Morgan
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Monel Mesh Strainer Mystery
I bought this strainer for 2 dollars because of the monel screen and it had a mysteriously strong attraction to a magnet but only in the center
region. So held it up to the light and could only make out a little bit of screen. I took a piece of fiberglass rod from an umbrella and found from
the wide end the rod went nearly all the way to the other end. From the crimped end it only went as far as the 4 channels.
So I figured what the heck, and forced the screen away from that end with some difficultly working all four channels until it I couldn't push the
obstruction any farther up the tube which was about half way. I thought for sure the screen would just tear. So then I used a pipe cutter and cut the
strainer in half. When apart the screen was severely crumpled but working it I was able to reform it into the basket shape. Monel seems very strong
and forgiving.
Anyway, there is this little spot welded inner ring/band inside the tube where the ferromagnetic properties come from and it's what held the monel
mesh sandwiched between the ring and tubing. The ferromagnetic section of tube was put on a red hot stove burner to see if it had a low or high Curie
point response. I had thought the tube copper, but it appears to have burned off or the thin plating alloyed with the base metal somehow. I took a
common 1 inch copper end cap fitting and heated that on the burner too, which produced a more bluish patina that didn't quite get picked up by the
camera - this was just to see how pure copper would change under similar heating.
The inner ring band appears the same coppery metal as the tube itself, but perhaps it's some kind of iron alloy. Or maybe the monel screen alloyed
with the two metals during the welding process but I think it's more likely the ferromagnetism is coming from the former. Again, it's just that one
spot that has an uncannily strong attraction, much more than any solid monel I have experimented with.
   
[Edited on 25-7-2014 by Morgan]
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aga
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How strong an attraction ?
Also, have you tested the tubing without the monel mesh in place ?
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Morgan
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The monel mesh itself is just weakly ferromagnetic but at dry ice temperatures I suspect it would increase greatly. The welded inner ring which
secured the monel basket you can see a bit better if you enlarge the one photo of it looking inside the tube. I think it's probably the inner ring
band that is causing the strong attraction. It takes a good pull to get a neodymium magnet to let go. There is a small amount of screen of course that
is sandwiched between the ring and outer tube that tore when I was trying to walk or force the screen out the wide end before I decided that wasn't
going to work. The ring is the only part of the tube that exhibits ferromagnetism.
Monel 400 is said to have a Curie temperature between 20 and 50 C but the fittings I played with only became strongly ferromagnetic when cooled to
something even lower than a refrigerator freezer would provide - dry ice worked like magic.
I'm curious why the outer tubing wasn't just made out of copper instead of copper plating.
I suppose too the inner ring could have been press fit in place instead of welded, I don't know.
[Edited on 26-7-2014 by Morgan]
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aga
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Iron band, and your Nd magnet is pretty strong i guess.
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