1. Starting Material
Titanium alloy tubing begins with an ingot produced by vacuum arc remelting, which is forged and rolled into billet. The billet is conditioned and inspected before it is made into a tube hollow. For welded tubing the starting material is titanium strip, which is produced by hot and cold rolling and supplied in coil form to ASTM B265. The quality of the starting material controls the quality of the finished tube, so every heat is traced and certified.
2. Extrusion of the Hollow
The billet is heated and pierced, then extruded into a thick-walled hollow through a die, using glass lubrication to reduce friction. The extrusion temperature is selected below the beta transus for alpha-beta alloys so that the microstructure remains suitable for subsequent cold work. After extrusion the hollow is cooled, conditioned and cut to length. Extrusion produces the initial tube geometry from which all smaller sizes are derived.
3. Cold Pilgering and Drawing
Cold pilgering reduces the outside diameter and wall thickness of the hollow in a single pass using a reciprocating roll and mandrel, achieving large reductions with good dimensional accuracy. Drawing, with a plug or a floating mandrel, further reduces the tube and sets the final bore. Between passes the tube is annealed to restore ductility, and the surface is cleaned and inspected. The cold work also refines the microstructure and improves the mechanical properties.
4. Annealing and Surface Treatment
Annealing is carried out in vacuum or inert atmosphere to recrystallise the worked structure and set the final strength and ductility. The annealed tube is then pickled to remove any surface oxide and to produce a clean, uniform finish. Surface treatment is critical for tube, because surface defects act as stress raisers and can shorten fatigue life. The finished surface condition is verified and documented.
5. Welded Tubing Route
Welded tubing is formed by roll-forming titanium strip into an open tube and seam welding it by gas tungsten arc welding under full inert gas shielding. The weld is then cold worked by passing the tube through a sizing operation, which consolidates the weld structure, and the tube is annealed to match the weld and base metal properties. Welded tubes are examined along the weld by eddy current or ultrasonic methods, and the finished product meets the same mechanical requirements as seamless tube.
6. Final Testing and Delivery
The finished tube is straightened, cut and tested. Tensile tests verify the mechanical properties, and the flattening and flaring tests confirm ductility. Hydrostatic or pneumatic pressure testing, or eddy current examination to ASTM E426, proves leak tightness and soundness. Dimensional checks cover the diameter, wall and ovality, and the mill certificate reports the composition, properties and heat treatment. The tube is then packed with protected ends for shipment.
Seamless tube is reduced by cold pilgering and drawing with intermediate annealing.
Welded tube is seam welded under inert gas and cold worked to consolidate the weld.
Eddy current and pressure testing confirm the quality of the finished tube.
Frequently Asked Questions
Q: What is cold pilgering?
Cold pilgering is a tube reduction process in which a reciprocating roll and mandrel reduce the diameter and wall thickness in one continuous pass.
Q: Why is intermediate annealing needed?
Cold working hardens titanium rapidly, so the tube is annealed between passes to restore ductility for the next reduction.
Q: Are welded tubes as strong as seamless tubes?
Welded tubes that are cold worked and annealed have properties close to seamless tubes and are accepted by the same specifications.
Q: What tests are done on the finished tube?
Tensile, flattening, flaring, hydrostatic or pneumatic pressure, eddy current and dimensional tests are the standard programme.
Q: Why is the surface finish of tubing important?
Surface defects concentrate stress and reduce fatigue life, so the tube surface is cleaned, pickled and inspected during production.
