Solution annealed steels are obtained through the solution annealing process, which is one of the most frequent heat treatments performed on austenitic steels.
The solution annealing treatment, also known as “solution hardening” or “hyper-hardening,” is a heat treatment specifically for austenitic steels (stainless and duplex steels), consisting in a heating and holding phases at a high temperature (between 1050 °C and 1. 200 °C, but more frequently between 1050 °C and 1100 °C) followed by rapid cooling with water (black finish requiring further pickling treatment) or inert gas (bright finish), to avoid chromium carbides precipitation, which would otherwise have time to form during slower cooling.
Chromium carbides are typically responsible for intercrystalline corrosion.
What is solution annealed process used for?
Solution heat treatment is usually intended for austenitic steels, such as stainless and duplex steels, and is particularly recommended to give steels elasticity, to protect the treated part from corrosion and to homogenize its constituents.
This type of heat treatment is also useful for improving steel properties, specifically:
- it maximizes its oxidation resistance
- improves its mechanical strength: both at room temperature and at high temperature
- improves its corrosion resistance
During the heating phase, the holding time must be long enough to ensure that the heating temperature reaches the core of the part, to eliminate structural changes caused by previous processing.
The mechanism of heating and holding at certain temperatures and rapid cooling is similar to the quenching treatment of martensitic steels, but in austenitic steels it confers the maximum softening state instead of increasing strength values.
For this reason, solution annealing is also called “negative hardening,” since it increases the malleability of the material, at the expense of hardness, i.e., “reverse hardening.”
