Page 95 - SSAB Welding Handbook Edition 2

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Welding handbook 15.0 Magnetic arc blow

If arc blow is encountered, equal polarity between This effect is brought on by:
the electrodes supports inward arc deflection and 4 Heating of the joint due to welding.
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unequal polarity supports outward arc deflection, 4 A redistribution of the magnetic field. This is
according to fig. 15.5.
facilitated if weld metal is present in the joint
because it acts as a bridge for the magnetic flux
flow.
Current direction
The following two steps can be utilized to plan
the welding sequences appropriately so that the
Direction of level of magnetism is below 40 Gauss in sections
magnetic flux
of the joint: First, mark the locations in the joint
where the local magnetism is below 40 Gauss.
Then place tack welds at suitable locations along
the marked areas. Long individual lengths of
Fig. 15.5: Arc deflections during welding with two electrodes.
tack welds are preferred and their individual
lengths should generally be set to a minimum

15.4 Measures to counteract of 50 mm each. Welding of joints with a plate
magnetic arc blow thickness up to 8 mm may be done with shorter
tack lengths than 50 mm. Repeat this procedure
When magnetic arc blow is encountered or until the magnetism is below 40 Gauss in the
suspected, steps can be taken to attain suitable complete joint, after which, the rest of the
conditions for welding. welding procedures can be performed.

The angle of the torch Welding with A.C.
A magnetic arc blow that deflects the arc at a Electrodes connected to A.C. are less prone to arc
specific direction can be counteracted by inclining blow in the joint compared to those coupled with
the electrode angle opposite to the direction of the D.C.
arc blow, see fig. 15.6.
The reasons are that:
4 The direction of the magnetic field around the
electrode continuously changes making it less
Before After
susceptible to external sources of magnetism.
4 Welding with A.C. introduces eddy currents
into the joint, see fig. 15.7, which reduces the
negative effect from an external magnetic field.

A.C.
Fig. 15.6: Straightening of the arc by angling the electrode.
Eddy current Magnetic flow
Welding sequences
Tack welds and root runs diminish and/or
redistribute the magnetic fields in the joint.

Fig. 15.7: Induction of eddy currents in the joint.

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. 95
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.