Thermal cycling: the real endurance test for refractory linings

The problem of thermal fatigue

Refractory materials are designed for high temperatures. They withstand 1,200, 1,400 or even 1,800 degrees Celsius without losing their structural integrity. What many operators underestimate is that the greatest enemy of a refractory lining is not the absolute peak temperature, but the repeated change between hot and cold. Every temperature change generates mechanical stresses in the material. The hot side expands while the cold side still has its original dimensions. This differential expansion produces shear stresses that work their way a little further into the material with every cycle.

Mechanisms of thermal damage

Thermal damage proceeds in several phases. First, fine microcracks form at the surface which are barely visible to the naked eye. With every further temperature change these microcracks grow deeper and join up into a network of cracks that divides the surface into individual segments. At an advanced stage these segments detach from the surface as spalling and the loss of material accelerates exponentially. In technical language this process is referred to as thermal fatigue or thermal shock.

• Heating phase: rapid temperature rises create compressive stresses at the surface and tensile stresses inside the material
• Cooling phase: the stress conditions reverse. The surface is under tension, the interior under compression
• Repetition: every cycle enlarges existing cracks and creates new ones. The damage is cumulative and irreversible
• Acceleration: beyond a critical crack density the heat loss rises, the surface temperature falls, the temperature gradient becomes steeper and the damage accelerates itself

Material selection for areas under thermal shock

The thermal shock resistance of a refractory material is determined by several material properties: low thermal expansion, high thermal conductivity, high fracture toughness and moderate strength. Materials with a high silicon carbide content or certain phosphate-bonded systems show excellent thermal shock behaviour. The choice of the right material always has to be made in the context of the specific operating conditions: how frequent are the temperature changes? How large is the temperature difference? How quickly do heating and cooling take place?

Operational measures to reduce thermal shock loading

Besides the choice of material, operational measures can reduce thermal shock loading considerably. Controlled heating and cooling rates are the single most effective measure. Wherever possible, a furnace should never be heated up or cooled down faster than 50 degrees per hour. Avoiding unnecessary shutdowns and optimising charging planning also help to protect the lining. A furnace operated as steadily as possible at a constant temperature achieves the longest service life.