In situ synthesis and interfacial bonding mechanism of SiC in MgO–SiC–C refractories

Adding SiC directly to MgO–C refractories possesses the disadvantages of low dispersion and interfacial bonding strength. Herein, the in situ synthesized SiC was introduced into the MgO–SiC–C refractories to maintain the original excellent performance of MgO–C refractories and reduce the carbon dissolution in molten steel. With the increase of Si and C content in raw materials, the morphology of SiC changed from whisker to network, whose growth mechanism was vapor–solid and vapor–liquid–solid. The network structure and uniform distribution of SiC improved the thermal shock resistance of MgO–SiC–C refractories. According to the analysis of molecular dynamics simulation by Materials Studio software, SiC strengthened the relationship between periclase and graphite to enhance the structure of the compound.     

Enhanced oxidation resistance of low-carbon MgO–C refractories with Al3BC3–Al antioxidants: A synergistic effect

The synergistic effects of Al₃BC₃–Al antioxidants on optimizing the oxidation resistance of low-carbon MgO–C refractories were investigated. The results indicated that the oxidation index and rate constant of low-carbon MgO–C refractories with optimized Al₃BC₃–Al additions were 13% and 1.10 × 10⁻⁴ cm² min⁻¹ at 1400°C for 3 h, respectively, which is much lower than that of Al or Al₃BC₃ containing ones. Single Al₃BC₃ is not a suitable antioxidant for low-carbon MgO–C refractories; however, if Al₃BC₃ was initially protected and Al reacted as the antioxidant, enhanced oxidation resistance at high temperature can be achieved. The formation of dense MgO–MgAl₂O₄–Mg₃B₂O₆ layer contributed to superior oxidation resistance, and the temperature for the generation of this layer was as low as 1100°C due to liquid and vapor phase–assisted reactions with Al₃BC₃–Al. Furthermore, a self-repairing function was achieved at 1600°C with the combination of Al₃BC₃–Al additions in spite of the faster oxidation rate.     

Slag resistance mechanism of MgO–Mg2SiO4–SiC–C refractories containing porous multiphase aggregates

The slag resistance of MgO–SiC–C (MSC) refractories should be improved because of the mismatch in the thermal expansion coefficient between the aggregates and matrix, as well as the defects caused by the affinity between periclase and slag. In this study, MgO–Mg₂SiO₄–SiC–C (MMSC) refractories were prepared using porous multiphase MgO–Mg₂SiO₄ (M-M₂S) aggregates to replace dense fused magnesia aggregates. Compared to MSC, the slag penetration index of MMSC decreased by 43.5%. The structure of the porous aggregates increased the surface roughness, and the multiphase composition of the aggregates decreased the mismatch of the thermal expansion coefficient with the matrix, thus reducing debonding between the aggregates and matrix. The aggregates and matrix in the MMSC formed an interlocking structure, which bound them more tightly to improve the slag resistance. The slag viscosity at different depths from the initial slag/refractory interface was calculated using the Ribond model. The M-M₂S aggregates increased Si_xO_y^z− in the slag, which increased the slag polymerization and slag viscosity. The aggregates and matrix in the MMSC reacted with the slag to form high melting point phases, which reduced the channel of the slag. In addition, the penetration depth and velocity derived from the Washburn Equation were modified for the CaO–SiO₂–Al₂O₃–MgO–FeO slag and magnesia based refractory to accurately evaluate slag penetration.     

Basic slag corrosion of alumina–magnesia–carbon refractory bricks containing Al and Si antioxidants

The slag-corrosion behavior of two alumina–magnesia–carbon refractories (AMC) with different antioxidants (Si and Al) has been comparatively studied by means of thermodynamic simulation and laboratory tests. A cup test (static) at 1723 K and 1873 K and dipping test (dynamic) at 1873 K were carried out using a steelmaking ladle slag. An iterative method that considers the change of the liquid's composition while it penetrates the refractory was employed for the thermodynamic calculation. The simulation as well as the static tests showed similar performance between both refractories (5% of wear at 1723 K and 6% at 1873 K). In spite of the type of antioxidant, the material with smaller particles of magnesia exhibited better performance in the dynamic test, with 55% less wear than the other AMC brick. This behavior was attributed to the faster MgAl₂O₄ spinel formation, which increased the material's cohesion, especially the matrix, during the heating stage. The Si antioxidant improved oxidation resistance, although it did not have a positive effect on the corrosion resistance of the material in any of the performed tests.     

