1. What is annealing?
Annealing is a metal heat treatment process in which the metal is slowly heated to a certain temperature, kept for a sufficient time, and then cooled at an appropriate speed. Annealing heat treatment is divided into complete annealing, incomplete annealing and stress relief annealing. The mechanical properties of annealed materials can be tested by tensile test or hardness test. Many steels are supplied in the annealed heat treatment state. The hardness test of steel can be tested by Rockwell hardness tester to test HRB hardness. For thinner steel plates, steel strips and thin-walled steel pipes, surface Rockwell hardness tester can be used to test HRT hardness.

2. The purpose of annealing is:
① Improve or eliminate various structural defects and residual stresses caused by steel in the casting, forging, rolling and welding process to prevent deformation and cracking of workpieces.
② Soften the workpiece for cutting.
③ Refine the grains and improve the structure to improve the mechanical properties of the workpiece.
④ Prepare the structure for the final heat treatment (quenching and tempering). 3. Common annealing processes are:
① Complete annealing.
It is used to refine the coarse overheated structure with poor mechanical properties of medium and low carbon steel after casting, forging and welding. Heat the workpiece to 30-50℃ above the temperature at which all ferrite is transformed into austenite, keep it warm for a period of time, and then slowly cool it with the furnace. During the cooling process, austenite will transform again, which can make the steel structure finer.
② Spheroidizing annealing.
It is used to reduce the high hardness of tool steel and bearing steel after forging. Heat the workpiece to 20-40℃ above the temperature at which steel begins to form austenite, keep it warm and then cool it slowly. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby reducing the hardness.
③ Isothermal annealing.
It is used to reduce the high hardness of some alloy structural steels with high nickel and chromium content for cutting processing. Generally, it is cooled to the most unstable temperature of austenite at a relatively fast speed, and kept warm for an appropriate time. Austenite is transformed into troostite or troostite, and the hardness can be reduced.
④ Recrystallization annealing.
It is used to eliminate the hardening phenomenon (increased hardness and decreased plasticity) 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 begins to form austenite. Only in this way can the work hardening effect be eliminated and the metal softened.
⑤ Graphitization annealing.
It is used to turn cast iron containing a large amount of cementite into forgeable cast iron with good plasticity. The process operation is to heat the casting to about 950℃, keep it warm for a certain period of time, and then cool it appropriately to decompose the cementite to form flocculent 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 the highest possible temperature without melting, and keep it warm for a long time, and then slowly cool it after the various elements in the alloy diffuse and tend to be evenly distributed.
⑦ Stress relief annealing.
It is used to eliminate the internal stress of steel castings and welded parts. For steel products, internal stress can be eliminated by heating them to a temperature below 100-200°C at which austenite begins to form and then cooling them in air after keeping them warm.