White cast iron: Just like the sugar we put in tea, the carbon dissolves completely in liquid iron. If this carbon dissolved in the liquid cannot be separated from the liquid iron while the cast iron solidifies, but remains completely dissolved in the structure, we call the resulting structure white cast iron. White cast iron, which has a very brittle structure, is called white cast iron because it exhibits a bright, white color when broken.
Gray cast iron: While the liquid cast iron solidifies, the carbon dissolved in the liquid metal, such as the sugar in tea, may emerge as a separate phase during solidification. When we examine such a structure under the microscope, we see that the carbon has decomposed into a separate structure visible to the naked eye, in the form of graphite. We call this type of cast iron as gray cast iron, because when this structure, in which the carbon appears in lamellae, that is, in layers, is broken, a dull and gray color emerges.
Spotted cast iron: The white cast irons we mentioned above appear in fast cooling conditions, while the gray cast irons appear in relatively slower cooling conditions. If the cooling rate of the poured part coincides with a range where the transition from white to gray occurs, it is possible to see that gray and white structures appear together. We call these cast irons mottled because when we break such a piece, gray islets appear on a white background.
Tempered cast iron: This type of cast iron is actually solidified as white cast iron. In other words, the solidification of the cast iron is ensured so that the carbon remains completely dissolved in the structure. Then, the solidified white cast iron is subjected to heat treatment so that the carbon dissolved in the structure is separated from the structure. After this heat treatment, we see that the carbon emerges as irregularly shaped spheres, clustered.
In addition to this classification, if the carbon was able to separate from the structure as a result of solidification (as in gray cast irons), we can make another classification by looking at the formal properties of the resulting graphite:
Gray (lamellar graphite) cast iron: If the carbon has solidified giving rise to a layered graphite structure like cabbage leaves, we refer to such cast irons as gray or lamellar graphite cast irons. We can solidify this structure, which occurs in alloys where oxygen and sulfur are relatively high, without showing much shrinkage tendency due to its high thermal conductivity.
Spherical graphite cast iron: As the name suggests, we see that in this structure, carbon appears as spherical graphite balls. In order for graphite to decompose into a spherical structure rather than a lamellar structure, the oxygen and sulfur in the liquid must be reduced below a certain level. That’s why when producing spheroidal graphite cast iron, we treat the liquid metal with magnesium, which can react very quickly with oxygen and sulfur, and then pour it into molds.
Vermicular graphite cast iron: If the magnesium treatment applied during the production of spheroidal graphite cast iron is insufficient and the graphite cannot be spheroidized completely, this graphite structure, which we call vermicular (or compact), may emerge. Vermicular graphite, which is a transitional form between lamellar and spheroidal graphite types, not only provides cast iron with the high mechanical properties of spheroidal graphite, but also reduces shrinkage tendency thanks to its high thermal conductivity. This structure, which is considered a mistake in the production of spheroidal graphite cast iron, is deliberately cast by many foundries due to the advantages mentioned above.
Post time: Mar-29-2023