ANNEALING | Heat treatment for steels


Annealing involves heating to a predetermined temperature holding at this temperature, and finally cooling at a very slow rate. Annealing can from either the final treatment or a preparatory step for further treatment. The various purpose of the treatment are to

1. Relieve internal stresses developed during solidification, machining, forging, rolling or welding
2. Improve or restore ductility and toughness.
3. Enhance machinability
4. Eliminate chemical non uniformity.
5. Refine grain size.
6. Reduce the gaseous contents in steel.

Depending on heat treatment temperature annealing treatment can be subdivided into three classes, namely
1. Full annealing : here steel is heated above the upper critical temperature A3 and then cooled very slowly.
2. Partial annealing : It also known as incomplete annealing or intercritical annealing. It involve heating of steel to a temperature lying between lower critical temperature A1 and upper critical temperature (A3 or Acm).

3. Subcritical annealing :

Subcritical annealing is a process in which the Maximum temperature to which steel is heated always, less than the lower critical temperature A1. In this process no phase transformation takes place, only thermally activated phenomenon such as recovery, recrystallization, grain growth, agglomeration of carbide and softening occurs. The rate of cooling from a subcritical temperature is of little significant since practically there is no variation as far as microstructure and final properties are concerned.
Annealing - heat treatment for steel

The various annealing processes are discussed below :

Full Annealing :

It consist of heating steel to austenite region, followed by slow cooling. Steel is heated to about 30 to 50℃ above the upper critical temperature A3 for hypoeutectoid steels. Then steel is held at this temperature for a predetermine period and then slowly cooled inside the furnace or a heated insulated container by which, equilibrium structure as predicted by equilibrium diagram are obtained in the steel. Due to this full annealing the steel get a homogeneous austenite structure.

Isothermal Annealing :

In this process hypoeutectoid steel is heated above the upper critical temperature A3 and held for some time at this temperature and then cooled rapidly to a temperature less than the lower critical temperature A1 (usually 600 to 700℃). Fast cooling can be achieved by rapidly transferring steel to another furnace and then maintained at the desired temperature. At which super cooled austenite has minimum stability within the pearlitic region, then held it until the all austenite gets transformed to pearlite. After all the austenite is transformed in to lamellar pearlite, steel is cooled in air. By which the magnitude of internal stresses developed within the steel will vary with cooling rate. By which this process not only improve the machinability but also results in a better surface finish by machining. So that it used more used for making alloys steel.

Diffusion Annealing :

Diffusion Annealing also known as homogenized annealing which is employed to removed any structural non uniformity, like dendrites, columnar grains and chemical inhomogeneous. Theses are generally observed in the case of ingots, heavy plain carbon steel casting and high alloy steel casting. These defect promote brittleness and reduce ductility and toughness of steel. In this type of annealing steel is heated sufficiently above the upper critical temperature (1000 to 1200℃) and is held at this temperature for usually 10 to 20 hours, then cool slowly. Heating to such a high temperature results in considerable coarsening of austenitic grains and heavy scale formation. The coarse austenite obtained further transforms to coarse pearlite on cooling. The coarse grain structure can be refined either by plastic working for invitations or by employing a second heat treatment for casting.

Partial Annealing :

Partial annealing is also referred to as intercritial annealing or incomplete annealing. In this process steel is heated between the lower critical temperature A1 and the upper critical temperature A3 or Acm, which is followed by slow cooling. Generally hyperutectoid steels are subjected to this treatment. The resultant microstructure consists of fine pearlite and cementite instead of coarse pearlite and a network of cementite at grain boundaries, as observed in the case of full annealing. As low temperature involve in this process, it is less expensive than full annealing. Hypoeutectoid steels are also subjected to this treatment in order to improve their machinability.

Recrystallization Annealing :

All steels, which have been heavily cold worked are subjected to this treatment. The process consists of heating steel above the recrystallization temperature, holding at this temperature and then cooling. It results in decrease in hardness or strength and increase in ductility. The desired extent of reduction in cross sectional area is possible with adoption of cold work recrystallization anneal cycle. The process is used both as an intermediate operation and as a final treatment. The treatment is frequently employed in manufacturing steel industries for making wires, sheets and strips. The final structure after the treatment consists of strain free grains produced at the expense of deformed original grains.

Process Annealing :

In process annealing steel is heated to a temperature below the lower critical temperature, and is held at this temperature for sufficient time and then cooled. Since it is a subcritical annealing, cooling rate is of little importance. The purpose of this treatment is to reduce hardness and to increase ductility of cold-worked steel so that further  working may be carried out easily. It is an intermediate operation and is sometimes referred to as in-process annealing. The process is less expensive than recrystallization annealing. It differs from recrystallization annealing in the sense that complete recrystallization of cold-worked steel may or may not take place in this treatment. Parts which are fabricated by cold forming such as stamping, extrusion, upsetting and drawing are frequently given this treatment as an intermediate step.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

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