Standard Basis and Product Scope
Titanium welded tube is a longitudinally welded product formed from titanium strip, welded, cold worked where the application requires tighter tolerance, and finally annealed. The relevant specifications are ASTM B338 for welded and seamless titanium tube for condensers and heat exchangers, ASTM B862 for welded titanium pipe, and GB/T 3625 for welded titanium tube in the Chinese standard system. Common grades are Gr1, Gr2, Gr7, Gr9 and Gr12, which correspond in the GB system to TA1, TA2, TA9, TA18 and TA10.
The two production routes that dominate commercial supply are high-frequency welding and argon arc welding. The choice between them is driven by tube size, wall thickness, required surface condition and the annual volume the line has to deliver.
High-Frequency Welding: Contact and Induction Routes
High-frequency welding uses the skin and proximity effect of a high-frequency current to heat the strip edges to welding temperature, after which the edges are squeezed together by rolls. Depending on how the current is introduced, the process splits into high-frequency contact welding and high-frequency induction welding.
Contact welding offers high production speed and continuous operation. The weld zone and heat affected zone are narrow, which limits grain growth and preserves the corrosion resistance of the parent strip. Its drawbacks are higher equipment investment, more complex tooling and a less smooth weld bead, so an external or internal scarfing step is normally fitted to remove weld burrs. Poor contact between the electrode and the strip can also cause arcing, which damages the tube surface.
Induction welding reaches similar production rates without direct electrical contact with the strip, so the risk of arcing damage is lower and surface marking is reduced. It is widely used for thin wall tube and for bright finish tube where surface quality matters.
Argon Arc Welding and Inert Gas Protection
Argon arc welding, normally tungsten inert gas welding for this product, gives a smooth weld bead, stable quality, simple equipment and low capital cost, which makes it attractive for smaller tube mills and for heavier wall tube. Its limits are lower welding speed and a cast weld microstructure with a wider heat affected zone, so the as welded joint is usually cold worked and annealed before delivery to restore a wrought structure and uniform mechanical properties.
Titanium is highly reactive above 450 °C, absorbing oxygen, nitrogen and hydrogen from the atmosphere. Oxidation shows as discolouration: a light straw tint indicates slight contamination, while blue and grey scale indicates a hardened, embrittled weld that will fail in service. Trailing shields, backing gas and fully enclosed welding chambers are therefore used, with argon of high purity as the shielding gas, and oxygen content in the weld zone is monitored.
Comparison of the Two Routes
| Item | High-frequency welding | Argon arc welding |
|---|---|---|
| Production efficiency | High, continuous | Moderate |
| Equipment investment | Higher, more complex | Lower, simpler |
| Weld bead | Needs burr removal, less smooth | Smooth, good formation |
| Heat affected zone | Narrow | Wider, cast weld structure |
| Typical use | Thin and medium wall, high volume | Heavier wall, flexible sizes |
Inspection, Delivery Condition and Fabrication Notes
Welded tube is normally supplied in the annealed and pickled or bright annealed condition, with the weld seam either left in place or rolled flush. Standard delivery inspection covers chemical analysis of the heat, tensile testing of the tube, flattening and flare tests on the weld, hydrostatic or pneumatic pressure testing, eddy current testing of the weld seam, and dimensional checks on outside diameter, wall thickness and straightness. A mill test certificate to EN 10204 3.1 accompanies each lot.
For buyers, the practical points are to state the governing standard and grade, the delivered condition, the weld seam condition and whether full penetration weld quality is verified by ultrasonic or radiographic testing. Tubes ordered for condensers should also specify the required wall tolerance and the maximum permissible ovality, because these govern how reliably the tube ends expand into the tubesheet.
Frequently Asked Questions
Q: Which welding route gives better corrosion resistance in the weld?
A: Both can reach equivalent performance when the weld is properly shielded; the narrow heat affected zone of high-frequency welding and the post-weld anneal used for argon arc welded tube both restore a favourable microstructure, so final property testing matters more than the route itself.
Q: What is the maximum wall thickness for welded titanium tube?
A: Commercially, welded tube is normally supplied from about 0.3 mm up to around 5 mm wall, and seamless tube is preferred for heavier walls and for higher pressure duties.
Q: Why is weld discolouration rejected?
A: A straw, blue or grey tint shows that oxygen and nitrogen were absorbed above 450 °C, which hardens and embrittles the weld and lowers its corrosion resistance.
Q: Is the weld seam normally removed?
A: For heat exchanger and condenser service the external burr is removed and the internal bead is often rolled flush to reduce flow disturbance and fouling in the tube bore.
Q: Can welded and seamless tube be mixed in one heat exchanger?
A: Yes, provided both conform to the same grade and standard and both meet the specified pressure and eddy current test requirements; the design pressure should be checked against the welded tube derating rules.
