Phase and microstructural evolution of low carbon MgO-C refractories with addition of Fe-catalyzed phenolic resin

In the present paper, phase and microstructural characterization of low carbon MgO-C refractories with addition of Fe-catalyzed phenolic resins as binder were investigated. Initially, phenolic resin was modified using various amounts of Fe particles as catalyst originated from iron nitrate ([Fe(NO₃)₃·9H₂O]). The MgO-C matrix compositions were prepared by using 7% of modified phenolic resin, shaped and cured at 200?°C for 24?h. The cured samples were coked in the temperature range from 800 to 1400?°C and then characterized by XRD and FE-SEM techniques. Based on attained results, in-situ graphitic carbons, particularly in carbon nanotubes (CNTs) network were gradually formed from Fe-catalyzed phenolic resin in the matrix of MgO-C refractory bodies. It was also clarified in comparison with sample containing as-received phenolic resin, more ceramic whiskers such as Al₄C₃, AlN, MgO and MgAl₂O₄ were formed in the matrix of MgO-C specimens with addition of Fe-catalyzed phenolic resin binder and significally increased with coking temperature. Microstructural observation showed the graphitic carbons like CNTs and ceramic whiskers mainly formed in the bonding phase between the aggregates, that certainly leads to enhancement of physical and mechanical properties of MgO-C refractories.     

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.     

Decarburisation behaviour of high-carbon MgO-C refractories in O2-CO2 oxidising atmospheres

High-carbon MgO-C refractories have been widely used in oxygen bottom blowing converters due to their excellent thermal shock resistance property. However, decarburisation is unavoidable in MgO-C refractory materials applied around oxygen bottom blowing tuyeres. To alleviate the decarburisation rate of MgO-C refractories and prolong the furnace life of oxygen bottom blowing converters, a new method consisting of mixing CO₂ in the bottom blowing oxygen was proposed by the authors. The decarburisation behaviour of MgO-C refractories in O₂-CO₂ oxidising atmosphere has not been studied before. Herein, the effects of the CO₂ ratio in the O₂-CO₂ oxidising atmosphere on the decarburisation amount and the microstructure of the decarburisation zone were investigated experimentally. The results show that as the CO₂ ratio increases, the weight loss of the MgO-C refractory decreases with a parabolic trend and the decarburisation zone depth decreases with a linear trend. Micrographs show that pores form in the decarburisation zone, and the amount of pores decreases as the CO₂ ratio increases. The results confirm that mixing CO₂ in the bottom blowing oxygen can effectively alleviate the decarburisation of MgO-C refractories.     

Oxidation behaviors of low carbon MgO-C refractories: Roles of Ti2AlC and Ti2AlN

In this study, Ti₂AlC/Ti₂AlN powders were first incorporated to fabricate low-carbon MgO-C refractories, and their oxidation behaviors were investigated. Computed tomography (CT) results indicated that stress cracks only occurred in the Ti₂AlC-added sample after exposure to 1100°C, and the anomalous oxidation behavior of Ti₂AlC powder at 578°C worsened the oxidation result at 1100°C for MgO-C refractories with Ti₂AlC. At 1500°C, the oxidation behaviors of MgO-Ti₂AlC/Ti₂AlN-C samples revealed a slight mass gain due to the disintegration of Ti₂AlC/Ti₂AlN, and their oxidation resistances increased by 18% as compared to their counterparts. In addition, the role of Ti₂AlC/Ti₂AlN was elucidated. The oxidation process was comprehensive and was mainly determined by the deterioration of carbon and MAX phases. The obtained results indicated that Ti₂AlN was more suitable for fabricating low-carbon MgO-C refractories as compared with Ti₂AlC.     

