Titanium Tube FAQ: Grades, Standards and Plant Selection

Titanium tubing is specified wherever a pressure-containing conduit has to survive seawater, chloride brine, oxidising acid or high-purity process fluid for decades without a corrosion allowance. It combines a density of about 4.51 g/cm3 with an air-formed, self-repairing oxide film that makes it effectively inert in chlorinated sea water and in most oxidising media. The answers below cover the questions that come up most often during grade selection, ordering and inspection of titanium condenser, heat-exchanger, hydraulic and process tubes, and every figure is tied to a published specification or to the commercial range in which titanium tube is normally supplied.

Titanium Tube Basics: Grades, UNS Numbers and Designations

Titanium tube is ordered by grade rather than by a single chemistry, because the same base metal is modified with oxygen, palladium, molybdenum, nickel or aluminium to shift strength, formability or corrosion resistance. Unalloyed grades are numbered one to four, commercially pure (CP) titanium; the difference between them is principally the oxygen and iron content, which raises strength and lowers ductility as it rises. Palladium-modified grades are produced for reducing-acid service, where the oxide film needs help to re-passivate. Alloy grades add aluminium and vanadium for strength, or molybdenum and nickel for resistance to crevice attack in hot brine.

The relevant designations are: Grade 1 (UNS R50250), Grade 2 (UNS R50400), Grade 3 (UNS R50550), Grade 7 (UNS R52400, Grade 2 plus 0.12-0.25% Pd), Grade 9 (UNS R56320, Ti-3Al-2.5V), Grade 11 (UNS R52250, Grade 1 plus Pd), Grade 12 (UNS R53400, Ti-0.3Mo-0.8Ni), Grade 5 (UNS R56400, Ti-6Al-4V) and Grade 23 (UNS R56407, Ti-6Al-4V ELI). In the Chinese designation system these correspond to TA1, TA2, TA3, TA9 (Ti-0.2Pd) and TA10 (Ti-0.3Mo-0.8Ni) under the composition tables of GB/T 3620.1, with GB/T 3620.2 covering permissible variations in chemical composition.

Product form matters as much as grade. ASTM B338 covers seamless and welded titanium and titanium alloy tubes for condensers and heat exchangers; ASTM B861 covers seamless pipe and ASTM B862 welded pipe; ASTM B337 is the older combined pipe specification that still appears on legacy drawings; ASTM B363 covers wrought welding fittings; ASTM B265 covers the strip, sheet and plate from which welded tube is formed; and ASTM B348 covers bar and billet. On the Chinese side, GB/T 3624 governs titanium and titanium alloy tube for general purpose use and GB/T 3625 governs tube for heat exchangers, while the JIS H 4630 series is used for heat-exchanger tubing in some Japanese-origin projects.

Dimensional Range, Tolerances and Surface Condition

Commercial seamless titanium tube is commonly offered from roughly 4 mm to 114 mm outside diameter with wall thickness from about 0.5 mm to 10 mm; welded tube covers a similar range and extends to larger diameters and thin walls, and can be supplied in long coils or straight lengths up to about 18 m where transport allows. Condenser and heat-exchanger programmes concentrate on 12.7 mm, 15.88 mm, 19.05 mm, 25.4 mm and 31.75 mm outside diameter with 0.5 mm to 1.65 mm wall, because these sizes match standard tubesheet layouts.

Permissible variations in outside diameter, wall thickness, length and straightness are tabulated in the product specification itself, and the purchaser should verify the tolerance class rather than assume it. As commercial practice, outside diameter is usually held within plus or minus 0.10 mm on small sizes, wall thickness within about plus or minus 10%, and straightness within roughly 1.5 mm per metre for cold-finished straight lengths. Where hydraulic or instrument tubing is involved, tighter concentricity and inner-surface cleanliness are normal requirements rather than options.

Surface condition must be stated explicitly: annealed and pickled, bright annealed in vacuum or inert gas, or ground and polished for pharmaceutical and semiconductor use. Titanium pickling uses a nitric-hydrofluoric acid mixture, and residual fluoride or embedded iron from tooling is a genuine contamination risk that shows up later as localised attack. Tube ends are normally supplied square cut, deburred and capped, with plastic or wooden end protection, and export packages frequently include desiccant and an inner liner to keep chlorides away from the surface during sea transport.

