The internal stress of aluminum alloy forgings during heat treatment has thermal stress and phase transformation stress.
1. Thermal stress
Thermal stress is the phenomenon that the forging is accompanied by thermal expansion and cold shrinkage during heating and cooling. When the temperature difference between the forging surface and the heart is caused by different heating or cooling rates, its volume expansion or contraction is different from that of the heart surface. The internal stress caused by volume change caused by temperature difference is called thermal stress.
In the process of heat treatment, the thermal stress changes of forgings are mainly shown as follows: when the forgings are heated, the surface heating speed is faster than the heart, the surface temperature is high, the heart temperature is low, there is no expansion, the surface pressure stress, and the heart tensile stress.
When the forging is diathermic, the heart temperature rises and expands, and the volume of the forging expands; When the workpiece is cooled, the surface cooling speed is faster than the heart, the surface should shrink, the heart temperature is high, and the surface stress, the heart stress, which still exists in the forging after cooling, is called residual stress.
2. Phase transformation stress
The transformation stress is that when the structure changes during the heat treatment of forgings, the quality and volume of forgings will inevitably change due to the different quality and volume of different structures. Due to the temperature difference between the forging surface and the heart, the change of the surface and the heart tissue is not timely, so the change of the internal and external mass volume will produce internal stress. This kind of internal stress caused by the difference of structure change is called transformation stress.
According to the order of austenite, pearlite, sorbite, troostite, lower bainite, tempered martensite and martensite, the mass volume of basic structure in steel increases in turn.
For example, when the forging is quenched and rapidly cooled, because the surface first cools to other points, the surface changes from austenite to martensite, but the heart is still in the austenitic state to prevent surface expansion, so the center of the forging is subject to tensile stress and surface compressive stress; When cooling continues, the surface temperature decreases and does not expand any more. The heart becomes martensite and the volume continues to expand, so the surface stops, so the heart is under compression stress and the surface tension stress. After cooling, the stress still exists in the forging and becomes residual stress.
During quenching and cooling of forgings, the changes of thermal stress and transformation stress are opposite, and the two residual stresses in forgings are also opposite. The combined stress of thermal stress and transformation stress is called quenching internal stress. When the residual internal stress in the forging exceeds the yield point of the steel, the workpiece will produce plastic deformation, resulting in distortion of the forging.