Quick Answer:
Annealing reduces the hardness of materials by relieving internal stresses, reducing structural defects, increasing grain size, and achieving room temperature-based equilibrium-based microstructure.
What is Annealing?
Annealing is a heat treatment process that alters a material’s physical and chemical properties. It is used to increase the ductility and reduce the hardness of materials by introducing controlled thermal processes that allow internal stresses to be relieved and structural defects to be reduced or eliminated.
Read more about Annealing and its effect on the microstructure of Steel in our detailed guide on Annealing of Steel.
How Annealing relieves stresses and defects present in materials?
Only point defects are removed when the temperature increases, below the Phase transition and recrystallization temperature. The grain size increases due to the migration of point defects, making it more homogenous with less strain energy and reduced potential driving force.
More information can be studied in Stress-Relieve Annealing.
What other benefits does Annealing offer?
By changing the microstructure of materials, Annealing also alters their physical properties. It is one of the most widely used techniques for improving the formability and machinability of metals by increasing ductility and reducing hardness.
Additionally, it increases toughness and fatigue strength, making them more suitable for various industrial and commercial applications. Moreover, it also enhances magnetic properties and electrical conductivity in certain materials compared to other heat treatments.
Did you know: Hardness and Strength of materials are two different things. If you are looking to see the impact of Annealing on a material’s strength, you better check our article, “Does annealing increase material’s strength?“.
How does Annealing decrease the hardness of materials?
Annealing is a commonly employed method to reduce the hardness of materials with the right heat treatment temperature and time in a proper furnace atmosphere. It reduces the hardness of materials due to the following reasons;
Annealing relieves internal stresses in Materials:
Internal stresses exist due to uneven cooling and internal strain caused by plastic deformation. Annealing relieves such internal stresses by slowly heating the material to a pre-defined temperature, then maintaining that temperature for a particular time and cooling it down slowly.
Internal stresses are relieved when the material is heated in the furnace due to the diffusion of atoms within grains. These internal stresses are the main reason for increased hardness in the cold worked steels.
Cold-worked steels are difficult to process because of increased hardness and lower ductility. Stress relief annealing is employed to relieve those stresses and prepare them for the next steps.
You will find more detail regarding Annealing on Cold work structure in article “Why annealing is done after cold working?“.
Annealing reduces structural defects in Materials:
Annealing also reduces structural defects such as dislocations, stacking, and grain boundaries by introducing several point defects like vacancies in the material.
With the diffusion of high-angle grain boundaries and associated defects, new strain-free grains start forming at the expense of old, elongated grain. These strain-free grains differ from elongated grains in size, shape, and dislocation density.
Defects present in a structure are due to manufacturing and cold working of the structure. Manufacturing results in various defects like vacancies and high-angle grain boundaries. These defects are reduced by polygonization, explained in the Annealing guide we created.
Annealing increases the grain size of materials:
Annealing results in recrystallization of cold worked steels which cause growth in grain size as well. As the temperature increases, below Phase transition and recrystallization temperatures, only point defects are removed. The grain size increases due to migration of point defects, making it more homogenous with less strain energy and reduced potential driving force.
Grain size in microstructure is achieved when the annealing temperature exceeds recrystallization temperature of the material. This increased grain size reduces grain boundary area, which lowers defects that can hinder dislocation movement. This results in a decrease in hardness.
Annealing achieves room temperature based equilibrium based microstructure in Materials:
When the cooling rate is slow enough, it ensures that new grains will grow to a size below the critical grain size, at which point further growth stops. This results in an equilibrium structure of the material at room temperature. The equilibrium structure has uniform grain size and low dislocation density.
This ensures that the material is less prone to external stresses and plastic deformation, resulting in lower hardness and increased ductility. This equilibrium structure also provides better properties to materials like improved strength, thermal stability, and corrosion resistance.
Annealing is often employed to achieve such an equilibrium structure of materials at room temperature. I tried to explain the equilibrium structure of Steel during Annealing in an article on the Annealing of Steel.
Conclusion
Thus, Annealing is an effective process to reduce materials’ hardness and achieve the desired properties. It provides several benefits, like relieving internal stresses, reducing structural defects, and achieving equilibrium-based microstructure at room temperature. This makes it a crucial step for many engineering applications.
