Research On The Key Technology Of Ultrasonic TOFD Testing Of Austenitic Stainless Steel Welds

Austenitic stainless steel is widely used in the fields of aerospace, machinery manufacturing, petroleum and chemical industry because of its excellent corrosion resistance and mechanical properties. Stainless steel weld is a weakness in full building, and usually used in some severe environment such as high temperature, high pressure, corrosion, erosive and radioactive. In addition, due to the influence of welding parameters, it may appear incomplete fusion, lack of penetration, slag, porosity, shape defects and cracks or other defects in the weld seam, which seriously affect its performance. During the production process and service process, nondestructive testing needs to be used as a technology to find the defects and damages in the weld seam. Advanced nondestructive testing technology is an important guarantee for the safe operation of equipment.Ultrasonic detection technology is one of the most widely used technology for weld detection, and it has the advantages of strong penetrating power, wide detection range, high efficiency, portability and harmless to the environment. However, due to the reason that the austenitic stainless steel weld has the structure of coarse columnar grain, ultrasonic wave will appears strong noise and sound beam distortion in the weld, lead to low SNR of the detection signal, difficult quantitative and positioning defects. Ultrasonic TOFD is a method of using the diffraction wave generated from end of defects to detect and evaluate the defection, and it has the advantages of the quasi quantitative, high efficiency, strong anti-noise ability and intuitively detection results. Because of above reasons, the paper proposes ultrasonic TOFD method to detect the defects of austenitic stainless steel weld.This study analyzes the propagation characteristics of ultrasonic wave in the weld and the interaction mechanism between ultrasonic and columnar grain, and through the way of optimizing the detection parameters to reduce the adverse effects of ultrasonic scattering to detection signal. Moreover, Synthetic aperture focusing technique(SAFT) based on ultrasonic TOFD-D scan is used to suppress the noise signal, and the method can highly improve the quality of ultrasonic TOFD-D scan imaging. The research contents are as follows:Firstly, research on the mechanism of interaction between columnar crystal and ultrasonic wave.The relationship between the SNR of defect detection and the angle between the columnar grain and ultrasonic beam is revealed by experiment. It found in the TOFD detection of austenitic stainless steel weld that the SNR of defect detection has a relationship with the angle between the columnar grain and ultrasonic beam. Using the method of metallographic analysis, it showed that the SNR of defect detection decrease with the increase of the angle between the columnar grain and ultrasonic beam, when the angle is small, higher SNR can be acquired.Secondly, optimized the testing parameters and proposed the secondary wave Inspection based on ultrasonic TOFD(time of flight diffraction) method.When using primary wave to detect, considered the SNR of the test signal affected by the angle between the columnar grain and ultrasonic beam and the pressure distribution, the probe need to be arranged at the root of weld side, and according to the organization structure of the stainless steel weld, adjust the probe wedge angle and probe spacing to ensure the angle between the columnar grain and ultrasonic beam is small, to improve the sound energy distribution in the detection area. Because the root of the weld is located in the inner wall of the pipe or vessel, the probe cannot detect. The secondary wave method based on ultrasonic TOFD can be used when the probes cannot put on the side of the weld root, and the diffraction wave of defects in stainless steel sheet can be abstracted by the method. The probe can be arranged in the reinforcement to obtain a smaller angle between beam and column crystal, using the method of secondary wave detection to extract the signal of defect diffraction in the weld. The research show that the pulse of each feature detection signal should be tested, include that variant primary bottom wave will lead to a non-detection zone of inspection surface. Detection of the acoustic energy distribution is an important factor which influences the SNR. To ensure a high distribution of detected acoustic wave energy in the weld, the research try to use small angle probe during the detection process, and set small PCS spacing to carry out detection work.Thirdly, designed the SAFT algorithm based on TOFD-D scanning, analysis and optimizes the amplitude change tracking window, so as to improve the SNR of detection signal, and enhance the TOFD scan image resolution. At first, research on the TOFD-D scanning signal of stainless steel weld, which processed by SAFT. Then optimized the internal parameters of the synthetic aperture algorithm to research on amplitude change tracking window. Comparing the SNR of the middle and end of defects to get a conclusion that the Gauss window(a=4), which can improve the central defect detection signals of defects on the premise end position, and the effect is better than rectangular window and Gauss window(a=1), it strengthen the ultrasonic TOFD-D scan image display capability in the defection of stainless steel weld.The mechanism research shows that the columnar crystal will cause the ultrasonic beam attenuation and noise interference. When the angle between the columnar grain and ultrasonic beam is small, ultrasonic attenuation and scattering become small. Based on the conclusion, when choosing primary wave to detect the weld, the probe should be arranged in the root side of the weld. However, when the probes can only be put on the weld enforcement, the secondary wave must be picked up to detect the stainless steel weld to obtain the small angle between the columnar grain and ultrasonic beam. The secondary wave method based on ultrasonic TOFD-D can be used when the probes cannot put on the side of the weld root, and to ensure inspect in the condition of larger acoustic energy distribution in the weld, try to use small angle probe and probe space. In the work of signal processing, SAFT can well combined with the TOFD detection technology, through analyzing and optimizing the amplitude change tracking window, to improve the defect display ability of ultrasonic TOFD-D scanning image in stainless steel weld.