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The effect of mullite on the mechanical properties and thermal shock behaviour of alumina–mullite refractory materials

Abstract Mechanical properties and thermal shock behaviour of slip-cast alumina-mullite refractories have been investigated. There was a marked improvement in strength, Youngs modulus and fracture toughness values with the addition of finest (∼5 μm) mullite particles. The shorter length needle-like mullite particles led to some pull-out and crack deflection. More fracture surface energy was therefore required to connect the cracks for propagation, resulting in a high value of fracture toughness. The resistance to fracture initiation and propagation caused by the thermal stresses was increased, as supported by the R and R st parameters. Therefore, the retained strength was improved by increasing the quench temperature, showing a high resistance to thermal shock.

The role of fine alumina and mullite particles on the thermomechanical behaviour of alumina-mullite refractory materials

Abstract Fine grain alumina and mullite particles (∼5 μm) were incorporated into slip-cast alumina–mullite refractories in order to investigate their effects on the microstructure, mechanical properties and thermal shock behaviour of the refractories. The incorporation of alumina particles improved both densification and mechanical properties markedly, compared to that of mullite particles, which resulted in high porosity and low mechanical properties. The crack initiation resistance increased with the addition of alumina, which is supported by R parameter. Therefore, more fracture surface energy was required to connect the cracks for propagation, associated with a high value of fracture toughness. The strength and Youngs modulus were also improved with increasing quench temperature, leading to a high thermal shock resistance.

Mechanical properties and thermal shock behaviour of alumina-mullite-zirconia and alumina-mullite refractory materials by slip casting

Strength and modulus values of slip-cast alumina–mullite–zirconia and alumina-mullite refractories have been determined as a function of quench temperature by air and water quenching. The resistance to fracture initiation and propagation caused by the thermal stresses was increased with the addition of zircon, as supported by the R and Rst parameters. There was no catastrophic decline in strength on thermal shock at high temperature or with repeated cycling, where a large proportion of intergranular fracture in the fracture surfaces was observed. There was ∼50% loss of strength after the 1st cycle, and further cycling did not markedly change the retained strength of the material to the 40th cycle. The inevitable cracking occurred after thermal shock testing was kept below the critical crack length due to the transformation toughening effect of zirconia, thus maintaining a high degree of integrity, associated with the high values of K1c and γi.

The influence of zircon on the mechanical properties and thermal shock behaviour of slip-cast alumina–mullite refractories

Abstract Mechanical properties of slip-cast alumina–mullite (AM) refractories were improved significantly by the addition of ∼2-μm zircon (ZrSiO 4 ) particles. Fracture behaviour showed mainly transgranular, with some intergranular, fracture. R parameter showed that there was a marked incline in resistance to fracture initiation. Thermal shock tests confirmed that the retained strength and Youngs modulus of alumina–mullite–zircon refractories were improved by increasing the quench temperature, indicating higher resistance to crack propagation.

Effect of the particle size distribution of spinel on the mechanical properties and thermal shock performance of MgO–spinel composites

Abstract The influence of varying the amounts of spinel with a similar median particle size, but with different distribution, on the mechanical properties and thermal shock performance of MgO–spinel composites was investigated. Mechanical properties of composites decreased significantly with increasing spinel content due to the thermal expansion mismatch. However, γWOF values of composites increased markedly, because of a significant change in the fracture mode from transgranular to intergranular fracture. A narrow distributed spinel A (Alcoa MR66) particles resulted in shorter initial crack propagation distances from the spinel particles, but spinel B (Britmag 67) particles with a significantly broader distribution were the origins of longer interlinked cracks. The improved resistance to thermal shock in MgO–spinel composites can therefore be attributed to the microcrack networks developed around the spinel particles, associated with the high values of γWOF, and not to an increased K1c. On the basis of theoretically calculated R‴ values and experimentally found γWOF/γi ratios, resistance to thermal shock damage would be more strongly favoured with materials containing spinel B particles, rather than spinel A, for which a much larger volume% was required to achieve a similar improvement.

