In the heating and cooling process, because the surface and the cooling speed and time does not agree, form the temperature difference, will cause the volume expansion and contraction stress of the accusation, i.e. thermal stress. In under the action of thermal stress, because the surface temperature below the start of heart, and the heart than contraction and make the heart in the tension, when the cooling end, because the mind department finally cooling volume contraction freedom not to pressure the surface tension and heart.
 
That is under the action of thermal stress in the final to make the work surface pressure and the heart in the tension. This phenomenon is the cooling speed, material composition and heat treatment process factors.
 
When the cooling speed as quickly, carbon and the higher the alloy component, cooling process in the influence of thermal stress the inhomogeneity of the plastic deformation, the final shape of the residual stress the big. On the other hand steel heat treatment process in organizational change that is because of the austenite to martensite transformation, because the increase of hematocrit will accompany with the volume of inflation, workpiece each place has phase change, causing volume inconsistent and produce organization grew up stress. The final results of the stress changes organization is surface tension stress, the heart of compression stress, just and thermal stress instead. Organization stress with the size of the workpiece in martensite transformation of the cooling speed, shape, the chemical composition of materials related to such factors.

Practice has proved, in heat treatment process any workpiece, so long as has the phase change, thermal stress and organizational stress will happen. Only the thermal stress in organization transformation ago has produced, and the organization is organization transformation in stress formed during the process of, in the whole cooling process, the thermal stress and organizational stress comprehensive effect of, is also in real stress. The two comprehensive effect of stress is very complex, by many factors, such as composition, shape and heat treatment process, etc. The development process is only two kinds of types, i.e. thermal stress and organizational stress, the function in opposite directions in both offset when, the same in each direction between superposition.
 
Whether mutual offset or a mutual superposition, two stress should have a dominant factor, the dominant thermal stress the role of the result is the heart of tension, the surface pressure. Organization stress dominated the role of the result is the heart of surface tension and pressure.




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