Nicolas Traon

Influence of Andalusite, Al 2 O 3 -ZrO 2 -SiO 2 and Al 2 O 3 -ZrO 2 Addition on Elastic and Mechanical Properties of High Alumina Castables

For many years, the optimization of refractory castable formulations has been a key factor in improving the service life of unshaped materials during their application in the steel and iron industry as well as in the metallurgical industry.The incorporation of functional aggregates in common low cement castables based on tabular alumina raw materials was investigated within the framework of the study presented herein. Such an addition of eutectic aggregates aims to modify the elastic and mechanical properties of the castable leading to eventual thermal shock resistance amelioration. Different high alumina-based formulations were investigated. Starting with a reference tabular alumina-based castable, the largest grain fraction of this first formulation was replaced by three different aggregates. Andalusite was incorporated to favour microcrack formation due to in situ mullite and ensuing glass formation during the sintering process. Both mechanisms may explain the depletion of elastic and mechanical properties and furthermore the decrease of the stored elastic energy of the castable. Al2O3-ZrO2 addition may also result in crack formation because of expansion mismatch between the calcium aluminate matrix and the zirconia-containing aggregate. The martensitic transformation of zirconia at high temperature characterized by a considerable volume expansion is the second factor explaining the increase in mechanical stresses at the level of the grain boundaries between aggregates and matrix. An alternative to these materials was the examination of Al2O3-ZrO2-SiO2 eutectic aggregates in order to study the influence of zirconia in the presence of a silica glassy phase. This study will focus on the modification of the elastic and thermo-mechanical properties of the andalusite-based castable in comparison with the reference castable (without functional aggregate incorporation) to discuss the impact of such aggregates on thermal shock resistance.

Estimation of Damage in Refractory Materials after Progressive Thermal Shocks with Resonant Frequency Damping Analysis

This work correlates damping measurements and the microstructural changes in refractory castables after these have been exposed to thermal shocks in air. In accordance with DIN EN 993 – 11, refractory samples based on tabular alumina with the addition of partially stabilized zirconia (PSZ) were progressively subjected to thermal shocks in air at different temperatures (750, 850, 950 and 1050 °C). White fused alumina samples were also exposed to the same thermal shocks at 950 °C. Evaluation of the thermal shock damage to the high-alumina refractory castables was based on the dynamic Young‘s modulus and damping characterization data obtained by means of the impulse excitation technique (IET), in accordance with ASTM E1876 – 07. Scanning electron microscopy (SEM) was also performed to enable understanding of the elastic changes in these refractory formulations. The results show that the damping increase in PSZ castables may be explained by crack nucleation and propagation while such phenomena do not occur in WFA castable.

Elastic and Mechanical Fatigue at High Temperatures of High-Alumina Castables with Addition of Partially Stabilized Zirconia

This work evaluates the impact of added zirconia on the thermomechanical and thermoelastic fatigue properties of high-alumina refractory castables. Three testing scenarios are presented in the study: 1) Resonant Frequency Damping Analysis (RFDA) after cyclic temperature changes with and without mechanical load, 2) Refractoriness under Load (RuL) tests, and 3) results of abrupt temperature shifts between two high temperatures in a special thermal shock furnace. The tested refractory formulations were a reference castable based on tabular alumina and two castables each containing 13.75 mass-% of zirconia. The formulations of the zirconia castables had an addition of either fully-stabilized zirconia doped with 8 mol-% yttria (Y-FSZ) or partially-stabilized zirconia doped with 3 mol-% calcia (Ca-PSZ). The Y-FSZ castable displayed elastic and mechanical behaviour by means of RFDA and RuL similar to the reference sample and had good recovery after thermal shock testing. The Ca-PSZ formulation showed some beneficial toughening through energy dissipation of progressing cracks, but after cooling exhibited the matrix fracturing influence of the martensitic zirconia phase transformation which would negatively impact service life of the refractory.

