Page 94 - SSAB Welding Handbook Edition 2

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15.0 Magnetic arc blow Welding handbook

15.1 Residual magnetism in steels Joint preparation
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Ferromagnetic steels, including the Hardox and
Strenx grades, can contain varying levels of
residual magnetism. If the intensity of the lines of
force is high enough, this magnetism can interfere = Direction of magnetic flux Magnetic field should
be measured at the point
with the magnetic field from the welding torch and of the next weld path
cause magnetic arc blow. Common sources that Fig. 15.3: Magnetic fields in a single-V prepared joint.
enhance the magnetism of the steel include magnets
used in jacking equipment or storage in a place
with high levels of magnetic flux densities.
The geometry in joints of larger plate thicknesses
can cause an increased risk for magnetic arc blow.
Low levels of magnetism in the joint will not Welding in this condition means that a substantial
influence the stability of the arc. Arc blow fraction of the arc can be surrounded by the parent
disturbances can occur when the magnetic flux metals at a close vicinity, especially during root
density is approximately 40 Gauss or above.
passes and tack welds. Since the magnetic field flows
easier through steel than air, an unsymmetrical
The Hardox and the Strenx grades are high magnetic field can be formed around the torch,
strength steels with favorable resistance to residual which may cause arc blow.
magnetism, even though there are instances when
they can obtain magnetic flux densities of 40 Gauss
or more. The sensitivity towards magnetic arc Similarly, arc blow can occur when welding plates of
blow, caused by residual magnetism in the steel, is different thicknesses, according to fig. 15.4. In this
generally more emphasized with Hardox grades of situation, the arc may deflect towards the thicker
higher hardness and higher strength Strenx grades. plate in the joint.

15.2 The appearance of the joint

The resistance to magnetic flux depends on the
characteristics of the medium through which it
flows. Steel has a substantially lower resistance in
this respect than air. Consequently, free ends of
plates can have a lower level of magnetism than a Fig. 15.4: Arc blow due to different plate thicknesses in the joint.
joint including the same plates.

The level of residual magnetism can vary throughout
the joint when the distances between the steels in 15.3 Welding with multiple arcs
the joint vary. For instance, in a single-V joint, the Multiple electrodes normally lead to increased
distance between the parent metals in the joint are welding productivity in comparison to welding
shortest at the root section. Therefore, there is a with a single electrode.
higher level of magnetism compared to other parts
of the joint where the space between the steels is Welding with multiple arcs using D.C., where there is
greater, see fig. 15.3. The magnetic flux density, only a small distance between the electrodes, may cause
therefore, should be measured at the location of the arc blow since the magnetic field from each electrode
next weld pass in order to determine the risk for can influence the performance of the adjacent arc.
magnetic arc blow. A hand held gauss meter is a
typical instrument used for this purpose.

94 This handbook contains general suggestions and information without any expressed or implied warranty of any kind. SSAB hereby expressly disclaims
all liability of any kind, including any damages, in connection with the use of the information and for their suitability for individual applications.
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.