What
Is It?
It goes by many names, there is differential tempering,
differential hardening, selective tempering, selective
hardening, zone treating, soft-back draw, and the list
goes on and on it seems. By whatever name is used, it
means that the blade has been heat treated to have a
hard cutting edge and a softer "springy" spine.
There are a few different ways to achieve this, so the
term I use depends on the process I used. More on this
later.
What Are the Benefits?
A blade in which the spine has been hardened to a lesser
degree than the cutting edge will exhibit a great deal
of strength and flexibility and still have exceptional
cutting ability. This is assuming of course that the
heat treating was properly done.
This is quite beneficial to larger blades which must
be able to handle a lot of abuse. Prime candidates
would be a bowie or camp knife which might be used
for chopping up firewood, clearing a path through the
woods, etc. This is also good for thinner blades that
require quite a bit of flexibility. Some knifemakers
do differential heat treating on all their blades.
One of the performance tests for a prospective Journeyman
or Master Smith in the American Bladesmith Society
is the bend test. The smiths blade is clamped vertically
in a vice and must be bent 90 degrees without breaking.
At first one might think that all you'd have to do
is temper the entire blade to a spring temper. Well,
the problem with this is that the cutting edge would
be too soft to pass the cutting tests. Prior to the
bend test the blade must cleanly sever a one inch free
hanging rope approximately six inches from the loose
end, in one movement. Next it must be able to chop
a 2x4 in half at least twice, with no edge damage,
and still be able to shave hair from the applicant's
arm. This is achieved by differential heat treatment.
How Is This Done?
There is more than one way to get the hard cutting
edge and tough, springy back. To cover the whole
spectrum I say that my blades have been differentially
heat treated. To be more specific as to the method
used, I use one of the terms mentioned above. Some
of the methods, as well as the terms I use to describe
the methods are listed below.
Differential Hardening
This is the term I use when the transition was performed
at the hardening phase of heat treating. This would
cover the edge quench, clay coating, and torch methods.
It is not my intention to give a full course in heat
treating, but here is a brief explanation of the three
methods.
Edge Quench - With this method, the whole blade is
brought to critical temperature, then the cutting edge
only is placed horizontally in the quench medium, and
carefully rocked upward toward the tip of the blade.
The steel exposed above the quench line will cool more
slowly and therefore reach a lesser degree of hardness.
Differential Tempering
This is the term I use when the transition was
performed at the tempering phase of heat treating.
That is, the
blade is fully hardened, then tempered,
and finally the spine is drawn back with a
torch. This is often
done with the cutting edge placed in
a tray of cold water. The depth of the water
is
usually set from 1/3
to 1/2 the overall width of the blade.
The spine is carefully drawn back with a "painting" action
of the torch on the spine.
Closing Statements
As previously stated, this was not intended to be a
lesson in heat treating. I have just scratched the
surface to give you an idea of what differential
heat treating is, where its application is beneficial,
and some of the methods used.
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