Key Properties Of Refractory Materials: Understanding Thermal Conductivity, Compressive Strength And Porosity
Crucial to the identity of the refractory material itself is how it performs in elevated temperatures - but this isn’t the only area where a refractory needs to excel. Certain applications require elevated levels of compressive strength and specific porosity profiles. The design of contemporary foundry processes leans heavily on the understanding of these factors to ensure a robust and productive system.
Introduction
Refractory materials (refractories) are materials that are, by their chemical composition, highly resistant to elevated temperatures, making them essential tools in the production of ferrous and non-ferrous metals, high precision castings and various smelting applications. Falling into three broad categories by chemical nature - acidic, basic and neutral - and further classified by size and form, the wide choice of refractory materials means they find the broadest applications worldwide. Production of refractories generally follows the sequence ‘raw material processing, forming and then firing’ with a variety of forming types available.
A broad overview of some common refractory materials and their normal service temperatures and compressive strength information is shown below(5L,M,N)
| Material | Service temperature (°C) | Compressive strength (MPa) |
| Fireclay brick (dense) | 1,400 max. | 15 - 60 |
| Fireclay brick (60% porous) | 1,400 - 1,600 | 10 - 20 |
| Magnesia-chromite 60/40 (13 - 22 % porosity) | 1,600 | 30 - 80 |
| Silica (sand, 23% porosity) | 1,630 | 10 |
| Magnesite | 1,600 - 1,800 | 35 - 60 |
| Chromite (14-21% porous) | 1,650 | 25 - 95 |
| Magnesia-chromite 35/65 (25 -60 % porosity) | 1,650 | 25 - 60 |
| Silicon carbide clay | 1,700 | 80 |
| Forsterite (20% porosity) | 1,730 | 22 |
| High alumina (18% porosity) | 1,800 + | 55 |
Thermal Conductivity
Perhaps the ‘headline figure’ when it comes to refractory materials is just how high a temperature they can withstand and how much heat they can absorb so as not to damage other tooling (for example in the case of tundish linings for aluminium smelting)(5C).
Thermal conductivity is influenced by several factors, but lead amongst them are melting point (which is dictated by the chemical composition of the refractory itself) and porosity (more below). Chromite and zirconia are common examples of widely used refractory materials and have a melting point of 1,700 to 2,000 °C and well in excess of 2,000 °C respectively. Mechanical or compressive strength rarely contributes to thermal conductivity.
Further classifications of refractory materials is by service temperatures, which are informed by porosity, in the main:
Heat resistant up to or equal to 1,100 °C
Refractory up to or equal to 1,400 °C
High refractory up to or equal to 1,700 °C
Ultra high refractory Over 1,700 °C
Pyrometric cone equivalents (PCEs) are measures of how much a ceramic will soften at temperature but not under load. Refractories may be grouped by PCE value, from super duty through high and intermediate to low duty, corresponding to values of 33-38, 30-33, 28-30 and 19-28 respectively. A low PCE value means that the refractory has a lower temperature at which it can operate before deforming and risking cracking. A detailed discussion of PCE values is beyond the scope of this article.
Compressive Strength
As with any industrial process, how much maintenance is required of a system is a crucial consideration alongside longevity. It stands to reason that any material that is physically stronger (in terms of compressive strength or otherwise) will last longer without breaking, therefore reducing maintenance requirements and enhancing the lifetime of the system(5B). With some refractory materials being relatively expensive, having to pause a metal casting operation, for example, for maintenance is a task that adds time, cost and complexity.
Compressive strength does vary with temperature, hence this is one of the guiding principles in refractory selection. ‘Creep’ tests have long been performed on refractory materials, with the tests applying pressure at elevated temperatures on refractory materials. ‘Creep’ in this sense refers to the notion of expansion or contraction (and thus elasticity, but flexural strength is beyond the scope of this article) with more movement being largely indicative of weakening at any particular temperature or pressure. Results from tests show that high purity silica refractory bricks are outperformed by certain types of fire bricks made from fire clay in moderate temperature testing up to 1,000 °C(5A).
