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.     

CaO含量对刚玉质浇注料性能与显微结构的影响

通过控制铝酸钙水泥加入量,研究了CaO含量分别为0、0.5%、1.3%、1.7%、2.5%、3%、3.5%、4.9%、5.6%和7.5%时对刚玉质浇注料烧结性能、抗热震性和显微结构的影响,同时借助XRD、SEM、EDS和高温热膨胀仪等手段对浇注料的物相变化、热膨胀系数和显微结构进行了分析和观察。结果表明:刚玉质浇注料中CaO含量显著影响着浇注料的烧结性能、抗热震性及其显微结构,当CaO含量低于2.5%时,随着CaO含量的增加,浇注料中片状CA6逐渐增多,并伴随着大量的体积膨胀,导致浇注料体积密度下降,显气孔率上升;当CaO含量超过2.5%时,随着CaO含量增加,浇注料中CA2相含量增加,CA6相含量逐渐减少,浇注料体积密度开始增加,显气孔率下降,线膨胀率减小,CaO含量高于5.6%的浇注料出现了收缩;刚玉质浇注料的抗热震性与显微结构密切相关,CaO含量为4.9%～7.5%的刚玉质浇注料具有很好的抗热震性,这是由于浇注料基质中粒状CA2与片状CA6相互穿插,刚玉骨料边缘片状CA6向基质中生长,强化了基质与骨料的结合,同时CA2相具有低膨胀性,导致浇注料具有较高的强度保持率和抗热震性。     

Heat resistance and thermomechanical behaviour of ultralow and zero cement castables

Abstract

Knowledge of relative heat resistance as well as thermo-mechanical behaviour of refractory castables is very important for their use as linings in high temperature furnaces and refining vessels in the metallurgical, cement, and petrochemical industries. The present work aims at studying these properties for different types of refractory castable. Two classes of castable were prepared, namely ultralow and zero cement, containing either high alumina cement or hydratable alumina as bonding agent. For each class, two different castable systems were prepared, one containing an alumina-silica mixture in its matrix and the other containing magnesia-alumina. In all castables studied, calcined alumina was used as aggregate. The prepared castable samples were subjected to firing temperatures up to 1500°C. Relative heat resistance, bending strength before and after thermal cycling, hot modulus of rupture, and creep deformation were measured according to international standard specifications. It was concluded that a limited content of cement (ultralow cement castables) is beneficial with the magnesia-alumina mix in the matrix owing to the formation of calcium hexaluminate-magnesium aluminate-corundum (matrix advantage system) that results in excellent relative heat resistance as well as thermome-chanical properties. Zero cement castables on the other hand are recommended for use with the alumina-silica mixture, since the absence of cement improves the chances of mullite formation without glassy phases, thereby enhancing the properties of such refractory castables.     

Microstructural changes and evolutions of elastic properties versus temperature of alumina and alumina–magnesia refractory castables

This paper deals with the evolutions of thermo-mechanical properties of castable refractories versus temperature. The measured properties are mainly Young's modulus evaluated by a high temperature ultrasonic technique and thermal expansion followed by dilatometry. Materials are alumina-based low cement castables with a fraction of alumina eventually substituted by spinel or magnesia. The granularity of the different used raw materials is chosen by using a packing model, in order to reduce the final porosity in the hydrated state. The interpretation of results is carried out by considering the materials as composites, constituted of aggregates (size > 100 μm) into a matrix where most of chemical and phase transformations occur. By using two-phase analytical models, it is shown that the evolutions of thermo-elastic properties of castables can be qualitatively predicted from measurements performed in a matrix-equivalent simplified material. Moreover, considering the elastic properties after heat treatment, the castable with magnesia is comparable to the castable processed with synthetic spinel.     