Synthesis of Al2O3-SiC powder from electroceramics waste and its application in low-carbon MgO-C refractories

Composite additives are an efficient means to improve the high-temperature stability and slag resistance of low-carbon MgO-C refractories. In this work, Al₂O₃-SiC powder was firstly synthesized from electroceramics waste by carbon embedded method at 1500°C, 1550°C, and 1600°C for 4 h, and then the as-synthesized Al₂O₃-SiC powder was used as an additive to low-carbon MgO-C refractories. The effects of its addition amounts of 0, 2.5 wt.%, 5.0 wt.%, and 7.5 wt.% on the properties of the refractories were investigated in detail. It was found that increasing the heat treatment temperature is beneficial to the phase conversion of mullite and quartz to alumina and silicon carbide in the electroceramics waste. Furthermore, the addition of Al₂O₃-SiC powder effectively improves the performance of low-carbon MgO-C samples, and the formation of spinel dense layer and high-viscosity isolation layer is the internal reason for the improvement of the oxidation resistance and slag resistance of low-carbon MgO-C samples. This work provides ideas for the reuse of electroceramics waste and presents an alternative strategy for the performance optimization of low-carbon MgO-C refractories.     

Improvements in the mechanical properties and oxidation resistance of MgO-C refractories with the addition of nano-Y2O3 powder

Nano-Y₂O₃ powder was added to the composition of MgO-C refractory to improve its mechanical properties and oxidation resistance. The changes in bulk density (BD), flexural strength (FS), cold crushing strength (CCS), and oxidation rate were studied with the phase and microstructural developments. The BD, FS and CCS of the MgO-C refractories were increased by adding the nano-Y₂O₃ powder, which contributed to the dispersion of the nano-Y₂O₃ in the gaps between differently sized MgO particles. Meanwhile, the oxidation rate of the MgO-C refractory added with the nano-Y₂O₃ was decreased owing to a dense oxidised layer was formed. The dense layer hindered the inward diffusion of oxygen and protected the refractory from oxidation.     

Combustion synthesis of B4C/Al2O3/C composite powders and their effects on properties of low carbon MgO-C refractories

To improve the dispersity and oxidation resistance of nano carbon black (CB) in low carbon MgO-C refractories, B₄C/Al₂O₃/C composite powders were prepared by a combustion synthesis method using B₂O₃, CB and Al powders as the raw materials. The phase compositions and microstructures of the synthesized products were characterized by X-ray diffraction (XRD), Raman spectroscopy, and a scanning electron microscopy/energy dispersive spectrometry (SEM/EDS). The results show that an 80 wt% excess of CB is the maximum amount of CB that can be added under the condition of a self-propagating combustion wave, and the phase compositions of the products are B₄C, α-Al₂O₃ and CB. B₄C particles with uniform sizes and cubic polyhedral structures are embedded in the Al₂O₃ matrix. The combustion-synthesized B₄C/Al₂O₃/C powders and mechanically mixed B₄C/Al₂O₃/C powders were added to the low carbon MgO-C refractories, and their corresponding properties were compared. The apparent porosity (AP) of the refractories with the synthesized powders (labelled as M3) is lower than those of the refractories with mechanically mixed powders (labelled as M2) and without composite powders (labelled as M1). The oxidation ratio and slag erosion depth of M3 were lower than those of M2 and M1. The thickness of the decarburized layer of M3 was 10.2% and 22.4% less than that of M2 and M1, respectively. The penetration depth of M3 was 12.0% and 27.9% less than that of M2 and M1, respectively. The thermal shock resistance of M3 was better than that of M2 and M1. The residual strength ratio of M3 was 15.8% and 17.2% more than that of M2 and M1, respectively. These results suggest that the combustion-synthesized B₄C/Al₂O₃/C composite powders can be used as new and promising additives for low carbon MgO-C refractories.     

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.     