Ultrasonic measurement of Young’s modulus MgO/C refractories at high temperature

The paper deals with the elastic behavior of MgO/C refractories used in BOF at temperatures up to 1400°C in air or inert atmosphere. Measurements have been made by the way of a high temperature ultrasonic technique. Heating-cooling cycles and long time aging in the range 700–1400°C show strong variations of Young’s modulus which have been interpreted with the aid of XRD analysis, SEM observations and EDS analysis. Carbon oxidation and sintering of MgO particles are found to be responsible of the major parts of the measured evolutions. ©     

Properties of MgO–Fe–C refractories as linings of vanadium-extraction converter

For the purpose to extend the service life of MgO–C bricks used as linings of vanadium-extraction converters, MgO–Fe–C bricks with different carbon content were designed and the properties of this novel refractory were investigated by comparing to the traditional MgO–C bricks. The results showed that the poor service life of MgO–C bricks was due to the poor sinterability of the oxidized layer at 1400 °C, whereas the oxidized layer of MgO–Fe–C brick was well sintered due to the oxidation of Fe particles in the oxidized layer and formation of MgO–FeO_ss in air atmosphere. Excellent oxidation resistance and corrosion resistance against vanadium containing slag were also obtained due to the increase of compactness of oxidized layer and concentration of FeO in the oxidized layer compared to MgO–C bricks, and it is considered that MgO–Fe–C brick is a favorable substitute of MgO–C refractory to be used as linings of vanadium-extraction converters.     

Effect of bimodal microstructure on high-temperature properties of nano-reinforced Al2O3–MgO–C refractories

The beneficial effects of adding nanostructured expandable graphite (EG) hybridized yttrium aluminium garnet (EG\YAG) powder as a composite reinforcement in improving the oxidation resistance, hot-strength, and microstructure development in Al₂O₃–MgO–C refractories were studied. The refractory components reinforced with EG\YAG exhibited more than 60% of oxidation resistance enhancement and as high as 200% increase in hot-strength performance over the standard refractories, formulated without EG\YAG. Correlating the damage parameter (D_E) calculations based on ultrasonic measurements with residual strength data (R_c, R_b) showed that there was a progressive increase in R_c and R_b values with consistent reduction in the oxidative damage of EG\YAG reinforced refractories. Analysis indicated that these beneficial features were majorly ascribed to the in-situ development of bimodal microstructure with EG\YAG sintered framework throughout the refractory interior in these new class of reinforced systems. Additionally, the mechanism of toughening and implications of these results to materials design are discussed.     

In situ formation of SiC in Al–40Si alloy during high-pressure solidification

The in situ formation of SiC in Al–40Si alloys during the fabrication of SiC/Al–Si composites by high-pressure solidification were investigated. The results demonstrate that the in situ formation of SiC occurs by a gradual conversion of Al₄C₃ and Al₄SiC₄ to SiC. In situ SiC can be formed in an Al–40Si alloy solidified under a pressure of 3 GPa at a temperature of 1373 K. The SiC particles (SiC_p) formed in situ was compatible with the α-Al matrix and the Si phases. The relative density of the SiC/Al-38.6Si composite was 99.9%. The bending strengths of the Al–40Si alloy and the SiC/Al-38.6Si composite obtained by solidification under a pressure of 3 GPa were 200.8 MPa and 322 MPa, respectively, which represents an enhancement of 60.3% as a result of reinforcement by the in situ-formed SiC.     

Formation of hollow MgO-rich spinel whiskers in low carbon MgO–C refractories with Al additives

MgO–C refractories with different carbon contents have been developed to meet the requirement of steel-making technologies. Actually, the carbon content in the refractories will affect their microstructure. In the present work, the phase compositions and microstructure of low carbon MgO–C refractories (1 wt% graphite) were investigated in comparison with those of 10 wt% and 20 wt% graphite, respectively. The results showed that Al₄C₃ whiskers and MgAl₂O₄ particles formed for all the specimens fired at 1000 °C. With the temperature up to 1400 °C, more MgAl₂O₄ particles were detected in the matrix and AlN whiskers occurred locally for high carbon MgO–C specimens (10 wt% and 20 wt% graphite). However, the hollow MgO-rich spinel whiskers began to form locally at 1200 °C and grew dramatically at 1400 °C in low carbon MgO–C refractories, whose growth mechanism was dominated by the capillary transportation from liquid Al at these temperatures.     

