What is caused by the sudden cooling of boiler tubes after they have been overheated?

The phenomenon that occurs due to the sudden cooling of boiler tubes after overheating is primarily cracking. When boiler tubes are subjected to excessive heat, the material expands and may develop various forms of stress. If they are then cooled rapidly, the uneven contraction can lead to the formation of cracks in the material.

Warping typically refers to a distortion in the shape of a material rather than the formation of cracks. While overheating and subsequent cooling can contribute to warping in some situations, it is the cracking that is more directly associated with the thermal shock experienced by the strain in the material from the rapid temperature change.

Additionally, fatigue generally refers to the weakening of a material caused by repeated cycles of stress, and corrosion involves the gradual degradation of metal due to chemical reactions with the environment. Neither of these conditions arises specifically from the sudden cooling of overheated tubes in the same direct manner as cracking. Thus, the correct answer is indeed linked to the direct consequences of thermal shock, which results in cracking.