High temperature mechanical behavior of Al2O3-MgO-C refractories for steelmaking use

The advantages of Al₂O₃-MgO-C (AMC) refractories are achieved mainly by the incorporation of graphite and the formation of spinel by solid reaction between alumina and magnesia. Regarding other members of oxide-C refractories (such as MgO-C bricks) and others properties (such as the slag corrosion resistance or the PLC), the information about the mechanical behavior of this type of refractories is scarce. In this work, the mechanical behavior of commercial AMC brick used in steelmaking ladles was studied by stress-strain curves in compression at RT and 1000 °C (nitrogen atmosphere). Before the mechanical testing, a comprehensive characterization of AMC materials was performed by several techniques: XRD, DTA/TGA, SEM/EDS, aggregate size distribution analysis and densities, porosities and thermal expansion measurements. Mechanical parameters such as fracture strength and strain, yield stress and Young modulus were determined together with the main characteristics of the fracture. In order to study the transformations occurred during the stay at high temperature, the specimens tested at 1000 °C were analyzed by the same techniques used for the as-received bricks characterization (with the exception of the thermal expansion analysis). The AMC refractories displayed differences in the mechanical behavior and its dependence on the testing temperature. These results were explained considering the differences in the composition and microstructure of both refractories and in their thermal transformations.     

Effect of the calcium alumino-titanate particle size on the microstructure and properties of bauxite-SiC composite refractories

Alkali resistance and thermo-mechanical properties of the transition zone of cement rotary kilns refractories are the key factors affecting their service life. Calcium alumino-titanate (CAT) containing bauxite-SiC composites were prepared using bauxite, SiC, CAT, Guang Xi white clay, α-alumina, and metallic silicon powder as starting materials and Al(H₂PO₄)₃ as the binder. The effects of the CAT particle size on the phase composition, microstructure, thermo-mechanical properties, and alkali resistance of the CAT-containing bauxite-SiC composites during firing were investigated. The results reveal that the CAT particle size strongly affects the composites’ microstructure and sintering densification. With decreasing particle size, the particle-particle and particle-matrix interfacial bonding deteriorates gradually. When CAT particles are added, the specimens show higher strength, refractoriness under load, and residual rupture strength than the specimens with fine CAT powders. Specimens with fine CAT powders show lower coefficient of thermal expansion compared to the specimens with CAT aggregates. The alkali attack test confirms that the bauxite-SiC composite refractories with CAT aggregates show better alkali resistance than those with CAT fine powder. According to the alkali mechanism, 1) K vapors penetrate the specimen through the open and connected pores and cracks, 2) K vapors react with anorthite and corundum to form kalsilite accompanied by the formation of a liquid phase and new cracks.     

Designing low-carbon MgO–Al2O3–La2O3–C refractories with balanced performance for ladle furnaces

In order to achieve high-quality and stable production of special steel, the performance of low-carbon MgO-C refractories needs to be further optimized. For this purpose, low-carbon MgO–Al₂O₃–La₂O₃–C refractories with enhanced thermal shock resistance and slag resistance were designed and successfully prepared by introducing Al₂O₃ as a reinforcer and La₂O₃ as a modifier. The results showed that the refractory samples with additives show better overall performance than those without additives. When 10 wt% of Al₂O₃ and La₂O₃ were added, the oxidation resistance, thermal shock resistance and slag resistance of the refractory samples coked at 1400 °C are increased by 13.57%, 17.75% and 43.09%, respectively. The analysis found that this can be mainly attributed to the formation of MgAl₂O₄, Mg₂SiO₄, and 2CaO·4La₂O₃·6SiO₂ and the consequent volume expansion effect and intergranular phase enhancement effect. Therefore, a low-cost and enforceable reinforcement strategy for low-carbon MgO-C refractories is proposed, which is expected to be applied in steelmaking.     

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.     

六铝酸钙加入量对MgO-C耐火材料性能的影响

为提高MgO-C耐火材料的寿命,以电熔镁砂(5～3、3～1、≤1、≤0.074 mm)、鳞片石墨(≤0.15 mm)、Si粉(≤0.044 mm)、Al粉(≤0.044 mm)、六铝酸钙(≤0.044 mm)为原料,以热固性酚醛树脂为结合剂,以150 MPa压力机压成型制备了MgO-C砖试样.试样经过1200、1400...     

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.     

Elucidating the role of Ti3AlC2 in low carbon MgO-C refractories: Antioxidant or alternative carbon source?

