TPi March 2016

An expert view on small diameter tubing Article supplied by Fine Tubes, UK, and Superior Tube, USA

How are they produced? Superior Tube and Fine Tubes offer two different forms of small diameter tubing – seamless and weldrawn. The manufacture of seamless tubing begins with either an extruded hollow tube or a solid bar that has been drilled out. It continues with a series of cold working steps to reduce the OD and make the wall thickness thinner. After each step, the tube is cut and then put through an annealing process to relax the stresses inside the tube and enable the next cold working stage to begin, until the product reaches the exact size, tolerances and temper required by the customer. The weldrawn process, originally developed and trademarked by Superior Tube, starts with a cold rolled strip of metal that is formed into a tubular shape with an open seam. TIG welding is applied to join the seam together before the same cold working and annealing steps are applied as for the seamless form. These steps have the effect of recrystallising the structure in the weld zone, creating a fine grain structure and making the weld invisible to the naked eye. One of the biggest challenges with small diameter tube processing relates to the cleaning of the interior diameter (ID). For the cold working of any tube, it is essential to use lubricants in order to prevent surface defects, but those lubricants have to be completely removed from both the OD and ID surfaces before the next annealing step is undertaken,

F or precision tube manufacturers Fine Tubes (UK) and Superior Tube (USA), small diameter tubing is viewed as tubing with an outside diameter (OD) of less than 9.5mm ( 3 / 8 "), typically with a correspondingly thinner wall thickness, although both businesses also produce small diameter tubes with thicker walls, depending on the requirements of the end applications.

What are they used for? The applications cover a wide range of markets, including aerospace, medical and nuclear. In the aerospace industry, for example, small diameter tubes are used extensively for hydraulic lines, instrumentation, engine components and heat exchangers. The products used generally have an OD of between 3.2 and 9.5mm ( 1 / 8 " to 3 / 8 ") but, when required, tubing with an OD as small as ten thousandths of an inch (0.01") – about three times the thickness of a human hair – can be manufactured. For the medical market, more than 80 per cent of the tubes that Superior Tube produces are ultimately implanted in a patient’s body, which means the tubing not only needs to meet the highest strength-to-weight ratios but also the highest

standards of microbiological corrosion resistance and fatigue life. Products from both Superior Tube and Fine Tubes form part of a wide range of medical devices such as coronary stents, artificial heart valves and components for implantable defibrillators. In the nuclear energy market, both tubemills have long records of supplying small diameter tubing for a range of in-core reactor components. Precision, quality and performance are crucial, and tubes are deployed in the fuel cans of advanced gas-cooled reactors in which they endure temperatures of 650°C, without fail, for five years non-stop.

High pressure tubes with 40-micron wall thickness

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March 2016 Tube ProducTs InTernaTIonal