I finished my diatrabe about temperng, then went into the OLDTOOLS
listserver. There was a message from Ron Hock, who makes custom
and semi-custom plane blades for woodworkers, on the subject. I
have take the liberty of forwarding this message to you. Although
Ron is talking about edge tools here, the process is exactly the
same.
Forwarded Message Follows
If the listmoms are getting tired of me posting this article,
maybe it could reside handily in an archive somewhere…
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 fire works.) 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 was easy, everybody’d
be doing it.
Okay, here we go again:
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, tho; 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).
Since we like our blades Good-n-Hard™, 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.
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. On any blade used bevel-down, the back IS
the Cutting Edge. Think about it. If the back hasn’t been honed
deeply enough, the blade will never work well.
Good luck and feel free to ask more questions.
Ron
Ron Hock (rhock@mcn.org)
http://www.hocktools.com
Hock Handmade Knives 16650 Mitchell Creek Drive Fort Bragg, CA 95437
(707) 964-2782 fax (707) 964-7816
Private replies: ron@hocktools.com
Public replies: oldtools@listserv.law.cornell.edu
To subscribe, signoff, to digest: listserv@listserv.law.cornell.edu
Other housekeeping: oldtools-owner@listserv.law.cornell.edu
Archive: http://mailmunch.law.cornell.edu/cgi-bin/archives/OLDTOOLS
When quoting, edit severely.
Marrin Fleet
@Marrin_and_Mary_Dell
Memphis, TN