6. Conclusions and outlook

The design of future thermal power plants and nuclear reactors has prompted numerous studies into the behavior of martensitic tempered steels subjected to fatigue and/or creep at high temperatures (450-650°C). Since the 1980s, the instability of their microstructure under certain loading conditions (fatigue, creep) has been highlighted in the literature. This instability leads to softening, which needs to be understood and predicted.

More recently, the effect of loads characteristic of the operating conditions to which the components of future thermal or nuclear power plants are subjected has been studied (low amplitudes of cyclic deformation, long creep periods). Experimental data are obviously rarer. Creep durations of up to 200 kh at 500-600°C have been explored (i.e. some twenty years).

Significant cyclic softening is demonstrated, even...