It is essential to understand the difference between the several classes of iron and steel, and the effects of the substances which they always retain from the blast furnace. These substances are frequently spoken of as impurities, but some of them are always more or less desirable in the iron in giving it certain properties. It may be stated that chemically pure iron is not known, except possibly as a laboratory curiosity.

The three general classes of iron are (1) wrought iron, (2) steel, and (3) cast iron. Within each class there are many grades, due to the varying quantities of the substances which each grade contains. These substances affect the metal adversely or otherwise according to their quantity and the use for which the metal is desired.

All grades and classes of iron and steel gradually merge one into another, and the difference between them is due primarily to carbon. The approximate limitations of carbon contained by each class are as follows:

Wrought Iron .......................................................................... Trace to .08

per cent

Mild Steel (also known as "Ingot Iron," "Low Carbon Steel," "Soft,Steel ") .................Trace to about 0.25

per cent

High Carbon Steel........................About 0.25 to 2.2

per cent

Cast Iron.......................................*2.2 to 4.5

per cent

Carbon in iron and steel has a direct effect upon their properties, and other elements usually contained govern somewhat the power of the metal to take up carbon, and their influence is both direct and indirect. Iron and steel are affected similarly by the same ingredients, and in discussing these the general term iron is meant to include steel.

It will assist, in the study of the several elements taken up by iron, to regard the metal in its molten state as a solvent which has a stronger tendency to dissolve some elements than others, and which, when certain elements have been dissolved, has a decreased or an increased power to dissolve other elements.