Mechanical and microstructural behaviour of isothermally and thermally fatigued ferritic/martensitic steels

Isothermal low cycle fatigue (LCF) and thermal low cycle fatigue results in the temperature range between room temperature and 823 K of the quenched and tempered reduced activation ferritic/martensitic steels F82H mod. and EUROFER 97 are reported. Under these test conditions both steels show, after the first few cycles and for both types of tests, a pronounced cyclic softening up to failure. The softening during LCF tests described by a simple empirical relationship is dependent on temperature but independent of the total strain amplitude of the tests. From the analysis of the hysteresis loops and corroborated by electron microscopy observations it can be concluded that the cyclic softening is produced by the softening observed in the internal stress as a consequence of the evolution of the microstructure. During cycling, the martensitic lath structure with high dislocation density and carbides along the lath interfaces evolves to a softer dislocation subgrain structure. This conclusion could be correlated with transmission electron microscopy observations.