Introduction to heat treatment workers

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, and after a certain period of heat preservation, the steel is cooled slowly with the furnace to obtain a nearly equilibrium structure.

Annealing types: according to the heating temperature can be divided into annealing above or below the critical temperature AC1, the former includes complete annealing, incomplete annealing, spheroidizing annealing, uniform annealing, the latter includes recrystallization annealing, stress-relieving annealing, according to the cooling mode can be divided into isothermal annealing and continuous cooling annealing.

Annealing purpose:

1. Complete annealing: complete annealing is a heat treatment process in which the steel is heated to 20-30℃ above AC3 for a long time enough to allow the austenitizing of the structure to be completed and then cooled slowly with the furnace to obtain a nearly equilibrium structure. It is mainly used in the subeutectoid steel, its purpose is to refine grain, eliminate internal stress and work hardening, improve plastic toughness, uniform steel chemical composition and structure, improve the cutting performance of steel, eliminate the defects in the medium carbon structure steel, such as the weytenstein structure and banded structure.

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

3. Spheroidizing annealing: spheroidizing annealing is a heat treatment process for spheroidizing carbide in steel to obtain granular pearlite. Mainly used for eutectoid steel, hypereutectoid steel and alloy tool steel. Its purpose is to reduce the hardness, improve the cutting performance, uniform structure, for quenching to do tissue preparation.

4. Homogeneous annealing: also known as diffusion annealing, it is a heat treatment process in which ingot, casting or forged rolling billet are heated to a temperature slightly lower than the solid-phase line for a long time for insulation, and then cooled slowly to room temperature. The aim is to eliminate dendrite segregation and regional segregation during solidification of ingot or casting, and to homogenize 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 cooled slowly to room temperature. The aim is to transform the deformed grain into uniform and equiaxed grain, eliminate the hardening and residual internal stress, and restore the microstructure and properties of steel to the state before cold deformation.

6. Stress-relieving 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 cooled slowly to room temperature. Its main purpose is to eliminate the residual internal stress (mainly the first type of internal stress) in castings, forging and rolling parts, welding parts and machining parts, so as to improve the dimensional stability and reduce the tendency of deformation and cracking of the workpiece.