Different approaches to develop low-carbon refractories have been recently proposed. Among them, the use of Ti₃AlC₂ can influence the corroded interface and improve the corrosion resistance of low-carbon containing refractories, but current understanding of the role of Ti₃AlC₂ is far from complete. In this work, the impact of the single additives Ti₃AlC₂ and Si as well as their combination on the microstructure and properties of MgO-C refractories were evaluated. The newly formed ceramic bonding phases contribute to superior mechanical properties. However, the high volume expansion of Ti₃AlC₂ led to adverse effect when using Ti₃AlC₂ as single antioxidant. The combination of Ti₃AlC₂ and Si could inhibit the rapid oxidation and excessive expansion of Ti₃AlC₂.     

Improvement of low carbon MgO-C refractories by MA-CA2 additives fabricated from metallurgical waste

In the present work, MA-CA₂ material was fabricated by adding industrial alumina into industrial waste residue, and its effect on the physical properties, oxidation resistance, slag resistance, and thermal shock resistance when it was added to the composition of a low carbon MgO-C refractory was discussed in detail. Although the introduction of MA-CA₂ material led to a slight inferior slag corrosion resistance, the volume stability and oxidation resistance of refractories were improved. Moreover, the samples containing MA-CA₂ addition showed significantly lower thermal expansion coefficients and increased thermal shock resistance performance. However, owing to the dissolution of SiO₂ impurity into the MA-CA₂ material, an excessive addition of MA-CA₂ material would increase the liquid phase amount in the sample during the heat treatment and slag attack, resulting in a performance degradation. In this study, the best comprehensive performance of the MgO-C refractory was obtained with 6 wt% MA-CA₂ addition.     

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.     

Degradation mechanisms of magnesia-carbon refractories by high-alumina stainless steel slags under vacuum

The corrosion behaviour of a pitch-bonded magnesia-carbon refractory by an Al₂O₃ rich (∼15 wt.%) stainless steelmaking slag was investigated by rotating finger tests in a vacuum induction furnace at high temperature (>1650 °C) and low oxygen partial pressure (1.5–4.3 × 10⁻¹⁰ atm). This study confirms the poor slagline behaviour of MgO-C bricks industrially observed in VOD ladles. Higher temperatures and longer exposure times lead to more severe slag infiltration and direct MgO dissolution. The intrinsic MgO-C reaction is the major decarburisation mechanism, while extrinsic decarburisation by oxygen from the atmosphere and/or reducible slag components (CrO_x, FeO_x) was limited. Three kinds of metallic particles with different size, shape, location, composition and origin were observed in the refractory specimens. Concurrently, the thermodynamic conditions for the formation of a protective Mg(Al,Cr)₂O₄ spinel layer at the slag/refractory interface are investigated. The industrial relevance of this spinel layer formation is discussed with respect to the chosen Al₂O₃ level. Guidelines are proposed to minimise MgO refractory dissolution in VOD slaglines.     

低碳MgO-C耐火材料结构和性能优化的研究进展

MgO-C耐火材料作为钢铁冶炼用关键基础材料,被广泛用作转炉衬砖、电弧炉炉壁和钢包渣线用砖。寻求制备高性能低碳MgO-C耐火材料的新方法对耐火材料及冶金行业的发展至关重要,本文从纳米碳源的引入、骨料表面的改性和镁基骨料的引入、酚醛树脂的改性、抗氧化剂的引入及陶瓷相的原位形成角度出发,综述了改善低碳MgO-C耐火材料结构和性能的研究进展,以期为进一步推动低碳MgO-C耐火材料的发展提供参考,并对MgO-C耐火材料未来的研究方向进行了展望。     

Strength of forsterite refractories at high temperatures

Conclusions The compressive strength of forsterite refractories is higher with the use of prefired dunite. The noncontinuous grading does not always produce high-strength products.The strength of forsterite refractories in the 100–200°C range drops 20–40%, then increases; at 1000°C it exceeds the original strength by 20–45%, after which it drops continuously.The strength of single-phase monomineral forsterite specimens (MgSiO₄) does not alter with temperature rise to 1100°C.