Characterisation and 3D modelling of Cast Duplex Stainless Steel microstructure: Application to ultrasonic wave propagation simulations

Abstract

Due to their high strength and corrosion resistance, Cast Duplex Stainless Steels (CDSS) are employed in the primary coolant piping of nuclear power reactors. In-service Non-Destructive Evaluations (NDEs) based on Ultrasonic Testing (UT) must be conducted to ensure their safe operation. However, accurately detecting and sizing flaws within CDSS components poses a significant challenge. This is primarily due to their manufacturing process, which results in metallurgical structures featuring coarse grains and complex microstructural features, including a dual-phase composition with various morphological scales. Thus, in these structures, ultrasonic waves undergo scattering at grain boundaries, leading to high attenuation and structure noise echoes that alter the inspection. Modelling these phenomena using 3D numerical simulation tools with a detailed description of the microstructure allows for a better understanding of the multiple wave/microstructure interactions by quantifying the influence of microstructural characteristics on NDE performance. This paper aims to present the results of numerical simulations applied to representative CDSS microstructures. Thanks to the use and development of numerical tools, virtual microstructures of these duplex steels can be generated with different levels of complexity in Representative Elements Volumes (REVs). These REVs are validated to varying scales through comparison with experimental metallurgical characterisations. They are then used to define the propagation media using dedicated Finite Element (FE) software to observe the impact of this microstructure on ultrasonic waves. These FE simulations will then characterise the effective homogeneous media by determining attenuation and phase velocity variations as a function of the wave frequency. The attenuation results obtained from these simulations are compared with experimental attenuation measurements for different frequencies.