Why Grade 12 Tube Is Specified for Aggressive Cooling Water
Grade 12 is a titanium alloy containing about 0.3 percent molybdenum and 0.8 percent nickel, registered as UNS R53400. The two alloying additions raise strength above the commercially pure grades and, more importantly, improve resistance to crevice corrosion in hot chloride environments. That combination is what makes it a common choice for heat exchanger tubing in cooling water service where the water is warm, brackish or chloride bearing and where tubes are rolled into tubesheets, creating the crevices in which titanium can otherwise suffer local attack.
The second half of the problem is the tubesheet. A solid titanium tubesheet is expensive and needs a thick plate because titanium is not a high strength structural material, whereas a carbon steel or low alloy steel tubesheet with a titanium facing gives the required pressure containment, stiffness and cost. The result is a titanium steel composite tubesheet clad with a Grade 12 or Grade 1 layer, into which Grade 12 tubes are expanded and welded.
The manufacturing challenge lies in joining two materials whose melting points, thermal expansion and metallurgical behaviour are entirely different, and in doing so with a joint that is both strong and leak tight. The accepted route is to expand the tube first to develop the mechanical strength of the joint, and then to weld for sealing.
Material Data for Grade 12 Tube and Titanium Clad Plate
The values below reflect the annealed condition typically supplied for tube and clad plate products, with chemistry and mechanical properties certified against the applicable specification.
| Item | Value or range |
|---|---|
| Grade and UNS | Grade 12, UNS R53400 |
| Nominal composition | Titanium with 0.2 to 0.4 percent molybdenum, 0.6 to 0.9 percent nickel |
| Tensile strength, minimum | 483 MPa |
| Yield strength, 0.2 percent offset, minimum | 345 MPa |
| Elongation, minimum | 18 percent |
| Density | 4.51 g/cm3 |
| Tube specification | ASTM B338, ASME SB338 for welded and seamless tube |
| Clad plate specification | ASTM B898 for reactive and refractory metal clad plate |
| Composite construction | Titanium or titanium alloy cladding on carbon steel or low alloy steel backing |
Clad plate is normally supplied with the cladding bonded over the entire surface and with a minimum cladding thickness that allows the full tubesheet face and hole depth to remain titanium after machining. The bond quality is verified by ultrasonic examination before the plate is released to the machine shop.
Composite Tubesheet Design and Bonding
Titanium clad steel tubesheets are produced by explosion bonding or by roll bonding. In explosion bonding, a titanium plate is accelerated against the steel backing plate at an angle, and the resulting high velocity collision creates a wavy metallurgical bond across the interface without melting the bulk of either material. Roll bonding achieves the same result through a hot rolling schedule that brings the two surfaces into intimate contact under high pressure.
From a fabrication point of view the designers care about three things:
Bond integrity - the cladding must remain bonded across the full plate, including the areas between tube holes, because an unbonded zone behind a tube hole can create a crevice and a leak path.
Cladding thickness - enough material must remain after facing and drilling to support tube expansion and welding without exposing the steel backing.
Thermal behaviour - titanium and steel have different thermal expansion coefficients, so the plate is machined and drilled under a sequence that limits the introduction of residual stress and distortion.
The Expansion Before Welding Sequence
For a titanium tube in a titanium clad tubesheet, the joint has to perform two separate functions: carry the mechanical load of the tube bundle and provide a leak tight seal. The practical manufacturing sequence separates those functions rather than asking a single operation to achieve both.
The tube is first expanded into the tubesheet by roller or hydraulic expansion. This provides the joint strength. The weld that follows is then required mainly to seal, and because it does not carry the full mechanical load, it can be made smaller and with less heat input, which reduces distortion and the risk of contaminating the weld pool. Expansion parameters are established by a procedure qualification on a mock up assembly, in which the wall reduction and the resulting pull out or push out strength are recorded. Wall reduction is normally kept in a controlled band and the achieved expansion is verified on sample joints because over expansion damages the tubesheet ligament and under expansion leaves the joint loose.
