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Middle East Industrial Training InstituteTel : +971 2582 7445 Fax: +971 25827994

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Demagnetization?

Middle ast Industrial Training Institute

http://www.meiti-uae.com/mailto:meitc@eim.aemailto:meitc@eim.aehttp://www.meiti-uae.com/http://www.meiti-uae.com/http://www.meiti-uae.com/

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DEMAGNETIZATION is defined as:

The removal of the residual magnetismby simultaneously or alternately reducingthe strength and reversing the direction ofa magnetic field

It is difficult to tell whether the circularly magnetizedcomponent is properly demagnetized since the flux lines

does not leave the component . Therefore it is oftenrecommended to established longitudinal field and thenproceed with demagnetization.

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1. The residual field is in the same direction as the magnetic field.

2. The residual field is weaker than the magnetizing field.

3. The original magnetizing force causes the residual field.

4. When an article has been magnetized in more than one direction,the second field applied completely overcomes the first field.However, this is only true if the second field is stronger than the first.When the second field applied is not stronger than the first field, thena combination Circular/Longitudinal residual field will result.

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A resident magnetic field may not be desirable in the part for

several reasons:

1. Residual fields will affect magnetic compasses or create

problems with delicate instruments.

2. Residual fields in rotating parts will attract metalparticles, causing excessive wear or binding.

3. Parts are also demagnetized so that all magnetic

particles can be removed for further processing.

4. Residual fields can cause arcblowwhich deflects themolten metal during DC welding operations.

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Each time the magnetizing force is reduce andreverse. The residual field is reduced.

Reversing the magnetic field

Reversing the part in the magnetic field.

Reversing the current through the coil.

Reversing the coil (turn the coil 180)

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Each time the magnetizing force is reduce andreverse. The residual field is reduced.

Reducing the magnetic field

Reduce the magnet current.

Move the part away from the coil /yoke.

Move the coil/yoke away from the part.

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1. On articles of soft steel or iron where retentivity is low

2. If, after the magnetic particle test , the article is to be heat treated.

3. On large castings, weldments or vessels where residual fields willhave no material effect.

4. If the materials is to be magnetized again in another direction at thesame higher amperage.

5. If the article is likely to become remagnetized during handling.

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1. Alternating Current Coil method:

Each time the current reverses indirection, the magnetic field of thecoil also reverses.

To complete the demagnetization,the part is place in the reversingmagnetic field and the current isslowly reduced which reduces thestrength of the magnetic field

Reducing the magnetizing field inan AC Coil is usually done byslowly moving the article awayfrom the coil

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1. Direct Current Demagnetization:

Direct Current demagnetization is usually more complete andeffective than alternating current.

To The article is placed in the coil connected to a source of directcurrent. The current is adjusted to a value greater than the initiallyused during magnetization

The process of reversing and reducing the current is continued untillowest value is reached.

For best results, the diameter of the demagnetization coil is justlarge enough to accommodate the article.

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1. Use of Residual FieldIndicators:

Compares the strength of theexternal field of the articlewith fixed field inside theindicator

Is used more to locate fluxleakage than to measurefield strength

Is used to show when parts isdemagnetized

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1. Use of Magnetic Field Indicators:

ASME FIELD INDICATORS

- Verifies the adequacy and direction of magnetic field

BERTHOLD SPOON

- Verifies the adequacy and direction of magnetic field

BURMAH CASTROL-STRIPS

- Verifies the magnetic field direction only.

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ASME FIELD INDICATORS

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BERTHOLD SPOON

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BURMAH-CASTROL STRIPS