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Technical Paper


The nominal notch tensile strength of castables after pre-firing at different temperatures is shown in Figure 20. The relationship of the σNT


www.ireng.org value with


respect to the pre-firing temperature is similar to that of the cold modulus of rupture and cold crushing strength, as shown in Figures 11 and 12. Namely, as temperature increases, σNT


the no-cement castable A4S26 exhibits a lower σNT castables at various temperatures.


increases accordingly. In addition, than the other two


The Young’s modulus results are shown in Figure 21. The Young’s modulus of a material is normally proportional to its mechanical strength. Therefore, samples pre-fired at higher temperatures demonstrate higher E values because the CMOR and CCS are greatly enhanced at higher temperatures, as observed in Figures 11 and 12. Therefore, a substantial increase of the Young’s moduli of all three materials is observed after the materials were pre-fired at 1500°C.


The characteristic length calculated by Eq. 3 is shown in Figure 22. The results show that the three castables perform differently at various pre-


firing temperatures. The no-cement material A4S26 exhibits the highest lch value among the tested materials, irrespective of the pre-firing temperature, mainly because the sample exhibits the lowest nominal notch tensile strength σNT


, as lch is inversely proportional to the squared σNT . Therefore,


the castable bonded with hydratable alumina displays the least brittleness compared to materials with a cement bond. In addition, the lch


value of


the A4S26 decreases as pre-firing temperature increases. These results suggest that higher pre-firing temperatures adversely affect the thermal shock resistance. However, without a higher pre-firing temperature, the no-cement castable may fail because of low mechanical strength at 1000°C. Therefore, the pre-firing temperature of approximately 1250°C for the Alphabond 300 bonded castable would possibly be applicable for optimising both mechanical strength and fracture resistance.


Among the three materials which were investigated, the material with cement and spinel (C5S26) showed the lowest lch


values at 1250 and 1500°C.


The main contributions to these values originate from the higher nominal notch tensile strengths of C5S26.


In the case of castable C5S0, the lch values are slightly increased with an increase of the pre-firing temperature, because the nominal notch tensile


strength is slightly increased with increasing temperature, whilst the Young’s modulus E is simultaneously significantly increased. For the two castables C5S0 and C5S26, the lch


temperature was increased from 1250°C to 1500°C. 4.4 Microstructural analysis


The microstructures of the different materials after sintering at 1650°C show distinct differences. The formation of new phases CA6


and C2 M2 A14


in C5S26 is shown in Figure 23, and EDS and XRD analyses are given in table 4 and Figure 24 respectively. CA6


sintered corundum grains and C2 M2 A14 also formed in C5S0 as platelets in the matrix


formed dense layers around the formed platelets growing into the


sintered spinel grains, intensively interlocking the aggregate grains with the surrounding matrix. CA6


(Figure 25), but the interlocking between aggregate grains and the matrix is much lower when compared to the spinel containing formulation. This microstructural difference could explain the clear difference in hot strength, e.g. HMoR, between cement bonded formulations with and without spinel. And it would also explain the higher specific fracture energy and nominal notch tensile strength of C5S26 when compared to C5S0 after firing at high temperatures.


In comparison to the cement bonded materials, the hydratable alumina bonded material A4S26 shows a less densified and more porous microstructure (Figure 26). During sintering, the transition aluminas from the bond will transform into the thermodynamically stable corundum phase and react with the other fine matrix aluminas. However, no binary or multicomponent new phases can form and accordingly the interlocking between aggregate grains and matrix is much lower. This explains the lower mechanical strength of the fired no-cement castable when compared to the cement bonded ones. On the other hand, it also explains the ductile, less brittle behaviour of this material during fracture behaviour testing. Crack propagation must be hampered in such a higher porous, less rigidly connected structure. As Sakai et al described [21]


, the crack would be


deflected by interface de-bonding. In addition, pores could act as the stress arrester, which dismiss the thermal stress by disturbing the route of crack propagation. Because of a much lower nominal notch tensile strength, the thermal shock resistance of the no-cement castable is higher than those of the other two castables.


slightly increased when the pre-firing


Figure 17: Load/displacement curves of the castables pre-fired at 1250°C for 5 h


Figure 18: Load/displacement curves of the castables pre-fired at 1500°C for 5 h


Figure19: Specific fracture energy of castables pre-fired at different temperatures


Figure 20: Nominal notch tensile strength σNT castables pre-fired at different temperatures


22


of


Figure 21: Young’s moduli of castables pre-fired at different temperatures


ENGINEER THE REFRACTORIES


Figure 22: Characteristic length lch pre-fired at different temperatures


of castables


July 2018 Issue


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