Thermal expansion of cementite and thermoelastic stresses in white cast iron

With X-ray diffraction the three microscopic differential thermal expansion coefficients of pure cementite were determined with high precision in the temperature range 30–320 °C. These data were used to calculate the average three-axial thermoelastic stresses in white cast iron with differently shaped and oriented cementite inclusions. Between 30 and 300 °C the orientation distribution of the (211) interplanar spacing of the ferrite phase in white cast iron was determined. An evaluation of these data according to the formalism of conventional X-ray stress analysis, however, yields completely wrong results because this formalism is based on the assumption that only biaxial average stresses parallel to the surface exist in the thin surface layer which is penetrated by the X-rays. It is demonstrated that what were observed in reality were the three-axial average phase-specific stresses from the interior of the specimen, which are complicated because of the preferred orientation of the cementite plates. The average phase-specific stress component σ33 perpendicular to the irradiated surface drops to zero only in a sub-micrometre range below the surface. Because of the equilibrium conditions it is exactly zero at the external surface.