Properties of Refractories...

Properties of Refractories Refractories are those materials which have high melting points and have properties which make them suitable to act as heat resisting barriers between high and low temperature zones. Refractories are inorganic, nonmetallic, porous and heterogeneous materials composed of thermally stable mineral aggregates, a binder phase and additives. The general requirements of refractories include (i) ability to withstand high temperatures and trap heat within a limited area such as a furnace, (ii) ability to withstand action of liquid metal, hot gasses and liquid slag by resisting erosion and corrosion etc. (iii) ability to withstand load at service environment, (iv) ability to resist contamination of the material with which it comes into contact, (v) ability to maintain necessary dimensional stability at high temperatures and after/during repeated thermal cycling, and (vi) ability to conserve heat. Important properties of refractories include chemical composition, bulk density, apparent porosity, apparent specific gravity and strength at atmospheric temperatures. These properties are frequently among those which are used as ‘control points’ in the manufacturing and quality control process. The chemical composition serves as a basic for classification of refractories and the density, porosity and strength are influenced by many other factors. Among these are type and quality of the raw materials, the size and fit of the particles, moisture content at the time of pressing, pressure at mould, firing temperature, duration of firing, and the rate of cooling. Properties of the refractories can be classified to resist four types of service stresses namely (i) chemical, (ii) mechanical, (iii) thermal, and (iv) thermo-technical. A suitable selection of the refractories for the furnace lining can only be made with an accurate knowledge of the refractory properties and the stresses on the refractories during service. The relationship between service stresses and important...