Annealing

          This is a kind of heat treatment after which a metal or alloy acquires a structure close to the equilibrium one. A material is exposed to an elevated temperature for an extended time period and then slowly cooled. Normally, annealing is carried out to:

1. To improve machinability,
2. To relative internal stresses,
3. To increase softness, ductility and toughness, 
4. To produce a specific micro-structure, 
5. To remove gases,
6. To refine grain size due to phase recrystallisation,
7. To modify electrical and magnetic properties.

          The approximate temperatures to which the steel-sample should be heated will depends upon its carbon content. The recommended temperatures are shown in the following table:

          There are a variety of annealing heat treatments are possible. These are characterized by the changes that are induced, which many times are microstructural and are responsible for the alteration of the mechanical properties.

An annealing process consists of three stages: 

      (i) heating to the desired temperature, 

      (ii) holding or 'soaking' at that temperature, and 

      (iii) cooling, usually to room temperature. 

             In these annealing procedures, time is an important parameter. There exist temperature gradients between the outside and interior portions of the piece during heating and cooling. The magnitudes of temperature gradients depend on the size and geometry of the piece. Soaking time may be given at the rate of 3-4 minutes for every one mm thickness of the cross-section of material. If the rate of temperature, change is too great, temperature gradients and internal stresses may be induced that may lead to warping or even cracking. Moreover, the actual annealing time must be long enough to allow for any necessary transformation reactions. An annealing temperature is also an important consideration. Since diffusional processes are normally involved and therefore annealing may be accelerated by increasing temperature. The various types of annealing operations are: 

1. Full annealing,
2. Isothermal annealing,
3. Process (or) sub critical annealing,
4. Spheroidise annealing,
5. Diffusion annealing, and 
6. Recrystallisation annealing.

Full Annealing

          If it is desired to reline the grain structure and produce a lamellar pearlite, a full annealing cycle should be used. This consists of heating the steel to a temperature above the transformation range, holding for one or two hours, and then cooling at a predetermined rate to obtain the desired microstructure. Grain refinement is accomplished in this instance by the re-crystallisation of the steel in passing through the critical range both in heating and in cooling. The microstructure obtained in cooling any steel from above the critical temperature range is dependent both upon the temperature range in that ranges. Thus, it is obvious that the rate at which any steel is cooled determines the final microstructure, since the degree of transformation will depend on the amount of time allowed for it to occur. Therefore, the slower the rate of cooling and the higher the temperature at which complete transformation occurs during full annealing, the coarse the pearlite will be with correspondingly lower hardness.

          Such treatment is performed usually on steel of 0.3 to 0.6 % carbon content which is to be machined.

Isothermal annealing

          It is a type of full annealing in which the steel first is cooled to the temperature at which it is desired to have transformation occurs, at a rate sufficiently rapid to prevent any structural change above that temperature. The steel then is held at the selected temperature for the time necessary to complete such transformation. Thus it possible, with this process to obtain a more uniform micro-structure that could be expected by continuous cooling. However, since it is necessary to drop the temperature rapidly to prevent any transformation above the desired temperature, there are definite limitations as to the mass that can be co-treated. It is applicable, therefore, only to small sections and would be suitable for large bars or large load in batch type furnace since in would be impossible to cool them at a rate sufficiently rapid to prevent some transformation.

          Isothermal annealing process not only improves machinability in general, but also results in a better finish by machining. However, it has the following important limitations:

          It is suitable only for small components. Heavy components cannot be subjected to this treatment because it is not possible to cool them rapidly and uniformly to the holding temperature at which transformation occurs. 

Process or Sub critical annealing

          Another type annealing called Process or Sub critical annealing consists of heating the steel to a temperature first under lower critical point and holding at this temperature for the proper time (2-4 hrs) followed by air cooling. This type of annealing results in softening the steel due to particle coagulation of the carbide to form the spheroids or small globules of carbide. It is not suitable when a close control of hardness or structure is desired, because the prior structure of steel determined to a marked degree the extent of spheroidisation which will occur.

          This process is quite satisfactory for rendering bars more suitable for cold sawing or shearing and is used to great extent for these purposes. Since the temperature to which the bars are heated is somewhat lower than in full annealing there is less scaling and warping can be controlled.   

Recrystallisation Annealing

• Practically all steels, which have been heavily cold worked, are subjected to this treatment.
• The process consists of heating steel above the recrystallisation temperature, holding at this temperature and cooling thereafter. It results in decrease in hardness or strength and increase in ductility. The process is used both as an intermediate operation and as a final treatment. The treatment is very important and is frequently employed in industries manufacturing steel wires, sheets and strips.
• As little scaling and decarburisation occurs in recrystallisation annealing, it is preferred over full annealing.
• High carbon steels and alloy steels require higher recrystallisation temperatures.