Due to the increasing scale of structures in the shipbuilding
and construction fields in recent years, adoption
of high strength and heavy thickness in the steel
materials used in those structures is progressing. In the
shipbuilding field, upscaling of container ships has progressed
to 18 000 TEU, considering efficiency in trans
portation of long distance cargos (TEU: Twenty-feet
equivalent unit; Container capacity converted to 20-foot
container unit) . Similarly, there is an orientation toward
designs reaching 80–100 mm in thickness of high
strength steels. Therefore, two-electrode electrogas
welding (EGW) is now applied to the hatch side coamings
and similar parts of ships that had been welded in
one pass by conventional large heat input welding11),
and the low heat input multi-layer welding by CO2 arc
welding is inevitably used welding of longitudinal stiffeners.
However, two-electrode EGW requires an
increased heat input and advanced welding technology.
On the other hand, various problems also arise in multilayer
CO2 arc welding, including increased welding time
and cost. To solve these problems, the development of
narrow gap welding technology which makes it possible
to reduce the heat input and improve welding efficiency
has been promoted. J-STARTM Welding12), which is a
type of CO2 arc welding, is one such effort. However,
J-STARTM Welding realizes droplet transfer in a spray
form, enabling deep penetration welding with minimal
spatter, and thus is an optimal method for narrow gap
welding. Figure 5 shows the setup for J-STARTM Welding.
In contrast to electrode positive (EP) welding,
which has been used until now, the opposite polarity,
electrode negative (EN), is adopted in J-STARTM Welding.
A wire with a trace amount of rare earth metal
(REM) as an arc stabilizer is used. Because J-STARTM
Welding utilizes a conventional welding power source
and enables narrow gap welding simply by selection of
the proper welding wire, low cost, high efficiency welding
is possible. Moreover, due to the smaller number of
welding passes and decrease in the amount of welding,
application to heavy gauge plates is considered possible
in the sectors of building and bridge construction.
Electroslag welding (ESW) is used in welding of diaphragms
of box columns, which is considered necessary
in the sector of building construction. Low heat input,
multi-layer welding is difficult to apply to this work, but