Working with metal processing can be complicated. One of its elements is the need to ensure that the processed materials maintain the appropriate properties. For this purpose, we can list several important procedures, but today we will focus on such a concept as annealing ofmetals. It is a type of heat treatment, most often used to reduce the hardness and brittleness of the material and to reduce the risk of its damage or cracking. We list the most popular types of annealing, briefly describe them and answer basic questions.
What is metal annealing and what does it involve?
In metalworking, annealing is a common heat treatment process used to alter the physical and mechanical properties of certain types of material in order to reduce their hardness, increase ductility, and eliminate internal stresses. This method is commonly used, for example, to reverse or reduce the effects of cold working, to enable further development of the material. Under certain circumstances, annealing can also affect the chemical and electrical properties of the metal.
Annealing involves heating a material above its recrystallization temperature, maintaining it at this temperature for a specified time, and then cooling it at a previously determined rate. This solution is used in many industries, from construction and manufacturing to automotive and jewelry.
Why is annealing used?
The primary goal of annealing is to change the mechanical properties of the metal, making it softer and more malleable. Annealing reduces the risk of cracks, deformations, and other material damages. Hard, brittle metals can also cause excessive wear or damage to tools and machines - full annealing reduces this impact.
As we mentioned, annealing of metals is often used after cold working processes such as drawing, grinding, rolling or bending, to reduce its effects and allow further processing of the material. It is also commonly used to remove stresses that occur after welding. In such a case, stress-relieving annealing is used.
Annealing types - what are the most popular?
Metals can be annealed in several different ways. The decision on which type of treatment should be applied to a specific steel alloy should be determined, among other things, by the properties we want to achieve through such a process. Let's take a look at a few popular types of annealing.
- Full annealing
During the application of this method, metal elements are heated to a temperature about 30°C higher than their critical transformation temperature. The material is heated for a certain period of time, after which it is allowed to cool naturally. The purpose of full annealing is to remove internal stresses, improve machinability, and reduce the hardness of steel.
- Softening (spheroidizing) annealing
Annealing is applicable to steels containing more than 0.8% carbon. The elements are heated to a temperature of about 650°C and maintained at this temperature for a certain time to transform the microstructure of the material. Then the metal is very slowly cooled to a temperature of 600°C, and beyond that, it can cool in any way. Softening annealing is usually performed on elements that have been hardened by strain to enable their further processing. The resulting product has improved ductility and strength with reduced hardness.
- Isothermal Annealing
In isothermal annealing, steel is heated above the upper critical temperature, which allows for uniform austenitization of the entire element. Then the material is quickly cooled to a temperature of about 550 °C and maintained at this level for some time. The final stage is the natural, slow cooling of the metal. This process reduces the hardness of the steel.
- Stress Relief Annealing
Often used in the case of stainless or austenitic steel. During stress-relieving annealing, the elements subjected to it are heated below the lower critical line, but without changing the material's microstructure, and then cooled very slowly. This process eliminates any casting, welding, and thermal stresses.
- Normalizing annealing of steel
This method is used to increase the ductility and strength of metal. The normalizing annealing of steel involves heating the material to a temperature about 30°C higher than their critical transformation temperature, soaking, and then cooling to room temperature.
Which metals can benefit from the annealing process?
Annealing is most commonly used in the case of steel, but it is also possible to use this process for the treatment of aluminum, copper, brass, and some alloys. Different types of annealing are applicable to different materials and in different situations.
A good example of the application of the described method are sheets that have undergone the cold rolling process and require further processing, such as bending or punching. Annealing can also be applied to many elements previously exposed to high temperatures (e.g. during welding), causing residual stresses. There are many examples, therefore, when commissioning heat treatment, it is always worth asking specialists whether the chosen method will be suitable for a specific material and its further use.