Titanium Tube Connection Methods: Welding, Flanging, Threading and Mechanical Joints
Jan 26, 2024
Codes and Tube Specifications Behind the Selection
Joint design starts with the tube itself. Seamless and welded titanium tube for condensers and heat exchangers is ordered under ASTM B338, seamless pipe under ASTM B861 and welded pipe under ASTM B862. Pressure piping is then designed to ASME B31.3, flanges to ASME B16.5, and threaded joints to ASME B1.1 for unified inch threads. The joining method is a consequence of the design pressure and temperature those documents set, not a free choice.
Welded Connections
Welding is the default for permanent, high-integrity joints between titanium tubes or between titanium tube and other metal components. Gas tungsten arc welding is the most widely used process, with gas metal arc, electron beam and laser welding applied where penetration, speed or distortion control justifies them. Aerospace fusion welding practice is defined by AWS D17.1, and titanium filler wire designations such as ERTi-2 and ERTi-5 are listed in AWS A5.16.
The controlling variable is atmosphere, not current. Titanium reacts with oxygen and nitrogen above roughly 400 degC, so the weld pool, the hot filler and the back side of the joint must be shielded until the metal cools below that level. Trailing shields and full-chamber purging are standard for high-purity work, and any discolouration beyond a light straw tint is evidence of contamination and normally rejects the joint. Weld quality is verified by visual inspection, penetrant or radiographic examination, and for critical circuits by hardness survey of the heat-affected zone.
Flanged and Mechanical Joints
| Method | Typical duty | Pressure/temperature suitability | Disassembly |
|---|---|---|---|
| Welded (GTAW) | Process and heat exchanger tubing | All classes within tube rating | Not intended |
| Flanged | Equipment and header connections | High pressure and high temperature | Yes, with gasket replacement |
| Threaded | Instrument and utility lines | Low pressure, non-critical | Yes |
| Ferrule / compression | Instrumentation, small bore | Medium pressure, controlled temperature | Yes, ferrule replaced |
| Quick coupling | Lines requiring frequent changeout | Low to medium pressure | Yes, tool-free |
| Expansion | Special installation cases | Dependent on tube wall | No |
Flanged connections fix the tube to a mating flange with bolts and a gasket, and are used where equipment must be removable. Flange dimensions and pressure-temperature ratings come from ASME B16.5, and bolt loads must be controlled to avoid crushing a thin-wall tube. Threaded connections are limited to low-pressure, non-critical circuits because a cut thread reduces wall thickness and creates a crevice that can initiate corrosion in chloride service.
Ferrule, Quick-Coupling and Expansion Joints
Ferrule connections seal by deforming a metal ring into the tube surface as the nut is tightened; the seal is created by controlled plastic deformation rather than by the thread, and the ferrule is discarded when the joint is remade. Quick couplings are used where lines are changed repeatedly, such as liquid transfer systems. Expansion joints are formed by expanding one tube end to receive the other, and are used for special installation cases rather than for routine pressure duty. In all three cases the tube outside diameter and wall tolerance from ASTM B338 govern whether the fitting will seal at the design pressure.
Joint Preparation and Personnel Requirements
Tube ends must be cut square, deburred and degreased before any joining operation; a chloride-bearing solvent or a contaminated abrasive will cause later cracking. Field welds and mechanical joints on titanium systems are normally restricted to personnel trained specifically on titanium, because the material behaves differently from stainless steel in heat input, shielding requirement and galling tendency. Threaded and ferrule joints should be assembled with a suitable anti-galling compound to prevent seizure during later maintenance.
Frequently Asked Questions
Q: Which welding process is used most for titanium tube?
A: Gas tungsten arc welding. Electron beam and laser welding are applied where deeper penetration or lower distortion is required.
Q: Why must the back of a titanium weld be purged?
A: Titanium absorbs oxygen and nitrogen above about 400 degC. Without inert shielding on both sides, the root oxidises and loses ductility and corrosion resistance.
Q: When should a flanged joint be chosen over a weld?
A: When the connection must be opened for maintenance or equipment changeout, and where the pressure and temperature class calls for a bolted flange to ASME B16.5.
Q: Can titanium tube be threaded like steel pipe?
A: It can, but threading reduces the wall and creates a crevice, so threaded joints are limited to low-pressure, non-critical circuits.
Q: What is the function of the ferrule in a compression fitting?
A: The ferrule is swaged into the tube surface and provides the seal; the fitting body and nut only apply the deformation, so the ferrule is replaced when the joint is remade.
