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Craze Cracking

Quench cracks form on the heat transfer surfaces when blasting relatively cold water onto the bare steel, resulting in sudden differential thermal contractions. Quench cracks are also referred to as thermal fatigue or craze cracks. The large temperature difference between the water and the tube metal causes rapid contractions of the affected area, resulting in fatigue cracks. Temperature fluctuations cause contractions or expansions in steels. If the temperature change is slow, then these changes in length occur gradually throughout the structure. However, if the temperature change is rapid, then temperature gradients are present and the material is internally stressed to accommodate the length differences. These stresses can fatigue the material or even tear it apart.  
External Appearance:
Scale-covered circumferential cracking consisting of multiple cracks that lay parallel to one another or perpendicular to the direction of maximum tensile stress. It is common that this condition is seen along with significant fire side wastage.

Probable cause:
Corrosion enhanced thermal fatigue, increased tube surface temperature, and cyclic stresses from load changes and startups. 
Corrective action:
Clean and inspect areas of the furnace to determine the presence of cracking.

This mainly occurs in supercritical units but can occur in sub critical boilers.  This condition is alternately known as circumferential cracking, horizontal cracking, transverse cracking, craze cracking, elephant hide, and alligator skin cracking.

Common Locations:
Waterwall Circuits