Author Topic: Heat Treating Magnetic Tester  (Read 987 times)

Offline sparky961

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Heat Treating Magnetic Tester
« on: March 25, 2016, 12:39:37 AM »
This is a bit of a half-baked idea that I'd like some input on.  It all rests on the idea that you only need to heat steel to the point where it loses its magnetism to harden it.  So if this is somehow mistaken or not fully accurate, do tell me.  If there are no problems with this, I really like the idea of using this property to determine when you've heated it just enough and not too much.

I don't like using the colour/brightness to judge temperature.  It's slightly different for different alloys and ambient light and your current mood makes it highly subjective.

What's so hard about testing a piece of steel with a magnet?  Well, I guess three things come to mind from my past experience:

- You've got a torch in one hand, possibly the part in the other, or you're passing the torch from hand to hand heat evenly.  Sometimes a third hand would be really great.  Some sort of stand, or attachment that doesn't require picking it up is needed.

- Ever used one of those long magnetic pickup tools to test whether a part is ready yet?  If you have, you inevitably have had the same experience as me: You're not quite there yet and the magnet STICKS.  You try to pull it off but instead the telescoping part extends, causing much less control at the business end.  You end up picking up the RED HOT part at the end of the flimsy rod, potentially dropping it in the least ideal location.  That, or you have to set down your torch, get your pliers, and separate the two.  Repeat this a few times before you finally get it.

- Magnets don't really like heat.  In fact, the heat that's transferred even from quick contact with the part is enough to kill a good neodymium magnet.  Looking at some specs, the maximum operating temperatures are pretty low (https://www.kjmagnetics.com/specs.asp - about half way down the page)  I've destroyed more than one magnet in this manner.

So, I started to think about maybe mounting a magnet inside an aluminum rod or thick walled tube, possibly with the working end fully enclosing the magnet.  Further brainstorming led me to think maybe to fill it with water or oil to help conduct and dissipate heat from the magnet and tool body.  Or maybe just a good fit into the housing with some heat-sink grease around it.  To counteract the problem of sticking, either the distance the magnet is set back into the housing would reduce the strength enough to detect magnetism but not pick up most small to medium sized parts, or it could incorporate a plunger that pulls the magnet back - similar to the magnetic pickup tools you see on the market.

Another thought was to use a compass, but most are plastic and this seems like a bad idea near an oxy-acetylene torch....

What are your thoughts?  Am I overthinking the problem?

Offline Lew_Merrick_PE

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Re: Heat Treating Magnetic Tester
« Reply #1 on: March 25, 2016, 02:01:56 AM »
Sparky,

1) The heat-treating temperature where steel loses it's magnetic properties is known as the Curie Point.

2) You are absolutely correct in that overheating magnets fall into the Bad Things category.

3) I normally hung an ALNICO magnet on a string close enough to where I am working such that I can see it "pull" if the piece is still magnetic.  Once the magnet hangs freely (with no "pull") I will hold the part at that temperature for about 20 minutes/inch of thickness -- and I do "check" every few minutes to assure that everything is still non-magnetic.

4) About a decade or so ago, I changed over to an optical pyrometer made from a PR-4 (torch) lightbulb with a variable voltage power supply and the magnet itself has been repurposed.