"Mechanical" Steel Square Tube (A513) is an electric resistance welded tube made from low carbon steel. Wall thicknesses 18 gauge and lighter are made from cold rolled steel, heaiver than 18 gauge is produced from hot rolled steel. Easy to finish and form, Flash is removed from Outside Diameter (OD), ID is flash-in. Meets ASTM A513 Type 1 (HR) or Type 2 (CR). "Structural" Steel Square Tube (A500 Grade B) is cold formed, electric resistance welded tubing, produced in larger, heavier wall sizes than the ASTM A513 tube. It offers high strength to weight ratio and is easily welded, drilled, formed and punched. Good appearance, Outside Diameter flash is removed. ID is flash-in.