It is well-known that steel expands or contracts when heated or cooled; in addition, it also expands or contracts during phase changes. When forging is heated or cooled, the internal and external parts of the forging cannot be uniformly heated or cooled at the same time, and there is a temperature difference between the inside and outside of the workpiece, leading to specific tolerances. Similarly, when the workpiece is heated or cooled, the core and surface of the workpiece cannot undergo tissue transformation simultaneously, which will also result in specific tolerances. These specific tolerances are the main cause of internal stresses during heat treatment.

"Within the workpiece, the internal stress caused by the difference in temperature between the inside and outside is referred to as 'thermal stress,' while the stress caused by the time difference in structure between the inside and outside is known as 'structural stress.' Additionally, there are also differences in structure along the cross-section of the workpiece. Emphasis is placed on this."

The residual internal stress in the workpiece after heat treatment is the result of the combined effect of the aforementioned internal stresses. When the workpiece is heated, with a longer heating time, sufficient holding time, and good plasticity at high temperatures, it can be considered that the thermal stresses and structural stresses during the heating process can be relieved through plastic deformation, recovery, and recrystallization during forging. Therefore, it can be concluded that what remains in the workpiece after heat treatment is just the superposition of thermal and structural stresses during the cooling process. Thus, the problem becomes simpler.

Three heat treatment processes for forgings include incomplete annealing, full annealing, and isothermal annealing.

Incomplete annealing is a heat treatment process where steel is heated to a certain temperature, held for a shorter duration, and then cooled slowly. The objective is to achieve a spheroidite and spheroidal carbide structure, reduce hardness, and enhance machinability. It is primarily used for cold die steels, tool steels, and bearing steels.

Complete annealing (commonly referred to as annealing) involves heating hypoeutectoid steel to 30~50°C, holding it to achieve full austenitization with a uniformly distributed composition, and then cooling it within the furnace, followed by a slow cool in sand or refractory earth powder to 600°C. It is air-cooled from the furnace at approximately room temperature to achieve a balanced microstructure.

The purpose of the forging is to reduce stress, lower hardness, increase plasticity, and enhance cutting performance; to remove coarse grains, modify the structure, and prepare the structure for subsequent heat treatment of the parts. Typically used in hypoeutectoid steels, such as 5CrMnMo, etc.

Isothermal annealing involves heating steel to 20~30℃ (hypoeutectoid steel), maintaining it fully austenitic and uniform, then rapidly cooling it below the temperature (i.e., the temperature at which austenite is unstable) while maintaining the temperature until the austenite completely transforms. Afterward, it is discharged from the furnace and air-cooled, or cooled together with the furnace, oil-cooled, or water-cooled. The purpose is to achieve a more uniform structure than complete annealing and to relieve forging stress. Compared to complete annealing, the annealing time is shortened and production efficiency is increased. It is suitable for hypoeutectoid, eutectoid, and hypereutectoid steels.

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