Refractories Review: The Importance of Microstructure
Microstructure can be defined generally as the structure of a material as determined by any of the available microscopic techniques, including reflected light metallography, thin sections, electron microprobe analysis (EPMA), scanning electron microscope (SEM), scanning/transmission electron (STEM), transmission electron microscopy (TEM) and others. Microscopic analysis of refractory microstructure is valuable for many applications, such as refractory development and improvement, quality control, reverse engineering, evaluation of refractory performance in service, and research.
The analysis and understanding of microstructure has played a vital role in the ongoing advancement of refractories technology with a direct contribution to the increased service life of refractories, allowing customers to realize longer lining life for effectively lower cost. For example, the refractory consumption rate in the U.S. and Japanese steel industries has decreased about 64% since 1970, from 28 kg of refractory per ton of steel produced to 10 kg/ton, with accompanying cost benefits for the customers.
As shown in Figure 1, the basic components of a refractory microstructure include size-graded aggregate of coarse through intermediate size, a fine-grained bonding matrix, an interfacial region between the aggregate and the matrix, and pores (rounded, irregular, channels and macro/micro cracks). The size-graded nature of refractory constituents directly affects the reaction kinetics, bonding and corrosion mechanisms, playing a very important role in their performance.
In the last 15 years, much greater attention has been paid to the size grading of refractories, especially the fine and ultrafine portion (bonding matrix) of the formulation, which has resulted in improved properties. The porosity of non-insulating industrial refractories typically falls in the broad range of 2 to 30%. Porosity provides the openings for the penetration and reaction of working face refractories that are directly exposed to service conditions such as molten metal and slag or dross, cement clinker, spray injected liquid alkali and molten glass. Therefore, mix changes that result in lower porosity will, in general, reduce or slow the reactivity in service and extend the service life.
Engineered Refractory Microstructures
It is a common practice to use the vast knowledge accrued during decades of materials research and microstructure analysis to optimize refractory properties and/or to innovate. Some of the techniques that have been used to engineer refractory microstructures, which have resulted in the ongoing advancement of refractory technology, include:
- Ultrafine materials such as silica and alumina have been added to the bonding matrix (subsieve fraction) of refractory brick and castable products, resulting in significant property and performance improvements.
- Single and double layer coatings have been applied to coarse aggregate grains to cause increased or decreased bonding in the interfacial region between the aggregate and bonding matrix.
- The shape and ratio of coarse aggregate (rounded vs. angular) can be controlled to effect desired property changes, such as increased density and strength.
- Higher purity constituents are used to reduce or eliminate the presence of low melting phases in the bonding matrix.
- The flexibility (or rigidity) of the bonding matrix can be controlled (increased or decreased) to fit the intended application.
Microscopy Education and Training
Knowing the great value of microscopic analysis, it is a concern to see that the use of the valuable older microscopic techniques, such as oil immersion mounts, thin sections and reflected light metallography, has declined. And ceramics and materials science students are receiving little or no training in the older techniques in favor of newer, more advanced methods like SEM, STEM and TEM.
Certainly there are many cases where the newer microscopic techniques can be used alone to fully analyze refractories and other ceramic materials. But there are other cases where the correct interpretation or information can be obtained more directly and easily by an older microscopic technique, although an analysis might be enhanced or expanded when used in conjunction with a newer technique. For example, in one case, the prime failure mechanism of a porous SiC material was completely missed by a foreign national laboratory that used only SEM analysis, while reflected light metallography clearly indicated that oxidation and cristobalite formation had caused the cracking and strength loss.
History has proven that the understanding and application of microstructure concepts are valuable for the improvement, custom design and innovation of refractories. To best use and fully benefit from microstructure control, it is important to properly apply the full range of available microscopic techniques. Hopefully, refractories companies will continue to recognize the importance of microstructure and microscopic analysis, and maintain their internal or external availability of the necessary equipment, facilities and experience, to profit from the associated benefits.