The primary purpose of an annealing treatment is to reduce the hardness of a material and facilitate the progress of subsequent manufacturing operations. Annealing is commonly used after casting, forging or rolling to soften materials and minimise residual stresses, improve machinability, and increase ductility by carefully controlling the microstructure. Many steels in strip form are annealed, as are most tool steels and stainless steels. Non-ferrous alloys are also annealed.
There are several process variations that qualify as annealing treatments:
- Full annealing is performed on steels by heating to a high temperature (typically 830-950°C), then cooling slowly to ambient temperature. Non-ferrous materials are softened and refined by full annealing at temperatures appropriate for each alloy.
- Isothermal/cyclic annealing is performed by heating steels to the full annealing temperature, cooling to an intermediate temperature (typically 550- 700°C) and soaking for a long period to allow transformation to proceed slowly, followed by cooling to ambient temperature.
- Inter-critical annealing is applied by heating steels to below the full annealing temperature (typically 723- 910°C) according to composition. A prolonged soak is followed by cooling to ambient temperature.
- Subcritical annealing takes place at a temperature for steels of typically 650 – 720°C, allowing a prolonged soak before cooling to ambient temperature.
- Homogenisation annealing can be applied to both ferrous and non-ferrous materials and is a prolonged high-temperature soak intended to break down segregation in the material’s structure.
- Solution annealing is applied commonly to austenitic stainless steels, typically at 1010-1150°C. With unstabilised grades, the treatment must be followed by fast cooling or quenching. It is applied as a softening process during manufacture or to optimise corrosion resistance (e.g. after welding).
Annealing prepares metals and alloys for further processing or for the intended service conditions. They control the ability of materials to be machined with ease, perform without distortion in service, be formed without cracking or splitting, be subsequently hardened or carburised with minimal distortion, or to resist corrosive environments.
All commercial alloys can be annealed whether in the form of billet, plate or rolled stock through to castings and forgings to provide properties required for their applications.
- Many austenitic stainless steels require fast cooling after high-temperature stress relief or solution annealing. A degree of distortion or reintroduction of residual stresses is inevitable in such cases.
- The size and shape of items that can be annealed depends on the type of equipment operated by the heat treater. For large items, check the availability of suitably-sized facilities at an early stage.
Annealing processes can take place in air or in protective media such as molten salt, controlled gaseous atmospheres or vacuum. Prolonged soaks, as required by some annealing operations, necessitate the selection of a protective medium.
All of the following information should be included if possible. If uncertain, ask your heat treater before producing a specification:
- The process: anneal in air or vacuum or protective atmosphere.
- Material: type, grade, and the standard from which it is drawn, with drawing, composition and mill certificate where available.
- Any general standards applicable (national, international or company) that contain relevant details which must be adhered to.
- Existing condition; e.g. details of any prior heat treatment, such as hardening and tempering, solution treatment and ageing, intended to establish mechanical or other properties.
- The level of mechanical properties required. Generally a hardness range and a maximum hardness level is often requested.
- The type(s) of testing required; e.g. hardness (Vickers, Brinell), tensile etc. and any special locations for testing or the removal of samples for test pieces.
- Requirements for any special certificates or data to be provided by your heat treater.
Guidance and information is always available from our experience heat treatment professionals.
Our procedures and work instructions are fully documented under our AS 9100 and ISO 9001 quality management systems.