On the Thermal Stress of Ingots during Heating Process

Study of thermal stress cracks in steel ingots-II

Abstract

During the heating process thermal stress arises in the steel ingot because of two main causes; i. e. the temperature difference within the ingot and the volume change caused by the transformation.
In this report author tried to relate theoretically the method of calculating the thermal stress under high temperature. As the deformation under high temperature is accompanied by not only elastic strain but plastic strain also, it is very difficult to determine the thermal stress. Here the author denotes the deformation in elastic-plastic range by a nonlinear integral equations and by introducing the conception of equivalent total strain he calculates the stress and the plastic strain through a comparatively easy successive integration.
For the calculation of the stress during transformation range the author regards the curve of the expansion values as three straight lines and these three lines denote the three stages of transformation i. e. the pre-, mid- and post-transformation stage and so he makes calculation according to these three stages. And he calculates the stress and plastic strain which occur when the transformation progresses from the surface to the core.
Two calculated examples are shown of high-carbon-chromium steel circular ingots at two stages; one is where the ingot is undergoing the transformation and the other is where the ingot has no relation with transformation.
The following facts have been discovered by this theoretical calculation;
(1) As for the stress distribution there is compression stress (σ_r=0, σ_t=σ_z) in the outer part of the ingot and tension stress (σ_z≅2σ_r=2σ_t) near its core and the stress is greatest at the core in the heating process, because of high surface temperature.
(2) During the transformation the tension stress which has been greatest at the core decreases in accordance with the progress of transformation for the time, but from the beginning of transformation of the core till its completion there occurs a violent increase of the stress and plastic strain and the elastic stress becomes the greatest of all the stages.
So if some crackings take place in this case, it can be inferred that discus-form crackings will occur normal to the axis near the core when the core completes the transformation.