Forging metal consists in raising it to a high temperature and hammering it into any form that may be required.
Repeated heating and reworking increases the strength of the iron up to a certain point; but overheating may ruin it (see below); the iron should therefore be brought to the required shape as quickly as possible.
The form given toforgings is also important; there should be no sudden change in the dimensions - angles should be avoided - the larger and thicker parts of a forging should gradually merge by curves into the smaller parts. Experiments have shown that the "continuity of the fibres near the surface should be as little interrupted as possible; in other words, that the fibres near the surface should lie in layers parallel to the surface." 1
If wrought iron be "burnt," i.e. raised to too high a temperature, its tensile strength and ductility are both seriously reduced. These qualities may, however, be to a great extent restored by carefully reheating and rerolling the iron.
This is well illustrated by the experiments made upon a specimen of bolt iron now before the writer - of which the results are shown below in a tabular form.
Tensile strength per square inch.
Original specimen as tested, 11/8 inch diameter
Fine fibrous fracture.
Overheated and fractured by slow tension .
Burnt leaden-looking fracture.
Reheated, rolled down to 1/4 inch diameter, and fractured by slow tension ....
Fine grey fibre.
Forging Steel requires still more care in order to avoid overheating. Each variety of steel differs as to the heat to which it can safely be raised. Shear Steel will stand a white heat. Blister Steel will stand a moderate heat. Cast Steel will stand a bright red heat.
"Welding is the process by which two pieces of metal are joined together with the aid of heat.
1 Rankine, Civil Engineering.
There are several different forms of weld.
It is not proposed here to describe the shape of the joint, or the process by which it is made, but merely to give an indication of the principles upon which the welding of metals depends. These are laid down in Dr. Percy's valuable work on Metallurgy, from which the information here given is extracted.
It will be sufficient to say that in welding generally the surfaces of the pieces to be joined having been shaped as required for the particular form of weld, are raised to a high temperature, and covered with a flux to prevent oxidation. They are then brought into intimate contact and well hammered, by which they are reduced to their original dimensions, the scale and flux are driven out, and the strength of the iron improved.