Thermal Shock Damage Resistance Research Articles

Microstructure, mechanical properties and thermal shock behavior of 2.5D oxide fiber preform reinforced oxide matrix composites

The 2.5D oxide/oxide composites become attractive for their excellent integrity and mechanical properties. The thermal shock resistance performance of the material is very important in the actual high temperature application. In this work, the evolution of microstructure and mechanical behavior of the 2.5D oxide/oxide composites from thermal shock at different temperatures and cycles was investigated experimentally mainly by optical microscope, X-ray computed tomography and macro-mechanical tests. The thermal shock resistance mechanism of oxide/oxide composites was qualitatively explored. The results showed that cracks propagation was the main manifestation of thermal shock damage in composites. Compared with 2D composites, 2.5D composites exhibited the finite domain effect on crack propagation due to its reinforcement had certain fibers in Z-direction. This directly caused the critical thermal shock temperature difference of the 2.5D composites (∼900 °C) was more than twice that of the 2D oxide/oxide composites (∼400 °C). In addition, the finite domain effect also significantly reduced the damage growth rate of the 2.5D composites from cyclic thermal shock, highlighting its excellent thermal shock damage resistance.

Calcium zirconate–based refractory with improved thermal shock damage resistance from Egyptian dolomite and zirconia through in situ spinel synthesis

Calcium zirconate–based refractory with improved thermal shock damage resistance from Egyptian dolomite and zirconia through in situ spinel synthesis

Structure Control and Evaluation of Effective Fracture Energy on Alumina-Magnesia Monolithic Refractories Prepared with CaO·6Al<sub>2</sub>O<sub>3</sub> Aggregate

Alumina-magnesium castable refractories are used for the inner walls of molten steel ladles. The main cause of damage to these refractories is thermal shock caused by rapid temperature changes during the loading and unloading of molten steel. Therefore, the thermal shock damage resistance coefficient R'''' is a parameter that expresses the resistance of refractories to thermal shock, and the evaluation of effective fracture energy is important for improving R''''. In this refractory, CaO·6Al2O3 (CA6) is formed from the raw materials alumina and alumina cement; CA6 has the advantage of inhibiting crack propagation and improving mechanical properties. On the other hand, CA6 has the disadvantage of volumetric expansion during formation. Therefore, microstructural changes caused by the formation of CA6 significantly affect the mechanical properties of refractories. In this study, CA6 was used as raw material and replaced with CA6 generated in-situ. The amount of coarse-grained alumina added was adjusted to control the microstructure. The effect of the microstructure change on the effective fracture energy was then evaluated. As a result, the effective fracture energy was improved in the sample with more coarse-grained alumina added, due to the replacement of CA6 aggregate.

Is the in-situ ZnAl2O4 formation an alternative for magnesia-alumina spinel zero-carbon shaped refractories?

The ever-increasing demand for cleaner steels is the driving force for developing zero-carbon bricks. These refractories have some advantages such as lower thermal conductivity and no direct CO2 emissions, reducing thermal losses and environmental impacts related to their use. Aiming at enhancing some properties of these refractories, the addition of spinel-like inducers (MgO or ZnO) was compared. These oxides react with alumina forming MgAl2O4 and ZnAl2O4, respectively. The spinel formation of the former phase is well-known to be a counter-diffusional solid-state reaction, in which the Kirkendall effect leads to residual porosity. This work assessed the diffusion mechanisms in the Al2O3–MgO and Al2O3–ZnO systems, addressing the role played by the Kirkendall effect on the physical and thermomechanical properties of shaped refractory. Supported by XRD analyses, the early gahnite formation, which starts close to 800 °C and is fully completed at 1200 °C, was attested. This fast spinelization, the thermal expansion mismatch between ZnAl2O4 and Al2O3, and the residual porosity due to the Kirkendall effect led to a higher thermal shock damage resistance. Further analysis of the microstructural features highlighted that a homogeneous sintering reaction was carried out between the MgO and Al2O3 oxides. However, for the ZnO-containing composition, substantial voids were present between the matrix and aggregates due to the Kirkendall effect, inhibiting the sample densification. Additionally, a coating was formed at the alumina-coarse boundaries, indicating a high ZnO diffusion from the matrix to the aggregates. This resulted in higher porosity, and thus diminishing the mechanical strength, which could not be suitable for the working linings of steel ladles. However, controlled generation of residual porosity enhanced the thermal shock damage resistance. Moreover, the addition of ZnO can help to anticipate the in-situ spinalization for applications in the aluminum industry or to produce pre-reacted spinel-like ZnAl2O4 raw materials.

