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.

渗碳是低碳钢的常见热处理工艺，但是渗碳深度的无损检测是一个尚未解决的热点问题。这项研究的目的是使用非线性超声表征渗碳渗层深度。采用渗碳法制备了渗碳层深不同的20钢和20CrMnTi钢样品，并用金相显微镜观察了热处理前后的显微组织。设计了穿透性纵波，临界折射纵波（LCR），折射剪切波和瑞利波的四种非线性超声检测模型。结果表明，在LCR波探测模型下，20钢和20CrMnTi钢的超声非线性系数均随渗碳层深度的增加而单调增加，非线性系数的增加与马氏体相变和表面应力的形成有关。这表明可以使用LCR波的超声非线性系数来估计渗碳渗层深度。