Balance of nuclei

The quantity of nuclei effective in solidification.

By strategic molten bath treatment it is possible to influence the balance of nuclei in the melt and any effects on the structure and the properties of a material.

In this context, the following factors are of major importance: overheating of the melt and inoculation.
Overheating of the melt results in impairment of the nucleation conditions in the cast iron melt. This is based on the principle that the solubility of nuclei in the melt is increased with increasing temperatures; and through reduced viscosity it is possible for the suspended particles to faster rise to the surface. If there is no possibility of repeated diffusion of nuclei, solidification of the primary austenite and the graphite eutectic commences at only a few centers and is generally performed with a great degree of supercooling. The result is radial growth of the primary dendrites, coarse eutectic cells and thus thoroughly supercooled graphite (D graphite). With considerably undereutectic compositions, the material also shows a tendency for formation of interdendritic E graphite.

Inoculation of such a type of melt, for example using FeSi inoculation alloys, causes local oversaturation due to the dissolved alloy, which has a positive effect on nucleation. This treatment method is the more effective the lower the temperature of the melt during inoculation.
Inoculation increases the count in heterogeneous nuclei which then greatly promote graphite diffusion during solidification and formation of an increased number of correspondingly small eutectic cells in the flake graphite (A graphite). This structure allows for an increase in tensile strength of up to 30 % and considerably minimizes the risk of chill or edge hardness (s. Edge hardness, Chill). The inoculation effect is significantly higher in undereutectic melts than in eutectic melts.

The inoculation effect does not remain stable over time. Fading of the effect results in the initial structural condition after 20 to 30 minutes (s. Fading effect). The cause for fading is increased coagulation of the heterogeneous nuclei which rise to the surface due to their buoyancy and are there absorbed in the slag.

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