| Austenitic stainless steel is widely used because of its excellent performance. Hot cracks easily emerge in welding process, so Non-Destructive Testing (NDT) is highly necessary for the reliability of welding structure. Ultrasonic testing is a kind of NDT applied widely in industry for its reliability and easily operating. But in the weld joint of austenitic stainless steel the microstructure is extraordinary coarse grains. Noise due to serious scattering of ultrasonic makes it difficult to identify defects. Consequently noise suppression using appropriate method becomes critical in ultrasonic testing of austenitic stainless steel.Ultrasonic Time of Flight Diffraction (TOFD) is a kind of ultrasonic NDT with high efficiency, good reliability and accurately positioning defects. In this research, TOFD is investigated and applied to test austenitic stainless. Simulating samples using in ultrasonic test were designed and machining originally. The examining parameters were determined by theoretical analysis and experiments. The influence of microstructure on TOFD testing was investigated by examining the base metal and weld joint in carbon steel and austenitic steel respectively. Weld structure noise signal was identified by destruction experiments. Defects signal and noise signal were extracted for subsequent signal analysis.By processing and analysis of defects signal and noise signal, it is proved that simple processing with time domain and frequency domain of ultrasonic testing results in austenitic stainless steel is bad and not appropriate. Therefore combination of time analysis and frequency analysis is applied. Here, the improved nonlinear wavelet threshold denoising method (NWS) is used to process testing images. The result demonstrates that improved NWS has excellent denoising effects in weld joint of austenitic stainless steel. Meanwhile it is found that there is obvious difference in results obtained using probes with varied frequency. |
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