Linde BOC Smootharc Advance MIG 425R Operating Manual page 47

Welding Progression Angle
Double 'V' Butt Weld
Used on plate of 12 mm and over in thickness when welding
can be applied from both sides. It allows faster welding and
greater economy of electrodes than a single 'V' preparation on
the same thickness of steel and also has less of a tendency to
distortion as weld contraction can be equalised.
Butt Weld with Backing Material
When square butt welds or single 'V' welds cannot be welded
from both sides it is desirable to use a backing bar to ensure
complete fusion.
Single 'U' Butt Weld
Used on thick plates an alternative to a single 'V' preparation. It
has advantages as regards speed of welding. It takes less weld
metal than a single 'V', there is less contraction and therefore a
lessened tendency to distortion. Preparation is more expensive
than in the case of a 'V', as machining is required. The type of
joint is most suitable for material over 40 mm in thickness.
Double 'U' Butt Weld
For use on thick plate that is accessible for welding from both
sides. For a given thickness it is faster, needs less weld metal
and causes less distortion than a single 'U' preparation.
Horizontal Butt Weld
The lower member in this case is bevelled to approximately 15°
and the upper member 45°, making an included angle of 60°.
This preparation provides a ledge on
the lower member, which tends
to retain the molten metal.
Layers Weld Beads
Electrode
Weld Pool
Slag
Weld Metal
Weld Beads
Electrode
70˚ - 85˚
Weld Pool
Arc Slag
Weld Metal
BOC Smootharc Advance III MIG 425R & 555R Operating manual
General notes on Butt Welds
The first run in a prepared butt weld should be deposited with an electrode
not larger than 4.0 mm. The angle of the electrode for the various runs in a
butt weld is shown.
It is necessary to maintain the root gap by tacking at intervals or by other
means, as it will tend to close during welding.
All single 'V', single 'U' and square butt welds should have a backing run
deposited on the underside of the joint; otherwise 50% may be deducted
from the permissible working stress of the joint.
Before proceeding with a run on the underside of a weld it is necessary to
remove any surplus metal or under penetration that is evident on that side of
the joint.
Butt welds should be overfilled to a certain extent by building up the weld
until it is above the surface of the plate. Excessive build-up, however, should
be avoided.
In multi-run butt welds it is necessary to remove all slag, and surplus
weld metal before a start is made on additional runs; this is particularly
important with the first run, which tends to form sharp corners that cannot
be penetrated with subsequent runs. Electrodes larger than 4.0 mm are not
generally used for vertical or overhead butt welds.
The diagrams following indicate the correct procedure for welding thick plate
when using multiple runs.
70˚ - 85˚
Arc
47