Precipitation hardening aka Ageing or Age Hardening, is a process which occurs after products have been solution treated and quenched (ie when products are in W or AQ temper).
The Technical details:
In the quenched state (SSSS), since the degree of solid solubility of alloy solute within the solid alloy solution decreases with decrease in temperature, precipitation zones of solid alloy solute occurs, over a period of time, at room temperature. As precipitation zones increases, stresses of metallurgical bonds between each precipitate zone and the solid solution, resulting in a progressive increase in alloy strength, hence the terms ageing / age hardening. At peak strength precipitation zones have grown to their critical size and are regarded as fully coherent with the solid solution.
The precipitation process occurs very slowly at room temperature, and can take many hours, days or even weeks to achieve desired mechanical properties. Temper designations for the solution treated and naturally aged states are: TB, T3, T4 (same as T3 but less cold work), T42 (same as T4 but not “mill” produced). It is common practice to cold work wrought products after quenching, resulting in additional digits to the codes eg T351, T352, T452 etc.
Age hardening time can be reduced from weeks / days to hours by soaking the alloy parts at elevated temperatures (instead of room temperature), typically between 100 and 200ºC, and is termed as artificial ageing. Generally, as ageing temperature increases, the required process time to arrive at peak strength, decreases. Temper designations for the solution treated and artificially aged states are: TF, T6, T8 (same as T6 but aged from T3), T62 (same as T6 but not “mill” produced).
However, when optimising age hardening process time, there is a critical temperature limit, which varies depending on alloy grade, where above this critical limit, peak mechanical properties start to fade, termed as over-ageing. At temperatures above the critical limit, further heat expansion of the product allows precipitation zones to grow beyond their limits of coherency. Beyond the limits of coherency, stressed metallurgical bonds between precipitates zones and solid solution break and reform. Breaking and reforming of metallurgical bonds releases alloy stress, resulting in a decline of strength, with increase in over-ageing time.
Certain alloys, 7xxx series grades in particular, require over-ageing to increase corrosion resistance at the expense of strength, where the degree of over-ageing is monitored by hardness and electrical conductivity testing. Temper designations for the solution treated and over-aged states are: TB7, TF7, T73, T74, T76.
Annealing is effectively caused by gross over-ageing, where in the annealed state (O), precipitation is regarded as fully incoherent, resulting in lowest strength and highest ductility.