Metallurgical and Mechanical Property Data

The oxygen, nitrogen, carbon, hydrogen and iron contents are not incidental; they are the alloying mechanism in CP grades and the principal reason two tubes of the same nominal grade can behave differently. Oxygen in particular moves tensile strength by tens of megapascals across the Grade 1 to Grade 4 range, while hydrogen is restricted because absorbed hydrogen can precipitate brittle hydride at moderate temperature and cause delayed cracking. Grade 7 carries palladium at 0.12-0.25% to widen the passive range in reducing acids; Grade 12 carries 0.2-0.4% Mo and 0.6-0.9% Ni to resist crevice corrosion under deposits and gaskets in hot chloride service; Grade 9 adds 3% aluminium and 2.5% vanadium to reach substantially higher strength while keeping reasonable formability for hydraulic lines.

The table below summarises the composition limits and minimum tensile properties that appear in the ASTM tube and pipe specifications for the grades most often ordered. Values are the specification minimums for annealed product, not typical mill results.

Grade (UNS)	Characteristic additions, wt%	Tensile min, MPa	Yield min, MPa	Elongation min, %	Typical duty
Grade 1 (R50250)	Pd none; O 0.18 max, Fe 0.20 max	240	170	24	Deep-drawn and highly formed tube, low-strength chemical service
Grade 2 (R50400)	O 0.25 max, Fe 0.30 max	345	275	20	General condenser, heat exchanger and process tube
Grade 3 (R50550)	O 0.35 max, Fe 0.30 max	450	380	18	Higher-strength unalloyed tube, moderate forming
Grade 7 (R52400)	Pd 0.12-0.25, O 0.25 max	345	275	20	Reducing acids, hydrochloric and sulphuric media, chemical exchangers
Grade 12 (R53400)	Mo 0.2-0.4, Ni 0.6-0.9, O 0.25 max	485	345	18	Hot brine, crevice-resistant tubesheet service, desalination
Grade 9 (R56320)	Al 2.5-3.5, V 2.0-3.0, O 0.15 max	620	483	15	Aerospace hydraulic and high-pressure fluid lines

Hydrogen is normally limited to 0.015% in commercial grades and to 0.0125% in the ELI alloy grade, and carbon to 0.10% or 0.08% depending on grade. Because these limits are measured on the finished tube, not on the ingot, a mill certificate that reports only heat chemistry is incomplete for critical service. Physical properties used in design are consistent across the CP grades: density 4.51 g/cm3, melting point about 1668 deg C, elastic modulus roughly 100-110 GPa, thermal conductivity around 16-22 W/m-K and a coefficient of thermal expansion near 8.6-9.2 micrometre per metre per kelvin between 20 deg C and 100 deg C. The low modulus relative to steel makes tube vibration and fretting analysis more important than the corrosion calculation in many condensers.

Where the tube forms part of a pressure boundary designed to an ASME code, the allowable stress is taken from the code tables rather than from the mill certificate; for Grade 2 tube in ASME BPVC Section II, Part D the ambient-temperature allowable stress is in the region of 60-65 MPa and falls with temperature, with the code placing a practical ceiling on unalloyed titanium in the region of 300 deg C.

Seamless and Welded Manufacturing Routes

Seamless tube begins with titanium sponge, which is consolidated and vacuum arc remelted, or melted by electron beam or plasma cold hearth when very low inclusion content is required, then forged to billet. The billet is extruded at elevated temperature to a hollow shell, and that shell is reduced to final size by cold pilger rolling or cold drawing with intermediate vacuum annealing, typically in the 650-750 deg C band, followed by pickling to remove the oxygen-enriched alpha case. The heavy cold reduction and repeated annealing give a uniform, fine-grained, fully dense wall, which is why seamless tube remains the default for high-pressure and highly stressed duty.

