After
the steel strip is cold rolled, finished
and oiled (see section on Cold Rolling)
the coil enter the ERW Mill for tubemaking.
Strip
edges are first flattened on a leveller
and the ends cut square by an upcutting
shear. Coil ends are then flash welded
to form a continuous length. The piece
of tube with the crossweld par is
scrapped later.
The
extra metal formed after flash welding
is removed by a stripper.
A
large loop is formed between the flash
welder and forming mill to operate
continuously even if the strip is
stopped during flash welding.
The
strip edges are now trimmed to the
exact width required and to ensure
the cleanest possible surface for
selding.
The
strip then enters a series of contour
rolls which bend it progressively
upwards into a cylindrical form with
the edges abutting.
The
abutting edges are welded together
electrically. An induction coil surrounds
the tube at a distance upstream of
the weld rolls. A high frequency current
is induced into the strip and flows
along the edges of the strip between
the point where the edges meet and
the point of application or induction
of the current. It establishes a complete
circuit by flowing around the edges
of the open 'V' up to the point at
which metal to metal contact is first
made. At the weld point the 3-squeeze-roll
assembly presses the edges together
with a controlled pressure and shapes
the tube to a welded form. The result
is a strong and consistent forged
weld.
After
welding a bead is formed inside and
outside the tube along the line of
the weld. the outside bead is immediately
removed while the tube is still fairly
hot, by a set of cutting tools. The
inside bead is removed (in the case
of boiler and mechanical tubes) by
suitable bead trimmers.
The
tube passes through a series of sizing
rolls to ensure an accurate outside
diameter.
As
the tube emerges from the sizing stands,
the Alpha shear, without stopping
the tube, cuts it to the required
length.
Where
tubes are required for high temperature,
high pressure applications they are
sent for normalising in a controlled
atmosphere furnace to relieve stresses
and ensure a homogeneous grain structure.
Various
physical tests are performed on specimens
of cut lengths to ensure that the
tubes meet the requirements specified
by the customer.
Straightening,
end treatment, hydraulic testing,
final inspection and application of
protective coating.
Strip
edges are first flattened on a leveller
and the ends cut square by an upcutting
shear. Coil ends are then flash welded
to form a continuous length. The piece
of tube with the crossweld par is
scrapped later.
