Short term overheating occurs in the absence of normal cooling effects caused by steam/water contact. The tube metal temperature quickly rises as a result. The tube surface bulges and eventually ruptures in a matter of seconds to minutes.

  

Tubes that have experience short term overheating have a thick, brittle, and dark oxide scale on external surfaces. Short term overheating produces considerable tube deformation in the form of metal elongation and reduction in wall thickness.  A fish-mouth type, longitudinal rupture with thin-edged fracture surfaces is typical for ferritic steel when it has failed before reaching its upper critical temperature. Microstructure examination can reveal evidence of grain plastic deformation near the fracture face and phase transformation to a hardened microstructure. This transformation occurs due to rapid overheating and quenching during rupture. Tubes exhibit relatively thick ID oxide scale at the failure locations along with possible hard quenched microstructure(s).