Which type of tubes are normally found to contain thermal cracks?

Superheated tubes are typically the ones that can contain thermal cracks due to the high temperatures and pressures they experience during operation. In a superheated state, the steam or fluid within the tube is at a temperature greater than its boiling point, which can lead to significant thermal stresses. This condition creates a scenario where rapid temperature changes, such as those that occur during start-up or shut-down, can lead to uneven expansion and contraction of the material. As a result, these tubes are more susceptible to developing thermal cracks.

Underheating and cold tubes usually do not generate the same level of thermal stress as superheated tubes. Insulated tubes are often designed to maintain temperature and prevent heat loss, but they do not inherently experience the same high thermal stresses that can contribute to the cracking seen in superheated tubes.