Young’s modulus measurements of magnesia–spinel composites using load–deflection curves, sonic modulus, strain gauges and Rayleigh waves

Abstract The extent of interlinking of the microcracking and a decrease in strength and modulus values were determined to be a function of both spinel particle size and volume fraction to allow calculation of thermal shock parameter, R‴. Measurements of Young’s modulus were carried out both at room temperature and after thermal shock testing by using the load–deflection curves (defined as “mechanical” modulus) and by the sonic modulus technique. The values obtained from these methods were significantly different for quenched/unquenched samples. To understand the basis for these differences, strain gauge and Rayleigh wave methods were also used to determine Young’s modulus of the composites. Modulus values obtained from these methods confirmed the differences measured, and provided a guide to the values to be used in calculating thermal shock parameters. The mechanical modulus technique was considered the most meaningful indicator of Young’s modulus for a situation in which large mechanical strains were to be applied to the materials during thermal shock.

The influence of zircon in a model aluminosilicate glass tank forehearth refractory

Abstract Standard aluminosilicate forehearth refractories are normally fabricated with a small proportion of zircon to improve their performance. To identify possible bases for the action of the zircon, fine-grain aluminosilicate materials of similar compositions were prepared and tested, to model the behaviour of the finer grain bond phase in the standard refractory. The reference materials contained a range of proportions of zircon, and further sets of materials were prepared in which the zircon was replaced by varying amounts of very fine alumina, silica or zirconia powders. The alumina and zirconia powders increased the strength at room temperature of the base aluminosilicate, with zirconia having the largest effect; silica had only a slight effect. It appears that the zircon increases strength through three mechanisms: the reduction in porosity brought about by improved efficiency of the particle packing, a faster rate of sintering of the fine grained bond phase, and a transformation toughening of the bond phase, caused by tetragonal zirconia formed in situ by high temperature dissociation of the zircon.

The Corrosion Resistance of Alumina-Mullite-Zircon Refractories in Molten Glass

Corrosion measurements on a set of alumina-mullite-zircon refractories have been carried out using a static crucible test consisting of a “hole in a block”. Rates of corrosion of the refractory at 1370 °C for 72 h by a standard soda-lime glass were evaluated by measuring selected dimensions within the hole, flux line loss, and the thickness of refractory penetrated by the molten glass. The extent of corrosion decreased with increasing refractory zircon content, and increased with increasing porosity. Wetting of the refractory by the glass decreased with increasing zircon content. Porosity and wettability are therefore believed to be major factors determining the susceptibility to penetration of the refractory by the glass, and thereby controlling the rate of corrosion. Microstructural features of the interfacial zone were examined by SEM: zirconia derived from the zircon appears to be much less reactive towards the molten glass than alumina. Undissolved zirconia particles may therefore be particularly effective in providing physical and energy barriers to penetration of the refractory pores by molten glass.

Mechanical Properties of Alumina-Mullite-Zircon Refractories

The relationships between mechanical properties and microstructure of alumina-mullitezircon refractories were investigated by varying zircon content. Various sintering temperatures were used for the slip-cast refractories and mechanical properties determined as a function of zircon content. Strength and modulus values increased significantly (by a factor of ~3) as a result of an increase in both sintering temperature and zircon content. The incorporation of fine zircon particles gave a significant improvement in densification through a reduction in porosity by faster sintering and by filling of interparticle voids due to the formation of a glassy phase. Fracture surface energy and critical crack size did not change significantly with increasing sintering temperature. However, the addition of increasing amount of zircon and the rise in the sintering temperature increased fracture toughness markedly, by a factor of ~3 and ~35%, respectively. Thermal shock parameter R′′′ showed a >10% improvement with increasing sintering temperature. Thermal shock tests confirmed that the refractory material, containing 20% zircon, sintered at 1650 °C showed the highest resistance to further thermal shock damage.

Thermal Shock Behaviour of Alumina-Mullite-Zircon Refractories

Thermal shock behaviour of alumina-mullite-zircon refractories was investigated by varying zircon content. Thermal shock parameters R and Rst based on the room temperature mechanical measurements were calculated as a function of zircon content in order to foresee thermal shock behaviour of refractory materials. The R parameter, predicting the resistance of a material to the initial shock and expressed as the difficulty of crack initiation, showed an ~1.5 times increase with the addition of zircon. Furthermore, Rst parameter also supported that there was a significant improvement (by a factor of >2) in the maximum allowable temperature difference required to propagate long cracks under severe thermal stress conditions. Thermal shock testing was then carried out based on the bend strength and Young’s modulus values as a function of quench temperature. The predictions made from R and Rst parameters were matched by thermal shock testing data. The refractory containing 20% zircon showed the highest resistance to both crack initiation and propagation, leading to a high resistance to fracture for a long-life performance in service for high temperature applications.