Research On Ultrasonic Simulation Modeling Of Austenitic Stainless Steel Weld In AP1000 Main Pipe

AP1000 belongs to third-generation pressurized water reactor nuclear power technology whose revolutionary character is the use of passive safety measurements, improving the security greatly. AP1000 main pipe belongs to class 1 nuclear device which is a significant part in pressure boundary of primary circuit reactor coolant system. AP1000 main pipe welding is a critical path in nuclear power station construction, and the welding quality influences on operation security of nuclear power units directly which cannot be ignored. Due to bulk columnar grain structure with complex gain orientations and elastic anisotropy of austenitic stainless steel weld of AP1000 main pipe, scattered noise is increased and detecting resolution and signal-noise ratio are decreased when using ultrasonic testing method. In order to solve the above problems, in this paper, a weld detection mathematical model was established based on grain characteristics of austenitic stainless steel weld of AP1000 main pipe, and the model was calculated by finite difference time domain method. Based on the validity verified by experiment, the influence of elastic anisotropic material on ultrasonic transmission was discussed. Besides, simulation of defects testing in weld was conducted and a signal processing mean was used to improve imaging quality and ability of defects testing. The main research work is as follows:(1) On the basis of understanding the produce mechanism of beam excursion and scattered noise when propagating in anisotropy media, metallographic structures were observed and acoustic parameters were measured. The study found that austenitic stainless steel weld was bulk columnar grain structure, however, the based metal was small isometric crystal structure. From micro metallographic graphs, weld was composed of columnar austenite and 8 ferrite which showed strong heterogeneity in columnar grain orientation. From Acoustic parameters measurements, structure noise and excursion of main frequency were more serious as the testing frequency increased. Velocity volatility, attenuation coefficient and SNR of bottom wave were 2.8%,0.082 dB/mm and 13.8 dB at 2.25 MHz.(2) A physical weld model was established based on grain orientation characteristics of austenitic stainless steel weld of AP1000 main pipe. And then a two-dimensional FDTD ultrasound simulation weld model was established which elastic stiffness matrix was calculated by Bond transformation. As a result, time-domain waveforms and ultrasonic B-scan images were calculated whose features were very close to those acquired from experiment. It could be seen from the simulation results that SNR difference value between simulation and experiment was 3.0 dB, velocity volatility difference was 50.2 m/s (0.88%), attenuation coefficient difference was 0.004 dB/mm (4.9%), main frequency was 0.04 MHz (1.8%).(3) An oblique-incident SAFT method with wedge was put forward. And some simulations were conducted with the model. In order to show advancement of oblique-incident SAFT method, the results were compared with those acquired from conventional B-scan and traditional SAFT method. The reconstructed images showed that defects at different depth of the weld could be detected effectively by oblique-incident SAFT method and the results held high detecting resolution and signal-noise ratio. Comparing with traditional SAFT method, of which?2 mm weld porosity at depth of 50 mm, API was decreased 1.27 and SNR was increased 8.05 dB. Then some typical volume and planer defects were added to the model. The simulation results showed that oblique-incident SAFT method can detect ?1 mm weld porosities and 2 mm height weld fusions. And the height of planer defects can be quantified and error reached to 0.8 mm. In addition, the amplitude ratio of the upper and lower end of defects could be used to make qualitative distinctions of volume and planer defects.