Blacksmithing
and Cutlery
by Gérard HEUTTE
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Quenching mediums
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In cutlery, several mediums can be used for the
hardening of the blades. The choice depends primarily on the chosen steel.
The steel supplier must normally indicate the quenching medium to be used for hardening.
Water
Water is the simplest of the quenching mediums.
Water will have to be heated beforehand around 40°C.
Use large metal bars heated in the forge then immersed in the
can of water.
Water hardening is hard and is appropriate for
slightly carburized steels (0.5 to 0.6 % of Carbon). More
carburized steels can (with some care) be also hardened in water, but
the interest is relatively weak.
It is possible to modify the water reaction by additives. Salt makes hardening harder.
It modifies the boiling point and the viscosity of the medium. The Soap makes hardening
softer. I do not have unfortunately information for the
proportioning of these additives...
Oil
Oil is one of the most used quenching mediums.
Use preferably vegetable oils rather than
mineral ones. Do not absolutely use old car oil (risk of toxicity)!
Envisage a can for hardening with a lid, because very often
oil ignites during hardening. The lid makes it possible
to choke this fire. Oil will be used warm, mainly to increase its fluidity.
You can easily heat it by plunging large metal rods heated beforehand in
the forge. The temperature will be carried to
approximately 50°C.
Air
Some steels harden in the air.
They are often called "self-hardening". It is enough to
heat them at the good temperature then to let them cool.
You can increase a bit the effect of hardening by using ventilated air.
Place the blade edge ahead in a airflow generated by a
large fan. You can more simply balance the blade held at
the end of tongs.
Goop
Goop is a mixture of 75% of vegetable fat and of 25% of
paraffin. It is used solid! Heat the blade at
the good temperature and gently plunge it in solid Goop, which
immediatly melt around the blade.
Its effect is almost identical to that of oil. However,
no need to heat it before use. It is the heat taken by
the solid-liquid transformation which is the key of its effectiveness!
Melted, it behaves like oil.
At ambient temperature, it is transported easily considering
its solid state. During hardening, it does not ignite
(to be checked however according to the quality of paraffin used).
The vegetable fat, used for french fries, is usually found in the oil department of the
supermakets. The paraffin is found in the jam department...
Scale
The quenching mediums are more or less hard.
By using a too hard medium, you can induce very strong internal
tensions and a risk of breaking the part. By
using a too soft medium, the cooling can be too slow,
Martensite is not created well and hardening risk "to not take"...
Here is the classification from the hardest to the softest:
- hardened Water (salt)
- Water
- softened Water (soap)
- Oil or Goop
- circulating Air
- Air
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