I posted a link to a site that has good about hardening
steel - then I realized maybe it’s over some people’s heads. It’s a
pretty important skill, so I thought maybe I better take it down to
basics for those who don’t know it. It’s going on 10:30 Saturday
night - don’t have a life, I guess. No, I just thought it was
important…
The physics of steel are deep, vast, and beyond what any of us here
really need to know. This is definitely a condensed version.
Tool steel must have something around 7 - 10% carbon to be
hardenable. I’m not going to do research while I write this, so
don’t shoot me if some of the details aren’t precise. Less carbon
will not harden (thus the uselessness of nails and other things) and
more carbon will not, and the why of that is definitely the stuff of
somebody’s research…They just won’t, for reasons that can be
learned, if wanted.
When you have a piece of steel - “plain carbon steel”, as they call
it - it is composed of iron and carbon in a mixture. That means they
don’t interact, they are mixed, like red and green jelly beans. When
you heat that steel to around 1500F - 1600F, depending on the steel,
which is red hot, BTW - not dull red, not orange, not yellow - red
hot. The iron and carbon atoms become more free in the metal. Not
free like a molten metal, but free enough to move about a bit. When
you reach that heat in most steel - again there are many steels and
this is generic - the iron and carbon move about a bit and form
crystalline lattices with each other. There are names for the
various types, the most important of which are martensite and
austenite. As I remember it is those two, but most especially
martensite, which are important. The most important thing of all to
understand is that if you heat steel to that temperature, the
lattices form. If you let the steel cool off, the lattices will
disintegrate and the metal will return to iron and carbon atoms in a
mixture.
So, here’s what happens - I’ll say again that it’s generic, because
there are many alloys of steel: You get your steel stock. You heat
it up to around 1500F, and then you let it cool off slowly. The iron
and carbon atoms have moved away from each other, the steel is soft
and you have just annealed it. Then you do your shaping, you’re
done. So, you heat the steel up again to around 1500F, which forms
hard crystals of iron and carbon called martensite and austenite
(and others), but this time while it’s still hot and those crystals
are formed, you dunk it in water and cool it in a moment. This means
that the crystals did not have time to disintegrate, and are still
intact, glass hard as martensite and austenite are. In fact they are
so hard that the steel will break like glass if you stress it, so
you need to temper it. You have caused the hard crystals to form,
and then you have cooled it so fast that they didn’t have time to
break apart, and are still in place.
Tempering steel means just what the word says - to make it “a bit
less so” - to temper something. So, you take the skin off the steel
so you can see it, and you warm it back up just a bit, for which
there are charts as to how much. As you warm it, the crystals of
austenite, especially, are disintegrating, leaving more martensite
behind, but just enough that the steel becomes useful - this is far,
far away from anything you would call annealing. It just takes the
edge off of the hardness, and gives a useful hardness and much
toughness - the ability to retain it’s shape under stress, and
resist stress.
Probably the most important things to understand about this whole
process:
It only occurs in steels with a certain carbon content. You can do
it all day long with nails and they will never harden.
You must reach a temperature of around 1500F for the crystalline
structure to form, to harden the steel. Heating steel to 600F from
an annealed state does nothing - it’s as though you never heated it.
You much first harden the steel by creating the iron/carbon
structures, which occur at a higher temperature. After that you can
temper it, and should.
Bottom line:
If you anneal steel so you can work it, and then “temper” it, you
still have annealed steel. You must first harden it at red heat,
which is where the crystals form, and quench so the crystals can’t
un-form. Then you can temper it to a specific hardness for your
needs.
Finally - the reason to use oil vs. water has a lot more to do with
the specific heat of the two than anything about getting carbon out
of oil. The oil cools steel much more slowly than water, and
sometimes that’s a good thing. And this whole process is really
pretty simple and pretty idiot-proof - newbies shouldn’t be put off
by it. It’s just important to do things somewhat properly to get
something resembling good steel. Toolmakers need to agonize over
exact times and temperatures because they want optimum results, and
yes, it’s best if we all do that too. But you can get decent results
in your garage, too, if you just understand some of the fundamentals
of it all.
http://www.donivanandmaggiora.com