Titanium Tube Applications by Grade, Service and Specification

Titanium Tubing as an Engineering Material

Titanium tubing occupies a narrow but decisive position in industrial procurement: it is specified where the combination of seawater-grade corrosion resistance, low density and moderate elevated-temperature strength cannot be met by stainless steel, copper-nickel or carbon steel at an acceptable service life. Commercially pure titanium has a density of approximately 4.51 g/cm3, roughly 57% that of carbon steel, and a modulus of elasticity near 105-120 GPa. Its melting point is close to 1668 C, and its thermal conductivity at room temperature is about 21.9 W/m-K, an order of magnitude below copper alloys. Those three numbers already explain most of the design rules that follow.

Two product families are normally quoted. Seamless tube is produced by extrusion followed by cold pilgering or cold drawing, and is governed by ASTM B861 for seamless tube and ASTM B338 for condenser and heat exchanger tube. Welded tube is formed from strip and continuously welded, then annealed, and is governed by ASTM B862, with ASTM B338 also covering welded heat exchanger tube. Pipe for general piping systems follows ASTM B337. For pressure equipment the ASME SB338 and SB861 designations apply, which are the ASME adoptions of the same ASTM specifications. A purchaser who mixes these designations in a single purchase order creates an acceptance dispute at the mill test report stage, so the specification and the product form must be fixed before the enquiry leaves the desk.

Grade Selection: Chemistry and Minimum Mechanical Properties

Grade choice is driven by the corrosive species, the design temperature and the required yield strength. Commercially pure grades are unalloyed titanium with controlled oxygen, nitrogen, carbon, hydrogen and iron. Oxygen is the principal strengthener and is the reason Grade 1 is soft and highly formable while Grade 4 is substantially stronger and less ductile. Palladium-bearing grades resist reducing acids and crevice attack in chlorides; the molybdenum-nickel Grade 12 is used for moderately reducing environments at lower cost than palladium grades; and the 3Al-2.5V Grade 9 alloy is chosen where the design pressure demands roughly twice the yield strength of Grade 2.

Grade	UNS Number	Nominal Composition	Min. Yield Strength	Typical Service Role
Grade 1	R50250	Unalloyed Ti, low oxygen	170 MPa (25 ksi)	Deep-drawn ferrules, high-formability tube, cladding liners
Grade 2	R50400	Unalloyed Ti, standard oxygen	275 MPa (40 ksi)	General heat exchanger tube, chemical and marine piping
Grade 7	R52400	Ti + 0.12-0.25% Pd	275 MPa (40 ksi)	Reducing acid service, crevice-resistant chloride duty
Grade 9	R56320	Ti-3Al-2.5V	483 MPa (70 ksi)	High-pressure hydraulic and instrument tubing
Grade 12	R53400	Ti + 0.3% Mo, 0.8% Ni	483 MPa (70 ksi)	Moderate reducing media, hotter chloride streams
Grade 16	R52402	Ti + 0.04-0.08% Pd	275 MPa (40 ksi)	Cost-optimised palladium grade for milder reducing duty

The mechanical values above are minimums taken from the applicable ASTM product specification and are confirmed on the mill certificate by tensile testing of each lot. Hydrogen is the element to watch during procurement: the specification limits it to 0.015% for most grades, because hydrogen pickup above that level promotes hydride formation and loss of ductility, particularly in welded tube that sees cathodic protection. A certificate showing hydrogen close to the limit is a legitimate reason to request retest rather than rejection, but the trend across successive heats is worth tracking.

Heat Exchanger and Condenser Service

The largest single volume application is tubular heat exchange in power generation, refining and heavy chemicals. Titanium tube in Grade 2 with an outside diameter between 19 mm and 25.4 mm and a wall of 0.5 mm to 1.24 mm is the standard configuration for surface condensers using once-through cooling water, including brackish and polluted seawater. The selection logic is not simply corrosion resistance: the very low thermal conductivity of titanium is offset by the thin wall that its corrosion allowance permits, so the overall heat transfer coefficient of a titanium condenser remains competitive with thicker-walled copper alloy bundles while eliminating the copper dissolution that contaminates boiler feedwater.

