Thermal ageing of cast duplex stainless steel components is a concern for long-term operation of EDF nuclear power plants. The thermal ageing embrittlement results from the microstructural evolution of the ferrite phase (spinodal decomposition), and can reduce the fracture toughness properties of the steel. In addition, it is necessary to consider manufacturing quality and the possible occurrence of casting defects such as shrinkage cavities. In a context of life extension, it is important to assess the safety margins to crack initiation and crack propagation instability. One major input of the assessment methodology is the toughness value of the thermally aged component. Recent work conducted at EDF R&D to improve the accuracy and the conservativeness of the toughness prediction has led to the development of new prediction formulae.
The toughness prediction relies on three steps:
• estimation of the Charpy impact test values at 20 and 320°C using the chemical composition of the steel and the aging conditions (temperature and duration),
• estimation of the J-R curve at 20 and 320°C - defined by a power law J = CΔan - thanks to correlations between n and C and the Charpy impact test values,
• estimation of the J-R curve at any temperature between 20 and 320°C thanks to interpolation formulae.
The paper presents the experimental data used to develop the formulae, the formulae themselves and some elements of validation.