Question

Explain three stages of annealing of metallic materials by sketching their microstructures. Why annealing is used? EXPLAIN.

Answer 1

Three stages of annealing process

1. Recovery
2. Recrystallization
3. Grain growth

1.Recovery

• It is a process by which deformed grains can reduce their stored energy by the rearrangement of defects in their crystal structure.
• The process is normally by reducing in a material strength and a simultaneous increase in the ductility. As a result, recovery may be considered beneficial depending on the circumstances. Recovery is related to the similar process of recrystallization and grain growth, each of them being stages anneling.
• Recovery occurs at the lower temperature stage of all annealing processes and before the appearance of new strain free grains.

2.Recrystallization

• The second stage of annealing is recrystallization process.
• During recrystallization , an impure solid compound is dissolved in a hot liquid until the solution is saturated and then the liquid is allowed to cool. The compound should then form relatively pure crystals.
• It new strain free grains nucleate and grow to replace those deformed by internal stressess.Once recrystallization has completed then third stages occurs.

3.Grain growth

• The microstructure starts to coarsen and may cause the metal to lose a substantial part of its original strength.
• Grain growth is the increase in size of grains in a material at high temperature.
• when recovery and recrystallisation are complete and furture reduction in the internal energy can only be achieved by reducing the total area of grain boundary.

Annealing uses

• Anneling is used to reduced hardness, increase ductility and help to eliminate internal stresses of metals.
• Annealing is heat treatment in which the metal is heated to a temperature above its recrystallisation temperature.
• Temperature for some time for homogenization of temperature followed by very slow cooling to develop equilibrium structure in the metal or alloy.