Why C12200 U-Bend Tube Is Used
Shell-and-tube heat exchangers need a bend at the end of each tube pair so that the coolant can reverse direction without extra fittings. ASTM B111/B111M covers these formed U-bends as part of the tube specification, and UNS C12200 - known as DHP, or deoxidised high phosphorus copper - is the workhorse material for the duty.
C12200 contains 99.90 percent minimum copper, including silver, and 0.015 to 0.040 percent phosphorus. The phosphorus deoxidises the melt, removing the oxygen that would otherwise cause hydrogen embrittlement, so the metal can be welded and brazed reliably. It also promotes a fine grain size and raises the softening temperature of work-hardened material, which helps the tube hold its shape after bending.
Chemical Composition and Mechanical Requirements
| Element | C12200 requirement |
|---|---|
| Copper (Cu), including silver | 99.90 % minimum |
| Phosphorus (P) | 0.015 - 0.040 % |
| Total other elements | 0.05 % maximum |
| Product standard | ASTM B111/B111M, ASME SB111 |
| European equivalent designation | Cu-DHP, seamless tube to EN 12451 |
| Japanese equivalent | C1220T to JIS H3300 |
In the annealed condition C12200 tube is specified at a minimum tensile strength of 207 MPa (30 ksi) with minimum elongation of 35 percent, and maximum hardness in the soft temper is about 50 HV. Drawn tempers raise the minimum tensile strength to around 250 MPa for light-drawn tube and 310 MPa for hard-drawn tube, with yield strengths of about 205 MPa and 275 MPa respectively.
Physical and Corrosion Properties
| Property | Typical value |
|---|---|
| Density | 8.90 g/cm³ at 20 °C |
| Melting point | 1083 °C |
| Coefficient of thermal expansion | 17.7 x 10^-6 per °C (20-300 °C) |
| Thermal conductivity | 290-365 W/(m·K) at 20 °C |
| Specific heat | 385 J/(kg·K) at 20 °C |
| Electrical conductivity | 70-90 % IACS, depending on temper |
| Modulus of elasticity | 115 GPa |
| Poisson's ratio | 0.33 |
Corrosion behaviour is similar to high-conductivity copper: excellent weathering resistance, very good resistance to a wide range of chemicals, and good resistance to pitting and impingement attack in natural waters. C12200 resists non-oxidising mineral and organic acids, alkaline solutions and saline solutions, and because it resists soil corrosion it can be used for buried pipework. It is not intended for use with oxidising acids, ammonia-bearing condensates or high-velocity aerated seawater without an agreed protective regime.
Fabrication and Joining Sequence
Bending: tube is bent on a mandrel in the soft annealed temper, with bend radius, ovality and wall thinning controlled to agreed limits.
Annealing: full anneal between 250 °C and 650 °C restores ductility after cold work; stress relief at 200 °C to 250 °C stabilises the bend.
Tube-sheet jointing: expansion, rolling or hydraulic expansion followed by seal welding where the design requires it.
Soldering and brazing: both rate as excellent, which is a key reason this grade dominates exchanger fabrication.
Welding: gas shielded arc welding rates good, and the phosphorus content makes the metal tolerant of normal workshop conditions.
Inspection Points for U-Bend Bundles
Verify bend radius, leg length tolerance and bend plane alignment, since these control whether the bundle will enter the shell.
Check wall thickness on the outside of the bend by ultrasonic measurement; thinning beyond the agreed limit reduces pressure rating.
Require residual-stress testing of straight tube before bending, and eddy-current testing after bending where specified.
Confirm the heat treatment condition: a partially annealed bend will crack during expansion into the tube sheet.
FAQ
Q: What does the designation DHP mean?
DHP stands for deoxidised high phosphorus. It describes copper such as UNS C12200 that is deoxidised with 0.015 to 0.040 percent phosphorus so that it can be welded and brazed without hydrogen embrittlement.
Q: Why is the tube supplied annealed?
The soft annealed temper gives the ductility needed to form the U-bend and to expand the tube ends into the tube sheet without cracking. Harder tempers are used for straight runs where stiffness matters.
Q: How much does phosphorus reduce conductivity?
Significantly. Electrical conductivity of C12200 falls into the 70-90 percent IACS band rather than the 101 percent IACS of oxygen-free grades, which is why C12200 is selected for strength, weldability and corrosion duty rather than maximum conductivity.
Q: Can U-bends be supplied with equal or unequal leg lengths?
Yes. Leg lengths, bend radius and bend plane orientation are made to the customer's bundle drawing, and each bend is inspected against those dimensions before packing.
Q: Which tests apply to these tubes?
Expansion, flattening, residual stress, grain size, eddy current and hydrostatic or pneumatic pressure tests are the usual ones, in line with the requirements of ASTM B111 and the purchaser's specification.
Q: How should U-bends be handled on site?
Support the bend, never the legs, and keep the bend area free of impact marks and abrasive damage. Store indoors and keep the tubes dry until installation to avoid surface staining.
