This
is an article to explain heat treating to a person
who is not really interested in steel structure
terms such as Martensite, Austensite and Pearlite.
They just want it explained in everyday language.
So
here goes. When we talk about “heat treating”
steel we are talking about the process of making
a piece of steel as hard or soft as needed to perform
a specific job. The following is the series of events
in order.
Heat
treating can vary bladesmith to bladesmith and steel
type to steel type. For example, one bladesmith
may have a different process for heat treating a
high carbon steel for a carving blade and yet another
for heat treating a filet knife. The bladesmith
can equate to a chef, they all have their“special
recipes” and methods to create their individual
product.
For
our purpose, critical temperature is when the molecules
are moving around without burning up or destroying
each other.
Annealing
Annealing is heating steel to a critical temperature
(sometimes 2000º) and letting it cool down
very slowly to room temperature in an oven, forge
or insulating mediums such as woodstove ashes, vermiculite,
or ceramic insulation.
Why
do we do this?
This is simply making steel as soft as possible.
When we start to form our blade, whether it is with
a file, sandpaper or grinder, it is more easily
formed when it is in a softer state. For example,
M2 steel is a high speed alloy steel that is used
for planer and molding blades. Very hard stuff.
It is sharpened and cut with hard ceramic stone.
To try and file it in its hardened state is like
trying to push a needle into glass. When properly
annealed, you can file it and sand it like a piece
of copper.
Picture in your mind a bundle of rubber bands that
you are grasping in both hands. We are taking the
bundle of super stretched rubber bands and putting
it in a totally relaxed state so that we are able
to easily change its form and remove rubber bands
from the bundle.
Normalizing
Normalizing is heating steel to a critical temperature,
holding it there for a few minutes, and then taking
it out of the forge to cool in still air.
Why
do we do this?
We do this to help the steel adjust to it’s
new form after forging, straightening or grinding.
When we change the form of the steel we create stresses
that are uneven in the steel.
To
use the rubber band analogy, when you try to stretch
them apart, some bands are very tight and some are
still relaxed. When we normalize we are trying to
get all of the steel in the same state of tension.
Hardening
Hardening is heating steel to a critical temperature
where all of the molecules of the steel are at their
best arrangement and stopping them abruptly using
a coolant or quench.
Why
do we do this?
This is making steel as hard as possible. A cutting
edge is best when we can form the hardest, finest,
smoothest edge possible.
Back
to the rubber band bundle. With the bundles of rubber
bands relaxed you can put them across your leg and
they don’t make much of a impression. Stretch
them as far as you can and push them into your leg
and they make a good impression.
Tempering
Tempering is removing hardness from steel with heat
to give it more toughness and flexibility.
Why
do we do this?
This process helps the blade become more stable.
If left in its hardened state the blade would be
as hard and brittle as glass. The blade will snap
like a potato chip when we try to carve with it.
When we temper it we make it relax to a more usable
state.
Rubber
bands again. When we stretch the rubber bands to
their limits and push them against our leg some
of them break. But if we can remove some of the
tension, and still make a dent in our leg without
breaking them we’ve achieved a perfect hardness.
Heat
treating can vary bladesmith to bladesmith and steel
type to steel type. For example, one bladesmith
may have a different process for heat treating a
high carbon steel for a carving blade and yet another
for heat treating a filet knife. The bladesmith
can equate to a chef, they all have their“special
recipes” and methods to create their individual
product.
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