The casting process typically involves pouring molten metal into a mold and allowing it to cool and solidify. This process creates inherent problems, which annealing aims to address:

1.Relieve Internal Stress (The Most Important Function):

Cause: Uneven thickness, varying cooling rates, and inconsistent shrinkage across the casting lead to significant internal stresses.

Harm: Internal stresses can cause deformation and cracking during processing or use, impacting dimensional stability and service life.

The Effect of Annealing: By heating to a certain temperature and holding the temperature, the atomic mobility within the metal is enhanced, allowing creep and recrystallization to occur, effectively eliminating these internal stresses.

2. Homogenize Microstructure and Composition:

Cause: Dendritic segregation occurs during casting solidification, indicating an uneven distribution of alloying elements.

Harm: This can lead to uneven performance across the casting and reduced mechanical properties.

The Effect of Annealing: Long-term holding at high temperatures allows for sufficient atomic diffusion, resulting in a casting with a more uniform composition and microstructure.

3.Reducing Hardness and Improving Machinability:

Cause: The as-cast structure of some metals (such as cast iron and high-carbon steel) is relatively hard, making it difficult to machine through processes like turning, milling, and drilling.

The Effect of Annealing: By changing the morphology of carbides (e.g., spheroidizing annealing transforms lamellar cementite into spherical ones), it significantly reduces material hardness, improves machinability, and extends tool life.

4.Improving or Modifying Mechanical Properties:

Adjusting the annealing process can refine the grain size, enhance the material's plasticity, toughness, and strength, and prepare the microstructure for subsequent strengthening processes such as quenching.