Frequently Asked Questions
Characteristics and causes
In some circumstances a welding arc may be deflected away from the point of welding, which can mean that the weld quality may suffer. Such arc deflection, usually forward or backward of the welding direction, is known as 'arc blow'.
Arc blow tends to occur if the material being welded has residual magnetism at a certain level, particularly when the weld root is being made, and the welding current is direct current (DC positive or negative). This is due to interaction between the directional magnetic field of the welding arc and the directional field of the residual magnetism.
Whilst any material in which a magnetic field can be induced could exhibit arc blow, it is a phenomenon that is most common in steels. As a rule of thumb, if the magnetic field strength is greater than ~50 gauss (50 x 10-4 tesla) arc blow may be experienced. Whilst individual items may have relatively low residual magnetism, when they are brought together for welding the magnetic field across the root gap may increase significantly.
Some steels are particularly prone to the induction of high levels of residual magnetism during their manufacture and de-magnetisation may be necessary as a final manufacturing operation. 9% Ni steel is an example of such a steel, and particular care is needed to prevent magnetic induction during transport, storage and subsequent handling and preparation for welding.
Overcoming arc blow
When arc blow is encountered, it may not be possible to totally eliminate it, but steps can be taken to reduce its disruptive effect. Such measures are:
- Use as short an arc length as possible (lower arc voltage) and the lowest current that is practical for the affected joint (possibly a smaller diameter electrode)
- Backward arc blow tends to occur when welding in a direction towards the current return connection, or earth connection, and forward arc blow when welding in the other direction. As it is likely to be easier to control a backward deflecting arc than a forward deflecting one during MMA welding, it is usually better to weld towards the return or earth connections. Use of multiple earth connections may solve the problem altogether.
- Use alternating current (AC) rather than DC for the root run. Note: this may be regarded by some Codes as change of an essential variable and require re-qualification of the welding procedure.
- If arc blow still persists when using AC for the root run, the power cable can be wrapped around the workpiece several times near to the joint. Care will be needed if preheat is applied.
- Reduce the residual magnetism in the steel to a tolerable level (less than ~50 gauss). This requires application of an auxiliary magnetic field with careful control. This is illustrated as follows - for pipe welding when the joint has been set up for welding:
- Measure the residual magnetism by inserting a 'blade' type gaussmeter (a Hall probe) in the weld gap, and move it around the circumference of the joint;
- Note the maximum reading and mark its position on the pipe;
- Wrap welding cable around each of the pipe ends with the edge of the extreme coil about 60mm from the joint line. Connect cable to the poles of a DC power source (the number of coils required will depend on the maximum level of magnetism recorded and the current-carrying capacity of the cable and may require some trials: there will probably be need for at least 2 turns but possibly significantly more for high residual magnetism);
- Gradually increase the current and monitor the magnetic field strength. If the strength increases, reverse the polarity of the current through the coil and then adjust the current to reduce the magnetic field strength in the weld gap to a low level (monitor the magnetic filed strength at the initial highest level position marked on the joint and around the circumference);
- Complete the root run with current applied to the de-magnetising coils. Once the root is complete, arc blow should not continue to be disruptive and the de-magnetising coils can be removed.