What is annealing of steel?
Annealing of steel: Annealing is a heat treatment process in which the steel is heated to a temperature above or below the critical point AC1, held for a certain period of time and then slowly cooled in the furnace to obtain a structure close to an equilibrium state.
Types of annealing:
According to the heating temperature, it 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 relief annealing. According to the cooling method, it can be divided into Isothermal annealing and continuous cooling annealing.
Annealing purposes:
1. Complete annealing: Complete annealing is a heat treatment process in which the steel is heated to 20-30°C above AC3, kept warm for a long enough time to completely austenitize the structure, and then slowly cooled in the furnace to obtain a structure close to an equilibrium state. It is mainly used in hypoeutectoid steel. Its purpose is to refine the grains, eliminate internal stress and work hardening, improve plastic toughness, uniform the chemical composition and structure of steel, improve the cutting performance of steel, and eliminate defects in medium carbon structural steel. Defects such as Widmanstatten tissue and band tissue.
2. Incomplete annealing: Incomplete annealing is to heat the steel to between AC1-AC3 (hypoeutectoid steel) or AC1-ACcm (hypereutectoid steel), hold it for a certain period of time and then slowly cool it with the furnace to obtain a state close to equilibrium. Tissue heat treatment process. For hypoeutectoid steel, if the original structure distribution of the steel is appropriate, incomplete annealing can be used instead of complete annealing to eliminate internal stress and reduce hardness. For hypereutectoid steel, incomplete annealing is mainly to obtain a spherical pearlite structure to eliminate internal stress, reduce hardness, and improve cutting performance.
3. Spheroidizing annealing: Spheroidizing annealing is a heat treatment process that spheroidizes carbides in steel and obtains granular pearlite. Mainly used for eutectoid steel, hypereutectoid steel and alloy tool steel. Its purpose is to reduce hardness, improve cutting performance, uniformize the structure, and prepare the structure for quenching.
4. Homogenization annealing: Also known as diffusion annealing, it is a heat treatment process in which steel ingots, castings or forging billets are heated to a temperature slightly lower than the solidus line and kept warm for a long time, and then slowly cooled to room temperature. Its purpose is to eliminate dendrite segregation and regional segregation produced during the solidification process of ingots or castings, and to homogenize the composition and structure.
5. Recrystallization annealing: A heat treatment process in which the cold-deformed metal is heated above the recrystallization temperature for an appropriate time, and then slowly cooled to room temperature. Its purpose is to transform the deformed grains into uniform equiaxed grains, eliminate work hardening and residual internal stress, and restore the structure and properties of the steel to the state before cold deformation.
6. Stress relief annealing: A heat treatment process in which the cold deformed metal is heated to a temperature below the recrystallization temperature, kept warm for a period of time and then slowly cooled to room temperature. Its main purpose is to eliminate residual internal stress (mainly the first type of internal stress) in castings, forgings, weldings and machined workpieces, so as to improve dimensional stability and reduce the tendency of workpieces to deform and crack.
