抄録
In a certain period during solidification of a metal casting cooling rate near the center is higher than that at the surface; the difference in the rate is particularly large at and immediately after the completion of salidification. This may give rise to a considerable magnitude of tensile stress at the center of the casting, and hence, in some cases, to internal hot tearing. A calculation was performed to estimate the extent of the stress of this type in sphere castings. Major assumptions made were (1) the material of casting was purely plastic without elasticity in the temperature range considered, (2) strain rate in a given material was dependent solely on temperature and three-axial stresses at the given place and moment and (3) cooling rate was linearly related to the distance from the center. Temperature, cooling rate, material constants and other variables were grouped into several non-dimensional numbers and equations were derived from the theory of steady creep under three-axial stresses. The equations were solved numerically by using a computer. Calculation was performed for both solid and hollow spheres, the latter corresponding to a casting with some liquid remaining in the core. The calculated stress and strain rate, expressed in a general form using the non-dimensional numbers, at various cooling conditions and material constants were plotted against the distance from the center. Interpretation of the graphs thus obtained was discussed. A sphere 100cm in diameter cast from 0.2%C steel in sand mold was taken as a practical example. The calculatad stress was relatively small when only the skin was solid, the remainder being still liquid. However, it rose to a value as high as 1 kg/mm2 at the center immediately after the end of solidification. It was shown that the tensile stress inside the casting could be minimized by appling external pressure; the pressure necessary for the purpose could be estimated from the result of the present calculation.
