The transformation of steel strip into Hollow Structural Sections (HSS) is the result of a series of operations including forming, welding and sizing. Currently three methods are being used for the manufacture of HSS.
1. Transforming the tube into the box column (Electric Resistance Welding (ERW) Process
In the tube mill, flat steel strip (1) is formed continuously around its longitudinal axis to produce a round tube. This is done by moving the strip through a progressive set of rolls (2-6). The strip edges (7) are heated by either high frequency induction or contact welding and then forged together by weld rolls to create a continuous longitudinal weld without the addition of filler metal. The weld seam (8) is then cooled and processed through a set of sizing shaping rolls which cold-form it into a circular (9), square (10) or rectangular (11) section.
tube to box tube to box
direct HSS production
 2. Straight-seam welding process (Form-Square Weld-Square (ERW) Process)
In the weld mill, driven forming dies progressively shape the flat strip (1) by forming the top two corners (2) of the square or rectangular tube in the initial forming station. Subsequent stations form the bottom two corners (3) of the shape. No cold working of the sides of the shape is performed, and the shape’s seam is welded by high-frequency contacts when the tube is near its final shape and size. The welded tube (4) is cooled and then driven through a series of sizing stations which qualifies the tube’s final dimensions.
3. Two-Piece Production (Submerge Arc Weld (SAW) Process)
Two identical pieces of flat strip (1) are placed in a press brake and formed into two identical halves (2) of a finished tube size. A backup bar is tack welded to each leg of one of the half-sections (3). The two half-sections are fitted together toe-to-toe (4) and welded by the submerged arc process to complete the square or rectangular section (5).
  two piece method