Qestion for Edstainless

[JerryRoy]

Howdy Ed,
I was researching the process for anealing 1018, and came upon your post of 25 Sept 09.


25 Sep 09 8:43

If this is a DC application who would care about the perm?
Saturation is all that matters.
Any low carbon steel grade with less than 0.2%C that has been annealed at 1400-1475F with a slow cool should give very nice high saturation levels.
= = = = = = = = = = = = = = = = = = = =

I was heating two 1018 pole pieces in the propane furnace, and got distracted, which allowed the temperature to exceed 1475 F.
By most color charts, and my thermocouple, I was in the 1750 F range.
I took the pieces out and buried them in Perlite, and am letting them cool.
The question is, will cooling from a higher temperature decrease the saturation density properties, (make it lower), and if so, how to recover from my faux pas.
Thank you for sharing your knowledge.

Jerry Roy

 

[EdStainless]

First, are you sure that your furnace has an oxidizing potential, and that you didn't carburize the material?
That would be bad.

I don't think that heating higher would really hurt.
You could either check the magnetic properties or the hardness of the material.
If it is too hard or has too low of saturation you can go back at 1400F and soak for a while, and then slow cool.
I don't recall of the top of my head what the hardness of annealed low C steel should be, but it is very soft, Rockwell B scale

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[JerryRoy]

Thanks for the reply, Ed,
I set the air shutter wide open on the burner, and didn't see any yellow in the flame, so I'll hope that the flame was Oxy rich. I'm at 5280 ft elevation, so we don't have a lot of extra Oxy.
The material is hot rolled, and I am going to surface grind it so that I get a small gap between the pole face and the pole piece.
Is there some easy way to measure the flux capacity, other than to measure the flux across a gap at various amps and see if it stops rising before I hit maximum amps? This is on a U shaped eighty thousand amp turn DC magnetizer, with eleven sq in cores.

 

[EdStainless]

The only air gap that you want is the working gap. Everything else should fit up very tight.
If you are removing enough material, say at least 0.060" then you should be OK.

The saturation will not change much with various anneal treatments.
The initial perm and residual magnetization will change.
Over time if you notice too high of a residual field with the power off you may want to remove the pole pieces and re-anneal them.
The second time I would wrap them in stainless tool makers foil to protect the surfaces since they will be at finished size.

sounds like some of the ones in our shop.
Large "C" shapes cut from 6' x 10' x 4" plate, 8 of them stacked.
The pole faces were tapered, made from 1002 steel with long term hydrogen anneal.
The windings were 1.5" x 2.5" hollow Cu bar brazed together, 16 turns on each side of the 16" air gap.
That gave us 320,000AT

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[JerryRoy]

Good info, Ed
You say that the core was stacked. And you put 10,000 amps through it! That will dim the lights. What voltage were you working with?
Does the lamination of the core impair the flux travel through it? Our core is the laminated core from a transformer, used in a DC magnet. (I couldn't justify the cost of pure iron for the core).
Also, from what I have been able to glean from the Internet, a 45 deg angle can be used to focus (concentrate) the flux going through a piece of steel. Is that included angle, or angle from central plane?
Thanks for your input, JR

 

[EdStainless]

Most of the time we ran at 8kA, 8V, at 10kA it was about 11 volts.
It was a nice solid state DC rectifier (Dynapower) and it never gave us any trouble.
The field did not cross the laminations.
I hope that the photos attach. I forgot that I had them.
I started to sketch it out and the decided that I needed something better.
It is used to heat treat Alnico permanent magnets, you do it in a field.
I don't recall the field strength that we got, I want to think that at 10kA we had 2.5kG in the middle of the 16" gap.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[JerryRoy]

Wowser! That must do some big Alnico pieces. I notice that the ends of the poles are beveled at what looks like a 45 deg angle. Is that to focus the flux down to a smaller area, thereby increasing it's density?
I was under the impression that the max flux density that you could get out of steel at saturation was around 2kG. Did the bevels allow more than that across the gap?
Thanks for sharing, JR

 

[EdStainless]

saturation of pure iron is near 20kG, 2T, with well annealed low carbon steels (1002 with 48 hr hydrogen anneal) we could get to 19kG.
The tapper is to help increase the field strength but also improve uniformity.
With flat faces the field near the edges drops off a lot.

