Correct Heat Treatment for ASTM B111 C70600 Copper-Nickel Tube
C70600 (90-10 copper-nickel, UNS C70600) is a seawater-resistant alloy widely used for heat exchanger and condenser tubes. Heat treatment of this alloy has three purposes - softening after cold work (annealing), residual-stress reduction (stress relief), and avoiding damage from overheating. This guide explains the correct practice and the common mistakes.
Does C70600 Need Post-Weld Heat Treatment? No.
C70600 does not require post-weld heat treatment. Unlike carbon steel or some stainless alloys, it retains its corrosion resistance and mechanical properties in the as-welded condition; PWHT can actually cause grain growth and reduce strength. One exception: if the tube was heavily cold worked (more than about 30% reduction) and then welded, a stress-relief anneal at 300–400°C for 30 minutes may be beneficial.
| Condition | Heat treatment needed? |
|---|---|
| As-welded, light cold work | No |
| As-welded, heavy cold work (>30%) | Optional stress relief |
| Annealed tube (O60) before welding | No |
| Hard-drawn tube before welding | No - but welding hard-drawn is not recommended |
| U-bend forming | Stress relief recommended after bending |
Full Annealing: 650–750°C
Full annealing restores softness and ductility after cold drawing, recrystallizes the grain structure and removes internal stress. Recommended cycle:
| Step | Temperature | Time | Cooling |
|---|---|---|---|
| Heat to temperature | 650–750°C | 30–60 min per 25 mm thickness | - |
| Soak | Hold at temperature | 30–60 min | - |
| Cool | Below 300°C | As needed | Air or water quench |
What annealing does: removes internal stress from cold drawing, restores ductility for bending and expansion, recrystallizes the grain structure, and does not change corrosion resistance (which comes from the 9–11% Ni and 1.0–1.8% Fe in the composition). What it does not do: it does not remove surface oxide (pickling is a separate step), does not change chemical composition, and cannot improve poor-quality material.
Stress-Relief Annealing: 300–400°C
Stress relief is used after cold bending (U-bends) or light cold working. It reduces residual stress without fully softening the tube. Typical practice: 300–400°C for 30–60 minutes, then air cool. Use stress relief for U-bent heat-exchanger tubes, tubes subject to vibration in service (compressor coolers) and after repair welding; it is usually unnecessary for straight tubes, light bends (radius > 5 × OD) or thin-wall tubes below about 1.0 mm wall (risk of distortion).
What Happens if C70600 Is Overheated?
| Temperature | Effect |
|---|---|
| 750°C | Normal annealing - acceptable |
| 800°C | Grain growth begins |
| 850°C | Excessive grain growth - reduced strength |
| 900°C | Surface oxidation, potential cracking |
| Above ~950°C | Melting begins (solidus about 1080–1100°C) |
Signs of overheating: the tube becomes brittle (fails a bend test), surface scale is thick and black, grain structure is visible to the naked eye after etching, or tensile strength falls below about 310 MPa. To avoid overheating, use a controlled-atmosphere furnace with thermocouples, do not use an open flame for annealing, and monitor the temperature during the cycle.
Effect on Mechanical Properties
| Condition | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness (HRB) |
|---|---|---|---|---|
| Hard drawn (H80) | 400–550 | 350–450 | 8–15 | 80–95 |
| Stress relieved | 350–450 | 250–350 | 15–25 | 65–80 |
| Annealed (O60) - standard | 310–380 | 105–150 | 30–45 | 40–65 |
| Over-annealed (>800°C) | Below 310 | Below 100 | 45–60 | Below 40 |
Specifying Heat Treatment in the Purchase Order
Always state the temper condition clearly. Example wording: "Tube to ASTM B111, alloy C70600, seamless, annealed (O60 temper). Full annealing at 650–750°C in a controlled-atmosphere furnace. Mill certificate to confirm temper and mechanical properties per ASTM B111." Common temper codes: O60 (soft annealed - standard), O61 (stress relieved - after cold forming), O30 (lightly cold drawn - higher strength), H80 (hard drawn - not for heat exchangers). For heat exchanger tubes, use annealed (O60) for rolling into tube sheets and stress-relieved for U-bend bundles; do not use hard-drawn tube where expansion is required.
Correct heat treatment of C70600 tube means: no PWHT after normal welding; full annealing at 650–750°C to restore ductility after cold work; stress relief at 300–400°C after heavy bending or cold work; strict avoidance of temperatures above 800°C; and specification of the O60 annealed temper for heat-exchanger service. Corrosion resistance comes from composition, not heat treatment, so annealed, stress-relieved and drawn tubes have similar seawater performance.
FAQ
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.
Testing methods
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
Packaging standards
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
Production equipment
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)
Our Copper Product Range
| Product form | Common alloys | Size range |
|---|---|---|
| Tube (seamless) | C10100, C10200, C12200, C70600, C71500, C44300, C68700 | OD 4mm–90mm, WT 0.3mm–5.0mm |
| Pipe (seamless) | C12200, C70600, C71500 | OD 10mm–108mm, WT 1.0mm–8.0mm |
| Rod / bar (round, hex, square) | C10100, C10200, C11000, C36000, C46400, C63000 | Dia 3mm–100mm |
| Wire | C10100, C10200, C11000, C16200, C19400 | Dia 0.1mm–8.0mm |
| Strip / coil | C10100, C10200, C11000, C19400, C26000, C26800, C52100 | Thk 0.1mm–3.0mm, width up to 400mm |
| Plate / sheet | C10100, C10200, C11000, C12200, C70600, C71500, C46400 | Thk 0.5mm–50mm, width up to 1000mm |
