Effect of thermal cycling on refractories: repeated heating and cooling primarily reduces resistance to which failure mode?

Introduction / Context:
Many furnaces operate in cycles: heat-up, soak, and cool-down. Repeated thermal cycling imposes thermal stresses due to temperature gradients and expansion/contraction mismatches. Understanding which failure mode is aggravated by cycling helps select materials with appropriate thermal shock resistance and microstructure.

Given Data / Assumptions:

• Refractory experiences multiple hot/cold cycles.
• No unusual slag chemistry is introduced by the cycling alone.

Concept / Approach:
Thermal spalling is the cracking, flaking, or exfoliation of refractory surfaces induced by thermal shock. Each cycle can propagate microcracks if the material lacks adequate toughness, low elastic modulus, or proper microstructural accommodation. While chemical corrosion (slag or CO attack) and hot deformation (fusion under load) are serious, they are not specifically caused by cycling; rather, they depend on chemistry and temperature under load.

Step-by-Step Solution:

Verification / Alternative check:
Thermal shock indices combine modulus, tensile strength, thermal conductivity, and thermal expansion. Materials with lower expansion, higher thermal conductivity, and adequate toughness show superior resistance to spalling in cyclic duty.

Why Other Options Are Wrong:

Common Pitfalls:
Blaming slag for surface cracking that actually began as thermal spalling; ignoring the role of controlled heat-up/cool-down ramps to mitigate shock.

Final Answer:
Thermal spalling (thermal shock cracking)