Five Performance Requirements for Refractory Materials Used in Furnaces

01 Thermal Conductivity
The thermal conductivity of refractory materials serves as an indispensable and critical basis for the design of ceramic kilns. It is of significant importance to determine the thermal conductivity of lightweight refractory materials—which are required to possess excellent thermal insulation properties—as well as that of flame-separating structural materials, which are required to exhibit good thermal conductivity. The utilization of kiln refractory materials with low thermal conductivity plays a vital role in lowering the surface temperature of the kiln body, minimizing heat loss, and thereby reducing the energy consumption associated with the production of ceramic products.

02 Flexural Strength
High-temperature flexural strength is crucial for reducing the thickness of ceramic kiln furniture—thereby lowering its weight—and achieving energy-saving objectives.

03 Refractoriness Under Load
Refractoriness under load is a critical quality indicator for refractory materials; to a certain extent, it reflects the material's structural strength under conditions analogous to its actual service environment. It serves as the primary criterion when designing ceramic kilns to determine the suitability of a specific refractory material. Typically, the refractoriness under load of the inner lining materials in a ceramic kiln should exceed the kiln's normal operating temperature by 100 to 500°C; for instance, in a kiln used for firing porcelain bricks, the refractoriness under load of the inner lining materials within the firing zone should exceed 1380°C. Failure to meet this requirement—particularly during kiln operation or the initial drying and firing process, when kiln temperatures may temporarily exceed the long-term service temperature—could lead to deformation of the kiln structure or even structural collapse and damage.

04 Reheating Linear Change
When refractory ceramic fiber boards are utilized in ceramic kilns, excessive reheating shrinkage at high temperatures can cause the joints between the furnace bricks to widen. This compromises the structural integrity and airtightness of the furnace body, and may even lead to structural damage—particularly regarding the kiln roof bricks, where shrinkage can induce sagging and deformation of the roof structure. Conversely, reheating expansion poses a lesser risk; indeed, moderate expansion often serves to effectively extend the service life of the kiln body. For lightweight insulating refractory bricks, reheating linear change is a critical parameter in determining their maximum long-term service temperature. According to international standards, the "maximum service temperature" (or "classification temperature") is defined as the experimental temperature at which the reheating linear change does not exceed 2% after a continuous holding period of 24 hours; however, this specific temperature point should not be adopted as the actual working temperature for lightweight refractory materials in practical applications. Since my country's standard test methods for reheating linear change do not explicitly specify a holding period of 24 hours, it is essential—when selecting lightweight refractory bricks—to specifically stipulate the reheating linear change value corresponding to a 24-hour holding period.

05 Thermal Shock Resistance
This property of refractory materials—the ability to withstand rapid changes in temperature without sustaining damage—is closely linked to the material's thermal expansion characteristics, thermal conductivity, microstructure, and physical shape. It serves as a reflection of the refractory material's overall performance and exerts a significant influence on the service life of the kiln. The specific requirements for the thermal shock resistance of refractory materials are dictated by the operational nature of the ceramic kiln. In the case of intermittent kilns, every operational cycle involves heating the kiln structure from ambient temperature up to the firing temperature required for the ceramic products; after being held at this temperature for a specific duration, the kiln is then cooled back down to ambient temperature. Given this repetitive cycle of heating and cooling, if the refractory materials possess poor thermal shock resistance, they become susceptible to defects such as cracking, spalling, and crumbling; in severe instances, this can even lead to the structural collapse of the kiln. Conversely, for ceramic kilns operating on a continuous basis—such as tunnel kilns and roller hearth kilns—although they typically run continuously around the clock, kiln shutdowns do occur during power outages, fuel supply interruptions, equipment malfunctions, or scheduled periodic maintenance. Each such shutdown entails a process of cooling the kiln from its firing temperature down to ambient temperature, followed by reheating it back up to the firing temperature; consequently, the service life of these kilns is also inextricably linked to the thermal shock resistance of their refractory materials.

Thermal shock resistance is of particular importance for the refractory materials used in kiln furniture and kiln cars. Regardless of whether the kiln operates continuously or intermittently, the materials comprising the kiln furniture and kiln cars are constantly subjected to thermal cycling. Furthermore, due to the specific requirements of the ceramic firing process—wherein ceramic products, immediately following sintering, must undergo rapid quenching from their peak temperature down to approximately 850°C—the refractory materials used in the kiln furniture and kiln cars (such as ceramic fiber boards) must likewise be capable of withstanding this same regimen of rapid cooling.