In refractory lining maintenance, recommendations for repairs and relines often consist of selecting a similar or equivalent material to replace the original. Sometimes that’s sufficient. Many refractory contractors and maintenance teams strive to use best practices by purchasing the same refractories that have worked on similar equipment in the past, but this carries the risk of assuming that nothing has changed in the process, production or maintenance of the equipment over time. This assumption can be a dangerous bet because furnace equipment is made to meet the immediate demands of each thermal processor, and these demands often change depending on factors like production orders and maintenance capacities.
Choosing an appropriate refractory lining for an application isn’t always a straightforward decision. Many times, it is part science and part art. Making an effective choice requires knowledge of the industrial application process, refractory performance expectations and potential refractory service failures. These factors must then be weighed against each other to find the right balance and best solution. While there are a number of important criteria to consider, refractory engineers focus on five aspects to make an evaluation and choose a refractory material for each specific application: thermal, mechanical, chemical, logistics and value – as well as connections among these aspects.
Thermal Requirements: Temperature
For any high-temperature industrial process, the primary piece of information to know is the operating and maximum temperatures. The refractory lining chosen must meet the operating temperature requirements.
Refractory linings are designed to maintain physical properties at very high temperatures – 932°F and above. Refractories used to line thermal equipment must have proper insulating properties to reduce the steel skin temperatures to acceptable levels, usually well below 300°F. The use of multi-component linings, which employ a dense refractory material at the hot face with an insulating refractory or ceramic-fiber board or blanket behind it, is known for achieving adequate cold-face temperatures with structural integrity for long thermal life.
Spalling and thermal shock are the most common thermal failure mechanisms in a refractory lining. These are due to crack formations caused by temperature cycling and high thermal loads.
In recent years, many thermal processors have experienced increased production demands. Meeting that need means their furnaces operate at higher temperatures. This can lead to overheating the refractory to the point that phase changes begin, forming low-temperature glassy phases, softening the refractory and shortening its life. To address this, refractory engineers may need to use materials with higher refractoriness—typically higher-alumina compositions.
Physical Properties: Mechanical
Most modern high-performance refractories have been developed to maximize physical properties that extend lining life and keep furnaces running efficiently. Technical Data Sheets often detail properties such as cold crushing strength, hot/cold modulus of rupture (MOR), and abrasion resistance.
These metrics are based on ASTM standards to allow fair comparisons between materials.
Refractory linings face mechanical and thermal loads that cause wear and eventual failure. These include:
Excessive expansion
Thermal cycling fatigue
Mechanical impact (dynamic loading)
Severe abrasion and erosion
Pinch spalling
Tensile loads
Large hydraulic loads (e.g., molten-metal containment)
Creep (deformation over time at high temperature)
Visual inspection of linings can indicate failure modes: crack patterns, bulges, discoloration, and spalling often point to specific issues such as thermal shock, poor expansion allowance, unsuitable materials, or anchoring failures.
Corrosion: Chemical
Chemical attack on the refractory matrix has always been a primary concern in ceramic engineering. At high temperatures, the refractory may react with furnace contents, changing its structure and weakening performance.
Common examples of chemical degradation include:
CO reaction in reducing atmospheres (e.g., CO boilers)
H₂ reaction reducing silica at high temperatures
Molten slag in coal-fired boilers
Alkali attack from ash in wood-burning furnaces
Corundum formation in aluminum furnaces, especially with MgO-containing alloys
These chemical or mineralogical changes can cause excessive volume changes or reduce oxide bonds in the matrix, degrading the refractory structure.
Installation: Logistics
Refractory contractors are often under time pressure to return thermal equipment to operation. This affects choices of anchoring, installation method, and bake-out schedules.
Brick linings may offer durability but are labor-intensive and time-consuming. Cast-in-place linings provide superior properties but require forming, casting, and stripping, extending downtime.
Low-cement castables (pioneered by Plibrico) offer excellent properties but require longer bake-out. Shotcreting or gunning may be preferred when speed or cost are priorities, as they can be applied quickly without forms.
Plastic ramming materials are also options when moisture curing isn’t feasible. New materials like Fast Track castables and gun mixes allow faster firing, reducing downtime, albeit with potential compromises in properties.
Price: Value
Refractory linings are major operational costs across the lifespan of industrial furnaces. Price is always a factor, but value goes beyond cost alone.
Economic context often dictates which material to choose. Everyone wants a refractory that is “good, fast, and cheap,” but achieving all three at once is rare. Instead, value is a trade-off: balancing performance, speed, and cost for the specific need.
Conclusion
What do refractory linings do? Fundamentally, they withstand high temperatures, contain heat, provide mechanical strength, and resist chemical degradation.
Choosing the right refractory for thermal-processing applications is a balancing act. Understanding the process, failure history, and application needs is key to choosing the best solution.
For more information about choosing the best refractory lining, contact AzarShid Pars Refractory Co.