There are various types of refractory (heat resistant) products including bricks, concretes, plastic/moldable materials, gunnites, mortars, and coatings. All of these products are typically installed in furnaces in one or more “layers” against a supporting structure that is usually a steel frame or “shell”. These products – once installed – may require a “thermal curing” treatment, and they are typically preheated to a service/process temperature at a defined “safe” rate. Both shaped (bricks) and unshaped (“monolithic”) products may be secured to shells by metallic or ceramic anchors. Brick linings in circular furnace structures may use special shapes (“keys” and “arches”) to turn a circumference.
Refractories are usually “classified” by the minerals used in their constitution. This gives rise to fireclay, high alumina, and basic types (magnesia, dolomite, etc.). Usually, fireclay and alumina refractories are used when acid or neutral corrosives/slags are present, while basic refractories are used when slags contain a significant amount of lime (calcium oxide).
Refractory “sintered” bricks are composed of aggregates and a intergranular “matrix” phase that gains strength and other useful physical properties by being heated/fired to temperatures well above 1000 degrees Celsius (1800 degrees Fahrenheit). Some refractories are fusion cast or contain fused aggregates. Refractory fiber products are based on alumino-silicate glasses and may contain additives to extend their upper temperature limit or increase their alkali resistance. Carbon/graphite and silicon carbide are special refractories, and their performance varies over a wide range of compositions and products.
Fundamentals of Performance
A fundamental property of refractories is their resistance to “shrinkage” or dilation when heated either in an unloaded or loaded/stresses condition. In most cases, the refractory is not supposed to shrink at the process temperature in the furnace. Various ASTM tests are employed to demonstrate the refractoriness of products. Some refractories including insulating products tend to shrink or creep near their use temperature, so a thorough understanding of ASTM Standards for the products is required.
Refractories are also exposed to corrosive materials in the process to include alkali vapors, slags, metals, glass, and other species. For maximum resistance to corrosion, lowest open porosities are usually desirable.
Refractory products also experience thermal shock and exhibit thermal spalling with rapid changes in process temperatures and unexpected furnace shut-downs.
Refractory mortars are usually thin applications of material intended to “seal” voids – as opposed to structural/clay bricks that typically utilize 3/8″ to 1/2″ mortar joints. Refractory mortars may be dry applied or used with trowel or slurry applications. Mortars have a duty or use limit as if found with bricks or monolithics. Some mortars can contain eye burn and respiratory hazards.
In today’s global situation, many refractory products are imported into North America from developing countries. Unfortunately, some of these products exhibit wide variability and may not meet ASTM Standards.
Any refractory forensic investigation usually involves the question, “does the refractory meet the typical chemical and physical properties given on the product data sheet?”
From these, a good understanding of the process/furnace is required. Forensic techniques include advances analytical investigations and microscopic studies.
Brickexpert.org has investigated the following general types of refractory failures:
- Supplied materials not meeting product data sheet values.
- Inadequate thermal expansion allowance exacerbating failures.
- Improper product recommendations.
- Refractory anchor failures.
- Inadequate curing or improper preheating of process vessels.
- Hot spots, bricks falling out, roof/crown collapse.
- Excessive slag corrosion.
- Alkali attack.
- Gas corrosion, particularly highly reducing atmospheres.
- Excessive shrinkage, joints opening up.
- Furnace/kiln explosions.
- Excessive thermal shock loss.