Tubing made to API specifications uses seamless or electric -weld processes.Seamless pipe is defined as a wrought steel tubular product made without a welded seam.It is manufactured by hot-working steel or if necessary, by subsequently cold-finishing the hot-worked product to produce the desired shape.dimensions, and properties. Because of the nature of the manufacturing,the cross section of the tubing wall area may be slightly eccentric and the tubing slightly oval and not perfectly straight.
Electric-welded pipe has one longitudinal seam formed by electric-resistance or electric-induction welding without the addition of filler metal.The edges to be welded are pressed together mechnically, and the heat for welding is genenated by the resistance to flow of electric current.The weld seam of electric welded pipe is heated -treated after welding to a minimum temperature of 1000F or processed so that no untempered martensite remains.See API Spec.5CT for exceptions.
ThE processes are accceptable for most oil and gas services, but some prefer seamless tubulars for sour service because the electric-weld process may result in a slightly different grain structure near the weld .Such difference are usually eliminated if the electric-weld tubing is heat-treated by the quenched-and-tempered process,which is mandatory for API grade L80, C90, T95 and P110. Coupling usually are made of seamless tubular product of the same grade and type as the pipe.