Ferritic-martensitic steels are the lead structural materials for next-generation nuclear energy systems. Due to increased operating temperatures required in advanced high-temperature reactor concepts, the high temperature performance of structural alloys and reliable high temperature structural design methodology have become increasingly urgent issues. Ferritic-martensitic steels experience significant cyclic softening at high temperatures, and this cyclic softening behavior affects consecutive stress relaxation response during hold time under creep-fatigue loading. It is found that the stress relaxation response during hold of the mod.9Cr-1Mo steel can be accurately described by a stress relaxation model. The creep damage associated with the stress relaxation during hold time can then be accurately calculated using the stress relaxation data and creep rupture data. It is shown that the unit creep damage per cycle in mod.9Cr-1Mo steel decreases considerably with increasing number of cycles due to cyclic softening, and the creep damage is sensitive to the initial stress of stress relaxation. Proper evaluation of the creep-fatigue damage in mod.9Cr-1Mo steel must consider the cyclic softening effect and its associated variations in creep damage from stress relaxation during the hold time.

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