CREEP RUPTURE FAILURES IN SUPERHEATER TUBES OF HIGH PRESSURE BOILERS.

A high input of heat or lack of sufficient steam flow in the tube caused relatively rapid creep deformation due to the biaxial stress (hoop stress and longitudinal stress) acting on the cross section with maximum value at the external skin of the tube. In the initial stages, the material bulged and gradually thinned down due to creep. The presence of stress at high temperature accelerated the building up of thick oxide layers at the external surface. This resulted in poor heat transfer from external surface to the steam. At a later stage longitudinal microfissures were created in the material due to rapid creep. Finally, the material failed by sudden bulging and bursting.