Hydrogen and Temper Embrittlement Effects on Fatigue Fracture Behaviour of 2.25Cr-1.0Mo Nuclear Reactor Pressure Vessel Steel

The Reactor Pressure Vessel known as RPV is an essential element of nuclear power plants. The vessel is a robust container with a thick wall that is designed to endure the internal pressure resulting from the activity of the reactor. This also serves a crucial function in establishing a requisite barrier to prevent the dispersion of radioactive substances into the surrounding environment. When evaluating the functional requirements, high strength low alloy (e.g., 2.25Cr-1Mo) steel is typically a suitable choice for fabricating the vessel. The working environment for the RPV is very harsh. Due to neutron bombardment the steel vessel, especially around reactor core, becomes hard and loses its ductility due to build-up of dislocations. Radiation can cause secondary and tertiary changesin microstructures. The working temperature (order of 300 ℃) also favours carbide precipitation as well as trace element segregation at various microstructural sites. The situation is further aggravated by the phenomenon known as hydrogen embrittlement (HE). The purpose of this study is to investigate how the classical temper embrittlement (TE) due to segregation of trace elements (like phosphorus and sulphur) alone or in combination with HE embrittlement changes the fatigue crack growth rate and morphologies of the fracture surfaces of 2.25Cr-1.0Mo pressure vessel steel under both pre- and post-thermal exposure conditions. The experimental findings indicate that both HE and TE mechanisms play an important role in augmenting the propagation of fatigue cracks, as well as altering the fracture morphology, which need to be considered in design and fabrication of less susceptible RPV via customizing the chemical compositions and various microstructural features of the steel used.