It is the ability of a material to sustain the action of heat without appreciable deformation or softening. Refractoriness of material is measured by its melting point, e.g. Alumine-silica composition Refractories are expected to resist mechanical abrasion, infusion of molten metals, slag or metallic vapours and also action of superheated steam, sulphurous oxides, chlorides and other gases.
Features of Refractories:
These materials have capacity of withstand high temperature without melting or decomposition, and the capacity to remain unreactive and inert when exposed to severe environments. These are good heat insulators, low electrical conductivity.
General applications are furnace linings, insulation coating, glass manufacturing metallurgical heat treatment and power generation.
Upon firing, the fine particles are involved in formation of bonding phase.
ADVERTISEMENTS:
This phase may be glass or crystalline.
Porosity has detrimental effect on brick properties strength, load bearing capacity and resistance to attack by corrosive material.
All of the above increase with porosity reduction at the same time thermal insulation and resistance to thermal shock are diminished.
Classification of Refractories:
(i) Fireclay Refractories:
Its melting point is 1587°C.
Below this temperature equilibrium phases present are mullite and silica (cristobalite).Upgrading the alumina (Al2O3) content increases the maximum service temperature.
Fireclay bricks are used primarily furnace construction to confine hot atmospheres and to thermally insulate structural members from excessive temperature.
Strength is not an important criteria but dimensional accuracy and stability are to be maintained.
ADVERTISEMENTS:
(ii) Silica Refractories (Acidic Refractories):
It is also termed as acid refractories have a major constituent as Silica.
These materials have high temperature load bearing capacity and are commonly temperature may reach as higher as 1600°C.
ADVERTISEMENTS:
Presence of even small quantities of composite on (7.7 wt% Al2O3) is very mean to silica extremity of phase diagram.
Even small additions of Al2O3 lower the liquidified temperature significantly, which means that substantial amount of liquid may be present at temperature around 1600°C.
These materials are resistant to acidic slags but are readily attacked by basic slags.
(iii) Basic Refractories:
Resistant to basic slags containing high quantities of CaO and MgO.
These find extensive use in steel making open hearth furnaces.
(iv) Special Refractories Zirconia (ZrO2):
It is white in colour has good resistance to thermal shock, wear and corrosion.
Partially and stabilized Zirconia (PSZ) has high strength and toughness and better reliability than Zirconia.
It is obtained by doping Zirconia with oxides of calcium, yttrium or magnesium.
This process form fine partials of tetragonal zirconia in a cubic lattice.
Typical applications are dies for hot extrusion grinding beads, for automotive primers and topcoats. Partially Stabilized Zirconia is very suitable for heat engines components such as cylinder liners and valve bushings.
(v) Neutral Refractories:
These neither acidic non basic with increasing alumina content, silica-alumina refractories may gradually change from an acidic to a neutral type. e.g., Sic, chromite & carbon.