Gas Tungsten Arc Welding and Shielding Control
Titanium is chemically active at temperature. At room temperature it carries a dense, self healing oxide film that gives it corrosion resistance, but above roughly 540 degrees Celsius the film formed in air is no longer protective and the metal absorbs oxygen, nitrogen and hydrogen rapidly, producing hard, brittle phases and a weld that may crack. Every titanium welding procedure is therefore built around excluding air from the weld pool and from all metal that is still hot.
Process - gas tungsten arc welding with direct current electrode negative is the standard process for tube to tubesheet joints, with a trailing shield and, where access allows, an internal purge of the tube bore.
Gas - high purity argon for both the torch and the trailing shield, with flow rates set so that shielding is maintained without turbulence that would draw in air.
Position - horizontal position welding with an argon filled internal shield gives good access and consistent protection for a full row of tube ends, whereas all position welding around a vertical tube face demands two operators working in close coordination and is far less tolerant of interruption.
Cleaning before welding - the weld zone is mechanically cleaned, degreased with a solvent such as acetone, and inspected for discolouration before the arc is struck. Machining oils, marking ink, dirt and iron particles from tooling all have to be removed because they become defects and contamination sources.
Cleanliness during welding - tube ends that have been prepared but not yet welded are covered so that dust and shop dirt cannot settle in the joint preparation.
Weld appearance is used as a first line acceptance check. A bright silver or light straw surface shows that shielding held throughout; any blue, purple, grey or white deposit indicates contamination, and the affected area is removed and re-welded under a corrected procedure.
Inspection, Leak Testing and Documentation
After welding, a titanium clad tubesheet and Grade 12 tube bundle is inspected in stages. Weld appearance and surface colour are examined visually. Dimensional checks confirm tube projection, hole pitch, tubesheet flatness and the finished cladding thickness on the tube face. Non destructive testing is applied to the welds where the specification demands it, using dye penetrant or a suitable volumetric method. Finally the assembly is pressure tested, commonly by hydrostatic test, with a helium leak test added for high integrity duty where a tracer gas can detect leakage at a far lower rate than a water test.
Documentation for such a bundle ties each tube and the clad plate back to its heat number, records the welding procedure qualification and the welder qualification, the expansion procedure and the achieved wall reduction, and the results of every pressure and leak test. That record set is what allows the equipment owner to demonstrate compliance during later inspection cycles.
Frequently Asked Questions
Q: Why is Grade 12 preferred over Grade 2 for cooling water service?
Grade 12 contains molybdenum and nickel, which raise strength and improve resistance to crevice corrosion in hot chloride bearing water, the condition most likely to attack tubes where they sit in the tubesheet crevice.
Q: Why expand the tube before welding instead of welding alone?
Expansion develops the mechanical strength of the joint so the weld only has to seal. That allows a lower heat input weld, less distortion and a lower risk of contamination and cracking.
Q: Which standard covers titanium steel composite plate?
ASTM B898 covers reactive and refractory metal clad plate, which is the usual specification for titanium clad steel used in tubesheets, with ASTM B338 covering the Grade 12 tube itself.
Q: What is the maximum temperature for welding titanium without contamination?
Protective shielding is required for the weld pool and all metal above roughly 540 degrees Celsius, because the oxide film formed in air above that temperature is not protective and the metal absorbs oxygen, nitrogen and hydrogen.
Q: How is weld quality judged on site?
By a combination of surface colour assessment, visual and dimensional inspection, non destructive testing where specified, and a final hydrostatic or helium leak test of the completed bundle.
Q: Why must the joint be cleaned with a stainless brush and solvent?
Iron contamination from tooling, cutting fluids or handling is drawn into the titanium weld pool and forms brittle phases, so the oxide layer, oil and dirt are removed immediately before welding and the cleaned area is protected from recontamination.
Q: What happens if the cladding is too thin after machining the tubesheet?
The steel backing can be exposed at the tube hole, which destroys the corrosion protection of the face; cladding thickness is therefore verified after facing and before drilling.
