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Heat
Treating Stainless Blade Steels at Ranger Original Handcrafted Knives
Special Thanks to Rob Ridley of
Ranger Original for this
Information
We have had a number of people
ask us for our “secrets” about heat treating knife blades.
While we appreciate the compliment, there really are no
secrets – and what works for us may not be your favoured solutions. Here
are our recipes. You may copy them, publish them or use them as you
please. We have drawn from various sources, including Crucible data
sheets, other steel suppliers and postings on Blade Forums as well as a
good dose of personal experience.
All of our stainless blades
get double wrapped in high temperature, 309SS foil envelopes – with double
folded seams pressed down firmly. Always put them in the envelope the same
way – so you can put them in spine down and pull them out by the handle,
instead of the tip. We use brown paper (no idea why brown – just what we
use) in pieces about 2 x 2 inches. 2 pieces for an average hunter and four
for a big bowie. The trick is to use enough paper to eat up the Oxygen in
the envelope – without blowing up the envelope like a balloon.
154CM, CPM154, ATS34
All three of these, heat treat
the same. After a full speed ramp up to temperature, they soak 45 minutes
to an hour in the Evenheat Oven at 1950F degrees. Then they get placed –
still in the foil – spine down – then flat - on a 1” aluminum plate. The
second aluminum plate is placed on top and pressure is applied. We used to
use weights for the pressure, but now we use clamps. You are looking for
good firm contact. If you are thinking hydraulics you are thinking WAY too
much pressure. After 2 minutes (or less) they will be hand cool, and ready
to remove from the foil for cryogenics. They don’t have to go straight
into cryo, but aim for something less than an hour from plate
quenching. We use liquid nitrogen for cryo but dry ice in acetone will
also do. The household freezer is no use at all. They probably only need
a couple hours in cryo, but we leave them overnight.
The next morning, they will be
in the range of RHC63+. After warming to room temperature, they get
tempered. We temper twice at 500F degrees for two hours each time – to get
about RHC61. You can experiment for other hardness's but 61 is a very good
target for these steels.
CPMS30V
Same wrap – same temperature
(1950F) - same soak time – same plate quench – same cryo as 154CM
above. This steel gets double tempered at 400F degree for about RHC60
440C
Same wrap – Harden at 1900F
degrees and only about 15 minutes at temperature – then plate quench and
cryo as above. Hardness out of cryo will be about 61. Temper (twice for 2
hours) at 275F for RHC60 – 325F for RHC59 – and 375F for an excellent
RHC57-58.
Questions and Answers provided by Rob Ridley of
Ranger Original
Q1: Does Cryo really make a difference.
A1: Yes! It increases initial hardness between 1.5 to 2 points and even
though you may temper back to softer levels, the steel has been more
completely transformed to a tough and desirable state.
Q2: Why do
you leave blades in the foil to plate quench them.
A2: The really hot blades
never see oxygen this way. That means no scale and very little oxide. Air
quenching the old way meant going back to the grinder to clean up the
mess. Blades done our way can be cleaned up with just a buffer.
Q3: Why do
you remove the blades from the envelope before cryo?
A3: Experience! We like
hanging the blades from a coat hanger in the cryo tank. Handle pin holes
make that easier. We also found that when a tight foil bag full of liquid
nitrogen, hits the warm summer air, bad things can happen. The low
tempering temperatures won’t cause you any scale or oxide problems anyway.
Q4: Can I
use my freezer for cryo?
A4: No. It isn’t nearly cold
enough to do any good at all. Dry Ice or Liquid Nitrogen work equally
well. Dry ice is easier to handle but only lasts a day or so. Liquid
nitrogen in a good dewar will last a month or more depending on use.
Q5: I
heard ATS34 etc can be tempered at a high or low range. Is this true?
A5: Sort of – but the high
range gives you reduced toughness and corrosion resistance. It should be
avoided.
Q6: Are
there other ways to heat treat these steels?
A6: Heck yes! Air quenching,
Oil quenching, Salt baths, Vacuum furnaces…. But
you’ll have to ask someone else about them. I haven’t seen anything that
gives more satisfactory results that the way we use now.
Q7:
Aluminum is expensive! Can I plate quench between thick steel plates.
A7: Yes – less desirable
because it absorbs heat slower – but workable.
Q8: How do
I know if it hardened and tempered properly.
A8: The correct answer is to
use a Rockwell C scale tester and then to do performance testing on your
blades. Some claim they can tell hardness pretty close by the way a file
skates over the steel. I can’t. We test every blade through here and write
the hardness on the blade before shipping it out. Having said that, we
are rarely out by more than 1 Rockwell point from where we aimed.
Q9: 321SS
foil is much cheaper. Can I use it instead of the 309SS?
A9: Depending on who you talk
to, the 321 is rated to something between 1800 - 2000 degrees. The risk is
that the 321 will either fail, allowing air in, or stick to the blade
inside. In practice, we have used it successfully with our own blades
without problem. We have seen 321SS stuck to a blade heat treated
elsewhere. Because we treat other maker’s blades, we choose to use only
309SS foil. Experiment at your own risk.
Q10: Does
it matter where in the oven I place the blade?
A10: Yes. Temperature inside
an oven can vary and generally is only measured at one point – where the
thermocouple is located. For consistent results, you should keep your
blades in a more or less consistent place in the chamber. Hint; Avoid
getting closer to heating coils than necessary.
Q11: How
do I know if my oven is accurate for temperature?
A11: Some will tell you that
you can test temperature with ceramic firing cones. We disagree. Cones do
not measure temperature. They measure heat work – a combination of time
and temperature. Orton makes a product called TempChek which is better but
still requires a rather complex cycle for a good result. An electronic
probe and thermocouple is the best way to check, though can be
expensive. Of course, unless you can calibrate the probe, you still don’t
really know do you?
Note:
Rob really nailed the question of using a pyrometric cone for temperature
measurement. It's apples and oranges. Save the pyrometric cones for the
potters.
Q12: How
far can a blade be finished before heat treat?
A12: Many of our customers
finish to 800 grit before heat treat. We ask makers to leave us 20 thou at
the edge and point – essentially a finished blade that has not been
sharpened. Although we have not had any stress fractures, it is also a
good idea to avoid sharp corners where possible and to deburr pinholes and
other irregularities.
Safety
Heat treating has obvious
hazards. Please wear safety glasses or face shield and decent gloves. The
day will come when you drop a hot blade. Make sure the floor around your
oven is free from combustibles and please resist the urge to catch the
falling blade.
Email
rob@rangeroriginal.com with questions.
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