Cracks: The cooling cracks of the wheel forgings after forging are caused by the internal stress generated during the cooling process. The causes of the internal stress during cooling are different: temperature stress, tissue stress and residual stress.
The temperature stress is caused by the difference between the internal and external temperature of the wheel forging during the cooling process, causing inconsistent shrinkage. The surface layer cools quickly at the beginning of cooling. The surface shrinkage is blocked by the heart. A tensile stress is generated on the surface, and a compressive stress is generated on the core. As the cooling continues, if the wheel forging is made of soft steel with a small resistance, the slight plastic deformation on the surface can relax the surface tensile stress, and the surface temperature drops to normal temperature in the later stage of cooling. The temperature of the heart is high, and it continues to shrink. The surface will hinder the contraction of the heart, resulting in compressive stress on the surface and tensile stress on the heart. In this case, there is a greater tendency to generate internal cracks. For hard steels with high resistance and difficult to deform, the surface tensile stress cannot be relaxed at the initial stage of cooling, and the additional compressive stress on the surface caused by the shrinkage of the core at the later stage of cooling can only reduce part of the surface tensile stress without changing the direction of temperature stress. The surface is still under tensile stress and the core is under compressive stress. In this case, there is a greater tendency for external cracking.
The structural stress is the phase change of the wheel forging during cooling. The surface and surface phase change time and phase specific volume are different to generate stress. For example, martensite specific volume is larger than austenite. When the surface of the wheel forging is cooled to horse When the martensite transformation temperature occurs, the martensite transformation first occurs on the surface, while the core is still in the austenite state. Therefore, the volume expansion of the surface of the wheel forging is restricted by the core. At this time, the structural stress generated is the surface compressive stress. The core is Tensile stress has high plastic toughness of the heart tissue, and the above stress can be alleviated through local plastic deformation. However, when the martensite transformation occurs in the core, the volume of the core expands and is hindered by the surface, causing the surface to be tensile stress and the core to be compressive stress. In actual production, the larger the size of the wheel forging, the smaller the thermal conductivity, and the larger the temperature stress and the tissue stress.
Residual stress is the stress caused by the uneven deformation and work hardening of the wheel forging during the forming process, which cannot be recrystallized and softened in time to be eliminated, and the stress remaining in the wheel forging after forging. When these three kinds of stresses exceed the strength limit of the steel, cracks will occur in the corresponding parts of the wheel forging, such as internal internal cracks and external cracks on the surface layer. Therefore, the forged wheel forgings cannot be arbitrarily cooled. If the steel with higher carbon content cannot be cooled quickly below 700 ℃, it is necessary to enter a slow cooling pit to cool slowly. Medium and low carbon steel, small and medium wheel forgings and low alloy steel wheel forgings can be air cooled.