Ultrasonic Time-of-Flight Diffraction Testing with Linear Frequency Modulated Excitation for Austenitic Stainless Steel Welds

Abstract

The main problems for ultrasonic inspection of austenitic stainless steel welds are low signal to noise ratio (SNR) and detection accuracy, caused by anisotropic and heterogeneous material structure with bulky columnar crystal. In order to overcome the problems mentioned above, a linear frequency modulated (LFM) pulse with a large time-bandwidth product can be used as the exciting signal in the ultrasonic inspection of the austenitic stainless steel welds, which can both enhance the average transmitted power and increase the SNR and time resolution by pulse compression process. Therefore, the ultrasonic time-of-flight diffraction (TOFD) technique with the LFM excitation was carried out to realize an accurate sizing and locating detection in austenitic stainless steel welds. Compared with the conventional TOFD testing, the SNR can be increased 12–15 dB, and the measured accuracy is also enhanced obviously. Accordingly the measured depth error is less than 0.81 % and the length error is less than 2.5 %. The proposed method can both enhance the capabilities of the ultrasonic inspection of austenitic welds and the measurement accuracy of ultrasonic TOFD testing.