The main defects are under burning, over burning, damage, pollution, black core, distortion, cracks and so on.
Under burning is caused by insufficient firing temperature, insufficient holding time or leakage of cold air. In general, the unburned brick has low mechanical strength, loose bonding, low sound when knocking, easy to damage when handling, and poor high temperature performance and corrosion resistance.
Overburning is caused by too high firing temperature, too long holding time, or direct contact of flame with brick surface. In general, the overburned brick has high mechanical strength, low apparent porosity, large size shrinkage, large deformation of brick body, too close bonding, serious vitrification, ringing when striking and poor thermal shock resistance. However, when the raw material contains high Fe2O3, TiO2 and other transitional metal oxide impurities, overburning may also make the refractory foamed, resulting in the decrease of density, increase of apparent porosity and increase of volume expansion.
The damage is the defect of missing edge, falling angle and falling grain after burning. The reasons may be that the amount of matrix is less, the bond strength is insufficient, the force is too strong during transportation, the mixing is uneven, the particle segregation, the molding pressure is not enough or the sintering temperature is low.
Contamination refers to defects such as melting holes and iron spots. The cause of the melting hole may be that the low melting materials such as cinder and sundries are mixed with brick making materials, and the refractories are melted during firing. Iron plaque is a kind of dark plaque caused by iron material mixed with raw materials during manufacturing and oxidation diffusion of iron material during sintering. The reason may be that iron materials are mixed with refractory materials during crushing and grinding.
The black core is a defect in the center of the fired refractory products. The cause of the black core may be that the surface of the product is sealed by the glass phase before the organic matter inside is oxidized. It may also be that the product is fired in reducing atmosphere and is affected by oxidizing atmosphere when cooling. In general, the corrosion resistance of black heart products is poor.
Distortion is the deformation of refractory products under high temperature, which is marked by the fact that the edges of the bottom of the refractory brick are not in the same plane. If the molding, loading and firing are not proper, the refractory brick may be distorted.
Crack refers to all kinds of cracks in the refractory products after burning, including surface, internal, visible and hidden cracks. The causes of the cracks are different, for example, spalling may occur if the forming is improper; Excessive shrinkage and expansion, or uneven shrinkage and expansion during drying and sintering will cause cracks; Uneven heating and cooling, inconsistent firing line changes or excessive thermal stress will also cause cracking. In addition, when firing refractories containing free calcium oxide, special attention should be paid to introducing as little water as possible (including free water, adsorbed water, crystal water and combined water), and the heating system should be strictly implemented to prevent the brick from cracking.
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