“Z” point, the intersecting point of the extension of the liquidus curve DC and that of the solidas curve IE in the equilibrium diagram of Fe-C system, is of significance on the spherodizing of graphite in hypoeutectic cast iron. Let the extension of the other liquidus curve BC be CR. Melt supercooled to the area between CZ and CR must be supersaturated with both carbon and iron. On the other hand, as are well known, separation of carbon from supercooled melt is rapid, While separation of iron from it is not so rapid, and the melt should be readily supercooled. Then it follows that;
1) In the cast of hypereurectic cast iron granular graphite precipitates before the repulsive force rises.
2) In hypoeutectic cast iron with composition between C and Z the repulsive force rises soon, when the melt is supercooled. Then carbon and iron must precipitate at the same time, that is the eutectic reaction takes place. So spherodizing of graphite is difficult in this case.
3) In hypoeutectic cast iron with composition between Z and E, melt may be supercooled readily, because the repulsive force does not rise, even if the melt is supercooled. Thus supersaturated solid solution “
γm” with the same carbon content as the melt may be formed, and “
γmax” with the maximum carbon content is obtained at the “Z” point. The carbon content being too high in “
γm” especially in “
γmax” carbon precipitates very rapidly, and results in the formation of granular graphite.
The situation of the “Z” point in the diagram assumed to be at about 3.2% carbon equivalent and 110°C lower than the eutectic point.
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