Silica bricks are prone to various cracks during the production process, such as spalling, vertical cracks, transverse cracks and network cracks. The rejection rate of silica bricks is high, and its yield rate is not as high as other refractory bricks. Please see the details below.
After the silicon brick is subjected to thermal shock, thermal stress will be generated inside, and the product will crack when the thermal stress is too large. The cracks of silicon bricks include surface cracks and layer cracks. Surface cracks are divided into horizontal cracks, vertical cracks and network cracks. Generally, spalling and vertical cracks are caused by mechanical pressure, and horizontal cracks and network cracks are caused by firing. The main factors affecting cracks are firing and mechanical pressure, but the role of other factors related to the production process cannot be ignored.
The cracks in the production process of products (including silica bricks) are one of the main reasons for the increase in the rejection rate. This article takes silica bricks as an example to analyze the causes of cracks in the finished silica bricks, and shows that product quality is related to the entire production process. Only by comprehensively inspecting all links of the production process, can we find the key factors to improve product quality from the clues. This experience can also be used for reference in the production of other products.
The generation mechanism and classification of cracks
One is the production mechanism
Refractory materials are subjected to the rapid changes in ambient temperature during the firing process, which is called thermal shock .
Under thermal shock, thermal stress is generated inside the refractory material, and when its value reaches the product strength limit value, the product will crack or break. Refractories are non-uniform brittle materials. Compared with metal products, due to its large thermal expansion rate, low thermal conductivity and low elasticity, low tensile strength, it is easy to crack during the firing process, expand continuously under thermal shock, and eventually break. When the silicon brick is fired, due to the crystal change, the volume expands greatly, and it is especially easy to crack or even break. However, the presence of microcracks in the microstructure can improve the elasticity of the structure, weaken the thermal stress, and suspend the propagation of cracks.
Visible to the naked eye or visible by conventional methods, cracks that affect the use of the product
The cracks in the finished silicon brick can be divided into surface cracks and internal cracks (also called layer cracks). Among them, the surface cracks are criss-crossed and have different lengths. In order to facilitate the analysis of the causes of cracks, it is necessary to classify them reasonably. Practice has proved that the shape of the crack is closely related to the pressure direction during the forming process, and the pressure direction is related to the shape of the brick, so it is usually classified according to the pressure direction related to the brick.
First, specify two directions related to classification, namely the horizontal and vertical (or vertical) directions of the bricks. For example, for standard ordinary bricks, the pressure direction is generally the thickness direction, and the other two directions are vertical.
(1) Vertical cracks, that is, perpendicular to the pressure direction of the product, usually along the height of the product. When semi-dry machine molding produces products, "layer density" will be generated along the pressing direction, resulting in uneven thermal expansion of the green body, large thermal stress, and cracks parallel to the density layer, so it is also called machine pressure cracking .
(2) Transverse cracks, that is, parallel to the pressure direction of the product, generally along the thickness direction of the product, usually caused by uneven heating of each part of the product during firing, and mainly appear on the outside of the brick stack, especially the surface of the top product .
(3) Reticulated cracks, that is, a closed curve composed of several cracks, are usually caused by high temperature, large fluctuations, large internal thermal stress of the product, large failure stress of the product, and microscopic unevenness of the green body itself. In addition, uneven mixing and changes in raw materials can also cause network cracks.
The reasons for the cracking of silica bricks are generally as follows:
(1) Use a large amount of fast-converting porous silica or vein quartz with excessive swelling in the ingredients;
(2) There are too many fine particles smaller than 0.5 mm or too many large particles in the mud, one of the reasons is that the critical particles are too large;
(3) Insufficient addition of silicon bricks, poor quality of lime milk, improper addition of mineralizers or too small water content of mud;
(4) Unreasonable model design or improper model operation method;
(5) The drying system is uncertain, the kiln loading direction of the bricks is incorrect, and the heating is too fast during firing, resulting in excessive volume expansion.
Silica brick is an acidic refractory material, mainly composed of phosphorous quartz, cristobalite and glass phases. It has strong resistance to acidic slag, but will be corroded by alkaline slag, and will not be corroded by oxides such as Al2O3, K2O, and Na2O. Its load softening temperature is high, between 1640~1680 ℃, but the disadvantage is low thermal shock stability, but the fire resistance is close to the load softening temperature, and it will not be deformed under high temperature for long-term use, which helps to ensure the structure of the masonry when used strength.
How to solve the problem of reticulated cracks in silica bricks?
Due to the high temperature, large fluctuations, large internal thermal stress of the product, large failure stress of the product, and microscopic unevenness of the green body itself. In addition, uneven mixing or raw material changes can also cause network cracks. Strict control of temperature and thermal stress, uniform mixing of materials, can reduce the occurrence of network cracks.