Aerospace Tubing: Seamless and Welded Tube by Specification

Updated October 13, 2026

Seamless, welded and porthole tube

How the tube was made is part of the specification, and the three constructions are not interchangeable on a drawing that names one. The Aluminum Association's terms guide gives clean definitions that apply to any metal:

Some stainless hydraulic specs, including AMS 5557 for 321 and AMS 5567 for 304, are titled seamless or welded, so a cert to those specs may cover either construction. If your design or your customer needs seamless only, say so on the PO, because the spec alone will not stop a welded delivery. Porthole tube is covered with other profiles on our aluminum extrusions page.

The specifications that govern aerospace tube

Order tube to the specification on the drawing, by alloy, condition and construction. The table lists the documents we checked on SAE's site and the revision shown at the time.

Two notes. AMS 4944 and AMS 4945 both cover Ti-3Al-2.5V hydraulic tube but in different conditions, cold worked and stress relieved versus texture controlled with a 105 ksi yield, so they cannot be swapped without engineering approval. And tube specs for alloy 625 and other nickel alloys exist as well; see the nickel alloys reference. How these tubes are bent, welded, proof tested and cleaned as assemblies is covered on our tube and fluid systems service page, and not repeated here.

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Hydraulic tube versus structural tube

Hydraulic tube is bought to carry pressure, and structural tube is bought to carry load. The spec title usually tells you which one you are looking at, and a structural tube cert does not qualify a pressure line.

• Hydraulic specs such as AMS 4083, 5557, 5567, 4944 and 4945 carry "hydraulic" in their titles. They are the tube for fluid lines on vehicles, GSE and test stands, where internal soundness, wall control and cleanliness matter as much as strength.
• Structural tube, such as 4130 to AMS 6360, typically goes into welded frames, trusses, handling fixtures and GSE structure. Weldability and section properties lead the choice.
• General service stainless to ASTM A269 is common in ground and facility fluid systems. The tube and fluid systems page notes when extra testing applies for piping code use.

The mistake we see most often is a structural or general service tube pulled from stock for a hydraulic line because the alloy and size match. Check the spec on the cert against the spec on the drawing, not just the alloy.

Wall thickness and dimensional tolerance

Order tube by outside diameter and wall, and know whether the wall is nominal or minimum. ASTM A269, for example, covers nominal wall thickness tubing, so a wall called out as a minimum on a pressure calculation is a different requirement that has to be stated.

• Nominal versus minimum wall. If the stress analysis uses minimum wall, put minimum wall on the PO or choose a nominal wall whose lower tolerance still meets it.
• Ovality and eccentricity. A tube can meet average wall and still be thin on one side. Ask how wall is measured on the cert when it is critical.
• Bending takes wall away. Outer wall thinning in bends comes out of whatever margin the raw tube had. Bend limits are covered on the fluid systems page.
• Straight lengths or coils. State random or cut lengths and length tolerance. Hydraulic tube is usually supplied in straight lengths for bending.

Eddy current and hydrostatic testing

Tube specifications prove each length sound with a nondestructive test, a pressure test, or both, and the product spec decides which. ASTM A269, for instance, requires each tube to receive either the nondestructive electric test or the hydrostatic test. The cert should say which test was performed and to what standard.

For welded tube the weld seam is the region these tests exist to prove. For seamless titanium and nickel tube, ask whether the spec requires ultrasonic inspection in addition to eddy current. Our nondestructive testing reference compares the methods.

Cleanliness, end caps and storage

A tube's inside surface is the one you cannot inspect easily later, so protect it from the mill onward. Drawing lubricants, chips from cutting, and dirt picked up in storage all end up in the system the tube feeds.

• Caps on both ends. Require tube to ship with both ends capped or plugged, and to stay capped in storage and after cutting. See caps, plugs and protective covers.
• Clean cut ends. Deburr and blow out after every cut, and recap. A burr inside a hydraulic line becomes a contaminant.
• Interior condition on the cert. Where the spec or PO requires a cleanliness condition, the cert should state it.
• Oxygen and propellant service. Raw tube is not oxygen clean. Precision cleaning happens after bending, welding and testing; see our precision oxygen cleaning service.
• Racking. Store long tube supported along its length so it stays straight, and keep stainless and titanium away from carbon steel dust and tools.

Materials and the trade-offs

Defense contracts that carry the specialty metals clause restrict the melt source of stainless, titanium, nickel and alloy steel tube; aluminum is outside the definition. See our DFARS specialty metals page. Fittings for these tubes are on the fluid fittings page.

Ordering checklist for tubing

• Alloy, condition and specification with revision, for example AMS 4944 Ti-3Al-2.5V cold worked and stress relieved
• Seamless only, if required, even when the spec allows welded
• Outside diameter and wall, stated as nominal or minimum, with length and quantity
• Nondestructive and pressure tests required, and that the cert must name them
• Mill cert for the delivered heat or lot, with heat number on every length and every cut piece
• Both ends capped, interior condition, and packaging that keeps lengths straight
• Country of melt and DFARS clause for stainless, titanium, nickel and steel tube on defense work
• Certificate of conformance from the seller; see our C of C guide

Counterfeit and quality risks, and how to receive tube

The common problems are general service tube supplied against a hydraulic spec, welded tube where seamless was required, wrong titanium condition, and certs that do not match the heat marking. At receiving, compare the spec on the cert with the drawing, confirm construction, measure outside diameter and wall at both ends, check that caps are intact and the bore is clean, and verify alloy on unfamiliar sources. See our receiving inspection guide and the counterfeit avoidance process.

How we source aerospace tubing

We do not stock tube or publish prices. We source it for your order from mills, service centers and distributors with heat traceability, or from certified surplus when the paperwork fully supports it, flow down the spec, construction, test and cleanliness requirements, and check the certs before shipment.

Raw tube can go on the same purchase order as the bent and welded assemblies, fittings and hose it becomes part of. For finished lines and manifolds, use our tube and fluid systems service or the fluid systems quote form. Other stock forms are on the raw material category, and obsolete tube sizes are handled through our obsolete parts process.

Request aerospace tubing

Send the alloy, specification, construction, outside diameter, wall, lengths and quantity through the certified material request form. Stating seamless or welded and nominal or minimum wall up front is what decides whether a given lot of tube is usable. A person replies within one business day, and rush requests are prioritized.

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Part, quantity and date are enough to start. Drawings and the mutual NDA follow by reply, and controlled programs never send technical data through this site.

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Sources

• SAE AMS 4083: Aluminum Alloy Tubing, 6061-T6, Hydraulic
• SAE AMS 5557: Steel Tubing, 321, Seamless or Welded, Hydraulic
• SAE AMS 5567: Steel Tubing, 304, Seamless or Welded, Hydraulic
• SAE AMS 4944: Titanium Alloy Tubing, Seamless, Hydraulic, 3.0Al-2.5V
• SAE AMS 4945: Titanium Alloy Tubing, Seamless, Hydraulic, 3Al-2.5V, Texture Controlled
• SAE AMS 5589: Alloy Tubing, Seamless, Nickel Base 718
• SAE AMS 6360: Steel Tubing, Seamless, Chromium Molybdenum
• ASTM A269/A269M-25: Seamless and Welded Austenitic Stainless Steel Tubing for General Service
• ASTM E426: Eddy Current Examination of Seamless and Welded Tubular Products
• Aluminum Association GAG Guidance Document 001: Terms and Definitions