Annealing

Annealing

Annealing is a metal heat treatment process that refers to slowly heating a metal to a temperature for a sufficient period of time and then cooling at a suitable rate. The purpose is to reduce hardness, improve machinability, eliminate residual stress, stabilize size, reduce deformation and crack tendency, refine grains, adjust microstructure, and eliminate tissue defects. Accurately, annealing is a heat treatment process for materials, including metallic materials and non-metallic materials. Moreover, the annealing purpose of the new material is also different from the traditional metal annealing.

Definition

A metal heat treatment process that heats the metal to a temperature for a sufficient period of time and then cools it at a suitable rate (usually slow cooling, sometimes controlled cooling).

Purpose

(1) Reduce hardness and improve machinability.
(2) Eliminate residual stress, stabilize the size, and reduce the tendency of deformation and crack;
(3) Refine the grains, adjust the structure, and eliminate tissue defects.
(4) Uniform material organization and composition, improve material properties or prepare for later heat treatment.
In production, the annealing process is widely used. Depending on the purpose of the workpiece to be annealed, there are a variety of annealing specifications, such as full annealing, spheroidizing annealing, and stress relief annealing.

Annealing process

Specific process of spheroidizing annealing

1 Ordinary (slow cooling) spheroidizing annealing, slow cooling is suitable for most steel types, especially when the furnace capacity is large, the operation is convenient, but the production cycle is long; 2 isothermal spheroidizing annealing is suitable for most steel types, especially difficult Spheroidized steel and steel with high spheroidal quality requirements (such as rolling bearing steel); its production cycle is shorter than ordinary spheroidizing annealing, but it needs a furnace capable of controlling the cooling rate before eutectoid transformation; 3-cycle spheroidizing annealing is suitable for The steel whose original structure is lamellar pearlite structure has a shorter production cycle than ordinary spheroidizing annealing. However, under the condition of large equipment loading capacity, it is difficult to change the temperature according to the control requirements, so it is not widely used in production; 4 low temperature spheroidizing annealing, suitable for cold deformed steel and quench hardened steel (the latter is usually called high temperature softening and tempering); 5 deformation spheroidizing annealing, deformation processing accelerates spheroidization, deformation processing Combined with spheroidization, the spheroidization time can be shortened. It is suitable for cold and hot shaped steel and steel (such as strip).

Recrystallization annealing process

An annealing method applied to metals and alloys subjected to cold deformation processing. The purpose is to make the internal structure of the metal into fine equiaxed grains, eliminate the deformation hardening, and restore the plasticity and deformation ability (recovery and recrystallization) of the metal or alloy. If the surface of the metal or alloy is to be kept bright, recrystallization annealing can be carried out in a controlled atmosphere furnace or in a vacuum furnace.
Stress Relief Annealing Casting, forging, and weldments generate internal stresses due to different cooling rates at various locations during cooling. Metals and alloys also generate internal stresses during cold deformation and during workpiece machining. If the internal stress is large and not removed in time, the workpiece often deforms or even forms cracks. Stress relief annealing is to slowly heat the workpiece to a lower temperature (for example, gray cast iron is 500 ~ 550 ° C, steel is 500 ~ 650 ° C), holding for a period of time, the metal inside relax, and then slowly cooled down. It should be noted that the stress relief annealing does not completely remove the internal stress, but only partially removes it, thereby eliminating its harmful effects.
There are also some special annealing methods, such as stainless acid-resistant steel stabilized annealing; soft magnetic alloy magnetic field annealing; silicon steel sheet hydrogen annealing; malleable cast iron forging annealing.