Creep–fatigue interaction and related structure property correlations of EUROFER97 steel at 550 °C by decoupling creep and fatigue load

Mechanical tests have been performed at 550 °C under vacuum on the ferritic–martensitic steel EUROFER97. These experiments included fatigue tests, creep tests and combined creep–fatigue tests. The latter showed significant cyclic softening in the fatigue stage and a remarkable break-down of creep strength in the creep stage. The cyclic softening behaviour was almost identical for all tests and therefore insensitive to the different strain amplitudes. SEM of the specimen’s fracture surfaces and free surfaces revealed that networks of coagulated surface cracks formed during creep–fatigue were not failure relevant. TEM imaging displayed a drastic drop in dislocation density, and a considerable formation of precipitates and subgrain-structures in all tests. Pure fatigue led to the strongest reduction of dislocation density, whereas creep–fatigue induced the most pronounced formation of precipitates. Obviously, the internal softening due to prior cycling led to accelerated creep. Hence, a modified damage model for creep–fatigue load cases was proposed.