Welding may be accomplished by various means, and the different systems employed may be divided into two main divisions :

The Autogenous Welding, by means of which the fusion or union of the metals is obtained direct, without the intermediary of any foreign material.

The Heterogeneous Welding, in which an additional foreign metal or alloy is introduced, the melting point of which is below that of the metals to be welded.

The autogenous welding is especially applicable to iron, steel and lead, while the heterogeneous welding is indispensable, or almost so, for zinc, brass, and, up to recently, also for aluminium, or, generally, for those metals which oxidise or volatilise at a temperature near that of the point of fusion.

In the autogenous welding of iron and steel two distinct processes may be distinguished: -

The Forging Process, which is based upon the property possessed by iron to become doughy or plastic at a temperature considerably below that of fusion.

The Welding by Fusion, which is carried out by means of compressed gases and blowpipes.

The other processes which have recourse to fusion of the metal, and are therefore also designated under the name of autogenous welding, are: -

The Alumino-Thermic Process.

The Electric Welding.

The Aluminium Welding.

The welding by means of compressed gases is generally effected by a mixture of various gases, amongst which oxygen plays the most important part, as being the gas of combustion.

Fig. 6. - Schematic Illustration. Low Pressure Oxy-Acetylene Welding Plant.

The various welding systems are generally recognised under the name of the combustible gas employed, as follows: -

The Acetylene welding.

The Blau-gas welding.

The Coal-gas welding.

The Hydrogen welding.

The Water-gas welding.

Most of the combustible gases employed are generated by automatic or non-automatic plants, and the gases may be used under low or high pressure.

A description of each welding system will be given below.