Titanium Pipe (Grade 2 / Grade 5 / Grade 7 / Grade 9 / Grade 12)

These four ASTM standards cover titanium tubular products for different applications with distinct dimensional, manufacturing, and inspection requirements. ASTM B337 (Titanium and Titanium Alloy Seamless and Welded Pipe) covers pipe intended for general industrial piping applications including chemical, marine, power generation, and process plant service. B337 pipe follows standard pipe dimensions (NPS designation system with nominal bore and schedule wall thickness per ASME B36.19M for metric sizes), available in both seamless and welded manufacture, grades 1 through 38. The standard specifies hydrostatic testing of every length at a test pressure calculated from the specified minimum yield strength, mandatory dimensional inspection, and either hydrostatic or non-destructive examination (eddy current) of the weld seam for welded pipe. ASTM B861 (Titanium and Titanium Alloy Seamless Pipe) covers seamless pipe only, with the same dimensional system as B337 but applicable specifically to the seamless manufacturing route — B861 replaces the seamless pipe provisions formerly combined with B337. ASTM B862 (Titanium and Titanium Alloy Welded Pipe) similarly covers welded pipe only, specifically the electric fusion welded or GTAW autogenous welded manufacturing route — B862 covers welded titanium pipe from large-diameter applications not efficiently produced by seamless methods. ASTM B338 (Seamless and Welded Titanium and Titanium Alloy Tubes for Condensers and Heat Exchangers) covers tube intended for installation in heat exchanger and condenser applications where the tube must be expanded into and sometimes welded to a tube sheet. B338 tube uses the OD × wall thickness dimensional system (rather than the nominal bore schedule system of B337/B861/B862) in the typical condenser tube range of 12.7–50.8mm OD with thin walls of 0.5–2.77mm, providing the thin-wall, high surface-area tubular geometry optimal for heat transfer. B338 requires eddy current or ultrasonic examination of the full length of every tube for internal and surface defects — mandatory because a single defective tube in a heat exchanger bundle cannot be individually replaced without complete tube bundle removal. The critical practical distinction: use B337/B861/B862 for titanium process piping systems; use B338 for heat exchanger tube bundles, condenser tube bundles, and other shell-and-tube heat exchanger applications.

Seamless and welded titanium pipe differ in manufacturing method, dimensional capability, mechanical property uniformity, weld zone characteristics, pressure rating philosophy, and cost — with each manufacturing route suited to different application requirements. Seamless titanium pipe is produced by hot piercing and rolling of solid titanium billet into a hollow tube without any longitudinal weld seam, followed by cold drawing and annealing to achieve final dimensions and properties. The defining advantage of seamless pipe is the absence of a longitudinal weld zone — the entire circumference of the pipe cross-section has uniform metallurgical structure and mechanical properties, with no potential weld defects, heat-affected zone microstructural changes, or residual welding stresses that must be managed in pressure system design. This makes seamless pipe the preferred specification for: high-pressure applications (above approximately 1,000 psi / 70 bar service pressure) where the weld zone margin is important; cyclic pressure and thermal fatigue applications where weld zone stress concentrations could initiate fatigue cracking; corrosive high-temperature service where weld zone sensitisation would accelerate local corrosion; critical aerospace structural tubing (AMS 4942 Grade 9 hydraulic tubing); and small-diameter precision tubing below approximately 25mm OD where seamless production is the only practical manufacturing route. Welded titanium pipe is produced by roll-forming titanium strip into a cylinder and joining the longitudinal seam by autogenous GTAW welding in argon atmosphere. Welded pipe offers: lower production cost versus seamless for medium and large diameters (above approximately 50mm OD) and thin wall thicknesses where seamless production is expensive or unavailable; better dimensional accuracy (OD and wall thickness tolerance) for the welded-and-drawn (W&D) production route; and availability in very large diameters (up to 610mm OD) where seamless titanium pipe is impractical due to ingot and equipment size limitations. The weld seam in premium welded titanium pipe (full penetration autogenous GTAW with argon backing gas purge) provides mechanical properties in the weld zone (ultimate tensile strength, yield, elongation) meeting or very closely approaching the parent material specification values — the titanium weld seam does not require filler metal and the weld chemistry is identical to the base material. Welded titanium pipe is the dominant commercial form for: heat exchanger condenser tubing (ASTM B338) in the 12.7–50.8mm OD range where high-volume welded-drawn production provides the closest possible dimensional tolerances for tube-to-tubesheet expansion; large-diameter process piping (DN100–DN600) for seawater and chemical service where the cost of seamless pipe in this size range is prohibitive; and medium-pressure piping systems (below approximately 700 psi / 50 bar) where the ASME B31.3 chemical plant piping code applies and welded pipe with the standard weld joint efficiency factor of 0.85 (or 1.0 for 100% radiography examined welds) provides adequate pressure rating.

