Quality assurance of sintered refractories through nondestructive testing with microwaves

For quality assurance of refractories for the glass industry an early nondestructive testing is required. A possibility for nondestructive testing, especially in the green product, is given by the transition of microwaves, as this requires no coupling medium like water. In this work, some basic physical characteristics were measured which determine microwave absorption through refractories and flaw detection. For different refractories the dielectric constant, specific attenuation coefficient as well as dependence of the attenuation on the frequency were determined. It is shown that a high dielectric constant improves the detection of flaws. In order to improve flaw-detection and resolution, lenses and frequency modulation were used successfully. Best results for the refractories investigated were achieved with a lens with a focal length of 86 mm. Measurements were then extended to other types of refractories, i.e. chromalumina, zirconmullite and zirconsilica, as well as to unfired bricks Here, especially one isostatic-pressed zirconsilica brick has shown the lowest specific attenuation coefficient due to low porosity and high dielectric constant. Furthermore, a test device for complete C-scans of large refractory samples was developed and tested successfully A number of refractories without or with natural and artificial flaws, respectively, were checked. Nondestructive testing with microwaves has now been sufficiently developed to be transferred to industrial use.