Heat treatments for stainless steels

Stainless steels can be heat-treated in a variety of ways.

Quality heat treatments are applied to stainless steels to give them their properties for use: mechanical properties or corrosion resistance. These quality treatments are carried out late in the range, either by the steelmaker or by the end user of the steel.

The aim of all treatments used to ensure good corrosion resistance is to make the chromium content of the metal as homogeneous as possible, avoiding any precipitation of chromium-rich phases (chromium carbonitrides, or chromium-rich intermetallic phases) and thus any reduction in chromium content in the surrounding zones. Indeed, it is chromium that ensures the rust-proofing of stainless steels, and a chromium content of over 10.5% at any point in the metal is essential in this respect. On the other hand, and even if the primary characteristic of stainless steels is their resistance to corrosion, it should be remembered that they can acquire significant mechanical characteristics or hardness through appropriate heat treatment, which can lead to savings in materials, lighter structures, or simply suitability for a given use. At the same time, the metal must retain sufficient ductility for its intended application. In the case of certain grades, the aim is to avoid embrittlement, which means that they cannot be maintained in certain temperature ranges.

Other treatments require a certain amount of deformation of the metal, and can therefore only be applied during shaping: these are thermomechanical treatments performed by the steelmaker. These thermal or thermomechanical treatments obviously differ according to the desired metallurgical structure at room temperature: ferritic, martensitic, austenitic, or mixed (ferrito-martensitic or austeno-ferritic). Treatments are therefore presented in this article according to the different families of stainless steels.

Stress-relieving treatments have mechanical rather than metallurgical effects.

The treatments applied after welding restore the metal's properties, particularly in heat-affected areas.

A number of precautions need to be taken when carrying out these various heat treatments.

As the nature of the heat transfer fluid used during heat treatment has an impact on the chemical composition of the part skin (loss or addition of chemical elements), and therefore on the surface properties of the stainless steel, in particular its brightness, hardness and corrosion resistance, the atmospheres to be used during heat treatment vary according to the desired product surface qualities, and the defects that can result from unsuitable atmospheres.

Certain heat treatment...