How To Improve The Thermal Shock Resistance Of Refractories

Thermal shock is the direct result of exposing the surface of refractory installations to rapid heating and cooling conditions which cause temperature gradients within the refractory blocks. Such gradients, in the case of uneven cooling or heating, may cause cracking. Thermal shock is one of the most important potential failure modes of refractory installations.

In many service conditions, refractories can undergo rapid temperature changes. These temperature fluctuations develop unequal thermal stresses, within the refractory, by causing either rapid expansion or contraction of a section of material. The failure occurs when the thermal stress exceeds the strength of the material in that mode of stressing. It is one of the common reasons of refractory lining damages, more dangerous, than chemical and mechanical tear and wear of the lining.

Thermal shock is a key property in refractory selection process. The most undesirable consequence of thermal shock is obviously spalling. Spalling is the loss of fragments or “spalls” from the face of a refractory brick or structure through cracking and rupture, which exposes inner portions of the refractory.

In many instances, a material properties or/and heat transfer conditions is taken to characterize thermal shock behavior of the refractories.

One of the most important parameters for thermal shock resistance is the coefficient of thermal expansion. Generally, the refractory with the lowest rate of thermal expansion (lowest coefficient of expansion) has the best thermal shock resistance. Inversely, the material with a high expansion has a low thermal shock resistance.

Aside from the thermal conductivity, another parameter which must be considered in thermal shock resistance, is the surface heat transfer coefficient.

Recommendations to improve the thermal shock resistance of refractories:
1) Use materials with a low thermal expansion coefficient, or a combination of a raw materials that would result in a low permanent linear contraction to reduce the thermally induced stresses.

2) Heat up or cool down the refractory line as slowly as possible. From an operational point of view, however, the faster heat-up or cool-down that can be achieved the less time is wasted in a furnace or vessel being not in operation and hopefully making money. These two considerations must be balanced: reducing downtime; against avoiding damaging the lining.

3) Use insulating refractories which play an insulation role and avoid too much weight and thickness.

4) Select refractories with high heat transfer coefficient.

Thermal shock resistance dictates refractory performance in many applications. The thermal shock occurs, then object temperature changes much in a short time. It can become a reason of a sudden failure of the lining at the very beginning. So it is important to take the thermal shock resistance into account when selecting and installing refractory linings.