Applied to some, but not all, engineering steels, normalising can soften, harden or stress relieve a material, depending on its initial state. The objective of the treatment is to counter the effects of prior processes, such as casting, forging or rolling, by refining the existing non-uniform structure into one which enhances machinability/formability or, in certain product forms, meets final mechanical property requirements.
A primary purpose is to condition the steel so that, after subsequent shaping, a component responds satisfactorily to a hardening operation (e.g. aiding dimensional stability).
Normalising consists of heating the suitable steel to a temperature typically in the range 830-950°C (at or above the hardening temperature of hardening steels, or above the carburising temperature for carburising steels) and then cooling in air. Heating is usually carried out in air, so subsequent machining or surface finishing is required to remove scale or decarburised layers.
Air-hardening steels (e.g. some automotive gear steels) are often “tempered” (subcritical annealing ) after normalising to soften the structure and/or promote machinability. Many aircraft specifications also call for this combination of treatments. Steels that are not usually normalised are those which would harden significantly during air cooling (e.g. many tool steels), or those which gain no structural benefit or produce inappropriate structures or mechanical properties (e.g. the stainless steels).
Normalising prepares the metal 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, and be subsequently hardened or carburised reducing distortion, or to resist corrosive environments.
Normalising is restricted to certain steels, these include case hardening alloys , engineering steels , hot rolled materials and ferrous castings or forgings.
The size and shape of items that can be normalised 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.
Normalising is usually performed in air on semi-finished steel products where scaling and decarburisation pose no problems because they are removed by subsequent machining.
A protective medium may sometimes be necessary; e.g. when normalising final-size products prior to localise surface hardening. These products can be normalized in protective mediums but size restrictions may be incurred.
All of the following information should be included if possible. If uncertain, ask your heat treater before producing a specification:
- The process: normalise indicate if bright treatment is necessary, or if treatment in air is satisfactory.
- 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 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.
- Other services required; e.g. straightening (with working limits), cleaning/blasting, laboratory or specialised NDT services etc.
- Heat Charts or Time Temperature Graph requirements or Certificates of Conformance
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.