What is the Correct Heat Treatment for ASTM B111 C70600 Copper Nickel Tube?

No. ASTM B111 C70600 does not require post-weld heat treatment.

Unlike carbon steel or some stainless alloys, C70600 copper nickel tube retains its corrosion resistance and mechanical properties in the as-welded condition. Post-weld heat treatment can actually cause grain growth and reduce strength.

But there is one exception: If the tube was cold worked heavily (more than 30% reduction) and then welded, stress relief annealing at 300–400°C for 30 minutes may be beneficial.

Full annealing: 650–750°C, followed by cooling in air or water.

Annealing restores softness and ductility after cold drawing. The complete annealing cycle:

What annealing does:

Removes internal stress from cold drawing

Restores ductility for bending and expansion

Recrystallizes the grain structure

Does not change corrosion resistance

What annealing does not do:

Does not remove surface oxide (pickling is separate)

Does not change chemical composition

Does not improve poor quality material

Stress relief: 300–400°C for 30–60 minutes, then air cool.

Stress relief is used after cold bending (U-bends) or light cold working. It reduces residual stress without fully softening the tube.

When to use stress relief:

U-bent heat exchanger tubes

Tubes that will be vibrated in service (compressor coolers)

After repair welding on existing systems

When to skip stress relief:

Straight tubes for general service

Tubes with light bending (radius > 5 x OD)

Thin wall tubes (below 1.0mm) – risk of distortion

Overheating above 800°C causes grain growth and loss of strength.

Signs of overheating:

Tube becomes brittle (fails bend test)

Surface scale is thick and black

Grain structure visible to naked eye after etching

Reduced tensile strength below 310 MPa

How to avoid overheating:

Use controlled atmosphere furnace with thermocouples

Do not use open flame for annealing

Monitor temperature every 15 minutes

Do not exceed 750°C for full annealing

Standard delivery for ASTM B111: Annealed (O60) unless specified otherwise.

For heat exchanger tubes:

Use annealed (O60) for rolling into tube sheets

Use stress relieved for U-bend bundles

Do not use hard drawn – will crack during expansion

Acceptance criteria for annealed tubes:

Hardness: 40–65 HRB

Elongation: minimum 30%

Grain size: ASTM 0.015–0.045mm average

Surface: clean, no heavy scale

Always write the temper condition clearly.

Sample purchase order wording:

Tube to ASTM B111, alloy C70600, seamless, annealed (O60 temper). Full annealing at 650–750°C in controlled atmosphere furnace. Mill certificate to confirm temper and mechanical properties per ASTM B111.

Temper codes for C70600:

If you need stress relief after U-bending:

*After U-bending, tubes shall be stress relieved at 300–400°C for 30 minutes minimum. No scaling or oxidation is permitted.*

What is the difference between annealing and stress relief for C70600?

Annealing is done at 650–750°C and fully softens the tube with recrystallized grain structure. Stress relief is done at 300–400°C and reduces residual stress without changing grain structure or hardness significantly.

Can I anneal C70600 tube in a field workshop with a torch?

No. Torch heating is uneven and will cause localized overheating. Annealing requires a controlled atmosphere furnace. For field work, use stress relief at lower temperature if absolutely necessary, or buy pre-annealed tubes.

Does heat treatment affect corrosion resistance of C70600?

No. Corrosion resistance of C70600 comes from its chemical composition (9–11% nickel, 1.0–1.8% iron), not from heat treatment. Annealed, stress relieved, and cold drawn tubes all have similar seawater corrosion resistance.

Why do some suppliers ship C70600 in hard drawn condition instead of annealed?

Hard drawn tubes are cheaper to produce because annealing is skipped. Some suppliers do this to cut costs. However, hard drawn tubes crack during expansion and should be rejected. Always require O60 annealed temper.

How to check if a tube is properly annealed without a lab?

Perform a bend test. Cut a 150mm sample. Bend it 90 degrees around a mandrel with diameter 2x tube OD. Annealed tube bends without cracking. Hard drawn tube cracks or shows surface tears.

What is the cooling method after C70600 annealing – water or air?

Both are acceptable. Water quenching gives slightly higher strength. Air cooling is slower and gives maximum ductility. For most applications, either method is fine. The standard does not specify cooling method.

Can I use a vacuum furnace for C70600 annealing?

Yes, vacuum furnace is excellent because it prevents surface oxidation. If using a vacuum furnace, hold temperature at 650–700°C for 30–60 minutes. Cool in vacuum or with inert gas backfill.

Does C70600 work harden during cold drawing?

Yes, significantly. Cold drawing reduces cross section by 30–70% and increases tensile strength from 310 MPa to 450–550 MPa. Elongation drops from 30–45% to 8–15%. Annealing is required to restore ductility.

What grain size is normal for annealed C70600 tube?

Average grain diameter of 0.015–0.045mm (ASTM grain size 9–6). Fine grains give better strength. Coarse grains give better creep resistance at high temperature. For most heat exchanger service, medium grain (0.025mm) is best.

How many times can C70600 be re-annealed?

Many times. C70600 does not lose its ability to be annealed even after multiple cycles. However, repeated heating causes surface oxidation and gradual grain coarsening. In practice, 2–3 re-annealing cycles are acceptable.

Eddy current test (ECT) to ASTM E243

Hydrostatic test up to 20 MPa

PMI (XRF) for alloy verification on every heat

Mechanical property test (tensile, hardness, flattening, expansion)

Microscopic examination for grain structure

Flattening test per ASTM B111

Reverse flattening test for welded tubes (where applicable)

Grain size measurement per ASTM E112

Plastic end caps on both ends to prevent contamination

Individual polybag wrapping for each tube

Wooden crates or steel spools for export

Moisture-proof paper and desiccant for sea shipment

Label with heat number, size, temper, and quantity on each package

Color-coded bands for temper identification (green for annealed)

Anti-rust paper between tube layers

Extrusion press (1500T and 2500T)

Cold drawing machines (10 lines, OD range 4mm to 90mm)

Annealing furnaces (controlled atmosphere, 650–750°C, 3 units)

Straightening and cutting lines

U-bending machine for heat exchanger tubes

Eddy current testing machines (3 units)

PMI gun for in-process alloy verification

Hardness tester (Rockwell and Vickers)

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