The pressure vessels of the primary circuit of French nuclear power plants are made by assembling low-alloy steel components, forged from high-tonnage ingots (> 100t) that solidify in a non-uniform manner. The high thickness of the component also implies that the evolution of temperature during post-forging heat treatments vary significantly depending on the position in the thickness of the component. These two effects contribute to producing heterogeneous microstructures that can significantly weaken the material.
The scientific objective of this thesis is to evaluate which elements within the microstructure are responsible, and in what proportion, for increased embrittlement of the material for certain unfavorable heat treatment conditions. Conversely, better identifying the range of heat treatment conditions for which this embrittlement of the material remains contained, for a given initial microstructure, is an objective with high industrial stakes. Several heat treatments have already been applied to coupons from a rejected industrial component before subjecting them to Charpy impact toughness tests, in the field of the brittle to ductile transition of the material. Instrumented mechanical tests will be conducted as well as advanced fractographic and microstructural analyses in order to identify the evolution of the nature of the initiation sites according to the heat treatment conditions. These elements will then be integrated into a local approach to fracture model developed specifically to account for the effects of microstructural variations on the resistance to brittle fracture of low-alloy steels.