matching degree between the large number of nuclei contained in the ferrofluid and the graphite lattice.

What about the graphite precipitation from cast iron liquid, which is difficult to achieve homogeneous nucleation by crystallization and solidification of metal liquid? Considering that the melting point of graphite is much higher than that of iron, it is of course very desirable to achieve homogeneous nucleation if there is microscopic graphite left in the ferrofluid, but the feasibility of this approach cannot be agreed so far due to the following reasons

The high solubility of carbon in the ferrofluid makes it difficult to control the number and size of residual graphite particles in the ferrofluid, and therefore the organization and metallurgical quality of the cast iron

When melting gray cast iron, if the size of the residual particulate graphite in the molten iron is slightly larger, it is very conducive to the graphite to its dependence and precipitation, it will lead to the appearance of coarse C-shaped graphite in the organization. Induction furnace melting gray cast iron, because there is no cupola high temperature superheat, the size of the larger graphite is not easy to completely dissolve into the molten iron, it is easy to lead to the organization of C-shaped graphite. xinyuanzhu group through experimental induction furnace melting cast iron, such as the furnace charge with a large number of pig iron ingots (more than 15), this situation often occurs.
Foundry has also put forward the idea of graphitization nuclei: liquid iron dissolution of carbon is much stronger than solid iron, the solidification of iron will occur when the carbon solubility of the plunge, if you can precipitate graphite nuclei, of course, very conducive to the precipitation of graphite. However, many experiments and research work have shown that the nucleation of graphite in cast iron by itself requires a subcooling of roughly 250°C, which is much lower than the sub-stable equilibrium temperature in Fe-C. Crystallization and solidification under such conditions can only produce carbides, and it is impossible to precipitate graphite. The nucleation of graphite in cast iron must also be done with the help of heterogeneous nucleation.

In the early days, the foundry used to add powdered crystalline graphite in the inoculant of gray cast iron, but now this method is no longer used.
The nucleation process of graphite precipitation in gray cast iron can be divided into two stages.
1. as the nucleus of graphite nuclei.
2. The formation of the outer layer of (Mn, x) S sulfide on the micro oxide, which is the heterogeneous nucleus on which the graphite precipitation depends, and its size is 5μm,generally 0.4~2.0μm.

Nuclei for graphite precipitation in ductile iron
The treatment of ductile iron is different from that of gray cast iron, in which the heterogeneous nuclei on which the graphite precipitates are based are also different from those of gray cast iron.
The purity of the spheroidized iron is high, and the sulfur and oxygen content is significantly reduced. From the point of view of thermodynamic energy potential, it appears that sulfides of some elements are more stable than oxides, and thus sulfides such as MgS, CaS and MnS are formed first as the cores of the nuclei. Then, a variety of oxides are formed on the microscopic sulfides, which in turn interact with SiO2 to form a composite silicate outer layer, which matches well with the graphite lattice, which is the heterogeneous nucleus precipitated from spherical graphite.

Based on this understanding, of course, it will think: if the original iron solution by spheroidization treatment with sulfur and oxygen containing inoculants for inoculation treatment, should have good results. This idea, has been more than a decade ago by the European counterparts of the research work confirmed that the use of sulfur, oxygen inoculant, can make the spheroidization rate increased, the number of graphite balls, graphite ball size reduction, and thus can improve the quality of ductile iron parts from many aspects.

Xinyuanzhu Group specializes in the production of Ductile iron castings, Pig iron gray iron casting, Steel casting, Brass Casting, Malleable Iron Casting, and undertakes batch casting processing of castings. Welcome guests to cooperate with pictures or samples.