Welding Compounds.—Elsewhere formulas are given of the compounds most desirable to use. It is obvious that the application of these substances on the heated surfaces, is not only to facilitate the heating, but to prepare the articles in such a manner that they will more readily adhere to each other.
Oxidation.—Oxidation is the thing to guard against in welding. The moment a piece of metal, heated to whiteness, is exposed, the air coats it with a film which is called an oxide. To remove this the welding compound is applied.
The next office of the substance thus applied, is to serve as a medium for keeping the welding parts in a liquid condition as long as possible, and thus facilitate the unity of the joined elements.
When the hammer beats the heated metals an additional increment of heat is imparted to the weld, due to the forcing together of the molecules of the iron, so that these two agencies, namely, the compound and the mechanical friction, act together to unite the particles of the metal.
Soldering.—Here another principle is involved, namely, the use of an intermediate material between two parts which are to be united. The surfaces to be brought together must be thoroughly cleaned, using such agents as will prevent the formation of oxides.
The parts to be united may be of the same, or of[p. 118] different materials, and it is in this particular that the workman must be able to make a choice of the solder most available, and whether hard or soft.
Soft Solder.—A soft solder is usually employed where lead, tin, or alloys of lead, tin and bismuth are to be soldered. These solders are all fusible at a low temperature, and they do not, as a result, have great strength.
Bismuth is a metal which lowers the fusing point of any alloy of which it forms a part, while lead makes the solder less fusible.
Hard Solder.—These are so distinguished because they require a temperature above the low red to fuse them. The metals which are alloyed for this purpose are copper, silver, brass, zinc and tin. Various alloys are thus made which require a high temperature to flux properly, and these are the ones to use in joining steel to steel, the parts to be united requiring an intense furnace heat.