Foreword
Refractory materials, as the name suggests, are materials resistant to high temperatures. If you simply believe that their sole function is to withstand heat, it’s time to delve deeper into their multifaceted roles. Strictly speaking, refractory materials have extensive applications, particularly in industrial production. Many modern industrial processes rely on high-temperature technologies, often referred to as high-temperature kiln sintering. However, with the growing popularity and market awareness of refractory materials in recent years, their use has expanded into various fields closely related to our daily lives, including pharmaceuticals,民生 (people’s livelihood), and more.
Depending on the application, refractory materials can be categorized into numerous types based on factors such as temperature resistance, acidic or alkaline combustion environments, high-corrosion environments, and high-erosion environments. The properties required for these applications are closely tied to the materials’ inherent characteristics. But do you know what these properties signify? Let’s explore them together today!
1. What is the Porosity of Refractory Materials?
Porosity, in simple terms, is a critical metric that measures the amount of pores present in refractory materials.
During the production of refractory materials, three primary types of pores are formed: open pores, closed pores, and through pores. These pores play a pivotal role in the structure and performance of refractory materials.
Specifically, apparent porosity refers to the volume of pores in refractory materials that are open to the atmosphere (i.e., open pores) relative to the total volume. This ratio directly influences key properties such as gas permeability and thermal insulation.
True porosity, on the other hand, is a more comprehensive concept. It encompasses the volume of all pores (including open, closed, and through pores) relative to the total volume. The magnitude of true porosity is crucial for assessing the overall quality, density, and stability of refractory materials under high-temperature conditions.
2. What is the Permeability of Refractory Materials?
Permeability is a characteristic value that reflects the ease with which a specific amount of gas passes through a refractory material under defined conditions. It is defined as the quantity of gas passing through a refractory material sample of a certain cross-section and thickness, under a given pressure, over a specific period.
With the exception of permeable bricks used in ladles, the permeability of other refractory materials is ideally as low as possible. This helps reduce the rate of slag erosion and lowers the thermal conductivity of the refractory material.
3. What is the Thermal Expansion of Refractory Materials?
During the use of refractory materials, as temperatures rise, the anharmonic vibrations of atoms within the main crystalline phase and matrix increase the interatomic distances, leading to volumetric expansion. This phenomenon is termed thermal expansion.
Thermal expansion in refractory materials is typically expressed in terms of linear expansion rate and linear expansion coefficient, defined as follows:
(1) Linear Expansion Rate: The relative rate of change in the length of a refractory material sample when heated from room temperature to a specified test temperature.
(2) Linear Expansion Coefficient: The relative rate of change in the length of a refractory material sample per degree Celsius (°C) increase in temperature when heated from room temperature to the test temperature.
Thermal expansion in 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 and Al₂O₃, where oxygen ions are closely packed, the mutual thermal vibrations of these ions result in a higher thermal expansion rate. Conversely, refractory materials with highly anisotropic structures exhibit lower thermal expansion rates, with cordierite being a typical example.
Thermal expansion is critical for the safe performance of refractory materials in steelmaking processes. For instance, refractory materials with poor thermal expansion properties may crack or spall during the baking stage, leading to material failure. Additionally, crack formation during use can significantly hinder the smooth operation of steelmaking.
4. What is the Thermal Conductivity of Refractory Materials?
When discussing refractory materials, one important physical property that cannot be overlooked is thermal conductivity. So, what is thermal conductivity? Simply put, thermal conductivity is a measure of a material’s ability to conduct heat, defined as the amount of heat transferred per unit time through a unit volume of the material under a unit temperature gradient. This property is particularly crucial for refractory materials as it directly impacts their performance in high-temperature environments.
Thermal conductivity is closely related to the porosity and mineral composition of refractory materials. Generally, the pores within refractory materials are filled with gases, which typically have low thermal conductivity. Therefore, it is understandable that refractory materials with higher porosity tend to exhibit lower thermal conductivity, as the gases in the pores effectively impede heat transfer, reducing the overall thermal conductivity of the material.
In addition to porosity, the mineral composition of refractory materials also affects their thermal conductivity. In refractory materials, more complex crystal structures tend to result in lower thermal conductivity. This is because intricate crystal structures obstruct the pathways for heat transfer, thereby reducing the material’s thermal conductivity. Similarly, a higher presence of impurities within refractory materials leads to lower thermal conductivity, as these impurities disrupt the crystal structure, further hindering heat transfer.
5. What is the Heat Capacity of Refractory Materials?
In simple terms, heat capacity is a measure of a material’s ability to absorb heat. Specifically, it refers to the amount of heat required to raise the temperature of 1 kg of a substance by 1°C under constant pressure, also known as specific heat capacity.
For refractory materials, specific heat capacity is a critical property. During use, it directly influences the heating and cooling processes of the material. Refractory materials with higher specific heat capacity will heat up more slowly when absorbing the same amount of heat, thus requiring longer baking times. This characteristic makes specific heat capacity an essential factor to consider when selecting and using refractory materials. Understanding and mastering the specific heat capacity of refractory materials can help us better control the baking process, improve production efficiency, and ensure optimal performance of the material.
6. What is the Refractoriness of Refractory Materials?
Simply put, refractoriness is a key indicator of a refractory material’s ability to withstand high temperatures without melting. Unlike most metallic materials, refractory materials do not have a fixed melting point. Instead, refractoriness refers to the temperature at which a refractory material softens to a certain degree under high temperatures.
