What Is Annealing? | Purpose, and Process of Annealing

Annealing is a metal heat treatment process, which refers to slowly heating the metal to a certain temperature, maintaining it for a sufficient time, and then cooling it at an appropriate speed. In a broad sense, annealing is a heat treatment process for materials, including metal materials and non-metal materials. In addition, the annealing purpose of new materials is also different from that of traditional metals. In this article, we’ll introduce what is annealing, and the purpose, and process of annealing.

What is annealing?

Annealing is a metal heat treatment process that slowly heats the metal to a certain temperature, maintains it for a sufficient time, and then cools it at an appropriate speed. Annealing heat treatment is divided into full annealing, partial annealing, and stress-relieving annealing. The mechanical properties of annealed materials can be detected by tensile testing or hardness testing. Many steel materials are supplied in the annealed heat treatment state. Steel hardness testing can use Rockwell hardness tester to test HRB hardness. For thinner steel sheets, steel strips, and thin-walled steel pipes, surface Rockwell hardness tester can be used to detect HRT hardness.

What’s the purpose of annealing?

The purpose of annealing is to:

① improve or eliminate various microstructure defects and residual stresses caused by casting, forging, rolling, and welding of iron and steel, and prevent workpiece deformation and cracking.

② soften the workpiece for machining.

③ refine the grain and improve the microstructure to enhance the mechanical properties of the workpiece.

④ prepare the microstructure for the final heat treatment (quenching and tempering).

What are the commonly used annealing processes?

The commonly used annealing processes are:

① Complete annealing. It is used to refine the coarsely large superheated structure with poor mechanical properties in medium and low carbon steels after casting, forging, rolling, and welding. The workpiece is heated to 30-50℃ above the temperature at which the ferrite completely transforms into austenite, held at this temperature for a period of time, and then cooled slowly in the furnace. During the cooling process, the austenite transforms again, which can refine the microstructure of the steel.

② Spherical annealing. It is used to reduce the excessively high hardness of tool steel and bearing steel after forging. The workpiece is heated to 20-40℃ above the temperature at which the steel starts to form austenite, held at this temperature for a period of time, and then cooled slowly. During the cooling process, the lamellar cementite in the pearlite transforms into globular, thus reducing the hardness.

③ Isothermal annealing. It is used to reduce the high hardness of certain alloy structural steels with high nickel and chromium content for machining. Generally, it is first cooled to the most unstable temperature of austenite at a relatively fast rate, held at this temperature for a suitable period of time, and then transformed into theta or so-theta phase, thus reducing the hardness.

④ Recrystallization annealing. It is used to eliminate the hardening phenomenon (increase in hardness and decrease in ductility) of metal wires and thin plates during cold drawing and cold rolling. The heating temperature is generally 50-150℃ below the temperature at which steel starts to form austenite. Only in this way can the processing hardening effect be eliminated to soften the metal.

⑤ Graphitization annealing. It is used to turn cast iron with a large amount of cementite into malleable cast iron. The process operation is to heat the cast iron to around 950℃, hold it at this temperature for a certain period of time, and then cool it appropriately to allow the cementite to decompose into flaky graphite.

⑥ Diffusion annealing. It is used to homogenize the chemical composition of alloy castings and improve their performance. The method is to heat the casting to as high a temperature as possible without melting, hold it at this temperature for a long time, and then slow cooling after various elements in the alloy have diffused evenly.

⑦ Stress relieving annealing. It is used to eliminate internal stress in steel castings and welded joints. For steel products, it is heated to below 100-200℃ above the temperature at which iron starts to form austenite, held at this temperature for a period of time, and then cooled in air to eliminate internal stress.

Conclusion

Thank you for reading our article and we hope it can help you to have a better understanding of what is annealing. If you want to learn more about stainless steel, we’d advise you to visit Sino Stainless Steel for more information.

As a leading supplier of stainless steel products across the world, Sino Stainless Steel provides customers with high-quality stainless steel products such as stainless steel strips, stainless steel coils, stainless steel plates, stainless steel sheets, stainless steel bars, and stainless steel wires at a very competitive price.