Hot-flame, or gas, welding and cutting has been a practical process for so long that there is no very clear record as to just when it first came into commercial use, but the development and exploitation of apparatus for the generation and use of gases for welding during the past ten or fifteen years has given an added impetus to the art and it is now used in many ways which were not originally contemplated. The process of gas welding consists merely in joining metals by fusing them together at the point desired through the use of a high temperature gas flame as the source of heat. Various gases and combinations of gases are used and several types of apparatus are now on the market for the purpose. Cutting by the flame is a later development than welding and is both rapid and economical.
There are two methods of welding in general use, the "Autogenous" and the "Heterogeneous", the names indicating clearly the main difference. The word "autogenous" signifies that the weld is made by the fusion or junction of the articles themselves, without the use of outside filling material to complete the joint. The word "heterogeneous" means a mixture, and signifies that the weld is made by fusing-in some sort of additional filling material, instead of depending entirely upon the metal of the pieces themselves. It is obvious that, if the filling material is of a metal different from the pieces welded, we still have heterogeneous welding; so this name can be correctly applied to brazing or soldering. Custom makes laws, it is said, and custom limits the term "welding" to cases where the same metal is used for a filler as is used in the article welded, and that is the way the term used here. The term "autogenous" has unfortunately come to be applied to all forms of gas welding, and some confusion has resulted because the statement is made that neither flux nor hammering is necessary with that process. The truth is that fluxes are very beneficial when welding some metals and hammering helps the strength of welds in heavy pieces because it tends to change the structure of the metal from a crystalline to a fibrous nature. Further reference will be made to this later, when discussing methods of welding.
The gases originally used for welding were probably oxygen and hydrogen, and efforts to liquefy them were made about one hundred years ago in order to simplify the means of storing them. It took about fifty years to develop a good process for doing this and resulted in the development of one of the principal present-day methods of producing oxygen. Most of the welding today is done with systems using oxygen in combination with another gas; and the processes take their names from the gases used. The leading processes are known as the oxy-acetylene, oxy-hydrogen (or oxy-hydric), oxy-pintsch gas, blau-gas, water-gas and coal-gas welding processes, the oxy-pintsch-gas process being the latest development. All of these processes depend upon the use of compressed gases, usually stored in strong cylinders and mixed in a burner or torch as used, and may be used for either cutting or welding.
The principal advantages claimed for gas welding are: the simplicity of the process; low first cost of the apparatus; wide range of applicability; light weight of the parts; ease of portability, if necessary; high temperature of the flame; and flexibility of the process for heating purposes. The limitations of the process are: the danger from using an exposed flame; the liability of explosion of the gas tanks and generators; oxidation or carbonization of the weld by the flame; crystallization and cracking of the weld when cooling; and high cost of operation as compared with electric welding. The danger from explosions is being reduced gradually by improved apparatus and the restrictions imposed by the Board of Fire Underwriters.
In manufacturing plants and large repair shops, stationary plants for the generation of acetylene are generally used and the oxygen is purchased from companies making it on a large scale, but some of the larger concerns also make their own oxygen. The same rule applies to the gases used for other processes of welding than the oxy-acetylene. Small portable outfits are also made for moving about shops which do comparatively little welding, tanks of gas of the proper kind being mounted on substantial trucks. In a few cases small gas generators are mounted on the trucks, but these offer but little advantage over the tanks and are more expensive and harder to handle.