Which type of damage causes deformation from semi-plastic flow of the tube metal due to mild overheat?

The type of damage described in the question is sagging. When a tube or pipe experiences mild overheating, the metal can undergo semi-plastic flow, leading to deformation primarily in the form of sagging. This occurs because the heat can weaken the structural integrity of the metal, causing it to lose its rigidity and sag under its own weight or pressure.

Sagging typically refers to the downward bending or drooping of a material as it loses strength when subjected to elevated temperatures, which doesn't reach the level of melting but is significant enough to cause changes in physical shape.

In contrast, bulging denotes a condition where the material expands outwards due to internal pressure or overheating, but this is not strictly due to a semi-plastic flow and is more aligned with conditions that exceed mild overheating. Cracking refers to the formation of fissures in the material, often as a result of thermal stress or fatigue, and corrosion involves the gradual degradation of material due to chemical reactions, not primarily due to overheating.