Annealing is a heat treatment process. The main goal of the procedure is to alter the microstructure and therefore the physical and chemical properties of metals. In steels, annealing reduces hardness, and internal stresses on the one hand and increases ductility on the other. Therefore, the process makes the material more formable and workable and prevents it from brittle failure.
What happens during the process?
Annealing decreases the number of dislocations which happens due to the fact that atoms migrate in the crystal lattice. This leads to changes in hardness and ductility of the material. With many metals, such as carbon steel, the phase composition and the crystal grain size determines the properties of the workpiece. Here, the cooling and the heating rate are the determining factors. After annealing, further heat treatment processes may be required to achieve the desired characteristics of the material. Therefore, hot and cold working is used to further alter the metal structure. However, even other heat treatments may follow.
Annealing helps to soften the metal. Therefore, the procedure prepares it for further machining such as shaping, stamping or forming.
The different stages during the process
The heat treatment process consists of three stages, which are:
- Recovery stage
- Recrystallization stage
- Grain growth stage
Stage 1: Recovery
The main goal of the first stage is to relieve the stress in the material. Therefore, it gets heated to a specific temperature. This is achieved with the help of a furnace or any other heating device.
Stage 2: Recrystallization
During this stage new grains without residual stress are formed. This is achieved by heating the material between the recrystallization temperature and the melting point.
Stage 3: Grain growth
The last stage leads to the full development of the new formed grains. The process can be controlled by adjusting the cooling temperature to achieve the desired results.
Different types of annealing
Annealing can be classified into several types. Which type is used depends on the desired result and the material used. Here are the most common:
- Full annealing: The objective is to relive internal stresses, reduce hardness and improve the machining of steel.
- Isothermal annealing: Compared to full annealing, isothermal annealing differs in the cooling temperature, but has the same outcomes. However, it is not suitable for large steel workpieces.
- Spheriodizing annealing: There are many types of spheriodizing annealing, but the object is always the same. This process breaks down lamellar pearlite into small spheroids of cementite in a continuous matrix of ferrite
- Diffusion annealing: The main goal is to reduce dendritic and regional segregation in the solidification process.
- Stress relieving: The process is used to eliminate residual internal stresses which occur due to plastic deformation or phase transformation.
The advantages and disadvantages of annealing
The heat treatment has many benefits. These include the following:
- The toughness of steel increases.
- Annealing increases ductility, as well.
- Consequently, machinability of any metal improves.
- Internal stress induced by processes such as grinding or machining get reduced by annealing.
- The process can enhance the magnetic characteristics of the material.
- Annealing reduces the brittleness of metals, such as steel.
- The procedure may be used to improve the electrical properties of the material.
Depending on the material, annealing can be very time consuming due to a long cooling period.