A312 Welded pipes are manufactured by cold-forming a flat sheet of stainless steel into a cylindrical shape and then finally joint on the extremities on both ends electrically via a process know as electric fusion weld. The welding seam will be visible in the inner surface of the pipe itself.
A312 Seamless pipes on the other hand are manufactured from stainless steel billets. In simplified terms, the billet is heated and then perforated to create the seamless pipe. Seamless pipes as the name implies, refers to the absence of a welding seam.
Along the years, with advancement of technology, so has the methodologies for welding been improving with improvements made along the way to reduce the technical superiority of seamless pipes versus ERW pipes. In some cases, EFW pipes might provide a suitable substitute to seamless pipes in applications that have low to medium pressure and temperature. One such is the A358 Class 1 specification which employs a double weld using filler metal together with a 100% radiography test of their weld seam to detect any defects during production.