Round tube gets the glory and the tonnage, but a large share of stainless fabrication runs on square and rectangular hollow sections. Flat faces accept fixings without saddle cuts, frames stay square during welding, and the clean lines are the reason architects keep specifying them for balustrades and facades. The catch is that hollow sections are ordered by outside dimension and wall, a convention that trips up buyers who quote pipe sizes, and the details around corner radius, finish and grade choice decide whether the sections arrive ready to weld or ready to complain about. Here is how to specify them properly.

The size range and how to write it down

Standard production covers square tube from 10 x 10 mm up to 300 x 300 mm, rectangular tube from 10 x 20 mm to 120 x 180 mm, and round tube with outside diameters from 8 mm to 680 mm. Wall thickness spans 0.5 mm to 75 mm across the range, which is wide enough to cover both a 1.2 mm handrail and a 20 mm structural column. The ordering convention is outside dimension first, wall second, then length: a 60 x 60 x 3 tube means 60 mm outside square with a 3 mm wall. Resist the temptation to borrow pipe language. Nominal bore sizes such as DN50 describe an internal flow diameter and translate only loosely to hollow sections, so drawings that mix the two invite the wrong material. Where the project needs round sections at the heavy end of the range, the stainless steel pipe programme takes over, including large diameter thick wall pipe for pressure and structural duty.

Wall thickness and the corner question

Light gauge sections up to about 3 mm are roll formed from strip and welded, which gives tight dimensional control and the best surface quality, and this is the route for almost all decorative and architectural work. Heavier walls are hot finished, with slightly looser tolerances but better toughness and no cold work in the section. The corner is where the two routes differ most visibly: roll formed square tube carries a defined corner radius, and a section with too large a radius will not sit flush in a slotted frame machined for sharp corners. Tolerance on wall and outside dimension runs around plus or minus 1 percent on standard production, which is comfortable for fabrication but worth tightening in writing when sections interlock. One more practical point: in rectangular tube, very high aspect ratios thin the wall on the wide faces during forming, so a 120 x 20 section at the extreme of the range behaves differently in bending than the wall figure suggests.

Choosing the grade for the environment

Grade selection for hollow sections follows the same logic as sheet. 304 stainless steel is the general purpose default for indoor architecture, furniture frames, kitchen equipment and most structural framing, and it is stocked as welded and seamless 304 and 304L pipe alongside the square sections. Where chlorides enter the picture, coastal railings, pool structures, food washdown and marine hardware, move to 316 or 316L stainless steel and accept the price difference as corrosion insurance. For cost-driven interior decoration, 201 stainless steel is the established compromise, but it belongs indoors and away from salt. High temperature work shifts the choice to 310 and 310S pipe and sections, while welded structures in the 450 to 850 degree band where carbide precipitation lives are better served by stabilised grades such as the ASTM A213 and ASME SA312 grade 347 programme, including 347H seamless pipe for pressure parts. In aggressive chemical duty the austenitic ladder continues through 317LMN tube, and where chloride stress corrosion cracking threatens, duplex grades such as duplex 2205 seamless pipe, S31803 pipe and S32205 pipe deliver roughly double the yield strength of 304 with far better cracking resistance. For spring-like or high strength small sections, 302 stainless steel tube carries the highest useful strength of the common austenitic grades.

Surface finish: match the section to the sheet

Available finishes run from pickled industrial through bright annealed to polished and mirror. The mill finishes sit behind familiar names: 2B for a dull uniform grey, BA for a bright reflective skin, and No.1 for the hot rolled and annealed face. Decorative sections are usually polished after forming, with No.4 brushed, HL hairline and 6K or 8K mirror as the common choices, and the practical rule is to match the section finish to the sheet or plate it will sit beside, because a mirror tube against a 2B panel reads as a defect even when both are in tolerance. Polishing also has a functional side: a smooth interior and exterior surface resists product build-up in food and pharmaceutical frames. Welding changes everything locally, and the heat tint left by a TIG arc must be removed by pickling paste or mechanical cleaning followed by passivation, or the weld zone will rust first in service while the rest of the frame stays bright.

Testing that actually matters for hollow sections

Standard test routines include the squash test, the extended flattening test, water pressure testing and intergranular corrosion testing. The flattening tests are the ones hollow section buyers should care about: a ring or coupon is crushed until the walls meet, and the weld must survive without cracking, which is the most direct proof that the tube will bend and flare on the fabrication floor. Water pressure testing matters where the section will ever see internal pressure, and the intergranular corrosion test verifies that sensitisation was controlled during annealing, which is the failure mode that shows up as weld-line corrosion years later.

Ordering practicalities

Supply covers ASTM, JIS and GB standards with production capacity around 20,000 tons per month, minimum order quantities of one ton, and delivery about seven working days against deposit for stock sizes. Standard export packing bundles sections with end protection, and custom cutting, bending and punching can be added before shipment so the sections arrive as parts rather than stock lengths. Specify grade, standard, outside dimension, wall, length and finish on one line of the purchase order, and confirm whether seamless or welded construction is expected, because the two are priced and marked differently even when the outside dimensions match. The full range of flats, coils and sections sits in the stainless steel catalogue, and a drawing sent with the enquiry settles most of these questions faster than a paragraph of description.