Thermodynamic calculation and microstructure characterization of spinel formation in MgO–Al2O3–C refractories

In this paper, by simulating the gas phase conditions inside the MgO–Al₂O₃–C refractories during continuous casting process and combining with thermodynamic analysis, as well as SEM analysis, the gas-gas and gas-solid formation of MA spinel were clarified in carbon containing refractories. Thermodynamic calculations showed that gas partial pressure of CO, O₂ and Mg could meet the formation and stable existence conditions of MA spinel in MgO–Al₂O₃–C refractories under service environment, and nitrogen could not affect the formation of MA spinel at 1550 °C in the thermodynamic condition. The formation processes of MA spinel were analyzed experimentally under embedding carbon atmosphere. The carbon-coated alumina powders in MgO–Al₂O₃–C refractories prevented the direct contact between magnesia and alumina. Mg gas was formed by carbon thermal reaction, then reacted with alumina (gas-solid) and gas containing aluminum (gas-gas) to generate MA spinel. Through gas-gas or gas-solid reaction, the formation of MA spinel was effectively controlled. By means of SEM analysis, a two-layer structure with dense outer spinel layer and loose inner layer was formed in MgO–Al₂O₃–C refractories.     

Enhanced performance of low-carbon MgO–C refractories with nano-sized ZrO2–Al2O3 composite powder

Low-carbon MgO–C refractories are facing great challenges with severe thermal shock and slag corrosion in service. Here, a new approach, based on the incorporation of nano-sized ZrO₂–Al₂O₃ composite powder, is proposed to enhance the thermal shock resistance and slag resistance of such refractories in this work. The results showed that addition of ZrO₂–Al₂O₃ composite powder was helpful for improving their comprehensive performances. Particularly, the thermal shock resistance of the specimen containing 0.5 wt% composite powder was enhanced significantly which was related to the transformation toughening of zirconia and in-situ formation of more spinel phases in the matrix; also, the slag resistance of the corresponding specimen was significantly improved, which was attributed to the optimization of pore structure and formation of much thicker MgO dense layer.     

Effect of nanocarbon sources on microstructure and mechanical properties of MgO–C refractories

A study of microstructural evolution, mechanical and thermo-mechanical properties of MgO–C refractories, based on graphite oxide nanosheets (GONs), carbon nanotubes (CNTs) and carbon black (CB), was carried out by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), three-point bending and thermal shock tests. Meanwhile, these results were compared to the conventional MgO–C refractory containing 10 wt% flaky graphite prepared under the same conditions. The results showed that higher cold modulus of rupture was obtained for the composition containing GONs, and the composition containing CNTs exhibited larger displacement after coking at 1000 °C and 1400 °C. Also, the addition of nanocarbons led to an improvement of the thermal shock resistance; in particular, both compositions containing CNTs and CB had higher residual strength ratio, approaching the thermal shock resistance of the reference composition containing 10 wt% flaky graphite, as it was associated with the presence of nanocarbons and in-situ formation of ceramic phases in the matrix.     

Microstructure evolution and performance of B4C–NdB6 composites fabricated by in situ hot pressing

B₄C–NdB₆ composites were fabricated by in situ hot pressing at different temperatures (1950–2150°C) with B₄C and Nd₂O₃ (2–4 wt%) as raw materials. The microstructure evolution of the composites with sintering temperature and Nd₂O₃ content was studied in detail, and the influence of pressure on the sintering of B₄C with different contents of Nd₂O₃ was also investigated. The performance of the fabricated composites was researched and the toughening mechanism was discussed. The results indicate that Nd₂O₃ can react with B₄C to form the thin-sheet intermediate products (Nd(BO₂)₃, Nd₂CO₅) first, which then transform to band-shaped NdB₆. Pressure can reduce the distance of B₄C and Nd₂O₃, accelerating the mass transfer and contributing to the formation of NdB₆. NdB₆ and intermediate products are first in agglomerate structure at 1950°C, and then the agglomerates are broken to form dispersive micron and submicron NdB₆ at 2000°C by the synergistic function of pressure, diffusion at high temperature, and liquid phase sintering. NdB₆ can enhance the densification owing to the bonding function. Excessive Nd₂O₃ content leads to residual pores, and excessive temperature (2150°C) results in the coarsening of phases. The coexistence of transgranular and intergranular fracture of NdB₆ promote the fracture toughness.     

