Three stage creep behavior of MgO containing ordinary refractories in tension and compression

In service tensile and compressive stresses occur in refractory linings, these stresses lead to creep of refractories. Ordinary refractories experience creep of the primary stage and may further proceed to the secondary and tertiary creep stage. For the development of advanced material models for finite element simulations it is necessary to investigate the creep behavior in all three creep stages under tensile and compressive loads. Hence, two advanced high temperature uniaxial creep testing devices, applying a wide range of tensile and compressive loads, were used to determine the three creep stages in a reasonable time under service related loading conditions. The Norton–Bailey creep equations and an inverse identification procedure were applied for the evaluation of the experimental results. A magnesia refractory was studied at elevated temperatures and its respective creep parameters for each stage were determined. The stress dependency on the creep behavior can be seen clearly on the creep curves and the corresponding creep parameters. Furthermore, a comparative study of creep parameters and creep rates was performed between the magnesia refractory and a magnesia-chromite refractory. The results demonstrate the significant asymmetrical creep behavior in tension and compression for both materials. The creep investigation in this paper favors the requirement for consideration of the three stage creep behavior and the asymmetrical creep behavior in thermomechanical modelling activities of industrial vessels.     

Compressive creep testing of refractories at elevated loads—Device,material law and evaluation techniques

In order to cost-effectively characterize the high temperature compressive creep behaviour of refractories a testing device was designed for application at elevated loads. Special measures have been taken necessary to enable an even stress distribution within the specimen. To identify Norton-Bailey strain hardening creep law parameters a general inverse procedure using a Levenberg–Marquardt algorithm was developed. Satisfying experimental results could be received from the creep measurement in a wide range of temperatures and loads for both shaped and unshaped materials. By fitting the strain/time curves the creep law parameters of refractories under various temperatures can be precisely identified. The measurements also reveal that at elevated loads all three creep stages can be observed.     

Advanced Measures to Characterize Mechanical Properties of Refractories

The thermomechanical modelling method is becoming an important tool nowadays for the refractory researchers,suppliers and end-users. On one hand,applications focus on the post-mortem thermomechanical analysis to interpret the occurred failure phenomena of refractories in service. On the other hand,a priori investigation is very helpful for the design of refractory lining concepts before putting them into effect; as a result it will minimize the probability of refractory lining premature failure and save costs for the refractory suppliers as well as for the end-users. For both investigation routines,suitable material constitutive models and testing approaches are of relevance. Existing material constitutive models often used for refractories are the fictitious crack model acting for tensile failure,the Mohr-Coulomb or Drucker-Prager model describing shear failure,and the Norton-Bailey model representing creep. To characterize the tensile and shear failure of refractories at room temperature and elevated temperatures,a wedge splitting test and a modified shear test can be applied,respectively. The creep behavior and corresponding creep parameters of refractories can be determined with an appropriate creep testing device at elevated loads. The proper application of material constitutive models and testing approaches allows for improving the thermo-mechanical modelling and the optimization of the lining design.     

Effect of carbon content on the oxidation resistance and kinetics of MgO-C refractory with the addition of Al powder

The effect of carbon content of MgO-C refractory was studied with respect to the physical properties, oxidation resistance and kinetics. The oxidation mechanism was investigated by calculating the relevant parameters of oxidation and analyzing the evolutions of phases and microstructures. The oxidation resistance of MgO-C refractory was influenced by the indirect oxidation of carbon through the formation of the MgO constituent layer. The effect of indirect oxidation resulted in the MgO-C refractory with 3 wt% carbon content having a lower oxidation rate and weight loss rate when fired at 1600 °C than when fired at 1400 °C. The oxidation kinetics results based on shrinking core model showed that the effective diffusion coefficient and full oxidation time increased with the increasing of carbon content. This seemingly paradoxical result was mainly due to the effect of the molar density of carbon prevailing over that of the effective diffusion coefficient in the oxidation process.     

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.     

Investigation of Compressive Refractory Creep

Creep of refractories at high temperatures may have significant influences on the thermomechanical behavior of refractory linings in industrial vessels,and thus the consideration of creep in finite element modeling of industrial vessels is necessary. The present paper introduces an advanced high temperature compressive creep device and a proper inverse procedure using Levenberg-Marquardt algorithm to identify Norton-Bailey creep parameters. In addition,finite element thermomechanical modeling results of a RH-snorkel are presented to illustrate the impact of creep,creep formulations and creep models on the circumferential stress and joint opening of the working lining.     

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.     

Method of raw materials selection for production of the MgO-C bricks of comparable properties using PCA and K-medoids

MgO-C refractories are zonally installed in most of the key heat devices of steel and iron metallurgy with the exposure to various corrosive factors. One of the most significant factors is low-temperature decarburization resulting in the oxidation of carbon in MgO-C refractories and loss of its properties. Moreover MgO-C materials are exposed for direct attack of liquid slag or steel melt which easily infiltrate the porous, decarburized hot face of the bricks. Cost of MgO-C materials of high quality is important for refractories user. For this reason researches try to develop materials of high quality but with lower cost by substituting fused raw materials with sintered ones or with recycled MgO-C. In this article, method of selecting the raw materials for production of MgO-C bricks with comparable properties is shown. Twenty different variants of MgO-C materials were prepared with the use of various quality raw materials. Basic properties of the industrially produced MgO-C bricks were measured. To select recipes with similar quality principal components analysis method and K-medoids algorithm were applied. To verify the results corrosion resistance of selected MgO-C materials were tested with the use of induction furnace of medium frequency.     

