Detailed Explanation Of Annealing Heat Treatment | Process, Furnace, Stages, Types etc.

Published 2024-04-30

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

It consists of heating the metal to a suitable temperature, holding it at that temperature for a certain time (called Soaking), and slowly cooling it in the furnace.

The annealing process works in three stages:

○ Recovery stage.

○ Recrystallization stage.

○Grain growth stage.

▶ TYPES OF ANNEALING:

Understanding the different types of annealing processes is essential for achieving the desired outcome when annealing metals.

The common types of annealing methods are:

1. Full annealing.

Heating the steel to an austenizing range of 30℃ to 50℃ above its critical temperature, maintained for a specific period and then cooling slowly in the furnace.

2. Process annealing.

The metal is heated below the critical temperature or very near to it, maintained at this temperature for a while, and then cooled slowly.

3. Spheroidizing annealing.

Steel is heated to a temperature below the eutectoid reaction maintained at that temperature for a while and then slowly cooled in the furnace.

4. Stress relief annealing.

In this annealing process, the steel metal is heated to a lower temperature of about 650℃ maintained at this temperature in the furnace for some time to remove the metal's internal stress. Then the metal is cooled slowly.

5. Isothermal annealing.

The steel is heated above the upper critical temperature where the structure transforms into an austenite structure. It is then quickly cooled to a temperature between 600℃ to 700℃ below the lower critical temperature.

The steel is then maintained at this new temperature for a specific period and then cooled slowly in the furnace.

6. Diffusion annealing.

In the diffusion annealing, the steel is heated to a relatively high temperature in the range of 1050℃ and 1300℃ above the upper critical temperature because the diffusion requires a higher temperature.

The steel is maintained for some time for diffusion of iron and carbon to occur and then cools slowly in the furnace.

7. Incomplete/partial annealing

This type of annealing is carried out by heating the steel sample just above the Lowe critical temperature and then holding it there for sufficient time. Finally, it is cooled slowly in the furnace.

Annealing is performed on a metal for any of the following reasons.

◐ To reduce hardness and brittleness.

◐ To alter the microstructure so that desirable mechanical properties can be obtained.

◐ To Soften metal for improved machinability.

◐To recrystallize cold work metals.

◐ To reduce residual stresses induced by prior processes.

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