Fatigue behaviour of the T91 martensitic steel at 300 deg. C in the lead-bismuth eutectic

The subject of this thesis aims at looking further into current knowledge on the interactions solid metal/molten metal and more particularly to study the fatigue behaviour of martensitic steel T91 in a liquid lead-bismuth environment. This study lies with the European project MEGAPIE-TEST (Megawatt Pilot Experiment) which aims to prove the feasibility of a lead-bismuth spallation target. We developed an experimental set-up which allowed fatigue tests experiments at 300 deg. C in the lead-bismuth eutectic (Pb 45%w, Bi 55%w). This eutectic contributes to decrease the fatigue life of the T91 steel and to increase the speed of cracking. The lead-bismuth eutectic changes the mechanisms of nucleation-growth of the short cracks through a suppression of effectiveness of the microstructural barriers. A change in the mode of cracking was also observed in the fatigue tests propagation when the crack tip is wet by the molten metal. We think that the molten metal modifies the crack tip by locating the plastic deformation. However, the harmful effect of the molten metal on the fatigue behaviour of material attenuates for low levels of stress and a pre-oxidation of the surface of material seems a promising way to explore to improve the fatigue resistance in the molten metal. (author)