Types of Annealing:

1. Full Annealing (آنیل کامل):

   • Objective: To reduce hardness, increase softness and ductility, improve the crystal structure, and reduce internal stresses.
   • Method: The workpiece is heated to a temperature above the critical temperature (Ac3 for steels) and held at that temperature for a sufficient time to form an austenitic structure. It is then slowly cooled in the furnace to form a coarse ferrite and pearlite structure.
   • Application: For parts that require maximum softness and ductility, such as metal sheets, wires, and formed parts.

2. Stress Relief Annealing:

   • Objective: To reduce internal stresses caused by forming, welding, or machining processes without significantly altering the mechanical properties.
   • Method: The workpiece is heated to a temperature below the critical temperature (usually between 500 and 650 degrees Celsius for steels) and held at that temperature for a short time. It is then slowly cooled in air or in the furnace.
   • Application: For parts that are under internal stress and are likely to crack or deform.

3. Normalizing Annealing:

   • Objective: To improve the crystal structure, increase hardness and wear resistance, and equalize the structure and properties.
   • Method: The workpiece is heated to a temperature above the critical temperature (Ac3 for steels) and cooled in air. The cooling rate is higher than in full annealing, resulting in a finer pearlite structure.
   • Application: For parts that require higher hardness and wear resistance, such as castings and forgings.

4. Isothermal Annealing:

   • Objective: To achieve the desired structure and properties by precisely controlling the temperature and time.
   • Method: The workpiece is heated to a temperature above the critical temperature and then rapidly cooled to a temperature below the critical temperature, where it is held for a certain period to allow the desired structural changes to occur. It is then slowly cooled.
   • Application: For parts that require specific mechanical properties and where precise process control is essential.

5. Spheroidizing Annealing:

   • Objective: To improve the machinability of high-carbon steels by converting the carbide structure into spherical particles.
   • Method: The workpiece is held for a long time at a temperature close to the critical temperature (Ac1 for steels) to allow the carbides to transform into spherical shapes. It is then slowly cooled.
   • Application: For parts that require precise and easy machining, such as tools and dies.

Factors Affecting Annealing:

• Temperature: The annealing temperature should be determined according to the type of material, its chemical composition, and the purpose of annealing.
• Time: The holding time at the annealing temperature depends on the type of material, the dimensions of the workpiece, and the annealing temperature.
• Cooling Rate: The cooling rate after annealing affects the final structure and properties. Faster cooling leads to a finer and harder structure, while slower cooling creates a coarser and softer structure.

Applications of Annealing:

• Automotive Industry: Production of engine, gearbox, and body parts.
• Aerospace Industry: Manufacturing of aircraft and spacecraft components.
• Construction Industry: Production of rebar, sheet metal, and other metal parts.
• Toolmaking Industry: Manufacturing of cutting tools, molds, and precision parts.

Annealing is an important process for improving the properties of materials. By precisely controlling the temperature, time, and cooling rate, the desired structure and mechanical properties can be achieved. Choosing the appropriate type of annealing depends on the type of material, the purpose of annealing, and the required properties.