Precipitation hardening is a heat treatment that significantly increases an alloy's yield strength by forming tiny precipitate particles within its microstructure. The same process is also called ageing or age hardening - in our Product Reference Guide we describe the condition as "Solution Annealed & Precipitation Treated" to avoid confusion.
How the process works
Solution annealing is applied to most alloys after final forging or rolling. Holding the alloy at a relatively high temperature for several hours - typically around 1000degC - absorbs the various elements into solid solution, and immediately quenching the bar or plate retains that structure at room temperature.
Time spent at elevated temperature then changes the solubility of certain elements. Precipitates - combinations of selected elements - come out of solution and appear as small particles distributed throughout the microstructure. These particles pin the grain boundaries, hindering their movement when the alloy is strained, which raises the yield strength.
The number, size and distribution of precipitates vary with the temperature and time of the ageing treatment, so a single alloy composition can be produced at a range of strength levels purely by altering the heat treatment.
Ageing may hold the bar or plate at a single temperature, or at different temperatures for different times. The cooling rate also matters: some alloys are slowly air cooled while others are quenched in oil or water. The process relies on complex thermodynamics - alloys were once developed through experimentation and testing, but can now be optimised by modelling how precipitates form under different thermal histories.
Why high-strength alloys are attractive
Designing components around higher-strength alloys lets customers reduce the overall material requirement, saving cost and time. Lighter components also bring savings elsewhere: suspended loads are reduced, which lowers the impact on ancillary systems.
Precipitation-hardened grades we supply
We supply a number of high-strength alloys that rely on precipitation hardening:
- Alloy K500 (2.4375, UNS N05500, Monel® K500) - a nickel-based alloy with copper. Aluminium and titanium are added to form gamma prime Ni3(Ti, Al) precipitates. The secondary heat treatment is a two-stage process followed by slow air cooling.
- Alloy 718 (2.4688, API 6ACRA, UNS N07718, Inconel® 718) - perhaps the most widely used precipitation-hardened nickel alloy. Niobium, titanium and aluminium form a series of precipitates with nickel, and it can be supplied at a variety of strength levels depending on the heat treatment. Most of our stock is the 120ksi version, with a minimum yield strength of 125ksi (827N/mm2).
- Alloy 725 (UNS N07725, API 6ACRA, Inconel 725) - the higher-strength variant of Alloy 625. The chemical composition is largely the same, apart from additions of niobium and titanium that precipitate with nickel after heat treatment. This roughly doubles the yield strength compared with Alloy 625 (UNS N06625, Inconel 625).
- Alloy 925 (UNS N09925, API 6ACRA, Incoloy® 925) - the higher-strength variant of Alloy 825. Additions of titanium and aluminium form Ni3(Al, Ti) gamma prime precipitates. Compared with Alloy 825 (2.4858, UNS N08825, Incoloy 825) the yield strength can be more than 3x higher.
- Hidurel® 5 (2.0855, UNS S64700, DTD498, ASTM B411, BS B25) - a copper-nickel alloy with high strength and good bearing and anti-friction properties. Annealing at elevated temperature creates a soft supersaturated alpha-phase structure, and subsequent age hardening at lower temperature precipitates finely dispersed nickel silicide in the matrix.