Annealing is a metal heat treatment process in which the metal is slowly heated to a certain temperature for a sufficient period of time and then cooled at a suitable rate. Annealing heat treatment is divided into complete annealing, incomplete annealing and stress relief annealing. The mechanical properties of the annealed material can be tested by tensile testing or by hardness testing. Many steels are supplied in an annealed heat treatment state. The hardness test of steel can be tested by Rockwell hardness tester. For thinner steel plates, steel strips and thin-walled steel pipes, surface Rockwell hardness tester can be used to test HRT hardness.
The purpose of annealing is to:
1, improve or eliminate various structural defects and residual stresses caused by steel during casting, forging, rolling and welding to prevent deformation and cracking of the workpiece.
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 organization for the final heat treatment (quenching, tempering).
Common annealing processes are:
1, fully annealed. It is used to refine the coarse superheated structure of medium and low carbon steel which has poor mechanical properties after casting, forging and welding. The workpiece is heated to a temperature of 30 to 50°C above the temperature at which all of the ferrite is transformed into austenite, and is kept for a period of time, then slowly cooled with the furnace, the austenite is transformed again during the cooling process, so that the microstructure of the steel is thinned.
2, spheroidizing annealing. It is used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to a temperature of 20 to 40°C above the temperature at which the steel begins to form austenite, and is slowly cooled after the heat preservation. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby lowering 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 cutting. Generally, it is cooled to the most unstable temperature of austenite at a relatively rapid rate, and the austenite is transformed into torsite or sorbite at a suitable temperature for a suitable period of time, and the hardness can be lowered.
4, recrystallization annealing. It is used to eliminate the hardening phenomenon (hardness increase, plasticity drop) of metal wire and sheet during cold drawing and cold rolling. The heating temperature is generally 50 to 150°C below the temperature at which the 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 make cast iron containing a large amount of cementite into a malleable cast iron with good plasticity. The process operation is to heat the casting to about 950°C, and after proper cooling for a certain period of time, the cementite is decomposed to form a group of 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 to keep the steel for a long time, and the various elements in the alloy tend to be uniformly distributed and then slowly cooled.
7, stress relief annealing. It is ued to eliminate the internal stress of steel castings and welded parts. After the steel product is heated, the austenite is formed at a temperature of 100 to 200°C below, and after cooling, it is cooled in the air to eliminate the internal stress.
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