1084 HT Question

F

foxtrot delta

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Jun 24, 2014
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I've been working with 1084 and had a question regarding the color of the metal during hardening. Everything I've read has said a dull to cherry red color is ideal however this does not seem to align with what I'm seeing. I harden more or less in the dark aside from the light from the forge. The steel is still magnetic while red in color. It is not until I reach a midtone orange that my magnet no longer has attraction. I'm using 1/8" thick stock and testing with a small workshop pickup magnet (the kind on an extensible stick.) Is something awry here?
 
Trust the magnet though it might be a good idea to get a bigger one. Even the light from the forge can effect how your eyes perceive the color.
 
FD....it doesn't sound like anything is wrong at all. Colors are subjective. Some see red.....others see orange. I am with you in the fact I remember starting out hearing some people say that the color of the steel at the proper temperature (1500F) is a bright red. I like to think I can make out colors very well....and I see more orange than red. So I would say trust your eyes....but definitely USE THE MAGNET. Color isn't THE indicator.....the magnet will let you know when you have reached approx 1414F, as at that temperature the magnet will no longer stick. However....1084 needs to go up to 1500F....so the general recommendation is "a shade or two more once the magnet stops sticking". Do NOT quench AT the point when the magnet releases....as this is about 75-85 degrees less than you need to be for full hardness. When using a forge or the like to heat treat.....I always do it in the same lighting conditions every time. That way you can train your eye to see the right colors.
 
The difference between daylight and night observation can be quite stunning. Blades that are being forged during the day seem red, while those done in the dimly lit smith at night seem brightly glowing. As Stuart said, color is objective and one person may call it a different shade than you do.

The color method I teach for HT of 1084 is to heat it evenly and check with the magnet often as it stars to get medium red. I place a cheap HF welding magnet on the side of the forge near the blade port. This keeps my hands free for handling the hot blade. As the blade gets red, I pull out and move it toward the magnet. It will stick to the magnet when below 1414F. As it heats more, I continue checking and when it gets to the Curie point, it suddenly stops sticking. At this point the steel structure has changed to austenite, which is not ferromagnetic. I tell people to observe the color and heat one shade red brighter. What I see I call cherry red, but you may call it something else. However, whatever we call it, one full shade brighter is going to be 1475-1500F range. Dunk the blade in a gallon or two of fast commercial HT oil or canola oil heated to 120F and it will harden just fine.
 
The first magnet I had literally exploded when I used it to check the blade. Not sure what the deal with that was. I switched to a neodymium magnet and it has been fine since.

I used a mixture of new 5w30 and atf fluid to quench. This was probably far from ideal. As this is only my second blade, should I go back and reharden using canola?

Thanks for the responses.
 
Yeah, it looks red at that temp- in a well lit workshop, perhaps with fluorescent lighting.
In my shop, with all natural light, it looks like a low orange. Getting ahold of a measuring device of some kind, whether it's a gun type thermometer or a kiln with an accurate thermostat helps a lot, then you know what 1500 looks like in your shop's lighting.
Other than that, "What Stacy said."
 
Gun type thermometers can be excellent if you take the appropriate steps to ensure the emissivity is correctly set for the materials and conditions. Otherwise, they can get you into all sorts of trouble.

A nice thick thermocouple (maybe 1/4" or so; enough visible area to properly see the colour) and a cheap pyrometer is usually more accurate and can be used to calibrate the mk1 eyeball for the lighting conditions.
 
timgunn,
I have to disagree with you on this. Almost all non-industrial grade "gun" type pyrometers are not worth a darn for reading 1500F steel. You would have to remove the blade and take several readings to get an accurate average...all the time while the blade is rapidly cooling . You can't read the blade while in the forge because the forge atmosphere would be what was being read. In the same way, a thermocouple and pyrometer won't be useful for taking the blade temperature. They are good for reading the forge temp. But you want the blade at 1500F and the forge will be well above that unless you build a PID controlled forge. I have a lab grade non-contact pyrometer that sells for $1800, and even with the emissivity set right I don't consider it accurate enough for HT.

Foxtrot,
You mentioned the magnet exploding??? Are you putting the magnet in the forge to check the blade? Pull the blade out briefly and check if still magnetic. That is why I stick my welding magnet on the forge shell near the port. You only need to touch the blade to the magnet for a second to check. Don't heat magnets too hot or they may break or be ruined. Todays magnets are mostly ceramic metal construction....and don't like heat.
 
As for the magnet, it was NOT put in the forge. I had it on the workspace near to the forge. When I touched it to the blade it shattered into 5 or 6 pieces. Kind of puzzled me. The neodymium has not done that.
 
Like others have said trust the magnet. You're probably working in low light so the blade appears brighter. I prefer to do heat treating after sun down since it is much easier to see how the heat is dispersed though the blade. Heat treating with a small single burner gas forge with no thermocouple can be a little tricky. Working in low light helps me keep an eye for hot spots and edge temp. A good magnet will tell you where that non-magnetic point is but the "shade higher" is something you'll have to learn for yourself through a little bit of trial and error. Think of it as "calibrating" your eye.
 
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