As none of the constituents except manganese are as heavy as iron, their volumes per unit of weight are correspondingly greater. Putting it into approximate figures we have the percentages by weight and by volume shown in Table C.
This means, of course, that of the cast iron plates of your cook stove or steam or water radiators fully one-quarter (26 per cent by volume) is not iron at all but brittle substances of little or no strength. These elements, silicon, sulphur, phosphorus, and carbon, are commonly called “metalloids.” While the first three named are not in “free” form in the alloy and therefore allow of some doubt as to just the space they require, we have good reason to suppose that the figures given are not far from correct.
With such a volume of weakening constituents and particularly with the graphite flakes cutting through and separating the iron grains as the photomicrographs show, can one wonder that cast iron is fragile—more so than steel or wrought iron?
To sum up, naming only the most familiar alloys and the two or three qualifying features of each which stand forth with particular boldness, we have:
Pig Iron—Very High Carbon. Brittle.
Gray Cast Iron—High Carbon. Brittle.
Malleable Cast Iron—High Carbon. Made Malleable by Annealing.
Wrought Iron—Slag. Little or no Carbon. Very Malleable without Annealing.
Mild Steel—Very Low Carbon. No Slag. Very Malleable without Annealing.
Carbon Tool Steel—Medium Carbon. No Slag. Of Medium Malleability.