They arrive at this composition, through breaking down of the austenite and cementite structures during the cooling,—just how not having been satisfactorily determined. Consistent study is being put upon this subject and several unique and long-studied possible explanations of this section of the full equilibrium diagram have been proposed and debated, all based upon the data so far available. While much information on this subject has been gained the matter is still so much in dispute that it is best for us to venture nothing definite in regard to just how the changes occur. The reference books for this chapter (see page [354]) give quite completely the data, theories and explanations so far available.
Compared with the steels, the cast irons are vastly complicated. In them we have elements which occur in practically negligible amounts in the steels. Commercial cast irons, for instance, have silicon ranging anywhere from ½% to 3%, phosphorus .10% to 2%, graphite 0% to 3.50%, and carbon in the combined form (pearlite or cementite) from 3.50% to .10%. If these represent the majority of cast irons what about our pig irons which have 2, 5, 8, 10 or even 15% of silicon, others with 1 or 2 and occasionally very much more of manganese, and still others which vary widely in phosphorus content?—for from the metallographic and physical chemistry standpoint pig irons are cast irons.
As we can never get perfectly pure iron-carbon alloys to experiment with, their content of other elements, silicon, nickel, phosphorus, etc., vitiate more or less the results obtained, but even could such pure alloys be secured we are not greatly helped since our serviceable alloys are never such. Each added element brings about greater complication and one does not wonder that in the short twenty years which have elapsed since study was seriously undertaken, metallurgical science has not entirely solved the big problem.
REFERENCES
GENERAL & VARIOUS
“The A B C of Iron and Steel,” Penton Publishing Co., Cleveland.
“The Steel Foundry,” J. H. Hall, McGraw-Hill Book Co., New York.
“The Metallurgy of Steel,” H. M. Howe, The Scientific Publishing Co., New York.
“The Manufacture and Properties of Iron and Steel,” H. H. Campbell, Hill Publishing Co., New York.
“The Metallurgy of Iron and Steel,” B. Stoughton, McGraw-Hill Book Co., New York.