R. Klueh (Sp), N. Hashimoto, P.J. Maziasz, Oak Ridge National Laboratory, Oak Ridge, TN (USA)
Martensitic steels are presently the preferred structural materials for elevated-temperature applications for fusion power plants. Their major advantage is good thermal properties relative to other elevated-temperature alloys. A major shortcoming is high-temperature strength, which places a limit on the maximum operating temperature. The reduced-activation steels developed for fusion applications have maximum operating temperatures of 550-600ºC. However, for increased efficiency of a fusion plant, designers require higher operating temperatures. This has led to work to develop oxide dispersion-strengthened (ODS) steels, which were introduced in the 1960s. These steels, strengthened by small oxide particles, are produced by complicated and expensive mechanical-alloying, powder-metallurgy techniques, as opposed to conventional processing techniques (casting followed by rolling, extruding, etc.) used for present-day elevated-temperature steels. ODS steels are also plagued by anisotropy in mechanical properties caused by the processing. Therefore, the need exists for elevated-temperature steels with higher operating temperatures that can be produced by conventional processing techniques (i.e., melting, casting, hot working, cold working, etc.)
Based on the science of precipitate strengthening (the need for large numbers of small precipitate particles) and using thermodynamic modeling to explore possible optimum compositions, a thermomechanical treatment (TMT) was developed that increased yield stress of commercial nitrogen-containing martensitic steels 88% at 700ºC. Preliminary creep-rupture tests indicate a commensurate increase in rupture life. Steels designed and produced specifically for the TMT have yield stresses at 700ºC up to 200% greater than conventional steels. Characterization of the precipitates in the new steels by transmission electron microscopy indicated the precipitates were eight-times smaller at a number density over three orders of magnitude greater than in the conventional steels they would replace.
