Heat treatments for metals change a metal’s physical and mechanical properties to make it perfect for a particular use. Producing high-precision components for any industry depends on heat treatments, the latest machinery and specialist staff.
Together, these three elements support the interpretation or design of the technical drawing, the machining of the product and the assessment of the finished part’s quality. Among the complementary services customers often overlook is annealing, a heat treatment that reduces a metal’s hardness and increases its ductility.
Annealing: what it is and what it’s for
What does annealing heat treatment involve? The material is brought to a high temperature, held there for a variable time and then cooled slowly.
The treatment removes internal mechanical stresses, makes the material’s composition uniform and achieves good ductility. In short, the metal is softened.
Annealing is usually associated with steel, but it can be carried out on various aluminium alloys, cast iron, bronze and brass.
Types of annealing
Annealing of metals must vary according to the structure and the desired hardness, to prepare them for subsequent machining at a temperature below the melting point.
The transformation of the material’s crystalline structure can achieve different results depending on the temperature range and the duration of the process. These are the most commonly used:
- Normalising – Used to make uniform the structure of parts with defects caused by overheating. It requires an annealing time calculated on the size of the part and cooling at room temperature.
- Full annealing – This type of annealing aims to even out differences in the material’s composition. It is often set aside in favour of isothermal annealing.
- Isothermal annealing – A form of full annealing in which the cooling phase changes and processing is faster.
- Recrystallisation – Recrystallisation annealing uses a low temperature, but one sufficient to eliminate the effects of cold plastic deformation, with slow cooling. The aim? To increase ductility at the cost of lower hardness.
- Subcritical annealing – Also called machinability annealing, it is used for structural steel alloys subjected to high thermal stress.
Broadly speaking, annealing is a heat treatment carried out with the same technique, but each type has its own temperature, holding time and cooling method.
In every case, the goal is to “soften” the structure of metal alloys and remove defects that could hinder the production of high-precision mechanical components.