Welded tubes

The welding process is one of the core ways in which we manufacture precision tubes. Our high quality tubing starts off as a flat strip of metal. The strip is formed into a tube through a series of rollers and the join, also called the seam, is then welded together.

Special non-destructive testing methods – including eddy current, ultrasonic and pressure testing - are used to ensure a consistent weld. Just like with seamless manufacturing, welded tubes can be drawn down by plug drawing or sinking. This method can be used to create very long lengths of coiled tube.

The main welded tube manufacturing processes used are strip welding and cold drawing, but other important processes in between, or at the end of the drawing process, include:

  • Degreasing/cleaning
  • Annealing
  • Straightening
  • Cutting and deburring
  • NDT testing
  • Printing
  • Packaging
  • Shipment

Extra optional processes include:

  • Polishing
  • Electropolishing

Our welding capabilities

  • Single torch TIG welding for high quality products with maximum reliability and process stability. A range of specialist welding gases are available for autogenous welding conducted in an inert atmosphere.
  • Multi-torch welding affords a very versatile torch set-up and enables us to weld with various configurations, on one line. We use this for specialist applications.
  • Plasma welding means lower heat input than TIG welding, and is used for specialist applications where there are specific metallurgical and/or weld profile requirements. A range of gas mixtures are available to meet specific technical and customer requirements.
  • Fine Tubes also has advanced post-weld bead rolling capability and advanced post-weld cross-sectional weld and parent metal cold work facilities.
  • We also have autogenous orbital welding equipment and facilities. Post weld heat treatment is an option. We can also control the orbital weld profile to give zero bore weld protrusion. All welding is conducted in an inert atmosphere and all welds are fully X-ray and pressure tested.
  • Orbital welding with cold-wire feed allows for weld reinforcement with a low heat input.

Drawing processes

After welding, we can draw the tube down to alter its size and shape. Both sink drawing and plug drawing are used. The plug is not attached in this case, but ‘floating’. This means it balances in position, being pulled forward by the tube being drawn, while at the same time it’s pushed back as it tries to pass through the dies.

Between each draw the tube is degreased, annealed and tagged for the next draw. Welded tube as well as long lengths of seamless tube can be drawn on to coils.

Coiled tubing

To make coils even longer, a series of welded or seamless coils can be joined by welding two coils together. The ends of each coil are very carefully cleaned, prepared and butted together. Then, using a special TIG orbital weld, the two coils are welded together. Using this method, coils of almost any length can be made, the only limit being the size of the drum. For example, single coils up to 8,000m long are not unusual for sub-sea oil drilling projects.

Each coil is non-destructively tested for cracks and other defects and the size is checked using laser micrometers. Also, each orbital weld is X-rayed within the manufacturing process before being coiled onto drums.

Fine Tubes use a continuous feed process. In-line welding, degreasing, roll reduction and annealing are all possible.

Degreasing - cleaning

We have recently switched to a non-VOC solvent for degreasing.

European Union environmental directives have meant that the trichloroethylene-based solvent used to degrease metal tubing products has not been available since December 2010. Working with specialist chemists and university labs, Fine Tubes has developed a degreasing process using a non-volatile, biodegradable solvent for degreasing which doesn’t harm the environment. We now use this solution across all market sectors and tests have proven it to be as good as, if not better than, trichloroethylene processes. It has a lower energy footprint as well

This represents a major step forward by Fine Tubes in terms of our commitment to the environment.

Annealing

The annealing process homogenises the metallurgical microstructure of the tube. As the grain structure of the tube after drawing is stressed, the material becomes hard and brittle. To be able to draw the tube again, the stress needs to be removed to return the material to its normal state.

During annealing, the tubes are exposed to a controlled temperature (up to 1200°C) and soak time. Through this process the tube remains in shape, but the grains in the structure of the tube reform into a regular unstressed pattern. The annealed tube is then softer and can be redrawn.

For high pressure tubes final heat treatment is not required. This leaves the material in a hard drawn condition enhancing the mechanical properties of the tube. Fine Tubes uses a variety of furnaces for the annealing process, such as hydrogen furnaces for bright annealing, vacuum furnaces, gas furnaces, coil furnace and inline annealer.

We have Nadcap accreditation for heat treatment to specification AMS 2750.

NDT testing and inspection

At Fine Tubes we inspect all tubes before shipment to ensure that all elements of the customer’s specifications have been met. Standard inspections include:

  • Positive material identification (PMI)
  • Visual inspection
  • Dimensional inspection
  • Laser micrometer inspection

Our laboratory tests the mechanical properties of tube samples and carries out chemical analysis and surface checks. The results are then recorded in test certificates.

Depending on the complexity of the specification, further tests can be applied which can be grouped into destructive and non-destructive testing (NDT) methods.

Destructive testing

This includes tensile, hardness, bending, flattening, flare and metallurgical tests. These tests are executed on samples from each tube batch.

Non-destructive testing (NDT)

We use eddy current testing and ultrasonic testing. Fine Tubes also performs pressure testing in oil or water up to a maximum of 2000bar (28,800psi) for coils and a maximum of 1600bar (23,200psi) for straight lengths, or air under water tests where tube coils are submerged in a water tank and tested up to 207bar (3,000psi). Radiographic (X-ray) and magnetic permeability testing are also used.

Precise information on individual testing methods used at Fine Tubes can be found on our Testing page.

Polishing - centreless grinding (optional)

Both polishing and centerless grinding are carried out only if the specification requires it. Polishing is an aesthetic consideration to improve the surface finish of either welded or seamless products. Centreless grinding is a high precision process to remove material from the OD of seamless tubes only. In addition to bringing the tube to the correct OD dimension, it also results in a surface finish of extremely high quality.

Electropolishing

This electro-chemical process is used to further refine the ID surface smoothness of straight lengths of tube. It achieves achieve a surface roughness of just 0.1 microns (4 μ inch) Ra.

Printing

All products are printed with information relevant to the job such as heat number, dimensions, material and part number. This is printed along the length of all products dispatched, both seamless and welded.

Packaging

Seamless straight lengths of tubing are individually sleeved by hand to ensure the highest quality is maintained and no damage occurs in the final stages of manufacture. Our precision tubes are either packed in wooden crates lined with wax craft paper, tri-wall clay-coated reinforced cases, standard cardboard boxes or tubes, ready for dispatch. Welded material is mainly coiled on wooden or steel drums of various dimensions before being covered with Lamiflex and moved ready for shipping. We can also pack tubes with ID or OD end caps if required.

Shipment

Fine Tubes can arrange shipment and delivery of your tubes to your doorstep including special deliveries. We currently export to over 30 countries all over the world and use the Incoterms® as our international framework for shipping.

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