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Precipitation hardening is the processof strengthening by precipitation of fineparticles of a second phase from asupersaturated solid solution.
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
Consider a system at 923K (650C) in whicha crystal of ferrite is in contact with a crystalof cementite. As shown in Fig.It is possible for carbon atom to leave thesolid solution (ferrite) and enter into thecementite .
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.
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.
When carbon enters in iron carbide the volume will increase and composition remains same but when the carbon enters the ferrite its composition change.
A suitable alloy is heated to a temperature atwhich a second phase (Usually present insmall quantities) dissolves in the moreabundant phase. The metal is left at thistemperature until a homogeneous solidsolution is attained, and then it is quenchedto a lower temperature to create a supersaturated condition. This heat treating cycleis known as solution treatment.
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.
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).
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.
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.
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.
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.
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.
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.
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.