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- Oct 2001
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Posted: 2/22/2013 11:22:47 PM EST
|Originally Posted By ratfink57:
A friend sent me this:
Originally Posted By CBR900:
What about salt bath heat treating as used in knife-making?
I've had a bunch done that way. Still working on getting it perfected. I use the services of a professional heater though. I don't know how you would do it your self.
All About Salt Baths
> I actually got an idea to write an informative post about salt baths about a
> month ago, when I saw some very puzzling post/ false assumptions regarding
> them, but the notes I started got set aside. However with the amount of
> recent posts regarding them I though it may be worth while to finish the
> writing and post the information that I have.
> Many of us would love to believe we are some sort of innovator or pioneer
> because we use salt baths, or that we were using them before others. The
> truth is that salt baths are VERY old news in industry. Records go back to
> their industrial use in America and places as distant as China to the turn
> of the 20Th century. By the 1930’s they had been around long enough to be so
> commonplace as to not even be notable.
> The first account I have of a custom knifemaker in America describing a use
> of salt baths is in 1984 at the first ABS hammer-ins in Wyoming, where
> German smith Heinrich Frank explained how he used a little table top salt
> bath to heat treat small blades after he had engraved them. By all accounts
> the mention was pretty much ignored by most knifemakers who found it too
> esoteric. The first full scale demonstration of salt baths, high and low
> temp, in use, that I am aware of, was many years later at the New England
> Bladesmith Guild Ashokan seminar, where smiths such as Dan Maragni, Phil
> Baldwin and Tim Zowada were quite familiar with the wide scale industrial
> uses and recognized the potential for knifemakers. Tim Zowada had designed a
> stackable columnar kiln to be built by the Evenheat Kiln Company in MI
> specifically for salt bath use. Folks like Al Pendray may have been working
> with the ideas in their shop at this time, but I believe Ashokan was the
> first full public demonstration by custom smiths.
> It wasn’t too long after this that I got set up with a high temp unit of my
> own due to my increasing work with swords. If none of my blades would have
> ever exceeded 10” I doubt I would have developed the motivation to take on
> the added expense and maintenance of the necessary equipment. For those who
> not familiar with my work, I have been using these tools for many years, I
> have helped many others, from individual knifemakers to large production
> companies, get set up with and properly use them. I didn’t invent anything
> about them, I am no innovator in their use, but I have plenty of experience
> and a good understanding of their use in knife/sword shops.
> What are salt baths anyhow?
> They are not brine solutions, brine is a very fast quenchant achieved by
> mixing salt (around 9%) with water. Salt baths are a different beast from
> the normal bladesmiths heat treating entirely. When steel is heated for the
> quench several problems arise at the necessary temperatures. Common issues
> in forges or ovens are times required for thorough heating, scaling and
> oxidation, decarburization, uneven heating and over heating of thinner
> parts. Most of these issues are connected to the atmospheres. Air is an
> insulator so heating is slow. Air contains oxygen so oxidation and scaling
> will always be present, even if you eliminate the oxygen in the heating
> chamber there will be momentary exposure when the part is removed. Most of
> the same applies to decarburization, and forges are very prone to
> overheating tips and edges.
> Salt baths were industry's (and eventually some knifemakers) answer to many
> of these issues. A salt bath is a volume of superheated molten salts that
> replace the atmosphere and heating chamber of the traditional oven, kiln or
> forge. The high temperature salts that most knifemakers would use are
> primarily NaCl based and begin to melt at around 1275F and have a working
> range up to around 1600F, but can go higher with other elements added to the
> chemistry. Most knifemakers find a columnar, tube type design to be the most
> convenient so I will focus on that. Picture a tube filled with crystalline
> NaCl salts that is sticking out the top of an enclosed heating chamber
> (either electric kiln, or gas fired). Heat is applied until the salts melt
> and then the liquid can be adjusted to any temperature you like within their
> working range.
> The blade is then placed not into an evacuated or gaseous open space, but
> immersed into this superheated liquid. Because it is a liquid (a conductor)
> the first thing you will note is the speed with which the steel will come up
> to temperature, indeed the only thing I have seen heat quicker is induction.
> The next thing you will notice is how evenly the heating will be. Unlike
> radiant heat traveling through an insulative space where thin sections can
> overheat and thick areas still not reach temperature, the totally even and
> conductive nature of the salts will not allow this. The cycling overshoot
> and undershoot nature of electrical elements is overcome by the conductive
> and convective effects of the mass of salts.
> Then there is the total absence of oxygen or other atmospheres under the
> surface of the salts. There is no scaling nor oxidation whatsoever because
> there is no oxygen involved in the heating. That is not to say that poorly
> maintained salts cannot tear up a blade, they are superheated NaCl salts
> after all. This is why proper salts prepared for, and intended for, the
> purpose are important along with regular monitoring and maintenance being
> essential. Contaminated or tired salts that get out of neutrality will begin
> to pit and decarburize steel on levels that will make you long for a forge
> or kiln again. However if you know what you are doing and pay attention to
> proper maintenance, salts pretty much eliminate oxidation, scaling or
> I take all of my blades to the final hand rubbed polish BEFORE heat
> treating, and could even sharpen the blades if I wanted to without any
> concerns that the blades would be just fine afterwards. All I do is a quick
> rub down with 800X on damascus blades before proceeding to etching, with no
> concerns about scaling, decarb, or overheated edges or tips.
> But why don’t you get decarb and scale when you pull the hot blade from the
> salts, you may ask? Because the blade is wet with a thin protective coating
> of salts, now if you left it exposed long enough you may have problems, but
> that length of time would also negate your entire quenching operation as
> The inherent benefits should already be apparent, but no salt bath should be
> complete without a digital controller to give the hand free, unequalled
> precision. Wired to a relay or valve to control the electricity or gas
> supply to the heating device is a PID electronic controller to which you
> attach a thermocouple probe which goes into the salts and sends temperature
> readings back to the controller. The controller uses this input to control
> the heat source and regulate the temperature of the salts to with 1F. or 2F.
> of the set temperature, to give one control over the austenitizing process
> that you may have never dreamed of.
> I have seen it fallaciously implied that things like salt baths can have
> negative effects on things such as grain size; this is totally false to a
> level that causes me to seriously consider the motives of anybody who would
> suggest something so erroneous. Serious steel industries turned to this
> technology to achieve controls that are still unparalelled in many
> applications. In the ranges for the steels we work, temperature is the key
> factor in final grain size, and with salt baths even someone new to heat
> treating with a few keystrokes can dial in any size they would like to
> maintain with temperature accuracy not possible with a forge or a torch. It
> is just a fact that I can punch in any number in the liquid range of the
> salts and hold it within 1-2 degrees for as long as I please.