Spalling, and Other Ways a Lining Falls Apart

Spalling is when fragments break off the hot face, as opposed to the face gradually dissolving or wearing thin. There are three kinds, they have different causes, and the broken piece usually tells you which one you are looking at.

Spalling is not the same as wearing out

If the hot face is smooth, glassy, and uniformly thinner, that is corrosion. If it has lost chunks and left sharp or layered fracture surfaces, that is spalling. The distinction matters because the fix is different: corrosion is a chemistry or temperature problem, spalling is usually a stress problem.

Thermal spalling

Cause: steep temperature gradients and thermal cycling. The hot face wants to be a different size from the material a few millimetres behind it, the stress exceeds the strength, and a crack runs roughly parallel to the face until a piece lets go.

Evidence: relatively clean, fresh fracture surfaces, often a curved crack sitting a short distance behind the original face. Worst at doors, roofs, and charge zones.

Fix: shock-resistant grades with low thermal expansion and some forgiving porosity, controlled heating and cooling rates, and keeping cold charge off the lining.

Mechanical spalling

Cause: stress from restrained expansion or from impact. The lining grows on heating, has nowhere to go because the expansion allowance was too small or absent, and shears or crushes, usually at the joints.

Evidence: crushed and spalled edges at joints, bulging courses, corners broken off. No sign of chemical change on the broken faces.

Fix: proper expansion joints, correct joint thickness, and anchor spacing that lets the lining move as intended.

Structural spalling

Cause: penetration. Slag, alkali vapour, or metal soaks into the open pores and chemically alters a band behind the hot face. That altered band has a different thermal expansion from the parent material, so it shears off as a slab and takes the penetrated layer with it.

Evidence: a distinct discoloured or glazed layer on the back of the spalled piece, and a flat crack at the boundary between altered and unaltered material. Common in blast furnace, lime kiln, and alkali-rich service.

Fix: lower porosity to slow penetration, matched chemistry, or a denser sacrificial hot face. Sometimes this is simply the wear mechanism, and the answer is to plan the campaign around it.

Reading the broken piece

• Clean and sharp fracture: thermal or mechanical, and recent.
• Glazed or discoloured band with a flat crack behind it: structural, driven by penetration.
• Crushed and rounded edges: mechanical or creep.
• Smooth glassy face, uniform loss: corrosion, not spalling at all.

One more to know: fireclay brick with iron oxide in it can burst from within under carbon monoxide at around 400 to 500 °C, as carbon deposits catalytically in the pores. Low-iron or dense high-fired grades avoid it.

The fracture surface is a witness statement. A clean crack blames the temperature swing, a glazed layer blames the slag, a crushed joint blames the missing expansion gap. Read it before you reorder the same brick.

The short version

The pattern tells you the cause, and the cause tells you whether to change the material, the operating practice, or the lining design. If you have spalled samples and are not sure what they are saying, send us photos of the fracture faces and the zone.