In this form it is sold to the founder or to the iron manufacturer.
By them it is subjected to various processes, which will hereafter be described.
The result of these processes is the production of materials which, though originally from the same ore, and still of nearly the same chemical composition, differ very widely in their mechanical properties and characteristics.
These materials may be divided into three general classes: -
The different processes required for the production of these three classes of material, and those connected with the conversion of the metals generally into the forms suited for the market - such as pigs or ingots, plates and sheets, bars of different sections, etc. - will be very lightly touched upon.
Pig-iron always contains foreign substances, among which are Carbon, Silicon, Sulphur, Phosphorus, and Manganese, besides many others in smaller proportion.
Of these foreign bodies that which plays by far the most important part is carbon.
The great differences (which will presently be pointed out) that exist between Cast iron, Steel, Wrought iron, depend chiefly upon the amount of carbon they respectively contain.
The other substances may generally be regarded as impurities.
Each, however, when present, plays an important part (see pp. 262, 263), and in some cases their presence is beneficial.
With regard to the influence of carbon, Dr. Percy makes the following remarks: -
"Of all the compounds of iron none are to be compared with those of carbon in practical importance. . . . When carbon is absent, or only present in very small quantity, we have wrought iron, which is comparatively soft, malleable, ductile, weldable, easily forgeable, and very tenacious, but not fusible except at temperatures rarely attainable in furnaces, and not susceptible of tempering like steel. When present in certain proportions, the limits of which cannot be exactly prescribed, we have the various kinds of steel, which are highly elastic, malleable, ductile, forgeable, weldable, and capable of receiving very different degrees of hardness by tempering, even so as to cut wrought iron with facility, and fusible in furnaces. And lastly, when present in greater proportion than in steel, we have cast iran, which is hard, comparatively brittle, and readily fusible, but not forgeable or weldable. The differences between these three well-known sorts of iron essentially depend upon differences in the proportion of carbon, though - as we shall learn hereafter - other elements may and do often concur in modifying, in a striking degree, the facilities of this wonderful metal." 1
It is very important for the proper understanding of this subject that the student should, from the outset, bear in mind the fact that Cast Iron contains a large percentage of carbon (about 2'0 to 6.0 per cent).
Steel contains a small percentage of carbon (about .15 to 1'8 per cent).
Wrought Iron, when perfectly pure, is quite free from carbon. Practically, however, it contains a small quantity - not exceeding 0.25 per cent.
Between these main classes there are several gradations, merging gradually one into the other, and to which no definite limits (as to percentage of carbon) can be assigned.
There are also several varieties of each class, varying according as the percentage of carbon varies within the limits of that class.
These minor distinctions will presently be referred to, but at present it will be only necessary to remember that the three great divisions - cast iron, steel, and wrought iron - differ chiefly according to the proportion of carbon they contain.