It is common for fixtures to be used for the age-hardening process because they help to prevent distortions. Salt baths provide users with the precise control needed for a short-time, high-temperature aging that will result in minimal distortion while also providing production economies for a short-cycle.
Residual stress arises from certain types of deformation that occurs after age hardening, which may be thermally relieved without sacrificing hardness. When heating materials to temperatures between 300 and 400 F for up to two hours, it is usually enough to create adequate stress relief.
For alloy 25, the standard age-hardening treatments require temperatures up to 600 F for two to three hours (cold work = two hours; annealed products = three hours). Representative Alloy 25 has age-hardening curves at 500, 600, 700, and 800 F in half-hard, in annealed, and in full hard conditions.
Keep in mind that age hardening at 600 to 625 F creates the highest strength for all tempers. A higher temperature will have materials achieve peak strengthen in a shorter time, but that quickness comes with some downsides. The quicker top strengths are met, the lower the peak strength is. Lower heats increase strength at slower times (excessive time). This is the case when working with beryllium copper and plated wire.
Cold work improves the possible strength levels for aging temperatures. As engineers increase levels of cold work, the heat needed to achieve peak strength goes down.
Keep in mind that as strength goes up, ductility will decrease. Overaging will improve ductility, but note that toughness gets reduced.
Give our team a call to discuss all your age hardening and strength needs.