- Time:Jul 28, 2022
The main defects of refractory bricks in the production process are under- burning, over-burning, damage, pollution, black heart, distortion, cracks, etc.
Under-burning is a problem caused by insufficient firing temperature, insufficient holding time, or leakage of cold air. Under normal circumstances, the mechanical strength of underfired bricks is low, the combination is loose, the sound is low when knocked, it is easy to be damaged when transported, and the high temperature performance and corrosion resistance are poor.
Overburning is a problem caused by the burning temperature being too high, or the holding time being too long, or the flame directly touching the brick surface. In general, over-fired bricks have high mechanical strength, low apparent porosity, large dimensional shrinkage, large deformation of the brick body, too tight bonding, serious porcelainization, clunking when knocked, and poor thermal shock resistance. However, when the raw material contains high transition metal oxide impurities such as Fe 2 O 3 and TiO 2 , over-burning may also cause the refractory to foam, resulting in a decrease in density, an increase in apparent porosity, and an increase in volume expansion.
Damage is the occurrence of defects such as missing edges, falling corners, and falling grains after burning. The reasons may be that the number of substrates is too small, the strength of the cement is insufficient, the force is too strong during handling, the mixing is uneven, the particles are segregated, the molding pressure is not enough, or the firing temperature is too low.
Pollution refers to defects such as melt holes and iron spots. The addiction of melting caves may be caused by the mixing of low-melting materials such as cinder and debris into the brick making materials, and the melting of the refractory materials during firing. Iron spots are dark spots caused by iron oxidation and diffusion during firing. The reason may be that iron substances are mixed into refractory raw materials during crushing and grinding.
Black heart is the defect that the center of the refractory product is black after burning. The cause of the black heart may be that the surface of the product is sealed by the generated glass phase before the organic matter inside the product is oxidized. It may also be that the product is fired in a reducing atmosphere and is affected by an oxidizing atmosphere during cooling. In general, the corrosion resistance of black heart products is not good.
Distortion is the deformation of the refractory product under pressure at high temperature, and its sign is that the edges of the bottom surface of the refractory brick are not on the same plane. If it is improperly shaped, kiln installed, and fired, it may cause the refractory brick to be distorted.
Cracks refer to various cracks in refractory products after burning, including surface, internal, obvious and hidden cracks. The causes of cracks are different, such as improper molding, spalling may occur; excessive shrinkage or expansion during drying and firing, or uneven shrinkage and expansion will cause cracks; during firing, the bricks are heated and cooled unevenly, resulting in inconsistent Firing line changes or excessive thermal stress can also cause cracking. In addition, when firing refractory materials containing free calcium oxide, special attention should be paid to introducing as little moisture as possible (including free water, adsorbed water, crystal water, and compound water), and strictly implement the heating system to prevent the brick from cracking.
