Refractory materials are essential in high-temperature industrial processes, and their performance depends on several critical properties. Below, we break down the key characteristics that define the quality and functionality of refractories.
There are three types of pores formed during the production of refractory materials: open pores, closed pores, and through pores.
The apparent porosity is the ratio of the volume of open pores, which are connected to the atmosphere, to the total volume of the refractory material. The true porosity is the ratio of the total pore volume—including open pores, closed pores, and through pores—to the total volume of the refractory material.
Permeability indicates how easily gases pass through a refractory material under specific conditions. It is defined by the volume of gas that flows through a material of a given thickness and cross-section under pressure over time.
Except for permeable bricks used in ladles, lower permeability is preferred in refractories to reduce slag erosion and improve thermal efficiency.
During use, as the temperature increases, the anharmonic vibrations of atoms in the primary crystal phase and matrix of refractory materials intensify, increasing the atomic spacing within the material. This results in volume expansion, which is referred to as the thermal expansion of refractory materials. The thermal expansion of refractory materials is typically expressed using the linear expansion ratio and the linear expansion coefficient, defined as follows:
Linear expansion ratio: The relative change in the length of a refractory sample when heated from room temperature to a specified test temperature.
Linear expansion coefficient: The relative change in length per 1°C temperature increases when a refractory sample is heated from room temperature to the test temperature.
The thermal expansion of refractory materials is closely related to their crystal structure. The bond energy within the crystal structure determines the thermal expansion coefficient. For example, in the crystal structures of MgO (magnesia) and Al₂O₃ (alumina), oxygen ions are densely packed. When heated, the thermal vibration of oxygen ions causes a significant thermal expansion. In contrast, refractory materials with a highly anisotropic crystal structure exhibit lower thermal expansion rates, such as cordierite.
Thermal expansion plays a crucial role in the safe application of refractory materials in the steelmaking process. For instance:
Refractory materials with poor thermal expansion resistance may crack and spill during the preheating stage, leading to material failure.
Crack formation due to thermal expansion during use can significantly impact the stability and efficiency of the steelmaking process.
Thermal conductivity refers to the amount of heat transferred through a unit volume of material per unit time under a unit temperature gradient. It is closely related to the porosity and mineral composition of refractory products.
Generally, the gas within the pores of refractory materials has very low thermal conductivity. Therefore, materials with higher porosity tend to have lower thermal conductivity.
In terms of mineral composition, the more complex the crystal structure, the lower the thermal conductivity; the higher the impurity content, the lower the thermal conductivity.
The heat capacity of a material refers to the amount of heat required to raise the temperature of 1 kg of the material by 1°C under normal pressure. It is also known as specific heat capacity. In the application of refractory materials, specific heat capacity affects the heating and cooling processes during baking. Refractory materials with a higher specific heat capacity require a longer baking time.
Refractoriness refers to the ability of refractory materials to resist high temperatures without melting. Since refractory materials do not have a fixed melting point, refractoriness is defined as the temperature at which the material softens to a certain degree. It is a key property of refractory materials, and their refractoriness should always be higher than their maximum service temperature. The refractoriness test involves heating a conical sample of the refractory material alongside standard samples. The temperature at which the cone bends until its tip touches the base is recorded as the refractoriness of the material.
The load softening temperature, also known as the load softening point, refers to the temperature at which a refractory material deforms under a constant load at high temperatures. While refractory materials exhibit high compressive strength at room temperature, their strength decreases under load at elevated temperatures, leading to deformation. The load softening temperature is the point at which a specified deformation occurs under these conditions.
Thermal stability refers to the ability of refractory materials to withstand rapid temperature changes without cracking or breaking. It also represents their resistance to spalling or fracturing during use. The thermal stability of a refractory material is measured by the number of cycles it can endure under rapid heating and cooling conditions, which is also known as thermal shock resistance.
Slag resistance refers to the ability of refractory materials to withstand slag erosion at high temperatures.
Molten slag comes into contact with refractory materials in liquid form, forming a liquid phase that causes material loss from the surface. It can also penetrate the refractory through its pores, leading to volume expansion and structural loosening due to temperature fluctuations. In some cases, slag infiltrates the refractory and reacts to form new high-melting-point spinel phases, which can render materials like ladle refractories unusable.
In addition to surface dissolution, slag can penetrate and react with refractory materials, increasing the reaction depth and area. This results in the formation of a degraded layer near the surface, which is more susceptible to dissolution, thereby shortening the refractory’s service life. The extent of slag erosion is closely related to the refractory’s porosity; even if two refractories have the same composition, differences in microstructure can lead to variations in erosion rates. The higher the porosity, the weaker the slag resistance.
The burn loss index of refractory materials represents the extent of arc-induced erosion on furnace walls. This concept was introduced by W.E. Schwabe in 1962 and plays a crucial role in determining metallurgical process parameters. For example, the secondary side voltage of ladle refining furnaces is determined based on the burn loss index of the refractory material.
The mineral composition of refractory materials refers to the mineral phases present in the product’s microstructure. For instance, in magnesia-carbon bricks, periclase is the primary crystalline phase, making it the main mineral component of the brick.
Even if refractory materials have the same mineral composition, differences in crystal size, shape, and distribution can result in variations in their properties. Refractory mineral phases can be either single-phase or multiphase structures, generally classified as crystalline phases or glassy phases. The primary crystalline phase, which forms the main structure of the refractory and has a high melting point, determines the material’s key properties. The remaining substances, which fill the gaps between large crystals or aggregates, are known as the matrix. For example, in magnesia-carbon bricks, carbon serves as the matrix. The properties, quantity, and bonding state of the primary crystalline phase directly determine the refractory’s performance.
