Stacy E. Apelt - Bladesmith
ilmarinen - MODERATOR
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Knifemaker / Craftsman / Service Provider
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- Aug 20, 2004
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I recently updated some of my info from the past and thought it would be good to bring this up again.
I used to suggest "a shade brighter than non-magnetic, but Larin and I discussed it and he suggests quenching as soon as it is non-magnetic. The reasoning is it will likely be hotter by the time it gets quenched and that the lower side of austenitization is less of a worry than overshooting it.
In teaching new smiths how to HT in a forge, I find that they frequently do one or more of these things:
They tend to pull the blade too soon when it is unevenly heated.
They greatly overheat the blade when judging the color by eye.
They only get the edge hot enough and by the time the blade gets to the quench it has dropped below critical.
Often these "lessons" are done over the internet and here in Shop Talk. This makes it impossible to physically show them what it should look like as you would in a hands-on session. Normalizing is pretty simple to teach online because temperatures can vary and a bit too high isn't disaster. Hardening is a bit harder when close temperature control is needed.
When using a magnet as the guide to when a blade has reached the transformation point while rising in temperature, the main point I try to instill is that the blade must be EVENLY heated so all the blade is the same color as it transitions from pearlite/ferrite/martensite to austenite and then converts in reverse as it cools and hardens in the quench. Uneven heating and cooling is the main cause of warp and twist. It is also important to watch the edge and avoid overheating it.
When using a HT oven, I use the charts in Larrin's book now. I used to use the ASM Heat Treaters Guide and used the minimum temperature listed. Larrin's figures are far better for knife blades. Larrin has also done research on hardening a blade in a forge.
When advising people doing any type of HT I recommend getting the pre-hardening condition right first and then using the lowest temperature needed for full austenitization. You can find great advice for normalizing and thermocycling a blade in Larrin's book or in a search using Shop Talk's custom search engine - https://cse.google.com/cse?cx=012217165931761871935:iqyc7cbzhci
How to use a magnet when doing a forge HT:
The magnet I use is a square welding magnet that I stick on the bottom or side of the forge near the port. Any strong magnet will work. Place it at a convenient location so you don't have to take the blade out for long.
Let the forge fully soak the refractory for a good 15 minutes. This time can be used to forge some small blades or such. No need to waste time and propane! The important thing is to get the forge interior fully soaked. This makes a more even and stable environment for the HT. I often shut the forge off as the blade starts to turn red and just use the internal heat from the refractory to bring it up to final temperature. This works well with most forges. A really useful addition to a venturi forge is a propane rated solenoid valve to turn the gas on and off. Run the valve with an On/OFF foot switch. This allows you to shut off the gas for a short while and then relight the forge with a step on the pedal again. The hot forge interior will immediately relight the flame. I use this in forging as well. A plus is you get to see the actual color of the blade and not have the flames in the way. I keep a butane candle lighter nearby for when I have it off too long to relight automatically. For the ultimate forge temperature control conversion , read my PID Controlled Forge plans in the forge info section of the Stickys. It can run a good forge almost as close as a HT oven.
Doing HT in a forge - Once the forge is evenly soaked, turn the forge down as low as it will go and hold a stable flame. In most forges this is still too hot for HT, but it is as low as it will go. (see above paragraph for options)
Let it run at this temperature for five minutes or so while you get the quench oil tank in position about 2 feet away from the forge and to the right or left. I recommend "smithy" temperature (50-100°) Parks 50 as the quenchant.
Well before the HT session, coat the blade with a coat of Turco-II or a slip wash of Satanite. I am sure ATP, Condursal, Brownell's, or NuClayer is just as good. Allowing the coating to fully dry on its own is also a key to success. (I won't go into all the reasoning for a coating here, but it has advantages.) I have experimented with NuClayer and find it excellent, but the cost is a bit high for some hobbyists. Satanite works almost as well and is cheap. Most of us with a forge already have a bag of it. Make a wash that is the consistency of a melted milkshake and coat the well cleaned blade with it. Let it dry for several hours. I dry them overnight. The thin coating evens out the heat transfer as well as cuts down on decarb.
