On 2018-08-08 14:48:54
Heat treatment stress and its classification
The stress of heat treatment can be divided into two kinds: thermal stress and tissue stress. The distortion of heat treatment is the result of the combined action of thermal stress and tissue stress. The state of heat treatment stress in workpiece and its effect are different. The internal stress caused by the uneven heating or cooling is called thermal stress; the internal stress caused by the uneven transformation of the structure 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. After heat treatment, the final stress state and size of the workpiece depend on the sum of thermal stress, tissue stress and additional stress, which is called residual stress.
The distortion and crack formed during heat treatment are the result of the comprehensive action of these internal stresses. At the same time, under the action of heat treatment stress, sometimes one part of the workpiece will be in the state of tensile stress, while the other part will be in the state of compressive stress, sometimes it may make the stress distribution of each part of the workpiece very complex. This should be analyzed according to the actual situation.
1. thermal stress
Thermal stress is the internal stress produced by the uneven expansion and contraction of the volume between the surface of the workpiece and the center or between the thin part and the thick part due to the different heating or cooling speed in the process of heat treatment. Generally, the faster the heating or cooling speed is, the greater the thermal stress will be.
2. Organizational stress
The internal stress caused by the change of specific volume caused by phase transformation is called the tissue stress, which is also called the phase transformation stress. In general, the larger the specific volume and the greater the time difference before and after the transformation, the greater the tissue stress.
3. Additional stress
In the process of heat treatment, in addition to the formation of thermal stress and tissue stress, internal stress can also be formed due to the inhomogeneity of the structure at the surface and center of the workpiece and the inconsistency of the elastic distortion inside the workpiece, which is called additional stress. For example, carburization or decarburization of workpiece surface, surface quenching or local quenching, and other factors that can lead to uneven structure of workpiece surface and center can generate stress near the heat treatment.
(1) When surface quenching or additional stress local quenching or surface quenching (such as induction quenching, flame quenching, laser quenching, etc.), martensitic structure is formed only in the quenched part, and the part without quenching is still the original structure, resulting in the difference of specific volume of the whole work piece. At this time, the expansion caused by the increase of specific volume due to martensite on the surface of the workpiece is limited by the central part, so that the surface is subject to compressive stress and the central part is subject to tensile stress.
(2) In case of additional stress carburizing workpiece formed during carburizing and quenching, due to its high carbon content in the surface layer and low carbon content in the interior (original carbon content of steel), the transformation temperature of the surface layer and the core (i.e. MS point) is different (the transformation temperature of the surface layer is lower than that of the core). Therefore, the internal organization changes first and expands. At this time, the surface structure is still austenite, still in the plastic state. At the initial stage, the surface is under the action of tensile stress, and the center is under the action of compressive stress. Because of the excellent plasticity of the surface layer, plastic distortion is easy to occur under the action of tensile stress, which leads to stress relaxation, that is, the stress value is reduced. Later, when the high carbon surface layer also undergoes martensitic transformation and expands, the stress in the surface layer and the center is just opposite, that is, the surface is compressive stress, and the center is tensile stress.
4. Residual stress
As long as the transformation process is accompanied by the heat treatment, the thermal stress and the tissue stress will be produced at the same time. The final stress state of the workpiece depends on the sum of thermal stress, structural stress and additional stress. The internal stress retained after heat treatment is called residual stress. It is divided into residual tensile stress (indicated by "+") and residual compressive stress (indicated by "-").