Design practice for these bundles concentrates on three failure modes. The first is inlet-end erosion from high-velocity water and entrained sand; the usual countermeasure is a thicker wall in the first 150-300 mm of the tube or the fitting of polymer ferrules. The second is vibration-induced fretting at baffle crossings, addressed by span limits and by the tube-to-baffle clearance specified in the exchanger design. The third is crevice corrosion under tube sheet deposits, which is the reason many specifications require periodic chlorination control and tube cleaning rather than a more highly alloyed grade. Where the cooling water is heavily reducing or the tube sheet is of a dissimilar metal, Grade 7 tube is frequently selected even though the bulk water chemistry would allow Grade 2.

Chemical Processing and Chlor-Alkali Duty

In chemical plants titanium tube serves as bayonet heaters, reboilers, absorption columns and thin-film evaporator internals. The material is essentially immune to wet chlorine, hypochlorous acid, chlorinated brine and sodium hypochlorite below the temperature at which pitting initiates, which is why chlor-alkali cell room piping and chlorine cooling towers standardise on Grade 2. Titanium is also used in acetic acid and terephthalic acid service where the process stream is oxidising; in strongly reducing acid the unalloyed grades corrode rapidly and Grade 7 or Grade 12 becomes mandatory rather than optional.

Several process-specific rules matter at the purchasing stage. Dry chlorine gas is handled in carbon steel because titanium can ignite in dry chlorine under certain conditions, so a specification calling for titanium in a dry chlorine header is an engineering error that should be questioned before quotation. Oleum and anhydrous methanol are similarly outside the safe envelope. Red fuming nitric acid is a known hazard for titanium and is excluded. In hydrogen-containing streams at elevated temperature, hydriding must be assessed against the operating partial pressure, and where hydriding risk exists the purchaser should specify a hydrogen limit at the lower end of the specification range and require the tube to be supplied in the annealed, pickled condition.

Desalination, Offshore and Seawater Cooling

Multi-stage flash and multi-effect desalination plants use titanium tube almost exclusively in the brine heater and heat recovery sections because the combination of hot aerated brine, high chloride content and residual chlorine destroys copper alloys within a few years. Typical specification is Grade 2 welded tube in 0.5-0.7 mm wall, with eddy-current examination of the weld seam on every tube. Offshore platforms use titanium for seawater cooling of generator and compressor packages, firewater systems and utility lines where weight reduction on the topside has a direct capital cost benefit and where maintenance access is expensive.

For welded tube in these services the acceptance criteria on the weld are the controlling quality characteristic. The strip must be welded in an inert atmosphere with trailing shielding, the weld bead is normally scarfed on the outside diameter and left flush inside, and the tube is annealed after welding to restore ductility in the weld zone and heat-affected zone. A flaring test, a flattening test and a hydrostatic or pneumatic test are standard, and the purchaser should state in the order whether the tube is supplied as-welded and annealed, or fully cold-worked after welding for higher strength.

Aerospace, Medical and Precision Instrument Tubing

Grade 9 Ti-3Al-2.5V seamless tube is the workhorse for aircraft hydraulic lines, fuel lines and pneumatic systems, specified in outside diameters from 3.2 mm to 38 mm with walls down to 0.5 mm and frequently in the stress-relieved condition for bendability followed by a final anneal after forming. The alloy was developed specifically to provide about 1.5 to 2 times the strength of Grade 2 with acceptable cold formability, and it tolerates the brazed or welded fittings used in aircraft plumbing. Tube for flight hardware requires lot traceability, heat identification and, in most programmes, a certificate of conformance in addition to the material test report.

Medical applications split into two distinct streams. Commercially pure Grade 2 or Grade 4 tube is used for cannulae, catheter shafts and surgical instrument components, with dimensional tolerance and surface finish, not strength, as the governing requirement. Implant devices use wrought titanium in ELI grades covered by the implant-specific ASTM specifications, which impose tighter limits on interstitial elements, particularly oxygen and iron, than the industrial tube specifications, and require validated cleaning and biocompatibility documentation. A supplier who quotes industrial Grade 2 tube for an implant application has misread the specification; the two product lines are not interchangeable even though the base metal is chemically similar.

