Heat treatment distortion may occur after forgings undergo annealing, normalizing, quenching, tempering and surface modification heat treatment. The root cause of the distortion is the internal stress generated during the heat treatment of the forging, that is, due to the internal and external temperature difference and the asynchrony of the microstructure transformation, the forging after heat treatment has residual internal stress.
When this stress exceeds the yield point of the steel at a certain moment in the heat treatment process, it will cause distortion of the forging. The internal stress generated during heat treatment includes thermal stress and phase transformation stress, and their causes and effects are different.
When the forging is heated and cooled, it is accompanied by thermal expansion and contraction. When the surface and the core of the forging are heated or cooled at different rates, resulting in a temperature difference, the expansion or contraction of the volume will be different between the surface and the core. The internal stress caused by the asynchrony of temperature difference and volume change is called thermal stress.
During the heat treatment process of the forging, the change of thermal stress is mainly as follows: when the forging is heated, the surface heating rate is faster than that of the core, the surface temperature is high and it expands, and the core temperature is low and does not expand. At this time, the surface is under compressive stress, and the core is tensile stress. When the forging is heated, the core temperature rises and expands, and the forging exhibits volume expansion; when the workpiece cools, the surface layer cools faster than the core, and the surface shrinks. , the core will generate compressive stress. When it cools to a certain temperature, the surface layer will no longer shrink after cooling, and the core will shrink due to continued cooling. At this time, the surface layer is subject to compressive stress, while the core is subject to tensile stress. After cooling, this stress still exists inside the forging, which is called residual stress.
When the structural transition occurs during the heat treatment of the forging, the mass volume of the forging must change due to the different mass and volume of the different structures. Due to the temperature difference between the surface and the core of the forging, the transformation of the surface and the core is not timely, so that the internal and external mass volume changes will generate internal stress. This internal stress caused by the asynchrony of tissue transformation is called phase transformation stress.
The mass volume of each basic structure in the steel increases sequentially in the order of austenite, pearlite, sorbate, troostite, lower bainite, tempered martensite, and martensite. For example, when the forging is quenched and cooled quickly because the surface layer is cooled to another point first, the surface layer is transformed from austenite to martensite, and the volume expands, but the core is still in an austenite state, preventing the expansion of the surface layer, so the forging The core is subject to tensile stress, and the surface is subject to compressive stress; when cooling continues, the surface temperature decreases and no longer expands, and the core will continue to expand due to the transformation into martensite, so it will be blocked by the surface, so the core will continue to expand. The part is subjected to compressive stress, and the surface layer is subjected to tensile stress. After cooling the junction, this stress still exists inside the forging as residual stress.
Therefore, in the process of quenching and cooling, the changes of thermal stress and phase transformation stress are opposite, and the two kinds of stress remaining inside the forging are also opposite. The combined stress of thermal stress and phase transformation stress is called quenching internal stress. When the internal stress remaining inside the forging exceeds the yield point of the steel, it will cause plastic deformation of the workpiece, resulting in the distortion of the forging.
