Welding with an electric arc creates a magnetic field around the electrode, according to the illustration below. The intensity of its lines of force, also called the magnetic flux density, gradually lowers as the distance from the electrode increases.
The direction of the magnetic field in relation to the current during welding with D.C.
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 several kinds of discontinuities in the joint, including lack of root penetration, lack of fusion, undercuts and pores. It may not be possible to eliminate arc blow but steps can be taken in order to reduce its negative effect.
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 below.
a) Forward arc blow. b) Backward arc blow.
Essential sources that can achieve, or contribute to, magnetic arc blow are:
Residual magnetism in the steels subjected to welding
The appearance of the joint
Welding with multiple electrodes
Residual magnetism is primarily an issue related to tack welds and root runs. After these sequences, the residual magnetism in the joint is normally low enough to reduce arc blow.