Grade Background and Scope
ASTM B111/B111M is the standard specification for seamless copper and copper-alloy condenser tubes, evaporator tubes, heat exchanger tubes and ferrule stock, and it covers tubes up to 3 1/8 in. (80 mm) inclusive in outside diameter for use in surface condensers, evaporators and heat exchangers. UNS C71640 sits within that specification as the copper-nickel-iron-manganese grade: nominally 30% nickel with about 2% iron and 2% manganese, which is why it is often written CuNi30Fe2Mn2 or described as a 30/2/2 cupronickel.
The extra iron and manganese compared with the standard 70/30 grade raise the alloy's tolerance to high seawater velocity and to abrasive conditions. That makes C71640 the choice where cooling water carries sand, where flow is high, or where the design demands the longest possible interval between tube bundle replacements.
Chemical Composition
| Element | Composition (%) | Function |
|---|---|---|
| Cu | Remainder | Base metal |
| Ni | 29.0-32.0 | Strength and marine corrosion resistance |
| Fe | 1.7-2.3 | High-velocity seawater film formation |
| Mn | 1.5-2.5 | Impingement resistance, structural stability |
| Zn | 1.0 max | Residual control |
| Pb | 0.05 max | Impurity limit |
Equivalent designations used in international tenders include CuNi30Fe2Mn2 in the ISO and German naming systems, CW353H in the EN 12451 series, CN108 in the British Standard tube series, C7164 in JIS H3300 and BFe30-2-2 in the Chinese GB system for heat exchanger tube.
Mechanical Properties and Delivered Tempers
| Temper code | Condition | Tensile strength min | Yield strength min |
|---|---|---|---|
| O61 | Annealed | 435 MPa (63 ksi) | 170 MPa (25 ksi) |
| HR50 | Drawn and stress relieved | 560 MPa (81 ksi) | 400 MPa (58 ksi) |
These minimum values, taken from the specification, are the design basis for pressure and mechanical calculations. Tubes are generally supplied annealed for roller expansion and U-bending, and drawn and stress relieved where higher strength is required for a supported span or a specific mechanical duty. Physical data used in design are a density of 8.94 g/cm³ and a thermal conductivity of approximately 29 W/m.K, with the alloy remaining ductile at cryogenic temperatures.
Service Applications
Surface condensers and heat exchangers in power generation and marine propulsion.
Condenser and cooler tubes exposed to high-velocity seawater carrying suspended solids.
Desalination evaporators, brine heaters and reject water systems.
Ship equipment parts and submarine piping subject to chloride attack and pressure.
Chemical plant condensers, evaporator tubes and heat transfer equipment handling aggressive media.
Cooling circuits on offshore platforms where maintenance access is limited.
Why the High Iron and Manganese Content Matters
Iron and manganese are the film-forming elements in copper-nickel alloys. In C71640 the ranges are much wider and higher than in the 70/30 grade: iron at 1.7% to 2.3% and manganese at 1.5% to 2.5%. The resulting surface layer is thicker, more uniform and quicker to repair after mechanical damage, which is why the grade sustains higher seawater velocities and tolerates sand-bearing water better than a standard 70/30 tube. Impingement attack, the most common failure mode of condenser tubes, is substantially suppressed.
The trade-off is a slightly lower thermal conductivity and a higher material cost than the 70/30 grade, so C71640 is normally reserved for the more severe duties or for tubes that are difficult and expensive to replace. In fresh water or in clean low-velocity seawater, a standard copper-nickel grade will usually be the more economical answer.
Fabrication, Heat Treatment and Inspection
C71640 is cold drawn and annealed, and it can be roller expanded, bent and U-formed in the annealed condition. Welding is carried out with matching copper-nickel filler metal under inert gas shielding, with clean, dry joint faces; carbon, sulphur and lead contamination must be avoided because they cause hot cracking. Stress relief is applied to drawn tube to the HR50 condition where the specification calls for it.
Inspection follows ASTM B111 and the purchase order: chemical analysis, tensile test, flattening or expansion test, eddy-current examination of each tube, hydrostatic or pneumatic pressure test, dimensional and straightness checks, and visual examination of the bore. Every bundle is labelled with the heat number so that the mill certificate can be reconciled at goods-in inspection, and tubes are cased with end caps for export shipment.
FAQ
Q: How does C71640 differ from C71500?
Both contain about 30% nickel, but C71640 has higher iron (1.7% to 2.3%) and manganese (1.5% to 2.5%) than C71500. The result is a stronger protective film and better resistance to high-velocity and sand-bearing seawater.
Q: What is the minimum tensile strength of C71640 tube?
ASTM B111 sets 435 MPa (63 ksi) for the O61 annealed temper and 560 MPa (81 ksi) for the HR50 drawn and stress relieved temper, with corresponding minimum yield strengths of 170 MPa and 400 MPa.
Q: Which industries normally specify C71640 tubes?
Power generation, marine and shipbuilding, desalination, offshore oil and gas, and chemical processing are the main users, particularly where seawater velocity or solids content is high.
Q: Can C71640 tube be U-bent?
Yes. Annealed tube is formed into U-bends with controlled radius, ovality and wall thinning, and the bent tube is inspected against the bend requirements of the applicable specification.
Q: Is C71640 suitable for chemical process fluids?
It handles many neutral, alkaline and non-oxidising media and is widely used in process condensers and evaporators. Each specific chemistry should be reviewed, because strongly oxidising acids are not suitable for copper alloys.
Q: What documentation is provided with a shipment?
A mill certificate with heat number, chemical analysis, tensile results, eddy-current and pressure test records and dimensional data, together with the packing list matched to the bundle labels.
