When welding ferromagnetic steels, lack of fusion imperfections can be caused through uncontrolled deflection of the arc, usually termed arc blow. Arc deflection (Figure 1) can be caused by distortion of the magnetic field produced by the arc current through:
- residual magnetism in the material as a result of using magnets for handling
- earth's magnetic field, for example in pipeline welding
- position of the current return
The effect of welding past the current return cable which is bolted to the centre of the place is shown in Figure 2.
The interaction of the magnetic field surrounding the arc and that generated by the current flow in the plate to the current return cable is sufficient to deflect the weld bead. Distortion of the arc current magnetic field can be minimised by positioning the current return so that welding is always towards or away from the clamp and, in MMA (SMA) welding, by using AC instead of DC. Often the only effective means is to demagnetise the steel before welding.
For more information on magnetic arc blow, see Job Knowledge for Welders 41