Heat treatment stress can be mainly divided into thermal stress and tissue stress. The heat treatment distortion of the workpiece is the result of the combined effect of thermal stress and tissue stress. The state of heat treatment stress in the workpiece and the effect it causes are different. The internal stress caused by uneven heating or cooling is called thermal stress; the internal stress caused by the anisochronism of tissue transformation is called tissue stress. In addition, the internal stress caused by the uneven transformation of the internal structure of the workpiece is called additional stress. The final stress state and stress magnitude of the workpiece after heat treatment depend on the sum of thermal stress, tissue stress and additional stress, which is called residual stress.

The distortion and cracks formed during the heat treatment of the workpiece are the result of the combined effects of these internal stresses. At the same time, under the action of heat treatment stress, sometimes a certain part of the workpiece will be in a state of tensile stress, while the other part will be in a state of compressive stress, which may sometimes make the distribution of stress states in various parts of the workpiece very complicated. In this regard, it should be analyzed according to the actual situation.

Thermal stress is the internal stress caused by the uneven volume expansion and contraction caused by the difference in heating or cooling speed between the surface and the center of the workpiece or between the thin part and the thick part during the heat treatment process. In general, the faster the rate of heating or cooling, the greater the resulting thermal stress.

The internal stress generated by the anisochronism of the specific volume change caused by the phase transition is called tissue stress, and the tissue stress is also called phase transition stress. Generally, the larger the specific volume before and after the transformation of the tissue structure, and the larger the time difference between the transformations of each part, the greater the tissue stress.

During the heat treatment process of the workpiece, in addition to the formation of thermal stress and tissue stress, the inhomogeneity of the surface and center of the workpiece and the inconsistency of the elastic distortion inside the workpiece can also form internal stress, which is called additional stress. For example, carburization or decarburization of the surface layer of the workpiece, surface quenching or partial quenching, and other factors that can cause uneven organization on the surface and center of the workpiece can all generate stress near the heat treatment.

(1) Additional stress formed during surface quenching or partial quenching When local quenching or surface quenching (such as induction hardening, flame quenching and laser quenching, etc.), only the martensite structure is formed in the quenched part, and the unquenched part is still The original tissue, resulting in a difference in specific volume across the workpiece. At this time, the expansion caused by the increase in specific volume due to the martensite on the surface of the workpiece is restricted by the central part, so that the surface is subjected to compressive stress and the center is subjected to tensile stress.

(2) Additional stress formed during carburizing and quenching When carburizing workpieces are quenched, because of the high carbon content in the surface layer and the low carbon content in the interior (the original carbon content of the steel), the phase transition temperature between the surface layer and the core ( That is, the Ms point) is different (the phase transition temperature of the surface layer is lower than that of the core). Therefore, the interior first undergoes organizational transformation and expands. At this time, the surface structure is still austenite and is still in a plastic state. The initial surface is subjected to tensile stress, and the core is subjected to compressive stress. Due to the excellent plasticity of the surface layer, plastic deformation is prone to occur under the action of tensile stress, resulting in stress relaxation, that is, the stress value is reduced. Subsequently, when the high-carbon surface layer also undergoes martensitic transformation and expands, the stress on the surface layer and the center is just opposite, that is, the surface is compressive stress, and the center is tensile stress.

As long as there is a phase transformation process during heat treatment, thermal stress and structural stress will be generated simultaneously. The final stress state of the workpiece depends on the sum of thermal stress, tissue stress and additional stress. The internal stress remaining after heat treatment is called residual stress. It is divided into residual tensile stress (indicated by “+”) and residual compressive stress (indicated by “-“).