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206                      METALLURGY   OF   CAST   IRON.
tained from the fuel used in smelting. The more carbon iron contains, the greater influence silicon, etc., can have in affecting or changing the 'c grade '' of iron. The carbon in gray iron is mostly in the form of graphite, and the iron may contain as much as three to four per cent, of it. Hard or l' white iron '' contains carbon in a different state from 'i gray iron.'' In white iron it is chiefly combined carbon, in which form it hardens the iron. The graphitic carbon in gray iron can have a large percentage made combined carbon, to harden iron, by casting it on a chill or suddenly cool ing it. By this action the carbon, which in melted iron is in the state of combination, does not have time to separate in the form of graphite.
Combined carbon is ascertained in true chemical exhibits of pig metal by the fracture being small grained, of a close, compact nature, and tending to a light gray color in Nos. i to 5, and in the higher numbers to a white color. The higher its percentage in combined carbon, the greater the approach to white iron. The faster the iron cools and the more combined carbon it contains, the finer the crystals or grain. The lowest combined carbon is found in castings having from three to four per cent, of silicon, and low in sulphur.
Graphitic carbon can be told in iron by the fracture being large grained and its crystals of a deep, brilliant color, from which flakes of graphite can often be extracted by hand or brushed out. A large percentage of graphite in iron will make it very soft, unless retarded by the presence of some hardening substance, like manganese. The more slowly a casting cools, the more graphite in the iron, and the larger the grain. For characteristic determinations of combined carbon in a fluid state, see Chapter LX, and the carbons, etc., in iron, as