Blacksmithing
and Cutlery
by Gérard HEUTTE
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Annealing
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Utility
Annealing is generally the first heat treatment
carried out on a forged blade...
The work of hot steel by forging, involves the
creation of strong internal stresses, especially in the case of strong
deformations. These constraints are not distributed in a
homogeneous way. Moreover, the hardness of steel is not
perfectlly homogeneous.
The whole of these factors limits the possibilities of work
thereafter (drilling, filing, abrasion and hardening).
Annealing aims to relax steel. At the end of
annealing:
> Internal stresses will be almost non-existent.
> Hardness will be homogeneous, and usually low!
An annealing operation cancels the effect of hardening. Thus,
before working on a recovery steel, the first operation to be made
is annealing!
Thermal Cycle
Here is schematically the thermal cycle of annealing:
And various phases:
(1) Heating
The heating will be done at moderate speed.
Some "difficult" steels will require intermediate stage
or precise speeds to rise in temperature!
The temperature of annealing (TA on the graph) is specific for
each steel. A range is generally given by the steel supplier.
If not, it will be always possible to use the technical
data of a close or similar steel.
In general, this temperature is about:
> For hypoeutectoid steels = AC3+50°C.
> For hypereutectoid steels = AC1+50°C.
(2) Stage
The temperature of annealing will be maintained
for a moment. The theoretical works give in general durations
like 1 minute per millimetre of the part. If the rise in
temperature were not too fast, the temperature is relatively
homogeneous (especially on a part of 3 or 4 millimetres thick) and
you can decrease the time for temperature stage.
(3) Cooling
Ideally, cooling should be as slowest as possible.
In practice, it must be adapted to steel.
Certain steels (especially those with Chromium) require
a very slow cooling!
The best compromise, influencing directly the means to
implement, comes from the experience gained with a steel.
Implementation
Ideally, the temperature of annealing is known!
A good steel supplier gives access to this information.
From a practical point of view, the panacea in this field is
the hardening oven! It is a furnace able to go rather high in
temperature and programmable. You can directly program there
the totality of the thermal cycle starting from ambient
temperature up to the return there at the end of the cycle!
Unfortunately, this equipment is out of reach for majority of people.
I will thus not speak about it longer, more especially as I do
not have one!
Heat the steel at the temperature of annealing then
maintain it during the required time. This operation could be
done directly with the forge. In a forge with
coal, move the blade in fire for a complete and homogeneous
heating.
For cooling, several possibilities:
> Dying fire in forge: It gives a slow cooling,
but poorly controlled. A draught or the wind can be enough
to revive embers and to heat the blade a little.
> Vermiculite: Insulating material sold in the do-it-yourself
stores. It is easy to use. The vermiculite
can be used cold or preheated for a slower cooling. Preheating
is done by plunging inside a large metal bar heated at red
in the forge. Remove the blade from the forge with tongs. Plunge it
swiftly and totaly in the vermiculite.
This is the method I use (simple, effective).
> Sand: Same use that the vermiculite, but a little less
effective. Cooling will be faster.
> Wood Ash: Same use that the vermiculite, but not very practical.
Ashes are light and disperse easily.
Notes
> Specialized literature described of many types
of annealing. The majority of them are used in industry.
In cutlery, the term "Annealing" indicates the annealing of
softening in general, of relaxation, total or complete.
> Annealing has a tendency to enlarge a little the grain of
steel, even if it is suitably made. In case of overheating, the
grain enlarges in a significant way because of the slowness of cooling.
> This tendency to enlarge the grain of steel requires a
normalization before hardening.
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