Seamless square and rectangular tubes are hollow profiles with no weld seams along their circumference. They are not formed by bending steel strip; the process begins with a solid round billet. After heating, the billet is pierced using a cross-rolling piercing mill to create a thick-walled round hollow shell. This shell is hot-rolled into a round mother tube and then subjected to multi-pass cold drawing through shaped dies, gradually transforming the circular cross-section into the square or rectangular shape specified in your order. Because the metal is formed by die compression rather than folding flat strip steel, there are no seams at the corners, and the tube wall remains continuous across the entire cross-section.
Since there are no weld seams prone to cracking, both the inner and outer surfaces of seamless profiles can undergo turning, boring, tapping, and honing without exposing a "soft line" on the machined surface. For hydraulic and pneumatic components, this is often the primary reason buyers specify seamless tubing over welded tubing, and it is why we stock phosphated and honed tubes.
Cold drawing ensures uniform strain hardening across the entire profile. In welded tubes, the weld seam and its heat-affected zone behave differently from the base metal under fatigue conditions, making welded profiles unsuitable for parts subject to cyclic loading; seamless square and rectangular tubes do not suffer from this discontinuity.
Standard cold-drawn products offer dimensional tolerances of ±0.3 mm and wall thickness tolerances of ±8%, while precision grades can achieve ±0.1 mm and ±5%. Straightness is typically 1.0 mm/m, with precision grades reaching 0.5 mm/m. Delivered straight, they allow machine shops to skip a straightening setup step.
Cold drawing increases yield strength without the addition of alloying elements; consequently, a "20#" grade seamless square amd rectangular tube can often replace a thicker welded tube while offering the same load-bearing capacity per unit length.
Seamless seamless square an d rectangular tubes can undergo flaring, upsetting, flattening, bending, and expanding without the risk of a weld line acting as a crack initiation point. This is crucial when parts require cold forming after delivery rather than direct machining.
Cold-drawn tubes arrive in a bright, pickled state, ensuring uniform thickness for chrome plating, phosphating, and painting. Welded tubes—even those with planed seams—often require additional pre-treatment before plating, and the weld seam may still show through thin coating layers.
Seamless profiles are machined into cylinder barrels, piston rod spacers, and valve blocks, as neither the inner nor outer surfaces can contain a weld seam. Such orders often specify phosphated tubes, honed tubes, or ready-to-machine stock.
Seamless square and rectangular tube are used for rollers, guide rails, and sliding carriages in printing, packaging, and textile machinery; a straightness of 0.5 mm/m allows carriages to operate without the need for shims.
Cold-drawn seamless tubes are commonly used for seat frames, brackets, and agricultural implement frames. Materials typically include 20# and 45# steel; since these parts are often welded to castings at one end, the tube body itself must not introduce a second weld seam.
Guide rails and curtain wall structural members require straightness levels that are difficult to consistently achieve with roll-formed tubes. Furthermore, as exposed surfaces often undergo anodizing or powder coating, a smooth, seamless surface is just as critical as dimensional tolerance.
Seamless square and rectangular tubes are used for frames requiring brazing or welded-in joints, where the tubing remains visible in the finished product.
For scenarios requiring transverse drilling or the use of the tube as a machined bushing or spacer, seamless square/rectangular tubing prevents drill bits from drifting into and piercing the weld seam.
For every heat number, the steel grade, chemical composition, and tensile test results are verified against the quality certificate; the outer diameter and wall thickness at both ends of the round parent tubes are measured. Tubes that do not match the certificate are not processed on the cold-drawing line.
After cold drawing, samples of seamless square and rectangular tube are taken from the beginning, middle, and end of each batch to measure dimensions, wall thickness, corner radii, and squareness. Since cold drawing work-hardens the metal, the wall thickness reduction per pass and the annealing parameters are recorded for each batch.
Eddy current or ultrasonic testing is performed on parent tubes and finished profiles as required to detect folds, hairline cracks, and internal defects; round parent tubes may also undergo hydrostatic testing prior to cold drawing. This inspection offers an advantage over welded tubing, as it examines the entire wall structure rather than just the weld line.
Tensile strength, yield strength, and elongation are verified according to the steel grade; flattening or bending tests are conducted on samples to ensure the cross-section does not crack. Before packaging, each bundle is inspected for length, straightness, twist, surface condition, and markings. Inspection records are archived by heat number, ensuring full traceability for specific projects.
We have over eight years of experience in the steel trade in Daqiuzhuang, serving markets in Southeast Asia and the Middle East; consequently, maritime logistics, packaging suitable for hot and humid shipping routes, and English documentation are standard procedures for us.
There is no minimum order quantity (MOQ) for stock specifications, and trial orders can be scheduled and shipped alongside mass production orders. Responses regarding processing drawings and cutting lists are provided within approximately two days, and customer-provided drawings can still be modified before production begins. We do not send installation teams abroad, but we can assist you in contacting local installation teams and provide the necessary assembly drawings.
A: In accordance with GB/T 3094-2012, we offer Ordinary, High, and Precision grades—with external dimension tolerances of ±0.5 mm, ±0.3 mm, and ±0.1 mm, respectively.
A: Specify seamless square and rectangular tube if the part requires machining, tapping, honing, pressure testing, or must withstand fatigue loads. For static structures like greenhouses, fencing, or shelving, cold-formed welded tubing performs the same function at a lower cost—we will be upfront about this and won't steer you toward more expensive options unnecessarily.
A: Cold-drawn square tubes range from 15×15 mm to 200×200 mm, and rectangular tubes from 20×10 mm to 300×200 mm, with wall thicknesses from 1.0 mm to 12 mm. Larger sizes up to 400×400 mm can be supplied via hot-rolling upon request; this involves longer lead times and minimum order quantities per size.
A: Yes, we can supply products compliant with EN 10210-1/-2, EN 10305-1, ASTM A500, ASTM A519, and JIS G3444. Please provide the standard number and tolerance grade when requesting a quote, and we will price accordingly.
A: Each shipment comes with a Quality Certificate detailing the heat number, chemical composition, and mechanical properties, along with our own dimensional and visual inspection records. If your project requires third-party inspection or specific non-destructive testing (NDT) methods, please specify the standard and the inspection agency when confirming your order; we will arrange the inspection based on the corresponding sampling plan.
A: Machining, phosphating, honing, and chrome plating can be arranged upon request. If the parts feature internal bores or threads, please provide drawings of the finished components rather than just the tube dimensions; this ensures we determine the correct wall thickness from the start.
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