It should be distinctly understood that the figures given in Table B are approximate only and are intended to be average, or rather, perhaps, typical.
There are all sorts of conditions which in practice modify the figures given in the table, and criticism may be maintained justly against some of the too specific statements which here it was necessary to make. The classification is given with considerable hesitation and only because, arranged in this way, it brings out existing relationships which otherwise would escape notice and which we cannot afford to overlook.
But please do not gain the impression that these alloys are divided into distinct classes. There are no dividing lines at all. One group merges into the next so gradually that it is impossible to tell where the one ends and the other begins.
It is to be hoped that no one will make himself miserable by trying to digest these rather formidable figures of Table B all at one sitting. They are given mainly for comparison and for reference. It is suggested that after noting carefully the similarities and differences to which attention is called, they be reserved until the processes of manufacture of the various alloys are taken up one by one. Reference to these figures on those occasions should be profitable.
The main points to be noted at this time are:
1. Open-hearth iron is practically pure iron, having no constituents or slag inclusions which materially affect its properties.
2. Wrought iron, for all practical purposes, is pure iron except for its content of slag. It is the only one of the iron family which does normally contain slag.
3. Neither open-hearth iron nor wrought iron contains carbon in appreciable quantities.
4. The distinguishing and active element of the steel family is carbon. With increase of carbon the hardness of the alloy increases as does its tensile strength, but the ductility (elongation or stretch) decreases.
5. Other conditions being equal, the more carbon the alloy contains the more easily it melts; i.e., at lower temperature. So the purer irons such as open-hearth iron, wrought iron, and mild steel (i.e., steel with low carbon, usually under 0.15 per cent) have relatively high and the cast irons lower melting points.