For refractory materials, refractoriness acts as a guiding light, helping us select and use suitable materials in high-temperature environments. To ensure that refractory materials perform stably and durably in practical applications, their refractoriness must be higher than their maximum operating temperature. This ensures that the material does not melt or soften excessively, thereby retaining its intended functionality.
So, how is the refractoriness of refractory materials tested? It is a relatively complex yet highly precise process. Typically, the refractory material to be tested is shaped into a cone specimen according to specified dimensions. This cone, along with standard specimens, is then placed in a high-temperature furnace for heating. As the temperature rises, the cone gradually softens and eventually bends. When the tip of the cone touches the furnace base, the temperature is recorded as the material’s refractoriness. Through this testing method, we can accurately understand the performance of refractory materials under high temperatures, providing a solid basis for their selection and application in real-world scenarios.
7. What is the Load Softening Temperature of Refractory Materials?
The load softening temperature, also known as the load softening point, is a crucial indicator of a refractory material’s ability to withstand loads under high temperatures. It is well known that refractory products typically exhibit high compressive strength at room temperature, resisting various pressures without deformation. However, when subjected to high temperatures and loads, the scenario changes. Elevated temperatures cause structural changes in the material, gradually diminishing its original strength and ultimately leading to deformation.
So, how is the load softening temperature specifically defined? Simply put, it is the temperature at which a refractory material begins to deform under a constant load at high temperatures. This temperature point is vital for the practical use of refractory materials, as it directly relates to their stability and durability in high-temperature environments. If the load softening temperature is too low, the material is prone to deformation and failure under high temperatures, compromising its intended performance. Therefore, when selecting and using refractory materials, it is essential to thoroughly consider this performance metric.
8. What is the Thermal Shock Stability of Refractory Materials?
Thermal shock stability, in simple terms, refers to the ability of a refractory material to withstand rapid temperature changes without cracking, damage, or failure, as well as its resistance to fragmentation or rupture during practical use. It is well known that refractory materials often operate in high-temperature environments, where temperature fluctuations can be extremely abrupt. If the material cannot endure such sudden temperature changes, it is likely to crack, degrade, or even disintegrate.
To measure the thermal shock stability of refractory materials, we typically use the number of rapid heating and cooling cycles it can withstand without damage. This metric, also known as resistance to thermal shock, directly reflects the material’s durability and stability under temperature variations.
If your furnace frequently undergoes shutdowns, thermal shock stability becomes a critical factor when selecting and using refractory materials. Only materials that remain stable under abrupt temperature changes can ensure long-term performance and safety in high-temperature environments. Therefore, if you are seeking high-quality refractory materials, be sure to pay close attention to their thermal shock stability!
9. What is the Slag Resistance of Refractory Materials?
The ability of refractory materials to withstand slag erosion at high temperatures is referred to as slag resistance.
Slag interacts with refractory materials in a liquid state, forming a liquid phase that peels away from the surface of the material or infiltrates its pores. During temperature fluctuations, this can cause volumetric expansion, leading to loosening and damage of the refractory material. Alternatively, the slag may penetrate the material, forming new high-melting-point spinel phases, rendering the refractory unusable and damaged. Furnace gases and other substances in contact with the refractory material can also cause similar forms of damage. In addition to surface dissolution, slag can invade or permeate the refractory material, expanding the reaction area and depth between the slag and the material. This leads to qualitative changes in the composition and structure of the refractory near its surface, forming a deteriorated layer that easily dissolves into the slag, thereby shortening the material’s service life. This mode of erosion is primarily related to the porosity of the refractory material. Even with identical compositions, refractory materials with different structures exhibit varying rates of erosion. The higher the porosity, the weaker the slag resistance.
10. What is the Burnout Index of Refractory Materials?
In the world of refractory materials, there is a crucial concept known as the “burnout index.” Perhaps this term is unfamiliar to you, but it plays a pivotal role in evaluating the performance of refractory materials.
Simply put, the burnout index is an indicator used to characterize the effect of electric arcs on the erosion of furnace walls. This concept was first introduced by W.E. Schwabe in 1962, marking a significant innovation in the field of refractory materials.
This index is vital because it plays a decisive role in determining the process route for smelting. For example, in ladle refining furnaces, the secondary voltage is meticulously calculated based on the burnout index of the refractory material. An inaccurate burnout index could lead to incorrect voltage settings, thereby affecting the quality and efficiency of the entire smelting process.
11. What are the Mineral Composition and Chemical Composition of Refractory Materials?
Mineral composition, in simple terms, refers to the mineralogical and petrographic structural components within refractory materials. It acts as the “genetic blueprint” of the material, determining many of its key properties. For instance, in the well-known magnesia-carbon bricks, the primary crystalline phase of periclase forms the “skeleton” of its mineral composition.
Did you know? Even refractory materials with the same mineral composition can exhibit vastly different properties due to variations in the size, shape, and distribution of mineral crystals. This is akin to humans—while we share similar genes, our appearances, personalities, and talents are remarkably diverse.
The mineral composition of refractory materials can be a “sole star” single crystalline phase or a “blooming garden” of multiple crystalline phases. These mineral phases are primarily divided into crystalline and glassy phases. The main crystalline phase, which constitutes the bulk of the refractory material and has a high melting point, is referred to as the primary crystalline phase. It acts as the “leader” of the material, guiding its functional properties. The substances present in the gaps between large crystals or aggregates are called the matrix, such as the carbon in magnesia-carbon bricks. Although the matrix may not be as “prominent” as the primary crystalline phase, it plays an indispensable role in the performance of the refractory material.
