Top 5 Annealing Process

Annealing involves heat treating a workpiece at a defined annealing temperature. The aim of this is to influence and optimise both the usage and processing characteristics of the material.

Annealing treatment consists of heating the material up to the required annealing temperature, holding the material at this temperature for a sufficiently long period and cooling it to a temperature that is appropriate to the respective objective of the annealing treatment. The annealing temperature of steels can also be estimated on the basis of the colours which occur during annealing, see table Annealing and temper colours, Page 296.

The most important heat treatment processes in the field of “annealing” are presented below.

Intrinsic stresses resulting from structural transformations or cold deformations can occur in workpieces. These are caused by irregular heating or cooling processes. In order to reduce these intrinsic stresses in workpieces, tools or blanks (as a result of plastic deformations), stress relief annealing is carried out at temperatures of between +450 °C and +650 °C. After an annealing time of 0,5 h to 1 h, the material must be cooled as slowly as possible to ensure that no new stresses arise.

In order to improve the deformability of C steels and facilitate machining, the material is soft annealed at temperatures in the range of Ac1. This also applies to workpieces that have been strengthened by hardening, precipitation hardening or cold forming. The temperature depends on the material (this is +650 °C to +750 °C for steel or a value lower than this for non-ferrous metals).

If a specific structural state, characterised by spheroidising of the carbides, is to be achieved, then “annealing to spheroidised cementite” (abbreviation: GKZ annealing) is applied. At the same time, a distinction is made between GKZ 2 (initial state martensite or bainite) and GKZ 1 (initial state normal structure). The spheroidal shape of the cementite can also be achieved by austenitisation and controlled cooling.

The possibility of cold forming a material is limited by the increase in hardness and the decrease in formability with the strain caused by deformation.
Recrystallisation annealing is applied to formed workpieces in order to eliminate any strain hardening that may have occurred and bring about a new formation of the grains. This re-enables or facilitates subsequent forming.

The temperature depends on the degree of deformation and, in the case of steel, is generally around +550 °C to +730 °C.

Normalising is carried out at austenitisation temperature, in other words at a temperature a little above Ac3 (in the case of hypereutectoid steels above Ac1). After an adequate holding period, the material is cooled at an appropriate rate so that a structure consisting of ferrite and pearlite is created at room temperature.

Normalising is used to refine a coarse-grained structure (for example in steel castings and welds) and to achieve as homogeneous a ferrite-pearlite distribution as possible. It should be applied instead of recrystallisation annealing if a coarse-grained structure is to be feared in the case of subcritically deformed workpieces.

If an excessive austenitisation temperature is chosen, the -mixed crystals grow, which also leads to a coarse-grained structure after transformation. An excessively slow cooling process can also result in a coarse ferrite grain.

Homogenising takes place at temperatures of between +1030 °C and +1150 °C above Ac3. It serves to eliminate segregation zones in ingots and strands. If the material is not subjected to hot forming after homogenising, it must be normalised in order to eliminate the coarse grain.