Ultrasonic backscattering model of lamellar duplex phase microstructures in polycrystalline materials

Carbon steel and low alloy steel are pearlitic heat-resistant steels with a lamellar microstructure. There are good mechanical properties and are widely used in crucial components of high-temperature pressure. However, long-term service in high-temperature environments can easily lead to material degradation, including spheroidization, graphitization, and thermal aging. This study proposes a theoretical model of ultrasonic backscattering with a lamellar structure in pearlite areas. It analyzes the effects of different pearlite area ratios and interlamellar spacing on ultrasonic backscattering signals. A Voronoi diagram is used to constructs a two-dimensional finite element (FE) model of the lamellar structure, and the effects of different pearlite area ratio and interlamellar spacing on the backscattering signals are analyzed to verify the correctness of the theoretical model. By preparing spheroidization samples of various grades, the change values of pearlite area ratio and interlamellar spacing are measured. The backscattering signals of different spheroidization samples are collected through the ultrasonic testing experimental platform, and the root-mean-square (RMS) maximum values of the ultrasonic backscattering are extracted. The observed trend is consistent with the theoretical model, finite element method (FEM), and experimental. Compared with the experimental results, the model results have some errors, but can be used to evaluate the performance degradation of metallic materials with lamellar pearlite structure.