1. What is annealing?
Annealing is a metal heat treatment process in which the metal is slowly heated to a certain temperature, kept for sufficient time, and then cooled at an appropriate rate.Annealing 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 annealed heat treatment. Rockwell hardness tester can be used to test HRB hardness. For thinner steel plates, steel strips and thin-walled steel tubes, rockwell hardness tester can be used to test HRT hardness.
2. The purpose of annealing is:
(1) improve or eliminate steel in the casting, forging, rolling and Welding process caused by various structural defects and residual stress, prevent the workpiece deformation, cracking.
(2) soften the workpiece for cutting.
(3) refine the grain, improve the structure to improve the mechanical properties of the workpiece.
(4) prepare the organization for the final heat treatment (quenching, tempering).
3. Common annealing processes include:
(1) complete annealing.It is used to refine the thick superheated structure of medium and low Carbon steel after casting, forging and welding.The workpiece is heated to 30 ~ 50℃ above the temperature at which all ferrite is transformed into austenite. The workpiece is kept warm for a period of time, and then slowly cooled with the furnace. In the cooling process, austenite is transformed again, which can make the microstructure of the steel thin.
(2) spheroidization annealing.Used to reduce the high hardness of tool steel and bearing steel after forging.The workpiece is heated to 20 ~ 40℃ above the temperature at which the steel begins to form austenite. After insulation, it is slowly cooled. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thus reducing the hardness.
(3) isothermal annealing.Used to reduce the high hardness of some alloy structural steels with high nickel and chromium content for cutting.In general, the austenite is cooled to the most unstable temperature of austenite at a relatively fast speed. After proper time of heat preservation, austenite is transformed into totorite or sostenite, and the hardness can be reduced.
(4) recrystallization annealing.To eliminate metal wire, thin plate in the process of cold drawing, Cold rolling hardening phenomenon (hardness increase, plastic decline).The heating temperature is generally 50 ~ 150℃ 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.
Graphitization annealing.Used to make cast iron with large amount of cementite into malleable iron with good plasticity.The process operation is to heat the casting to about 950℃, hold the casting for a certain time, and then cool it appropriately, so that the cementite can decompose to form flocculent graphite.
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. After the diffusion of various elements in the alloy tends to be evenly distributed, the casting is cooled slowly.
Stress annealing.To eliminate the internal stress of steel casting and welding parts.For iron and steel products, the temperature of austenite after heating is less than 100 ~ 200℃, and the austenite can be cooled in the air after heat preservation to eliminate the internal stress.
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