Heat treatment stress can be 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 unequal timing 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 size of the workpiece after heat treatment depends on the sum of thermal stress, tissue stress and additional stress, which is called residual stress.
The distortion and cracks formed by the workpiece during heat treatment are the result of the combined effect of these internal stresses. At the same time, under the effect of heat treatment stress, sometimes one part of the workpiece is in a state of tensile stress, and the other part is in a state of compressive stress, and sometimes the distribution of the stress state of each part of the workpiece may be very complicated. This should be analyzed according to the actual situation.
1. Thermal stress
Thermal stress is the internal stress caused by uneven volume expansion and contraction caused by the difference in heating or cooling rate between the surface of the workpiece and the center or thin and thick parts during heat treatment. Generally, the faster the heating or cooling rate, the greater the thermal stress generated.
2. Tissue stress
The internal stress generated by the unequal time of specific volume change caused by phase transformation is called tissue stress, which is also called phase transformation stress. Generally, the larger the specific volume before and after the transformation of the tissue structure and the greater the time difference between the transitions, the greater the tissue stress.
Post time: Jul-07-2020