Fabrication, Welding and Installation Practice

Titanium tube is welded by gas tungsten arc welding with a trailing shield and, for thicker sections, a purge chamber; the metal must be protected from oxygen and nitrogen above roughly 400 C or it takes up interstitials and embrittles. Colour is the field indicator: a silver or light straw weld is acceptable, a blue or purple tint indicates contamination and a white flaky oxide means the joint must be cut out. Filler wire of matching grade is used for groove welds; orbital welding is standard for instrument and hydraulic tubing in diameters up to about 25 mm.

Cold bending is practical to a centreline radius of about 2.5 to 3 times the outside diameter for annealed Grade 2 tube with an internal mandrel, and about 3.5 times for Grade 9. Springback is significant because of the low modulus, so bend tooling must be compensated accordingly. Threading is not recommended; connections are made by welded, flared or mechanically gripped fittings. Titanium is susceptible to galling during assembly, so threaded fasteners and flared joints require a suitable anti-seize compound that is compatible with the process fluid. Tube supports and clamps should be lined or of a compatible material to avoid fretting, and the tube should not be allowed to rub against carbon steel supports in wet service, where a galvanic couple accelerates attack on the steel rather than the titanium.

Inspection, Testing and Documentation at Acceptance

A purchase order for titanium tube should state the specification and grade, the manufacturing route (seamless or welded), dimensions with tolerances, the heat treatment condition, the testing regime and the documentation package. Standard tests include chemical analysis of each heat, tensile testing of each lot, flattening and flaring, hydrostatic testing at a stated pressure or an equivalent non-destructive test by agreement, and eddy-current examination. Dimensional inspection covers outside diameter, wall thickness, length, straightness and squareness of ends. Surface condition is usually specified as pickled and annealed, with a limit on depth of permissible imperfections and a prohibition on embedded iron contamination, which is detected by ferroxyl testing.

Documentation should include a mill test report traceable to the heat number, a certificate of conformance, and a dimensional and visual inspection record. For pressure equipment the material certificate is normally required to EN 10204 type 3.1. Buyers should verify that the heat number on the tube stencil, the bundle tag and the mill certificate agree; mismatches are the most common documentation defect found at goods receipt and are far easier to resolve before shipment than after. Wall thickness should be checked at several points around the circumference, because eccentricity in a seamless tube can leave the minimum wall below the nominal minus tolerance even when the average wall appears correct.

FAQ

Q: What is the difference between ASTM B338, B861 and B862 titanium tube?
B338 covers seamless and welded tube for condensers and heat exchangers, B861 covers seamless tube for general service, and B862 covers welded tube for general service. The correct choice depends on product form and end use, and all three are adopted in the ASME code as SB338, SB861 and SB862.

Q: Why is Grade 2 the default grade for seawater heat exchangers?
Grade 2 provides sufficient strength for thin-wall condenser tube, has good weldability and formability, and resists flowing seawater and chloride solutions without the pitting that affects stainless steels. It is also the most widely stocked grade, which shortens lead time.

Q: What wall thickness tolerance applies to titanium heat exchanger tube?
Wall thickness is normally specified to a percentage tolerance of the nominal wall, commonly plus or minus 10%, with the minimum wall being the governing figure for pressure design. The exact tolerance is stated in the applicable ASTM specification and should be quoted explicitly on the order.

Q: Can titanium tube be used in dry chlorine gas?
No. Titanium can ignite in dry chlorine under some conditions and is therefore not specified for dry chlorine service; carbon steel is the conventional material there. Titanium is used for wet chlorine and hypochlorite solutions.

Q: How is titanium tube weight per metre calculated?
Mass per metre in kg/m equals 0.01417 multiplied by wall thickness in mm multiplied by the mean diameter in mm, that is outside diameter minus wall thickness. The constant derives from a density of 4.51 g/cm3 and applies to commercially pure grades.

Q: What weld colour is acceptable on titanium tube joints?
Silver or light straw indicates adequate shielding. Blue or purple discolouration shows oxygen contamination and requires removal or rejection of the joint, and a white flaky oxide indicates severe contamination that cannot be repaired by cleaning.