Seamless tubes

The high precision seamless tubes manufactured at Fine Tubes begin their journey as seamless tube extrusions, in a range of different sizes. This material is reduced in size a number of times until it gradually reaches the exact specifications our customers require. Each time we perform a reduction process, the tube needs to be cut, cleaned and heat treated before the next one can commence.

The main processes we use when manufacturing seamless tubes are tube reduction (or pilgering) and cold drawing.

Other important processes in between or after the drawing process are:

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

Optional processes include:

  • Polishing
  • Electropolishing

Seamless tube production methods

 At Fine Tubes we use multiple combinations of different processes to manufacture high quality seamless tubing to the exact specifications needed by each of our customers. The tubes can be created in a variety of different sizes and shapes using a whole range of alloys. Here are descriptions of the methods we employ.

Tube reduction - pilgering

Also known as cold pilgering or tube rocking, this process reduces the size of a metal tube across three dimensions: outside diameter, internal diameter and wall thickness. A typical first reduction elongates the tube by factors of greater than 8.

Our cold pilger tooling is designed in house and involves very complex geometry. This is tested using Finite Element Analysis (FEA) - an accurate and flexible technique to predict the performance of a structure, mechanism or process - to improve the efficiency of the design and quality of tubing produced.

Cold drawing

This is the second level of cold working tubes to reduce their size. Here the tube is pulled through a die which is smaller than the tube, so the end of each tube needs to be machined in order to fit. It will be ‘swaged’ or ‘tagged’ before drawing. Following this procedure, the tube end can pass through the die and is clamped to a drawing trolley, also called a pulley, which draws the tube through the die.

The machine used for this process is called a draw bench, and Fine Tubes has them in various sizes. A smaller one is used each time the tube is reduced. They are equipped with different tools so they can perform different types of cold drawing such as:

Sink drawing

This is the simplest drawing procedure. The tube is drawn through a hardened die made of polished tool steel or, for smaller sizes, an industrial diamond. With the correct lubrication, when the tube surface comes into contact with the polished die surface it is worked smooth. The inside bore surface is not constrained so the wall thickness of the tube will always increase when it’s drawn. Large cross sectional area reductions can be performed using this technique.

Rod drawing

Rod drawing is one of the main intermediate drawing stages. The outside diameter and wall thickness are reduced at the same time.

The tube is loaded over a hardened steel mandrel rod and both are then drawn through a die. This squeezes the tube onto the rod and reduces the wall thickness. The die and mandrel determine the size of the drawn tube which is then slightly expanded using pressure rollers so that the rod can be removed. Rod draws are used as intermediate stages to reduce tube sizes prior to the final drawing cycle.

Plug drawing

This is the type of drawing used for the highest quality output. The outside and inside diameters of the tube are worked at the same time. A plug made of high grade tool steel with a polished surface is located precisely in the middle of the outside diameter drawing die. The tube is loaded over the plug attached to a fixed rod. As the tube is drawn through the die the burnishing action of the metal flowing over the stationary plug imparts a high tolerance surface finish inside the tube.

When properly lubricated and prepared the ID will show very few defects and finishes of 0.20microns Ra are achieved. Therefore, plug drawing is chosen to significantly improve the ID surface condition.

We have developed specialist procedures to manufacture tubes with the highest quality ID surface finishes, and these are used specifically on our semiconductor, medical and aerospace products. Improved ID surfaces have several potential advantages from limited contamination and high cleanliness to improved high pressure performance, increasing the fatigue life of the tube.

Shaping

For both welded and seamless products, tubing can be drawn to almost any profile. For further details please see our page on Tube shapes and profiles.

Degreasing - cleaning

We have recently switched to using non-VOC solvent in our tube degreasing and cleaning facilities.

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 in the Fine Tubes environmental policy.

Annealing

Tube annealingThe 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 furnaces 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 (optional)

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 35 countries all over the world and use the Incoterms® as our international framework for shipping.

Request a quote