Heat treaters like precision. When asked to achieve a given hardness value after heat treating a piece of steel, there is comfort in designing a recipe that achieves this hardness and then, by control of process and equipment variability, repeat the process and results time after time.
However, it is too often the case when heat treating nonferrous materials such as aluminum and titanium that this repeatability of hardness is not always possible. If it turns out the hardness is too high, one of the solutions heat treaters talk about is to overage the material. Is this a good practice or not? Let’s learn more.
Age hardening (aka precipitation hardening or aging) is a heat treatment used to strengthen certain alloys. Resultant hardness and mechanical properties are a function of aging temperature, time and alloy (Fig. 1). Overaging is aging at a higher temperature or for a longer time than is required to reach peak aging (i.e., that required for critical particle dispersion), thus causing particle agglomeration of the precipitating phase and, as a result, loss of hardness (and strength). Overaging has been deliberately performed, for example, in certain extreme-service applications (supersonic aircraft, engines subject to high-temperature exposure) so as to avoid further loss of mechanical properties.
Aging Effects on Mechanical Properties
As mentioned, mechanical properties are influenced by aging. In one study, A355 (Al-Si-Mg) alloy castings were solution treated at 540°C (1004°F) for 10 hours, water quenched then refrigerated at 15˚C (60°F) for 24 hours prior to aging. The aging times were varied up to 100,000 hours, the result of which were changes in mechanical properties (Table 1, Fig. 2).
Other Comparative Examples
Engine blocks and other complex casting shapes produced from cast aluminum alloys such as A356-T6 or A357-T6 (Al-Si-Mg alloys) are prime examples of materials that can be affected by overaging. Their mechanical properties can become compromised after prolonged high-temperature exposure in service. Lower hardness and tensile strength values for A356 castings, for example, were found after exposure to temperatures up to 200°C (390°F), which are commonly exceeded in certain automotive racing engine components.
One way in which to overcome these limitations is to use other casting alloys, such as C355 (Al-Si-Cu-Mg). The effect on hardness has been studied comparing A356-T6 and C355-T6 castings after overaging in the range of 170-305°C (350-580°F) for times up to 168 hours. For these alloys, the hardness was found to be basically unaffected up to 185°C (365°F) and experienced only a slight drop-off for times greater than 130 hours.
The study also compared the effect of overaging A356-T6 and C355-T6 alloys (Fig. 3, online) at 175°C (350°F) and 205°C (400°F). While both alloys had comparable hardness at the peak aged condition, the A356-T6 alloy experienced a hardness reduction of approximately 34% (113 HB to 76 HB) after over 126 hours, while the C355-T6 alloy’s hardness was essentially unchanged. At 205°C (400°F), the A356-T6 alloy reached a minimum hardness value (45 HB), while the C355 alloy experienced a progressive loss of hardness with increasing aging time, reaching a minimum hardness value of 80 HB after approximately 165 hours.
The change in mechanical properties (strength and hardness) from peak aging to overaging found that the peak-aged C355-T6 alloy exhibited slightly higher mechanical properties than A356-T6 after exposure at 210°C (410°F) for 41 hours (Fig. 4). The C355-T6 alloy showed a decrease in ultimate tensile strength (UTS) and yield strength (Y.S.) of 8% and 9%, respectively, while the A356-T6 alloys revealed a more dramatic loss. UTS decreased 32%, while Y.S. fell approximately 40%. Similarly, the hardness of C355-T6 decreased about 15% as a result of overaging, while the reduction of A356-T6 was in the order of 37%. Elongations to failure were comparable at approximately 5%.
Overaging Temper Designation
Finally, the -T7 temper is assigned to thermally heat-treated wrought aluminum alloys (mainly 7xxx), which are artificially overaged to obtain a compromise between exfoliation corrosion resistance, stress corrosion resistance, fracture toughness and tensile strength. The additional x digit for T7x (Table 2) indicates how much the alloy is overaged. Understanding the degree to which overaging has taken place can be especially important in preventing stress corrosion cracking and general corrosion in these high-strength 7xxx alloys.
To Overage or Not to Overage? That is the Question.
While overaging appears to be a simple solution for reducing hardness, one must always remember that the process is tricky, and its effect on microstructure, mechanical properties and ultimately in-service performance/ultimate life must be carefully evaluated. As the Doctor is fond of saying, you have climbed the mountain and reached the summit but are now sliding, a bit uncontrollably, down the other side.
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