Advanced Welding Processes: Technologies and Process Control

Narrow-gap welding techniques 177
Table 9.3 Applications of narrow-gap submerged arc welding
Weld type Application
Circumferential rotated pipe Offshore oil platform tubular fabrication
Circumferential rotated shafts Power generation turbine components.
Propeller shafts
Rotated thick wall vessels Nuclear reactor containment vessels
applying the techniques normally applied to conventional submerged arc
welding to the narrow-gap process, i.e.:
∑ extended stick-out;
∑ twin wire;
∑ hot wire;
∑ metal powder addition;
∑ flux-cored consumables.
These techniques are not widely used at present although some development
work has been reported. [192, 193]
Applications
The standard NGSAW process has been in use in many commercial applications
since the early 1980s. [194] Some of these are summarized in Table 9.3; they
range from nuclear reactor containment vessels in 600 mm thick Ni/Cr/Mo
alloy steels to the welding of 60 mm material for offshore tubulars.
9.4 Summary and implications
The use of narrower joint gaps and reduced preparation angles can result in
significant improvements in productivity. The use of processes which involve
the inherent use of a narrow gap (EBW, laser, plasma, friction, MIAB)
automatically exploit these advantages, whilst systems have been developed
to allow narrow gaps to be used with GTAW, GMAW and SAW processes.
The potential reduction in running costs must be evaluated against the capital
cost of the equipment, although it is reported [195] that high cost, sophisticated
narrow gap submerged arc systems have been justified for welding 350-mmthick
high-pressure feed-water heater shells. The minimum economic thickness
for narrow-gap technology varies with the process and operating mode. Narrowgap
GTAW welding may be justified on thicknesses down to 15 mm. Narrowgap
type configurations have been used for GMAW in thicknesses from 15–
22 mm upwards. Narrow-gap SAW is normally considered to be viable at
thicknesses above 60–70 mm, but, if conventional equipment is used, this
lower economic limit may be reduced until it overlaps conventional square