On the Detection of Inhomogeneities in Steel by Computed Tomography

The detection of inhomogeneities such as shrinkage pores, gas pores or cracks in light metals or plastics by means of microfocus x-rays computed tomography (μXCT) has become a common non-destructive testing method in recent years. The evaluation of CT-data of Fe-based materials is more difficult, since they are usually noisy, of poor contrast and the interpretation is affected by measurement artefacts. Thus inhomogeneities as mentioned above and non-metallic inclusions are much more difficult to detect in continuously cast steel slabs, because of the higher x-ray absorption coefficient of the base metal. Suitable measuring parameters and evaluation routines are presented and rules for the detectability and classification regarding contrast and shape are specified. Verification is provided by target scanning electron microscopic (SEM) -metallography. Various XCT-methods are applied to correlate the information gained at different size scales. Measurements of non-metallic inclusions and pores in steel slabs are performed by using highresolution μXCT and synchrotron tomography (sXCT) providing spatial resolutions of (6.1 μm)3/voxel, (1.7 μm)3/voxel and (0.3 μm)/voxel respectively. These types of inhomogeneities can be distinguished clearly via the grey value only with monochromatic synchrotron radiation. A rough classification of non-metallic inclusions consisting of different phases can be achieved by μXCT assisted by discrimination via the 3D shape of the inhomogeneity.