ERW (Electric Resistance Welded) stainless steel pipes start as a flat strip of stainless steel (coil). The strips are rolled and an electric current is passed through the edges of the strip. The resistance to this current generates enough heat to fuse the edges together. 

No filler metal is added. It’s just the steel bonding to itself, creating a clean, high-strength seam.

Seamless stainless steel pipes are crafted by heating a solid cylindrical billet to extreme temperatures until it reaches a malleable state. Once heated, a pointed mandrel is forced through the center of the billet to "pierce" it, effectively hollowing out the solid metal into a tube. This hollow shell is then stretched to refine its wall thickness and diameter, resulting in a continuous, single-piece structure without any weld seams.