| As narrow-gap TIG welding has advantages of narrow gap weld seam, high quality weld and short welding period, it has replace the traditional shielded metal arc welding in nuclear power plant coolant pipe. Ultrasonic testing with TRL (Transmit-Receive Longitudinal Angle-beam Probe) is widely used in non-destructive testing for primary coolant in Pressurized water reactors. Compared with the weld of shielded metal arc metal, the distance of wave propagated is longer than in the weld of narrow-gap TIG welding. Take a example of weld seam of66mm-thick CCASS (Centrifugal Casting Austenitic Stainless Steel, CCASS) with narrow-gap TIG welding, when using a TRL probe to detect a30mm-depth defect with incident angle of45°, the distance of wave propagated in base metal (37mm) is far longer than in weld (45mm). Meanwhile, as the CCASS coarse-grained are heterogeneity and elastic anisotropy, thus causes ultrasonic beam distortion and serious scattering attenuation, reducing the signal-to-noise ratios. With the advantages of good visibility and easy parameter adjustment, the numerical simulation has great significance because it can simulate ultrasonic propagation in materials and improve detection efficiency when combined with experiments. As the present study is major in the modeling of weld and the research on the base metal is less studied, this study take a research on the modeling of CCASS base metal with Voronoi diagram of Z3CN20-09M austenitic stainless steel and numerical simulation with finite difference method. The main contents are as follows:(1) The CCASS macroscopic structure indicates that the base metal is mainly composed of columnar region and equiaxed grain region. The equiaxed grains are in different sizes and show clear stratifications. The macroscopic structure analysis of96mm-thick CCASS base metal shows that the aspect ratio of columnar grains is approximately11.5and the equiaxed grains form outside to inside are with different average diameters of1.2mm,1.8mm and2.8mm with measurements.(2) The acoustic parameter measurements of CCASS show that the average longitudinal velocity of left base metal of the weld is5714m/s, while the right base metal is5770m/s. The different value of two sides is56m/s and the fluctuation of sound velocity is0.98%. The attenuation coefficient velocity of left base metal of the weld is0.098dB/mm, and the right base metal is0.074dB/mm. The different value is0.024dB/mm and the fluctuation of attenuation coefficients is a great value of24%. (3) Based on the Voronoi diagram, the columnar grain model and the equiaxed grain model CCASS are respectively constructed using MABLAB, and the base metal model of CCASS is also constructed on the base of these models, the comparison of the simulated and experimental time domain waveforms, longitudinal wave velocity and attenuation coefficient show good agreements to the experiments, which verifies the feasibility of the modeling method. Then the Φ5mm CCASS hole defect model is constructed on the base of the CCASS base metal model. The waveform of simulation and experiment show good agreements. Finally, the66mm-thick CCASS defect model with bottom groove height of1～40mm is also constructed and the simulation shows that the amplitude of the echo signal of groove signal is increasing with the increase of groove height. Once the amplitude of echo reaches to the peak, if the height of groove continues to increase, the amplitude of bottom groove echo tends to a fixed value. |
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