Austenite
Named for Sir W.C. Roberts Austen this is a solid solution structure in which
gamma iron is the solvent with carbon or iron carbide as
the solute.
Austenite is the state of iron/carbon that most of the structures
(martensite, pearlite, bainite etc...) used in bladesmithing are derived from.
Upon heating to the temperature designated ac1 alpha
iron makes the allotropic shift from Body centered cubic to face centered cubic
gamma iron. Gamma iron
is capable of holding much more carbon in solution and begins to accept carbon
into the iron atomic matrix.
Holding higher amounts of carbon in solution in the FCC
configuration causes austenite to be unstable at temperatures below ar1.
Upon slow cooling carbon will diffuse to form pearlite from the parent austenite.
If rapidly cooled austenite will be unable to diffuse carbon sufficiently enough
to form pearlite and the result will be martensite or bainite depending upon the
rate of cooling.
In many ways austenite is the parent of the other microstructures, not only
from the standpoint that the other structures arise from its decomposition but
also that it provides the framework for some of their characteristics. Austenite
leaves its affects and "finger print" in the form of the austenite grain boundary.
Shape and size of the austenite grain will determine the rates of transformation
through points of nucleation, or "toe holds" for transformations to begin. And
this in turn will affect the formation of new austenite upon reheating to
ac1. Steel with larger austenite grains tends to harden more deeply
due to the reduction of nucleation points for the diffusion of pearlite to
begin. The drawback is that larger grains make steel much more weak and brittle.
For bladesmithing the greater of these two evils is the brittleness so large
grains and grain growth is to be avoided whenever possible.
Austenite grain growth occurs when the steel is heated beyond ac1
and acm (the point at which the extra cementite is dissolved), and
increases with temperature. The larger austenite grains will grow at the expense
of the smaller grains. So time at these elevated temperatures should be carefully
watched and kept to a minimum.
When heating a steel to ac1 the shift to gamma iron will allow any
pearlite to be dissolved and form new austenite grains. These new grains will
be slightly finer and within the previous boundaries due to increased nucleation.
As temperature increases the proeutectoid ferrite (if the steel is hypoeutectoid)
or proeutectoid cementite (if the steel is hypereutectoid) will be dissolved
until ac3 or acm and there is complete austenite.
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