The deflection of an arc from its normal path because of magnetic forces. See STANDARD WELDING TERMS.

Arc blow, when it occurs, is encountered principally with direct-current welding of magnetic materials (iron and nickel). It can be encountered with alternating current under some conditions, but these cases are rare, and the intensity of the arc blow is always much less severe. Direct current flowing through the electrode and base metal sets up magnetic fields around the electrode which tend to deflect the arc to the side at times, but usually the arc is deflected either forward or backward along the joint.

Back blow is encountered when welding toward the workpiece connection near the end of the joint or into a corner. Forward blow is encountered when welding away from the lead at the start of the joint. In general, arc blow is the result of two basic conditions:

(1) The change of direction of the current flow as it enters the work and is conducted toward the work lead.

(2) The asymmetric arrangement of magnetic material around the arc, a condition that normally exists when welding is done near the end of ferromagnetic materials.

Although arc blow cannot always be eliminated, it can be controlled or reduced to an acceptable Level through a knowledge of the above two conditions. Except in cases where arc blow is unusually severe, certain corrective steps may be taken to eliminate it or at least to reduce its severity. Some or all of the following steps may be necessary:

(1) Place the workpiece lead connections as far as possible from the joints to be welded.

(2) If back blow is the problem, place the workpiece connection at the start of welding, and weld toward a heavy tack weld.

(3) If forward blow causes the trouble, place the workpiece connection at the end of the joint to be welded.

(4) Position the electrode so that the arc force counteracts the arc blow.

(5) Use the shortest possible arc consistent with good welding practice. This helps the arc force to counteract the arc blow.

(6) Reduce the welding current.

(7) Weld toward a heavy tack or runoff tab.

(8) Use the backstep sequence of welding.

(9) Change to ac, which may require a change in the electrode classification.

(10) Wrap the workpiece lead around the workpiece in the direction that sets up a magnetic field which will counteract the magnetic field causing the arc blow.

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