High temperature damage as an indicator of operating conditions and residual creep life

Damage due to creep and associated mechanisms is typically life-limiting in the pressure retaining hot sections of power boilers, requiring inspections to support the condition and life assessment. Since a single local failure can result in a shutdown, it is important to find the critical hot spots that will determine the time to next failure, and to evaluate the condition at those spots. This paper aims to demonstrate the characteristic damage in terms of in-service condition and related clues for life assessment. In particular, the appearance of microstructural degradation, internal oxidation and intergranular creep cavitation can indicate the path and extent of deviations from the expected status in components. Examples are shown on ferritic boiler steels, with degradation and damage proceeding through somewhat comparable stages regardless of the material grade. Hence although there are large differences e.g. in the creep strength between alloys, the operating conditions should largely compensate for the differences in the mechanical properties, and the predicted life fraction is not a strong function of the material grade for the same level of in-service damage. However, uncompensated residual differences between materials also occur and are discussed with examples