magnetic particle inspection - simple technique with magnet

Hi everyone
Just had a useful result using a permanent magnet for Magnetic Particle Inspection.
http://www.weldsmith.co.uk/tech/radiography_ndt/1107_MPI_Tjoint_welds/110705_magpart.html
Thanks to those whose advice enabled me to get this far.
Told: - the magnet may be a bit too strong, - blowing the indication should see the particles flow back to genuine defect... not test of all particles will not blow off if genuine defect - technique should improve with experience
Anyway, how have you folk coming from the welding perspective found mag. part.?
Any techniques you like? Good applications which work well for you?
Richard S
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I have studied Mag Particle Inspection and taught it. It is very useful for surface and near surface defects, but is heavily dependent on the magnetic qualities of the base metal, and surface texture. Dry method is very easy, but a liquid suspension of finer particles shows better detail. Wet method uses a kerosene base for the suspension, and phosphorescent particles to be read under a UV light. Mag particle is even used underwater using a thicker oil suspension.

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Hi Ernie, everyone
These fatigue cracks are always surface-breaking, so OK on that. System used in pix is kerosene based - spray-can has "Magnaflux" logo. Wondering about dry system. Hoping to mag.part. samples as seen in pic while in fatigue-test machine http://www.weldsmith.co.uk/dropbox/cranu/110602_fatiguetest/110602_fatigue_test.html Can't have kerosene going into machine's grips - would lube. Heard of dry sys and water-based, but never seen - don't know realities of them. Water-based? Cloth around sample would absrob water and water-only getting through would evaporate away? Dry sys...? Sounds ideal. Meeting one comment - could have vac cleaner beside sample so surplus drawn away.
I'd much appreciate knowing the way ahead and what to ask for.
Richard S.

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Ernie, all
Explaining last posting...
The reason I want to mag. part. in the testing machine is to see when cracks first appear. Would tell us about what's going on.
Hoping to try weld dressing treatments like TIG-remelt and peen. Which are claimed to produce a significant crack initiation time (as-welded - prior defects claimed to give zero crack init time). Hoping works well for some hybrid GMAW/laser welds we've done.
So the promise of mag. part. is to inspect these claims.
Got to do everything within July, so all guidance appreciated.
Have 2million cycle test right now running over weekend - all go. About to jump on bicycle and get around airfield to check machine all happy. So lot to play for...
Richard S
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There are several methods of creating the magnetic field. A portable yoke, a portable coil, a stationary machine, or prods.
The yoke looks like a horse-shoe magnet with a on button. They are very common in field inspections for small parts. With a yoke you are only testing the areas between the legs. You can reposition the yoke for multiple shots quickly. The dry powder is dispensed with an air puffer that creates a little cloud of particles that drifts onto the part. The dry powders come in many colors for the best contrast. The yoke will show indications at a right angle to a line between the 2 legs of the yoke.
A portable coil is used for showing transverse cracks in shafts and other linear parts.
A stationary machine is huge and very expensive, but works the best for small high quality parts. Stationary machines use a kerosene suspension and phosphorescent particles. A stationary machine can do everything a yoke and coil and do, faster.
Prods are used for large rough parts where it doesn't matter if you put arc strikes on the surface of the material.
All of these toys can be purchased from NDT suppliers.
http://www.ndtsupply.com/shop/default.asp?keyword=Magnetic%20Particle

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