Legend has it that a Syrian king once wanted to
punish a criminal in particularly gruesome manner.
He ordered that he be impaled upon a red-hot
sword. To the astonishment of the executioner,
the blade proved to be harder and more flexible
afterwards. What can we learn from history? Even
before this modern technological age, our predecessors
– particularly the more experimentally
minded swordsmiths – discovered that through
heating and quenching the hardness of steel can
be markedly increased. This is called hardening.
A cast iron pan which falls on to a stone floor,
shatters – because cast iron is not only hard,
it is also brittle. By contrast, a steel pan will,
in most cases, survive its fall without problem.
This is because steel is elastic, i.e. softer and
more ductile than cast iron. The aim of hardening
is to confer cast iron hardness on steel without
any loss in elasticity.
However both elasticity and ductility are immediately
sacrificed when hardening is carried out.
By carrying out so-called tempering, i.e. re-heating
the hardened steel, the elasticity and ductility
are partially regained. Moreover, from the ‘annealing
and tempering colours’ which the steel takes
on during heat treatment, blacksmiths have long
known the correct temperatures for this process.
The hardening of steel is therefore a well established
technique which has always required
the experience, knowledge, and sensitivity of
a master craftsman. Today, computer controlled
industrial
furnaces are used in the process and
Ipsen is the world’s number one when it comes
to the manufacture of such systems.
We didn’t
discover hardening.
350 °C –
Tempering temperature
450 °C –
Tempering temperature
550 °C –
Tempering temperature
850 °C –
Hardening temperature
for tempered steels,
case-hardened steels
and bearing steels
1.050 °C –
Hardening temperature
for tool steels and
stainless steels
1.250 °C –
Hardening temperature
for high speed steels
