Welded Pipe in Grade 7 Titanium
Heat exchanger pipe and tube are supplied welded when the wall is thin, the diameter is large or the quantity is high, because roll forming and seam welding use less material than boring or pilgering a seamless shell. Grade 7, UNS R52400, is the palladium bearing version of Grade 2 and is selected when the process side is chemically aggressive. Its mechanical behaviour is close to that of Grade 2, which means existing design rules, expansion procedures and welding procedures transfer directly, so upgrading from Grade 2 to Grade 7 for corrosion reasons rarely requires requalification.
Composition, Strength and Physical Data
Grade 7 is controlled to 0.12 to 0.25 percent palladium with maximum oxygen 0.25 percent, nitrogen 0.03 percent, carbon 0.08 percent, hydrogen 0.015 percent and iron 0.30 percent, the balance being titanium. ASTM B338 requires a minimum tensile strength of 345 MPa (50 ksi), a minimum 0.2% offset yield strength of 275 MPa (40 ksi) and a minimum elongation of 20 percent in 50 mm. Density is 4.51 g/cm3, the melting point is about 1668 C, and the modulus of elasticity is approximately 103 GPa. The low carbon content and iron limit support good weld ductility and predictable corrosion behaviour.
| Item | Grade 7 (R52400) Requirement |
|---|---|
| Tensile strength, min | 345 MPa (50 ksi) |
| Yield strength, 0.2% offset, min | 275 MPa (40 ksi) |
| Elongation in 50 mm, min | 20% |
| Palladium | 0.12-0.25% |
| Oxygen, max | 0.25% |
| Melting point | approx. 1668 C |
Duty Compared With Other Titanium Grades
Grade 2 and Grade 7 have similar melting points and strength, but Grade 7 is markedly more corrosion resistant in reducing conditions, so it is chosen where Grade 2 would be marginal. Where higher strength is needed rather than higher corrosion resistance, Grade 3 and Grade 12 are the usual alternatives, and for the most aggressive chloride and oxidising combinations the higher palladium or ruthenium bearing grades are considered. Selection should be made from the actual process chemistry, temperature and velocity rather than from a generic resistance chart.
Service Applications
Heat exchangers, reboilers and coolers in chemical production plants.
Pulp and paper bleaching circuits and chlorine dioxide stages.
Seawater and brine cooled condensers and desalination preheaters.
Pollution control and flue gas scrubber cooling banks.
Power generation auxiliary coolers and chemical dosing lines.
Welded pipe in this grade is commonly combined with seamless tube in the same bundle, with welded lengths used for the larger diameters and seamless for small bore positions.
Fabrication and Quality Control
Welded pipe is formed from strip, seam welded in an inert atmosphere and annealed, then sized and straightened. Tubes are cut to length, deburred and roller expanded or welded into the tube sheet, and each joint is leak tested by hydrostatic or pneumatic means before assembly. Standard inspection includes spectrographic chemical analysis, tensile and flattening tests, hardness checks on the weld, eddy current examination of the full length and a pressure test of each finished tube. Delivery conditions are annealed, pickled or bright annealed, with plain, bevelled or expanded ends. Marking shows standard, grade, size, wall and heat number, and documentation is issued as an EN 10204 3.1 mill test certificate with heat traceability. Additional requirements such as intergranular corrosion testing or third party witness are quoted on request, and bend or flare tests are added when the customer will form the tube after delivery.
Frequently Asked Questions
Q: What is UNS R52400 equivalent to in other systems?
A: It is Grade 7 titanium and carries the German material number 3.7235. Its composition is unalloyed titanium with a controlled palladium addition.
Q: Does Grade 7 require different welding procedures from Grade 2?
A: No. The procedures are the same, with full inert gas shielding and a monitored purge. Only the corrosion performance of the finished joint differs.
Q: Is welded pipe as corrosion resistant as seamless?
A: Yes, provided the weld is properly shielded and annealed. A discoloured or contaminated seam is the one place where corrosion can initiate, so weld appearance and hardness are checked.
Q: What schedules are produced?
A: Thin wall heat exchanger tube is supplied by outside diameter and wall, while pipe is available in schedules 5, 10, 40, 80, 160 and heavier on request, in random or cut lengths.
Q: Can the pipe be supplied with tube sheet joints already made?
A: Fabricated bundles with expanded or welded and expanded joints are produced to drawing, including baffles and tie rods, and pressure tested before shipment.
