Page 89 - Ship Construction.DJ Eyres 6Ed

Ch09-H8070.fm Page 78 Wednesday, October 18, 2006 7:36 AM
78 Ship Construction
welding can be employed in the fabrication of ventilation and air con-
ditioning trunking, cable trays, and light steel furniture; some plumbing
and similar work may also make use of gas welding. These trades may also
employ the gas flame for brazing purposes, where joints are obtained without
reaching the fusion temperature of the material being joined.

--- ใช้เพื่อการศึกษาเท่านั้น---
Electric Arc Welding
The basic principle of electric arc welding is that a wire or electrode is
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
connected to a source of electrical supply with a return lead to the plates to
be welded. If the electrode is brought into contact with the plates an electric
current flows in the circuit. By removing the electrode a short distance from
the plate, so that the electric current is able to jump the gap, a high temper-
ature electrical arc is created. This will melt the plate edges and the end of
the electrode if this is of the consumable type.
Electrical power sources vary, DC generators or rectifiers with variable
or constant voltage characteristics being available as well as AC transformers
with variable voltage characteristics for single or multiple operation. The
latter are most commonly used in shipbuilding.
ึ
Illustrated in Figure 9.2 are the range of manual, semi-automatic, and
automatic electric arc welding processes which might be employed in ship-
building. Each of these electric arc welding processes is discussed below
with its application.

SLAG SHIELDED PROCESSES Metal arc welding started as bare wire
welding, the wire being attached to normal power lines. This gave unsatis-
factory welds, and subsequently it was discovered that by dipping the wire in
lime a more stable arc was obtained. As a result of further developments
many forms of slag are now available for coating the wire or for deposition
on the joint prior to welding.

Manual Welding Electrodes The core wire normally used for mild steel
electrodes is rimming steel. This is ideal for wire drawing purposes, and
elements used to ‘kill’ steel such as silicon or aluminium tend to destabilize
the arc, making ‘killed’ steels unsuitable. Coatings for the electrodes nor-
mally consist of a mixture of mineral silicates, oxides, fluorides, carbonates,
hydrocarbons, and powdered metal alloys plus a liquid binder. After mixing,
the coating is then extruded onto the core wire and the finished electrodes
are dried in batches in ovens.
Electrode coatings should provide gas shielding for the arc, easy striking
and arc stability, a protective slag, good weld shape, and most important of
all a gas shield consuming the surrounding oxygen and protecting the molten
weld metal. Various electrode types are available, the type often being defined