A method to characterize asymmetrical three-stage creep of ordinary refractory ceramics and its application for numerical modelling

During operation, thermomechanical stresses occur in refractory linings. Under elevated stress and temperatures, these ceramics experience primary creep, which can further proceed to the secondary and tertiary creep stages. This necessitates a characterization of their three-stage creep behavior. Hence, two advanced uniaxial tensile and compressive creep testing devices are utilized. The Norton-Bailey creep equations and an inverse identification procedure are applied for the evaluation of the creep curves. To account for the full three-stage creep behavior in thermomechanical modelling activities, a creep-stage transition criterion is identified and subsequently implemented together with the Norton-Bailey creep-strain rate representations in a new developed creep model. The finite element simulation results from different creep testing procedures are in accordance with the corresponding experimental results of a magnesia-chromite refractory ceramic. The study also reveals the temperature-dependent asymmetrical creep behavior of the material in terms of the creep-strain rates and critical creep strains.     

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.     

Study on low carbon containing MgO-C refractory: Use of nano carbon

Development of nano carbon containing magnesia carbon refractories has been studied to reduce the total carbon content, thereby reducing the heat loss from the metallurgical process and producing more eco-friendly refractories. The carbon contamination from refractory to liquid metal will be minimized using low amount of nano carbon. Different percentages of nano carbon are used in combination with graphite as carbon source and the total carbon is maintained below the half of the total carbon of the conventional MgO-C refractories. The compositions were processed as per the conventional manufacturing techniques and the properties were evaluated and compared against the conventional refractory prepared under exactly similar conditions. Also elemental mapping of carbon was done to study the distribution of the nano carbon in the matrix.     

Nano carbon containing MgO-C refractory: Effect of graphite content

Development of low carbon containing MgO-C refractories has been studied containing a fixed 0.9 wt% of nano carbon and 1–9 wt% of flake graphite. Refractory compositions were prepared and processed as per the conventional manufacturing techniques of MgO-C refractory. Properties of the different compositions were evaluated and also compared against the conventional MgO-C refractory containing 10 wt% graphite prepared under exactly similar conditions. Addition of 3 wt% of flake graphite in combination with 0.9 wt% of nano carbon black was found to be optimum and resulted in better/comparable properties to that of conventional MgO-C refractory.     

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。     

几种钢包用含碳耐火材料对IF钢增碳的比较

通过感应炉试验分析了钢包渣线用3种碳含量不同的镁碳砖(C的质量分数分别为8.3%、15.5%和17.9%),钢包底用蜡石-碳化硅砖(C的质量分数为3.71%)以及实验室开发的MgO-Al2O3-SiC质钢包渣线浇注料(C的质量分数为4.07%)对IF钢增碳的影响,并对其增碳的机理进行了初步分析讨论。试验结果表明:渣线镁碳砖的碳含量越高,对IF钢造成的增碳量越大;包底蜡石-碳化硅砖对IF钢水的增碳量达到渣线镁碳砖的7.73倍;MgO-Al2O3-SiC质浇注料也对IF钢产生明显的增碳效果,不宜用作冶炼超低碳钢的钢包渣线材料。     

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.     

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.     

Effect of different metal powder anti-oxidants on N220 nano carbon containing low carbon MgO-C refractory: An in-depth investigation

Effect of three different metal powder antioxidants, namely, Al, Si, and Mg was studied on the development of N220 nano carbon containing low carbon MgO-C refractory. The study was also extended to conventional MgO-C refractory compositions with 16% graphite content. All the refractory compositions were processed as per conventional MgO-C refractory manufacturing techniques and were evaluated for physical, mechanical, thermo-mechanical properties, oxidation resistance and static corrosion resistance against steel converter slag. Microstructural developments and in-situ ceramic phase formation in the compositions were also studied. The results show that nano carbon containing low carbon MgO-C refractory compositions have better properties than conventional composition and among the three anti-oxidants, the aluminum metal powder has shown the improved properties.     

α-Al_2O_3微粉对低碳MgO-C材料性能的影响

以电熔镁砂及鳞片石墨为主要原料,热塑性酚醛树脂为结合剂,研究了α-Al_2O_3微粉对低碳MgO-C材料性能的影响.结果表明:(1)Al_2O_3微粉的加入提高了低碳MgO-C材料的体积密度,降低了显气孔率,提高了强度,显著改善了MgO-C砖的抗氧化性;(2)高温下Al_2O_3微粉与MgO原位反应生成连续的尖晶石相,改善了材料基质的显微结构,增强了陶瓷结合,将MgO颗粒与鳞片石墨紧密地结合起来,提高了组织结构的整体性,从而提高了材料的致密度和强度;(3)高温下原位反应生成连续的尖晶石相堵塞脱碳层中的部分气孔,降低了氧的扩散速度,同时提高了MgO的沉积速度,使加入Al_2O_3微粉的低碳MgO-C材料抗氧化性能得以提高.     

Observed and Theoretical Creep Rates for an Alumina Ceramic and a Silicon Nitride Ceramic in Flexure

Observed creep curvature rates are compared to theoretical rates for both an alumina ceramic at 1000°C and a silicon nitride ceramic at 1200°C in four-point flexure. The observed rates have been calculated from published rise-displacement rates, and the theoretical rates have been calculated from published power-law parameters for compressive and tensile creep, which differ appreciably for these ceramics. Although both compressive and tensile creep measurements are easier to analyze than flexural creep measurements, the latter are usually less expensive and easier to conduct. The present work shows the usefulness of flexural creep tests to verify the accuracy of compressive and tensile creep tests.     

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.