The manufacturing process for welded steel tubing requires elaborate precision production equipment. Flat steel strip is fed continuously from coils through forming rolls, which gradually shape it into a circular form. At the beginning of the operation, the top and bottom forming rolls are opposite in contour, but the final forming rolls are of the same contour top and bottom. From the forming rolls the butted tube is passed through the welding section of the machine. When the resistance process is used, the welding unit consists of a high-amperage current supply connected to a pair of copper alloy discs which serve as electrodes and make contact with the two edges of the formed strip. When the tubing passes under the electrodes, the current is automatically applied and travels from one electrode to another across the seam cleft, creating heat through the resistance offered to the flow of the current by the edge surfaces.
The moment the current is applied, the side rolls exert sufficient pressure to bring the edges together to form a welded joint. The material at the extreme edges of the butted joints, having been heated somewhat beyond the plastic state, is squeezed outward, and the union is made with unexposed metal which is in the plastic condition. No extra metal is added, since the weld is a complete union of the butted edges. The manufacturing process results in a weld free from inclusions, oxides, overheated structures and similar defects, and one which has the same composition as the base metal.
Historical Background
The first electric welded tubing is said to have been produced in 1896 in Cleveland, Ohio. The first commercial oxyacetylene-welded tubing in this country was believed to have been in 1912. The Johnson Process, which uses resistance welding for producing welded steel tubing, was patented in the early 1920s.
In 1929, the National Bureau of Standards completed an investigation of the properties of electric resistance welded steel tubing which clearly established the integrity of the product. The results of these tests were reported in Research Paper No. 161, Bureau of Standards, April, 1930.
Progress in this field is evidenced by the fact that welded tubing meets the rigid requirements and specifications of the automotive industry, those required for boilers, condensers and heat exchangers; and the requirements of the United States Army, Navy and other government departments.