2. •
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
3. •
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.
4. •
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.
5. •
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.
6. •
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.
7. •
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.
8. •
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).
9. •
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.