Atmosphere and carbon effects on microstructure and phase analysis of in situ spinel formation in MgO–C refractories matrix

The effects of carbon, air and reducing atmospheres on microstructure and phase evolution of in situ MgAl₂O₄ spinel (S) formation in the matrix of MgO–C refractories were investigated by X-ray diffraction powder analysis (XRD) and scanning electron microscopy (SEM)/energy-dispersive spectroscopy (EDS) techniques. The formation of spinel started under 1000 °C in both air and reducing atmospheres. The morphology of in situ spinel and its formation mechanism were however different and dependent upon the atmosphere. The solid-state reaction was clarified to be the dominant mechanism of spinel formation in oxide atmosphere, while the gas–solid reaction was found to play a vital role in reducing atmosphere. Reaction of MgO and C in reducing atmosphere led to the formation of Mg_(g) which was found to be partially controlling the in situ spinel formation in the carbon containing samples fired in reducing environment. The results which were necessary are explained with emphasis on MgO–C refractories applications.     

Corrosion resistance of Al–Cr-slag containing chromium–corundum refractories to slags with different basicity

Al–Cr slag is the solid waste generated by the smelting of Cr metal. It presents a range of environmental hazards. This study addressed the corrosion resistance of Al–Cr slag containing chromium–corundum refractories to slags with different basicity. Herein, we provide suggestions for the use of Cr–corundum of different basicity in kilns. Al–Cr slag, brown fused Al₂O₃, and chrome green were used as the raw materials, with pure calcium aluminate cement being used as a binder. The brick samples, prepared using different blends of chrome green and corundum, were fired at 1600?°C, and subsequently subjected to a slag corrosion test. After corrosion by slag of different basicity, the phase composition and microstructure of the sample were analyzed by X-ray diffraction, energy dispersive spectrometer and scanning electron microscopy. There were two major findings. First, Cr–corundum brick made from Al–Cr slag has a better slag corrosion resistance than that made from Cr₂O₃ and brown fused Al₂O₃. Second, Cr–corundum brick made from Al–Cr slag has superior corrosion resistance to slag with a CaO:SiO₂ ratio of 2:1.     

Microstructure and mechanical properties of multi-walled carbon nanotubes containing Al2O3–C refractories with addition of polycarbosilane

Carbon nanotubes (CNTs) are a promising reinforcement for fabricating Al₂O₃–C refractories. However, CNTs are prone to agglomerate or react with antioxidants or reactive gaseous phases such as Al (g), Si (g) and SiO (g), etc. at high temperatures. To overcome the problems above, polycarbosilane (PCS) and multi-walled carbon nanotubes (MWCNTs) were firstly mixed with micro-alumina powder in a liquid medium and then incorporated into Al₂O₃–C refractories. Then the microstructure and mechanical properties of Al₂O₃–C refractories fired in the temperature range from 800 °C to 1400 °C were investigated in this work. The results showed that the MWCNTs were well dispersed in the specimens with addition of PCS in contrast to the specimens without PCS due to the PCS adsorption on the surface of MWCNTs during the mixing process. And the mechanical properties, such as cold modulus of rupture (CMOR), flexural modulus (FM), forces and displacements of Al₂O₃–C refractories with PCS were much higher than those without PCS, which was attributed to more homogeneous dispersion of MWCNTs, more residual MWCNTs as well as different morphologies of ceramic whiskers. Meanwhile, the oxidation resistance of Al₂O₃–C refractories with PCS was improved greatly, which was supposed that the in situ formed SiC_xO_y coating prevented the oxidation of MWCNTs to some extent.     

