High Temperature Volume Stability of Refractory Materials
Refractory materials are materials that are resistant to heat and can withstand extremely high temperatures. They are widely used in many different industries and applications, such as steelmaking, petrochemical, and oil & gas production. However, no matter how robust and reliable a refractory material is, it will not be as successful as expected if it fails to maintain its form and volume at elevated temperatures. Therefore, it is critical to understand the high temperature volume stability of a refractory material before specifying it for an application.
The high temperature stability of a refractory material is closely related to the physical and chemical properties of the material, as well as its processing characteristics, such as grain size and Bond composition. In general, the higher the bulk density and the finer the grain size, the better the refractory material’s High Temperature Volume Stability (HVS). Refractories with high bulk densities and grain sizes of less than 80um typically have better HVS than those with low bulk densities and coarse grain sizes. In addition to the bulk density, the porosity of the material affects its HVS. A material with higher porosity has a greater chance of experiencing volume changes when exposed to very high temperatures.
The changes in volume of a refractory material at high temperatures are typically due to the combination of thermal expansion, shrinkage and sintering. Thermal expansion occurs when the material absorbs heat and causes an increase in the volume. In contrast, shrinkage occurs due to material losses such as decomposition, vaporization and volatilization occurring at elevated temperatures. Sintering on the other hand is a phenomenon related to the formation of dense, interconnected networks of crystallites or grains. During sintering, the refractory material undergoes compaction and densification, hence resulting in volume reduction.
The most commonly used test to measure the HVS of a refractory material is the coefficient of thermal expansion (CTE). This test is conducted in an oven where the material is heated and cooled slowly while measuring the volume changes that occur in between. Tests such as the linear thermal shrinkage (LTS) and the modulus of elasticity (MOE) are also used to evaluate the volume stability of materials. The LTS test measures the amount of shrinkage that occurs at a given temperature, while the MOE test measures the amount of elasticity or flexibility that the material has when it is subjected to temperature variations.
In conclusion, there are a number of factors that affect the high temperature volume stability of a refractory material, such as grain size, Bulk density and Porosity. By understanding these parameters and testing the material appropriately, engineers will be able to select the best refractory material for their application and ensure that it is able to withstand the extreme temperatures present during operation.
