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MPI - Magnetic Particle Testing

Coatings 

Unfortunately, ASTM E-1444 specifies that Magnetic Particle testing shall not be performed with nonmagnetic coatings (paint, etc.) in place that exceed 0.003" (0.08mm) in thickness, or ferromagnetic coatings (electroplated nickel, etc.) that exceed 0.001" (0.03mm) in thickness.

That is not very thick! So be careful on those painted surfaces.......don't forget the chance of poor bonding on those thin coatings.

Current Types

ASTM E-1444 recommends:

Half-wave rectified AC is best for the dry particle method.

For defects open to the surface use AC only.

When using Wet particle method for subsurface defects, use full-wave rectified AC.

Yokes

Funny, but look at the difference, Standards can make on a procedure:

ASTM E-1444 requires DC yokes to have a lifting force of at least 30 lbs.(small spacing) and a 50 lbs. lift for a large spacing of legs.

ASTM A-275 requires DC yokes have a lifting power of at least 40 lbs at 3 to 6" spacing of legs.

The ASME Boiler Code requires a DC yoke to have at least 40 lbs of lift at the maximum pole spacing.

There is a misconception in the 'field' that one can take a yoke and proceed with magnitizing an object by dragging the yoke along the object's surface. Wrong!!!

If you are dragging an AC yoke, you are in fact not magnetizing the object but rather you are doing a lovely job of DEMAGNETIZING the object. As in ASTM E-1444 above, paragraph 6.7.1.1, demagnetizing using a coil (yoke) by moving the object through........(which is what you are doing).

To correctly magnetize an object, position the yoke and apply magnetizing force for 2 - 5 seconds before moving the unit to the next position.

So watch which specification and procedure is being applied to your project.

 

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