Titanium heat exchanger condenser tube installation in tube sheets requires specific mechanical expansion techniques adapted to titanium's unique work-hardening characteristics and the necessity of achieving leak-free, pull-out-resistant joints without exceeding titanium's ductility limits. The tube-to-tubesheet expansion process for titanium tubes follows the same mechanical rolling or hydraulic expansion methodology used for brass and stainless steel condenser tubes, with specific parameter adjustments required for titanium's higher yield strength (Grade 2: 275 MPa yield versus ~170 MPa for 90/10 cupronickel) and higher work-hardening rate. Mechanical roller expansion uses a mandrel-driven multi-roll expander that radially expands the titanium tube against the tube sheet bore, creating a mechanical interference fit joint — the tube OD is increased by 1.5–2.5% beyond the tube sheet bore diameter to generate sufficient contact pressure for leak-free sealing and adequate tube pull-out resistance. For thin-wall titanium condenser tube (wall 0.5–0.89mm, typical for high thermal performance condensers), the permitted tube thinning during expansion (typically expressed as percentage of wall thickness reduction) must be carefully controlled to prevent over-expansion that reduces wall thickness below the minimum required for tube integrity, or under-expansion that provides insufficient joint seal integrity — titanium tube expansion practice typically uses 5–10% wall reduction as the target, compared to 3–7% for brass and 8–12% for stainless steel, reflecting titanium's higher yield strength requiring more expansion force but its faster work-hardening rate limiting maximum permissible expansion. Hydraulic expansion (also called hydroswage or Swagelok-type expansion) uses an elastomeric plug inflated with pressurised hydraulic fluid to expand the tube uniformly around its full circumference without the localised roll contact marks that can cause crevice corrosion initiation sites in corrosive tube sheet environments — hydraulic expansion is preferred for Grade 7 (Ti-0.15Pd) and pharmaceutical-grade titanium tube installations where crevice corrosion resistance and cleanliness are priorities. Tube wall thickness selection for ASTM B338 titanium condenser tubes follows TEMA (Tubular Exchanger Manufacturers Association) standards, with 0.5mm (0.020 inch) wall being the minimum practical for Grade 2 tube in clean seawater service, 0.7mm (0.028 inch) as the standard conservative selection for most power station condenser service, and 0.9mm (0.035 inch) for higher-velocity or slightly more aggressive seawater conditions — the thin titanium walls, even at 0.5mm, provide adequate corrosion allowance because titanium's corrosion rate in seawater is effectively zero (less than 0.002 mm/year measured corrosion rate in clean seawater), so no corrosion allowance addition to the structural wall thickness is required unlike steel tube sizing calculations. Tube sheet bore diameter and tube-to-tubesheet clearance for titanium tubes per TEMA standards: standard groove (two grooves per tube sheet bore are standard for expanded tube joints) with 0.4mm (1/64 inch) diametral clearance for Grade 2 tube; the grooves lock the expanded tube against longitudinal pull-out forces from thermal expansion differential and hydraulic pressure end thrust in the condenser.

Grade 9 titanium (Ti-3Al-2.5V, UNS R56320, ASTM Grade 9) is an alpha-beta titanium alloy containing 3% aluminium and 2.5% vanadium — a composition specifically engineered to balance three competing requirements for aerospace hydraulic tubing: sufficient strength to contain hydraulic system operating pressures of 3,000–5,000 psi (207–345 bar); adequate cold-workability to enable cold-drawing to final tube dimensions in the small OD range (3–19mm) required for aircraft hydraulic lines; and minimum weight for equivalent pressure containment versus the stainless steel tubing it replaces. The Ti-3Al-2.5V alloy composition was specifically chosen over the stronger but less cold-workable Ti-6Al-4V (Grade 5) for hydraulic tubing because the lower aluminium (3% vs 6%) and lower vanadium (2.5% vs 4%) content maintains sufficient beta phase stability and ductility at room temperature to enable cold drawing of thin-wall small-diameter tube to the tight dimensional tolerances (OD ±0.05mm, wall ±0.05mm) required by aerospace hydraulic line specifications — Ti-6Al-4V cannot be cold-drawn to small OD thin-wall dimensions without multiple intermediate anneals, making tube production impractical for high-volume aerospace hydraulic system production. Grade 9 per AMS 4942 (annealed condition) achieves minimum tensile strength 620 MPa (90 ksi) and yield strength 483 MPa (70 ksi) — approximately 30% higher than Grade 2 CP titanium, enabling thinner tube walls for equivalent pressure rating. Per AMS 4944 (cold-worked and stress-relieved condition, the standard for aircraft hydraulic tubing), Grade 9 achieves 690 MPa tensile and 483 MPa (minimum) yield — providing the required burst pressure safety factor of 4:1 above working pressure in the typical 3mm–12mm OD hydraulic tube sizes used in aircraft hydraulic circuits. The weight saving of Grade 9 titanium hydraulic tubing versus the 304L stainless steel tube it replaces is approximately 43% (titanium density 4.51 g/cm³ versus stainless 7.95 g/cm³) at equivalent hydraulic pressure rating — for a wide-body commercial aircraft with 2,000–3,000 metres of hydraulic tubing throughout its hydraulic system, this represents 50–100 kg of structural weight saving per aircraft, which over a 30-year commercial life at current fuel economics represents millions of dollars of fuel saving per aircraft. Grade 9 titanium is fully compatible with phosphate ester hydraulic fluid (Skydrol LD-4 and equivalent), the aviation-standard hydraulic fluid required by aircraft certification for fire resistance — unlike aluminium alloy hydraulic tubing which reacts with phosphate ester in the presence of moisture, titanium hydraulic tubing is inert to all approved aviation hydraulic fluids at all operating temperatures. Grade 9 is supplied per AMS 4944 with mandatory eddy current inspection of the full tube length (100% ET to ASTM E309 methodology detecting surface and near-surface defects ≥0.076mm depth in 3–6mm OD tubes), hydraulic pressure testing to minimum 110 MPa (16,000 psi) for 3,000 psi rated tubing, dimensional inspection of OD and wall at minimum 15 positions per tube, and grain flow macroexamination to verify tube forming quality.

