Considerations for design with cyclic softening materials in fusion reactor applications

Bainitic and martensitic steels have received considerable attention as structural materials in fusion reactor systems. Both magnetic and inertial confinement systems are expected to operate in a cyclic (pulsed) mode. While design stresses are usually low, certain physical or metallurgical stress concentrations can lead to changes in local materials properties inducing high local damage during cyclic loading. In particular, the bainitic and martensitic steels lose a considerable amount of their initial high strength when subjected to cyclic loading. Limitations in design with these cyclically softening materials are examined in this paper. The effects of strain range, fatigue cycles, hold times, strain rates, and environment on the elevated temperature fatigue behavior of bainitic 2.25Cr-lMo, modified 9Cr-lMo, and HT-9 are evaluated. These alloys are all found to cyclically soften, and these changes can also influence materials creep strength at elevated temperatures, leading to unacceptable strains and premature creep failure.