Normalising And Annealing

Normalising

Normalising aims to give the steel a uniform and fine-grained structure. The process is used to obtain a predictable microstructure and an assurance of the steel’s mechanical properties.

During normalising, the material is heated to a temperature approximately equivalent to the hardening temperature (800-920°C). At this temperature new austenitic grains are formed. The austenitic grains are much smaller than the previous ferritic grains. After heating and a short soaking time, the components are cooled freely in air. During cooling, new ferritic grains are formed with a further refined grain size. In some cases, both heating and cooling take place under protective gas to avoid oxidation and decarburisation.

Benefits

After forging, hot rolling or casting, a steel’s microstructure is often unhomogeneous consisting of large grains and unwanted structural components such as bainite and carbides. Such a microstructure has a negative impact on the steel’s mechanical properties as well as on the machinability. By undergoing a normalising process, the steel can obtain a more fine-grained homogeneous structure with predictable properties and machinability.

Application

Normalising is mainly used on carbon and low alloyed steels to normalise the structure after forging, hot rolling or casting. The hardness obtained after normalising depends on the steel alloy content, carbon content and the cooling rate used (approximately 100-250 HB).

Annealing

Annealing is a heat treatment process which alters the microstructure of a material to change its mechanical or electrical properties. Typically, in steels, annealing is used to reduce hardness, increase ductility and help eliminate internal stresses.

Annealing is a generic term and can be further classified according to temperature and atmosphere.

For steels, subcritical annealing takes place at 500°C – 680°C, so there is no crystal structure change. Intermediate annealing is carried out at 680°C – 760°C, so there is some transformation to austenite and full annealing involves completely austenitising the work at 800°C – 950°C

Parts can be annealed in a vacuum or protective atmosphere where a bright surface finish is needed. Annealing in air is employed where surface finish is not an important factor.Alternatively an endothermic/neutral atmosphere may be used during annealing to control decarburisation.

Benefits

Annealing will restore ductility following cold working and hence allow additional processing without cracking. Annealing may also be used to release mechanical stresses induced by grinding, machining etc. thereby preventing distortion during subsequent higher temperature heat treatment operations. In some cases, annealing is used to improve electrical properties.

Application

One of the main applications of annealing is reversing the effects of work hardening. During cold forming, drawing, bending etc. the material can become hardened to the point where further working can be impossible or result in cracking. An annealing operation at this stage will make the material more ductile, permitting further forming. In a similar manner, annealing is utilised to remove the internal stresses which occur when welds solidify.

Besides steels, other metals may also benefit from annealing such as copper, aluminium, and brass.