Mechanical Properties And Thermal Shock Resistance Of MgO-C Refractories Containing Calcium Magnesium Aluminate

MgO-C refractories have been widely used in the linings of converter,electric arc furnaces and the slag line of steel ladles due to their excellent properties.And they have become the important materials to support the safe and stable operation of steelmaking industry.However,with the development of steelmaking technology and the acceleration of steelmaking pace,the service environment of MgO-C refractories is becoming increasingly severe,resulting in the reduction of their service life.In recent years,a new type of calcium magnesium aluminate(CMA)aggregate was successfully applied to prepare MgO-C refractories,and they showed better application effect than the traditional MgO-C refractories.However,there are few studies on mechanical properties and thermal shock resistance of MgO-C refractories with CMA aggregates,so it is necessary to conduct the systematic research.In this paper,the effect of CMA aggregates on the mechanical properties and thermal shock resistance of MgO-C refractories was firstly studied,and the relationship between liquid phase formation at high temperature and the properties of MgO-C refractories containing CMA aggregate was revealed.Secondly,spinel or alumina coated CMA aggregates were prepared by coating technology,and the effect of coating aggregates on their mechanical properties and thermal shock resistance was systematically studied.Finally,the influence of CMA powder addition on the structure and properties of MgO-C refractories was studied.The distribution of CMA powder was adjusted by optimizing the mixing sequence.The relationship between the content of CMA powder and the mixing sequence on the microstructure,mechanical properties and thermal shock resistance of MgO-C refractories was investigated.Based on the above research work,the main conclusions are as follows:1.The introduction of CMA aggregates significantly affects the mechanical properties and thermal shock resistance of MgO-C refractories.After heating at 200-1400℃,the cold modulus of rupture(CMOR)of the specimen was slightly lower than that of the specimen without CMA aggregates.but the CMOR of the specimen increased when the heating temperature increased to 1600℃.The thermal shock test showed that the specimen containing CMA aggregates after coking at 1400℃ possessed higher residual strength ratio than CMA-free specimen.After thermal shocks and reheated to 1600℃,the strength of specimen containing CMA aggregates recovered significantly,and the CMOR increased from 13.9MPa to 14.7MPa.This was due to the higher porosity of CMA aggregates than magnesia aggregates,which affected the CMOR of the specimen coked at 1400℃.However,after heating at 1600℃,the liquid phase of calcium aluminate with the low melting point was formed in CMA aggregates,which promoted the formation of Mg Al2O4 whiskers and improved the strength of the materials.2.The introduction of spinel or alumina coated CMA aggregates can improve the mechanical properties and thermal shock resistance of MgO-C refractories.After heating at1600℃,the CMOR of specimen containing spinel or alumina coated CMA aggregates increased from 13.3MPa to 14.3MPa and 14.9MPa,respectively,compared with those of specimen directly introduced into CMA aggregate.The thermal shock test showed that the strength reduction of the specimen containing spinel or alumina coated CMA aggregates coked at 1600℃ decreased,their CMOR increased from 7.6MPa to 8.7MPa and 8.8MPa,respectively,compared with those directly introduced into CMA aggregate.This is mainly because the coated CMA aggregate possessed relatively low porosity,and the coated layer limited the penetration process of the calcium aluminate liquid phase with the low melting point in CMA aggregates at high temperature,while a small amount of liquid phase can repair the cracks in the structure through the cracks in the coated layer after quenching.3.The content and distribution of CMA powder have obvious effect on the mechanical properties and thermal shock resistance of MgO-C refractories.When 2.5wt% CMA powder was introduced,the CMOR of specimen was increased,especially the specimen where the CMA powder was distributed around the aggregates.After heating at 1400℃,the CMOR of MgO-C specimen containing 2.5wt%CMA powder increased from 13.1MPa to 14.6MPa and 14.6MPa,respectively.When the content of CMA powder was increased to 5wt%,the CMOR of MgO-C specimen decreased obviously.This was owing to the introduction of a small amount of CMA powder formed liquid phase at high temperature,promoting the formation of Mg Al2O4 whiskers,but when the amount of CMA powder increased,too much liquid phase at high temperature led to the reduction of their hot modulus of rupture.The thermal shock test showed that the MgO-C specimen with 2.5wt% CMA powder distributed around the aggregates possessed the best thermal shock resistance.