How the Processing Route for Titanium Tubing Is Chosen
Titanium tubing reaches its finished dimensions through hot working, cold working or a combination of the two. Forging, hot rolling and hot extrusion supply the coarse shape, while cold rolling, cold drawing and cold extrusion deliver the final wall thickness, diameter and surface finish. The route that is chosen fixes the grain size, wall tolerance, ovality, residual stress level and internal surface condition of the delivered tube, so it is decided at the quoting stage rather than adjusted on the shop floor.
A seamless tube normally follows one sequence: sponge, compacted electrode, melting, forging, bar billet, extrusion, rolling, straightening, finished tube. Welded tube takes a different path from hot rolled coil through cold rolling, forming, longitudinal welding and sizing. The two routes behave differently in service, which is why the applicable product standard, such as ASTM B338 for seamless and welded tube, is quoted together with the process route.
Material Properties and Their Processing Consequences
Titanium offers high strength and outstanding corrosion resistance, but its room-temperature plasticity is limited and its thermal conductivity is low. Both characteristics shape the process. Low plasticity restricts how much reduction can be taken in a single cold pass before cracking, so intermediate annealing is built into the schedule. Poor conductivity keeps the heat generated by cutting, drawing or grinding close to the surface, promoting local overheating, galling and tool wear unless cutting speeds, feeds and cooling are chosen carefully.
The metal also has a strong affinity for interstitial elements. Above roughly 400 °C it absorbs oxygen, nitrogen and hydrogen readily, producing a hard, brittle surface layer and, in the case of hydrogen, an internal embrittlement risk. Every step of the schedule therefore needs either a controlled atmosphere, a protective coating or an allowance for removing the contaminated layer afterwards.
Condition of the Billet, Plate and Coil
Tubing is made from sheet, coil or a bored billet, so the quality of the previous processing step carries directly into the tube. Cutting, forming and machining operations that were performed without adequate control leave cracks, folds, laps, inclusions and locked-in internal stress. A surface that was not fully conditioned may still carry an oxygen-rich alpha case layer, and that layer will crack rather than deform during extrusion or drawing.
The practical countermeasure is to inspect and condition incoming material before it reaches the tube mill: machine or grind away surface defects, pickle off scale and contamination, and verify chemistry and hardness so that the deformation behaviour of the batch is predictable.
Temperature and Atmosphere Control
Titanium melts at approximately 1668 °C, which is high enough that furnace design and tooling life both matter, but the more important thresholds are much lower. The alpha-to-beta transformation temperature lies near 882 °C for Grade 2 and near 995 °C for Grade 5, and working above or below that temperature produces a different microstructure, a different flow stress and therefore a different finished property set. Forging and extrusion temperatures are set with that boundary in mind.
Atmosphere control runs alongside temperature control. Vacuum or high-purity argon is used for annealing, and furnace dew point and residual oxygen are monitored during hot working. Excess humidity and airborne dust also matter at the cold end of the plant, where they affect surface finish and can lead to pick-up on the tube bore. A clean, dry, temperature-controlled workshop is a process variable, not a housekeeping detail.
Deformation Control, Equipment, Skills and Safety
Deformation is the lever that converts a specification into a property. Too little reduction leaves the structure coarse and the mechanical properties below target; too much reduction raises forming resistance, creates local hot spots, produces uneven strain and can trigger abnormal grain growth. A graded schedule that increases reduction step by step refines the structure progressively and keeps the tube inside its forming window.
| Stage | Key control point | Typical practice |
|---|---|---|
| Melting | Chemistry, segregation, inclusions | Double or triple vacuum arc remelting |
| Billet conditioning | Surface defects, alpha case | Machining, grinding and acid pickling |
| Hot working | Temperature against the beta transus | Forging or extrusion under controlled atmosphere |
| Cold working | Pass reduction, lubrication, tooling wear | Plug or mandrel drawing with intermediate anneal |
| Annealing | Temperature, soak time, atmosphere | Vacuum or argon cycle to relieve work hardening |
| Finishing | Straightness, tolerance, soundness | Straightening, pickling, hydrostatic proof test |
Equipment capability and operator skill sit behind every line of that table. Heating, forging and cold drawing each demand trained operators who can read the process rather than follow a fixed recipe, and the high-temperature, high-energy environment requires strict safety management covering furnace handling, hot work, fine metal dust and pressure testing.
The process is finally verified rather than assumed: dimensional and ovality checks, hardness and flattening tests, eddy current or ultrasonic inspection where specified, a proof pressure test, and a mill certificate to EN 10204 3.1 that records the chemical composition and mechanical properties of the batch.
Frequently Asked Questions
Q: Which manufacturing route is best for titanium tubing?
Seamless tube suits high-pressure and demanding service, while welded tube is more economical for thin walls and long lengths; the choice depends on pressure, code and surface requirements.
Q: At what temperature does titanium tubing stop absorbing oxygen?
Practical risk begins above roughly 400 °C, so heating for forging, extrusion or annealing is carried out in vacuum, argon or another controlled atmosphere.
Q: Why is intermediate annealing needed during cold drawing?
Cold working hardens titanium quickly and reduces ductility; an anneal between passes restores elongation so the next reduction can be taken without cracking.
Q: How does pass deformation change tube properties?
Graded, moderate reductions refine the structure and raise strength evenly, while excessive reduction causes local heating, uneven strain and abnormal grain growth.
Q: Do defects in the incoming billet affect the finished tube?
Yes. Cracks, folds, inclusions, stress and residual alpha case all propagate through tube making, so incoming material is inspected and conditioned first.
Q: How is titanium tubing qualified after manufacture?
By dimensional and surface inspection, hardness and flattening tests, eddy current or ultrasonic examination where required, a proof pressure test and a 3.1 mill certificate.
