Magnetic arc blow

Magnetic Effect on the Welding Process

DIVERSE Technologies

Cambridge

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One of the most frustrating causes of weld failure is magnetic arc blow. A magnetic field present in the weld preparation region of mating steel components can cause the type of weld problem that could never pass inspection.

Magnetic arc blow is not an uncommon problem if the entire welding industry is considered, but it is rare enough for individual fabricators to operate for years without any serious difficulties and then be caught by surprise when it does occur.

 

What is magnetic arc blow?

All welding processes that use either an arc or a beam of electrons are subject to disruption in a magnetic field. This is caused by the electrons being forced to follow a curved path when they pass through the magnetic field, deflecting the arc and causing it to behave erratically.

Some welding processes are more sensitive to arc blow than others. The effects of arc blow are reduced by welding at higher currents which produces a stiffer arc. Welding carried out in hyperbaric conditions is more prone to arc blow because the electrons in the arc are slowed down and scattered by the extra gas molecules in the high pressure atmosphere, allowing them to turn more in the magnetic field. TIG welding tends to be more sensitive than MIG or MMA because of the lower arc voltages used. Additional difficulty is sometimes found when two or more welding heads are used at the same time on the same joint because the magnetic field produced by the weld current at one head can interfere with the arc at another head.

The following table lists the most common effects of magnetism on welding processes. The table shows how welding arcs can be expected to behave in increasing magnetic field levels.

Welding process

0 - 10 gauss

10 - 20 gauss

20 - 40 gauss

over 40 gauss

TIG

normal welding

arc instability

arc blow

severe arc blow

Manual metal arc

normal welding

normal

arc instability

arc blow

Submerged arc

normal welding

normal

normal

arc instability

Problems begin from 10 - 40 gauss, yet when even weakly magnetic fields are present in parent material the value in the joint preparation can easily be several hundred gauss and fields over 1000 gauss are not unusual.

 

Reducing magnetic problems.

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 or gauss meter 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 passed through a demagnetising 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 demagnetisation is virtually impossible to achieve. However, it is possible to remove magnetism in a joint preparation, enabling welding to take place.

 

The Zeromag Demagnetising System

Zeromag Z100 is a demagnetising 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. Z100 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: Zeromag100 power supply, demagnetising cable, magnetic field meter, magnetic field probe.

Time to demagnetize: Typically 10 seconds.

Versions: Manual version. Level of magnetism in the joint is adjusted manually.

Automatic version. Level of magnetism is automatically and continually adjusted to less than 10 gauss.

Subsea. Zeromag can be used topside or subsea. Customised versions are available to meet particular subsea requirements.

Options. Gas cooled magnetic field probes are available to enable the magnetic field to be continually monitored and removed close to the point of welding.

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.

Conclusions

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.


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