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A quenched alloy increase its hardness with aging time
this is known as Age-Hardening (Precipitation
Steps for age- hardening
The aging treatment accomplished by nucleation &
First we nucleate cementite crystals & cementite
crystals grow in size due to the diffusion of carbon
from surrounding ferrite.
Removal of carbon from ferrite done in two ways;
1. The formation of new nuclei
2. Growth of previously formed nuclei
Precipitation not begin immediately it require a (to
) before it
Here we use Logarithmic units for time because
spontaneous reaction of this nature start rapidly & finish
The speed at which precipitation occurs varies
with temperature this is shown qualitatively in
fig. At very low temperatures, long times are
required to complete the precipitation because
the diffusion rate is very slow. Here the rate of
reaction is controlled by the rate at which atoms
The rate of precipitation is also very slow at
temperatures just below the solvus line(point e
Effect of free energy
In this cause the solution is only slightly over
saturated and the free-energy decrease resulting
from precipitation is very small. Nucleation is,
accordingly, slow, and precipitation is controlled
by the rate at which nuclei can form.
The combination of moderates diffusions and
nucleation rate makes precipitation rapid.
Precipitation of second phase
The most important effect of the precipitation of the
second phase (cementite) is that the matrix (ferrite) is
Figure shows a typical hardening curve for a dilute
Holding, the specimen for too long a period at a given
temperatures causes them to lose their hardness. This
effect is known as over aging.
Effect of different temperatures
The curve mark T1 represents aging at too low a
temperature. In this cause, atomic motion is so slow
that no appreciable precipitation occurs and
hardening occurs slowly.
A further lowering of the aging temperature below T1
will effectively stop all precipitation and prevent
Temperature T2 corresponds to an optimum
temperature, a temperature at which maximum
hardening occurs within a reasonable length of time.
At T3 hardening occurs quickly due to rapid diffusion.
However, softening effects also are accelerated,
resulting in a lower maximum hardness.