Effect of ZnO Addition on Effective Fracture Energy of Alumina-Magnesia Castable Refractory

Alumina-magnesia castable refractory is used for the inner wall of molten steel ladle. The main cause of damage to this refractory is thermal shock caused by a sudden temperature change when putting in and out molten steel. Therefore, there is a thermal shock damage resistance coefficient R'''' as a parameter that represents the resistance of refractory materials to thermal shock, and the evaluation of effective fracture energy is important for improving R''''. The purpose of this study is to evaluate the effect of ZnO addition on the effective fracture energy of alumina-magnesia castable refractories. ZnO is reported to improve the mechanical properties of spinel (MgAl2O4). Samples containing 0 to 3 mass% ZnO were prepared, and the effective fracture energy, crystalline phase, microstructure, and apparent porosity were evaluated. The effective fracture energy increased with the increase of the amount of ZnO added. The apparent porosity shows a constant value regardless of the amount of ZnO added, and the promotion of spinel sintering and the increase of spinel amount by the solid solution of zinc oxide contributed to the improvement of the effective fracture energy.

Study of the castable selection for blast furnace blowpipe

The contradictory properties required of castable refractories makes selecting castable refractories for industrial applications challenging. This paper seeks to describe the material selection for a blast furnace blowpipe application that is subjected to sudden temperature changes and must prevent heat loss. Three commercial high alumina castables containing andalusite or mullite from different manufacturers were characterized. Thermal shock damage resistance was evaluated using thermal shock damage resistance theory and experiments. The castables’ coefficient of thermal expansion was estimated using quantitative X-ray diffraction. Crack propagation resistance was measured using the work-of-fracture technique. Thermal shock damage was experimentally evaluated by measuring the modulus of elasticity and rupture prior to and after thermal cycles. Ultimately, the microstructure of the castables was related to the thermal shock damage behavior by estimating the aggregate size and the fracture toughening mechanisms using light optical and scanning electron microscopes. Heat loss was evaluated by calculating the blowpipe shell temperature using a one-dimensional steady-state heat conduction model. The best commercial castable refractory for blowpipe showed high thermal shock damage resistance and low thermal conductivity. The results in this study agreed with thermal shock damage resistance parameters and showed a correlation between coarse microstructure with large aggregate and higher thermal shock damage resistance.

EFFECT OF PORE COMPOSITION ON THERMAL SHOCK RESISTANCE OF POROUS CERAMICS

Porous alumina ceramics with different pore composition (intrinsic pores and extrinsic pores) were prepared by gel injection molding process combined with pore forming technology and effects of intergranular pores and extrinsic pores on thermal shock fracture resistance and thermal shock damage resistance were studied. The results show that, when the solid phase content or total porosity is the same, the porous ceramic material with extrinsic pores has higher damage tolerance, thermal shock critical temperature difference (ΔTc) and residual strength retention rate. A ratio parameter (KIC/E) for evaluating the thermal shock stability of materials is deduction. The larger the ratio is, the better the thermal shock resistance of materials.

Thermal shock resistance of porous ZrB2–SiC ceramic after oxidation

ABSTRACTThermal shock water quenching tests on porous ZrB2–SiC (ZS) ceramics with 8.7, 17.9 and 27.3% porosity were conducted after these specimens were oxidised in static air at a constant temperature of 1200°C for 30 min. The effect of oxidation on their thermal shock resistance was investigated in detail. The oxidation improved the material’s thermal shock fracture resistance, but reduced their thermal shock damage resistance (TSDR). The thermal shock critical temperature difference (ΔTc) of the oxidised porous ZS ceramics decreased as the porosity increased, which was the opposite trend to that observed in the case of unoxidised porous ZS ceramics. ZS ceramics with higher porosity had higher TSDR with or without oxidation treatment. Thickness of the oxidation layer, degree of oxide layer coverage and internal oxidation were observed to affect the thermal shock behaviour of the porous ZS ceramics.

Effects of porosity and pore size on mechanical and thermal properties as well as thermal shock fracture resistance of porous ZrB2–SiC ceramics

Porous ZrB2–SiC (ZS) ceramics with different volume fractions of porosity, i.e. 8.7%, 17.9% and 27.3% and different mean pore diameter, i.e. 2.21, 3.65 and 4.23μm, were fabricated using partial hot pressing, in order to investigate the effect of the porosity and pore size on their mechanical and thermal properties as well as thermal shock performance. Particular emphasis was placed in the study on thermal shock fracture resistance. The critical temperature difference, ΔTc, of porous ZS herein was found to increase slightly with increasing porosity. This phenomenon is inconsistent with most previous researches on other material systems. However, it was well explained from the viewpoints of both physical properties and microstructures of the material, which provides new perspectives and a deeper understanding of how porosity and pore-size affect ΔTc. The ΔTc was also observed to increase as pore size decrease. It could tailor the thermal shock resistance of ZS ceramic and obtain porous ZS ceramic with both high thermal shock fracture resistance and high thermal shock damage resistance, using the above porosity and pore-size dependence of physical properties.

Effect of Specimen Size on the Resistance to Thermal Shock of Refractory Castables Containing Eutectic Aggregates

Thermal shock is one of the most severe conditions to which a refractory lining can be subjected to during its industrial application. Thus, there are several methods available for testing thermal shock damage resistance of refractories. Among them, a very common method is the quench test, which consists of quenching a hot refractory bar into water. After that a retained mechanical property is determined. Considering this, the aim of the work herein is to compare the thermal shock damage resistance between two specimen sizes among several materials. The dimensions 150 mm × 25 mm × 25 mm and 160 mm × 40 mm × 40 mm were used. The small bars are generally used for mechanical characterization in the refractory field (recommended by ASTM C1171-05). The large bars, on the other hand, are a requirement of DIN 196-1, which regards procedures for testing cement materials. Experimental results have indicated that the thermal shock damage is bigger for the large bars, as predicted by theoretical aspects. Although the size difference between the specimens was not so big, it was possible to observe the size effect using a statistical treatment. Five different castable formulations, of which three contained eutectic aggregates, were tested. The highest variation found in thermal shock damage resistance because of the size was about 15%.

Fracture energy and thermal shock damage resistance of refractory castables containing eutectic aggregates

One of the main reasons to investigate fracture energy of refractory castables is their need for superlative performance in resisting to thermal shock damage, which is one of the most crucial thermomechanical requirements in face to their industrial application. The fracture energy depends on toughening mechanisms, which will vary according to the material׳s microstructure. Considering this, the addition of eutectic electrofused aggregates is an interesting choice to increase toughness, since eutectic microstructures can deflect cracks during the fracture process. In that case, the crack deflection is likely to occur inside the aggregates during the transgranular fracture, which would increase the fracture energy. In the work herein, castables containing eutectic electrofused aggregates were investigated by the means of a comparison to blank materials, based on aggregates of white electrofused alumina. The fracture energy results were sensible to the variation of aggregate type, pointing out that the eutectic ones have a better performance regarding thermal shock damage resistance.

Thermal shock behavior of porous ZrB2–SiC ceramics

Porous ZrB2–SiC (ZS) ceramics with different microstructures were fabricated by partial hot-pressing or pressureless-sintering, in order to obtain high thermal shock damage resistance. Their thermal shock behavior was investigated by water-quenching. The influences of various microstructural characteristics: porosity, pore size and sintering degree, on mechanical properties were investigated in detail, and their effects on critical crack size, critical thermal shock temperature (ΔTc) and residual strength retention rate after thermal shock were also discussed. The pressureless-sintered ZS with 11% porosity had an optimum configuration of porosity and pore size, and exhibited the best combination of mechanical properties and thermal shock damage resistance. No cracks were visible under the scanning electron microscope on the surfaces of this specimen after thermal shock, showing its excellent structural stability.

Mechanical Properties of Solid‐Sintered Porous Silicon Carbide Ceramics

AbstractThe freeze‐drying combining solid sintering is a good method to fabricate porous SiC ceramics with good high temperature mechanical properties. Though SiC ceramics are sintered at a high temperature of 2150 °C, there are still large quantity of macropores generated by sublimating large ice crystals and micropores in the ceramic wall caused by small ice crystals sublimating, SiC particles stacking, and burning out of polyvinyl alcohol (PVA). The content of PVA affects the microstructure and strength. As increasing the content of PVA, the size of pores caused by sublimating the ice crystals decreases while both open porosity and room temperature strength increase. After being treated at a temperature of 1200 and 1500 °C, the ceramics exhibit good thermal shock damage resistance.

Extension of Hasselman's thermal shock theory for crack/microstructure interactions in refractories

Thermal shock damage resistance in advanced refractories depends on the crack interactions with the microstructure. These energy dissipation mechanisms during crack propagation are not directly considered in the original classical thermal shock model of Hasselman. They are imbedded within the N and γ terms of his derivations. In this extension of Hasselman's work, an expression is presented, which estimates the final crack size (ℓf) as the fracture surface energy ratio between γNBT and γWOF. That expression directly considers the crack interaction mechanisms with the refractory microstructure as it includes the R-curve behavior effects. In addition, the equation presented allows a quantitative evaluation of the volumetric density of cracks in refractories.

The effect of SnO2 on the improvement of mechanical properties of MgO–MgAl2O4 composites

Incorporation of SnO2 into MgO–spinel (M–S) increased mechanical properties significantly. Relationships between the parameters improving mechanical properties and microstructural variables were examined. Basic parameters improving the mechanical properties of M–S–SnO2 composites were identified as follows: (a) when microcracks come across with either Mg2SnO4 particles or pores; crack branching and deviation of interlinked microcracks or crack arresting occurred more effectively than those of spinel particles, (b) fracture type was converted to intergranular fracture with incorporation of spinel into MgO, and transgranular fracture with addition of SnO2 to M–S; additionally with the incorporation of additives, (c) critical defect size, (d) work of fracture values increased, and (e) MgO grain size decreased. Rst thermal shock parameter values of M–S–SnO2 composites were markedly higher than those of M–S materials, associated with low strength loss, high thermal shock damage resistance and thus longer service life of M–S–SnO2 composites for high-temperature industrial applications.

Improvements on the mechanical properties and thermal shock behaviours of MgO–spinel composite refractories by ZrO 2 incorporation

Microstructural features and improvements on the mechanical properties and thermal shock behaviours of MgO–spinel composite refractories with ZrO 2 addition were examined. ZrO 2 incorporation into MgO–spinel led to improvements around ∼1.5-fold ratios on mechanical properties, R st values and thermal shock results. The basic parameters improving mechanical properties and thermal shock resistance of MgO–spinel–ZrO 2 composite refractories were determined as follows: (i) propagation of microcracks for a short distance by interlinking each other, (ii) stopping or deviation of microcracks when reaching pores or ZrO 2 particles, (iii) concurrent occurrence of mostly intergranular and some transgranular cracks on fracture surfaces, and with the addition of ZrO 2 (iv) the increase in bulk density, and (v) a significant decrease in MgO grain size. The improvements observed in thermo-mechanical properties confirmed that MgO–spinel–ZrO 2 refractories showed a low strength loss and high thermal shock damage resistance at high temperatures, leading to longer service lives for using industrial applications.

Teor de cimento e seus efeitos sobre a resistência ao choque térmico de concretos espinelizados in-situ

Concretos refratários aluminosos contendo espinélio são amplamente utilizados como revestimentos de panelas siderúrgicas, devido a sua elevada resistência à corrosão. Além do bom desempenho anti-corrosivo, estes materiais devem suportar as variações bruscas de temperatura causadas pela ciclagem térmica, responsáveis pela diminuição da vida útil do revestimento. Neste contexto, a análise da resistência ao dano por choque térmico se torna parâmetro fundamental para a correta seleção do revestimento refratário. Esta propriedade pode ser melhorada por meio da incorporação de mecanismos de tenacificação à estrutura do concreto. Uma fase que auxilia na tenacificação do material é o CA6, que está associada ao cimento de aluminato de cálcio (CAC), matéria-prima usada como ligante. Sendo assim, o objetivo deste trabalho é caracterizar o efeito do teor de cimento na microestrutura do concreto e suas conseqüências nos danos causados por choque térmico. Os resultados obtidos indicam que concretos com valores intermediários de cimento possuem melhor desempenho em relação ao choque térmico, nas temperaturas de uso das panelas. Dessa forma, o controle do teor de cimento possibilita aumentar a resistência ao dano por choque térmico e, consequentemente, a vida útil do revestimento.

Stability of crack propagation during bending tests on brittle materials

Applications of ceramic materials usually involve thermal stresses that can reduce their lifetime considerably. One of the most relevant mechanical properties for evaluating thermal shock damage resistance is the energy of fracture. In order to obtain this property, a stable crack growth regimen must be achieved throughout the test. In this paper, three-point bending tests are analyzed to determine how machine stiffness influences load point displacement rates. Understanding the differences between recorded and adjusted displacement rates is important in determining the stability of crack growth. This statement is corroborated by experimental data on sintered high-alumina, low-cement refractory castables.

Thermal shock behaviour of mullite-bonded porous silicon carbide ceramics with yttria addition

Thermal shock resistance of mullite (3Al2O3 · 2SiO2)-bonded porous silicon carbide (SiC) ceramics with 3.0 wt% yttria (Y2O3) addition was evaluated by a water-quenching technique. The thermal shock damage was investigated as a function of the quenching temperature, quenching cycles and specimen thickness. The residual flexural strength of the quenched specimens decreases with increasing quenching temperature and specimen thickness due to the larger thermal stress caused by thermal shock. However, quenching cycles at the temperature difference of 1200 °C have no effect on the residual strength since the same thermal stress was produced in repeated thermal shock processes. The good thermal shock damage resistance of the specimens is contributed mainly by the low strength and moderate elastic modulus. Moreover, the pores prevent the continuous propagation of cracks and alleviate further damage.

Proposal of new damage model for thermal shock based on dynamic fracture on the brittle materials

In the current paper, we propose that the mechanical resistance of the brittle materials under dynamic loading can be used to predict the thermal shock damage resistance via a new proposed damage criterion. The increase of the mechanical resistance based on high loading rate has been investigated by experimental methods for ‘Lead Crystal Glass’ including the static and dynamic loadings by universal ‘INSTRON’ and ‘split Hopkinson pressure bars (SHPB)’. The relationship between the measured static and dynamic strengths of materials has been compared and developed via Tuler and Butcher’s dynamic criterion. ‘Modified Brazilian disk (MBD)’ specimens are used to obtain indirect tensile stress concentration at notch tip. The fracture process under various loading rates has been observed by optical and scanning electronic microscope (SEM) to study the fracture phenomenon according to the high loading rate effect for Lead Crystal Glass. The dynamic stress due to high loading rates can be considered as the thermal stress during thermal shock phenomenon with high heat convection coefficient values which leads to high gradient temperature, low duration time and intrinsic damage feature like cracks and flakes for concerned material. The transient thermal stress analysis has been carried out by numerical method for MBD specimen subjected to rapid cooling condition. The numerical results and experimental dynamic resistance of selected material are utilized to develop a new semi-empirical damage criterion based on the continuum damage mechanics (CDM) including dynamic material resistance, critical thermal shock time which corresponds to maximum transient thermo-mechanical stress and heat convection coefficient evolution according to the thermal shock severity.