Page 91 - Ship Construction.DJ Eyres 6Ed

Ch09-H8070.fm Page 80 Wednesday, October 18, 2006 7:36 AM
80 Ship Construction
by the nature of the coating. The more important types are the rutile and
basic (or low hydrogen) electrodes. Rutile electrodes have coatings containing
a high percentage of titania, and are general purpose electrodes which are
easily controlled and give a good weld finish with sound properties. Basic
or low hydrogen electrodes, the coating of which has a high lime content,
are manufactured with the moisture content of the coating reduced to a
minimum to ensure low hydrogen properties. The mechanical properties of
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weld metal deposited with this type of electrode are superior to those of
other types, and basic electrodes are generally specified for welding the higher
tensile strength steels. Where high restraint occurs, for example at the final
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erection seam weld between two athwartships rings of unit structure, low
hydrogen electrodes may also be employed. An experienced welder is
required where this type of electrode is used since it is less easily controlled.
Welding with manual electrodes may be accomplished in the down-
hand position, for example welding at the deck from above, also in the hor-
izontal vertical, or vertical positions, for example across or up a bulkhead,
and in the overhead position, for example welding at the deck from below
(Figure 9.3). Welding in any of these positions requires selection of the cor-
rect electrode (positional suitability stipulated by manufacturer), correct
current, correct technique, and inevitably experience, particularly for the
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vertical and overhead positions.

Automatic Welding with Cored Wires Flux cored wires (FCAW) are often
used in mechanised welding allowing higher deposition rates and improved
quality of weld. Basic or rutile flux cored wires are commonly used for one-
sided welding with a ceramic backing.

Submerged Arc Welding This is an arc welding process in which the arc
is maintained within a blanket of granulated flux (see Figure 9.4). A con-
sumable filler wire is employed and the arc is maintained between this wire
and the parent plate. Around the arc the granulated flux breaks down and
provides some gases, and a highly protective thermally insulating molten
container for the arc. This allows a high concentration of heat, making the
process very efficient and suitable for heavy deposits at fast speeds. After
welding the molten metal is protected by a layer of fused flux which together
with the unfused flux may be recovered before cooling.
This is the most commonly used process for downhand mechanical weld-
ing in the shipbuilding industry. Metal powder additions which result in a
30–50 per cent increase in metal deposition rate without incurring an increase
in arc energy input may be used for the welding of joint thicknesses of 25mm
or more. Submerged arc multi-wire and twin arc systems are also used to
give high productivity.
With shipyards worldwide adopting one-side welding in their ship panel
lines for improved productivity the submerged arc process is commonly used