Preliminary Study On Non-destructive Testing Of 45# Steel Quench Depth Based Ultrasonic

Non-destructive characterization of quenched depth has important significance in both performance improvement and life extension of the components. Ultrasonic NDT is the main method for identification of organizational structures. However, it is difficult to realize the hardening layer quantitative depth characterization as the echo signal characteristic information extraction in hardening layer is hard to identify, which are influenced by ultrasound imaging characteristics, the complex internal organizational structure of material and detection system. This paper emphasizes on the structural properties of hardening material, sound propagation mechanism, experimental design and data processing on echo signal via describing the interaction between ultrasonic waves and hardening material organization. And ultimately implement the hardening layer quantitative depth characterization.Firstly, do research on the crystallinity and the microstructure transformation before and after hardening of 45 steel. By analyzing material crystallization process and organization transition during hardening process can be achieved based on iron-carbon alloy phase diagram. And the depth of hardening layer of the test piece can be measured through analyzing the composition and organization transition of hardening material based on microscopic metallographic method. Combining with three orthogonal surface speed of sound measurement and microscopic metallographic method, the measured 45 steel can be simplified as arbitrary randomly oriented, equiaxed, flexible multi-phase polycrystalline material model.Secondly, do research on ultrasound hardening layer detection theory. Characteristic quantities and propagation mechanism of ultrasound in an elastic medium are sketched,and the elastic wave characteristic in an isotropic medium is analyzed by using wave equation. Simulating the multilayer material in acoustic wave propagation using finite element software, and studying particle backscatter mathematical based rigid sphere model.Thirdly, do research on experimental design in ultrasound hardening layer detection of 45 steel. The hardening layer detection system can be built and the parameters can bedetermined through utilizing OmniScan MX2, thus the related experiment results processing can be carried out.Finally, data processing in collected echo signals from the experiments. By the time domain filtered signal analysis, frequency domain analysis and time-frequency domain analysis, the grass-like echo in echo signal can be simplified as superimposed signal of instrument noise and microstructure scattering signal. The hardening layer depth characterization can be obtained through both the analysis in the corresponding relationship between frequency components of echo signal and sound path of acoustic wave, and the comparison in power spectral density of echo signal between hardening and unhardened test block to measure quenched depth.As the research shown, ultrasound can be used for characterizing 45 steel hardening layer depth. And the 10 MHz probe has a better outcome than 20 MHz probe in the hardening layer detection. Data analysis also indicates water immersion focused probe has a better effect in the hardening layer characterization compared with unfocused probe,and the characterization has high accuracy when probe focal point is within the interior hardening layer. On the contrary, the method can not characterize quenched depth.