My basic home-brew Turco-II recipe for the wash is a cup of denatured alcohol, a tablespoon of Satanite, a tablespoon of boric acid, a teaspoon of yellow ochre, a teaspoon of powdered charcoal, a teaspoon spoon of gum of tragacanth.
A simpler mix of equal amounts of Satanite and yellow ochre with water works quite well. I mix a pound of Satanite and a pound of ochre, then add water slowly as I mix it well. If not thick enough, add more Satanite. If too thick, add more water. You can keep this in a jar and use it for a long time. Adjust the thickness as needed.
As you heat the blade, turn it over and over, move from side to side in the forge, shove it in and out, and try to heat the spine more than the edge to allow the heat to run to the edge. If a spot gets brighter color than the rest, move that part of the blade to a cooler area in the forge.
As it gets an even dull red color, start checking the magnetic state. Pull the blade out and drag it across the magnet from ricasso to tip and right back in the forge. If it is still magnetic the feel is a resistance from pulling to the magnet. As it hits the right shade of red (far less bright-red than most folks realize!), it suddenly stops being attracted to the magnet and just slides across it smoothly. Immediately quench when it reaches non-magnetic. Hold it in the quench while pumping it up and down for at least 10 seconds before pulling it out. If it is a large blade and still smoking a lot, leave it under the oil until the smoke has stopped.
(once out of the oil you have a short window to check for warps/twist and correct them before the martensite starts to form at 400°)
Side note - if your quench is smoking or flaming it is not the blade causing it - it is that you are getting the tongs too hot. After forging, straightening, and then doing normalization, the tongs can get pretty hot. Switch to fresh cool tongs before starting the HT. I often switch tongs just before the quench when the blade gets evenly red and is close to non-magnetic.
Lots of smoke or a puff of flame, à la FIF, is a sign that something is WAY too hot. From HT temps around 1500° the blade drops to 800-900° in a second or two, so it is almost surely the hot tongs above the oil igniting the oil vapors.
A quench tank that is not deep enough or large enough is the other cause. The optimum tank should be 12" deeper than the blade is long. A minimum volume for small blades is 2 gallons of oil. For big or long blades, 5 gallons is usually needed.
I used to suggest "a shade brighter than non-magnetic, but Larin and I discussed it and he suggests quenching as soon as it is non-magnetic. The reasoning is it will likely be hotter by the time it gets quenched and that the lower side of austenitization is less of a worry than overshooting it.
In teaching new smiths how to HT in a forge, I find that they frequently do one or more of these things:
They tend to pull the blade too soon when it is unevenly heated.
They greatly overheat the blade when judging the color by eye.
They only get the edge hot enough and by the time the blade gets to the quench it has dropped below critical.
Often these "lessons" are done over the internet and here in Shop Talk. This makes it impossible to physically show them what it should look like as you would in a hands-on session. Normalizing is pretty simple to teach online because temperatures can vary and a bit too high isn't disaster. Hardening is a bit harder when close temperature control is needed.
When using a magnet as the guide to when a blade has reached the transformation point while rising in temperature, the main point I try to instill is that the blade must be EVENLY heated so all the blade is the same color as it transitions from pearlite/ferrite/martensite to austenite and then converts in reverse as it cools and hardens in the quench. Uneven heating and cooling is the main cause of warp and twist. It is also important to watch the edge and avoid overheating it.
When using a HT oven, I use the charts in Larrin's book now. I used to use the ASM Heat Treaters Guide and used the minimum temperature listed. Larrin's figures are far better for knife blades. Larrin has also done research on hardening a blade in a forge.
When advising people doing any type of HT I recommend getting the pre-hardening condition right first and then using the lowest temperature needed for full austenitization. You can find great advice for normalizing and thermocycling a blade in Larrin's book or in a search using Shop Talk's custom search engine - https://cse.google.com/cse?cx=012217165931761871935:iqyc7cbzhci
How to use a magnet when doing a forge HT:
The magnet I use is a square welding magnet that I stick on the bottom or side of the forge near the port. Any strong magnet will work. Place it at a convenient location so you don't have to take the blade out for long.
