Seamless tubes are as defined - they do not have a welded seam. The tubing is manufactured through an extrusion process where the tube is drawn from a solid stainless steel billet and extruded into a hollow form.Seamless steel pipe is manufactured by the round steel billet, through hot rolling, cold rolling or cold drawn processes, no seam on the pipe body. Welded steel pipe is manufactured by flat steel plate, with welding seam on the pipe body. The only significant difference between 309 and 309S is the carbon content. Alloy 309S has a much less carbon composition which minimizes carbide precipitation and improves weldability. Type 310S Stainless Steel is identical to Type 310 except for a lower carbon content that minimizes carbide precipitation and improves weldability. They are essentially nonmagnetic as annealed and become lightly magnetic when cold worked.