Page 110 - Ship Construction.DJ Eyres 6Ed

Ch10-H8070.fm Page 99 Wednesday, October 18, 2006 7:37 AM
Welding Practice and Testing Welds 99
Tack welds are used throughout the construction to hold plates and
sections in place after alignment and prior to completion of the full butt or
fillet weld. These are short light runs of weld metal, which may be welded
over, or cut out in more critical joints during the final welding of the joint.
Fillet welds may be continuous or intermittent depending on the structural
effectiveness of the member to be welded. Where fillets are intermittent
they may be either staggered or chain welded (see Figure 10.2), the member
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may also be scalloped to give the same result when continuously welded.
On thicker plates it becomes necessary to bevel the edges of plates which
are to be butted together in order to achieve complete penetration of the
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weld metal (Figure 10.2). This operation may be carried out whilst profiling
or trimming the plate edges which must be aligned correctly. Most edge
preparations are made by gas or plasma heads having three nozzles out of
phase which can be set at different angles to give the required bevels. Alter-
natively the edge preparation may be obtained by mechanical machining
methods using either a planing or milling tool. For very high quality welds
in thick plate, particularly of the higher tensile types of steel, mechanical
machining may well be specified. It is worth noting that there is little to
choose between the two as far as metallurgical damage goes, but mechani-
cal methods provide a better finish.
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Plates of varying thickness may be butt welded together at different loca-
tions, a good example being where heavy insert plates are fitted. Insert
plates are preferred to doubling plates in welded construction, and the
heavy plate is chamfered to the thickness of the adjacent thinner plate
before the butt edge preparation is made.
To ease the assembly of welded units it is common practice to make use
of what is known as an ‘egg box’ construction. Within the double bottom
unit the floors and side girders may be slotted at their intersections so that
they fit neatly together prior to construction.

Welding Automation
Larger shipyards with a large production line throughput of welded panels
use automated welding systems to produce the stiffened panels. To join the
plates high speed one-sided submerged arc welding (see Figure 9.4) is used.
The required welding parameters are set in advance in the operation box
and linked to a computer. The operator selects the plate thickness and
starts the machine. The machine automatically controls the welding para-
meters for the weld crater and stops when the run-off tab is reached at the
end of the plates. The welded plate panel is then moved over floor mounted
rollers to the next stage where the stiffening members are to be attached.
Each stiffener is lowered onto the plate panel and tack welded using metal
arc gas shielded welding. The plate panel with tacked stiffeners is then