Heat treatment stress of steel parts

Brief introduction of stress produced in heat treatment of steel parts

The stress produced in heat treatment of steel parts includes the first, second and third types of internal stress, that is, the macro internal stress (the first type of internal stress) between various parts of steel parts, the stress within the scope of grain (the second type of stress), and the internal stress of lattice (the third type of stress). Only the first type of internal stress is enough to cause distortion and crack of the workpiece. This chapter discusses the internal stress within the first type Stress.

In the process of heating and cooling, there will be volume change of expansion and contraction, and volume change due to the tolerance between the old and the new when the structure changes. Due to the process of heat conduction, the surface of the workpiece is heated or cooled before the center, resulting in the temperature difference between the parts on the section, resulting in the above volume change of the surface and the center of the steel workpiece at the same time. The internal stress is formed by the volume change of each part. The greater the heating or cooling speed is, the greater the temperature difference on the workpiece section is, and the greater the internal stress is.

The internal stress formed in the process of heat treatment is the changing instantaneous stress. The internal stress existing in the workpiece after heat treatment is the residual internal stress. The instantaneous stress changes its size and direction and distribution state with the change of temperature and the process of structure transformation.

According to the cause of internal stress formation, Changzhou heat treatment divides it into thermal stress caused by uneven expansion and contraction caused by temperature difference and tissue stress caused by different phases of phase transformation and uneven structure.

In fact, the stress of heat treatment has been active, which is one of the important factors of deformation and cracking of steel parts; however, it also has its advantages, for example, after surface strengthening heat treatment, shot peening, rolling and other processes, it can create beneficial residual stress distribution on the surface of the workpiece, which can effectively improve the mechanical properties, especially greatly improve the fatigue strength and prolong the service life of the workpiece.

It is of great significance to find out the cause of internal stress and its changing law for giving full play to its favorable role, controlling and eliminating its harmful role deformation and cracking.