If you ever wanted to know how to heat treat a sword this episode of a Day at the Forge
may answer some of your questions.
For those of you who have never watched me before, I create different kinds of videos.
With a Day at the Forge, you get a window into a four hour slice of my typical Saturday
morning.
Usually that means you will not get to see finished products but you get to observe my
process where I share some of what I have learned over the last couple of years.
Heat treating is the process in which the edge of the sword is hardened so that it can
hold a sharp edge.
With a sword as long as this one, symmetry becomes very important.
You might ask why?
So, let me try to explain to you.
While I am by no means an expert in metallography, I can tell you that a minimum amount of carbon
is required to allow the hardening of steel.
Steel starts to be hardenable over .35% of carbon but for swords,
I usually work from anywhere above .6% to about 1% of carbon.
The carbon atoms can have different arangements in the steel.
There are roughly three: pearlite, austenite and martensite.
At a gross over simplification, soft steel contains pearlite, when it is heated above
it's critical temperature the transitions to austentive and quick cooling arrests the
carbon atoms so that the steel forms martensite.
Martensite is hard and that's what we want to get for the edge of the sword.
However, the transition to martensite is quite stressful and if not done carefully can destroy
or terribly warp the sword.
This brings us back to symmetry.
Every place in the sword that is not symmetrical creates an opportunity for unequal stress
and thus warping and bending of the sword when quenching.
I am getting ahead of myself though.
In the previous episode I was done with all my forging work.
Before the sword can be heat treated, it's final geometry needs to be created.
That means the sword has a well defined and symmetrical cutting edge as well as a fuller
in the middle to reduce its weight.
Establishing the fuller on the grinding wheel is a delicate operation.
I usually apply layout fluid and then carefully scribe a line that shows me where I need to
stop grinding.
Once the initial fuller has been established, it becomes a little bit easier to follow the
lines by hand.
Since the wheel will create a circular fuller shape, I sometimes employ a rocking motion
to flatten the bottom of the fuller.
All of this takes patience and also creates a lot of grinder dust.
Instead of using a blower to move the dust away from me, I am wearing my powered air
respirator today.
It will provide with a constant flow of fresh air
and is a healthy practice when I expect to be in front of the grinder for a long time.
Now that I have gotten some practice with fuller grining and feel back in the swing
of things, I reapply the layout fluid to the other side of blade and keep going.
Scratching the line can be done very lightly by just removing the layout fluid and does
not require actually marking the steel.
As with the other side, I initially take very light passes on the grinder.
This gives me a feel for the positioning of the blade to the grinding wheel and sets me
up for more aggressive grinding later.
As you may remember this sword has quite complex pattern welding and has the additional challenge
that I need to keep the layer transition in the middle of the blade which of course is
something that I cannot see.
Now that I am reasonably happy with the fuller I created, it's time to grind down the cutting
edge.
I usually do that on the platen and a small table for support.
The table is not necessary but at least for the initial passes, it makes it easy to aggressively
grind down the edge.
After heat treating, I usually employ a light touch and may forgo the table.
That of course assumes that the sword is going to survive the heat
treat.
Unfortunately, that is never guranteed.
I mentioned before that carbon is necessary for steel to harden.
The amount of sparks created on the grinder are a good indication that this steel has
a lot of carbon.
With a sword of this length, it is somewhat difficult to maintain a completely straight
edge.
So, I often need to pause and take a look.
Usually, I am reasonably close to straight but every now and then I need to compensate.
You may notice that I am wearing gloves.
That is because grinding creates a lot of heat and before the sword has been hardened
heat is not really a problem.
However, even with gloves it occasionally needs to be cooled down.
For me at least, every step in creating a sword is iterative.
With grinding, I switch between grinding the fuller and then grinding the edge.
At first very aggressively but then more carefully as I am getting closer to the final shape.
So, while you just saw me grind the fuller and the cutting edge, I am not done yet.
I need to do another light pass on the fuller and thus it is back to layout fluid.
One technique that makes grinding easier and more controllable is to rock back and forth
with the body instead of using the arms.
Rocking back and forth is slightly more difficult when working on a sword, so I am usually doing
a little bit of both.
However, if you are working on a knife or a short sword, there is a lot of control to
be had by resting your arms at your side and solely moving your whole mass.
After a few hours on the grinder, the process becomes a little bit too repetetive and I
usually need a break or switch over to a different technique.
Fortunately, I almost done and can grab lunch while normalizing the sword.
Normalizing is the process where the sword is heated up to critical temperature and then
allowed to slowly cool down.
This relieves stressed in the blade and also reduces the grain size.
Since metal has a crystal structure it can form grains in which the structure aligns
and creates macroscopic grains.
These are brittle and cean lead to fractures around grain boundaries.
Before heat treating, it usually is a good practices to reduce the size of these grains.
That's what I am doing now.
I'll heat the sword up to its critical temperature and then let it cool in the air.
Usually this process does not have a lot of surprises but as I am pulling the sword out,
I am noticing that it has a strong bend to one side.
While it's still hot some impromptu counter bending comes to the rescue.
With disaster averted, I am getting to the final part in preparing the sword for heat
treating.
I have been operating the grinder so that it cuts perpendicular to the edge, this leaves
grinder marks along which stresses during quenching can travel and potentially fracture
the steel.
To remove these vertical grinder marks, I will draw file the cutting edges and remove
them.
Draw filing removes material quite quickly but also leads to alot of sweating especially
during a hot California day.
Fortunately, my nieces from Portland just stopped by and left me some of their extra
water.
The moment of truth has arrived.
The electric oven will heat the sword up to 1500F (815C) and then I will quickly move
the sword into room temperature quenching oil.
At this point, I have invested somewhere between 40 - 50 hours into making the sword and if
I am unlucky may waste that time in a matter of seconds.
As I pull the sword out of the quenching oil, a bend is clearly visible.
Unfortunately, the low light I have not quite noticed it yet.
Another surprise, also clearly predictable is that the sword is too long for my quench
tank.
In practice this does not really matter since the cutting edge close to the hilt will not
see any action.
In the heat of the moment, a little correction may help but I will address the bend more
systematially during tempering.
I am almost at the end of this episode and for those who stayed with me I hope you enjoyed
watching it.
If you have any questions, please ask them in the comments.
I would also really appreciate your help by sharing these videos with your friends and
social media.
Getting more subscribers motivates me to make more videos.
But for now, let's take a look at the revealed pattern.
Remember the other side will look completely differently but that's something you may see
in the next video.
So, subscribe to my channel and I will see you next time.