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Precipitation hardening

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Precipitation hardening

  1. 1. Muhammad Umair Bukhari Engr.umair.bukhari@gmail.com www.bzuiam.webs.com 03136050151
  2. 2. Precipitation hardening is the processof strengthening by precipitation of fineparticles of a second phase from asupersaturated solid solution.
  3. 3. 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
  4. 4. 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 .
  5. 5.  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.
  6. 6.  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.
  7. 7.  When carbon enters in iron carbide the volume will increase and composition remains same but when the carbon enters the ferrite its composition change.
  8. 8. 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.
  9. 9.  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.
  10. 10.  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).
  11. 11.  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.
  12. 12.  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.
  13. 13.  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.
  14. 14.  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.
  15. 15.  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.
  16. 16.  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.
  17. 17.  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.

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