Precipitation hardening, also known as age hardening and particle hardening, is a heat treatment process that is applied to increase yield strength of malleable materials, such as aluminium, magnesium and some select stainless steel grades.
The effect of Precipitation hardening
In simple terms, we have already established that precipitation hardening is performed on parts to improve their yield strength. But what is the mechanism behind that?
During the process, a new phase is formed, which consists of small atoms, known as precipitates. Precipitates are homogeneous particles that are formed within the original phase matrix.
The Process of Precipitation Hardening
Precipitation hardening is a heat treatment technique that takes place in low temperatures and makes use of alloying materials, such as aluminum and titanium. This causes increased yield strength as well as improved corrosion resistance, depending on the alloying metals.
Precipitation hardening takes place in three steps:
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.
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.
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.
Suitable Materials for Precipitation Hardening
- Stainless Steels: Stainless steels that contain high levels of chromium and nickel are suitable for precipitation hardening. Stainless steels that have been age hardened are also referred to as Precipitation Hardening steels, or PH steels. Their structure can be classified as martensitic or semi-autensitic.
- Aluminium alloys: Aluminium plays an important part in the automobile and aircraft and aerospace industry. Its clear advantage, even towards stainless steel: it does not rust and is quite soft and easy to work with. To harden aluminium, precipitation hardening is performed on the material. One of the most common aluminium alloy subjected to PH is aluminium-copper (Al-Cu).
- Magnesium alloys: Magnesium is a relatively light and malleable type of metal. Its low melting point and abundance make the processing and treatment of magnesium and its alloys both cheap and easy. To harden magnesium (alloys), precipitation hardening is performed to form solid-state precipitates.