Signal Processing of Ultrasonic Testing of Hardened Layer Depth Based on Wavelet Transform Theory

The hardened layer depth of metal parts is one of the important factors to determine its wear resistance and fatigue strength. Ultrasonic backscatter method can realize the nondestructive testing for the hardened layer depth, but the backscattering signal is affected by noise, which leads to the inaccurate test results. In order to improve signal-to-noise ratio and detection accuracy, the wavelet threshold denoising method is used to process the backscattering signal and four threshold selection methods are adopted to compare the denoising results in this paper. The hardened layer depth before and after the signal processing is compared with the result measured by the traditional metallographic method. Ultrasonic backscatter experiments are performed on two kinds of material samples including 45 steel and 42Crmo. The result shows that the scattering echo signal submerged by noise can be revealed after the "rigrsure" principle of wavelet threshold denoising and the hardened layer depth obtained by this principle is closer to the metallographic method in both materials.