As can be seen in the examples in the table above, compressive strength bears little influence on refractoriness.
Looking at examples where refractories include coal dust - such as many fire bricks, it has been stated that thermal conductivity decreases with increases in compressive strength and pore size (i.e. more porous)(5H). When considering refractory binders (that is, materials that hold together the refractory material prior to firing), the use of anthracite as such a binder is far superior in terms of adding compressive strength than is a resinous or fibrous organic material(5K).
Compressive strength is important too in the manufacture of the refractory, not just when the refractory is deployed. Considerations should be made with regards to compressive strength if the pressing method of forming refractory bricks is used. There is little to no risk of problems during the pressing process if the refractory material is itself resistant to the highest pressures.
Porosity
Porosity is the measure of how porous a material is, i.e. how many small channels are present throughout the bulk of a material that may allow liquids or gasses to pass through. Highly porous materials are prized in many industrial areas, not least in catalysis where highly porous zeolites are regarded as some of the most productive catalysts around, but also porosity has a valuable part to play in designing and implementing the refractory material.
As a general rule of thumb, the greater the level of porosity in a material, the less dense it is and the poorer a thermal conductor it is. Both of these phenomena are explained by the holes in the bulk material being filled with air (or another gas - gases are poor conductors of heat). It stands to reason, then, that more holes means more air, which means poorer heat conductivity(5I). This when combined with a material that has already a very high tolerance for temperature makes for an excellent refractory material. One further advantage of more porous materials is that transport costs are reduced, owing to their lower weight per given unit volume.
Tundish linings are prime examples of where porosity is important, where often elevated structures need to be not so heavy as they may collapse under load. Common magnesia chrome refractories are used here for this reason, amongst other reasons(5D). Their porous structures aid in the purification of the molten metal, preventing oxidation and absorbing non-metallic impurities as the molten metal passes through(5F). Porosity is an important factor for refractory plaster, too, which cannot be heavy or very dense or else it will simply not function well as a ‘glue’ to bind together refractory bricks, or patch up cracked refractories(5E).
Furthermore, in the case of green sand castings, the porosity of the chrome-based refractory material is crucial in allowing built up gases to escape(5G) - failure to achieve this could lead to surface defects or wetting. Porosity, as mentioned, contributes to thermal insulation, as air is a poor temperature conductor. Additionally, it is widely acknowledged that porosity is directly related to permeability. Permeability is one of the major governing factors in the longevity of refractory materials(5J).
To summarise, a network of uniform, small and evenly distributed pores is advantageous and offers the lowest thermal conductivity.
Idealised Refractories
Combining the above knowledge, a conclusion can be drawn about what an idealised refractory material could be. Certainly, the material would need to be chemically composed of a high melting point material; be excellent in resisting heavy weights and strong pressures placed upon it without breaking; and be relatively porous, so as to dissipate the heat effectively.
It should be noted, the choice of refractory based on chemical identity, compressive strength and porosity needs to be made alongside the nature of the refractory (acidic, basic, neutral) and whether the shape of the refractory is suitable for the process overall. Thankfully, modern production techniques allow for a wide diversity of materials in virtually any shape and size, so as to be ideally suited to a given industrial task at elevated temperature.
Summary
- The choice of refractory materials is dependent on many factors including - but not limited to - thermal resistance, compressive strength and porosity
- These factors are often interdependent and need to be considered in the whole when selecting a refractory material, or compound refractory material
- Highly thermal resistant materials are the obvious first choice for a refractory material, as it is a high temperature environment in which nearly all refractories operate
- Materials with high compressive strength values are useful because they can be used for virtually any process (especially in metal casting) where large weight forces are placed on the refractory - enhanced capacity for extreme loads means that the material won't break over time, providing longevity
- Highly porous materials are prized because of their low weight to volume ratio, and additionally being porous itself means that the material is a poorer conductor (i.e. better insulator of heat)
References
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