Precipitation Hardening

Solution TreatmentAge-Hardening

Age-HardeningA quenched alloy increase its hardness with

aging time this is known as Age-Hardening (Precipitation hardening)

Steps for age- hardeningThe aging treatment accomplished by

nucleation & growth process.GrowthFirst we nucleate cementite crystals &

cementite crystals grow in size due to the diffusion of carbon from surrounding ferrite.

Continue…..Removal of carbon from ferrite done in two ways;1.The formation of new nuclei2.Growth of previously formed nuclei

Incubation PeriodPrecipitation not begin immediately it require a (to)

before it detectable.Here we use Logarithmic units for time because

spontaneous reaction of this nature start rapidly & finish slowly.

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 can migrate. The rate of precipitation is also very slow at temperatures just below the solvus line(point e fig).

Effect of free energyIn 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.

ConclusionThe combination of moderates diffusions and

nucleation rate makes precipitation rapid.

Precipitation of second phaseThe most important effect of the precipitation

of the second phase (cementite) is that the matrix (ferrite) is hardened.

Figure shows a typical hardening curve for a dilute iron-carbon alloy.

Over agingHolding, 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 temperaturesThe 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 hardening.

Continue…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.

T2 ( 40.5 o C)

T1 ( 0 o C )

T3 ( 99 o C)

Effect of composition

Solubility at room temperature = 8.2 x 10 – 12

percent