PVD Coating of Mg–AZ31 by Thin Layer of Al and Al–Si

Although magnesium alloys have the advantage of high specific strength, they have poor atmospheric corrosion resistance. An important method of improving the corrosion resistance is by applying a coating layer. In this work, the physical vapor deposition (PVD) technique is used for coating a magnesium (Mg) AZ31 sheet substrate with a thin layer of high purity aluminum (Al) and Al–12.6% Si. Aluminum is expected to be suitable as a coating layer on Mg sheets, due to its corrosion resistance and its formability. Before coating, the substrate was subjected to several consecutive surface preparations, including sand-blasting, mechanical grinding, mirror-like polishing, ultrasonic etching, and finally ion etching by magnetron sputtering (MS). PVD coating was conducted using a PVD machine with max electron beam power and voltage of 100 kW and 40 kV, respectively. This was either with or without plasma activation, and with variable substrate speeds ranging between 10 and 70 mm/s. During MS ion etching and coating, the substrate temperature increased. The substrate temperature increased with the application of plasma activation and with lower substrate speeds. The coating-layer thickness varied inversely with substrate speed. A thinner coat with finer morphology was obtained in the case of plasma activation. Other results included coating layer characteristics, diffusion between the AZ 31 substrate and the Al coating layer, adhesion of the coating layer to the substrate, and corrosion resistance by a humidity test.     

Raw materials,microstructure, and properties of MgO–C refractories: Directions for refractory recipe development

A range of steel making vessels and continuous casting components use graphite containing MgO-C refractories that work from ambient to 1600 °C or higher. In the current study, a detailed review on the key importance behind the rightful selection of raw material quality in the development of MgO-C refractories with improved high-temperature microstructure stability is provided. Special cases of carbon\ceramic reinforcements (SiC, nanocarbon, EG, CNT’s, Zircon, Titania) are also included in this review study with the combination of regular raw materials used in refractory formulation such as magnesia, graphite, resin binder, antioxidant additives, and alloys thereof. Additionally, the material design concept based on strength factor (f_s) has been applied to implicate the raw material quality analysis in the development of carbon containing refractory recipe exhibiting satisfactory hot-strength performance with the recyclable MgO-C grog over the commercially available carbon\ceramic reinforcements is discussed.     

Effect of electromagnetic field on slag corrosion resistance of low carbon MgO–C refractories

Using MgO–C refractories containing 6% carbon and the slag with a basicity (CaO/SiO₂) of around 0.8, the melting slag resistance experiments of low carbon MgO–C refractories were carried out in induction furnace and resistance furnace, respectively. The microstructure of low carbon MgO–C refractories corroded by slag under the different conditions was analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDAX). The results show that in induction furnace having electromagnetic field (EMF), there are MgFe₂O₄ spinel with a little of Mn ions generated in the interfacial layer. Part of the solid solution is monticellite [CaMgSiO₄] containing a little MnO and FeO. While under the condition of EMF free, there is not MgFe₂O₄ spinel in the interfacial layer and the solid solution is monticellite (CaMgSiO₄). At a high temperature, EMF increases the diffusion coefficient of Fe^2+/3+ ions, which displaces Mg²⁺ and forms MgFe₂O₄ with a little of Mn ions. There are MgAl₂O₄ spinel in the penetration layers under the conditions of both EMF and EMF free. EMF speeds up corrosion of low carbon MgO–C refractories.     

Fracture behavior of low carbon MgO–C refractories using the wedge splitting test

The fracture behavior of low carbon MgO–C refractories containing various carbon sources were investigated by means of the wedge splitting test and microscopic fractographic analysis to evaluate quantitatively their thermal shock resistance in the present work. The results showed that the addition of various nanocarbons in MgO–C specimens can lead to more tortuous crack propagation path during the wedge splitting test and much better thermal shock resistance compared to the specimen with flaky graphite as carbon source; particularly, the specimen containing carbon nanotubes had the most outstanding thermal shock resistance. Also, it was suggested from the correlation analysis that the increase of the specific fracture energy and interface crack propagation as well as the decrease of the modulus of elasticity, coefficient of thermal expansion and transgranular crack propagation can contribute to an improvement of thermal shock resistance of MgO–C refractories.