EFFECT OF EXPANSION ON SHRINKAGE, ETC'. 395
pect soft irons, which exhibit after solidification more graphite, to show the greater expansion.
The formation of graphite is confessedly promoted by silicon, and hindered by the metalloids which *' harden '' the iron. When these metalloids are present in such proportions as to overpower the effect of the silicon, combined carbon, instead of graphite, is produced in the solidified metal, and the individual grains, crystals, or structural elements of the cast iron are consequently smaller and more densely packed in hard than in soft grades of such iron. Expansion (and, perhaps, also contraction,) would be, therefore, exhibited by a larger number of such structural elements in a given volume of metal, to be effected by changes in their form and size. This may explain the greater expansion shown by the hard grades in Tests Nos. 2, 4, 6, and 8 in Fig. 74, where the largest percentages of the antagonistic constituents, silicon and sulphur, are presented. (See page 420.)
But any theory on the subject may be premature. Far more important at this time is the fact itself, which affects so directly our foundry practice. I attribute the failure to detect it heretofore to the circumstance that in the every-day work of the founder, the expansion of solidification does not force itself upon his attention. The shrinkage of the liquid mass, requiring '' feeding,'' is obvious enough; and so is the final contraction of the solid mass, for which allowance has to be made in the pattern. But the intervening expansion, not being marked by the final contraction, has been overlooked. *
I may here observe that the tests illustrated in Fig. 74 refute the opinion heretofore advanced, that the
*The subject of shrinkage is continued at the close of this chapter on pages 404 to 414. ? ? f ? ? i !iJt'ft<?M