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Posted by on September 15, 2007, 3:35 pm
Please log in for more thread options Neon John wrote:
> The boiling point elevation from the salt is so slight that it
> doesn't make any difference. I'm not sure what function the brine
> actually performs. Perhaps salt crystals plate out on the hot
> metal and help form steam bubble nucleation sites or something.
> Salt water is more dense than pure water so the static head down
> in the tank would be a bit higher and THIS will raise the boiling
> point a bit. Or maybe the salt is in the formula because brine's
> always been used :-)
Anybody still around here with a copy of Dell K. Allen's
"Metallurgy Theory and Practice"?
http://www.Amazon.com
At 54c (but with $3.99 shipping) is it too much? ;)
Page 205, Fig.7-26A shows the salt cystals formed (NaOH or NaCl) at
first plunge.
It's been said that when "water" is mentioned that they really mean
"brine" or something based on water since pure water is such a
crummy quenchant. ??
Anyway...
MT&P page 203 to 204...
"The salt addition materially decreases the duration of the vapor
film stage. It has been shown that crystals of salt are
precipitated momentarily on the steel surface and then explosively
thrown off during the intial stages of brine quenching. (Fig.7-26A)
[real nice photo of that action on page 205 along with oil's
reaction in photo Fig.7-26B] This causes a continuous disruption
of the vapor film with improved wetting of the steel and increased
heat removal. Brine quenching, because of its vigorous action,
removes heat treat scale from specimens much more readily than when
a water quench is used."
Pure water can leave soft spots in plain carbon steel because of the
bubbles formed and so is seldom used by those that know what they
are doing. ;) So naturally I've BTDT! :/
A little Mo or Cr or a little extra Mn can fix that.
A little W or V -by themselves- can make it worse.
Also I've gotten better results from quenching oil, made for the
purpose, for (thin) knife blades (even when quenching 1095) than I
got from pure water or brine. Less warping for one. My blades and
springs have been hardness tested and are right on the money too,
according to the tables and graphs for each type of steel. So why
not use "real" quenching oil in that case? :) YMMV
"A 3-5 percent sodium hydroxide quenching bath has also been found
to be a good medium for [straight] carbon steels. This bath cools
even faster than the sodium chloride bath; however, more caution
must be exercised..."
"Table 7-5 Quenching severity for common media with various degrees
of agitation. Still water is 1.0."
BTW, on the metallurgy newsgroup no one has ever come up with a
better metallurgy book than Allen's 1969 MT&P for teaching yourself
metallurgy. :)
The few books that were mentioned I didn't already own I found at
the UofA's library. Funny how a couple suggested were just plain
crap. ;) My metallurgy class text is crap too (Neely/Bertone).
I'll buy your copy of MT&P if you don't like it. :)
> Because the latent heat of vaporization of a liquid is many times
> it specific heat. It takes one BTU to raise the temperature of a
> pound of water 1 deg F but it takes 970 BTU to vaporize that same
> pound of water. If we take water from 70 deg to 212 degrees then
> we input 142 BTU. To convert that pound of water at 212 deg into
> a pound of steam at 212 requires 970 BTU.
>
> This holds true for any substance that has a liquid phase. Water
> has one of the highest latent heat of vaporization of any common
> liquid so it's an ideal coolant in addition to being common and
> cheap.
>
> The problem is to get the heat to the water. The film of steam
> that forms on very hot metal is a very poor conductor of heat so
> once the steam film forms, heat transfer slows dramatically.
> These various chemicals are evidently designed to help the water
> stay in contact with the hot metal and reduce film boiling.
>
> The effect is small, if any, as evidenced by the steel cooling
> about as fast without them. The major effect will be in the low
> temperature region where the steel is still hot enough to boil the
> water but not hot enough to form a resilient steam film.
> >Would replacing the water with motor oil, in the super quench
> >recipe, be of benefit, or work for that matter?
> No, no benefit. Oil has almost no surface tension to begin with
> so a wetting agent isn't needed. It also has a very high
> (relative to water) boiling temperature so no boiling point
> elevation is necessary.
>
> The main difference between oil and water, and the reason oil is
> slower, is that both it's specific heat (btu per degree rise) and
> its latent heat of vaporization (btu necessary to boil it) are
> much lower than water. I don't have numbers handy but as a
> general rule, oil has from about a third to a quarter the specific
> heat of water. The latent heat difference is probably in the same
> range.
Cool stuff there John. :)
And Pete's stuff too. :)
> If you want a really fast quench, much faster than water, then try
> a liquid metal. If chemophobia doesn't paralyze ya, mercury would
> be lightning fast.
Hg is mentioned for quenching carbon steel taps, in Brownell's
Gunsmith Kinks books "to make them harder than factory". Not sure
of the real-world benefits tho.
> It might be interesting to see how one of the low melting point
> solders would work. Something that melts in the 300 deg range.
> John De Armond
> http://www.neon-john.com
> http://www.johndearmond.com
Cool idea but at those sorts of expenses, O1, 4340 or other alloyed
steels would be cheaper and better both?
Using scrap/unknowns is about cheap, I understand "cheap". ;)
There is some new stuff (that I don't know anything about) called
"polymer quenchants".
Alvin in AZ
ps- Pine for email, Tin & Pico for NGs and SSH to access both :)
pps- Ubuntu is perfect for this dumb ol'doze-hatin' Dos 3.3 guy :)
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