不同结合系统刚玉-方镁石-尖晶石浇注料的性能

对比研究了水泥结合和水合氧化铝结合的刚玉 -方镁石 -尖晶石浇注料的性能。结果表明 :二者相比 ,水合氧化铝结合浇注料的需水量较大 ,流动性较差 ,烧结致密化温度高 ,在 1 5 5 0℃下烧结后的强度和致密度低 ,但其抗热震性能及抗渣性能较好。     

Fabrication of aluminium titanate castable

Abstract

A castable was made using an aggregate of aluminium titanate (AT) of composition 95%Al_1·57 Fe_0·43 TiO₅ , 2% corundum, and 3% aluminosilicate glass with 10% high alumina cement, deflocculant, and water. Cast blocks after firing at 1400°C for 6 h were used to determine physical and mechanical properties at ambient temperature. The castables had a crushing strength of 125 MPa, a bend strength of 14 MPa, and a Young's modulus of 9·6 GPa. They had a coefficient of thermal expansion (CTE) of 1·3 × 10^-6 K^-1 from ambient temperature to 1200°C. The low Young's modulus and CTE indicate that the castable should have excellent thermal shock resistance. The properties of the AT castable when compared with those of commercial high alumina castables showed it to have a similar strength, but much lower porosity (10%), Young—s modulus, and CTE. Phase analysis of the cast blocks showed that the major phase was iron aluminium titanate with minor phases (<10%) of corundum and a calcium aluminosilicate, which is probably a glass.     

The impact of bonite aggregate on the properties of lightweight cement-bonded bonite–alumina–spinel refractory castables

The lightweight bonite–alumina–spinel (CA₆–Al₂O₃–MA) refractory castables with bonite aggregate and different spinel sources (pre-formed and in situ formation) were prepared in this study. The phase composition, microstructural features, and mechanical and thermo-mechanical properties of CA₆–Al₂O₃–MA castables treated at various temperatures were investigated by techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), three-point bending method, and thermal shock test. The results indicated that the incorporation of bonite aggregate had a positive influence on the strength, thermal shock resistance and slag corrosion resistance. It especially decreased the thermal conductivity and had a slightly negative influence on the refractory under load and slag penetration resistance of the castables. For the in situ spinel-containing castable, the formation of in situ spinel with finer particle sizes and acicular CA₆ grains led to higher overall volume expansion, resulting in higher thermal expansion (∆L/L₀), linear change and the apparent porosity of castables. Also, the heat insulation, thermal shock and slag penetration resistance of castables with in situ spinel improved, while the strength, displacement, refractory under load and slag corrosion resistance decreased sharply.     

h-BN加入量对低水泥刚玉质浇注料性能的影响

为了提高低水泥刚玉质浇注料中温处理后的强度和抗热震性,在其配料中引入不同量的h-BN粉(其质量分数分别为0、0.5%、1%、1.5%和2%),经成型、养护、脱模、烘干、750℃热处理后,检测试样的体积密度、显气孔率、常温抗折强度、常温耐压强度、抗热震性,并进行XRD分析。结果表明:随着h-BN加入量的增加,750℃处理后试样的显气孔率有所减小,体积密度逐渐减小,常温耐压强度逐渐增大,常温抗折强度也呈增大趋势,抗热震性逐渐改善。可见,加入h-BN有助于提高低水泥刚玉质浇注料中温处理后强度和抗热震性。     

Thermomechanical and thermophysical characteristics of alumina-carbon monolithic refractory containing surface-modified graphites in matrix

The thermophysical and thermomechanical behavior of graphite-containing refractory castable matrix with 20.0?wt% of graphite had been compared with graphite-free high alumina based castable matrix analogous to it. The thermomechanical properties of similar type of castables with and without 5.0% of graphite had also been evaluated. Graphite was incorporated both as coated and as-received forms, the former having a thin sol-gel derived calcium aluminate coating on graphite flakes. The influence of thermal conductivity, thermal diffusivity and pore size distribution had been critically estimated. The changes in flexural strength, porosity and density of the matrices had also been determined to interpret the castable performance, namely refractoriness under load (RUL), spalling resistance and conventional physical properties. The role of coated graphite on improved densification and thermal tolerance of refractories was further explored by microstructure and phase evolution studies of all kinds of fired samples at 1500?°C. It was further supplemented with the scanning electron microscope (SEM) studies of surface-modified graphites at uncalcined and calcined condition.     