The trays that we heat treated in were about 8" wide.
Most of our parts were 1" or less, so they were stacked in rows.
We solution annealed in a furnace at 2200-2250F, then moved into the field and force air cooled to about 1500F.
Then we pulled the furnace over the parts and held them at 1500-1550F, in the field, for about 20 min.
Then they were fan cooled.
The final aging was done in pit draw furnaces.
I don't recall the temp, maybe 800F, for 3 days.

I loved making Alnico 9, the grain oriented grade.
We would etch pieces and save the ones that were nearly single crystal.
The properties were incredible.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[JerryRoy]

Fun stuff! What was the support framework made out of? To withstand 1500f, it would have to be steel, but that would interfere with the field.
You mentioned that the field did not cross the laminations. Yet if you have a coil around a square core, the portion of the coil that is facing the faces of the outer laminations can only impart it's flux into the outer laminations unless the flux crosses a lamination gap. That was my concern about using laminated transformer core as the core for our magnet.
Is there a higher efficiency obtained in making magnets by using a round core and windings?
I hope you don't mind all the questions. I can't seem to find an online or library source for answers to them. ( The reason for the sloped faces explains "why" for one of my questions. I'll give that some consideration in the configuration of our pole pieces.)
Thank again, JR

 

[MagBen]

Ed,
Yes, it is fun to work with AlNiCo from a viewpoint of both application and manufacturing. One of amazing things is this alloy can be orientated in multiple directions/fashions. AlNiCo 9 is an exception, which is only orientated in one direction. We use directional solidfication tehcnique trying to make 100% columnar structure (not necessarily single crystal). We do a 100% magnetic inspection for AlNiCo 9, when a magnet fails, one can easily see the grain structure simply breaking the magnet (no need to etch), it either contains considerable equiaxed grains or the grain direction is off the center.

The needed magnetic field for anneal is only 1-2 kG, since the purpose is not to magnetize. Also note the coercivity of AlNiCo is controlled by shape anisotropy.

 

[EdStainless]

When we did small diameter discs in Alnico9 we would cast large blocks and EDM the round slugs out of them, OD grind, then slice to final length.
We would always take a few of the long slugs and etch them, keeping ones with very few grain boundaries.

I may be biased but Alnico are real magnets, everything else is just temporary.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[JerryRoy]

The magnets I am trying to deal with were made in England and used in Lucas magnetos. I was told that the Alnico used in the late 50's was Alnico 5, but I really don't know.
It was also stated in the Lucas manual that if the armature, which housed the magnets, was removed from the housing, that some of the magnetism was lost, and the only way to restore it was to assemble the mag, and magnetize the magnets in situ. Does this sound right to you?
With a tapered face on the pole pieces, with a 2 sq in face area, we can get 1400 mT in a 1/2 in gap. Does that sound like enough to re magnetize Alnico 5?
Thanks for your input,

Jerry Roy

 

[EdStainless]

The first part all sounds right.
The magnetic path is longer in the assembly, therefore the operating point is higher on the curve, meaning that there is less self demagnetization. And often the act of removing the magnets forces them to partially demag.
Most Alnico 5 units have to maged as an assembly.
You have two magnets, with full steel return path, and a 1/2" gap, and tapered poles.
yes, 14kG is possible. The magnets will have Br of 12 - 13 kG, and you may get enough concentration from the tapered poles.
They sound magnetized to me.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[MagBen]

Ed, very interesting for making AlNiCo 9, but it may have some difficulty for mass production. How high (BH)m can you Alnico 9 go with single crystal? can you get nominal 11MGOe?

with a yoke and two taped pole pieces and two RE magnets, it is not difficult to create as high as a 2T field as long as the gap is small enough, but I never tried AlNiCo. It is very tricky to use AlNiCo, for a horseshoe or large L/d AlniCo 5 magnet, no need to worry much about demag, since the operating point at open circuit is already high, much above the knee, it can almost recoil back to the original point.

Jerry, so are you trying to re-magnetize your AlNico 5? a rule of thumb is to use a field 3-4 times the intrinsic coercivity to saturate the magnet. For AlNiCo 5, you just need 2-3 kG