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 mechanical properties 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 cooling rate depends on the type of metal. Ferrous metals, such as steel, are usually cooled down to room temperature only by air. Other metals, such as silver or copper, can also be quenched in water.
The different stages of the annealing 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 internal stresses in the material. Therefore, it gets heated to a specific temperature. Typically, this is achieved with the help of an annealing furnace. However, any other heating device can be used.
Stage 2: Recrystallization
During this stage, new grains without residual stresses are formed. This is achieved by raising the temperature of 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 rate to achieve the desired results.
Different types of annealing
Annealing can be classified into several types. Which type is used depends on the desired properties of used the material. Here are the most common:
- Full annealing: The objective is to relive internal stresses, reduce hardness and improve the machining of steel. In most cases, the treated material is furnace cooled.
- 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.
Why is annealing important?
Materials are subject to annealing to improve the following properties:
- Reduction of hardness for cold working: When it comes to cold working, one of the biggest challenges is stress cracking. Therefore, some ductility has to be restored again, which can be provided by annealing.
- Reduction of residual stress: A side effect of annealing can be the reduction of residual stress. Heating materials causes yield strength to be reduced. Stress-locations within would deform as a result, releasing internal stress. The proper method to de-stress locations within materials is called stress relief annealing. This method is aimed at reduction of residual stress only as opposed to changing mechanical properties, which is the case of annealing.
- Improvement in machinability: The reduced hardness results in improved malleability and machinability.
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.
- The effects of work hardening such as internal stress induced by grinding or machining get reduced by annealing.
- The process can enhance the magnetic properties of the material.
- Annealing reduces the brittleness of metals, such as steel.
- The procedure may be used to improve the electrical conductivity of the material.
Depending on the material, annealing can be very time consuming due to a long cooling period.
The most common type of material that is used in the annealing process is of course steel. However, brass, copper, and silver are also suitable. They are heated and then cooled down to room temperature. Unlike steel, which is cooled relatively slowly in still air, these metals can also be quenched in water. Steel is not cooled down quickly because annealing enables it to be processed more easily. After being processed, steel may be subject to further heat treatment that involves quenching.