Page 106 - Ship Construction.DJ Eyres 6Ed

Ch09-H8070.fm Page 95 Wednesday, October 18, 2006 7:36 AM
Welding and Cutting Processes used in Shipbuilding 95
mixture being required to cut the greater thickness sections. A water-injection
plasma-arc cutting system is available for cutting materials up to 75mm thick
using nitrogen as the carrier gas. A higher cutting speed is possible and
pollution minimized with the use of water and an exhaust system around
the torch.
Water cutting tables were often used with plasma – arc cutting but more
recent systems have dispensed with underwater cutting. Cutting in water
--- ใช้เพื่อการศึกษาเท่านั้น---
absorbed the dust and particulate matter and reduced the plasma noise and
ultraviolet radiation of earlier plasma cutters.
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
GOUGING Both gas and arc welding processes may be modified to
produce means of gouging out shallow depressions in plates to form edge
preparations for welding purposes where precision is not important. Gouging
is particularly useful in shipbuilding for cleaning out the backs of welds
to expose clean metal prior to depositing a weld back run. The alternative to
gouging for this task is mechanical chipping which is slow and arduous.
Usually where gouging is applied for this purpose what is known as ‘arc-air’
gouging is used. A tubular electrode is employed, the electrode metal con-
ducting the current and maintaining an arc of sufficient intensity to heat the
workpiece to incandescence. Whilst the arc is maintained, a stream of oxygen
ึ
is discharged from the bore of the electrode which ignites the incandescent
electrode metal and the combustible elements of the workpiece. At the
same time the kinetic energy of the excess oxygen removes the products of
combustion, and produces a cut. Held at an angle to the plate the electrode
will gouge out the unwanted material (Figure 9.9).
A gas cutting torch may be provided with special nozzles which allow
gouging to be accomplished when the torch is held at an acute angle to
the plate.

LASER CUTTING Profile cutting and planing at high speeds can be
obtained with a concentrated laser beam and increasingly employed in a
mechanised or robotic form in the shipbuilding industry in recent years. In a
laser beam the light is of one wavelength, travels in the same direction, and
is coherent, i.e. all the waves are in phase. Such a beam can be focused to
give high energy densities. For welding and cutting the beam is generated in
a CO laser. This consists of a tube filled with a mixture of CO , nitrogen,
2
2
and helium which is caused to fluoresce by a high-voltage discharge. The
tube emits infra-red radiation with a wavelength of about 1.6 μm and is
capable of delivering outputs up to 20 kW.
Laser cutting relies on keyholing to penetrate the thickness, and the molten
metal is blown out of the hole by a gas jet. A nozzle is fitted concentric with
the output from a CO laser so that a gas jet can be directed at the work
2
coaxial with the laser beam. The jet can be an inert gas, nitrogen or in the
case of steel, oxygen. With oxygen there is an exothermic reaction with the