Nonlinear ultrasonic characterisation and microscopic modelling of pearlite spheroidisation damage in 15CrMo steel

This study employs a nonlinear ultrasonic testing technique to evaluate spheroidisation damage in 15CrMo steel. The experimental results demonstrate that as the degree of spheroidisation increases, the acoustic nonlinearity parameter exhibits a trend of accelerating decline, with a total reduction of 15.6%. Based on metallographic analysis, the spheroidisation damage of pearlite is classified into five characteristic stages, including micro-damage, complete fracture, multiple fractures, complete spheroidisation, and carbide segregation. Using this characterisation, a microscopic acoustic model for pearlite spheroidisation was developed to investigate the ultrasonic nonlinear effects arising from interfacial damage. Molecular dynamics simulations indicate that spheroidisation damage reduces the acoustic nonlinear parameter by 3.4%. When scaled to match the experimental pearlite area fraction, this reduction becomes 16.0%, showing excellent agreement with the measured data. This decrease is attributed to the reduction in the cementite lamellae interfacial area and the carbide segregation. This study utilised experiments and microscopic simulations to demonstrate the potential of nonlinear ultrasonic techniques for characterising spheroidisation damage in pearlitic steel.