The Performance of Creep-Strengthened Ferritic Steels in Power Generating Plant

Creep-strengthened ferritic steels, such as Grade 91, offer the potential for excellent high-temperature performance. To realize the benefits for these alloys requires careful control of original composition and manufacturing processes, such as welding and bending, as well as the associated heat treatments. Laboratory tests indicate that long-term lives may be below the original estimates made based on Larson Miller extrapolation. Furthermore, accelerated rates of damage accumulation in in-service Grade 91 components can occur due to a number of factors including: • Problems associated with design, for example with reinforcement at nozzles and with stress concentrations in piping systems. • Incorrect heat treatment, in addition to proper instrumentation appropriate heat treatment schedules should consider specific compositions. • Bending, problems may be introduced following both hot or cold bending. • High-temperature operation in tubing leading to excessive scale formation and overheating. • Type IV cracking in welds which results from the local reduction in the heat affected zone strength resulting from welding thermal cycles. Review of key information regarding the high-temperature performance of creep strengthened ferritic steels shows that the long-term creep strength may not achieve the levels expected from simple extrapolation of short term data. The problems experienced are highlighted with reference to actual case histories. The additional challenges associated with the development of creep-fatigue damage in high-temperature plant operated in a cyclic mode are also discussed.