The stress of heat treatment of steel

Artifacts in the process of heating and cooling, due to the surface and center of the cooling rate and time inconsistency, formation temperature, will lead to uneven volume expansion and contraction stress, thermal stress. Under the action of thermal stress, as surface temperature is lower than the core, and make the heart of contraction is greater than the core department in tension, when at the end of the cooling, due to the heart of the final cooling volume shrinkage can't free and make the surface tension of compression of the core. Eventually make the workpiece under the action of thermal stress in the surface tension of compression and the core. This phenomenon is by the cooling rate, material composition and the influence of factors such as heat treatment process. When the faster the cooling rate, the higher the carbon and alloy elements, under the action of thermal stress during the cooling process of inhomogeneous plastic deformation is larger, the final formation of the residual stress is big. Steel in the process of heat treatment on the other hand due to the change of organization of austenite to martensite transformation, due to the increase of the volume will be accompanied by the expansion of the size of workpiece, workpiece parts successively phase transition, grew up volume caused by inconsistent and organization stress generation. Organizational stress changes of the end result is that the surface tensile stress, heart of compressive stress, just opposite to the thermal stress. Organizational stress and the size of the workpiece in the cooling speed of martensite phase transformation area, shape, material chemical composition and other factors. Practice has proved that any artifacts in the process of heat treatment, as long as there is phase change, the thermal stress and organization stress will occur. Only the thermal stress in the organization transformation had produced, and organizational stress is produced in the process of organizational change, in the cooling process, a result of the combined action of thermal stress and organization stress, is the real stress in the workpiece. A result of the combined action of these two kinds of stress is very complex, influenced by many factors, such as composition, shape, heat treatment, etc. In terms of its development process there are only two types, namely, the thermal stress and organization stress, role in the opposite direction when the offset, the direction phase superposition on each other at the same time. Offset each other and mutual superposition, the two stress should be a dominant factor, the thermal stress results when the dominant role in the tension of workpiece is the core and surface pressure. Organizational stress results when the dominant role in the compression of the center of workpiece surface tension.