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Welding handbook 15.0 Magnetic arc blow

15.0 Magnetic arc blow
©SSAB

Welding with an electric arc creates a magnetic field
around the electrode, according to fig. 15.1. The
intensity of its lines of force, also called the magnetic
flux density, gradually lowers as the distance from Welding
direction
the electrode increases. If the magnetic field is
disturbed by external sources, it may cause deflection
of the arc during welding. This phenomenon is
called magnetic arc blow, or arc blow. It can lead to
A)
several kinds of discontinuities in the joint, including
lack of root penetration, lack of fusion, undercuts
and pores. It may not be possible to totally eliminate
arc blow but steps can be taken in order to reduce Welding
its negative effect. direction

B)

Fig. 15.2: Forward arc blow is illustrated in the upper figure A) and
backward arc blow in the lower figure B).
Direction of magnetic flux

Essential sources that can achieve, or contribute
to, magnetic arc blow are:
4 Residual magnetism in the steels subjected to
welding
4 The appearance of the joint
4 Welding with multiple electrodes
Direction of current

Fig. 15.1: The direction of the magnetic field in relation to the Residual magnetism is primarily an issue related
current during welding with D.C.
to tack welds and root runs. After these sequences,
the residual magnetism in the joint is normally low
enough to reduce arc blow.
The deflection of the arc can be more or less
stable during magnetic arc blow. The source of
the interference is what influences its behavior. A
deflection of forward arc blow and backward arc
blow is illustrated in fig. 15.2.

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