Heat Treating the Knife
Do you need a forge to heat treat your knife?
Nope! Turns out, there are lots of places that will do your heat treating for you. For those of you that don’t have a forge and don’t want to get into that side of things yet, it’s a small expense to guarantee a proper job! Try Google-ing “heat treat knife your city” and go from there. Alternatively, check out this simple Soup Can Forge build that you can do yourself.
Types of Forges
You can really see a knife maker’s creativity shine when it comes to constructing a forge. Because commercial forges are a bit cost-prohibitive, DIYers are creating forges out of firebricks, firepits, BBQs, coffee cans, and much more. The preferred method in the community is a homemade 2-brick forge supplied with heat by a torch. There are many great tutorials online for creating your own forge (see here, here, or here), but the general idea is to create an environment that you can heat up to above 1450°F and hold there while you wash your knife in heat until it’s non-magnetic. Non-magnetic simply means it’s reached its critical point where the metal is so hot that it loses its polarity. Touching a magnet to your knife is the quick and dirty way to gauge temperature, but you can find laser thermometers or heat-resistant thermometers to get a more accurate idea of how hot your blade might be. This is useful if your steel requires particular heat treating instructions.
Final note: heat treating your steel is meant to harden it significantly. Without this step, the steel would be too soft to retain an edge for long. That said, it’s also going to make it a lot harder to file or sand down, so make sure you’re happy with your knife before you treat it.
Let’s get into it!
Heating treating the knife to its critical point
There are many ways to heat treat your blade and it will vary with the steel you’ve chosen as well as the equipment you have at your disposal. That said, use this as a guide to heat treating, but not a verbatim blow-by-blow. You must refer to the directions for heat treating your particular type of steel.
The basic process is as follows: heat your forge up to the critical point for your steel (+1450°F depending on your steel). Using a pair of industrial sized tongs, stick your knife in the heat until it’s a consistent cherry red (sometimes 10-15 minutes). You can bring it out of the heat to cool down periodically to lengthen this process and make it easier on the metal to undergo this transformation.
Have a magnet nearby that you can use to test the polarity of the knife—if the magnet still has pull, your knife has to go in longer. Heat the metal to non-magnetic (the critical point). For O1, hold it in there for another 4 minutes or so. Other steels like the 1095 can be quenched the second they hit critical. If your blade starts to glow yellow, it’s too hot. Please refer to the instructions for heat treating your particular steel.
Quenching your knife in oil
Quenching is the process of rapidly cooling down your knife. Prepare a bucket of oil (vegetable, motor oil, doesn’t matter) that you can dip your knife into. When your knife reaches the critical point, quench it in the oil. Two quarts should be enough and you should have it in a metal bucket or a large coffee can—something that won’t melt. Don’t stick the blade in all at once though. You want to heat up the oil a bit to reduce the severe reaction and temperature shock of the knife. You can either prime your bucket of oil by dipping some hot scrap metal like rebar in, or you can dip the tip of your knife into the oil and wait for the flames to die out. Ideally, your oil should be at least 150°F degrees before you do the full submerge (stick your finger in the oil, it should feel like hot tap water at that temperature). Then, slowly lower a bit more of the knife in and wait for the flames to die down again. Once they do, slowly lower the rest (it wouldn’t be a bad idea to have a fire extinguisher or a powdered fire retardant handy). Once it’s submerged, swish it slowly side-to-side horizontally as it cools. After it sits in the oil for 30 seconds to a minute, you can dip it in some water to bring down the temperature enough to hold.
OPTIONAL: instead of heating your blade straight from room temperature up to its critical point, some knife makers “normalize” the blade once or twice to help warm up the metal up before going all the way. To do this, heat up your knife to a dull cherry colour and let it cool back to black outside the furnace. Normalizing your knife reduces the chance of warping, cracking, and the overall stress of this change in temperature.
Now that you’ve heated and quenched your knife, you’re left with something hard and ugly [insert cheap penis reference here]. This is just left of scale and built up carbon that we’ll scrape off shortly. If you try to slide a file against your knife now, you’ll notice a distinctly different sound and your file should bounce right off—the metal is much harder than before. Give yourself a pat on the back. Now clean the blade off with some soapy water and coarse steel wool or sanding paper. Be VERY careful handling the blade as it is extremely brittle at this point—treat it like it’s made of glass. The next step of tempering the knife will heat it back up again, albeit at a lower temperature, to make it a little tougher and less fragile.