Nonlinear ultrasonic characterization of carburized case depth

Abstract Carburizing is a common heat treatment process for low carbon steel, but non-destructive detection of carburized case depth is a hot issue that has not been solved. The aim of this study was to characterize the carburized case depth using nonlinear ultrasonic. Samples of 20 steel and 20CrMnTi steel with different carburized case depth were prepared by gas carburizing method, and the microstructure before and after heat treatment was observed by metallographic microscope. Four nonlinear ultrasonic detection models of penetrating longitudinal wave, critically refracted longitudinal (LCR) wave, refracted shear wave and Rayleigh wave were designed. It was found that the ultrasonic nonlinear coefficients of both 20 steel and 20CrMnTi steel increase monotonously with the increase of carburized case depth under the LCR wave detection model, the increase of nonlinear coefficient was related to martensite transformation and the formation of surface stress. This indicates that there is possible to use the ultrasonic nonlinear coefficients of LCR wave to estimate the carburized case depth.