Solution treatment

1. Solvus Curve
• From the fig. at temp 923k(650c), a crystal of ferrite is in
contact with a crystal of cementite.

2. • In a system of this type, it is possible for carbon atom to leave ferrite i-e
solid solution and enter the cementite.
• It is quite possible that a carbon atom can transfer from cementite to solid
solution (ferrite).
• When a carbon atom leaves cementite or ferrite and enter the solid
solution (ferrite) or cementite three iron atoms must also leave the
compound with a carbon atom.

4. • When iron atoms leave cementite and enter into ferrite, composition of
ferrite does not change.
• But when carbon atom transfer from cementite to ferrite, composition of
ferrite will change.
• It must be kept in mind, however, that as carbon enter from ferrite to
cementite the latter phase grows in volume and that the composition of this
phase does not change.
• On the other hand, when carbon enters from cementite to solid
solutions(ferrite), the composition of the latter changes.

5. • Precipitation hardening leads two treatments:
i) The Solution Treatment
ii) The Aging Treatment
• Hardening caused by the precipitation of a constituent from a super
saturated solid solution on rapid cooling.

6. The Solution treatment
• A suitable alloy is heated to a temperature at which a 2nd phase dissolve 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 super saturated condition. This heat-treating cycle is
known as solution treatment.

7. Cont….
• Consider a specific dilute iron carbon alloy, one with a total carbon content
of 0.008%.
• If this alloy is in equilibrium at room temperature, nearly all of the carbon
will be in the form of cementite because the solubility of carbon in ferrite at
room temperature is 8.2exp-12%.
• Suppose that this same alloy is heated to 923k(650c), indicated by point d in
figure.
• At this temperature the equilibrium the concentration of carbon in solid
solutions is

8. Cont….
0.010%, which is more than the total amount of carbon in the metal.
• The cementite which is stable at room temperature is no longer stable at
923k (650c) and dissolve by giving carbon atom to the solid solution.
• Because the equilibrium concentration is greater than the total carbon
content of the alloy, the cementite must disappear completely. If the alloy is
maintained for a sufficiently long time at elevated temperature i-e 650c.

9. Cont….
the alloy that originally contain two phases (cementite and ferrite) is thus
converted into a single phase (ferrite).
• However, the solid solution attained by maintaining the alloy or specimen
at 923k is not a saturated solution.
• Because it contains carbon concentration less than the equilibrium
concentration.
• Rapidly cool the above alloy after it has been transformed into a
homogeneous solid solution at 923k(650c).

10. Cont….
• Very rapid cooling of heated metal specimen can be accomplished by
immersing in a liquid cooling medium for example water. This operation is
generally known as Quench.
• In the present case a rapid quench will prevent an appreciable diffusion of
carbon atom, so it can be assumed that solid solution that existed at
923k(650c) is brought down to room temperature essentially unchanged.

11. Cont….
• The alloy which was slightly unsaturated at higher temperature
923k(650c), will now be extremely supersaturated. Its 0.008% carbon in
solution is roughly exp9 times greater than the equilibrium value (8.2exp-12).
• The alloy which is now supersaturated at room temperature is in very
unstable condition.
• As we heated this alloy at about 200c the carbon start diffusing, to make
this alloy stable aging treatment is done.