If the phase change of the crane wheel forging during the cooling process occurs, it will produce tissue stress. As with heating, the tissue stress is also caused by the change in the specific volume of the tissue before and after the phase change, and the phase changes of the outer layer and the heart are different.
Take the martensite transformation of the crane wheel forging during cooling as an example. As the temperature of the forging decreases, the outer layer undergoes martensite transformation first. The specific volume of martensite is larger than the specific volume of austenite, the outer layer expands, but the core has not yet been transformed, and it is used for the expansion of the outer layer. Therefore, the structural stress is: the outer layer is compressive stress, and the core is tensile stress . However, at this time, the heart temperature is relatively high, and it is still in the austenite state with good plasticity. Through local plastic deformation, the structural stress can be relaxed.
As the crane wheel forging continues to cool, the heart also begins to undergo martensitic transformation and expansion, but the transformation of the outer layer has ended and the volume no longer changes, so it limits the expansion of the core. Therefore, the structural stress generated at this time: the core is compressive stress, and the outer layer is tensile stress, and it continues to increase until the end of the martensitic transformation is completed. The late stage of crane wheel forgings is in a low temperature elastic state with poor plasticity, so the risk of cracking caused by this tissue stress is very large. The specific volume of all phases in the steel is larger than that of austenite. Therefore, when other structural transformations occur in the cooling of crane wheel forgings, the generated structural stress also has the above-mentioned law.
The tissue stress during cooling is also a three-dimensional stress, and the circumferential stress is the largest, which is the main cause of longitudinal cooling cracks on the surface.