Annealing steel: theory and processes

Annealing steel is a heat treatment, at which steel is heated:

  • above the upper critical temperature Ac3 – complete annealing;
  • between critical temperatures Ac1 and As3 - incomplete annealing;
  • below critical temperatures - low annealing.

After heating, slow cooling follows, most often with a stove.

Heating above the Ac temperature3 provides complete recrystallization of steel, and slow cooling ensures the decomposition of austenite into a completely ferrite-pearlite structure (picture 1).

struktura-otzhigaPicture 1 - Medium carbon steel 40 after complete annealing.
Ferrito-pearlitic structure.

Annealing steel targets

The main goals of annealing steel are recrystallization of steel and elimination of internal stresses. Annealing, like normalization, is the initial heat treatment operation. The purpose of annealing is to eliminate defects in previous metallurgical operations (casting, rolling, forging) or prepare steel for subsequent technological operations, eg, cutting or hardening. Annealing is often the final thermal operation., when the properties of the steel after annealing meet the requirements for the part or product.

Complete annealing of steel

Full annealing - heating above the upper critical temperature followed by slow cooling - solves both of these problems. When heated, the ferrite-pearlite structure of steel transforms into austenitic, and then, upon cooling, austenite turns back into ferrite and pearlite - complete recrystallization occurs. Coarse-grained ferrite-pearlite structure, characteristic of steel after casting or forging, after complete annealing, it turns into a structure of fine grains of ferrite and pearlite. Full annealing temperature ranges are shown in the figure. 2 for steel with different carbon content.

temperatura-otzhiga-stali
Temperature range for complete annealing of steel.

Incomplete annealing of steel

Incomplete annealing of steel - heating between above temperature Ac1, but below temperature Ac3 - spend, when the steel structure is not too coarse and there is no Widmanstätt ferrite structure. This annealing is sometimes called intercritical.. In this case, only the recrystallization of the pearlite structure occurs., and ferrite remains unchanged. clear, that incomplete annealing is more economical, than full.

Low annealing of steel

Low annealing is carried out below both critical points. Therefore, this annealing is also called subcritical.. No austenite is formed during this annealing.. Low annealing of steel is carried out in those cases, when the original structure does not require correction and there is no need to recrystallize it. The purpose of low annealing is only to reduce internal stresses in the part through return mechanisms, recrystallization, grain growth and agglomeration of carbides. If the original steel structure is bainitic or martensitic, then this operation is called non-annealing, but on vacation.

Low tempering is one way to spheroidize steel.

Diffusion annealing

Diffusion annealing is a variant of full annealing. It is carried out for steel ingots. Cast steel is characterized by heterogeneity of chemical composition, as well as dendritic liquation. Diffusion annealing operation, which is also called homogenization, carried out at high temperature, usually before 1000-1100 ° C. Such heating with exposure leads to the elimination or softening of the dendritic inhomogeneity.. However, such high heating results in a coarse-grained structure., which requires additional heat treatment, usually - annealing. If diffusion annealing was applied to ingots, which are intended for the processing of metals by pressure (rolling, forging), then there is no need for annealing - the grain will be crushed by subsequent plastic deformation.

Cooling of steel during its annealing

The cooling rate during annealing of steel should not be more than 50-100 ° C per hour, which can only be achieved by cooling with an oven. This ensures the transformation of austenite with a minimum degree of supercooling and guarantees the formation of an equilibrium ferrite-pearlite structure.

Isothermal annealing

To avoid the difficulty of controlling the cooling rate of steel during annealing, and also to reduce the duration of annealing, instead of classical annealing with slow cooling, in practice, the so-called isothermal annealing is often used. It differs from conventional full annealing in that, that the steel from the annealing temperature is cooled quickly to a temperature of 50-100 ° C below the critical point Ac1 and withstand her so much, how much is required for the complete transformation of austenite.

Sources:
1. Gulyaev A. P. Metallurgy, 1986.
2. The Heater’s Guide: Practices and Procedures for Irons and Steels, AMS International, 1995.