Annealing

The Annealing process:-

Annealing is a heat treatment process that changes the physical and sometimes also the chemical properties of a material to increase ductility and reduce the hardness to make it more workable

Annealing steel consists of heating a metal above a critical temperature and then cooling at a rate that will produce a refined microstructure.

Annealing is most often used to soften a metal after cold working, to improve machinability, or to enhance properties like electrical conductivity.

Annealing works in three stages – the recovery stage, recrystallization stage and the grain growth stage. These work as follows:

1. Recovery Stage

This stage is where the furnace or other heating device is used to raise the temperature of the material to such a point that the internal stresses are relieved.

2. Recrystallization Stage

Heating the material above its recrystallization temperature but below its melting point causes new grains to form without any residual stresses.

3. Grain Growth Stage

Cooling the material at a specific rate causes new grains to develop. After which the material will be more workable. Subsequent operations to alter mechanical properties can be carried out following annealing.

Advantages

The main advantages of annealing are in how the process improves the workability of a material, increasing toughness, reducing hardness and increasing the ductility and machinability of a metal.

The heating and cooling process also reduces the brittleness of metals while enhancing their magnetic properties and electrical conductivity.

Disadvantages

The main drawback with annealing is that it can be a time consuming procedure, depending on which materials are being annealed. Materials with high temperature requirements can take a long time to cool sufficiently, especially if they are being left to cool naturally inside an annealing furnace.

Recrystallisation

Recrystallisation is a process accomplished by heating whereby deformed grains are replaced by a new set of grains that nucleate and grow until the original grains have been entirely consumed.

Recyrstallisation annealing is an annealing process applied to cold-worked metal to obtain nucleation and growth of new grains without phase change. This heat treatment removes the results of the heavy plastic deformation of highly shaped cold formed parts. The annealing is effective when applied to hardened or cold-worked steels, which recrystallise the structure to form new ferrite grains.

Benefits

• allows recovery process by reduction or removal of work-hardening effects (stresses)
• increases equiaxed ferrite grains formed from the elongated grains
• decreases the strength and hardness level
• increases ductility

Application & materials

• The annealing of stamped parts in cold-rolled steel is designed to produce a recrystallised ferrite microstructure from highly elongated, stressed grains resulting from cold work.
• The annealing of forged parts is performed to facilitate subsequent operations, like machining or cold forming.

Sub-critical annealing / intercritical annealing

Sub-critical annealing (or sub-critical treatment) is annealing carried out slightly below the eutectoid temperature (Ac1 point = eutectoid transformation (723°C for carbon-steels)). Sub-critical annealing does not involve the formation of austenite, while intercritical annealing involves the formation of ferrite and austenite (< 0.8%C carbon-steels).

Benefits

The aim of the soft annealing process is to form an even distribution of spheroidal carbides in the steel, which will make the material softer and tougher. Normally, increasing the size of the spheroids will increase the steel’s machinability.

Application & materials

Spheroidised structure (globular carbides in a ferrite matrix) on carbon steels improves the cold formability of steels and permits severe deformation like cold drawing or deep drawing. But, a spheroidised structure of carbon-steels (< 0.4%C) is not always adapted for certain machining operations because the material is too soft and can stick on the cutting tools.

Soft annealing

Soft annealing is a high temperature heat treatment process performed around A1. As the name suggests the aim of the process is to make a material as soft as possible. After soft annealing the material will have a soft and easy to machine structure.

Benefits

Steels with higher carbon content, and most high-alloy steels, which are allowed to air cool after hot working, such as forging or hot rolling, are usually hard to machine. Soft annealing reduces the hardness and makes the material easier to machine. Soft annealing of low carbon steels < 0,35% C will normally result in a structure too soft and sticky for cutting operations.

The risk of hardening cracks during re-hardening of quenched and tempered steel can be reduced by soft annealing prior to the hardening and tempering process.

Application & materials

Soft annealing can be performed on steel as well as on copper and brass alloys.

Soft annealing is normally performed on steels with a high carbon content and on tool steels to provide a softer and easier to machine structure.

During cold forming of copper and brass components, the material will become harder. The greater the cold forming is, the harder the material will become. The effect of cold forming can be eliminated by soft annealing.