How does internal cracks of forging blank occur during heating
Internal Crack of Flange Blank
When the blank is heated, if its internal stress exceeds its strength limit at a certain temperature, the cracks will be generated. Usually, there are temperature stress, structural stress, and residual stress.
Temperature Stress
The temperature difference is generated due to the inconsistency of the temperature of the outer layer and that of the inner layer during the heating process. Because the expansion of the outer and that of the inner are uneven - The outer layer of the blank will be heated and expanded ahead, and because the inner layer will be heated later, it will limit the expansion of the outer layer. This causes compressive stress in the outer layer and tensile stress in the inner layer. This kind of stress generated by uneven temperature on the blank is called temperature stress, or thermal stress.
The level of the temperature stress is related to the heating rate, the temperature difference in the blank, and the properties of the metal. The faster the heating rate is, the worse the thermal conductivity of the material is. The larger the cross-sectional dimension of the blank, the greater the temperature difference, and the bigger the temperature stress generated. When the heated metal is not sufficiently ductile, and basically in an elastic state, the temperature stress may exceed the strength limit of the metal at a certain temperature, causing cracks inside the metal. However, as the temperature rises, if the plasticity of the metal is increased, the temperature stress will be partially weakened by the local plastic deformation. Take carbon steel and normal alloy steel for an example, accelerating the heating process before heating to 550℃-650℃ is dangerous because in this time, they are in an elastic state and their plasticity is low. Whereas the plasticity of austenitic alloy steel will drastically reduce from 600℃ to 900℃, in this range, the heating should be slow and even, especially for blanks with large cross section.
Structural Stress
During the heating process, metal with phase transition will also produce structural stress. Like when the steel is converted from α-iron to γ-iron, its volume will shrink by 1%. The outer layer will shrink first after phase transition, so the phase transition of the inner layer will hinder the shrinkage of the outer layer and cause tensile stress in the outer layer, the compressive stress in the inner layer. Similarly, when the pearlite turns into austenite, the direction of the tensile stress in the outer layer and the compressive stress in the inner layer caused by specific volume reducing is opposite to that of the temperature stress, so that the total stress is reduced. When a phase transition occurs due to the temperature of the inner layer increasing, the plasticity of metal is greatly increased. At this time, the cracking will not occur if the heating temperature is appropriately accelerated. Therefore, the structural stress has little effect on the formation of cracks inside the blank.
Residual Stress
During the solidification and cooling of the ingot, due to the cooling sequence of the outer layer and the center, the mutual restraint between the parts will generate residual stress. When the ingot is solidified, since the out layer is cooled first (compressive stress) and then the center (tensile stress), the metal will crack if the residual stress exceeds the strength limit. So pay enough attention to residual stress.
When the blank is heated, if its internal stress exceeds its strength limit at a certain temperature, the cracks will be generated. Usually, there are temperature stress, structural stress, and residual stress.
Temperature Stress
The temperature difference is generated due to the inconsistency of the temperature of the outer layer and that of the inner layer during the heating process. Because the expansion of the outer and that of the inner are uneven - The outer layer of the blank will be heated and expanded ahead, and because the inner layer will be heated later, it will limit the expansion of the outer layer. This causes compressive stress in the outer layer and tensile stress in the inner layer. This kind of stress generated by uneven temperature on the blank is called temperature stress, or thermal stress.
The level of the temperature stress is related to the heating rate, the temperature difference in the blank, and the properties of the metal. The faster the heating rate is, the worse the thermal conductivity of the material is. The larger the cross-sectional dimension of the blank, the greater the temperature difference, and the bigger the temperature stress generated. When the heated metal is not sufficiently ductile, and basically in an elastic state, the temperature stress may exceed the strength limit of the metal at a certain temperature, causing cracks inside the metal. However, as the temperature rises, if the plasticity of the metal is increased, the temperature stress will be partially weakened by the local plastic deformation. Take carbon steel and normal alloy steel for an example, accelerating the heating process before heating to 550℃-650℃ is dangerous because in this time, they are in an elastic state and their plasticity is low. Whereas the plasticity of austenitic alloy steel will drastically reduce from 600℃ to 900℃, in this range, the heating should be slow and even, especially for blanks with large cross section.
Structural Stress
During the heating process, metal with phase transition will also produce structural stress. Like when the steel is converted from α-iron to γ-iron, its volume will shrink by 1%. The outer layer will shrink first after phase transition, so the phase transition of the inner layer will hinder the shrinkage of the outer layer and cause tensile stress in the outer layer, the compressive stress in the inner layer. Similarly, when the pearlite turns into austenite, the direction of the tensile stress in the outer layer and the compressive stress in the inner layer caused by specific volume reducing is opposite to that of the temperature stress, so that the total stress is reduced. When a phase transition occurs due to the temperature of the inner layer increasing, the plasticity of metal is greatly increased. At this time, the cracking will not occur if the heating temperature is appropriately accelerated. Therefore, the structural stress has little effect on the formation of cracks inside the blank.
Residual Stress
During the solidification and cooling of the ingot, due to the cooling sequence of the outer layer and the center, the mutual restraint between the parts will generate residual stress. When the ingot is solidified, since the out layer is cooled first (compressive stress) and then the center (tensile stress), the metal will crack if the residual stress exceeds the strength limit. So pay enough attention to residual stress.
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