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How to Build a Knife

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Step 5: Heat Treating--for the little pyro in all of us

Here's probably the most technical part of the entire project--heat-treating the blade. You can use either a coal forge (as I did), a gas forge, or a torch. The last should only be used on small knives--maintaining high heat on a big blade would be hard with just a torch. See picture one below to see me starting the fire.

Heat-treating consists of two steps, hardening and tempering. In hardening, you heat the blade to a critical temperature and then quench it. This changes the structure of the steel so it's extremely hard but also pretty brittle. A knife in this stage, if dropped, can crack or shatter like glass. The next step, tempering, is done by heating the knife to a lower temperature, around four hundred degrees. This makes the knife less brittle, while still keeping a relative amount of strength.

Now, You'll need a hardening bath. For 01 steel, you should use oil. Different types of steel require different methods of quenching--oil quench, water quench, air quench, etc... again, I recommend 01 steel because it's easy to heat treat and doesn't require anything more complicated than a bucket of motor oil. See picture two. You should be able to immerse the blade completely. The second thing you'll need for hardening is a magnet. This will help you determine the hardening temperature, because at that point the steel becomes non-magnetic. See picture three--I keep the magnet on the hood of my forge, specifically for this purpose.

Now to start. Make a fire on your coal or gas forge or light up your torch--heat the blade by the spine, so as not to burn off the edge. Steel will burn off or melt into an unusable foam-like metal mousse if it's heated too high.
So, you're going to heat the metal to a medium-high orange heat, until the steel becomes non-magnetic. Just tap it against the magnet while it's glowing, and if it doesn't stick, it's ready. At this point you'll want to let the steel cool slowly in the open air, a couple times. This is called annealing, and relieves stresses in the steel cause by the rolling and milling process. After you've annealed (three times is a good round number), heat it to the same temperature you have been, but instead of annealing it, plunge it into the oil bath. Wear gloves because there's going to be some fire here. See picture eight. When you take the knife out it'll be smoking and the entire room should smell like the French fry tent at the county fair. To test the edge, run a sharp file over it. If it's hard, the file should skitter over the edge without making a mark, as in picture ten. You've hardened the blade at this point, so be careful. It'll break if you drop it.

Now, there's not much you can do with the blade until you temper it. Put out your fire, go inside, and preheat the oven. Your steel might have come with tempering information on it. If it did, chose your hardness from the sheet and use that temperature. You'll want a medium hardness for a knife. The eleventh picture of this step is an illustration of the tempering colors--these are a visual aids for measuring the temper of the blade. The higher the temperature, the softer and springier the blade will be. Try to shoot for a brown or purplish color, which will usually show up at about 400-450 degrees. If you don't know exactly what temperature to use, go for 425 degrees fahrenheit. Put the blade on the middle rack and let it cook for one hour. When the hour's up, the knife is ready. Congratulations. You've officially made a blade--though to turn the blade into a fine piece of cutlery, you'll need to do a little more work.
 
