What is annealing of steel? What are the types and USES of annealing?

- Oct 28, 2019-

What is annealing of steel? What are the types and USES of annealing?

Annealing of steel: annealing is a heat treatment process in which the steel is heated to the temperature above or below the critical point of AC1 and then slowly cooled with the furnace after holding for a certain period of time in order to obtain nearly equilibrium structure. Annealing type: according to the heating temperature, annealing can be divided into annealing above or below the critical temperature AC1. The former includes complete annealing, incomplete annealing, spheroidization annealing and homogenization annealing; the latter includes recrystallization annealing and stress removal annealing. According to the cooling mode, annealing can be divided into isothermal annealing and continuous cooling annealing.

 

Annealing purpose:

1. Full annealing: full annealing is a heat treatment process of heating steel to 20-30℃ above AC3, holding the steel for a long enough time to make the structure completely austenitized and then slowly cooling with furnace to obtain nearly equilibrium state of structure. It is mainly used in hypoeutectoid steel, whose purpose is to refine grain, eliminate internal stress and work hardening, improve ductility, chemical composition and structure of uniform steel, improve the cutting performance of steel, and eliminate the defects such as weil structure and banded structure in medium carbon structural steel.

2. Incomplete annealing: incomplete annealing is a heat treatment process in which the steel is heated to ac1-ac3 (hypoeutectoid steel) or ac1-accm (hypoeutectoid steel). After a certain period of heat preservation, the steel is slowly cooled in the furnace to obtain nearly equilibrium structure. For hypoeutectoid steel, if the original structure distribution of the steel is suitable, incomplete annealing can be used instead of complete annealing to eliminate the internal stress and reduce the hardness. For hyperprecipitation steel, incomplete annealing is mainly to obtain spherical pearlite structure, in order to eliminate internal stress, reduce hardness and improve cutting performance.

3. Spheroidization annealing: spheroidization annealing is the heat treatment process of spheroidization of carbide in steel to obtain granular pearlite. Mainly used in eutectoid steel, eutectoid steel and alloy tool steel. Its purpose is to reduce hardness, improve cutting performance, uniform structure, and prepare for quenching.

4. Homogenization annealing: also known as diffusion annealing, which is a heat treatment process that heats steel ingots, castings or forging billets to a temperature slightly lower than the solid phase line for a long time and then slowly cools them to room temperature. Its purpose is to eliminate dendrite segregation and regional segregation in the solidification process of ingots or castings so as to homogenize the composition and structure. 5. Recrystallization annealing: heat the cold deformed metal above the recrystallization temperature for an appropriate time, and then slowly cool it to room temperature. The purpose is to convert the deformed grain to uniform equiaxed grain, eliminate work hardening and residual internal stress, and restore the structure and properties of steel to the state before cold deformation.

5. Recrystallization annealing: a heat treatment process in which a cold deformed metal is heated above the recrystallization temperature for an appropriate period of time and then slowly cooled to room temperature. The purpose is to convert the deformed grain to uniform equiaxed grain, eliminate work hardening and residual internal stress, and restore the structure and properties of steel to the state before cold deformation.

6. De-stress annealing: a heat treatment process in which the cold deformed metal is heated to a temperature below the recrystallization temperature, held for a period of time, and then slowly cooled to room temperature. Its main purpose is to eliminate residual internal stress (mainly the first kind of internal stress) in castings, forged and rolled parts, welded parts and machined parts, so as to improve dimensional stability and reduce the tendency of deformation and cracking of the parts.

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