Einfluss des Zusatzes von Andalusit, Al2O3-ZrO2-SiO2 und Al2O3-ZrO2 auf die elastischen und mechanischen Eigenschaften eines hochtonerdehaltigen Feuerbetons

ZusammenfassungDie Optimierung der Zusammensetzung von Feuerbetonen ist seit Jahren der Schlüssel zur Verbesserung der Standzeiten von ungeformten Feuerfestprodukten im Einsatz in der Eisen- und Stahlindustrie.In dieser Studie wird der Zusatz von funktionalen Aggregaten in zementarmen, hochtonerdehaltigen Feuerbetonen mit dem Ziel untersucht, die elastischen und mechanischen Eigenschaften des Feuerbetons zu modifizieren. Dies kann zu einer Verbesserung der Thermoschockbeständigkeit führen.Es wurden verschieden zusammengesetzte Feuerbetone auf Basis von Tonerde betrachtet. Als Referenzmaterial diente ein reiner Hochtonerde-Feuerbeton. Für die anderen Zusammensetzungen wurde jeweils die größte Kornfraktion (2,24–3,0 mm) der Referenzmischung durch eines der drei zu untersuchenden Aggregate ersetzt. Andalusit wurde mit dem Ziel eingesetzt, die Bildung von Mikrorissen durch in-situ-Mullitbildung zu begünstigen. Zudem entstehen beim Sintern des Andalusits Glasphasen. Beide Mechanismen erklären die verringerten elastischen und mechanischen Eigenschaften und darüber hinaus die Verringerung der eingespeicherten elastischen Energie im Werkstoff. Der Einsatz von Al2O3-ZrO2-Aggregaten kann ebenfalls zu Rissbildung führen, da das zikonoxidhaltige Aggregat und die Kalziumaluminat-Matrix unterschiedliche thermische Ausdehnungskoeffizienten aufweisen. Des Weiteren kommt es bei hohen Temperaturen zu einer martensitischen Umwandlung des Zirkonoxids, die mit einer erheblichen Volumenänderung einhergeht. Dies könnte die erhöhte mechanische Belastung an den Korngrenzen zwischen Aggregaten und Matrix erklären. Das dritte eutektische Aggregat besteht aus Al2O3-ZrO2-SiO2 und wird eingesetzt, um den Einfluss von Zirkonoxid im Beisein einer Glasphase zu untersuchen.Diese Arbeit beschäftigt sich hauptsächlich mit der Modifikation der elastischen und thermomechanischen Eigenschaften durch Einsatz von Andalusitaggregaten im Feuerbeton und dem Einfluss dieser auf die Thermoschockbeständigkeit.AbstractFor many years the optimization of refractory castable formulations has been a key factor in improving the service life of unshaped materials during their application in the steel and iron industry as well as in the metallurgical industry.The incorporation of functional aggregates in common low cement castables based on tabular alumina raw materials was investigated within the framework of the study presented herein. Such an addition of eutectic aggregates aims to modify the elastic and mechanical properties of the castable leading to eventual thermal shock resistance amelioration.Different high alumina-based formulations were investigated. Starting with a reference pure tabular alumina-based castable, the largest grain fraction (2.24–3.0 mm) of this first formulation was replaced by three different aggregates. Andalusite was incorporated to favour microcrack formation due to in situ mullite and ensuing glass formation during the sintering process. Both mechanisms may explain the depletion of elastic and mechanical properties and furthermore the decrease of the stored elastic energy of the castable. Al2O3-ZrO2 addition may also result in crack formation because of expansion mismatch between the calcium aluminate matrix and the zirconia-containing aggregate. The martensitic transformation of zirconia at high temperature characterized by a considerable volume expansion is the second factor explaining the increase in mechanical stresses at the level of the grain boundaries between aggregates and matrix. An alternative to these materials was the examination of Al2O3-ZrO2-SiO2 eutectic aggregates in order to study the influence of zirconia in the presence of a silica glassy phase. This study will focus on the modification of the elastic and thermo-mechanical properties of the andalusite-based castable in comparison with the reference castable (without functional aggregate incorporation) to discuss the impact of such aggregates on thermal shock resistance.

Study of Reactive Impregnation and Phase Transformations during the Corrosion of High Alumina Refractories by Al2O3-CaO Slag

Corrosion of refractories results from reactive transport namely, transport of agents and chemical reactions of these agents with impregnated medium. On one hand, the transport involves either diffusion or impregnation depending on the state of the corrosive agents and the microstructure of the host media. On the other hand, chemical reactions may be very numerous and complex. This study focused on the reactive impregnation of Al2O3-CaO slag into porous high alumina refractory.Transport properties of the porous media have been assessed by performing wicking test. Chemical reactions between the solid high alumina skeleton and Al2O3-CaO slag involve successive dissolution/precipitation mechanisms forming aluminates of lime. Contrary to the thermodynamic properties of the binary system, the kinetics of these solid/liquid reactions is not well known. Corrosion tests associated with quenching method, XRD and high temperature XRD were performed for a better understanding of the kinetics.