Blacksmithing
and Cutlery
by Gérard HEUTTE
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Tempering
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Utility
After hardening, the blade is very
hard but also very fragile. It breaks easily if it falls on
the ground! The Tempering consists in heating slightly the
part. For the knives, the range of temperature generally goes
from 180° to 240°C, but here also, it is necessary to refer to the
recommendations of the manufacturer of the steel. The Tempering gives:
> Light loss of hardness (i.e. some points HRC)
> Large gain in strength and flexibility.
These effects are usually all more marked as the temperature of the
Tempering is high.
Thremal Cycle
Here is schematically the thermal cycle of the Tempering:
And various phases:
(1) Heating
The heating will be done at moderate speed.
Speed does not have impact on the results.
The temperature of Tempering (TT on the graph) is specific for each
steel and is selected in accordance with desired final hardness. 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, these temperatures are about 200°C to 250°C.
(2) Stage
The temperature of Tempering will be maintained for
a long moment, with a one hour minimum.
(3) Cooling
Cooling is not a critical factor. It is
necessary however to return at ambient temperature before making the
second cycle.
The first Tempering causes to transform a (small) part of residual
austenite into martensite. This transformation is done at the
time of the descent in temperature. Martensite thus created is not
tempered. This explains why two identical successive
temperings should be made.
Implementation
This heat treatment is simple to carry out
since a simple domestic oven is enough. It must have a
thermostat and a timer is useful. The thermostat will have to
be calibrated using a thermometer.
In the absence of a calibration of the oven, you can assess
the temperature reached by the color of steel at the exit of
the oven. Attention with the special steels which react
differently...
Set the thermostat on the desired temperature.
Place the blade in the heated oven. Set
the timer on the required time. You can let the blade cool
quietly in the furnace...
For the second tempering, it is not necessary to remove the
blade from the oven during heating.
To maintain the blades in the furnace you can use a
support. Opposite, a rack made with a fine metal sheet (PC power
supply!) with the help of an angle grinder. It enables
to keep the blades vertical and stable
in the oven during tempering.
And now, your blade is ready for the completion!
Notes
> You can use the forge to
make tempering directly, but I find this method less reliable in term
of effectiveness and convenience.
> You can make the tempering with a blowtorch or a burner.
Same remarks as above!
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