It is well known that expansion or contraction occurs when heat or cooling is in the steel, and in addition to expansion and contraction in phase transition. When the workpiece is heated or cooled, it is impossible to be heated or cooled evenly both inside and outside, and there is a temperature difference inside and outside the workpiece, which causes the ratio tolerance. Similarly, when the workpiece is heated or cooled, it is not possible for the heart and the surface to occur at the same time, thus causing the ratio tolerance.
These specific tolerances are the main causes of internal stress during heat treatment.
Among them, the internal stress caused by the temperature difference between the workpiece is called "thermal stress", and the stress caused by the change of the internal and external microstructure of the workpiece is called "tissue stress", in addition to the stress caused by the difference of microstructure along the section of workpiece. The residual internal stress after heat treatment of forgings is the result of the comprehensive action of the above internal stress. When the workpiece is heated, the heating time is longer, the heat preservation time is sufficient, and the workpiece has good plasticity at high temperature, so it can be thought that the thermal stress and the tissue stress can be relaxed by the process of plastic deformation, recovery and recrystallization in the workpiece. Therefore, it can be concluded that the residual of the workpiece after heat treatment is only the superposition result of thermal stress and tissue stress in the cooling process. In this way, the problem is easier.