What is the Annealing?

A nnealing (heat treatment) is a combination of heating and cooling operations applied to an alloy or metal for obtaining the desired prop...

Annealing (heat treatment) is a combination of heating and cooling operations applied to an alloy or metal for obtaining the desired properties. Annealing for steel is a precise heat treatment of metal in which it is heating slightly above the critical temperature of steel (723 degrees Centigrade) and allowing it to cool down very slowly to make it ductile and malleable.

Heat treatment involves the heating and cooling without changing the physical state and shape of the metal. Annealing is vital metal processing step that helps in improvement of product properties and machinability in many ways.

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The Process of Annealing

The Annealing process of metals consists of three major steps.

1. The metal is heated to required temperature (Varies with metal).
2. Holding it at the specified temperature for some time.
3. Cooling at a proper rate (slow cooling).

The annealing process is not same for all the metals; the difference lies in the cooling mechanism of the annealing process. An example, steel is heated to 723ᵒC (1333ᵒF) and cooled slowly for achieving desired properties of the metal. The combination of heating and cooling is used to obtain desired mechanical properties of metal.

Annealing Application

The application of annealing is that the resulting metal will be soft, can be cut and shaped easily. The annealing process helps in grain refinement and improvement in mechanical properties of the steel. In other words, the annealing heat treatment process makes the steel malleable, ductile and workable.

Types of Annealing

There are various types of annealing.

1. Full Annealing
• The process involves heating the steel to 30 to 50 degrees Centigrade above the critical temperature (austenitic temperature) of steel and maintaining the temperature for a specified period of time, then allowing the material to slowly cool down inside the furnace itself without any forced means of cooling.
• Once the metal is reached at 50 °C (122 °F) it can further be cooled to room temperature using air with natural draft.
• The basic heat treatment processes for carbon steel involve the decomposition or conversion of austenite.
• Hot Worked sheets, forgings, and castings made from medium and high carbon steels need full annealing. 
2. Process Annealing
• This process is mainly suited for low carbon steel. The material is heated up to a temperature just below the lower critical temperature of steel. Cold worked steel normally tends to posses increased hardness and decrease ductility making it difficult to work. Process annealing tends to improve these characteristics. This is mainly carried out on cold rolled steel like wire drawn steel, etc. 
3. Stress Relief Annealing
• Large castings or welded structures tend to possess internal stresses caused mainly during their manufacture and uneven cooling. This internal stress cause brittleness at isolated locations in the castings or structures, which can lead to sudden breakage or failure of the material. This process involves heating the casting or structure to about 650 Degree centigrade. The temperature is maintained constantly for a few hours and allowed to cool down slowly. 
4. Spherodise Annealing
• This is a process for high carbon and alloy steel in order to improve their machinability. The process tends to improve the internal structure of the steel. This can be done by two methods
1. The material is heated just below the lower critical temperature about 700 Degree centigrade and the temperature is maintained for about 8 hours and allowed to cool down slowly.
2. Heating and cooling the material alternatively between temperatures just above and below the lower critical temperature. 
5. Isothermal Annealing
• This is a process where is steel is heated above the upper critical temperature. This causes the structure of the steel to be converted rapidly into austenite structure. The steel is then cooled to a temperature below the lower critical temperature about 600 to 700 Degree Centigrade. This cooling is done using a forced cooling means. The temperature is then maintained constant for a specified amount of time in order to produce a homogenous structure in the material. This is mainly applicable for low carbon and alloy steels to improve their machinability.
6. Soft Annealing
• The soft annealing heat treatment can be performed on steel and alloys of copper and brass. Steel with high carbon content is typically treated with soft annealing process which gives it softer and easier to work structure. 
• The process takes about 12 to 48 hours and can be performed in continuous or batch-wise in the oven. The load is heated to the temperature of 800 °C (1472 °F). The metal is held at this temperature for 2-4 hours so that the structure fully converts into austenite. 
• The temperature of the metal is quickly brought down to 790 C (1454 °F). Further cooling of this steel is performed at a controlled and steady rate of 10 °C (50 °F) per hour until the temperature reaches 690 °C (1274 °F). The metal is then cooled to ambient temperature. The structural changes in the steel make it soft. 
• The cooling condition defines the degree of softness attained. The advantage of this process is that a soft and ductile carbon steel is obtained that has a good ability to form.