Page 94 - Ship Construction.DJ Eyres 6Ed

Ch09-H8070.fm Page 83 Wednesday, October 18, 2006 7:36 AM
Welding and Cutting Processes used in Shipbuilding 83
with a fusible backing, using either flux or glass fibre materials to contain
and control the weld penetration bead.

Stud Welding Stud welding may be classed as a shielded arc process, the arc
being drawn between the stud (electrode) and the plate to which the stud is
to be attached. Each stud is inserted into a stud welding gun chuck, and a
ceramic ferrule is slipped over it before the stud is placed against the plate
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surface. On depressing the gun trigger the stud is automatically retracted
from the plate and the arc established, melting the end of the stud and the
local plate surface. When the arcing period is complete, the current is auto-
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
matically shut off and the stud driven into a molten pool of weld metal so
attaching stud to plate.
Apart from the stud welding gun the equipment includes a control unit
for timing the period of current flow. Granular flux is contained within the
end of each stud to create a protective atmosphere during arcing. The
ceramic ferrule which surrounds the weld area restricts the access of air to
the weld zone; it also concentrates the heat of the arc, and confines the
molten metal to the weld area (see Figure 9.5).
Stud welding is often used in shipbuilding, generally for the fastening of
stud bolts to secure wood sheathing to decks, insulation to bulkheads, etc.
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Apart from various forms of stud bolts, items like stud hooks and rings are
also available.

GAS SHIELDED ARC WELDING PROCESSES The application of bare
wire welding with gas shielding was developed in the 1960s, and was quickly
adopted for the welding of lighter steel structures in shipyards, as well as
for welding aluminium alloys. Gas shielded processes are principally of an
automatic or semi-automatic nature.

Tungsten Inert Gas (TIG) Welding In the TIG welding process the arc is
drawn between a watercooled non-consumable tungsten electrode and the
plate (Figure 9.6). An inert gas shield is provided to protect the weld metal
from the atmosphere, and filler metal may be added to the weld pool as
required. Ignition of the arc is obtained by means of a high frequency dis-
charge across the gap since it is not advisable to strike an arc on the plate
with the tungsten electrode. Normally in Britain the inert gas shield used
for welding aluminium and steel is argon. Only plate thicknesses of less
than 6 mm would normally be welded by this process, and in particular alu-
minium sheet, a skilled operator being required for manual work. This may
also be referred to as TAGS welding, i.e. tungsten arc gas-shielded welding.

Metal Inert Gas (MIG) Welding This is in effect an extension of TIG welding,
the electrode in this process becoming a consumable metal wire.