Field welding of titanium pipe is significantly more demanding than welding stainless steel or carbon steel due to titanium's extreme reactivity with oxygen, nitrogen, and hydrogen at welding temperatures — contamination of the weld metal or heat-affected zone by atmospheric gases produces brittle oxide, nitride, and hydride phases that dramatically reduce the toughness and corrosion resistance of the welded joint, potentially causing immediate weld cracking or in-service failure. The critical welding requirement is complete exclusion of all atmospheric gases from the weld metal, heat-affected zone, and the cooling titanium surface throughout the entire welding cycle and cooling period. Shielding gas requirements: high-purity argon (99.999% minimum purity, dew point ≤−60°C) is the standard shielding gas for all titanium welding; helium or argon-helium mixtures are used for higher heat input when penetration requirements demand it; nitrogen, CO2, and mixed gas blends used for steel and stainless welding are absolutely prohibited for titanium. Three shielding zones must be established simultaneously: (1) Primary arc shielding — the GTAW torch cup delivers argon gas directly around the tungsten electrode and weld puddle from the front; (2) Trailing gas shield — a separate trailing shield (often called a 'shoe' or 'sled') supplies additional argon coverage over the completed weld and HAZ as the torch advances, protecting the hot titanium surface as it cools below 400°C — the colour of the titanium weld surface is the indicator of atmospheric contamination: bright silver (acceptable, no contamination), light gold (borderline acceptable in non-critical areas — minor surface oxidation), purple or blue (reject — significant oxidation, loss of corrosion resistance), grey-white (reject — severe oxidation, embrittlement), white chalky (catastrophic contamination — weld must be completely removed). (3) Purge gas — the interior bore of the pipe must be continuously purged with argon throughout the welding operation to prevent oxidation of the pipe bore weld root pass, which is exposed to atmospheric air if not purged. The bore purge requirement distinguishes titanium pipe welding from stainless steel pipe welding — stainless steel pipe is purged to prevent carbide precipitation (sensitisation) at the weld root, but titanium pipe purging prevents catastrophic oxidation embrittlement that would occur without purge. Practical purge requirements: argon purge flow rate minimum 10–20 SLPM introduced at one end of the pipe section, with monitoring of the exit end purge gas quality using an oxygen analyser (oxygen level must be below 50 ppm, ideally below 20 ppm, before welding commences and maintained throughout); use dedicated titanium pipe purge plugs or inflatable pipe dams at both ends of the weld section to contain the purge gas; allow minimum 5 pipe volume changes of purge gas before welding. For orbital (automatic) GTAW welding of titanium hydraulic and process tubing in controlled environments (fabrication shops), dedicated titanium welding glove boxes or local inert atmosphere enclosures eliminate the atmospheric contamination risk entirely and are the preferred approach for high-quality titanium pipe spool fabrication. Filler metal for titanium GTAW should match parent material composition — ERTi-2 for Grade 2 parent pipe, ERTi-9 for Grade 9 parent pipe — and must be stored in sealed dry containers to prevent hydrogen pick-up from atmospheric moisture. Weld joint design should specify 100% penetration butt joints; fillet welds on titanium should be avoided as they create crevice geometries vulnerable to the crevice corrosion mechanism in titanium in reducing acid environments.