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did you have your own coal forge or did you have to make one and if so how did you make it.
rstumph2 years ago
ok i read this instructable its a little confusing ok , the step need to be step by step for example, you harden the steel to a temperature a couple a times, in tell non magnetized then the blade is ready For Annealing After Annealing then you Quench it ok ... i really like it though . I Have AD/HD so thought id let you know cause i hate to go searching and end up missing a step or the knife looks, works crappy its really irritating. thank you for trying you did a good job.
fishcake273 years ago
Thanks basta, im gona make my knife over thanksgiving break, it kinda looks like jay fisher's PJLT design. i'll post it when i get the chance, and whats the cheapest way to heat treat the blade (like to get it that hot)....
dogianto4443 years ago
do i really have to put it in the oven? what is its purpose of doing it?
Basta (author)  dogianto4443 years ago
Tempering. Read the 2d and last paragraph on this step.
i already made it hardened in a charcoal fire and it is completely black just like the 10th picture. how do i know when it is is a brown or purplish color when its in the oven? does the black skin eventually get replaced?
Basta (author)  dogianto4443 years ago
It doesn't get replaced. I sand the hardened blade a little bit so I can see the white metal underneath. This is where the colors show up.
one last question when you take it out of the oven do u dump it in water or do you let it sit in the open air?
Basta (author)  dogianto4443 years ago
Either.
zerrodach3 years ago
When you were annealing, how long did you let the steel cool in the open air?
sculptur3 years ago
can you use used motor oil?
Basta (author)  sculptur3 years ago
Yes, apparently that's even better because of its carbon content. Quenching in unused oil is fine, but it zips away a little of the carbon in the metal.
sculptur Basta3 years ago
thanks because i have lots of old used oil
knibbles4 years ago
"If it did, chose your hardness from the sheet and use that temperature. You'll want a medium hardness for a knife." I think you meant If it did, CHOOSE not chose. P.S. Other than that, great 'ible. I would give you six stars if I could.
lobo_pal4 years ago
How soft is the metal if you don't heat treat it. I want to, but I don't really have a means right now besides simply using a gas torch.
Basta (author)  lobo_pal4 years ago
Pretty soft. Edges won't hold up to normal use if the steel is completely untreated. Hardness is a question of ease of sharpening and flexibility vs. edge retention and brittleness, as far as knife makers are concerned. Untreated steel will be extremely resistant to breaking damage, and will be extremely easy to sharpen to a point of usability, but it will not stay sharp for long as you will probably not be able to reach the blade's full cutting potential even immediately after sharpening. That said, it's fine if you don't want to heat treat it, but it won't be a knife in the usual sense of the word--it'll be more like a spear head. Just keep that fact in mind. I'd still try to use tool steel if you can as it will be more wear-resistant than mild steel even untreated.
lobo_pal Basta4 years ago
If I were to try to treat it with a gas torch, How do you think it would turn out? Or should I just see about a ceramic oven, which I might have access to in the future.
magiccowy4 years ago
Is it 450 farenheight? Not celsius?
An Villain4 years ago
picture 6, epic zombie weapon.
 Agreed.
that gives real meaning to the phrase "too hot to handle."
 Could you harden the blade in a fireplace?
Whales4 years ago
I can't understand the tempering part, can someone explain.. soon hopefully..
kerns Whales4 years ago
Here's some initial info. If you're really interested look into the many knifemaking forums online as well.

http://en.wikipedia.org/wiki/Heat_treatment#Hardening_and_tempering_.28quenching_and_tempering.29
apod135 years ago
finished hardening and tempering this weekend!
how do u make it shiny i know i sound stupid but im jw
reddutch5 years ago
hey I was just wondering if you where using the can of water for cleaning or for cooling the iron or maybe not at all. cheers, reddutch
i got 1 last question about the hardening in oil how much fire are we dealing with. and does the oil bubble or splatter cause burning oil is no joke and i can be as careful as possible and still end up burned. How long does it take for the fire to go out? i may be worrying over nothing but please understand my (and my mothers) concerns. btw i cant thank you enough for your help :)
thanks i think im ready to get on ith my knife i got my sketches and im going to order my steel thanks for your help and ill be sure to be very careful.
So i should be able to do the hardening and annealing in my charcoal grill right? And what is the file test exactly (shown in pic 10). also do you know where i can get some tongs or whatever you use to grab the hot blade or will a large pair of pliers work?
Basta (author)  i_build_stuff5 years ago
No, not your charcoal grill. A soft coal fire or a really, really hot wood fire will work. The file will not bite if your blade is successfully hardened, but will bite if it is still soft. Big pliers are fine but for god's sake be careful.
tibbaryllis5 years ago
something that is fun to play with when making blades involves the hard/temp process and taking a note from extremely well skilled cultures. One of the single greatest features of GREAT knives/swords involves a hard edge that holds an edge well but also a good strong body that is tough but bends rather than breaks. A fun technique (taken from skilled katana makers) is to, after you get your blade all ready to be fired up and heat treated, cover any part of the blade that you do not intend on sharpening with an even layer of clay. Then do all the steps as listed. After dousing in the oil bath, simply break the clay off and proceed the oven.
danoliveri5 years ago
if i dont have a forge what can i use?
Just for clarification, you heat the blade and let it cool on its own about three times before heating it again and quenching it in oil, right? And you heat it with the edge up?
Basta (author)  shinigami5026 years ago
Yup, that's it. You heat with the edge facing up so that it's in the cooler part of the fire--otherwise the edge would heat too fast and burn away.