Age Harden
Is age hardening the same as precipitation hardening?
Yes. In manufacturing literature, age hardening, aging, and precipitation hardening are used interchangeably for the same strengthening mechanism. The process involves solution treatment, quenching, and controlled heating to precipitate fine particles that increase strength and hardness.
Why does hardness increase during aging?
Aging forms fine precipitates in the metal's crystal lattice. These precipitates block dislocation motion, which makes plastic deformation more difficult and raises yield strength and hardness.
When should a shop order material already age-hardened?
When the job has very tight tolerances, thin walls, or geometry that cannot tolerate post-heat-treatment shrinkage. For Inconel 718, one machining source recommends age-hardened stock for features tighter than ±0.05 mm or thin walls under 1 mm.
Age hardening, also called precipitation hardening or aging, is a heat-treatment process that strengthens a metal by forming fine precipitates that block dislocation movement. The common cycle is solution treat, quench, and age. In CNC machining, age hardening raises strength and hardness but can cause slight shrinkage, so dimensional allowances must be planned before finishing.
On the shop floor, age hardening is usually scheduled between roughing and finishing. A common traveler reads: rough in solution-treated stock, age to final hardness, then finish machine critical datums and bores. Because aging creates strength through fine precipitates rather than heavy quench-and-temper deformation, it preserves geometry better, but it still can cause roughly 0.1% shrinkage in alloys like Inconel 718. A machinist must leave stock allowance on thin walls, bores, and locating features so post-age movement does not push the part out of tolerance. Cutting tools also see a harder matrix after aging, so feeds, speeds, and tool grades need to be adjusted. For aluminum parts, aging is used to increase wear resistance and load capacity after solution heat treatment. The key is to treat age hardening as a planned process step, not an afterthought.
Post-aging dimensional drift: Cutting critical bores and datums before aging leaves no allowance for the roughly 0.1% shrinkage that occurs in alloys like Inconel 718, pulling precision features out of tolerance and causing rework.
Stress-release distortion: Asymmetric stock removal leaves residual stresses that relax during aging, causing bow, twist, or hole shift in parts that looked dimensionally correct after roughing, leading to scrapped workpieces.
Hardened-material tool failure: Using softer-stock feeds and speeds on an age-hardened matrix increases tool wear sharply and worsens chip control, especially in superalloys where the cutting strategy must be revised for the harder material.