Cooling of forgings refers to cooling from the final forging temperature to room temperature after forging. If the cooling method is not chosen properly, the forgings may be scrapped due to cracks or white spots, and productivity may be affected by extending the production cycle. Therefore, forging cooling is also an important link in forging production. Forging internal stress in the cooling process: the billet will produce internal stress in the heating process, and the forging will also cause internal stress in the cooling process. Because the forgings are in the elastic state with low temperature in the later cooling period, the risk of cooling internal stress is greater than that of heating internal stress. According to the different causes of internal stress during cooling, there are temperature stress, tissue stress and residual stress.

1. Temperature stress in the early stage of forging cooling, surface cooling fast, large volume shrinkage; The cooling of the core is slow and the volume shrinks. As the shrinkage of the surface is hindered by the heart, the temperature stress is generated inside the forging, the surface is tensile stress, and the heart is compressive stress. If the forging material is mild steel with small resistance and easy deformation, with continued cooling, the tensile stress generated on the surface at the early stage of cooling is gradually reduced to zero by deformation relaxation. At the later stage of cooling, the surface temperature is very low, and the volume shrinkage stops, while the core volume shrinkage is restricted by the surface layer. As a result, the temperature stress symbol changes, the surface layer becomes compressive stress, and the core becomes tensile stress. If hard steel forgings materials for resistance to great deformation, at the beginning of the cooling surface of the tensile stress can not relax, late is cooling, the volume shrinkage of although the core attached on the surface pressure stress, but also can make the surface early produces tensile stress decreases, and won't cause changes in the temperature stress of symbols, the surface is still the tensile stress, the heart is still the compressive stress. Therefore, mild steel forgings may appear internal crack when cooling, and hard steel forgings are easy to produce external crack when cooling.

2. Organizational stress forgings in the cooling process, such as phase transformation, in addition to causing temperature stress, will also produce organizational stress, is also due to the change of the specific capacity of the organization before and after the phase transformation, and the result of different phase transformation time in the forging table. Such as forgings in the cooling process of martensite transformation, with the temperature of the forgings reduced, the surface of the martensite transformation, due to martensite specific capacity is greater than the specific capacity of austenite, then the organizational stress caused by the surface is compressive stress, the heart is tensile stress. But at this time, the core temperature is relatively high, in a good plastic austenite state, through local plastic deformation, the above stress is quickly relaxed. Then the forging continued to cool, martensitic transformation occurred in the heart, then the organizational stress, the heart is compressive stress, the surface layer is tensile stress. The stress increases until the martensite transformation is complete. As the specific volume of all phases in the steel is larger than that of austenite, the microstructure stress produced by other microstructure changes during the cooling of the forging also has the above law.
3.residual stress forgings in the forging process, due to uneven deformation of the internal stress caused by work hardening, such as the end can be timely recrystallization softening to eliminate it, after forging will become residual stress retained. The distribution of residual stress in the forging inside the declaration party according to the uneven deformation in the declaration party. It may be tensile stress on the surface and compressive stress in the center, or vice versa. It can be seen that there are three kinds of internal stresses mentioned above in the cooling process after forging, and the total internal stresses are superimposed by the three. When the superimposed stress value exceeds the strength limit, it will cause cracks in the corresponding parts of the forging, cooling cracks often occur when the temperature is low and the plasticity is poor:. Such as the superposition of internal stress did not cause damage, cooling end will be retained, known as the residual stress of the forging.