Nanocarbon Containing Alumina-carbon Refractories

Abstract

The functional refractories used in steel casting operations are usually made up of alumina- carbon based compositions due to their comprehensive properties. Conventionally, these refractories contain 25-30 wt% carbon, with graphite as the main carbon source, particularly to impart and improve corrosion and thermal shock resistance. The presence of alumina provides excellent mechanical and thermomechanical properties along with other conventional refractory properties. Owing to its low thermal expansion and superior thermal conductivity, carbon addition provides excellent thermal shock resistance. Because of its non-wetting nature, it avoids the adhesion of steel/slag melts on the refractory thereby improving corrosion resistance. As carbon imparts these two extremely important properties in the refractory compositions, the amount of graphite usage is considerably high for these refractories compared to other carbon-containing ones. However, the presence of high carbon encompasses several problems viz. a) higher heat loss due to the increased thermal conductivity of the refractory which increases the specific energy consumption per unit of steel production b) enhances the chances of carbon pick-up by steel, which affects the quality and properties of steel negatively c) oxidation of carbon causing a porous refractory structure which can be easily penetrated and corroded by steel/slag melts d) releases a higher amount of COx gases into the atmosphere. Considering these factors carbon content in the Al2O3-C refractories is to be reduced. However, minimizing the carbon content reduces the non-wetting behavior and also decreases the thermal conductivity of the refractory which causes an increase in thermal stress within the system, which damages their comprehensive properties and results in poor service life. Hence, the development of low-carbon Al2O3-C refractories without conceding any beneficial properties is a challenge to refractory technologists. Such a challenge is intended in the present work with the use of nanocarbon as a carbon source to reduce/replace graphite. Literature shows different studies are available to develop low carbon-containing Al2O3-C refractories using various carbon sources, replacing graphite partially or entirely, and further evaluation of the properties developed. But hardly any study is available on making low carbon Al2O3-C refractories with systematic optimization of nanocarbon and graphite content in the composition. Also, the comparison of developed properties of experimental compositions against conventional composition is done in the current work which is rarely found. In the present work, the variation in physical, mechanical, and thermo-mechanical properties with variation in the amount of nanocarbon in the composition is studied. Phase analysis and microstructural developments are also evaluated along with the oxidation resistance at different temperatures. Thermal shock resistance and corrosion resistance are also examined for optimized and selected batches. Nanocarbon possesses greater reactivity due to its high surface area in comparison to graphite and helps in the formation of in-situ ceramic phases like carbides. Formation of aluminum carbide in nanocarbon-containing compositions enhances strength. It can easily disperse in the gaps between coarse, medium, and fine alumina particles thereby reduces porosity and contributing to strength development. Well dispersed nanocarbon results in comparable corrosion properties even at much lower amounts than graphite. These nanocarbon particles can absorb and relieve stresses caused during thermal cycling, thereby improving thermal shock resistance.