Two Copper-Zinc Alloys for Condenser and Heat Exchanger Service
ASTM B111 (identical in content to ASME SB111) is the standard specification for seamless copper and copper-alloy condenser tubes, evaporator tubes and ferrule stock. Within that specification, UNS C44300 admiralty brass and UNS C68700 aluminium brass are the two most widely ordered copper-zinc grades for power station condensers, process heat exchangers and distillers.
C44300 is a copper-zinc-tin alloy - the classic admiralty metal - in which a small arsenic addition suppresses dezincification. C68700 is the aluminium brass, in which about 2% aluminium replaces part of the zinc and forms a protective surface film that resists impingement attack. The two grades are frequently quoted together because they are selected for the same duty and are interchangeable in terms of tube-sheet practice, but they differ in the cooling-water velocities they tolerate.
Chemical Composition
| UNS number | Cu % | Sn % | Al % | As % | Fe % max | Pb % max | Zn % |
|---|---|---|---|---|---|---|---|
| C44300 (admiralty brass) | 70.0-73.0 | 0.9-1.2 | - | 0.02-0.06 | 0.06 | 0.07 | remainder |
| C68700 (aluminium brass) | 76.0-79.0 | - | 1.8-2.5 | 0.02-0.06 | 0.06 | 0.07 | remainder |
The arsenic range is mandatory rather than optional: both alloys rely on a controlled arsenic addition to inhibit dezincification in cooling waters, and a heat outside the specified range is normally rejected on chemistry alone, however good its mechanical results may be.
Mechanical Requirements in the Annealed Temper
Tubes to ASTM B111 are normally supplied in the annealed temper (O61, formerly O50-style annealed tube) so that they can be rolled or expanded into tube sheets without cracking. For C44300 in the annealed temper the standard specifies a minimum tensile strength of 310 MPa (45 ksi) and a minimum yield strength of 105 MPa (15 ksi). The aluminium brass C68700 is specified with higher minimum tensile and yield strengths in the same temper because the aluminium in solid solution strengthens the matrix; the exact values are those tabulated in ASTM B111 for the ordered temper and tube size.
| Property | C44300 annealed |
|---|---|
| Minimum tensile strength | 310 MPa (45 ksi) |
| Minimum yield strength | 105 MPa (15 ksi) |
| Typical delivery temper | annealed, O61 |
| Hardness / grain size | checked as required by the ordered specification |
B111 also requires that each tube pass an eddy-current examination in accordance with ASTM E243, and that residual stress be verified where specified by a mercurous nitrate test to ASTM B154. Hydrostatic or pneumatic pressure testing is applied when the purchase order requires it. These are the tests that turn a dimensionally correct tube into an accepted condenser tube, and their results belong on the mill test certificate.
International Designations and Equivalent Standards
| System | Standard | C44300 | C68700 |
|---|---|---|---|
| USA | ASTM B111 / ASME SB111 | C44300 | C68700 |
| China | GB/T 8890 | HSn70-1 | HAL77-2 |
| UK | BS 2871 | CZ111 | CZ110 |
| Japan | JIS H3300 | C4430 | C6870 |
| Germany | DIN 1785 | CuZn28Sn1 | CuZn20Al2 |
The composition ranges are aligned across these systems, so a tube produced to GB/T 8890 HSn70-1 can normally be certified as C44300 as well, provided the tin and arsenic limits of the tighter specification are respected.
Typical Service Applications
Steam turbine condensers and auxiliary coolers in power plants.
Shell-and-tube heat exchangers, evaporators and distiller tubing in process and desalination plants.
Condenser tube plates, ferrules and oil-well pump liners.
Bourdon tubes, strainers and marine plumbing components that see moving or aerated water.
General industrial tubing where cooling water is brackish, mildly polluted or variable in quality.
C44300 is the standard choice for clean or moderately aggressive fresh and brackish cooling water. C68700 is preferred where water velocities are higher or where local impingement attack is likely, because the aluminium-rich surface film resists the mechanical scouring that removes the protective layer from plain admiralty brass.
Selection, Rolling and Handling Guidance
Success with these alloys begins with water chemistry. Chloride level, oxygen content, velocity and the presence of suspended solids all influence grade selection, and a velocity chosen for C44300 may be conservative or inadequate for C68700. As a working rule, aluminium brass tolerates higher cooling-water velocities, admiralty brass is the safer selection in low-velocity, low-oxygen circuits.
Tube rolling into tube sheets should follow a controlled expansion procedure with the correct torque and wall reduction, keeping the expanded length inside the tube-sheet thickness. Over-rolling produces residual tensile stress that leads to stress-corrosion cracking in service; under-rolling produces leaks at the joint. Heat treatment or stress relief after any bending operation is advisable, and bending radii should be generous.
Storage and installation matter as much as manufacturing. Tubes should be kept dry and clean before installation, protected from contact with carbon steel, and free of chlorides on the surface. Contamination with salts or with sulphur-bearing compounds is a common cause of premature failure that is later wrongly attributed to the material.
Inspection Points and Common Pitfalls
Confirm arsenic content on the certificate - it is the single most frequently mis-stated element in C44300 and C68700 orders and the main reason for dezincification failures.
Require eddy-current test reports to ASTM E243 with the acceptance level stated on the purchase order.
Check the delivered temper and grain size against the expansion operation planned in the workshop.
Verify that tube ends are free from burrs, ovality and drawing marks, which reduce joint reliability.
For welded tube-sheet joints, confirm the filler metal is compatible with a copper-zinc base metal; many failures in these grades originate in a mismatched filler rather than in the tube itself.
FAQ
Q: What is the difference between C44300 and C68700?
C44300 is an admiralty brass with about 1% tin and a controlled arsenic addition, while C68700 is an aluminium brass containing about 2% aluminium. Both resist dezincification, but aluminium brass forms a protective film that makes it more resistant to impingement attack at higher cooling-water velocities.
Q: What tensile strength does ASTM B111 require for C44300 tube?
In the annealed temper the standard specifies a minimum tensile strength of 310 MPa (45 ksi) and a minimum yield strength of 105 MPa (15 ksi). Acceptance is based on tests performed on the finished tube, not on the ingot.
Q: Which standard governs seamless heat exchanger tubes in China?
GB/T 8890 covers seamless copper alloy heat exchanger tubes, with HSn70-1 corresponding to C44300 and HAL77-2 corresponding to C68700. These designations are routinely dual-certified against ASTM B111 for export orders.
Q: Why is arsenic added to these alloys?
Arsenic is a deliberate dezincification inhibitor. Without it, these copper-zinc alloys can lose zinc selectively in certain waters, leaving a weak, porous copper sponge that eventually perforates the tube.
Q: How should the tubes be tested before delivery?
ASTM B111 calls for eddy-current examination to ASTM E243 on each tube, with hydrostatic or pneumatic testing and residual-stress verification (mercurous nitrate to ASTM B154) applied when the order specifies them. Dimensional and visual inspection completes the release.
Q: Are these tubes suitable for seawater?
They are widely used in brackish and moderately aggressive waters, but for full-strength flowing seawater at high velocity, copper-nickel grades such as C70600, C71500 and C71640 are usually the preferred selection because of their higher tolerance to erosion-corrosion.
