The thermo-mechanical properties of refractory castables for applications in the metallurgical and steel industry are continuously investigated in order to improve their performance against thermal shocks. In order to optimize the service life of refractories, the thermal shock resistance is currently determined based on the analysis of the Young’s modulus after different thermal shock cycles with the help of the non-destructive impulse excitation technique (IET).

Damage characterization after thermal shocks

The impulse excitation technique is a non-destructive technique used to measure material properties. After the sample is mechanically excited, the resonance frequencies and internal friction are recorded. These material properties are a fingerprint of the sample and monitoring them in time gives valuable information regarding failure mechanisms. Typically, measurements are performed at room temperature after the samples are exposed to different cyclic loading schemes or specific heat treatments.

Traon et al. studied the thermal shock behavior of high aluminia castables composed of tabular alumina and partially stabilized zirconia (PSZ). They were pre-fired at 1300 °C for 6 hours at a heating and cooling rate of 1K/min. The grains of the sample had a maximum size of 3mm and the sample had a rectangular shape with a length of 160 mm, a width of 40 mm and a thickness of 40 mm.

The samples were subjected to thermal shocks in according to DIN EN993-11 at four different temperatures (750 °C, 850 °C, 950 °C and 1050 °C). After every thermal shock, the Young’s modulus and internal friction were measured according to the ASTM E1876-07 standard with the RFDA professional.

Fig. 1 shows the retained Young’s modulus values of the PSZ samples as a function of the number of thermal shock cycles at different tested temperatures (750, 850, 950 and 1050 °C). During the first thermal shock cycles, the Young’s modulus is degrading significantly. This can be explained by the crack nucleation in the grain boundaries which becomes more noticeable as the temperature changes increases.

As shown in Fig. 2, the internal friction data is highly correlated with the Young’s modulus data as the lower the Young’s modulus, the higher the internal friction due to the formation of cracks and microcracks. Damping values do not change as a two-step process. Indeed, damping increases proportionally with the damage increase. Based on the conclusions of Traon et al., the experiments conducted at a thermal shock temperature change of 1050 °C leads to the conclusion of a four-step evolution of the microstructural changes. Up to and including the second cycle, crack nucleation at the level of the grain boundaries causes the first increase of the damping measurements up to 1000 %. The second increase reaching 2000 % of the initial value occurring up to and including the fifth cycle can be explained by the propagation of the cracks within the matrix and throughout the Al2O3 aggregates. This step is followed by a steady state up to and including the eighth cycle while the cracks have to consume a lot of energy in order to bypass the ZrO2 aggregates.

Finally, the unification of cracks occurs during the ninth cycle, this network of cracks explains the depletion of the elastic properties and the further difficulties in achieving realistic damping values over 2500 % of the initial value.

Based on the obtained results, it is clear that the combination of elastic properties and damping data obtained by the impulse excitation technique leads to a better understanding of the microstructural changes within the refractory castables after progressive thermal shocks.

References

  1. N. Traon et al., Estimation of Damage in Refractory Materials after Progressive Thermal Shocks with Resonant Frequency Damping Analysis. J. Ceram. Sci. Tech., 07 (2016) 165-172