普通烧结镁砂骨料中杂质相对镁碳耐火材料抗热震性和高温断裂行为的影响

Influence of Impurities in Ordinary Sintered Magnesia Aggregates on Thermal Shock Resistance and High Temperature Fracture Behavior of MgO–C Refractory

镁碳砖是炼钢过程中重要的耐火材料之一。为进一步提高其抗热震性,本文利用普通烧结镁砂存在少量低熔点相特性,在镁碳材料制备过程中部分替代电熔镁砂骨料,使之在服役条件下(1 000~1 700℃)材料基体内产生局部液相,缓解材料中应力集中的问题。结果表明:经1 000℃热处理后镁碳材料内普通镁砂骨料保持完整;温度升至1 400℃,部分普通镁砂骨料中的杂质溶出并向周围基质中扩散,骨料瓦解呈多孔结构。此外,基质内因单质硅的引入,除形成大量的晶须状和颗粒状Mg_(2)SiO_(4)相外,还出现了Si C晶须;1 600℃热处理时,杂质溶出扩散加剧,基质中存在颗粒状Mg2SiO4相和Si C晶须。上述材料在高温下的结构变化,提高了材料的强度,甚至高温抗折强度,改善了材料的抗热震性能。高温劈裂实验表明,烧结镁砂的孔隙结构缓解了应力集中,而1 400℃条件下液相的形成促进了烧结和基质中晶须的形成,增加裂纹沿骨料–基质界面扩展的比例,最终导致材料的断裂能和抗热震因子增加。

