On the cyclic behaviour of fusion reactor ferritic-martensitic steels at intermediate temperatures

The low cycle fatigue response of EUROFER 97 has been investigated at different total plastic strain ranges and temperatures between 20-450 C. The cyclic behaviour has been analyzed through the decomposition of the flow stress in back stress and friction stress, and different microstructural mechanisms associated with cyclic softening were proposed to explain the differences observed in those stress components. At 20 C, the cyclic softening could be attributed to a progressively unpinning of dislocations from carbon atoms. At higher temperatures, the mechanical softening effects agree with the evolution from initially very fine martensitic laths to a coarser equiaxial cells structure. A decrease of dislocation density inside the micrograins is observed for all the temperatures, to a higher degree for higher temperatures. (orig.)