Introduction to Annealing Process

Annealing is a crucial metal heat treatment process that involves heating the material to a specific temperature, maintaining it for a certain period, and then cooling it slowly to improve material properties. The essence of annealing is to heat the steel to an austenitic state and then undergo a pearlite transformation, resulting in a structure that is close to equilibrium.

The purposes of annealing are mainly threefold: first, to reduce the hardness of steel and increase its plasticity for ease of machining and cold deformation processing; second, to homogenize the chemical composition and structure of the steel, refine the grains, thereby improving the performance of the steel or preparing the structure for subsequent quenching treatment; and lastly, to eliminate internal stresses and work hardening to prevent deformation and cracking during processing and use.

Annealing and normalizing are commonly used for preliminary heat treatment, but they can also serve as final heat treatment for parts that are not subject to significant stress and have low performance requirements. Annealing methods can be divided into two categories based on the heating temperature: one is phase transformation recrystallization annealing above the critical temperature (Ac1 or Ac3), which includes full annealing, diffusion annealing, incomplete annealing, and spheroidizing annealing; the other is annealing below the critical temperature, such as recrystallization annealing and stress relief annealing.

Here is a detailed explanation of the seven types of annealing methods:

1. 1. Full Annealing: Suitable for hypoeutectoid steel, it involves heating to 20-30°C above Ac3, maintaining the temperature, and then slowly cooling with the furnace to obtain an equilibrium structure. The purpose is to refine grains, homogenize the structure, eliminate internal stresses, reduce hardness, and improve machinability.
2. 2. Isothermal Annealing: Suitable for steels where austenite is stable, it involves heating to a temperature above Ac3 (or Ac1), quickly cooling to the pearlite region for isothermal holding, and then air cooling. The purpose is to shorten the annealing time and control the structural transformation.
3. 3. Incomplete Annealing: Mainly used for hypereutectoid steel, it involves heating to between Ac1 and Accm, maintaining the temperature, and then slowly cooling to obtain a spheroidized pearlite structure, reduce hardness, and improve machinability.
4. 4. Spheroidizing Annealing: Used to spheroidize the carbides in steel, suitable for eutectoid and hypereutectoid steel, it involves heating to 20-30°C above Ac1, maintaining the temperature, and then furnace cooling or isothermal treatment to reduce hardness, homogenize the structure, and improve workability.
5. 5. Diffusion Annealing: Used to eliminate dendritic and regional segregation in cast ingots, it involves heating to below the solidus temperature for long-term holding and then slow cooling.
6. 6. Stress Relief Annealing: Involves heating to 500-650°C, maintaining the temperature, and then cooling with the furnace to eliminate residual internal stresses without causing structural changes.
7. 7. Recrystallization Annealing: Suitable for metals after cold deformation, it involves heating to above the recrystallization temperature, holding for an appropriate time to eliminate work hardening and residual stresses.

The principles for selecting annealing methods include: hypoeutectoid steels generally use full annealing, hypereutectoid steels use spheroidizing annealing, recrystallization annealing is used to eliminate work hardening, stress relief annealing is used to eliminate internal stresses, and diffusion annealing is often used for large castings of high-quality alloy steels to improve the structure and composition uniformity.