1. State of the art

1.1 Mechanical behavior at high amplitudes

Martensitic steels are commonly used in thermal power plants due to their favorable thermal fatigue design characteristics (high thermal conductivity and low thermal expansion, respectively 28.5 W/m.K and 13.5.10 ^-6 /K at 500°C). Their mechanical behavior in fatigue and creep was therefore first studied under the effect of high amplitudes or stresses in order to reduce test times. Significant softening was observed during deformation at high temperatures (450-700°C).