Page 112 - Ship Construction.DJ Eyres 6Ed

Ch10-H8070.fm Page 101 Wednesday, October 18, 2006 7:37 AM
Welding Practice and Testing Welds 101
rolled within reach of a fillet welding gantry with double fillet welding
machines. The gantry moves parallel to the stiffeners at the same speed as
the welding heads and carries packs containing the welding wires and
power sources for the double fillet welding heads. Welding is carried out
using flux-cored wires with CO2 gas shielding.
The use of robotics for welding is referred to in Chapter 13.

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Welding Sequences
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During the welding operation heat is applied to the plate, and because of
this the metal will expand, and on cooling contract. A weld on cooling and
contracting tends to pull the plate with it. This results in a structural
deflection, the restraining action of the plate preventing the weld from
contracting fully. The actual distortion of a welded structure is difficult to
predict owing to the lack of knowledge of the degree of restraint. It is
known however that shrinkage in butt welds does occur principally along
the length of the weld, and to a lesser extent across it. If a high restraint is
provided in an effort to control distortion the structure will contain high
residual stresses, which are to be avoided.
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In order to minimize distortion in manual welding the ‘backstep’ and
‘wandering’ methods of welding are often used, the length of each step
being the amount of weld metal laid down by an electrode to suit the
required cross section of weld (see Figure 10.3).
To reduce distortion and limit the residual stresses in the structure it is
important that a correct welding sequence should be utilized throughout
the construction. This applies both during the fabrication of units and at
erection and joining on the berth.
Of the more important welds in the construction of the ship the sequences
involving welding of butts and seams in plating panels may be considered
(see Figure 10.4). At T intersections it is necessary to weld the butt first
fully, then gouge out the ends to renew the seam edge preparation before
welding the seam. Welding the seam first would cause high restraint across
the plate strake and when the butt was finished a crack might occur. General
practice when welding shell panels is to start by welding the central butts and
then adjacent seams, working outwards both transversely and longitudinally.
Ships’ structural panels have various forms of stiffener attached to the plate
panels, these generally being welded to the panel after completing the weld-
ing of the panel plates. These stiffening members are left unwelded across
the butts and seams of the plates until these are completed, if they are
attached at some intermediate stage.
Erection welding sequences generally follow the principles laid down for
plating panels. In welded ships the lower side plating seams should not be
welded before the upper seams, particularly the deck and gunwale seams. If
this sequence of welding the side shell were adopted the upper portion of