骨料对铝镁质浇注料性能的影响

研究了白刚玉、棕刚玉及矾土熟料三种骨料对铝镁质浇注料性能的影响。结果表明棕刚玉骨料浇注料和矾土熟料骨料浇注料具有较好的流动性、高温烧结性和抗渣性,但它们的热震稳定性都较差。     

Hot-erosion of nano-bonded refractory castables for petrochemical industries

Cold-erosion tests are usually applied for refractory castable evaluation in petrochemical industries, mainly for materials used in cyclones and risers of fluidized catalytic cracking units (FCCU). Due to the fact that in the operation, the refractory material is subjected to high temperature, at first, hot-erosion tests could be more representative. Erosion wear has a great influence on the FCCU refractory working life and the lost income associated with a production halt can reach values close to US$500,000 per day. These aspects are strong enough to justify hot-erosion resistance characterization. The test viability, which could result in selecting better material for the application, was evaluated using commercial products and nano-bonded castables from 200 up to 815 °C. At this temperature range, a significant difference between the cold and hot-erosion loss was not observed. Therefore, it indicates that the cold-erosion test is suitable and sufficient for selecting refractory castables for FCCU applications. The cold-erosion test also presented a higher data correlation with the splitting tensile strength and the hot modulus of rupture. Regarding the different evaluated materials, the nano-bonded castables showed a higher erosion resistance and were less susceptible to the temperature effect than the calcium aluminate cement bonded commercial products.     

陶瓷梗对致密浇注料性能的影响

对比研究了添加莫来石质陶瓷梗和不锈钢纤维分别对刚玉质超低水泥浇注料和高铝质超低水泥浇注料性能的影响 ,并从显微结构分析了其影响机理。研究表明 :莫来石陶瓷梗能显著改善烧成后致密浇注料的抗热震性和体积稳定性 ;而含不锈钢纤维的浇注料烧成后 ,纤维氧化成氧化铁和氧化铬 ,促进了烧结以及纤维与基体之间的结合 ,使浇注料的刚性增加 ,高温抗折强度提高 ,但抗热震性下降 ,同时 ,高温下钢纤维氧化生成更大体积的氧化铁 ,导致了浇注料本体的膨胀 ,使其烧成线变化增加。     

Preparation and characterization of HA bonded Al2O3–MgO based castables with superior slag resistance for the working lining of Si-killed stainless steel ladles

The study prepared Al₂O₃–MgO based castables bonded by hydratable alumina (HA) instead of calcium aluminate cement (CAC) for the working lining of Si-killed stainless steel ladles. The microstructure, phase composition, mechanical properties, and slag resistance of castables were investigated by SEM, XRD, and thermodynamic software FactSage®. The results indicated that the HA bonded castables showed superior hot flexural strength, thermal shock resistance and slag resistance than the CAC bonded castables, due to the optimized pore characteristics, less liquid content, and higher liquid viscosity of the castable matrix and the formation of a continuous insulating layer.     

Use of alumina dispersant in alumina-spinel castable: Comparison between in situ and preformed spinels

The role of alumina dispersant as a flow modifier, completely replacing fume silica, on the properties of preformed and in situ spinel-containing low cement high alumina castables was investigated with variations in spinel content and granulometry. Both 10 and 20 wt.% spinel-containing compositions were prepared using preformed spinel or magnesia in alumina castable compositions with vibratable and self-flowing consistency. The castables underwent conventional processing methods and accessed for various refractory-related parameters post-heat treatments. No glassy phase was observed in the castables upon firing. The presence of self-flowing consistency led to improved characteristics. Preformed spinel-containing compositions showed improved density, strength, hot strength, and thermal shock resistance properties, whereas slag corrosion and penetration resistances were found to be higher for in situ spinel-containing castable.     