Introduction MgO–C refractories are extensively utilized as lining materials for smelting equipment(i.e.,converters,ladles,and electric arc furnaces)due to their excellent resistance to slag erosion and thermal shock.Nevertheless,under high scrap ratio smelting conditions,MgO–C refractories are subjected to mechanical stress due to the introduction of scrap steel and thermal stress caused by frequent fluctuations in service temperatures,resulting in a substantial reduction in their operational lifespan.It is thus crucial to further enhance the mechanical properties and thermal shock resistance of MgO–C refractories.In recent years,effective approaches on the composition and structural design of materials are explored,which include the construction of nanostructure matrices,light-weighting and strengthening of magnesia-based aggregates,the introduction of novel additives,and the optimization of secondary carbon in binders.Among them,calcium magnesium aluminate(CMA)aggregates as a novel porous aggregate have attracted much attention.At elevated temperatures,it induces a partial liquid phase within the material matrix,effectively mitigating internal thermal stresses,repairing micro-cracks resulting from thermal shock,and facilitating the formation and growth of spinel whiskers.Consequently,a substantial enhancement in both toughness and thermal shock resistance of the material is achieved.In fact,ordinary sintered magnesia aggregates contain certain impurity phases,such as merwinite(with a melting point of 1550℃)and andradite(with a melting point of 1170℃).When conventional fused magnesia aggregates partially are substituted with ordinary sintered magnesia aggregates in the preparation of MgO–C refractories,localized liquid phases can be generated within the matrix at high temperatures,which may serve a similar function to CMA aggregates.Compared to CMA aggregates,ordinary sintered magnesia aggregates are more affordable,resulting in a substantial reduction in production expenses and facilitating the efficient utilization of low-grade magnesia.In this paper,ordinary sintered magnesia aggregates were selected to partially substitute conventional fused magnesia aggregates in the preparation of MgO–C materials.The microstructure evolution,mechanical properties,thermal shock resistance,and high-temperature fracture behavior of MgO–C refractories containing ordinary sintered magnesia aggregates were investigated.Methods MgO–C refractories were prepared via substituting 10%of ordinary sintered magnesia aggregates(w(MgO)=93.33%,w(SiO_(2))=1.72%,w(CaO)=3.12%,and w(CaO)/w(SiO_(2))=1.81,w is mass fraction)for conventional fused magnesia aggregates(w(MgO)=97.35%,w(SiO_(2))=0.5%,w(CaO)=1.12%,and w(CaO)/w(SiO_(2))=2.24)with a flake graphite as carbon source,metallic silicon powder and boron carbide as antioxidants,and thermosetting phenolic resin as a binder.After thoroughly blending the aforementioned materials,bar-shaped specimens with the sizes of 140 mm×25 mm×25 mm and standard brick specimens with the sizes of 230 mm×110 mm×70 mm were shaped at 150 MPa and then cured at 200℃for 24 h.The cold modulus of rupture(CMOR)of the bar-shaped specimens,treated at different temperatures(i.e.,200,1000,1400℃and 1600℃)in a coke bed was measured by a model XD-117A three-point bending test machine.The microstructure and elemental composition of the specimens were determined by a model Nova Nano 400 field emission scanning electron microscope,combined with a model IE 350 PentaFET X-3 energy dispersive spectrometer.The bar-shaped specimens(140 mm×25 mm×25 mm)coked at 1400℃were selected for thermal shock test by an oil quenching method.Namely,these specimens were initially heated in a coke bed at 900℃for 30 min,followed by quenching in an oil bath.After undergoing three thermal shock cycles,the residual strength index(CMOR after thermal shocks/CMOR before thermal shocks)of specimens was calculated.Subsequently,the bar-shaped specimens after thermal shock test were further heated in the coke bed at 1600℃for 3 h,and then the recovery strength index(the change of CMOR after reheating to 1600℃/CMOR after thermal shocks)of MgO–C specimens was calculated.In addition,the standard brick specimens were processed into the wedge splitting specimens(100 mm×100 mm×70 mm),and heated in the coke bed at 1000℃.Subsequently,high-temperature wedge splitting tests were conducted at 1000℃and 1400℃for 30 min,respectively.The specimens were surrounded in alumina crucibles filled with coke in a high-temperature furnace to establish a reducing atmosphere.Fracture parameters such as the nominal notch tensile strength(σNT),specific fracture energy(Gf),characteristic length(lch)and thermal shock resistance(Rst),were calculated to further quantitatively characterize the thermal shock resistance of specimens.Results and discussion Ordinary sintered magnesia aggregates remain intact in MgO–C refractories coked at 1000℃.However,the impurities within some ordinary sintered magnesia aggregates dissolve and diffuse into the surrounding matrix as the temperature increases to 1400℃,disintegrating the aggregates into a porous structure.In addition,Mg_(2)SiO_(4) particles and whiskers,as well as SiC whiskers also appear in the matrix due to the introduction of silica powder.The dissolution and diffusion of impurities increase as the temperature further increases to 1600℃,generating Mg_(2)SiO_(4) particles and SiC whiskers in the matrix.These structural changes in MgO–C refractories at high temperatures enhance the strength,even the high-temperature flexural strength,and improve the thermal shock resistance of MgO–C refractories.The results of high-temperature wedge splitting test show that MgO–C specimens containing ordinary sintered magnesia aggregates exhibit an equivalent peak load to those of the control specimens,while their fracture displacements significantly enlarge.As evident from various fracture parameters,MgO–C specimens containing ordinary sintered magnesia aggregates exhibit higher Gf,lch,and Rst at 1000℃and 1400℃,indicating that the introduction of ordinary sintered magnesia aggregates effectively reduces the material brittleness due to improving the thermal shock resistance of specimen Also,the specimens containing ordinary sintered magnesia aggregates exhibit a higher proportion of cracks along the aggregate-matrix interface in the crack propagation path.The enhancement of fracture toughness and thermal shock resistance of MgO–C refractories with ordinary sintered magnesia aggregates addition can be attributed to the stress relief provided by the porous structure of ordinary sintered magnesia,the promotion of sintering at 1400℃by the formation of liquid phases,and the formation of whiskers in the matrix.Conclusions The addition of ordinary sintered magnesia effectively enhanced the thermal shock resistance of MgO–C refractories due to the porous structure of ordinary sintered magnesia aggregate and its ability to form a liquid phase at high temperatures,thereby reducing the overall thermal expansion rate of the specimens.MgO–C specimens containing ordinary sintered magnesia aggregates exhibited higher Gf,lch,and Rst at 1000℃and 1400℃due to the higher porosity of the ordinary sintered magnesia aggregates,effectively mitigating the stress concentration.At 1400℃,the impurities in the ordinary sintered magnesia aggregates dissolved and diffused into the matrix to form a liquid phase network that was conducive to absorbing stress at crack tips.The formation of this liquid phase promoted the whiskers formation and slowed down the crack propagation.