Welded tube is formed from annealed and pickled strip, rolled into a cylinder and welded autogenously, without filler metal, using gas tungsten arc, plasma, laser or electron beam welding. The weld bead is scarfed or worked, the tube is then cold drawn or pilgered to final dimensions to refine the weld zone, and it is vacuum or inert-gas annealed. Well-made welded tube carries the same grade and the same minimum mechanical properties as seamless tube because the cold work and annealing steps bring the weld into the parent metal condition, and modern heat-exchanger specifications allow it for the majority of surface-condenser and general exchanger duty. It should not be assumed equivalent for every application: services with severe thermal cycling, high pressure or a requirement for maximum wall uniformity are usually written around seamless product, and the purchase specification should state which one is acceptable rather than leaving it to the mill.

Joining Titanium Tubes to Tubesheets and Support Plates

Tube-to-tubesheet joints in titanium exchangers are normally made by rolling, by explosive or hydraulic expansion, or by a combination of rolling and seal welding. The rolled joint relies on controlled wall reduction in the tubesheet hole; hydraulic expansion produces a more uniform residual contact pressure than roller expansion and is preferred where tubesheet ligaments are thin or where the tube is thin-walled. Where the joint must be leaktight against hydrogen or against a toxic process stream, a seal weld is added around the tube end, and the welding procedure is qualified in accordance with ASME BPVC Section IX with the examination method agreed between purchaser and fabricator.

Titanium welding is intolerant of atmospheric contamination. The molten and hot metal must be shielded with high-purity argon, including the weld root and the trailing zone, until the metal has cooled below roughly 400 deg C; an oxygen or nitrogen pickup shows immediately as a colour change on the finished weld. Silver or light straw is generally accepted, while a deep blue, purple or powdery white weld indicates contamination and reduced ductility, and such joints are normally rejected and cut out rather than accepted on pressure test alone. Filler wire, when used, is of matching or slightly higher purity grade, and consumables, backing gas dewpoint, arc travel speed and interpass temperature are all recorded on the welding procedure. Support plates, baffles and tube spacers interact with the same low elastic modulus that affects vibration, so clearances and hole diameters are usually specified with tighter tolerances than the tubesheet itself.

Inspection, Testing and Mill Certificate Review

Finished tube is verified by a combination of destructive and non-destructive tests. Tensile testing follows ASTM E8/E8M, grain size is measured by the methods of ASTM E112, and chemical analysis is performed by inductively coupled plasma optical emission spectrometry or an equivalent method on the finished product. The tube specifications themselves require or permit hydrostatic testing, pneumatic testing, eddy-current testing, flaring and flattening tests, and dimensional verification, with the hydrostatic test pressure calculated from an allowable fibre stress stated in the specification rather than selected by the supplier. Eddy-current examination is the fastest full-length method for detecting through-wall and subsurface defects in titanium tube, and it also confirms the degree of cold work in the wall.

A usable mill certificate for titanium tube should identify the grade and UNS number, the specification and its edition, the heat and lot numbers, the product analysis with oxygen, nitrogen, hydrogen, iron, carbon and any alloying additions, the tensile and yield results with elongation, the heat treatment condition, the results of the hydrostatic or pneumatic test and of the eddy-current examination, and the quantity and dimensions supplied. Where the order is placed against a code construction, the documentation is frequently required in the form of an EN 10204 type 3.1 certificate issued by the manufacturer with the results of the specific tests. Positive material identification by portable X-ray fluorescence is a practical field check that will confirm the presence of palladium, molybdenum and nickel in the alloyed grades, but it cannot measure carbon or hydrogen, so it supplements rather than replaces the certified product analysis.

Three failure modes deserve explicit attention in the inspection plan: loss of ductility caused by hydrogen pickup during pickling or welding, iron contamination on the surface from fabrication tooling, and excessive grain growth from annealing above the beta transus, which is roughly 880-900 deg C for CP titanium. Any of the three can pass a pressure test and still produce premature failure in service, which is why visual and metallographic checks belong in the acceptance criteria alongside the mechanical results.

Procurement, Quotation Basis and Life-Cycle Cost

Titanium tube price is not a single market number, and quotes should be compared on an identical technical basis. The dominant variables are the titanium sponge and scrap input, the grade itself because palladium and alloying additions are priced separately and palladium is a precious metal, the melting and extrusion yield, the outside diameter and wall combination, tube length and straightness class, seamless versus welded construction, and the scope of testing and documentation. Long thin tubes and short thick tubes are both more expensive per kilogram than the standard condenser sizes, and any extra eddy-current, corrosion, flaring or third-party inspection requirement carries a cost that must be priced in from the start.

