The three qualities that most effect the selection of a steel for a
hand-tool application are edge-holding, sharpenability, and
corrosion-resistance. For metallurgical reasons, you can only
have two of the three. We at
that in woodworking, corrosion-resistance is the least important of
the three, and prefer an edge that is easily sharpened and long
A steel's carbon content determines its ability to harden with
heat treatment. That hardness determines a tool's ability to
hold a sharp cutting edge under abrasive pressure (wear).
Generally, the harder the metal the better its edge holding, but it
will be more brittle. Tempering reduces that brittleness,
although it also reduces the tool's hardness and wear resistance. So
a balance must be struck to decide how hard a blade should be.
Our blades are hardened to Rc62 for long edge life. This is
harder than most available replacement blades yet not as hard or
brittle as most Japanese blades.
"Tool Steel" refers to a class of steels that are metallurgically
very "clean" and fall within strict limits for alloy proportions.
Vanadium, tungsten, and molybdenum are often added to tool steels to
make the steel resist annealing (softening) when used in
"high-speed" (high heat) applications. Chromium is added in very
large quantities for corrosion resistance ("stainless").
steels are essential in metal-working tools (drills, milling
cutters, etc.) and "stainless" steels can be cost effective by
resisting rust during the manufacture, shipping, and storage of the
tool itself. Correctly heat-treated, tools made from high-speed,
stainless, and "chrome-vanadium" steels may hold an edge well in
woodworking applications, but, due to the large, hard carbide
particles that form during hardening, they are difficult to sharpen
and cannot be honed as sharply as a blade of plain high-carbon
Our choice of High-Carbon Tool-Steel (.95% Carbon) offers the
finest, sharpest edge possible. Its chromium and vanadium additions
amount to only 1/2% each allowing quick, clean honing with
traditional techniques. High-carbon steel holds and takes an edge
better than anything else. We guarantee it.
Some thoughts on Do-It-Yourself Heat Treating of Tool Steel
I posted this some time ago when the group project was the St.
James Bay plane "kits" and some were (bravely) doing their own
blades for them. It's doable; get some extra pieces of the same
steel to practice on...
The only addition this time around has to do with the great
question of which quenchant to use with which steel. The steel used
in any given blade is not an easy thing to determine. A
metallurgical lab charges a fair amount to test for alloy and there
is no home test kit that I know of ("Look, Honey, it turned blue!")
And there is some risk in quenching, say, an oil hardening steel in
water. It could fracture at worst or warp like crazy at least. The
old-timers "sparked" steels to tell what was in them. The sparks
generated from a grinder will burn with different visual
characteristics depending on the alloying elements. (Like the
different colorants in fireworks.) So you can grind a corner,
observe the sparks, then grind a known steel and try to compare the
little spark-flares for shape, brightness, complexity, etc. and
attempt a match.
Mostly we're talking oil vs. water hardening steels. The air
hardening ones are the Cr-V and stuff that us Galoots don't use too
much and that weren't used in old tools at all. It is safer to
quench an unknown, perhaps water-hardening steel in oil than vice
versa. The water-hardening steel may not harden in the oil and if
that is the case, you can try again in water. I don't mean to muddy
the water with all this but, hey, if it were easy, everybody'd be
The first step is to get the metal to its critical temperature,
which with good old O-1 (the oil hardening stuff) is 1450 - 1500F.
Got a good pyrometer? No problem. For some reason (let it be a
mystery; there are so few left) steel ceases to be magnetic at that
temp. This phenomenon is called the "Curie Point" after the
discoverer, Pierre. So one can simply heat the metal till the magnet
is no longer attracted to it then quench in oil. I like to use
peanut oil because the flash point is very high which minimizes the
risk of fire (the risk is still there, though; be prepared: use long
tongs to handle the work to keep your hand out of the way, wear
gloves and keep the fire extinguisher handy) and it smells nice(r)
when it smokes.
How to get the blade to the Curie point is probably
the biggest problem for the DIYer. When the metal is glowing red,
the carbon behaves as if it's in a liquid and can therefore migrate
around as it pleases. This is necessary for the hardening to occur
but near the surface of the metal those unfaithful little carbon
atoms would just as soon run off with any available oxygen-sluts it
runs into (oxygen is soooo seductive) and they're lost then forever.
We hate that.
We attempt to prevent this by: heating the metal in an
inert (oxygen free atmosphere) and/or limit the time at red-heat (in
air) to as little as possible. A torch makes both of those very
difficult. It's very hard to heat something as large as a
Norris-type blade evenly with a small torch-generated spot of heat.
A forge fire is better because of its uniformity and it can be
starved for air a bit to decrease the oxygen in its immediate
vicinity. A small lab-type test oven works quite well. (Also used
for ceramic glaze tests.) Toss in a charcoal briquette to scavenge
some of the oxygen.
When it's hit critical temp, remove it from the heat and quickly
dunk it into a sufficient quantity of oil (preheated to about 150F.)
Swish it around a bit until it's cooled then let it cool to ambient
in the air. It should now be very hard and too brittle to use. (If
you attempt to file it, the file should skid on the blade.)
Two ways to temper to a useable hardness/toughness: by colors or
by temp. If you have a very accurate oven in the kitchen, just heat
it to 325F and you're done. An accurate deep-fryer will do the same.
But without the accurate temp control, you'll have to use the
surface oxide colors to know when enough is enough. First, clean
some part of the blade (probably the flat area back from the bevel)
till it's bright metal again. When heated, that spot will change
colors (you've seen the rainbow of colors on any overheated steel)
starting with a very faint yellow (called light straw).
like our blades Good-n-Hard(tm), stop there (remove from the heat,
quench if necessary to stop any further increase.) Any color beyond
the faintest straw is too much. (The blade will still work, it just
won't hold the edge you want.) Be overly cautious with tempering.
You can always re-temper a too-hard blade, but if you go too far and
soften it too much, you have to re-harden it all over again. So if a
blade seems too hard, just toss it back in the oven and go a little
You're done. If the blade looks awful, you can sandblast or grind
it pretty but it should work well regardless. Before honing, be sure
to grind back the bevel a bit. That thin section probably took more
than its fair share of carbon burn-out abuse and you need to get to
the good stuff. Same for the back.
Doing a good job on the back is
at least if not more important than the work on the bevel. A little
extra elbow grease will remove the de-carbed layer and get to good
metal. Don't forget: the back IS the Cutting Edge. Think about it.
If the back hasn't been honed deeply enough, the blade will never