Let the forge fully soak the refractory for a good 15 minutes. This time can be used to forge some small blades or such. No need to waste time and propane! The important thing is to get the forge interior fully soaked. This makes a more even and stable environment for the HT. I often shut the forge off as the blade starts to turn red and just use the internal heat from the refractory to bring it up to final temperature. This works well with most forges. A really useful addition to a venturi forge is a propane rated solenoid valve to turn the gas on and off. Run the valve with an On/OFF foot switch. This allows you to shut off the gas for a short while and then relight the forge with a step on the pedal again. The hot forge interior will immediately relight the flame. I use this in forging as well. A plus is you get to see the actual color of the blade and not have the flames in the way. I keep a butane candle lighter nearby for when I have it off too long to relight automatically. For the ultimate forge temperature control conversion , read my PID Controlled Forge plans in the forge info section of the Stickys. It can run a good forge almost as close as a HT oven.
Doing HT in a forge - Once the forge is evenly soaked, turn the forge down as low as it will go and hold a stable flame. In most forges this is still too hot for HT, but it is as low as it will go. (see above paragraph for options)
Let it run at this temperature for five minutes or so while you get the quench oil tank in position about 2 feet away from the forge and to the right or left. I recommend "smithy" temperature (50-100°) Parks 50 as the quenchant.
Well before the HT session, coat the blade with a coat of Turco-II or a slip wash of Satanite. I am sure ATP, Condursal, Brownell's, or NuClayer is just as good. Allowing the coating to fully dry on its own is also a key to success. (I won't go into all the reasoning for a coating here, but it has advantages.) I have experimented with NuClayer and find it excellent, but the cost is a bit high for some hobbyists. Satanite works almost as well and is cheap. Most of us with a forge already have a bag of it. Make a wash that is the consistency of a melted milkshake and coat the well cleaned blade with it. Let it dry for several hours. I dry them overnight. The thin coating evens out the heat transfer as well as cuts down on decarb.
My basic home-brew Turco-II recipe for the wash is a cup of denatured alcohol, a tablespoon of Satanite, a tablespoon of boric acid, a teaspoon of yellow ochre, a teaspoon of powdered charcoal, a teaspoon spoon of gum of tragacanth.
A simpler mix of equal amounts of Satanite and yellow ochre with water works quite well. I mix a pound of Satanite and a pound of ochre, then add water slowly as I mix it well. If not thick enough, add more Satanite. If too thick, add more water. You can keep this in a jar and use it for a long time. Adjust the thickness as needed.
As you heat the blade, turn it over and over, move from side to side in the forge, shove it in and out, and try to heat the spine more than the edge to allow the heat to run to the edge. If a spot gets brighter color than the rest, move that part of the blade to a cooler area in the forge.
As it gets an even dull red color, start checking the magnetic state. Pull the blade out and drag it across the magnet from ricasso to tip and right back in the forge. If it is still magnetic the feel is a resistance from pulling to the magnet. As it hits the right shade of red (far less bright-red than most folks realize!), it suddenly stops being attracted to the magnet and just slides across it smoothly. Immediately quench when it reaches non-magnetic. Hold it in the quench while pumping it up and down for at least 10 seconds before pulling it out. If it is a large blade and still smoking a lot, leave it under the oil until the smoke has stopped.
(once out of the oil you have a short window to check for warps/twist and correct them before the martensite starts to form at 400°)
Side note - if your quench is smoking or flaming it is not the blade causing it - it is that you are getting the tongs too hot. After forging, straightening, and then doing normalization, the tongs can get pretty hot. Switch to fresh cool tongs before starting the HT. I often switch tongs just before the quench when the blade gets evenly red and is close to non-magnetic.
Lots of smoke or a puff of flame, à la FIF, is a sign that something is WAY too hot. From HT temps around 1500° the blade drops to 800-900° in a second or two, so it is almost surely the hot tongs above the oil igniting the oil vapors.
A quench tank that is not deep enough or large enough is the other cause. The optimum tank should be 12" deeper than the blade is long. A minimum volume for small blades is 2 gallons of oil. For big or long blades, 5 gallons is usually needed.
