Volume 8, Issue 8 (September 2011)
Creep Fatigue Behavior of Creep Strength Enhanced Ferritic Steels
Creep Fatigue knowledge is necessary for meaningful reliability assessment of existing power generating plant because cyclic operation of an aging power plant fleet requires better understanding of factors affecting damage initiation and propagation, as well as technologies and methods to predict damage accumulation for individual components and overall safe/economic life. Utilities typically adopt operating practices which involve more severe cycles, either in terms of number or magnitude or both, so that there is, and will continue to be, a requirement to assess the creep fatigue performance of traditional boiler and turbine components. The recent installation of large numbers of combustion turbines and the associated heat recovery steam generators has significantly increased the number of materials and components which are operating under cyclic conditions at high temperatures. Indeed, the materials and component types susceptible to creep-fatigue is increasing since much of the new high efficiency coal plant will be operating with steam/metal temperatures much higher than those of traditional plant. Recent expert review has reemphasized that with most practical scenarios regarding future generation mix, fossil plant will continue to be important. Thus, there is the expectation that creep-fatigue damage will increase in occurrence, and affect an increased range of components. Different challenges will be faced since the new components will involve a greater range of alloys, methods of manufacture, and types of operation than conventional fossil generating stations. It is therefore apparent that operation under cyclic conditions at temperatures where creep can occur the potential for creep/fatigue damage is, and will continue to be, a matter of significant concern. The present paper considers specific challenges associated with creep/fatigue in creep strength enhanced ferritic steels such as Grade 91 and Grade 92.