Increased strength which is usually the purpose of heat treatments is developed during aging or precipitation hardening. Aging means allowing enough time for a large number and high density of small dislocation-pinning precipitates to provide the alloy with strength and hardness before it used for its intended applications. Heat treatment solutions and quenching will establish the necessary conditions for dislocation pinning to occur; aging allows this to take place. Natural aging takes place at room temperature. Some alloys reach their maximum strength through natural aging in a few days or weeks while others will gain precipitation hardening for years. For example, alloy 2024 will complete natural aging in about 4 days at room temperature while alloys 2014 and 6061 will reach fairly stable precipitation conditions in one month at room temperature.
Since many alloys harden slowly at room temperature and require years to strengthen appreciably, they are treated at Aging ovens to accelerate precipitation through artificial aging. Holding the alloy for a limited time at a moderately raised temperature will increase the mobility of the dissolved elements and will allow them to precipitate more quickly than if left at room temperature. This is the reason why artificial aging is also referred to as precipitation heat treatment.
Even alloys that can naturally age rapidly may be artificially aged to ensure the development of their maximum strength. The strength of certain alloys can only be maximized through the application of a combination of treatments like a minimal amount of cold working to be followed by artificial aging. The heat treatment temperature in the Aging ovens generally ranges from 240oF to 375oF (115oC to 190oC) with soak times varying from 5 hours to 48 hours. The cycle for time and temperature is selected based on the most suitable compromise between the conflicting demands of product strength and ductility and other characteristics that may be required like corrosion resistance.
Artificial aging can be carried out beyond the point of maximum alloy strength in a procedure called “over-aging”. In the procedure of over-aging, the controlled growth of large precipitates is promoted and thus reducing strength and hardness to achieve certain specified product properties. However, it should also be noted that precipitation hardening can be slowed down to a certain extent by reducing temperature in the Aging ovens. Another option to slow down or delay aging for a few days is by refrigerating the metal at zero degrees Fahrenheit or -18oC or even lower.
In some instances, procedures like forming and straightening may induce more desirable changes in properties if done before precipitation have occurred. If the procedures cannot be carried out right after quenching, the alloy can be refrigerated to delay aging until the metal can be worked on.