Grade 7 Titanium Seamless Tube to ASTM B338 / ASME SB338 (R52400)
Sep 09, 2025 ASTM-B338-Titanium.pdf
Specification Identity: ASTM B338 and ASME SB338
Grade 7 titanium seamless tube is standardised in ASTM B338, the specification covering seamless and welded titanium and titanium alloy tubes for condensers and heat exchangers. The ASME version, ASME SB338, is technically identical and is the document cited for pressure-retaining service under the ASME Boiler and Pressure Vessel Code. Buyers occasionally meet a mistaken reference to "ASTM A338 titanium"; A338 is not a titanium tube specification, and any enquiry written that way should be corrected to B338 before a purchase order is placed, because the wrong callout will produce quotations for material that cannot be certified to the required standard.
Grade 7 is also available as pipe under ASTM B861 and as welded pipe under ASTM B862, while fittings follow ASTM B363. Selecting the correct document for the product form matters as much as selecting the grade.
Chemistry and Metallurgy of Grade 7
Grade 7, UNS R52400, is commercially pure titanium modified by a deliberate palladium addition of 0.12 to 0.25 percent. It is not an alpha-beta alloy and it contains no aluminium or vanadium; the percentage figures that circulate for those elements belong to other grades and are a frequent source of confusion in secondary literature. The controlled elements are nitrogen 0.03 percent maximum, carbon 0.10 percent maximum, hydrogen 0.015 percent maximum, iron 0.30 percent maximum, oxygen 0.25 percent maximum and palladium 0.12 to 0.25 percent, with titanium as the balance.
The palladium addition works electrochemically. In reducing acids such as hydrochloric and sulphuric acid, the corrosion reaction generates hydrogen, and the metal can absorb it and embrittle. Palladium raises the cathodic efficiency of the surface, so the corrosion potential shifts toward the passive region and hydrogen evolution occurs at a lower overvoltage; the metal then remains passive instead of dissolving at a high rate. A higher-palladium variant, Grade 7H, exists for more severe reducing conditions.
Mechanical Properties and Design Values
| Property | Grade 7 value | Grade 2 value for comparison |
|---|---|---|
| Tensile strength, minimum | 345 MPa | 345 MPa |
| Yield strength, minimum | 275 MPa | 275 MPa |
| Elongation, minimum | 20 percent | 20 percent |
| Density | 4.51 g/cm3 | 4.51 g/cm3 |
| Modulus of elasticity | about 103 GPa | about 103 GPa |
The strength and ductility requirements are the same as those for Grade 2, which is the point: designers specify Grade 7 for the corrosion allowance rather than for load capacity. The low modulus relative to steel, roughly half, means deflection rather than stress often governs tube layout, and support spacing in a heat exchanger must respect that flexibility.
Where Grade 7 Tube Is Selected
Heat exchangers and condensers cooled by seawater or brackish water, where chloride pitting and crevice attack would defeat conventional steel.
Chemical process equipment handling hydrochloric acid, sulphuric acid or mixed reducing acids, especially where traces of chlorides are present.
Flue gas desulphurisation and scrubber circuits, where acidic condensates form on cold surfaces.
Pulp and paper bleach plant piping and heat recovery equipment exposed to chlorine-based liquors.
Desalination and brine handling, and any duty where an accidental reducing condition could otherwise attack Grade 2 tube.
Hydrogen-bearing process streams where absorption and hydriding of unalloyed titanium are a concern.
Fabrication, Welding and Testing
Weld Grade 7 with the gas tungsten arc process under inert shielding, using matching palladium-bearing filler where the specification requires filler metal.
Protect the weld pool and the hot bead with trailing and backing purge; titanium at elevated temperature picks up oxygen and nitrogen readily.
Heat treatment and subsequent descaling follow the applicable practice, and the surface must be free of iron contamination from tooling or handling before it enters service.
Eddy current or hydrostatic testing is performed as specified on the order. The test method, acceptance level and reference standard must be stated on the purchase order.
Do not mix Grade 7 and Grade 2 tube in a tube bundle without marking; visual identification is impossible once installed, and the corrosion benefit is lost where Grade 2 inadvertently replaces Grade 7.
Store and handle tube with non-metallic slings; surface iron pick-up is the most common cause of premature pitting in an otherwise correct installation.
Common Misconceptions
The most damaging error is describing Grade 7 as an aluminium-vanadium alloy. That description belongs to Grade 5 or Grade 9 and leads to entirely wrong expectations about strength and heat treatment. A second misconception is that palladium makes titanium immune to every acid; Grade 7 is intended for reducing media and offers no advantage in strongly oxidising service, where the cheaper Grade 2 performs equally well. Third, the small palladium addition does not change the mechanical property minimums, so substitution between Grade 2 and Grade 7 must be judged on corrosion, never on strength.
FAQ
Q: What is the difference between Grade 2 and Grade 7 titanium tube?
The mechanical requirements are identical. Grade 7 adds 0.12 to 0.25 percent palladium, which improves resistance to reducing acids and to hydrogen embrittlement.
Q: Is ASME SB338 the same as ASTM B338?
Yes. SB338 is the ASME designation for the same specification, published for use in ASME code construction. Either callout is acceptable if the project specifies it.
Q: Does Grade 7 contain aluminium and vanadium?
No. Grade 7 is a palladium-modified unalloyed grade. Aluminium and vanadium belong to other titanium grades.
Q: Which filler metal is used for Grade 7 tube welds?
A palladium-bearing titanium filler matching the parent metal is used where filler is required, so that the weld retains the corrosion resistance of the tube.
Q: Can Grade 2 be substituted for Grade 7 to reduce cost?
Only if the service is fully oxidising and no reducing excursions are credible. In reducing or chloride-bearing environments the substitution removes the protection that was paid for.
Q: What tests are typical on Grade 7 seamless tube?
Chemical analysis, tensile testing and either hydrostatic or eddy current testing, with the specific acceptance criteria and any additional examination stated on the order.
