Square and rectangular hollow sections are made on the same tube mill that makes round pipe: the difference is the roll set and the sizing pass. That single fact explains most of the questions buyers ask, including the one that matters most — why the brochure says 168 mm round but only 125 mm square.
It is not a derating for marketing. A round section is formed by bending strip round a circle, where the load is spread over the whole circumference. A square section is formed by bending the same strip round four corners, and the corners carry the load.
Why the square maximum is lower
Across the HG series published on this site the pattern is consistent: HG32 makes 45 mm round but 35×35 square, HG127 makes 127 mm round but 100×100 square, HG165 makes 168 mm round but 125×125 square. The square figure is what the roll set and the sizing stands can hold, not what the welder can join.
So buy from the square figure if your mix is mostly square. A mill chosen on its round maximum will disappoint on a square order, and the fix is a different machine, not a setting.
Strip width is where the arithmetic happens
Strip width for a round tube is roughly the circumference; for a square it is roughly four times the side (plus corner and weld allowance). That is why a heavy square needs a wider coil than the same weight of round tube, and why coil width — not just pipe size — belongs in the specification.
Rectangular sections sit between: the same perimeter logic applies, but the roll set has to handle two different side lengths, so the rectangular range is usually narrower than the square range on the same mill. On this site, for example, HG127 is published for 30×30–100×100 square and 20×40–120×80 rectangular — wider in one direction, shorter in the other.
Corner radius, weld position and tolerances
Square and rectangular hollow sections are bought to a standard, and the standard fixes the corner radius, the tolerance on side length, the squareness and the mass tolerance — not the mill. EN 10219 and ASTM A500 are the usual references for cold-formed hollow sections; which one applies, and which grade, is settled with the order and confirmed at quotation rather than assumed here.
The weld position is a separate decision: it can sit on a flat face or on a corner, and that changes how the section is used and how it is finished. Say which you need — it is a roll-set question, not a post-processing one.
What to put in the enquiry
Side lengths (square) or width × height (rectangular), wall thickness, corner radius if it matters, weld position, cut length and tolerance, material, and the standard you sell against. Plus the annual tonnage by size — a mix of many small sizes is a different purchase from one dominant size.
Where to go next
Related pages: HG series model comparison (PDF), square and rectangular hollow sections (PDF), steel pipe machinery, HF pipe welding machines, how to choose a welded tube mill
Frequently asked questions
Can the same mill make round and square tube?
Yes, with a different roll set and sizing pass. The maximum square size is lower than the maximum round diameter on the same machine — for example 127 mm round against 100×100 square on the HG127.
Why does a square section need wider strip than a round one?
Strip width follows the perimeter. A square section's perimeter is roughly four times its side, so a heavy square needs a wider coil than a round tube of similar weight.
What standard are hollow sections bought to?
EN 10219 and ASTM A500 are the usual references for cold-formed hollow sections. The applicable standard and grade are confirmed with the order at quotation.