PLC stands for Permanent Linear Change—the irreversible expansion or contraction of a refractory material after exposure to high temperature. It is one of the most critical properties to check before selecting a refractory, because a poor PLC value can lead to cracked linings, joint failure, or even structural collapse.
What PLC Actually Means
When a refractory is heated, two types of dimensional changes occur:
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Reversible change: thermal expansion that disappears upon cooling.
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Permanent change: the material does not return to its original size after cooling.
PLC is expressed as a percentage, with a negative value indicating shrinkage and a positive value indicating expansion.
Typical PLC values for common refractories (after firing at rated temperature):
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Clay bricks: –0.1% to –0.5% (slight shrinkage)
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High-alumina bricks: –0.3% to +0.5% (depending on grade)
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Magnesia-based bricks: 0% to +1.0% (often expand slightly)
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Castables (monolithics): –0.5% to +0.5% (after drying and firing)
Why PLC Matters
A refractory with excessive permanent change will cause serious problems:
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Excessive shrinkage opens gaps between bricks or between the lining and steel shell, allowing hot gases to bypass and overheat the shell.
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Excessive expansion creates compressive stress, leading to spalling, buckling, or crushed joints.
In monolithic linings (castables, gunning mixes), PLC determines whether the lining will pull away from anchors (shrinkage) or push against them (expansion)—both can cause premature failure.
How PLC Is Determined
PLC is measured by a standard test (e.g., ASTM C179, ISO 2477). A sample is heated to a specified temperature (typically the material’s rated service temperature), held for a set time, cooled, and measured. The result is reported as:
PLC (%) = (L_final – L_initial) / L_initial × 100%
A negative value = shrinkage.
A positive value = expansion.
What to Look For When Selecting Refractories
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Match PLC to application.
For tight-joint brickwork, choose materials with PLC as close to zero as possible.
For monolithic linings with anchors, slight expansion (positive PLC) is often preferred to maintain anchoring contact. -
Check PLC at actual operating temperature.
A material may show acceptable PLC at 1200°C but shrink excessively at 1400°C. Always verify the test temperature matches your process. -
Consider combined effects.
PLC is not an isolated property—it interacts with thermal expansion, creep, and hot strength. A material with near‑zero PLC can still fail if its thermal expansion is poorly matched to the steel shell.