A comparison of offers is only meaningful when every quotation states the same delivery basis and the same quantity basis. Offers are typically made ex-works, free on board or delivered to port, priced per kilogram or per metre, with a stated quantity tolerance, a stated currency and, for long-lead material, a stated validity period and a metal-price adjustment clause. The technical enquiry should carry the grade and UNS number, the product specification and edition, the dimensions with tolerance class, the length range, the heat treatment and surface condition, the test scope, the documentation requirement, the packing and marking requirement and the required delivery. That list removes the ambiguity that otherwise reappears as disputes at goods receipt.

On a life-cycle basis the absence of a corrosion allowance usually outweighs the higher first cost. A titanium tube of 0.7 mm wall can replace a carbon steel or copper alloy tube of 2 mm or more in seawater service, the mass per metre is lower despite the higher price per kilogram, and the replacement interval moves from a few years to the two or three decades typical of well-run seawater-cooled condensers. The real economic comparison is therefore retubing cost plus downtime plus output loss against first cost, and it is decided long before any negotiation over price per kilogram.

FAQ

Q: What is the difference between a titanium tube and a titanium pipe?
Tube and pipe are separate product families. Pipe is specified by nominal pipe size and schedule wall and is governed by ASTM B861 for seamless and ASTM B862 for welded product; tube is specified by exact outside diameter and wall thickness and is governed by ASTM B338 for condenser and heat-exchanger service. Tube is normally cold finished to tighter dimensional tolerance and is supplied in fixed lengths for exchanger assembly, whereas pipe follows the dimensional conventions of the piping line it belongs to.

Q: Which titanium grade should be used for a seawater-cooled heat exchanger?
Grade 2 covers the great majority of seawater and brackish-water exchangers, including chlorinated cooling water, because the passive film is stable in oxidising chloride environments. Grade 7 is chosen when the stream is reducing or contains acid, and Grade 12 when the risk is crevice corrosion under deposits, in tubesheet holes or under biological fouling in hot brine. Grade 1 is reserved for heavily formed parts where maximum ductility is needed rather than for pressure duty.

Q: Is welded titanium tube acceptable instead of seamless tube?
For most surface-condenser and general heat-exchanger duty, welded tube made to ASTM B338 is acceptable and is widely used, provided the weld is autogenous, the tube has been cold worked and annealed after welding so that weld properties match the parent metal, and full-length eddy-current examination and a hydrostatic or pneumatic test have been performed. Seamless tube is normally specified for higher pressures, severe thermal cycling and services where maximum wall uniformity is critical.

Q: How are titanium tubes joined to the tubesheet?
By rolling, by hydraulic or explosive expansion, or by expansion followed by a seal weld. The rolling or expansion step creates the mechanical and leak-tight joint; the seal weld is added for critical or toxic service and is qualified under ASME BPVC Section IX. Because titanium must be shielded from air above about 400 deg C, weld colour is inspected as an acceptance criterion, with silver or light straw acceptable and blue, purple or white contamination rejected.

Q: What should be checked on the mill certificate before accepting a delivery?
Confirm grade and UNS number, specification edition, heat and lot identification, product analysis including oxygen, nitrogen, hydrogen, iron, carbon and alloying elements, tensile and yield strength with elongation, heat treatment condition, hydrostatic or pneumatic test result, eddy-current examination result, dimensions and quantity, and the documentation type required by the order. Spot checks by positive material identification, dimensional measurement and visual surface inspection should accompany the document review.

Q: What temperature limits apply to titanium tube in service?
Unalloyed grades maintain useful strength and corrosion resistance in the region of 300 deg C, and code allowable stresses for Grade 2 fall away above that, so pressure-boundary design generally stops around that level. Titanium also absorbs hydrogen at elevated temperature and can form brittle hydride, so high-temperature service in hydrogen-bearing environments requires specific evaluation rather than a simple extrapolation of the seawater data.