What Does Age Hardening Mean?
In metallurgy, age hardening is a heat treatment technique used to increase the hardness of an alloy by a relatively low-temperature heat treatment that causes precipitation of components or phases of the alloy from the supersaturated solid solution.
It is used to increase the tensile and yield strength of materials, including most alloys of aluminum, magnesium/titanium/nickel and some stainless steel. In superalloys, it is known to cause yield strength anomaly, providing excellent high-temperature strength.
Age hardening is also known as precipitation hardening.
Corrosionpedia Explains Age Hardening
Age hardening is a heat-treatment process used to strengthen metal alloys. Unlike ordinary tempering, alloys must be kept at elevated temperature for hours, or "aged," to allow precipitation to take place.
Age hardening creates changes in physical and mechanical properties by producing fine particles of a precipitate phase, which impede the movement of dislocations, or defects in a crystal's lattice. Dislocations serve to harden the material.
Properties of age hardening include:
- Creates high tensile strength and hardness
- Improves wear resistance
- Reduces ductility/toughness
- Allows for easy machinability in soft state prior to age hardening
- Performed at low temperatures
- Allows for little to no distortion of the part or component
In age hardening, metal is heated to a high temperature, which varies according to the materials being used and the desired properties of the final result. Alloying materials are added and allowed to diffuse through the metal until the heated metal is supersaturated with them.
Age hardening gives moderate to good corrosion resistance. For example, age hardening stainless steels are used where high strength and good corrosion resistance are required as well as for applications requiring high fatigue strength, good resistance to galling and stress corrosion resistance.
Alloys made by the age-hardening process have many uses, especially in applications where high strength and good performance at high temperatures are needed. They can be used for metal parts and components which call for an increased yield strength, such as:
- Gate valves
- Engine parts
- Processing equipment
- Shafts, gears, plungers
- Valve stems/balls and bushings
- Turbine blades/molding dies
- Nuclear waste casks
- Aircraft parts