TPi September 2016

63 / 68

september 2016 Tube ProducTs InTernaTIonal 61
Not all tubes are created equal
By Stewart Jones, Tata Steel
Specification of any product is largely based upon the
available budget and ensuring compliance with key
legislation. Therefore, a product’s manufacturing process can
often be overlooked, with more influential drivers leading a
specification. In the tube and pipework industry, this is often
the case, with a tube’s manufacturing process not taken into
consideration.
Produced by two methods, either hot-finished or cold-formed,
the two processes create products with similar specification
information. However, there are several underlying factors and
properties that should be considered.
Formed from a flat coil, cold-formed steel tubes are shaped
when cold, then welded to produce the tube
shape. During this process the material undergoes
work hardening, interrupting the grain structure of
the material and introducing internal stresses.
This occurs in the region surrounding the welded
seam and is often referred to as the heat affected
zone (HAZ). Within this region the steel has a
distorted gain structure, producing a harder,
less malleable material to work with. During
fabrication and installation this distortion can
be increased by additional working and product
manipulation. This heightens the level of internal
stress, consequently increasing the risk of the
product cracking and failing.
Used in heating, air conditioning and ventilation
applications as well as for general conveyance
purposes, a cracking and failing tube product
Stewart Jones, senior customer technical services
field engineer, conveyance tubes at Tata Steel,
outlines the distinctive
differences between
hot and cold formed
tube products, and
discusses the training
required to resolve
confusion surrounding
British and European
standards.
can have a serious impact. Therefore reducing the risk of
failure and consequently improving the health and safety of
employees is of significant importance.
While the manufacturing process for hot-finished tubes is
very similar to the cold-formed process, the addition of a vital
step relieves any internal stress. To provide a full-bodied hot-
finish process route, the steel tubes are heated to very high
temperatures using a furnace or induction process. As the
temperature of the steel rises, its internal stresses are relaxed
and the HAZs are eradicated.
Removing the internal weakness within the steel tube means
that hot-finished tubes present several benefits over their
cold-formed counterpart throughout the product’s lifespan.
For example, with a uniform microstructure and no internal
stress, the product has consistent and reliable mechanical
properties throughout the entire product.
Furthermore, the heating process also improves the final
product’s structural integrity and ductility, its toughness
and its ability to withstand high pressure. This means that
hot-finished steel tubes will not suffer any loss of strength
from additional welding and heating, and have an increased
performance against corrosion. These characteristics are
essential for tube products used in environments where
product failure is not an option and employee health and
safety is paramount.
As well as being structurally sounder and stronger, hot-
formed tubes have added practical advantage over seamless
Stewart Jones