At elevated temperatures and stresses, much less than the high-temperature yield stress, metals undergo permanent plastic deformation called creep. Creep may be defined as a time-dependent deformation at elevated temperature and constant stress. The temperature at which creep begins depends on the alloy composition. Creep failures occur from an extended period of slight overheating above the design metal temperature, a slowly increasing level of stress, or the accumulation of periods of excessive overheating (during start-ups, for example). The ASME Boiler and Pressure Vessel Code recognizes creep and creep deformation as a high-temperature design limitation, and provides allowable stresses for all alloys used in the creep range. One of the criteria used in the determination of these allowable stresses is 1% creep deformation in 100,000 hours of service. Thus, the code recognizes that over the operating life, some creep deformation is likely.

  

High temperature creep produces a longitudinal fracture. The extent of the fracture and its appearance may vary. A small fracture will form a blister like opening, while a large fracture creates a wide fish mouth appearance.  The fracture surface is thick-edged with extensive secondary cracking adjacent to the primary fracture. Creep damage appears as intergranular voids near the fracture face. The voids are generally oxide coated due to the time scale associated with this failure.