展望新一代优质高效耐火材料

展望了新世纪为适应高温新技术的发展而必将兴起并迅速发展的新一代高效耐火材料 ,它们包括 :(1 )具有优良的高温强度、抗热震性和抗侵蚀性的氧化物 -非氧化物复合材料 ;(2 )能够净化金属熔液和吸收废气中有害杂质的含游离CaO的碱性材料 ;(3 )高性能自流浇注料以及基质成分和结构逐渐变化的“梯度”浇注料     

In situ spinel bonded refractory castable in relation to co-precipitation and sol–gel derived spinel forming agents

This paper deals with the preparation and characterization of two types of in situ spinel bonded low cement high alumina based castable refractories. Semidried magnesium aluminate mass was prepared from cheaper precursors via coprecipitation and sol–gel routes for application in a refractory castable composition in different concentrations. The pH, average particle size, solid content, DTG analysis and XRD patterns of those two additives were observed. After being fired at elevated temperatures those two kinds of in situ spinel bonded castables were characterized and compared in terms of bulk density, apparent porosity, cold crushing strength, flexural strength, volume shrinkage, spalling resistance, and XRD phase analysis. Scanning electron microscopy of some selected fired samples was done to analyse the mode of interaction of in situ spinel bonds in castable microstructure. The corrosion resistance of the castables was estimated by heating with blast furnace and converter slags.     

Al2O3-SiC-C质湿式喷射浇注料的研究和应用

以高纯莫来石、碳化硅、SiO2微粉、Al2O3微粉、纯铝酸钙水泥、白刚玉细粉和球沥青等为原料,研究了q值为0.23、0.25、0.27、0.29和0.31时湿式喷射浇注料的流变性能;进一步研究了q值为0.27时,SiC加入量(质量分数分别为12%、15%、18%、21%)对其流变性能、常温物理性能和高温抗折强度、抗热震性能、抗渣侵蚀性能的影响。结果表明:根据Andreassen方程,当q=0.27时,配制的浇注料具有较好的流变性能;加水量为6.5%(w)时,研制浇注料的流变性能类似于宾汉姆流体,适于管道输送;SiC加入质量分数为18%时,浇注料具有较高的高温抗折强度、抗渣侵蚀性能和抗热震性能;经对鱼雷罐和出铁沟的喷补使用证明,研制的湿式喷射浇注料施工性能好,使用性能优异。     

Alumina-spinel castable for steel ladles: An overview

Alumina-spinel castable has excellent refractory properties, including high volume and thermal stability, good hot strength, and excellent resistance against corrosion, erosion, and thermal spalling. Excellent refractory properties have made it a preferred lining for steel ladles, reducing the erosion and corrosion of molten metal and slag and improving the ladle life and efficiency for decades. These castables are also significant for the replacement of chrome-bearing refractories that cause several health hazards. The current review paper covers preformed and in situ spinel containing alumina castables. It focuses on the importance of these castables in the sidewall and bottom portion of the steel ladle in detail, the effect of spinel and MgO content, and the factors affecting castable properties. The review also covers the final properties of castable and the application areas of alumina-spinel castable in steel ladle, purging plugs, and RH-degasser.     

Nano mullite bonded refractory castable composition for high temperature applications

Development of high pure alumina castable is studied by using synthesized mullite sol as the sole binder. Mullite sol is prepared by wet chemical route and is characterized by its solid content, particle size, thermal analysis, phase development with temperature, microstructure, etc. This sol is used at two different percentages in high alumina castable compositions with two different particle size distribution patterns. Conventional castable processing is done on the compositions and the characterizations are done after heat treatment at three different temperatures. Finally, the best composition is also compared with the commercially available silica sol containing high alumina castable with similar particle size distribution. Considerably improved hot strength, high corrosion resistance and flexural strength (hot modulus of rupture) are obtained for the mullite sol containing composition but with relatively lower thermal shock resistance.     

碳化硅对低水泥结合氧化铝空心球浇注料性能的影响

以3种粒度级别的氧化铝和氧化铝微粉为原料,以铝酸钙水泥为结合剂,采用振动浇注成型的方法制备了低水泥结合氧化铝空心球浇注料。利用耐火材料检测设备和扫描电子显微镜(SEM)研究了加入碳化硅对低水泥氧化铝空心球浇注料性能及微观结构的影响。研究结果表明,适量加入碳化硅对提高该种浇注料的体积密度、常温耐压强度和热震稳定性有利。