ASTM B111 C68700 is an arsenical aluminum brass seamless tube grade normally supplied in the annealed O61 temper. With 76.0–79.0% copper, 1.8–2.5% aluminum and a controlled arsenic range of 0.02–0.10%, the alloy forms a thin, tenacious oxide layer that shields the tube wall from fast-flowing seawater. That behavior is the reason C68700 tube is a standard selection for cooler shells, condenser tube bundles and heat exchanger tube banks in power generation, marine and desalination service.
Why Aluminum Brass C68700 Suits Cooler HT / ET Service
Coolers and heat exchangers built around HT and ET tube families place two demands on the tube at the same time: efficient heat transfer and resistance to aggressive cooling water. Aluminum brass satisfies both. The copper-rich matrix keeps thermal conductivity high, while the aluminum-rich surface film resists impingement attack and the erosive action of high water velocity in the tube inlet region.
Good resistance to seawater and brackish cooling water
Strong resistance to impingement and erosion at elevated flow velocity
Stable mechanical strength across the working temperature range of shell-and-tube coolers
Straightforward roller expansion into tubesheets and ferrule joints
Smooth bore that limits fouling and keeps cleaning cycles short
Chemical Composition of C68700 Wire and Tube Stock
The table below lists the specified ranges for aluminum brass C68700 together with the composition limits of the other condenser tube alloys most often quoted alongside it.
| UNS No. | Cu (%) | Al (%) | As (%) | Pb, max (%) | Fe, max (%) | Zn (%) |
|---|---|---|---|---|---|---|
| C68700 | 76.0–79.0 | 1.8–2.5 | 0.02–0.10 | 0.07 | 0.06 | Remainder |
| C44300 | 70.0–73.0 | – | 0.02–0.06 | 0.07 | 0.06 | Remainder |
| C70600 | Remainder | – | – | 0.05 | 1.0–1.8 | 1.0 max |
| C71500 | Remainder | – | – | 0.05 | 0.40–1.0 | 1.0 max |
Mechanical Properties in the O61 Annealed Temper
| Temper | Annealing temperature | Tensile strength, min | Yield strength, min | Hardness |
|---|---|---|---|---|
| O61 (annealed) | above 650 °C | 345 MPa (50 ksi) | 125 MPa (18 ksi) | 150 HV |
Because the tube is delivered annealed, it can be cut, bent and rolled into tubesheets without intermediate heat treatment. Where a cooler duty requires a stiffer tube, the same alloy is also produced in lightly worked tempers.
Equivalent and Comparable Designations
| Standard body | Designation | Scope |
|---|---|---|
| ASTM | ASTM B111 | Copper and copper-alloy seamless condenser tubes and ferrule stock |
| GB/T | GB/T 8890 | Seamless copper alloy heat exchanger tubes |
| BS | BS 2871 (CZ110) | Copper and copper alloy tubes |
| JIS | JIS H3300 | Copper and copper alloy seamless pipes and tubes |
| DIN | DIN 1785 | Wrought copper and copper alloy tubes for condensers and heat exchangers |
Typical Applications of C68700 Tube
Surface condensers and central cooling water condensers in power stations
Cooler HT and ET tube bundles in air-conditioning and refrigeration plants
Evaporator and distiller tubes in multi-stage seawater desalination trains
Heat exchanger tubes in chemical, petrochemical and process cooling loops
Ferrules and tube inserts where a copper alloy transition is required
The alloy is widely applied in seawater desalination plants, shipboard cooling systems and onshore installations where cooling water salinity is high or variable.
Testing and Inspection of Every Heat
Tubes are released against a documented test programme that covers both the metal and the finished tube:
Tensile testing of the tube section
Hardness testing in the delivered temper
Chemical analysis by spectrographic methods
Positive material identification of the finished tube
Flattening and flaring tests to confirm ductility for expansion work
Micro and macro examination of the tube section
Pitting resistance assessment for seawater duty
Eddy current testing of the tube body
Tube ends are deburred, the bore is kept free of drawing lubricant, and the material is packed for sea transport so that the annealed surface reaches the fabrication shop clean.
Frequently Asked Questions
Q: What does the O61 temper mean for a C68700 tube?
O61 is the annealed condition in which the tube is supplied. Tensile strength is a minimum of 345 MPa and yield strength a minimum of 125 MPa, with annealing performed above 650 °C.
Q: How does C68700 differ from C44300 admiralty brass?
C68700 is an aluminum brass: aluminum at 1.8–2.5% replaces the tin-bearing chemistry of admiralty brass and gives better resistance to impingement and higher water velocity in seawater coolers.
Q: Which cooling media is C68700 tube suitable for?
It is selected for seawater, brackish water and fresh water loops, and is most often found in condensers, evaporators, distiller tubes and heat exchanger tube bundles.
Q: Why is arsenic added to C68700?
The controlled arsenic addition of 0.02–0.10% suppresses dezincification and stabilizes the protective surface film in chlorinated and polluted cooling water.
Q: Can C68700 tubes be roller expanded into tubesheets?
Yes. The annealed temper provides the ductility needed for roller expansion, and flaring and flattening tests are performed on each lot to confirm that ductility before shipment.
Q: How is the tube identified before installation?
Positive material identification and spectrographic chemical analysis are carried out on the finished tube, and the lot travels with its test record so that it can be matched to the bundle position on site.
