ASTM B338 Grade 7 Titanium Tube: UNS R52400 Chemistry and Sizing
Oct 17, 2025 ASTM-B338-Titanium.pdf
What ASTM B338 Grade 7 Tube Actually Is
ASTM B338 covers annealed titanium and titanium alloy tube, seamless and welded, for condensers, heat exchangers, evaporators and comparable heat-transfer service. Grade 7 is the palladium-bearing grade of commercially pure titanium, registered as UNS R52400. Its base chemistry is that of Grade 2, with a deliberate addition of 0.12-0.25 % palladium that shifts the corrosion potential of the metal into the passive region even when dissolved oxygen is scarce.
Two statements that circulate widely about this grade are incorrect and worth correcting. Grade 7 is not an aluminium-vanadium alloy: it contains no intentional aluminium or vanadium and belongs to the alpha, single-phase hexagonal close-packed family. The palladium addition also does not raise strength. Grade 7 sits at the same annealed strength level as Grade 2, and its entire value lies in corrosion resistance under reducing-acid and crevice conditions.
Chemical Composition of Grade 7 (UNS R52400)
| Element | Grade 7 | Grade 2 (reference) |
|---|---|---|
| Titanium | Balance | Balance |
| Palladium | 0.12-0.25 % | Not specified |
| Iron | 0.30 % max | 0.30 % max |
| Oxygen | 0.25 % max | 0.25 % max |
| Nitrogen | 0.03 % max | 0.03 % max |
| Carbon | 0.08 % max | 0.08 % max |
| Hydrogen | 0.015 % max | 0.015 % max |
Because the interstitial limits are identical, the two grades are metallurgically interchangeable in terms of fabricability. Choosing between them is a corrosion-engineering decision, not a strength decision.
Mechanical Properties and Heat Treatment
Tube supplied to ASTM B338 in the annealed condition must satisfy minimum tensile properties of 345 MPa (50 ksi) tensile strength, 275 MPa (40 ksi) yield strength at 0.2 % offset, and 20 % elongation. Typical mill values run comfortably above those minima. Physical data normally used for design: density 4.51 g/cm3, elastic modulus about 102-105 GPa, thermal conductivity near 16-17 W/(m*K) at room temperature, and a coefficient of thermal expansion of roughly 8.6 x 10-6 per K.
Heat treatment is normally a recrystallisation anneal carried out in vacuum or in high-purity inert atmosphere. Stress relief falls in the 540-650 C band; full annealing to control grain size typically sits between about 650 and 760 C. Time and temperature are selected to keep the grain size fine enough for eddy-current examination and for bending, and to limit alpha-case formation. Hydrogen pickup must be avoided during every thermal operation, because a palladium-bearing grade loses its advantage if the hydrogen limit of the specification is exceeded.
Why Palladium Changes the Corrosion Story
Commercially pure titanium depends on a thin, self-repairing oxide film. In aerated neutral chlorides that film is stable and Grade 2 performs well. In reducing acids such as hydrochloric or sulphuric acid, or inside tight crevices where oxygen cannot be replenished, the film can break down and active dissolution begins. Palladium on the surface catalyses hydrogen recombination, which raises the potential of the alloy and re-establishes passivity.
| Grade | Alloying addition | Typical selection driver |
|---|---|---|
| Grade 2 | None, commercially pure | Seawater, oxidising media, general chemical service |
| Grade 7 | 0.12-0.25 % Pd | Reducing acids, crevice and under-deposit conditions |
| Grade 11 | 0.12-0.25 % Pd, lower oxygen | Same as Grade 7 with better formability |
| Grade 12 | 0.2-0.4 % Mo, 0.6-0.9 % Ni | Hot chlorides, moderate reducing-acid service |
| Grade 16 | 0.04-0.08 % Pd | Lower-cost option where the full palladium range is not required |
The lower-palladium Grade 16 is often evaluated first on cost grounds. Grade 7 is specified when corrosion data or service experience call for the full palladium range.
Dimensions, Testing and Tube-Specific Requirements
Grade 7 heat-exchanger tube is commonly produced in outside diameters from about 12.7 mm to 60 mm with walls of 0.5 mm to 3.0 mm, in straight lengths or as U-bends. Ordering practice follows B338 for diameter, wall and length tolerances, straightness and permissible ovality. Commonly required tests are tensile testing, flattening or reverse-flattening, flange testing, full-length eddy-current examination, and hydrostatic or pneumatic pressure testing. Where the tube will be expanded into a tubesheet, hardness much above 200 HV slows rolling, which is one more reason the annealed condition is preferred.
Welding, Forming and Cleanliness
Grade 7 welds much like Grade 2. Gas tungsten arc welding with a trailing shield, or welding inside an argon-purged chamber, is standard practice. A bright silvery weld indicates adequate shielding, while straw or blue tinting means contamination and reduced corrosion resistance. Filler metal is matched as ERTi-7. The work area must be kept free of iron and copper contamination, cutting tools should be dedicated to titanium, and pickling is performed with nitric-hydrofluoric acid mixes followed by thorough rinsing. Iron pickup on the surface is one of the most common causes of premature corrosion in field service, and it is invisible after machining.
Applications and Common Misconceptions
Typical uses include chemical-process heat exchangers handling chlorides and dilute reducing acids, desalination and brine circuits, flue-gas desulphurisation and scrubber components, pulp and paper bleach-plant equipment, and marine hardware. In aerospace programmes, palladium-bearing commercially pure titanium appears in fluid systems, ground-support equipment and specialised hardware rather than in primary airframe structure, where higher-strength alpha-beta grades dominate. A purchase specification that names Grade 7 for a stressed airframe part should be reviewed with that in mind.
Recurring misconceptions: that Grade 7 is an aluminium-vanadium alloy; that palladium increases strength; that Grade 7 and Grade 12 are interchangeable; and that better corrosion resistance removes the need for proper weld shielding. Each of these leads either to an over-specified or to an under-protected component.
FAQ
Q: Is Grade 7 stronger than Grade 2?
No. The minimum annealed tensile and yield strengths are identical at 345 MPa and 275 MPa. The difference is corrosion resistance from the palladium addition.
Q: Can Grade 7 replace Grade 12 in hot chloride service?
Not automatically. Grade 12, UNS R53400, with molybdenum and nickel is often the better choice in high-temperature chloride and crevice environments; selection should follow corrosion data for the actual medium and temperature.
Q: Must Grade 7 tube be supplied annealed?
ASTM B338 tube is ordered in the annealed condition, which supports the required elongation, flattening performance, eddy-current inspection and tubesheet expansion behaviour.
Q: How is Grade 7 tube inspected?
Typical requirements are full-length eddy-current examination, tensile and flattening tests, dimensional verification, and hydrostatic or pneumatic pressure testing, with weld radiography where the tube is welded.
Q: What filler metal matches Grade 7?
ERTi-7 filler is used for welded joints in Grade 7 components. ERTi-2 would dilute the palladium content in the weld pool and reduce local corrosion resistance.
Q: What wall thickness is normal for heat-exchanger service?
Common practice is 0.9 mm to 2.1 mm for tube diameters between about 19 mm and 25.4 mm, adjusted for design pressure, corrosion allowance and expansion requirements.
