Hi everyone!
Aaron from Gough custom here.
I'm going to show you today how to heat-treat an O1 tool-steel knife in your own shop, with
just a few simple tools.
The very first thing you'll need is quench oil.
The quench oil is what you put the blade into when it's been heated up to red hot to cool
it down fast and make the blade harden.
In this case we're just using vegetable oil, which is cheap and easily available and works
very well.
The next thing you're going to need is a pair of tongs to hold the hot steel.
I use these big vise-grips, they were quite cheap.
I think they were only about $6 or $7 and they work quite well.
If you can find blacksmith's tongs then that's great but they're not necessary as something
like this works well.
The offset head keeps your hand out of the path of any possible flame from when you're
quenching the steel in the oil.
The next thing you need is a container to hold your quenching oil.
This is actually a small wine-bucket that I got from a kitchen supply place.
Kitchen supply places are great for this kind of thing because they have lots and lots of
different things that would work.
Anything that you buy needs to be either aluminum or stainless steel...
No plastic because you'll melt through it.
Anything you can find that's tall enough to fit your knife in is perfect.
Then you're going to need a torch.
I use a Bernzomatic TS4000, this is a relatively expensive torch, it costs about $40.
It works very well, and it works with both MAPP gas and propane.
So if you need a hotter flame for brazing or whatever then this is a great torch.
If you are looking for cheaper torches there is one thing that you need to be aware of
and that is: if you look at this torch you can see that the inlet holes for the burn
tube are right down here which means that they're almost 6 inches away from the head
of the flame.
On this cheaper torch the air inlet holes are only about an inch away from the head
of the flame, what happens is that the burnt gases will recirculate into the air inlet
hole when the torch is used with a forge because something else is in such close proximity
with it.
If you're looking to buy a less expensive torch is to make sure that the air inlet holes
are as far away from the head of the torch as possible.
If it too close, like this one, then it will work fine as a blowtorch but it won't work
with a forge or for heat-treating.
The next thing you need is a forge.
This one is a very simple home-made one it's basically just made of 2 fire-bricks that
have semi-circular channels cut into them.
And an inlet hole at the front.
The flame goes in at an angle, angled toward the rear, then swirls around the burn chamber
before it exits at the back.
Having the flame right at the front lets you move the blade in and out while you're heat-treating
it to get an even heat from the handle of the blade all the way to the tip.
If you have the flame entering at the back then you'll be able to get the tip very hot,
but you'll have a hard time getting the rear of the blade, the tang and the ricasso hot.
So I recommend having the hole at the front, it works really well.
The next thing you're going to need are fire-proof gloves.
I use welding gloves, and these work pretty well.
They're not the most comfortable but you need something that's quite flame retardent because
if your quenching oil catches on fire or spits up a big burst of flame when you first put
the hot blade in it can burn your hands quite badly if you don't have gloves on, so I recommend
wearing gloves.
And then finally, unless you like burning your house down, I highly recommend that you
have a fire extinguisher on hand.
Using water to extinguish an oil fire is extremely dangerous and you shouldn't do it so make
sure you have a dry powder fire extinguisher at hand that's suitable for use on oil fires.
Here you can see how the propane torch is setup in relation to the forge.
The flame is angled upward, and toward the rear of the forge.
This helps ensure that the heat is distributed as evenly as possible by causing the flame
to form a spiral around the inside of the forge.
After letting the forge heat up for several minutes, the next step is to pre-heat our
quench oil.
Warm quench oil will actually cool the steel faster, because it forms less vapour bubbles
around the blade during the quench.
To pre-heat the quench oil I am simply heating up a piece of scrap steel and then plunging
it into the quench container.
Now it's time to start heating our knife blade.
Notice that I'm constantly moving the blade around in the forge.
This helps ensure that I don't over-heat any particular part of the blade.
What we're looking for is an even dull cherry-red colour.
If parts of the blade get hotter than dull cherry then they may end up being brittle.
You can see that the thin parts of the blade heat up the fastest.
Keep them away from the direct flame and make sure to continuously move the blade in order
to get the heat as even as possible.
It will take a few minutes.
After the blade is an even, dull, cherry red colour, remove it quickly from the forge and
plunge it straight down into the quench oil.
You only have a few seconds to get it out of the forge and into the oil.
Be careful of where your hands are, as the oil will often catch fire on the surface for
a few seconds.
After the blade is in the oil, make gentle slicing motions with it to help cool the blade.
Do not move the blade sideways as this may cause warping.
After the blade has completely cooled from the quench, we need to test that it has hardened
properly.
To do this we run a file over the edge and spine of the knife.
If the knife has hardened properly then the file will simply skate over the surface of
the steel.
At this point the knife will be extremely hard, but also quite brittle.
If you were to drop it accidentally it would probably shatter.
So the last thing we need to do is to temper the knife.
This process gives up some hardness in the steel in exchange for much more toughness.
To temper the knife we simply heat it to an elevated temperature and keep it there for
a while.
In this case we'll be heating it to approximately 400 degrees farenheit and keeping it there
for 2 hours.
I'm using a toaster oven for this process, but you could also easily use a regular household
oven.
After the tempering process is complete, your knife blade will be hard and hold a great
edge, but it will also be tough and resilient.
At this point the blade is ready for final finishing and use.