In what circumstance is radial cracking most likely to occur in boiler operations?

Radial cracking in boiler operations is most likely to occur due to thermal expansion because of the nature of materials and how they respond to temperature changes. As a boiler heats up, the metal expands, and if it does so unevenly, or if there are constraints that limit this expansion, stresses can build up in the material. These stresses can lead to cracks radiating outwards from a central point, hence the term "radial cracking."

Materials tend to expand when heated; however, if the constraints prevent equal expansion or if there are pre-existing flaws, the internal stress can surpass the material's tensile strength, resulting in cracks. This cracking is most commonly seen in areas where there are sudden changes in temperature or where there is a large thermal gradient, which can occur during the heating phases.

While high pressure, startup phases, and cold conditions may contribute to pressure stresses and other forms of fatigue, they are not as directly correlated with the specific mechanism of thermal expansion causing radial cracks as the thermal effects themselves. Therefore, understanding thermal expansion and its impact on materials is crucial in preventing radial cracking in boiler operations.