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Determining temps with magnets?

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Okay so, I've made a knife and it comes to the point where I need to harden the blade. This is where I am stumped. The knife is made from a grobet file that some guy broke.
So I have a few questions.

  • 1 How do I gauge the temperature of my piece in the fire without some sort of pyrometer?

  • 2 How do I figure out what the right temperature is? What am I aiming for?

I read in "The Art of Blacksmithing" by Alex Bealer, that

"In recent times the proper hardening color may be realized by touching the steel with a small magnet as the heat rises. The magnet will stick until the metal reaches a red heat. At the exact moment that the magnet ceases to stick, the steel should be quenched for maximum hardness."

Is it really that simple? I'm always reading about hardening temps always around 1500 degrees. But they're always a bit different? why doesn't everyone just use the magnet technique?

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The none-magnetic test is just a sign the blade has transformed to austenite (it's none magnetic, this is the curie temperature). Quenching at the critical temperature (usually around the curie temperature.) will ensure the greatest formation of martensite from austenite.
Is it that simple? Yes it is. No it isn't. (Very helpful right?)

I've honestly had a rough time with heat treating. This book helped me at least understand how much I don't understand :P “Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel” written by John D. Verhoeven.

I've had better results using a magnet on a scrap piece of desired steel at night to identify and learn the visuals. I then heat treat at night... I do this because I found I'd over heat the edge, where it counts, when depending on the magnet... All that said I'm not a real knife maker. But I've made some decent tools. If you want to make something perfect, use perfect tools and techniques. Otherwise don't sweat imperfection and learn from your mistakes.

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It is best to hang the magnet from a small wire. Like a strand out of electrical wire. If your doing a small part that will not hold heat maybe you should heat it a bit past non magnetic so by the time you quench it it is still above non mag.
The hanging magnet should be swung close to the part and when it no longer deflects.....

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Files are generally a simple steel and take simple heat treating.

High alloy steels are quite complex and can require extremely complex heat treating with ramping temperatures, soak times, special quenchants, etc.

For judging temps, smiths generally work by the colour of the steel which is very different to different people, ambient light, etc; so learning what the proper temperature looks like to *you* in *your* set up is a good practice!

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One thing to watch is that a lot of magnets stop being magnetic when they get hot, particularly the powerful modern ones. The "stick a magnet on the workpiece and quench when it drops off" approach is probably not a good one.

In most cases, the magnet will stop being a magnet at a much lower temperature than the workpiece will cease to be attracted by a magnet.

It's better to heat the workpiece and touch it with the magnet occasionally, letting the magnet cool in between tries.

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Be carful with the magnet trick. The magnet won't stick after 1414f and that's not the temp you want to quench at. Most steels need to be quenched at 1450-1475 so I like to use the magnet and go a shade brighter, hold for a minute and quench. If your using a file I would use a light oil like canola or water both should be warmed up to 120-160.
Oil is safer then water because of the rate of cooling.

Another point about the magnet is when the steel reaches 1414 it won't stick BUT it will continue to not stick till it is nearly black.

To bad you don't have another one to test with. Don't forget to normalize it a few times too.

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