Tempering is a heat treatment technique for metals and
alloys, most often the toughening of martensitic steel. Most steel
blades (from knives to swords) are hardened by quenching (which
produces a martensitic transformation), but this hardening effect
generally must be reduced by tempering to avoid brittleness.
The heat-treatment process of a typical steel involves heating
the object (usually in a forge) (austenizing) and then
causing a quick and sharp drop in its temperature (quenching).
Together, these two processes produce an extremely hard
microstructure in medium-carbon or high-carbon steels, which can
then be "tempered" (normalized, moderated) to prevent the material
from shattering.
Chemically, the process of tempering is a transformation from
metastable martensite to ferrite and cementite. This change is
accomplished by annealing at a temperature below the austenizing
temperature, but high enough that nucleation of cementite
particles can occur. The formation of cementite draws carbon from
the surrounding alloy, allowing it to transform into ferrite.
Cooling the object ends the annealing process, stopping cementite
formation by slowing down the diffusion of carbon.
A uniform alloy will always see a tradeoff between softness and
brittleness. This delicate balance highlights many of the
subtleties inherent to the tempering process: precise control of
both time and temperature are critical to avoid the useless
extremes of mechanical properties. Although the temperatures used
in tempering are often too low to be gauged by the color of the
workpiece, the lengths of time involved can still be measured
using music. That is, the time of the tempering process might have
been measured by the time it took the blacksmith to sing a
familiar song. The cumulative effects of time and temperature can
also be gauged by monitoring the color of the oxide film formed by
tempering a well-polished blade.
The exquisite properties of ancient Japanese swords are in
great part owed to the blacksmiths' traditional understanding and
control of these parameters. However, they also reflect a means of
transcending the brittleness tradeoff, by creating a blade of
nonuniform composition. Great pains were taken to create a layered
structure, so that some (high-carbon) areas are fully martensitic,
while other (low carbon) areas are unresponsive to heat-treatment
and so remained soft to provide toughness. Differential quenching
was also used to reduce the need for tempering, which was only
used to a very limited extent on such blades. This process
involved holding a hot block of copper against different points
along the blade's spine to locally adjust its curvature.
Taken to an extreme, similar strategies can allow extremely
hard and tough blades to be made without any austenization,
quenching, or tempering at all, as in materials such as Damascus
steel.
