Over time, the metal molecules slowly migrate to a stable pattern,
a condition known as aging, with two effects:
- Ductility is increased slightly, and tensile strength, impact
strength and hardness are somewhat decreased.
- There is slight shrinkage.
Changes in ductility and tensile strength occur over a period of
approximately 10 years at room temperature. Shrinkage is 0.0007
in./in., of which 0.0005 in. occurs within five weeks at room
temperature. If the castings are quenched, an additional 0.004
in. shrinkage occurs.
If desirable, the aging process can be accelerated by oven heating
according to the following schedule:
3 hours at 212°F
(100°C)
5 hours at 185°F
(85°C)
10 hours at 158°F
( 70°C)
