KEY POINTS IN SOLIDIFICATION PROCESS OF GRAY CAST IRON AND NODULAR CAST IRON (6)

  1. EUTECTIC TRANSFORMATION OF NODULAR CAST IRON

The eutectic transformation of spheroidal graphite cast iron is not as obvious as that of gray iron, although graphite is first precipitated and austenite is then precipitated, graphite plays a less dominant role in the Eutectic Solidification process coexisting and coexisting under conditions of common and liquid contact.

During the eutectic transformation of nodular cast iron, the graphite spherules nucleate in the liquid phase close to the eutectic composition, and there is a growth process. The spheroidal graphite grows to a certain size, the carbon equivalent of the surrounding liquid phase is very low, and the austenite nucleates and grows on the surface of the graphite, gradually forming a ‘Halo’ around the spheroidal graphite, which blocks the contact between the graphite and the liquid phase. The growth rate of graphite is much lower than that of graphite flake in gray cast iron, which can only be achieved by the outward diffusion of iron atoms at the graphite-austenite interface and the diffusion of carbon atoms to graphite through the austenite halo.

Because the graphite ball is out of contact with the liquid phase and does not have the conditions for co-growth with Austenite, it can not be said to be a normal eutectic transition, and the graphite and austenite are precipitated from the liquid phase near the eutectic temperature, therefore, it is commonly referred to as “Divorced eutectic” and its eutectic transformation is shown in figure 5.

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             Fig. 5 Schematic diagram of EUTECTIC TRANSFORMATION PROCESS OF DUCTILE IRON

Professor Zhou Jiyang of the Dalian University of Technology, who has systematically studied the divorced eutectic of ductile iron by means of color metallography, has put forward another view The results show that graphite and austenite can be separated from the liquid phase at different positions and at different times. Therefore, the EUTECTIC transformation of nodular cast iron may have many forms.

During eutectic transformation, austenite can grow and nucleate at the interface of graphite spheres, or at the interface of graphite spheres depending on other heterogeneous crystals.

There is a eutectic grain composed of graphite balls and austenite, and there are austenite grains surrounded by several graphite balls.

What’s more, several austenite grains nucleate and grow around the EUTECTIC graphite ball, forming a halo around the graphite ball, as shown in figure 6.

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A) the nucleation and growth of graphite spherules,
B) the carbon-deficient region around the graphite spherules;
C) austenite nucleation on the surface or outside of the graphite ball;
D) austenite grows into dendrites; e) forms a closed Halo

4. GRAPHITE NUCLEUS IN DUCTILE IRON
The treatment of nodular cast iron is different from that of grey cast iron, and the Heterogeneous Crystal Nucleus supported by precipitated graphite is also different from that of grey cast iron.

The spheroidized molten iron has high purity, and the content of sulfur and oxygen in the molten iron decreases significantly. 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 the crystal nucleus.

Then, a variety of oxides are formed on the micro sulfides. These oxides react with SIO2 to form a composite silicate outer layer, which matches the graphite lattice well.

With regard to graphitizing nucleation of Ductile Iron, the following points should be noted:

Due to the high purity of molten iron after strong treatment, the number of heterogeneous nuclei is reduced, and the amount of inoculant required is more than that of gray cast iron

Generally, the sulfur content in molten iron should be as low as possible, but it should not be too low, especially not high or low, from the aspect of graphitization; it is better to keep between 0.005% and 0.015% ;

The molten iron should still have a certain oxygen content.

Based on this understanding, it will be thought: If the original iron after spheroidizing treatment with sulfur, oxygen inoculant inoculation treatment, should have a good effect. This assumption has been confirmed by the research work of European colleagues more than 10 years ago that the use of inoculants containing sulfur and oxygen can increase the spheroidization rate, increase the number of graphite balls and decrease the size of graphite balls, Therefore, the quality of ductile iron castings can be improved from many aspects.