Joel Heisler, age 14, of Santa Rosa, Calif., for his question:
CAN YOU EXPLAIN WELDING?
Welding is any of about 40 different processes in which two or more pieces of metal are joined together by the application of heat, pressure or a combination of both. Most of the processes may be grouped into main categories: the pressure welding process and the nonpressure welding process. Brazing and soldering are other methods of joining metals.
Welding processes are generally classified according to the sources of heat and pressure used. The processes most commonly employed include gas welding, arc welding and resistance welding.
Gas welding is a nonpressure process using heat from a gas flame. In many applications, the flame is obtained from the combustion of oxygen and acetylene.
The oxyacetylene torch is applied directly to the metal edges to be joined and to a filler metal in wire or rod form, called the welding rod, which is melted to the joint.
Gas welding has the advantage of involving apparatus that is readily portable and independent of an electric power source. The surfaces to be welded and the welding rod are coated with a fusible material called flux, which prevents the formation of oxides or nitrides that would result in a defective weld.
Arc welding processes, which are the most important welding processes, particularly for joining steels, require a continuous supply of either direct or alternating electric current. They have several advantages over other welding methods. The welding speed is faster because of high heat concentration, which also tends to reduce distortion. Also, in certain methods of arc welding, fluxes may be eliminated.
Four different processes of arc welding are commonly used. Metal arc, carbon arc and atomic hydrogen arc welding have been used for some time. Inert gas arc was developed after World War II.
In resistance welding, a pressure process, heat is obtained from the resistance of metal to the flow of an electric current. Electrodes are clamped on either side of the parts to be welded, the parts are subjected to great pressure and a heavy current is briefly applied. Fusion results from the combined effect of heavy pressure and high heat.
Resistance welding is extensively employed in many fields of manufacturing and is particularly adaptable to repetitive welds made by automatic or semiautomatic machines.
In Thermit welding, heat is generated by the chemical reaction that results when a mixture of aluminum powder and iron oxide, known as Thermit, is ignited. The aluminum unites with the oxygen at high temperatures and the iron is released in the form of liquid steel, which serves as filler metal for the weld.
Thermit welding is employed chiefly in welding breaks or seams in heavy iron and steel sections. It is also used in the welding of railroad tracks.