Determining temps with magnets?

[CaptainBruno]

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?
Thanks.

[jmccustomknives]

It's really that simple, on simple carbon steels.

[GregDP]

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.

[r smith]

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.....

[ThomasPowers]

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!

[CaptainBruno]

Great thanks a whole lot. I'll see how that goes.

[timgunn1962]

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.

[r smith]

That is why it is a good idea to hang the magnet from a thin copper wire. Very easy to tell if it is "sticking" or not.

[metal99]

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.

[Will73]

I’ve been heating carbon steel blade with coal and shop vac. The blade had the orange glow , but is still magnetic.  I’ve done this twice now.  Any help is appreciated.  Thanks 

[Frosty]

Welcome aboard Will, glad to have you. If you put your general location in the header you'll have a better chance of meeting up with members living within visiting distance.

You aren't getting it hard enough. Forget the idea of "The Orange glow," you don't have the experience to know what THE color actually is. That is what the magnet is for so you don't have to judge temperature by color. Having a reasonably accurate gauge like a magnet helps mitigate the effects of lighting changes. Even if you're indoors a cloud covering or clearing the sun shining in the window WILL throw off how you see heat colors.

Make sense?

Lose the shop vac, it produces more air than 3-4 forges need. A blow drier is a better choice. I use mattress inflater blowers and they produce 2-3x the air  needed for a good size coal fire and they're easy to control with a choke plate on the intake side. 

Frosty The Lucky.

[George N. M.]

Welcome aboard from 7500' in SE Wyoming.  Glad to have you.

I agree with everything Frosty said.  The magnet is the indicator, not the color.  You could be color blind and still judge the correct temperature with the magnet.

I will say that you can use a shop fac blower but you have to have it on a rheostat to slow it WAY down.  One way or another, you HAVE to be able to control the amount of blast to your fire.  It can be electrically, something like a butterfly valve that blocks the flow of air, how hard or slowly you crank a hand blower, or anything else.  There are times you want a hard blast and there are times you want just a gentle puff.

Also, this is a world wide forum and we don't know if you are in Ohio, Siberia, or South Africa.  A surprrising number of answers to questions is geography dependent.

"By hammer and hand all arts do stand."

[anvil]

Yup, what Frosty and George said.

[Irondragon Forge ClayWorks]

On 6/9/2023 at 6:48 PM, Frosty said:

You aren't getting it hard enough

Shouldn't that be Hot enough?

I can't control the wind, all I can do is adjust my sails. ~ Semper Paratus

[Frosty]

Yes, HOT not hard. <sigh> I think faster than I type and get ahead of myself. 

Thanks for catching that Randy, I hope Will didn't think I was messing with him. 

Frosty The Lucky.

[Buster Bolster]

There is some nuance here. If you have something near the eutectoid limit with no carbide issue to deal with (how much carbon will be in solution, etc), you can pretty much get book by going to nonmagetic or ever so slightly past. Book as in book hardness and toughness that you'd expect with a furnace. 

If you're using a grobet file, it depends a little on what it is. If it's 1.2-1.3% carbon steel, it will harden at nonmagnetic, but it will need to get a little hotter than that to be able to see hardness out of the quench like you'd expect, and get sort of the biting edge you'd expect out of file steel. Hardness out of quench on files for me without grain bloat - presuming they're something simple  - is 68 or so. 

Short story long, if working only to thorough nonmagnetic and quenching quickly to avoid grain growth, 1075/1084-ish stuff is going to be just as good as a file, and maybe better. If willing to experiment with how much more heat for a file, the fine edge quality and post-temper hardness of a file or something like 26c3 (almost identical to older file and razor steels) will blow away 1084 and 1075.

Is it that important? Only if you plan to make a lot from files. 

By the way, putting a relatively inexpensive pickup tool magnet on the end of a hollow steel or brass tube is usually what I do for something that's sitting in heat. You can push the magnet tool into the fire very briefly and back out off and on to test things and you won't end up with a hot magnet, etc. It's a little easier than handling just a magnet or a magnet on wire. It may be TMI, but with higher carbon steels, you'll find a partial or near complete loss of magnetism and then sort of a transition in higher temps before all of the magnetism is gone. if you're working without computer control, you want the journey through this range to be quick to avoid bloating the grain. Files are nice because they do have more functional protection from carbides if we're talking about running the temperature up in a very short period of time.