TPi March 2016

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March 2016 Tube ProducTs InTernaTIonal 83
An expert view on
small diameter tubing
Article supplied by Fine Tubes, UK, and Superior Tube, USA
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.
For 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.
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,
High pressure tubes with 40-micron wall thickness