Precipitation hardening involves heating an alloy to dissolve a secondary phase, then quenching to form a supersaturated solid solution. Upon aging at an intermediate temperature, the secondary phase precipitates out of the solution, strengthening the material. The rate of precipitation is fastest at an intermediate temperature, where there is sufficient driving force for precipitation but not too slow diffusion. Precipitation hardening provides a heat treatment method to enhance the hardness of alloys.

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3. Precipitation hardening is the process
of strengthening by precipitation of fine
particles of a second phase from a
supersaturated solid solution.

4. The precipitation hardening is only possible for
a certain alloys which posses the following
two features:
 An appreciable level of solid solubility of the
alloying element in the host metal.
 A solid solubility limit that decreases rapidly
with decreasing temperature

5. Consider a system at 923K (650C) in which
a crystal of ferrite is in contact with a crystal
of cementite. As shown in Fig.
It is possible for carbon atom to leave the
solid solution (ferrite) and enter into the
cementite .

6.  To maintain the stoichiometric ratio
characteristic of Iron carbide ,three iron
atoms must leave the ferrite and join the
cementite with transfer of one carbon
atom.
 Similarly when a carbon atom leave the
Fe3C to enter the solution three iron atoms
must leave the compound.

7.  At 923K (650C) temperature the solution
contains 0.01% carbon so the removal of
iron simultaneously with carbon will not
affect the concentration of solution.
 If there is only the transformation of
carbon atoms then the concentration of
the solution will change.

8.  When carbon enters in iron carbide the
volume will increase and composition
remains same but when the carbon
enters the ferrite its composition
change.

9. A suitable alloy is heated to a temperature at
which a second phase (Usually present in
small quantities) dissolves in the more
abundant phase. The metal is left at this
temperature until a homogeneous solid
solution is attained, and then it is quenched
to a lower temperature to create a super
saturated condition. This heat treating cycle
is known as solution treatment.

10.  Consider a specific iron carbon alloy
containing 0.008% carbon.
 Due to low solubility (8.2x10^-12) of
carbon in ferrite at room temperature
(300K) all the carbon is present in the
form of cementite.
 At the temperature 923K the equilibrium
concentration is 0.010% carbon which
was 0.008% at room temperature.

11.  At 923K the cementite phase is not
longer stable so it dissolves by yielding
its carbon atoms to the solid solution
because the equilibrium concentration is
change.
 By holding the alloy (
Cementite+Ferrite) at 923K for long
period of time then all the cementite
dissolves into the ferrite and alloy
changes into a homogeneous solution
(Ferrite).

12.  At this high temperature the solid
solution is not saturated because there is
no carbon available to get equilibrium
concentration.
 If we rapidly cool (Quench) this solution
in some cooling medium (Water) there
will be no time available for carbon
atoms to move and form cementite.

13.  So the solution which was existed at
923K is brought down to the room
temperature essentially unchanged and
the solution will be extremely super
saturated.
 Its 0.008% carbon in solution is roughly
10^9 times greater than the equilibrium
value (8.2x10^-12) so this alloy is
accordingly in very unstable condition.

14.  Precipitation of cementite from a super
saturated solid solution (Ferrite) occur by
nucleation and growth process.
 It is necessary that stable nuclei of
cementite are located followed by there
growth in size as a result of diffusion of
carbon from surrounding ferrite towards
the nucleus.

15.  It is however possible that solid solution
may lose its carbon in two ways, either
due to growth of cementite particles
already formed or due to formation of
more nuclei.
 In other words nucleation may continue
simultaneously with the growth of
particles already formed.

16.  The amount of precipitation of time shown
in figure. The curve shows that precipitation
does not start spontaneously it requires
some time "To".
 "To" time is termed as nucleation period.
 Rate at which precipitation occurs in
function of temperature.
 At very low temperature long times are
required to complete the precipitation.

17.  Rate of precipitation is also very slow at
room temperature just below the solvus
line (Point e in figure).
 In this case the solution is only slightly over
saturated and the free energy decrease
resulting from precipitation is very small.
Therefore, the precipitation rate is very
slow.
 At an intermediate temperature between
two precipitation rate increases to a
maximum so that the time to complete the
precipitation is very small.

18.  Such hardening curves obtained a
number of specimen are given solution
treatment to convert there structure into
supersaturated solid solution.
 Immediately following by quenching the
sample is placed at an intermediate
temperature below the solvus for
different period of time.