The residual force of steel heat treatment refers to the final residual stress of the workpiece after heat treatment, which has a very important influence on the shape, size and performance of the workpiece. When it exceeds the yield strength of the material, it causes the deformation of the workpiece, and when it exceeds the strength limit of the material, it will crack the workpiece, which is its harmful side and should be reduced and eliminated.
However, under certain conditions, controlling the stress to make it reasonably distributed can improve the mechanical properties and service life of the parts, and turn harmful into beneficial.
Analyzing the distribution and variation of stress in the process of heat treatment of steel and making it reasonably distributed has far-reaching practical significance for improving product quality.
1. Heat treatment stress formation principle of steel
During the heating and cooling process of the workpiece, due to the inconsistent temperature difference between the cooling speed and time of the surface layer and the heart, it will lead to uneven volume expansion and contraction and produce stress, that is, thermal stress. Under the action of thermal stress, because the surface temperature is lower than the core, the contraction is greater than the core and the core is strained. When the cooling ends, the surface compression core is strained because the final cooling volume contraction of the core cannot be carried out freely. That is, under the action of thermal stress, the surface of the workpiece is finally compressed and the heart is strained.
This phenomenon is affected by factors such as cooling rate, material composition and heat treatment process. The faster the cooling rate, the higher the carbon content and alloy composition, the larger the non-uniform plastic deformation under the action of thermal stress during the cooling process, and the larger the residual stress.
On the other hand, due to the change of the structure of steel in the heat treatment process, that is, when the austenite is transformed into martensite, the increase of the specific volume will be accompanied by the expansion of the volume of the workpiece, and the phase change of each part of the workpiece will result in inconsistent volume growth and organizational stress. The end result of the structural stress change is the surface tensile stress and the core compressive stress, which is exactly the opposite of the thermal stress. The size of the structure stress is related to the cooling rate, shape and chemical composition of the workpiece in the martensitic phase transformation zone.
Practice has proved that any workpiece in the heat treatment process, as long as there is a phase change, thermal stress and organizational stress will occur. But the thermal stress has been generated before the organizational transformation, and the organizational stress is generated during the organizational transformation process, in the entire cooling process, the result of the combined effect of thermal stress and organizational stress is the actual stress in the workpiece.
The result of the combined action of these two stresses is very complex, affected by many factors, such as composition, shape, heat treatment process and so on. In terms of its development process, there are only two types, namely thermal stress and organizational stress, which cancel each other when the direction of action is opposite, and overlap each other when the direction of action is the same. Whether they cancel each other or overlap each other, the two stresses should have a dominant factor, and the effect of thermal stress when it dominates is that the workpiece is strained and the surface is compressed. The effect of the predominant structural stress is that the compression surface of the workpiece core is strained.
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