Property Improvement By In-situ Formed Non-oxide Bonding Phase Via Nitridation And Carbonization Of Alumina Castables

Oxide-nonoxide composites are of impportant appraoch to high performances ofrefractories. However, there are few published works on in situ formed non-oxidebonding phases in castables. In order to achieve high performance of refractories forBF’s tuyere and meet the requirement of lining with precast shapes of castables, thiswork tried a new approach to properties improvment by incorporating silicon powdersin alumina based castables and then nitridation or carbonization to form in situ non-oxide bonding phases. It consists of three parts, i.e. firstly the investigation of theeffect of Si powder addition on properties of ULC alumina based castables afternitridation, then the investigation of the influence of carbonization by carbonembedding on properties of the castables added with Si powders, and finally theinvestigation of the effect of binding system on properties of the castable added withcertain Si addidtion after nitridation.Using sub-white fused alumina and tabular alumina respectively as aggregates,and tabular alumina, SiC, CA cement, microsilica and ultrafine α-Al2O3powders as thematrix, alumina besed ULC castables were prepared. Influences of Si powder additionsat0%,4%,6%,8%and10%on physical properties, cold strengths, hot modulus ofrupture (HMOR), thermal shock resistance (TSR), abrasion resistance, alkali resistanceand microstructure of the castables after nitridation and carbonization respectivelywere investigated. The effect of different CA cement addition on properties, phasecomposition and microstructure of the castables with10%of Si addition was alsoinvestigated after nitridation.The in situ formed nonoxide bonding phases including β-Sialon, O’-Sialon andSiC are found in the castables incorporated with Si powders after nitridation orcarbonization, leading to significant improvement in high temperature properties. TheHMOR at1200℃and1400℃increases from10.5MPa and0.4MPa respectively without Si addition to32.0MPa and5.7MPa respectively at10%Si addition after thenitridation. The residual CMOR ratio after thermal shock increases from46.7%without Si addition to102.6%with10%Si addition. After the carbonization, theHMOR at1400℃increases from1.6MPa without Si addition to8.3MPa at10%Siaddition；the residual CMOR ratio after thermal shock increases from64.9%for S0to137.9%for S10. The improvement in HMOR and TSR is attributed to themicrostructure featured by in situ formed needle-or prism-like nonoxide crystalsinterlocked in matrix. Meanwhile, the nonoxide crystals connecting to the corundumgrain boundaries grow into the matrix, which can enhance the bonding between thematrix and the aggregate, contributive to increased HMOR. Glassy phase and anorthite(CAS2) are formed in the cement containing bonding system, benefitiable to coldstrength but unfavorable to hot strength due to softening at high temperature.Hydratable alumina in replace of cement can reduce CaO and remarkably improveHMOR of the castables. The HMOR at1400℃in the cement free system reaches21.1MPa at10%Si addition after the nitridation, compared to4.5MPa in the ULC system.The present work has confirmed that in situ nonoxide or oxide-nonoxidecombined bonding in the alumina based castables can be formed by incorporatingsilicon powders and then nitridation or carbonization, leading to high performace ofthe materials. Cement free binding system is more favorable to maintain high HMORat1400℃. The achievements by this work deserve to be a reference and guidance tofurther work in the connection of property optimization and application of suchcastable precast shapes.