Arc blow reduction

 

First it is necessary to know how strong the magnetic field is in the weld preparation.  Fields of 4mT (40 gauss) will exert a noticeable pull on a welding rod (except for an austenitic rod).  A magnetic field meter (e.g. Diverse MF300H+) will give a quantitative reading and show if the magnetic field is uniform or along the joint. If the field is relatively low it may be possible to reduce the problem to manageable levels by adjusting the welding parameters.

Small components can be demagnetized by passing them through a degaussing coil powered from the mains.

If a component is large, such as a pipeline or tubular structure then there is so much stored magnetic energy that demagnetization of the whole component is virtually impossible to achieve.  However, it is possible to remove magnetism in a joint preparation, enabling welding to take place.

Zeromag ZM100A is a demagnetizing system for steel tubes and plates.  It is used to reduce the magnetic field levels in welding joint preparations to levels which are low enough to enable welding to proceed.  ZM100A is used to eliminate arc blow which can occur when joint preparations are magnetic.  Zeromag will reduce magnetic field levels to below 10 gauss even with residual field levels of over 1000 gauss.  The equipment uses a Hall probe to monitor the magnetic field a produces a compensating magnetic field to cancel out the field in the joint.

• Maximum magnetic field cancelling capability: ±2000 gauss in most pipeline steels.
• Maximum diameter tubulars: 4 metres.
• System includes: Zeromag power unit and controller, demagnetizing cable, magnetic field meter, magnetic field probe.
• Time to overcome magnetism: Typically <10 seconds.
• Modes: Manual: Level of magnetism in the joint is adjusted manually.
• Automatic: Level of magnetism is automatically and continually adjusted to <10 gauss.
• Subsea: Zeromag can be used topside or subsea. Customised versions are available to meet particular subsea requirements.
• Options: Demagnetizing kit for pipe welders, joint and pipe end degauss unit, clam coils, pipe end bobbins, ZeroZone, versions for smelter environments.

Once the weld is completed the Zeromag is switched off and the intrinsic magnetic field in the component returns but is of no further concern.

Conclusion

The ultimate result of magnetic arc blow, even in its milder forms, is a reduction in quality and an increase in cost.  Removing magnetism from the weld preparation quickly and efficiently frees the welders to do the job they do best and enable the fabricator to complete the work within budget.