Scope of ASTM B68 and the Aerospace Grade
ASTM B68 describes seamless copper tube produced in the bright annealed temper, a condition in which the tube is annealed and cooled under a protective atmosphere so that the bore and the outside surface stay clean and oxide free. Bright annealed tube is used in equipment where cleanliness, uniform grain size and reliable expansion or bending behaviour matter, and aerospace hardware is a typical example. ASME SB68 is the matching boiler and pressure vessel code edition.
UNS C11000, electrolytic tough pitch copper, contains a minimum of 99.90 % copper plus silver and is the standard high-conductivity grade. It carries roughly 100 % IACS electrical conductivity, which is why hollow round tube in this grade is used for buswork, waveguides and thermal distribution in aircraft and spacecraft systems where weight and conductivity are both at a premium.
Chemical Composition
| Element | Requirement |
|---|---|
| Copper plus silver | 99.90 % min |
| Oxygen | 0.02 - 0.04 % typical, controlled |
| Tin, zinc, lead, iron | Each 0.005 % max in the high purity grades |
| Other elements, total | 0.10 % max |
Oxygen-free grades such as C10100 and C10200 are specified when the tube has to withstand hydrogen-bearing atmospheres or when welding and brazing are involved, because oxygen in the C11000 matrix can react with hydrogen at high temperature and cause embrittlement. Where a welded assembly is unavoidable, a deoxidised grade such as C12200 should be evaluated first.
Physical, Electrical and Mechanical Properties
| Property | Value |
|---|---|
| Density | 8.94 g/cm³ |
| Melting point | 1,083 °C |
| Electrical conductivity | 100 % IACS nominal |
| Thermal conductivity | 394 W/(m·K) |
| Coefficient of thermal expansion | 17.0 um/(m.K), 20 - 300 °C |
| Condition | Tensile strength | Yield strength | Elongation | Hardness |
|---|---|---|---|---|
| Annealed (soft) | 220 - 275 MPa | about 180 MPa | 45 - 50 % | 35 - 45 HRB |
| Cold worked (hard) | 275 - 345 MPa | 250 - 300 MPa | 5 - 10 % | 60 - 80 HRB |
Annealed tube at 220 - 275 MPa tensile is easy to bend and flare, which suits fuel, hydraulic and instrument lines routed through tight airframe spaces. Cold worked tube at 275 - 345 MPa gives greater stiffness and crush resistance but only 5 - 10 % elongation, so bending must be carried out with a controlled radius and a soft mandrel or after a local anneal.
Dimensional Range of Hollow Round Tube
| Parameter | Range |
|---|---|
| Outside diameter | 5 mm to 100 mm |
| Wall thickness | 0.5 mm to 10 mm |
| Length | Straight lengths and cut lengths to drawing |
| Temper | Bright annealed as standard, light drawn or hard drawn on request |
Wall thickness is selected from pressure, stiffness and weight considerations. Because copper is roughly three times denser than aluminium, aerospace designers use the thinnest wall that satisfies stiffness and pressure requirements and support the tube at close intervals to prevent vibration fatigue.
Aerospace and High-Technology Applications
Electrical busbars, current return paths and bonding straps in avionics bays.
Thermal control loops and heat pipes for avionics cooling and instrument thermal management.
Waveguide and RF cavity components that need smooth, high-conductivity internal surfaces.
Antenna feeders, lightning protection conductors and earthing hardware.
Fuel, hydraulic, pneumatic and instrument lines in utility and ground support systems.
Vacuum and laboratory equipment where a clean, non-magnetic bore is required.
Copper is also chosen because it is non-magnetic and does not spark, both of which matter around sensitive instrumentation and fuel systems. Its resistance to atmospheric corrosion keeps surfaces stable over long service intervals, and its high thermal conductivity spreads heat away from concentrated sources quickly.
Processing, Joining and Handling Guidance
Bright annealed tube should be handled with clean gloves, kept in its protective wrapping until installation and never stored in contact with carbon steel, which can transfer iron onto the surface and initiate corrosion. Tube is bent on a mandrel bender or with a filled and capped bore for tight radii, and flaring is carried out with dies matched to the annealed temper.
Joining methods include soft soldering, silver brazing, orbital welding with deoxidised copper filler and mechanical fittings with elastomeric seals. Purging with dry nitrogen protects the bore during brazing and welding, and any flux residue must be removed by flushing because residual flux attacks copper in the presence of moisture. Annealing is carried out at about 400 °C to 650 °C in a protective atmosphere when local softening is required, and a stress relief is applied to bent sections where vibration is expected.
Inspection and Acceptance Tests
Typical inspection for aerospace work includes dimensional verification, wall thickness measurement, tensile testing, grain size assessment, eddy current examination of the bore wall, and a cleanliness check on the internal surface. Chemical certification by heat number, a mill test certificate to EN 10204 3.1 and, where specified, positive material identification complete the documentation package. Surface condition is checked visually against a defined standard because scratches and iron contamination, not bulk properties, are the usual cause of rejection in hydraulic and fuel line applications.
FAQ
Q: Why is bright annealed tube preferred for aerospace use?
Bright annealing removes internal stresses while keeping the bore clean and oxide free. That combination gives predictable bending and flaring behaviour and avoids loose scale that could later contaminate a hydraulic, fuel or instrument system.
Q: Can C11000 be welded?
It can be welded with matching deoxidised filler metal and an inert shield, but the oxygen in the grade makes it more sensitive than C10200 or C12200. Welded assemblies are normally specified in a deoxidised or oxygen-free grade.
Q: Is copper tube non-magnetic?
Yes. Copper is effectively non-magnetic, which makes it suitable for use close to compass systems, magnetometers and other sensitive instruments.
Q: How should tube be protected before installation?
Keep the original wrapping in place, store indoors in a dry area, support the tube along its length so that it cannot sag, and avoid contact with carbon steel racks or offcuts that could transfer iron.
Q: What wall thickness is typical for instrument lines?
Instrument and hydraulic lines generally use the thinnest practical wall for the working pressure, commonly in the 0.5 mm to 1.5 mm range, with support spacing set by vibration and stiffness analysis rather than by the pressure rating alone.
Q: Which temper should be ordered for a bent tube assembly?
Annealed tube is used where tight bends and flared ends are needed. Light drawn or hard drawn tube is chosen for straight runs that must resist crushing and vibration, with bending carried out after a controlled local anneal.
