Welded pipe starts out as a long, coiled ribbon of steel called a skelp. The skelp is cut to the desired length, resulting in a flat rectangular sheet. The width of that sheet's shorter ends will become the pipe's outside circumference, a value that can be used to calculate its eventual outside diameter.
Welded pipe is made by forming flat products (strip, sheet or plate) into the desired shape, in this case, normally round. Once the desired shape has been achieved, a high energy source is used to melt the metal locally at the weld joint. It is squeezed together and allowed to solidify, forming a weld bead. The high energy source may be an electric arc, a plasma arc, a laser beam, or even an electron beam. The as-welded weld bead is typically somewhat thicker than the adjacent base metal and needs to be modified to match the base metal thickness, and to correct the undesirable physical, chemical and corrosion resistance attributes of the weld.
At the start of the welded pipe manufacturing process so called slabs or billets are cast in a steel mill. Dependent on the external dimensions and wall thickness there are different methods of producing welded pipes and pipes: Electric Resistance Welding (ERW), Fusion Welding (EFW) and Double Submerged Arc Welding (DSAW). In the ERW and EFW manufacturing process welded pipes are formed by hot or cold rolling plate and welding the seam. In order to keep the outside (O.D.) surface of a welded pipe smooth and uniform a cutting tool called scarfing blade is used to remove the weld flash. Scarfing from the inside (I.D.) welded flash is also possible. With a heat treating at the end of the manufacturing process the weld zone of the steel pipe can be made less visible. Due to the weld seam lower operating pressures are stated in accordance with ASME compared to seamless pipes. In general welded pipes possess tighter dimensional tolerances than seamless pipes and are less costly if produced in same quantities.
