The Process of Precipitation Hardening

Precipitation/Age Hardening and Heat Treatment of Metals

Precipitation Hardening
Titanium nitride precipitates in a precipitation hardened HSLA steel. Image copyright: University of Nevada, Reno

Precipitation hardening, also called age or particle hardening, is a heat treatment process that produces uniformly dispersed particles within a metal's grain structure. These particles hinder dislocation motion and thereby strengthen the metal, particularly those that are malleable.

How Precipitates Are Formed

The formation of these precipitates is accomplished by using a solution treatment at high temperatures prior to a rapid cooling process.

The solution heat treatment results in a single-phase solution while the rapid cooling results in a stable material by preventing the creation and propagation of lattice defects. This greatly strengthens the metal matrix.

Precipitation hardening is typically performed in a vacuum, inert atmosphere at temperatures ranging from between 900º and 1150° F. The process ranges in time from one to four hours, depending on the exact material and the characteristics specified.

As with tempering, a balance must be struck when precipitation hardening between the resulting increase in strength and the loss of ductility and toughness. Additionally, it is possible to over-age the material by tempering for too long, resulting in large, spread-out and thus ineffective precipitates.  

Metals Treated by Precipitation 

Metals that are often treated by precipitation or age hardening include:

  • Aluminum—The chemical element of atomic number 13, it does not rust or magnetize.
  • Magnesium—Regulates biochemical reactions in the human body. 
  • Nickel—The chemical element of atomic number 28, nickel can be used in everything from food preparation to building hi-rises and transportation infrastructures.
  • Titanium—This is a metal of the transition series and is often found in alloys. It has a chemical element of atomic number 22.
  • Stainless steels—Stainless steels are alloys of iron and chromium that are resistant to corrosion. 

Other alloys that are hardened by precipitation treatments include: