Mechanical behavior and thermal shock resistance of MgO-C refractories: Influence of graphite content

The mechanical and thermo-mechanical properties of MgO-C refractories are of major importance in the industrial applications, and highly depend on the optimization of their microstructural design. In the present work, the influence of flaky graphite content on mechanical behavior and thermal shock resistance of such refractories was investigated with the aid of the wedge splitting test, fractal and microscopic fractographic analysis. The results showed that the increase of graphite content in the specimens led to an enhanced non-linear fracture behavior, a reduced nominal notch tensile strength (σ_NT), and a higher specific fracture energy (G_f), characteristic length (l_ch) and thermal shock resistance parameter (R_st). The fractal analysis of the crack propagation path of the specimens after the wedge splitting test indicated that increasing graphite content in the refractories can enhance their irregularity of the crack propagation path during fracture. Also, it was suggested from microscopic fractographic analysis that the improvement of thermal shock resistance of MgO-C refractories was positively correlated with the increase of interface crack propagation.     

Tensile behaviour of magnesia-spinel refractories: Comparison of tensile and wedge splitting tests

In some industrial applications, the need to improve the thermal shock resistance of refractories by optimisation of their microstructural design is of major importance. Refractories with enhanced thermal shock resistance usually present a rather low resistance to crack initiation but high resistance to crack propagation (rising R-curves), as well as a mechanical behaviour deviating from pure linear elastic fracture mechanics (LEFM), often qualified as nonlinear. The present work aimed at studying the influence of thermal micro-damage within the microstructure released during the cooling process on the nonlinearity of the mechanical behaviour in tension. The two-phase composites considered were magnesia-spinel refractories with different spinel inclusions content allowing to modulate the micro-damage level. Two different destructive mechanical tests, namely tensile and wedge splitting tests, were performed and their results were compared. The influence of thermal damage on different relevant mechanical parameters was investigated, and a quantitative correlation analysis between these parameters was proposed.     

Effect of ceramic bonding phases on the thermo-mechanical properties of Al2O3–C refractories

Ceramic bonding phases of non-oxide whiskers can enhance the hot strength and the thermal shock resistance of Al₂O₃–C refractories. In this paper, the effect of different metals on the microstructure and thermo-mechanical properties of Al₂O₃–C refractories has been investigated. Thermodynamic calculation of Al–Si–O–C–N systems shows that Al₄C₃, AlN, SiC and β-Sialon are stable at elevated temperature. AlN with the shape of short column can be generated in Al₂O₃–C refractories with metallic Al, which leads to high hot modulus of rupture (HMOR) and poor resistance to thermal shock. SiC whiskers formed in Al₂O₃–C refractories with metallic Si give rise to low HMOR and good resistance to thermal shock. When metallic Si and Al are added together in the refractories, β-Sialon (z=2) with plane structure can be generated under the action of catalyst (nano-sized Ni). The existence of the catalyst promotes the diffusion of Al and O in Si₃N₄ crystals and contributes to the generation of plane-shaped β-Sialon. The corresponding HMOR and residual cold modulus of rupture respectively increase to about 20 MPa and 10.3 MPa. The plane-shaped β-Sialon can significantly enhance both hot strength and thermal shock resistance of Al₂O₃–C refractories.     

Characterisation and properties of low-conductivity microporous magnesia based aggregates with in-situ intergranular spinel phases

Development of microporous magnesia based aggregates serving as working-line refractories have great significance in reducing energy loss and saving resource. Microporous magnesia-based aggregates were fabricated at 1780 °C by in-situ decomposition of magnesite with addition of nano-sized Al₂O₃. Intergranular MgAl₂O₄ phases formed in situ decreased the closed-pore size, thermal conductivity and improved the ceramic bonding and thermal shock resistance. Furthermore, the results suggested that pore size distribution was the dominate factor affecting thermal conductivity. Thermal contact resistance owing to networks of intergranular spinel in magnesia could improve thermal insulation performance effectively. The mismatch of thermal expansion coefficient between spinel and magnesia and the micro-scale closed pores enhanced thermal shock resistance by accommodating thermal stress and suppressing crack propagation. Microporous magnesia-based aggregates with 3 wt% nano-sized Al₂O₃ presented a mean pore size of 3.42 μm, thermal conductivity of 5.76 W m^?1 k^?1 (800 °C), a cold compressive strength of ~285 MPa, and a residual strength retention rate of 65.0% after thermal shock cycles. The low-conductivity microporous magnesia-based aggregates with excellent thermal shock resistance show promise for future application in working-lining lightweight refractories.     

Advanced refractories for titanium metallurgy based on calcium zirconate with improved thermomechanical properties

Calcium zirconate refractories exhibit a promisingly high corrosion resistance in contact with titanium alloy melts. In the present article, we improved the thermomechanical properties of calcium zirconate based refractories by altering their microstructure and chemical composition. In a full factorial experimental design we investigated the effects of an insitu phase formation, of the addition of coarse grained MgO aggregates as well as the addition of SrCO₃. Based on these factors, the chemical, physical, mechanical and thermomechanical properties as well as the resulting microstructure were thoroughly characterized. Strikingly, the addition of SrCO₃ resulted in significantly improved mechanical properties before and after thermal shock. The improved thermal shock resistance can be attributed to a lower determined thermal expansion coefficient, a homogeneous pore size distribution with a reduced pore size and a better bonding between the matrix and the coarse grained aggregates.     

Nanoscale toughening of low carbon Al2O3-C refractories by means of SiCnw/Al2O3 composite reinforcement

To improve the thermal shock resistance of low carbon Al₂O₃-C refractories, SiC nanowires (SiC_nw) containing SiC_nw/Al₂O₃ composite reinforcement were introduced. The specific fracture energy of the Al₂O₃-C refractory matrix was obtained by statistical grid nano-indentation. The reinforcement mechanism of SiC_nw/Al₂O₃ on thermal shock resistance of refractories was investigated. The results revealed that the matrix-specific fracture energy of A6 (6 wt% SiC_nw/Al₂O₃ added) was 217 N/m, which was 58.4% higher than reference sample A0 (137 N/m) and 18.6% higher than MA6 (183 N/m, 6 wt% SiC/Al₂O₃ added). A6 showed the highest residual strength ratio of 49.8%, which was 114.7 % higher than A0 (23.2%) and 82.4 % higher than MA6 (27.3%). The components with different morphology in SiC_nw/Al₂O₃ cluster, especially SiC nanowires, promote the generation of microcracks, crack multi-deflection, and branching, which toughen the matrix and improve the thermal shock resistance of refractories. In comparison to the literature, A6 showed a higher rising in residual strength ratio than those with higher graphite content (4 wt% and 20 wt%), which will greatly reduce the consumption of carbon-containing refractories and contribute to the reduction of CO₂ emission.     

The microstructure evolution and mechanical properties of MgO-C refractories with recycling Si/SiC solid waste from photovoltaic industry

In the present work, the recycling of Si/SiC solid waste from photovoltaic industry for MgO-C refractories preparation has been introduced. The influence of solid waste powders as antioxidant additive on microstructure evolution, mechanical properties and thermal shock resistance of MgO-C refractories has been investigated systematically. With 4?wt% Si/SiC rich solid waste addition, the MgO-C refractories exhibited the highest strength (4.39?MPa) and residual Young's modulus (7.86?GPa) after firing at 1400?°C, compared to only Si or SiC-addition. The presence of iron in the solid waste also promoted the formation of MgO and Mg₂SiO₄ whiskers via catalyst-assisted method. Moreover, a dissolution-saturation-precipitation growth mechanism was used to explain the formation process of the whiskers. The improvements in strength as well as thermal shock resistance can be attributed to the microstructural evolution.     

R-curves determination of ordinary refractory ceramics assisted by digital image correlation method

As a figure-of-merit, the rising ratio of crack propagation resistance to fracture initiation resistance indicates a reduction of the brittleness and enhances the thermal shock resistance of ordinary refractory ceramics. The significant nonlinear fracture behaviour is related to the development of a fracture process zone (FPZ). The universal dimensionless load–displacement diagram method is applied as a promising graphical method for the determination of R-curves for magnesia refractories showing different brittleness. By applying digital image correlation (DIC) together with the graphical method, the problems arisen with accurate determination of the fracture initiation resistance and the crack length are overcome. Meanwhile, the R-curve is subdivided with respect to the fracture processes, viz the fracture initiation, the development of FPZ and the onset of traction free macro-crack. With the simultaneous crack lengths evaluated from DIC, the contribution of each fracture process to the crack propagation resistance at certain loading stage is quantitatively presented.     

Oxidation resistance,residual strength,and microstructural evolution in Al2O3-MgO–C refractory composites with YAG nanopowder

The decisive role of nanostructured yttrium aluminium garnet (YAG;Y₃Al₅O₁₂) powder addition on oxidation resistance, residual strength and microstructural evolution were studied in Al₂O₃-MgO–C refractory composites. Oxidation index and rate constant calculations indicated that the oxidation resistance was almost 70 % improved for the nano-YAG containing refractories oxidized in air at 1600 °C. Residual compressive strength (R_c) estimations showed that there was nearly 75 % strength retained in these oxidized refractories fortified with nano-YAG. Residual bending strength (R_b) estimations based on cyclic thermal shock, exhibited that there was almost 70 % thermal shock resistance enhancement in refractories reinforced with nano-YAG, showed a good agreement between R_b and R_c values. These beneficial properties were attributed to the formation of a well-sintered framework of YAG/Spinel bonding grains throughout the dense oxidized layer microstructure of these new class of refractories. The concept of interfacial toughening and implications of these results to practical applications are discussed.     

Thermal shock damage evaluation of refractory castables via hot elastic modulus measurements

When refractory castables are submitted to continuous thermal changes, crack nucleation and/or propagation can take place resulting in a loss of mechanical strength and overall degradation of such materials. This work evaluates the thermal shock damage cycling of high-alumina and mullite refractory castables designed for petrochemical application. Hot elastic modulus analyses were carried out after 0, 2, 4, 6, 8 and 10 thermal cycles (ΔT=800 °C) in order to investigate the microcracking evolution due to the temperature changes. Additionally, apparent porosity, hot modulus of rupture, erosion and work of fracture measurements were also performed. According to the attained results, it was detected at which temperature range the stiffening or embrittlement took place in the mullite-based refractory (M-SA) microstructure. Nevertheless, the damage induced by the thermal shock tests was not permanent, as further increase of the elastic modulus results was observed for all evaluated samples after annealing. On the other hand, the alumina-based composition containing a sintering additive (TA-SA) presented enhanced mechanical strength, high thermal stability and E values. Simulations indicated that refractories with high E values (∼140 GPa, such as those attained for alumina-based castable) showed a reduced amount of stored elastic strain energy even under severe thermal stresses, which seems to be a key aspect for the engineered design of thermal shock resistance materials.     

Cr7C3: A potential antioxidant for low carbon MgO–C refractories

Magnesia carbon (MgO–C) refractory, one of the most commonly used refractories in the steelmaking system, relies on graphite to improve the thermal shock resistance and slag corrosion resistance. The oxidation of graphite carbon in a MgO–C brick usually leads to the destruction of the carbon network in the brick, which causes the structure of the brick to become loose and easily eroded. At present, metal powders, carbides, and borides are used as antioxidants to prevent the oxidation of carbon in MgO–C bricks. The metal carbide Cr₇C₃ can be prepared from aluminum chromium slag through a simple synthetic process and at a low cost. In this work, we investigated the oxidation resistance of low carbon MgO–C refractories with different amounts of Cr₇C₃ powder (1, 2, 3, and 4 wt%). The refractories with 3 wt% Cr₇C₃ powder showed optimal resistance to oxidation. The microstructure indicated that oxygen reacts with Cr₇C₃ preferentially over carbon to form chromium oxide and magnesium chromium spinel, blocking the pores and hindering oxygen diffusion. Carbon arising from the reduction of carbon monoxide by Cr₇C₃ can act as a supplementary carbon source. The better oxidation resistance also contributed to the improvements in slag corrosion and thermal shock resistance of the refractories.     

Improvements on the mechanical properties and thermal shock behaviours of MgO-spinel composite refractories by ZrO2 incorporation

Microstructural features and improvements on the mechanical properties and thermal shock behaviours of MgO-spinel composite refractories with ZrO₂ addition were examined. ZrO₂ 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-ZrO₂ 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 ZrO₂ particles, (iii) concurrent occurrence of mostly intergranular and some transgranular cracks on fracture surfaces, and with the addition of ZrO₂ (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-ZrO₂ refractories showed a low strength loss and high thermal shock damage resistance at high temperatures, leading to longer service lives for using industrial applications.     

Fracture behavior of Al2O3–SiO2 castables by the wedge splitting test and digital image correlation technique

The fracture mechanisms are helpful for the optimization and design of toughness and microstructure of refractories. Fracture behavior of ultra-low cement bonded Al₂O₃–SiO₂ castables was researched using the wedge splitting test coupled with digital image correlation technique (WST-DIC). Flexibility of Al₂O₃–SiO₂ castables is improved by introducing andalusite aggregates into the castables. The characteristic length L_CH, a parameter used to assess flexiblity of materials, was observed to reach 287.2 mm in andalusite-containing Al₂O₃–SiO₂ castables, more than 5 times that of reference castables. Microcracks toughening is the main toughening mechanisms for flexibility improvement of the Al₂O₃–SiO₂ castables containing andalusite. Microcrack network in the Al₂O₃–SiO₂ castables could be designed by exploiting the volume expansion caused by mullitization of andalusite and the coefficient of thermal expansion (CTE) mismatch between the andalusite aggregate and the matrix. Unlike andalusite-free castables, castables containing andalusite possess a distinct fracture process zone (FPZ), the crack branching and deflection can be observed around the main crack during the fracture process, which leads to the prolong of the crack propagation path, the increase of the dissipation energy during the fracture, and the enhancement of resistance to crack propagation.     

Corrosion of oxide bonded silicon carbide refractories by molten salts in solid waste-to-energy facilities

Oxide bonded silicon carbide refractories are used successfully in solid waste-to-energy facilities (WtE). They are submitted to severe thermo-chemical stresses that limit their performance. Even if the corrosion resistance of silicon carbide is high, wear and failure of refractory lining are currently observed.For a better understanding of corrosion mechanisms, oxide bonded silicon carbide refractories, collected in the combustion chamber of several WtE facilities, were examined. The main mechanisms of corrosion, according to the environment of refractories, were determined. The chemical composition and the nature of the corrosive agents were calculated from the thermo-chemical modeling. They are mainly condensed phases of sulfates and chlorides (CaSO₄, K₂SO₄, Na₂SO₄, KCl, and NaCl). In service conditions, these molten salts react with the SiC aggregates and the matrix of the refractories to form low melting compounds.The post-mortem analyses showed the formation of para-wollastonite in the porosity and around the SiC grains, on the hot face of refractory tiles. Other phases such as cristobalite and microline (KAlSiO₈) were also formed down to the core of refractories. The volume expansion created by the formation of new mineral phases (cristobalite, para-wollastonite) causes the formation of micro cracks in the refractory lining.In this paper, the degradation mechanisms of oxide bonded silicon carbide refractories are presented and the main research developments for the future are discussed.     

Effect of h-BN on the microstructure and fracture behavior of low carbon Al2O3-C refractories

h-BN is an ideal substitution candidate for graphite due to its similar crystal structure, better oxidation resistance. In this work, the effect of h-BN on microstructure and comprehensive properties of Al₂O₃-C refractories are investigated, and the specimen containing 0.5 wt% h-BN (G0.5N0.5) possesses the best comprehensive properties. The addition of h-BN could reduce the diameter of SiC whiskers, which leads to the highest strength of specimen G0.5N0.5 (42.63 ± 3.10 MPa). Moreover, the fracture behavior of the specimens is demonstrated using wedge splitting test. The results show that the specimen G0.5N0.5 possesses the highest crack initiation and propagation resistance, which could be attributed to the collaborative effect of h-BN and SiC whiskers. Noteworthily, the addition h-BN could improve the thermal shock resistance. The specimens containing h-BN possess the higher residual ratio, compared with the specimen containing no h-BN (G1N0), and the specimen G0.5N0.5 shows the highest residual strength (14.12 ± 0.67 MPa). Furthermore, the oxidation resistance could be enhanced with introducing the h-BN.     

Mechanical properties and microstructure evolution of MgO–Al–C slide plate refractories in presence of Al powder-modified magnesia aggregates

MgO–Al–C slide plate refractories were fabricated using sintered magnesia and modified sintered magnesia as aggregates, fused magnesia aggregates and fines, Al powder and carbon black (N220) as fines, and thermosetting phenolic resin as the binder. Al powder-modified magnesia aggregates were prepared and characterized and were introduced into the MgO–Al–C slide plate refractories. The effects of the modified aggregates on the properties, phase composition, and microstructure were investigated. 1) The Al powder-modified magnesia aggregates exhibited considerably high bonding strengths and low Al powder shedding ratios, thus meeting the preparation requirements of MgO–Al–C slide plate refractories. 2) At high temperatures, more needle-like and fibrous Al₄C₃, AlN and octahedral MgAl₂O₄ were generated on the surface of the modified magnesia aggregates, which enhanced the bond between the matrix and the aggregates and increased the hot modulus of rupture of the material. 3) Non-oxide Al₄C₃ and AlN phases were formed in situ and had high thermal conductivity and low coefficient of expansion; this could relieve the internal thermal stress of the material and create a toughening effect, improving the thermal shock resistance of the material.     

Comparative study of B4C,Mg2B2O5, and ZrB2 powder additions on the mechanical properties,oxidation, and slag corrosion resistance of MgO–C refractories

Boron-containing additives are used to improve the oxidation resistance of carbon-containing refractories; however, their effects on the mechanical properties and slag corrosion resistance of the refractories have rarely been studied. In this work, B₄C, Mg₂B₂O₅, and ZrB₂ powders were incorporated into low-carbon MgO–C refractories to study their effects on the mechanical properties, oxidation resistance, and slag corrosion resistance of the refractories. The relationships between these properties and the microstructure and phase evolution were also studied. The results show that the flexural strengths of the MgO–C refractories at high temperatures are closely related to the apparent porosity and formation of an Mg₃B₂O₆ phase. The oxidation resistances are greatly improved after the introduction of boron-containing additives into the MgO–C refractories in terms of both thermodynamical aspects and the filling of voids and pores. The most effective antioxidant is B₄C, followed by the ZrB₂ and Mg₂B₂O₅ powders. The mechanisms through which the vanadium-containing slag attacks the MgO–C refractories mainly include the dissolution of magnesia to form melting phases, penetration through pores, and redox reaction with carbon.     

Evaluation of aluminum dross waste as raw material for refractories

An aluminum dross waste from plasma processing for Al metal reclamation was tested as a replacement raw material in refractories. The main phases of the starting Al dross waste material were MgAl₂O₄ and AlN. The waste was tested to replace calcined alumina in castables and refractory clay in a molded refractory at levels below 6.5%. The results of physical and mechanical tests indicated that the waste may be applied directly, without prior calcination, as a substitute for fine structural components in refractories. The waste and water contents used in processing, however, must be optimized to avoid the formation of crack-like defects in the microstructure. The origin of these defects is related to the generation of gas from the waste at high temperatures. It is not known if these crack defects impact physical properties. The waste was also tested as a replacement for anti-oxidant elements (Al and Si powders) in a resin-bonded refractory. Oxidation tests, however, indicated a negative effect on oxidation resistance.     

Influence of NaX (X=Cl,Br, I) additive on phase composition,thermostability and high infrared performances of γ-La2S3 transparent ceramics

A simple method to synthesize Na⁺-doped La₂S₃ powder with only γ phase was developed where NaX (X = Cl, Br, I) served as sodium source. In initial molar ratio Na/La ≥ 0.8 of the mixed raw materials, γ-La₂S₃:Na⁺ tended to reach a constant ratio Na/La = 0.2 and could be represented as Na_0.5La_2.5S₄, which exhibited enough high-temperature stability available for sintering. A modified process was designed to prepare γ-La₂S₃ ceramics with NaCl additive. The microstructure, typical grain boundary composition, photoluminescence and IR performance of as-sintered γ-La₂S₃ ceramics were investigated in details. The optical transparency with mean transmittance 61.3% in 8–14 μm and maximum 64.03% at 13.1 μm was achieved for pellet with Na:La = 0.2 and 1.0 wt% NaCl nanocrystals. NaCl was proved to play multiple roles in governing phase composition, enhancing stability, optimizing microstructure, improving IR properties and affecting luminescence of γ-La₂S₃ ceramics. The mechanisms of action of NaCl additive are proposed by combining the defects reaction, ionic radius and the diffusion behavior of related ions with the cation-deficient structure of γ-La₂S₃.     

Nanocarbon containing low carbon Al2O3–C refractories: Comparison between N220 and N990 nanocarbons

Continuous casting process is the majorly used solidification process in steel fabrication. The refractories used in this process are most commonly made up of alumina-carbon-based compositions. Generally, these functional refractories consist of about 30% residual carbon after coking. Improvements in steel industries, such as attaining clean steel and ultralow-carbon steel, require alumina-carbon refractories with low carbon content. In the present work, low carbon-containing Al₂O₃–C refractories are studied by using two different grade nanocarbons, namely, N220 and N990 with varying amounts, along with fixed 3-wt% graphite in the batch composition. The physical, mechanical, and thermomechanical properties along with the oxidation resistance are evaluated and compared. Phase analysis and microstructural developments at different temperatures were also characterized. Optimized compositions of both the nanocarbons are further studied for hot strength and oxidation resistance measurement. Based on all the obtained results, one batch composition is finalized for the thermal shock and corrosion testing. All the results are compared against a reference batch composition containing 25% graphite as a carbon source. The formation of in situ ceramic phases like aluminum carbide in nanocarbon-containing compositions provides a dense compact microstructure that improves strength, helps to inhibit oxidation, and contributes to corrosion resistance.