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Working With Steel - HARDENING AND TEMPERING BLADES
Okay, so you've shaped a blade... BUT... you're nowhere near done. Now comes the
tricky (and technical) part. Don't be intimidated. It's really not that bad when properly
explained, and I think I can convey it in one easily understandable page.
Here goes...
TOOL TIPS
I use a forge to heat my blades. it's a commercially available forge (NC Tool
Knifemaker model).
HOWEVER, if all youre doing is blades, you can MAKE a mini blade forge
using a blowtorch and some fire bricks:
Keep the interior as small as possible and heat until the bricks are glowing,
and then youre ready to heat your blade.
Here's another couple of money saving tips:
Vise-Grip pliers make great fire tongs for moving blades in and out of the
forge.
Show up at your local Jiffy Lube with a 5 gallon pail and ask for waste
oil. Well used motor oil makes a wonderful quench medium!
HEATING THE STEEL
Now to determine how
much to heat it. When I
use 10 series steels
(1080, 1085, 1090, 1095,
etc) and even some of
the spring steels, (5160,
6150), I do a simple heat
to between bright red
and orange/red color
(1500F-1600F), and then
I quickly do an oil
quench.
Heres a handy chart I
keep on my shop wall.
As you can see,
temperature can be
easily determined by the
color of the steel or its
glow:
Heres the other chart I keep on the wall for reference. In the phase diagram
below, youll see that to get a carbon steel like the ones I mentioned to full
austenitic, that 1500F-1600F will bring the steel to full Austenite (carbon is fully
in solution). This means CARBIDE which is what were looking for!
If your blade cools slowly, the carbides go away. If you quench it, the carbides
are locked in place exactly what you want!
TEMPERING YOUR BLADE
Okay, so now you have a REALLY hard blade. It is in fact it is WAAAAY too
hard, and is thus very brittle. It will need to be tempered back down to a
manageable hardness. The third and final chart youll need is this one. This
particular chart is for 1095, but will yield similar results from other 10 series
high carbon steels.
For smaller blades, you can simply put them in the oven in your kitchen at 350F
and bake them for an hour or so. That will bring them to the torsion peak you
see in the chart, while retaining a Rockwell hardness of about 62 incredibly
good edge holding.
For larger blades, this is too brittle and youll want to shoot for about a 58 on
the Rockwell scale. For this, we need to shoot for about 600F more than you
can do in the oven! Refer back to the color chart and youll see that the steel
turns bright blue at about 575F. Remember... the blue isnt a GLOW, just an
oxide color, so DON'T wait for it to glow blue! At which point the blue starts to
fade to a duller blue/gray, I remove it from heat. I cant do this for an hour, so
instead I do repeated tempering cycles, usually 2 or 3 times to blue and
beyond.
if you have access to an oven that will get to a stable 600F, consider yourself
lucky. Right now I'm considering building my own with fire bricks, a digital
temperature controller, a thermocouple and some resistive heat elements. Not
easy, but cheap!
DIFFERENTIALLY HARDENING YOUR
BLADE
Your other option (and this takes a bit of practice) is differential hardening. For
this, I take the blade to my cherry/orange austenitic point, and then quench
only the sharpened edge. I really like doing this because I can then let the rest
cool slowly. I do the 350F temper cycle in the oven. This makes a large blade
that has an extremely tough spine, yet with an incredible edge holding ability
you have to try to believe! Polish this up, and you can see the "hamon" or the
wavy lines that are the result of the steel's structure changing from a fine
grained carbide to a larger grained lower hardness steel.
I hope all this info helps.
TO BE CONTINUED...
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