Nondestructive evaluation of spheroidization grades based on entropy characteristic parameters method

Due to its excellent mechanical properties, 15CrMo steel is widely used in critical components exposed for hightemperature and high-pressure conditions. Long term high-temperature will increase the spheroidization possibility and even cause serious safety accidents. However, characterization of the spheroidization grades is a very difficult problem. To quantitatively evaluate the spheroidization grades of 15CrMo steel, destructive testing methods are used to determine spheroidization grades. This study uses the ultrasonic backscattering method to detect the spheroidization grades. The ultrasonic testing platform collects the backscattering signals, and the typical characteristic parameters and entropy characteristic parameters of the backscattering signals are extracted. New entropy characteristic parameters, including the information entropy, conditional entropy, sample entropy, fuzzy entropy, permutation entropy, approximate entropy, power spectral entropy, and singular spectrum entropy, are introduced to evaluate the spheroidization grades of 15CrMo steel. It is found that the proposed entropy characteristic parameters can reflect the changes in the microstructure under different spheroidization grades. Therefore, the entropy characteristic parameters of ultrasonic backscattering signals are advantageous for evaluating the spheroidization grades of 15CrMo steel.