Ferrite: Pure iron, body centered cubic crystal
at ordinary temperatures.
Cementite: Fe3C, iron carbide. Where the carbon goes in steel.
Austenite: The solid solution of carbon in iron. In low alloy
steels it
can normally only exist at elevated temeratures. Face centered
cubic
crystal, allowing lots of room for carbon atoms. The solubility
of carbon
is austenite is about 2 %.
Pearlite: The lammelar structure that results from slow cooling
from a
high temperature. It consists of alternating plates of ferrite
and
cementite. The spacing of the plates, or relative
"fineness" is
determined by the temperature at which it was formed, as is the
hardness.
Fine pearlite can be as hard as HRC 48-50, while coarse pearlite
formed
by an anneal is usually only HRC 20-25 (numbers given refer to
1075-1095).
Martensite: The usual condition that is referred to as
"hardened" steel.
It is the result of rapid cooling of austenite, trapping the
carbon atoms
inside the face centered cubic structure, causing them to be
greatly
distorted into a tetragonal shape that is highly stressed, and
thus quite
hard. Normally tempered to achieve a comprimise between hardness
and
"toughness".
Hardening: Heating the workpiece to a temperature above A1
("critical")
to make austenite, then cooling rapidly in some kind of quenchant
to
produce martensite.
Tempering: Re-heating the as quenched martensite to a lower
temperature
to relieve some of the stress. This lowers the hardness some, but
greatly
increases the impact resistance, or "toughness". Also
precipitates small
"temper carbides" out of the martensite as a function
of the stress
relief.
Annealing: Softening the material so that it can be easily
fabricated/machined. There are a number of methods and structures
that
can be described as annealed. Classic "full anneal"
requires that the
workpiece be heated to austenitic condition, then cooled very
slowly
under near equilibrium conditions to produce a structure of
coarse
pearlite. This is the normal condition for hot rolled bar. Tool
steels
are usually shipped from the mills in a condition called
"spherodize
annealed". This consists of a ferrite matrix, with all the
carbon as
spheroidal carbides of relatively large size scattered evenly
throughout
the ferrite matrix. This is the most easily machinable condition
for
hypereutectoid (above .8%C) steels.