Some thoughts on Do-It-Yourself Heat Treating
of Tool Steel
First, know your steel. If you bought commercially
available tool steel you should know precisely what it is. But if
you are using something found, scavenged or of otherwise uncertain
provenance you may have problems hardening it. 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 doing it.
The first step is to get the metal to its critical
temperature, which with good old O-1 (the oil hardening stuff) is
1450° - 1500°F. Got a good pyrometer? No problem. During
the crystal transormation from ferrite to austenite 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.
Update: There are coatings that prevent oxidation
and carbon loss at www.rosemill.com that promise to make home heat
treating a more successful endeavor.
When it's hit critical temp, remove it from the
heat and quickly dunk it into a sufficient quantity of room temperature
oil. Swish it around a bit until it's cooled throughout to below
150°F. 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 temperature. If you have a very accurate oven in
the kitchen, just heat it to 325°F and you're done. An accurate
deep-fryer will do the same but use a good thermometer to double
check on the oven or deep fryer's thermostat. Without accurate temperature
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). Since we 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 higher. The oven/deep fryer
method is preferred, however because you can leave the part at tempering
temperature long enough for true tempering to occur. The torch method,
using the surface colors, may leave some of the transformation undone.
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. (it could take as much
as .025" to get through the de-carbed layer.) 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 work well.
Ron Hock ©2009