All
of my blades are heat-treated using salt bath technology. For years I did it the
old fashioned way; in the forge with the most sophisticated measuring device
being my eyes. For years heat treaters in the industry have worked to find a
better way to get the heat treat as fool proof as possible. This goal has been
realized in salt bath technology. The benefits of this method are many; three of
which are absolute temperature control and accuracy, complete evenness of
heating and no atmosphere to contend with.
Absolute temperature control
is achieved through the use of extremely accurate thermocouple devices to read
the temperature and electronic controllers to regulate in accordance with the
input from the thermocouples. My controllers are programmed with fuzzy logic
which is an elementary form of artificial intelligence. This allows them to
learn and adapt to the conditions under which I use them. They tell me the
temperature as it is and accept my input to adjust the temperature to where I
want it.
Evenness of heating is a result of the conductive nature of the
molten salts. The edge, tip, spine and ricasso all come up to the exact
temperature that I wish and can get no hotter. Upon switching to salt baths,
distortion was all but eliminated due to the total evenness of the austenitizing
heat. Some of my tests have shown a remarkable difference between blades
austenitized in a forge and blades austenitized in salts. Of blades dropped from
the height of 5 feet, point first, onto 1/2" steel plate, the few that showed
damage to the tip were all austenitized in the forge. You just can't hope to get
the edge, spine and tip all up to the same accurate temperature, all at the same
time, using convective heat from a forge. Conductive heat from molten salts, by
its very nature, is quick and even.
No atmosphere is a real plus because
of carbon's (and iron's, to some extent) affinity to oxygen. When steel is heated
in an open atmosphere, oxygen causes large amounts of scaling and
decarburization, resulting in an ugly soft skin that must be removed from the
outer surface of the piece. If you do not compensate for this you get a blade
that goes dull very quickly until it has been sharpened quite a few times and
good steel is exposed. With salt baths the blade never comes in contact with the
atmosphere. It is immersed in the molten salts for austenitizing and a thin film
of salts coat it when it is removed to quench so oxygen can do no harm. You can
take the edge down as close to finish as you like and finish the blade to the
same extent. It will come out looking pretty!
Until now I have
described the high temperature side of salt baths; now for the low temperature
part! The high temperature salts heat the blade to the austenitizing temperature
in preparation for the quench. The low temperature salts act as the quenching
medium. Now only a bladesmith would refer to 400oF. as "low
temperature" but compared to the other, they are low temperature. Why
400oF. and not as fast a quench as possible to get the steel as hard
as possible? A little more insight in the way the process of hardening works
will eliminate the desire for the fastest, coolest quench possible. You see, all
one has to do is cool the steel very quickly from the high temperature to below
approximately 900 oF. (this varies depending upon the steel so 900
oF. is a good ballpark) in order to get a successful quench. Steel
doesn't actually begin to harden until it reaches 450-400oF. The
less stress it is under while hardening the less brittle and distortion or crack
free it will be. So if you equalize the temperature at 400 oF. in the
quench and then allow it to cool gently through the hardening phase, you gain
impact strength and toughness. The added benefit is that you can put on good
insulated gloves and straighten any warps or problems that may have developed
before it sets! It must be pointed out, however, that the cooling rate of this quench
is insufficient for water hardening steels and will result in a lack of
hardness. Marquenching is the realm of oil hardening steels.
The two most common ways to go about salt bath systems are electric and gas. I
work with both. I have a gas-fired unit for high temperature and an electric unit
for low temperature. Since I have switched to gas for high temperature salts, I
feel foolish for ever putting up with the hassles of electric with them. The high
temperature salts are very corrosive and this spells expensive trouble for
electric kiln elements. They are also very conductive which makes for exciting
new tingling experiences when they vaporize in the heating chamber and complete
a circuit between the 220V elements and the steel blade you are holding! But
then gas can also be pretty dangerous if you are not careful. The high temperature
salts themselves must be respected. Not only can they burn you beyond belief but
if anything volatile or containing moisture comes in contact with them they pop and explode,
sending 1230oF.+ salts flying about! The low temperature salts are much tamer.
They are less corrosive and less reactive, but 400oF. can still burn you bad!
The electric setup is a set of stackable kilns from Evenheat Kilns inc. in which the steel tube full of
salts is set. In mine the salts are a nitrite/nitrate base salts called Thermo-
Quench from Heatbath Corporation that melt at
288oF. The temperature of the salts is measured by a thermocouple probe that is
attached to a CN7630 series controller from Omega Company. It
interprets the temperature readings in comparison to the desired temperature that
I entered into it via its keypad. It is equipped with a relay feature that sends
power to a mercury displacement relay that controls the 220v power going into the
kilns. The CN7630 is in complete control of the kilns' power and does all of the
temperature regulating for me.
The gas unit is the same concept except that instead of a kiln there is an enclosure
that is basically a long gas forge sat on end. The enclosure is a 10" steel pipe
with a ceramic fiber lining. The ceramic fiber or "wool" is superior, in this
application, to a solid refractory because it does not hold the same thermal mass
and overshoot your desired temperature, making temperature control much easier.
Instead of a mercury displacement relay, a solenoid valve controls the gas flow
to the burner tip. The tube containing the salts is a thick walled stainless steel
to resist the corrosive effects of the salts and high temperatures. The high
temperature salts are a neutralized sodium chloride called Nu-Sal from Heatbath Corporation
and melt at 1230oF.
To heat treat, I determine the proper austenitizing temperature for my steel and
enter it into the keypad. The controller takes it from there. When the Nu-Sal
salts reach the exact desired temperature I immerse the blade into them. In
just a couple of minutes the blade will be up to the temperature of the salts
(remember salts are a much greater conductor than air). I soak for the time
required to get the best internal structure for the steel and then pull the
blade out and plunge it into the 400oF. Thermo-Quench salts, moving it about
in them to help the cooling. After a minute or two the blade has equalized at
400oF. and I remove it from the salts and then check it for straightness. If
there are any warps I easily straighten them with my gloved hands (the blade is
still very soft and moveable at this temperature). I then just let it gently air cool.
For steels that are not oil hardening I still use the high temperature salts, but
I quench into a fast oil.
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