Precipitation Hardening (Age)

Precipitation hardening, also called age hardening is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels and stainless steels.

Unlike ordinary tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called “aging”.

1. Solution annealing

The first step of precipitation hardening is called “solution annealing”. Its goal is to precipitate (or withdraw) metastable phases of alloys. In this process, inhomogeneities are transformed into homogeneities.

The metal is treated with a solution at high temperatures, slightly below the eutectic point of the material. Too low and the solution annealing is ineffective; too high and metals reach their melting point.

2. Quenching

Once the alloying materials are dissolved into the surface of the part, rapid cooling takes place until the solubility limit is exceeded. This prevents alloys from discharging. The solid solution after quenching is metastable, oversaturated and single-phased.

Typical quenching agents in precipitation hardening are water, oil and gas.

3. Aging

Aging is the last and most time consuming step of precipitation hardening. The solution is heated to an intermediate temperature. At temperatures between 150°C to 190°C (400°C- 500 °C for stainless steels), the part is held in a constant heat. The exact aging temperature depends on the composition of the material. The oversaturated solid solution transforms into two-phase alloys. The dominating phase is also known as matrix.

Precipitation Hardening Properties
* Creates high tensile strength and hardness properties * Improves wear resistance * Reduces ductility/toughness properties * Process allows for easy machinability in soft state prior to age hardening * Age hardening is performed at low temperatures * Allows for little-to-no distortion of the part or component * Not all alloys are age hardenable * Alloy must be properly solution-treat annealed prior to aging
Precipitation Hardening Applications
For metal parts and components which call for an increased yield strength Aluminum, magnesium, nickel, titanium and stainless steel parts Gate valves Engine parts Processing equipment Shafts, gears, plungers Valve stems Balls and bushings Turbine blades Moulding dies Nuclear waste casks Fasteners Aircraft parts