Most of the refractories are used under mechanical constraint. In order to simulate the conditions of actual use, we made the test equipment that could evaluate to thermal shock resistance of samples under constraint. The digital photos of samples were taken with a camera equipped on the side during test and it was possible to evaluate the amount of strains and the extent of cracks by the obtained images. In the tests under constraint, cracks parallel to the heating surface were generated and the cracks enlarged with the repeated heating and cooling. In the tests under unconstraint, generated cracks parallel to the heating surface were small only. By the thermal stress analysis, cause for the crack generation will be explained as follows. The sample deforms into a fan shape by the thermal strain difference between the heating surface side and the rear side and stresses leading to cracks parallel to the heating surface are generated.

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We have been carrying out an investigation using x-ray diffractometry（XRD）with a high sensitive semiconductor detector in order to determine the orientation of graphite in MgO-C bricks. Test samples of three MgO-C bricks with different graphite content were used. Two types of test pieces were prepared. One type was taken for measurement parallel to the direction of the press force used in manufacturing the brick, FD（forming direction measurement）. The other type was taken for measurement perpendicular to the direction of force exerted by the press, PD（perpendicular direction measurement）. They were examined by bulk XRD method. We determined the orientation of the flaky graphite from the FD/PD ratios, which were calculated by dividing the FD x-ray intensity by the PD x-ray intensity. 8 lattice planes in the C（00l）and C（10l）groups were represented by the 0 °- 90 °inclination angle of the graphite layer plane against the test face. The FD/PD ratios were nearly constant in the 0 °- 30 °range of the inclination angle. To investigate graphite orientation, we calculated the average of the C（002）and C（004）FD/PD plane ratios, which were determined with high sensitivity x-ray intensities, the orientation index. The orientation of the flaky graphite was expressible with the orientation index. This value increased with the increase in the graphite content of the MgO-C bricks. In the case of MgO-C bricks with a graphite content of 20 mass%, the orientation index was 5.4.

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