he higher-level objective of this master thesis was the creation of a more detailed working instruction after implementing optimization measures for refractory corrosion tests in a high frequency induction furnace. Therefore, several issues had to be worked on.
One main point was the question how to deal with the preheating procedure of the to be tested refractory material specimens which also included plant-engineering aspects for controlling the required atmosphere. The protection of carbon containing specimen against oxidation during preheating plays a significant role for gaining meaningful results. Therefore, several trials with a particularly designed and inert gas purged refractory lid as protection against ambient atmosphere during preheating were conducted. The results were assessed by reflected-light microscopy and compared to samples of a trial with no precaution against oxidation. The results showed that with the usage of a refractory lid the decarburized layer of the specimens could be decreased by up to >90%. For assessment of the corrosion resistance, it is necessary to eliminate the influence of thermal shock to a certain degree. The temperature inside the specimens, which were positioned above the melt, was measured during preheating for different refractory materials. This led to a preheating time of 10 minutes to guarantee sufficient specimen temperature before immersing them into the melt.
A further important part of this thesis was the optimization or new fabrication of the crucible itself especially referring to steel ladle slags. For this reason, thermochemical solubility calculations by FactSageTM were conducted for Cr2O3 and ZrO2 in a steel ladle slag. The results led to the selection of a ZrO2 mix as suitable material for the crucible insert. This also included the practical manufacture of the crucible insert. As the produced insert variants showed cracks after firing the decision was taken to stop the activity at this point. Finally, Cr2O3 crucible inserts were chosen to conduct the trials. These trials with a steel ladle slag clearly showed the advantage of using this type of crucible material compared to the before used carbon containing crucibles regarding change of the slag composition and reduction of iron oxide in the slag.
Consecutively a major part was the topic repeatability of experiments. Several trials were conducted with different materials and slags. One main finding was that performing trials with carbon containing specimens in an iron oxide bearing slag is only feasible in a restricted repeatable way. The carbothermal reduction of the iron oxide leads to uncontrolled foam formation after immersion of the specimens into the slag bath. The repeatability of the test method was therefore assessed for trials with specimens out of a magnesia chromite material and fayalitic slag. This trial was repeated three times and led to an interval of repeatability of 2,8% for these three trials.
All these aspects included convenient examination methods. Furthermore, new assessment methods for the trial results like computed tomography (CT) and Micro-XRF were used and evaluated. Examination by CT offers accurate, 2- or 3-dimensional and rather fast possibilities to assess the corrosion resistance of refractory materials after the finger test compared to the existing assessment methods. The Micro-XRF measurement can be used for assessment of the infiltration depth. This method was also evaluated with reflected-light microscopy and reached good agreement.
In the end it should be emphasized that this test method is only a comparable method as it is not possible to adjust the same parameters as given in a real situation in a vessel. All these findings and optimizations were summarized and integrated in the existing working instruction.