Shouldn't it be how strong does an AC field need to be to attract iron filings? Classic doorbells use AC solenoids to move the plunger for striking the chime

I assume VIM means vacuum induction melting? If so, then probably, since it would take a fair amount of power, so pretty large field, I'd imagine, although the peak field is probably centered on the workpiece

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doorbell must be low frequency, so it strikes the bell back and forth?

yes, VIM=vacuum inducting melting, the power level is high at 100KW. The frequency is actually about 1000Hz, the polarity change is such quick, not sure the solenoid coil can attract any ferromagnetic subject.

Doorbells operate off of 16 to 24 VAC (60-Hz); old school doorbells have zero electronics, only on/off switch; which is why some of them would rattle or buzz if the switch was held on for a long time.

Similar is true of the standard garden irrigation valve, which run off 12 VAC@60Hz; energize, and the plunger pulls in and stays pulled in for the duration of the watering cycle.

Nevertheless, the frequency question is interesting; however, I would point out that sufficiently strong AC fields, particularly in the case of audio tape, are able to move magnetic domains in the tape well into the tens of kHz for audio, and VCR tapes were imprintable up to around MHz, so the "fixed" particles in the tapes are able to switch orientations pretty quickly.

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No, it will not attract Fe from the dust inside the shell.
These fields have very little reach. The field is meant to be contained in the melting coils.
Some larger furnaces actually have buck turns on the ends of the coil.
So you are melting about 100lbs?
If you are picking up Fe it is either from the refractory or the raw materials.

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@IRstuff, the polarity change in ferromagnetic materials can be up to microwave range. In a typical FeCoB/Mgo/FeCoB STT-MRAM, magnetization switch is up to 10GHz. however, the spinning of magnetic moment or domain movement doesnot seem to mean it will exert a force. Even in doorbell case, if the distance between teh plunger and the chime is twice as large, could the back-and-forth forces, generated by the solenoid, be canceled out?

@EDStainless, yes it is a "baby" VIM at about 100lbs, compared with tens of tonnage VIMs in my previous Steel industry. our small VIM doesnot include a buck turns on the coil. I didnot get a chance to look at the coil design in large VIM, but i can imagine the design needs to contain flux inside the coi at least inside the chamber, for the safety in the minimum.
I tend to believe the Fe contamination (tens of ppm level) was from the steel mold, but wanted to investigate any possibility.

Quote:

Even in doorbell case, if the distance between teh plunger and the chime is twice as large, could the back-and-forth forces, generated by the solenoid, be canceled out?

Hence, my earlier question about field strength. Unless there is some sort of shielding around the coil, the field would extend out from the coil ala the classical images such field lines, AND if the particles are small enough to be lifted, then something might happen.

However, since this is VIM, presumably, there is some moving of air around, which might suggest that the turbulence might simply blow dust around and cause it to land in random places.

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Use a BN wash for you molds.
Saint-Gobain sells BN slurries.
You can make your own but you safety people would have a fit (microwave BN powder in 100% isopropanol).
We used solid BN for TC sheaths, and BN wash in crucibles and on molds.

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Have tried spraying BN and Y2O3 (and baked) on the mold, but inconsistently got B, Y, O pickups. B, O are even less tolerable than Fe for material applications (asking for 5N purity). It is believed the coatings could be mechanically peeled off at casting (pouring). BN could also decompose. we are kind of hesitating to use coatings on mold.

@IR, for sure the field extends out of the coil to some degree. we did see the moly foil (to cover the open area of mold) underneath the coil turned red, heated by AC field. I am thinking perhaps the attraction of an AC magnetic field to a ferromagnetic subject is a function of frequency, distance and field strength?

We would coat with BN, then bake in vacuum, and then burnish the surface to remove any loose material.
What worked best for molds though was water cooled Cu. they weren't that durable but worked well.

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@Ed, do you think water-cooled Cu mold limits copper contaminations? we tried Cu mold but without water cool, turned out quite a lot of Cu (>10ppmw) was picked up. We have to melting and casting in vacuum, putting a water cool system in the melting chamber is not an easy task, but if have to, we could try.

Yes, on both accounts.
The water-cooled molds may show a little erosion at the impingement point so you need to be careful how you pour, but they greatly reduce Cu pickup.
And running water lines inside a high vac environment is total PIA. No quick connects, and no pipe threads. Everything has to be straight threads with O-ring seals.
We ran some antifreeze in the cooling water so that we could use a blacklight to search for leaks in the vac side.

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To answer the original question, regarding 1KHz magnetic fields attacting iron particulates.

It all depends on the size and mass of the particles and the proximity to the pole pieces.

The fields themselves do not attract the particles. Consult Chikazumi or more recent handbooks.

@hacksaw,

Quote (The fields themselves do not attract the particles.)

, then why does it depend on size and mass of the particles? in the VIM case, the distance from the cover to the top of the induction coil is about 1.5 feet. There is literally no pole piece, especially the charge materials are no ferromagnetic.

Stated Parameters

(a)A practical question: could the magnetic field in a VIM cause iron contamination (pick-up) from the iron particle residuals (from previously iron-based melt) on the melting chamber?

Depends on particle size and rate of heating.

(b)VIM=vacuum inducting melting, the power level is high at 100KW. The frequency is actually about 1000Hz, the polarity change is such quick, not sure the solenoid coil can attract any ferromagnetic subject.

Here again paricle size is important, due to skin depth and rate it temperature increase

(c) the polarity change in ferromagnetic materials can be up to microwave range. In a typical FeCoB/Mgo/FeCoB STT-MRAM, magnetization switch is up to 10GHz. however, the spinning of magnetic moment or domain movement doesnot seem to mean it will exert a force. Even in doorbell case, if the distance between teh plunger and the chime is twice as large, could the back-and-forth forces, generated by the solenoid, be canceled out?

With microwave range drive field, switching not likely with metal particulates, more likely just heating, which for small paricles results in results in rapid temperature changes.