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A Quick Guide to Welding and Weld Inspection

Figure 5.11 SAW equipment

does enable some other positions to be used. Root control is
critical and some form of root backing is required.
Polarities
Polarities typically used are as follows.
. Alternating current is used for multihead systems using
high currents to prevent magnetic arc-blow problems. Arc-
blow is where the magnetic field surrounding the electric
arc is affected by another magnetic field causing the arc to
deflect. This is not a problem with a.c. because the
magnetic field changes direction with each cycle.
. DCEP is used for welding applications to give deep
penetration.
. DCEN is used for surfacing or cladding applications
(DCEN gives shallow penetration).
. Multihead systems of two or more heads often use
combinations of polarities to increase penetration and
deposition rates without causing arc-blow. It is common
for the leading head to be d.c. while the trailing head(s) are
on a.c. The power source normally has a ‘flat’ or constant

64

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200905QG Welding chap5.3d Page 64 of 66