Thermal spalling in furnaces: at which stage of operation is thermal spalling most likely to occur due to steep thermal gradients?

Introduction / Context:
Thermal spalling is the fracture or flaking of refractory surfaces caused by thermal stresses generated during temperature changes. Because ceramics have limited tensile strength and thermal shock resistance, the rate and uniformity of heating and cooling critically affect spalling risk.

Given Data / Assumptions:

• Furnace linings experience heat-up and cool-down cycles.
• Thermal gradients produce stress; tensile stress near the surface promotes spalls.
• We seek the stage most prone to spalling.

Concept / Approach:
On rapid cool-down, the hot interior remains expanded while the surface contracts as it cools, inducing tensile stresses at or near the exposed face. Ceramics are weak in tension, so this condition is especially damaging. While rapid heat-up can also cause spalling (compressive-to-tensile reversals and differential expansion), the classic and more frequent problem is during cooling because the outer skin sees strong tensile stress.

Step-by-Step Solution:
Relate cooling to tensile stress at the surface due to contraction.Recognize ceramic brittleness → low tolerance for surface tension.Conclude that spalling is mainly associated with cooling down.

Verification / Alternative check:
Thermal shock parameters (R and R′) and case studies show more failures attributed to quenching/cool-down events than controlled heat-ups, especially in silica and high-alumina linings without fiber back-up.

Why Other Options Are Wrong:
Warming up can cause damage if too fast, but is generally less severe given controlled ramping and compressive states.“Both” and “Neither” do not capture the principal mode observed in service.

Common Pitfalls:
Assuming symmetric behaviour in heating and cooling; ignoring thermal mass and conductivity differences across the lining.

Final Answer:
Cooling down